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Numerical Investigation on the Thermal Behaviour of a LOx/LCH4 Demonstrator Cooling System

Numerical Investigation on the Thermal Behaviour of a LOx/LCH4 Demonstrator Cooling System aerospace Article Numerical Investigation on the Thermal Behaviour of a LOx/LCH Demonstrator Cooling System Daniele Ricci * , Francesco Battista and Manrico Fragiacomo CIRA, Centro Italiano Ricerche Aerospaziali, Via Maiorise, 81043 Capua, Italy; f.battista@cira.it (F.B.); m.fragiacomo@cira.it (M.F.) * Correspondence: d.ricci@cira.it; Tel.: +39-0823-623096; Fax: +39-0823-623100 Abstract: Reliability of liquid rocket engines is strictly connected with the successful operation of cooling jackets, able to sustain the impressive operative conditions in terms of huge thermal and mechanical loads, generated in thrust chambers. Cryogenic fuels, like methane or hydrogen, are often used as coolants and they may behave as transcritical fluids flowing in the jackets: after injection in a liquid state, a phase pseudo-change occurs along the chamber because of the heat released by combustion gases and coolants exiting as a vapour. Thus, in the development of such subsystems, important issues are focused on numerical methodologies adopted to simulate the fluid thermal behaviour inside the jackets, design procedures as well as manufacturing and technological process topics. The present paper includes the numerical thermal analyses regarding the cooling jacket belonging to the liquid oxygen/liquid methane demonstrator, realized in the framework of the HYPROB (HYdrocarbon PROpulsion test Bench) program. Numerical results considering the nominal operating conditions of cooling jackets in the methane-fuelled mode and the water-fed Citation: Ricci, D.; Battista, F.; one are included in the case of the application of electrodeposition process for manufacturing. A Fragiacomo, M. Numerical comparison with a similar cooling jacket, realized through the conventional brazing process, is Investigation on the Thermal addressed to underline the benefits of the application of electrodeposition technology. Behaviour of a LOx/LCH Demonstrator Cooling System. Keywords: liquid rocket engine; numerical analyses; thermal control; cooling jacket design; regener- Aerospace 2021, 8, 151. ative cooling; methane transcritical behaviour; electrodeposition technology; brazing process https://doi.org/10.3390/ aerospace8060151 Academic Editor: 1. Introduction Konstantinos Kontis In the last few years, an increasing interest has arisen in the utilization of the LO /CH X 4 Received: 14 April 2021 propellant combination for space propulsion applications as testified by the efforts spent by Accepted: 24 May 2021 several academic and research institutions, international agencies and private companies. Published: 27 May 2021 The utilization of the LO /CH combination for space propulsion applications provides X 4 many advantages, as indicated by several authors [1–3]: Publisher’s Note: MDPI stays neutral high specific impulse; with regard to jurisdictional claims in thrust-to-weight ratio performances; published maps and institutional affil- good cooling capability; iations. engine reusability and throttlability; fewer storage, handling and insulation concerns; reduced pollution impact on ground, atmosphere and space; compatibility with ISRU (in situ resource utilization) purposes for lunar/Martian missions. Copyright: © 2021 by the authors. These capabilities result in a large number of applications and missions enabled Licensee MDPI, Basel, Switzerland. by methane-based propulsion systems, from in-space systems (landing or descent ve- This article is an open access article hicles, service modules, etc.) to space launchers (main stages or upper stages). In fact, distributed under the terms and oxygen/methane couple represents a potential candidate to substitute hypergolic and conditions of the Creative Commons solid propellants in the future. Thus, its versatility makes methane a good candidate Attribution (CC BY) license (https:// for several applications, from in-space propulsion systems (service modules, landing or creativecommons.org/licenses/by/ descent vehicles, and ascent stages) to accessing to space (first stages of launchers or upper 4.0/). Aerospace 2021, 8, 151. https://doi.org/10.3390/aerospace8060151 https://www.mdpi.com/journal/aerospace Aerospace 2021, 8, x 2 of 21 Aerospace 2021, 8, 151 2 of 20 applications, from in-space propulsion systems (service modules, landing or descent ve- hicles, and ascent stages) to accessing to space (first stages of launchers or upper stages) [3]). Some important programs have recently been launched in Europe like the LYRA pro- stages) [3]. Some important programs have recently been launched in Europe like the ject (ASI/Italian Space Agency-Avio) [4], which provided the first impulse to the develop- LYRA project (ASI/Italian Space Agency-Avio) [4], which provided the first impulse to ment of future generation of VEGA upper stage engines, currently on-going due to AVIO the development of future generation of VEGA upper stage engines, currently on-going efforts [5]; Prometheus engine (100-t-thrust) project, led by CNES (Centre National due to AVIO efforts [5]; Prometheus engine (100-t-thrust) project, led by CNES (Centre d'Etudes Spatiales)/Airbus-Safran Launcher (France), is inserted in ESA FLPP Neo Pro- National d’Etudes Spatiales)/Airbus-Safran Launcher (France), is inserted in ESA FLPP gram (European Space Agency Future Launchers Preparatory Programme) and is devoted Neo Program (European Space Agency Future Launchers Preparatory Programme) and is to developing the future launcher family after Ariane 6 [6]. In the Russian Federation, devoted to developing the future launcher family after Ariane 6 [6]. In the Russian Federa- several projects, like Energomash, KBKhM (KB KhimMash/Volga staged combustion de- tion, several projects, like Energomash, KBKhM (KB KhimMash/Volga staged combustion rived engine with up to 10 t of thrust), KBKhA (KB KhimAvtomatika) and Starsem (So- derived engine with up to 10 t of thrust), KBKhA (KB KhimAvtomatika) and Starsem yuz), are historically active with this kind of issue and involved in several studies [7]. (Soyuz), are historically active with this kind of issue and involved in several studies [7]. Japan has reached high levels of readiness [8] through the consolidated cooperation be- Japan has reached high levels of readiness [8] through the consolidated cooperation be- tween JAXA (Japan Aerospace Exploration Agency) and IHI. In the United States, a lot of tween JAXA (Japan Aerospace Exploration Agency) and IHI. In the United States, a lot players are involved in several projects, oriented to different applications. It is worth re- of players are involved in several projects, oriented to different applications. It is worth membering t remembering he SpaceX the SpaceX Raptor Project [9], Bl Raptor Project ue-O [9],rig Blue-Origin in (240-t-thr (240-t-thr ust BE-4 ust engine BE-4 , based on engine, based staged combustion cycle) [10] and NASA, developing a 20-kN pressure-fed engine, in- on staged combustion cycle) [10] and NASA, developing a 20-kN pressure-fed engine, tended for the Morpheus lunar lander (Armadillo Aerospace, Mesquite, Texas, USA) [11]. intended for the Morpheus lunar lander (Armadillo Aerospace, Mesquite, TX, USA) [11]. Besides the aforementione Besides the aforementioned d initiatives, in It initiatives, aly there is a strong in Italy there is a str interest in chem ong interest in chemical ical space propu space pr lsopulsion ion issues issues and in t and he l iniq the uid liquid oxygen/ oxygen/liquid liquid methane c methane ombinat combination. ion. In fact In , tfact, he the Italian Ministry of University and Research is funding a specific program devoted to the Italian Ministry of University and Research is funding a specific program devoted to the consoli consolidation dation of current of curr teent chnologi technologies, es, methodologie methodologies s and m and anu manufacturing facturing capabilit capabilities ies as as well as the development of future propulsion systems. The program is named HYPROB well as the development of future propulsion systems. The program is named HYPROB (HYdrocarbon PROpulsion test Bench) and has been assigned to the Italian Aerospace (HYdrocarbon PROpulsion test Bench) and has been assigned to the Italian Aerospace Research Centre (CIRA) [12]. Among the different goals, the most significant objective Research Centre (CIRA) [12]. Among the different goals, the most significant objective is is the design, realization and testing of a LO /LCH demonstrator (DEMO), capable the design, realization and testing of a LOX/LCH4 dem Xonstrator (DE 4 MO), capable of 30 kN of 30 kN of thrust. An incremental approach strategy has been adopted to enlarge the of thrust. An incremental approach strategy has been adopted to enlarge the comprehen- comprehension of critical physical aspects through the design, manufacturing and testing of sion of critical physical aspects through the design, manufacturing and testing of specific specific test articles. Basic activities like the design and experiments on injectors or material test articles. Basic activities like the design and experiments on injectors or material char- characterizations have been accomplished. However, central issues were represented by the acterizations have been accomplished. However, central issues were represented by the experimental campaigns on igniters and subscale mono-injector engines (to investigate the experimental campaigns on igniters and subscale mono-injector engines (to investigate comprehension of supercritical combustion, heat release, injection and mixing phenomena, the comprehension of supercritical combustion, heat release, injection and mixing phe- etc.) as well the experimental studies on methane transcritical behaviour as depicted by nomena, etc.) as well the experimental studies on methane transcritical behaviour as de- Figure 1 [13]. picted by Figure 1 [13]. Figure 1. Logical steps of LOX/CH4 demonstrator (DEMO) development. Figure 1. Logical steps of LO /CH demonstrator (DEMO) development. X 4 The final object is represented by a 30-kN-class-thrust demonstrator. The baseline The final object is represented by a 30-kN-class-thrust demonstrator. The baseline thrust chamber assembly concept is depicted in Figure 2: it includes an igniter, an injector thrust chamber assembly concept is depicted in Figure 2: it includes an igniter, an injec- tor head with 18 injectors and a regenerative cooling jacket with 96 axial channels and inlet/outlet manifolds. In Table 1 the main parameters are detailed. Aerospace 2021, 8, x 3 of 21 Aerospace 2021, 8, 151 3 of 20 head with 18 injectors and a regenerative cooling jacket with 96 axial channels and in- let/outlet manifolds. In Table 1 the main parameters are detailed. 1—igniter; 2—injector head; 3—outlet manifold with collector; 4—thrust chamber including the cooling jacket; 5—inlet fuel collector and distributor. Figure 2. DEMO thrust chamber assembly view with the main components. Figure 2. DEMO thrust chamber assembly view with the main components. Table 1. HYPROB (HYdrocarbon PROpulsion test Bench) DEMO main performance parameters. Table 1. HYPROB (HYdrocarbon PROpulsion test Bench) DEMO main performance parameters. O/FO/F 3.4 3.4 PPCC 5. 5.55 MPaMPa CC Reaction efficiency 0.98 Isp 286 s Reaction efficiency 0.98 I 286 s sp Thrust 30 kN A /A 4.0 Thrust 30 kN A cc cc/At t 4.0 Based on a counter-flow architecture, methane enters the channels in the nozzle re- Based on a counter-flow architecture, methane enters the channels in the nozzle gion in liquid phase, is heated by the combustion gases along the chamber and, then, in- region in liquid phase, is heated by the combustion gases along the chamber and, then, jected into the combustion chamber as a supercritical gas through the injection head. The injected into the combustion chamber as a supercritical gas through the injection head. cooling jacket represents the most critical component: it is composed of an inner liner, The cooling jacket represents the most critical component: it is composed of an inner made of high thermal conductive materials (generally copper alloys), and by a close-out liner, made of high thermal conductive materials (generally copper alloys), and by a structure, made up of robust alloys, like Inconel or nickel, and consists 96 axial channels, close-out structure, made up of robust alloys, like Inconel or nickel, and consists 96 axial surrounding the combustion chamber. The design activities have been internally con- channels, surrounding the combustion chamber. The design activities have been internally ducted by means of in-house codes, as reported by [14], and supported by CFD analyses conducted by means of in-house codes, as reported by [14], and supported by CFD analyses (to characterize the thermal and fluid-dynamic behaviour of the cooling jacket) and (to characterize the thermal and fluid-dynamic behaviour of the cooling jacket) and thermo- thermo-structural verifications. In the process of developing the final DEMO version, a structural verifications. In the process of developing the final DEMO version, a firing test firing test campaign is needed in water-cooled mode to characterize the cooling jacket campaign is needed in water-cooled mode to characterize the cooling jacket behaviour and to behavio accomplish ur and t qualification o accompliof sh the qual innovative ification ofmanufacturing the innovative m method anufact (i.e., uring electr met oplating) hod (i.e., selected electroplating to join ) selected the copper to j liner oin the copp with the nickel er liner wi close-out. th the ni In fact, ckel CIRA, close- after out. In accomplishing fact, CIRA, aafter accomplishin specific technological g a spec activity ific techno , decided logical ac to substitute tivity, de the cided to planned subuse stitute the planned use of a conventional pr of a conventional proc ocess, based on brazing ess, of based on non-homogeneous brazing of non-h components, omogeneous with galvanic components, deposition with gal- of copper and nickel layers for the realization of the cooling jacket. In fact, large difficulties vanic deposition of copper and nickel layers for the realization of the cooling jacket. In were encountered in the repeatability of the brazing process for chambers with tens of fact, large difficulties were encountered in the repeatability of the brazing process for cooling channels working in tough conditions: they have to withstand pressure values up chambers with tens of cooling channels working in tough conditions: they have to with- to 16.0 MPa, are required to keep methane liquid as long as possible and huge thermal stand pressure values up to 16.0 MPa, are required to keep methane liquid as long as pos- loads (fluxes up to tens of MW/m ); walls should be as thin as possible for efficient thermal sible and huge thermal loads (fluxes up to tens of MW/m ); walls should be as thin as exchange and weight reduction purposes, as depicted in Figure 3a. According to the new possible for efficient thermal exchange and weight reduction purposes, as depicted in Fig- process, grooves in the liner part are milled and then overlaid with copper and nickel [15]: ure 3a. According to the new process, grooves in the liner part are milled and then over- in this way, channels are generated by a combination of two special galvanic depositions of laid with copper and nickel [15]: in this way, channels are generated by a combination of pure copper and nickel, as depicted by Figure 3b. two special galvanic depositions of pure copper and nickel, as depicted by Figure 3b. Aerospace 2021, 8, 151 4 of 20 Aerospace 2021, 8, x 4 of 21 Aerospace 2021, 8, x 4 of 21 (a) (b) (a) (b) Figure 3. Example of a typical liquid rocket engine (LRE) cooling jacket: (a) milled channels brazed onto the external close- Figure 3. Example of a typical liquid rocket engine (LRE) cooling jacket: (a) milled channels brazed onto the external close- Figure 3. Example of a typical liquid rocket engine (LRE) cooling jacket: (a) milled channels brazed onto the external out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. close-out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. The advantages are summarized in: (a) brazing and welding free process since cop- The advantages are summarized in: (a) brazing and welding free process since cop- The advantages are summarized in: (a) brazing and welding free process since copper per and alloys could be applied without thermal stresses and deterioration of base mate- per and alloys could be applied without thermal stresses and deterioration of base mate- and alloys could be applied without thermal stresses and deterioration of base materials rials and (b) high reliability because the process repeatability can be controlled, improving rials and (b) high reliability because the process repeatability can be controlled, improving and (b) high reliability because the process repeatability can be controlled, improving mechanical resistance, thermal and electrical conductivity of the deposited copper. Thus, memechanical chanical resist resistance, ance, ther ther mamal l and and elect electrical rical con conductivity ductivity of the of the deposited copp deposited copper er. Th . us, Thus, the use of this advanced process has been motivated by the possibility of avoiding any the use of this advanced process has been motivated by the possibility of avoiding any the use of this advanced process has been motivated by the possibility of avoiding any deterioration of base materials, and by a high level of repeatability and reliability [16]. deterioration of base materials, and by a high level of repeatability and reliability [16]. deterioration of base materials, and by a high level of repeatability and reliability [16]. However, this choice led to a plan to optimize activity on the cooling system arrangement However, this choi However, this choice ce led to a pla led to a plan n to optimi to optimize ze actactivity ivity on the coolin on the cooling g system system arrarrangement angement to adapt the brazed configuration to the electroplated one. At this point, the DEMO-0A to a to da adapt pt the bra the brazed zed confi configuration guration to the el to theectropla electroplated ted one. At thi one. At this s poi point, nt, the DEMO-0 the DEMO-0A A manufacturing phase has been completed and the product has been accepted by means of manufacturing phase has been completed and the product has been accepted by means of manufacturing phase has been completed and the product has been accepted by means of leak and proof test (Figure 4). The next step will be the integration with the injector head, leak and proof test (Figure 4). The next step will be the integration with the injector head, leak and proof test (Figure 4). The next step will be the integration with the injector head, alr already eady v validated alidated in in a a p pr reviou evious s f firing iring c campaign, ampaign, a and nd the the igniter igniter in in order order to to sta start rt the the final final already validated in a previous firing campaign, and the igniter in order to start the final test test a activity ctivity. . test activity. Figure 4. Pictures of DEMO-0A in the manufacturing phase and after the completion of realization phase [17]. Figure 4. Pictures of DEMO-0A in the manufacturing phase and after the completion of realization phase [17]. Figure 4. Pictures of DEMO-0A in the manufacturing phase and after the completion of realization phase [17]. Concerning the description of the cooling jacket thermal behaviour, in a regeneratively Concerning the description of the cooling jacket thermal behaviour, in a regenera- Concerning the description of the cooling jacket thermal behaviour, in a regenera- cooled LRE, the coolant is represented by the propellant. Moreover, in the case of cryogenic tively cooled LRE, the coolant is represented by the propellant. Moreover, in the case of tively cooled LRE, the coolant is represented by the propellant. Moreover, in the case of fluids like methane or hydrogen, the propellant behaviour in terms of phase and working cryogenic fluids like methane or hydrogen, the propellant behaviour in terms of phase cryogenic fluids like methane or hydrogen, the propellant behaviour in terms of phase conditions evolves rapidly in the cooling jacket. Thus, fluid is injected into the cooling and working conditions evolves rapidly in the cooling jacket. Thus, fluid is injected into and working conditions evolves rapidly in the cooling jacket. Thus, fluid is injected into system in liquid state (characterized by values of pressure and temperature higher and the cooling system in liquid state (characterized by values of pressure and temperature the cooling system in liquid state (characterized by values of pressure and temperature lower than critical values, respectively) and generally undergoes a “pseudo-phase change” higher and lower than critical values, respectively) and generally undergoes a “pseudo- higher and lower than critical values, respectively) and generally undergoes a “pseudo- from liquid-like state to a vapour-like one, as carried out by [18,19]. Thermo-physical phase change” from liquid-like state to a vapour-like one, as carried out by [18,19]. phase change” from liquid-like state to a vapour-like one, as carried out by [18,19]. properties change quickly around the near critical zone (T = 190.56 K and P = 4.59 MPa cr cr Thermo-physical properties change quickly around the near critical zone (Tcr = 190.56 K Thermo-physical properties change quickly around the near critical zone (Tcr = 190.56 K for methane), as reported in Figure 5a, but gradually if subcritical phase change phenomena and Pcr = 4.59 MPa for methane), as reported in Figure 5a, but gradually if subcritical phase and Pcr = 4.59 MPa for methane), as reported in Figure 5a, but gradually if subcritical phase are compared [20–22]. As aforementioned, methane thermo-physical properties inside change phenomena are compared [20–22]. As aforementioned, methane thermo-physical change phenomena are compared [20–22]. As aforementioned, methane thermo-physical the LRE channels may change rapidly and a heat transfer deterioration phenomenon properties inside the LRE channels may change rapidly and a heat transfer deterioration properties inside the LRE channels may change rapidly and a heat transfer deterioration may be observed from to the axial coordinate where the specific heat at constant pressure phenomenon may be observed from to the axial coordinate where the specific heat at con- phenomenon may be observed from to the axial coordinate where the specific heat at con- exhibits the peak (pseudo-critical conditions, T > T ) [23–25]. Pseudo-critical temperature pc cr stant pressure exhibits the peak (pseudo-critical conditions, Tpc > Tcr) [23–25]. Pseudo-crit- stant pressure exhibits the peak (pseudo-critical conditions, Tpc > Tcr) [23–25]. Pseudo-crit- increases as pressure increases while the specific heat peak value reduces, up to vanishing ical temperature increases as pressure increases while the specific heat peak value re- ical temperature increases as pressure increases while the specific heat peak value re- at very high values of pressure, as depicted by Figure 5b. Designers generally pay a lot of duces, up to vanishing at very high values of pressure, as depicted by Figure 5b. Designers duces, up to vanishing at very high values of pressure, as depicted by Figure 5b. Designers attention to properly conceiving the cooling passages to avoid the aforementioned critical generally pay a lot of attention to properly conceiving the cooling passages to avoid the generally pay a lot of attention to properly conceiving the cooling passages to avoid the phenomenon. In fact, due to the thermal stratification and disadvantageous physical aforementioned critical phenomenon. In fact, due to the thermal stratification and disad- aforementioned critical phenomenon. In fact, due to the thermal stratification and disad- conditions, low convective heat transfer coefficient values can be detected because a thin vantageous physical conditions, low convective heat transfer coefficient values can be de- vantageous physical conditions, low convective heat transfer coefficient values can be de- layer (behaving as a vapour and having low thermal conductivity) may divide the heated tected because a thin layer (behaving as a vapour and having low thermal conductivity) tected because a thin layer (behaving as a vapour and having low thermal conductivity) Aerospace 2021, 8, 151 5 of 20 Aerospace 2021, 8, x 5 of 21 wall from the core, composed of liquid-like layers. In the view of developing robust may divide the heated wall from the core, composed of liquid-like layers. In the view of design solutions for cooling jackets, risks, linked to the heat transfer deterioration on- developing robust design solutions for cooling jackets, risks, linked to the heat transfer set, can be reduced either by increasing the coolant pressure or by increasing the surface deterioration on-set, can be reduced either by increasing the coolant pressure or by in- roughness [24]. creasing the surface roughness [24]. ρ (density) [kg/m ] ρ [kg/m ] 18.0 16.0 Supercritical vapour 14 0 . 350 12.0 300 10.0 8.0 6.0 4.0 Critical Point 2.0 1 100 150 200 250 300 350 400 450 T[K] (a) (b) Figure 5. Methane thermo-physical properties, as a function of temperature and pressure: (a) density 3 Figure 5. Methane thermo-physical properties, as a function of temperature and pressure: (a) density (kg/m ); (b) specific (kg/m ); (b) specific heat (J/kg K). heat (J/kg K). Given this background, the coolant flow analysis and the deep comprehension of fluid Given this background, the coolant flow analysis and the deep comprehension of transcritical behaviour represent central points in the design activity of LO /LCH rocket X 4 fluid transcritical behaviour represent central points in the design activity of LOX/LCH4 engines: the prediction of surface temperature and heat flux from the combustion gases to rocket engines: the prediction of surface temperature and heat flux from the combustion the engine wall is directly dependent on heat transfer capabilities of the coolant [25]. More- gases to the engine wall is directly dependent on heat transfer capabilities of the coolant over, classical semi-empirical correlations for the evaluation of heat transfer coefficients do [25]. Moreover, classical semi-empirical correlations for the evaluation of heat transfer co- not work in the deteriorated mode and relatively high values of roughness, which may efficients do not work in the deteriorated mode and relatively high values of roughness, P [MPa] Aerospace 2021, 8, 151 6 of 20 Aerospace 2021, 8, x 6 of 21 occur since the typical dimension of channels (in the order of some mm) also represent which may occur since the typical dimension of channels (in the order of some mm) also represent challenging chaconditions llenging cond from itions a numerical from a numeric point of al poi view nt. of view Thus, an . Th ac us, curate an acc investigation urate inves- about configurations of rocket-engine-like cooling channels was needed before designing tigation about configurations of rocket-engine-like cooling channels was needed before the DEMO jacket. For this reason, a specific test article, named MTP-BB (Methane Thermal designing the DEMO jacket. For this reason, a specific test article, named MTP-BB (Me- Properties Breadboard) and shown by Figure 6, has been designed by CIRA and tested at thane Thermal Properties Breadboard) and shown by Figure 6, has been designed by different conditions [26]. The breadboard, made of a copper alloy, was provided with a CIRA and tested at different conditions [26]. The breadboard, made of a copper alloy, was single rectangular channel on the top, connected to the facility supplies through mechanical provided with a single rectangular channel on the top, connected to the facility supplies flanges, where temperature and pressure sensors were accommodated. Liquid methane through mechanical flanges, where temperature and pressure sensors were accommo- was injected in the rectangular passage (mass flow rate ranging from 0.01 to 0.06 kg/s, dated. Liquid methane was injected in the rectangular passage (mass flow rate ranging T ranging from 120 to 140 K and P in the range 6.0–16.0 MPa) and gradually heated in in from 0.01 to 0.06 kg/s, Tin ranging from 120 to 140 K and Pin in the range 6.0–16.0 MPa) and by means of the electrical cartridges, placed on the bottom part, to reach transcritical gradually heated by means of the electrical cartridges, placed on the bottom part, to reach conditions before exiting. Several internal thermocouples were placed inside the body transcritical conditions before exiting. Several internal thermocouples were placed inside to collect temperature data at different axial positions and at a distance from the channel the body to collect temperature data at different axial positions and at a distance from the bottom up to 4 mm. channel bottom up to 4 mm. Figure 6. MTP breadboard: sketch of the test article. Figure 6. MTP breadboard: sketch of the test article. The collected experimental data were useful to conduct a rebuilding activity in order The collected experimental data were useful to conduct a rebuilding activity in order to set the thermal numerical models and support the design of the DEMO cooling jacket. to set the thermal numerical models and support the design of the DEMO cooling jacket. In fact, the channel located on the top of the MTP breadboard has similar dimensions In fact, the channel located on the top of the MTP breadboard has similar dimensions with with respect to the DEMO cooling jacket; as well as, input heat flux (imposed from the respect to the DEMO cooling jacket; as well as, input heat flux (imposed from the base- basement), inlet conditions and mass flow rates are in the typical range of DEMO operating ment), inlet conditions and mass flow rates are in the typical range of DEMO operating conditions. Details on the experimental rebuilding and validation activity are reported conditions. Details on the experimental rebuilding and validation activity are reported in in [26]. [26]. In this paper, both methane-cooled and water-cooled modes of the DEMO cooling In this paper, both methane-cooled and water-cooled modes of the DEMO cooling system have been analysed through a 3-D CFD model regarding a single channel. The system have been analysed through a 3-D CFD model regarding a single channel. The validation of the numerical procedure has been accomplished indirectly through the exper- validation of the numerical procedure has been accomplished indirectly through the ex- imental results obtained in the MTP breadboard campaign for methane while waiting for perimental results obtained in the MTP breadboard campaign for methane while waiting future firing tests. However, the present investigation allows describing of the transcritical for future firing tests. However, the present investigation allows describing of the tran- behaviour in an LRE typical cooling jacket, on one hand, and to compare the response of such scritical behaviour in an LRE typical cooling jacket, on one hand, and to compare the re- systems, manufactured by means of different technological process, on the other one. In fact, sponse of such systems, manufactured by means of different technological process, on the comparisons between the brazed configuration of the demonstrator and the electroplated other one. In fact, comparisons between the brazed configuration of the demonstrator and one, named DEMO-0A, are carried out to underline the achieved improvements under the electroplated one, named DEMO-0A, are carried out to underline the achieved im- the thermal point of view. CFD results were adopted as input for the thermo-structural provements under the thermal point of view. CFD results were adopted as input for the simulations, needed to evaluate the lifecycle of the thrust chamber assembly. thermo-structural simulations, needed to evaluate the lifecycle of the thrust chamber as- sembly. 2. Materials and Methods The final demonstrator architecture provides a regeneratively cooled ground engine, 2. Materials and Methods which implements a typical counter-flow architecture. The cooling jacket is characterized The final demonstrator architecture provides a regeneratively cooled ground engine, by several narrow axial channels, defined in the bottom part by a copper alloy liner, covered which implements a typical counter-flow architecture. The cooling jacket is characterized with electrodeposited layers of pure copper and pure nickel, or by a brazed nickel part, by several narrow axial channels, defined in the bottom part by a copper alloy liner, cov- which represent the close-out part. ered with electrodeposited layers of pure copper and pure nickel, or by a brazed nickel An in-house design tool, based on typical correlations, was developed to design the part, which represent the close-out part. cooling jacket in terms of channel dimension, liner and rib thickness [14]. The adopted An in-house design tool, based on typical correlations, was developed to design the strategy considers a constant number of channels and a constant value for the rib width cooling jacket in terms of channel dimension, liner and rib thickness [14]. The adopted (w) while a variable value of the rib height (h) has been considered, according to Figure 7a. strategy considers a constant number of channels and a constant value for the rib width In this way, an optimization of the cooling performances was achieved, taking into account Symmetry Symmetry Aerospace 2021, 8, x 7 of 21 Aerospace 2021, 8, 151 7 of 20 (w) while a variable value of the rib height (h) has been considered, according to Figure 7a. In this way, an optimization of the cooling performances was achieved, taking into some account significant some sign sections, ificant se such ctions, s as the uch a nozzle s the nozz (NZ), thr le (oat NZ), t (CT) hro and at (C cylindrical T) and cylin part dric(CP), al part (CP), highlighted in Figure 8. Furthermore, Figure 8 depicts the dimensionless profile of highlighted in Figure 8. Furthermore, Figure 8 depicts the dimensionless profile of the thr th ust e th chamber rust cham , the bevariations r, the variat ofion cooling s of cooling ch channel hydraulic annel hydr diameter aulic diam (d et ) er along (dh) a the long the axial axis axiand al axis the and applied the ap heat plie flux d he pr atofile flux p (defined rofile (d as ef “nominal” ined as “nom in the inal “Results ” in the “ and ResDiscussion” ults and Dis- section), representing the design profile. It has been calculated through reactive simulations cussion” section), representing the design profile. It has been calculated through reactive inside the combustion chamber [15]. simulations inside the combustion chamber [15]. Considering L, the overall thrust chamber length, as the reference length, the geometric Considering L, the overall thrust chamber length, as the reference length, the geo- parameters are equal to: metric parameters are equal to: channel height (h/L), ranging from 0.0018 to 0.0061; • channel height (h/L), ranging from 0.0018 to 0.0061; channel width (b/L), ranging from 0.0019 to 0.0107; • channel width (b/L), ranging from 0.0019 to 0.0107; rib width (w/L) = 0.0032; • rib width (w/L) = 0.0032; liner thickness (h /L) = 0.0020; • liner thickness (h1/L) = 0.0020; copper layer height (h /L) = 0.0023; cu • copper layer height (hcu/L) = 0.0023; nickel layer height (h /L) = 0.0034. ni • nickel layer height (hni/L) = 0.0034. In the case of the DEMO brazed version, the copper layer and nickel close-out are In the case of the DEMO brazed version, the copper layer and nickel close-out are substituted by a unique Inconel part, joined with the liner rib through the brazing process. substituted by a unique Inconel part, joined with the liner rib through the brazing process. Close-out Close-out Copper Layer Fluid Fluid Liner Liner Thermal input Thermal input (a) (b) Figure 7. Sketch of the model concerning: (a) geometry description of the DEMO-0A cooling jacket—cross-section; (b) Figure 7. Sketch of the model concerning: (a) geometry description of the DEMO-0A cooling jacket—cross-section; (b) Aerospace 2021, 8, x 8 of 21 materials and boundary conditions for electroplated (left) and brazed (right) DEMO versions. materials and boundary conditions for electroplated (left) and brazed (right) DEMO versions. 0.12 thrust chamber profile Inlet q (reactive CFD results) hg,input 0.1 Section NZ 0.08 Section CP 0.06 Outlet 0.04 0.02 Section CT 0 0.2 0.4 0.6 0.8 1 x/L Figure 8. DEMO assembly details on chamber profile, hydraulic diameter of cooling channels and Figure 8. DEMO assembly details on chamber profile, hydraulic diameter of cooling channels and input heat flux profile. input heat flux profile. The numerical investigations on a single DEMO cooling channel, extracted from the complete model, were performed by means of ANSYS Fluent v17 (Canonsburg, Pennsyl- vania, USA) [27]. The solution of the governing equations, such as continuity, momentum and energy in three-dimensional form were accomplished in a steady state regime and considering an NIST (National Institute of Standard and Technology) real gas model and turbulent flow with thermo-physical properties, calculated through REFPROP v7.0 data- base [28]. Conduction effects have been contemplated. Rough channel walls have been taken into account while k-ω sst turbulence model was assumed [27,29]. A pressure-based method was selected to solve the energy and momentum equations and a second-order upwind scheme was chosen together with PISO (Pressure-Implicit with Splitting of Oper- ators) coupling to couple pressure and velocity, respectively. Concerning the convergence −6 criteria, residuals of velocity components and energy values were considered equal to 10 −9 and 10 , respectively. Initialization was performed at inlet section conditions, Pin = 16.0 MPa and Tin = 110 K for methane-cooled mode (overall mass flow rate is equal to 1.92 kg/s). For the water-cooled mode (overall mass flow rate is equal to 4.5 kg/s and 5.0 kg/s) Pin = 16.0 and 12.0 MPa (to take into account different test conditions offered by the test facility) and Tin = 293 K, respectively, were adopted. A NIST real gas model was applied and the single-species flow form was selected to eventually handle both liquid and vapour phases in supercritical pressure conditions [27,28], in the case of methane. In fact, if me- thane is considered, it is fundamental to the transcritical operating conditions of the work- ing fluid since both the simulation initialization and the convergence history can be influ- enced. According to Figure 7a,b, the considered solid materials were: (a) a copper alloy (Cu- CrZr) for the liner, chosen because of its high thermal conductivity and good mechanical properties, for both modes; and (b) Inconel or pure nickel, due their mechanical perfor- mances, for the close-out part in the case of brazed and electroplated versions, respec- tively. For DEMO-0A, a copper layer was deposited between the liner and the external close-out. The thermo-physical properties of all the materials were assumed to be depend- ent on temperature and they were calculated through a characterization campaign [14]. y/L, d [m] throat q [MW/m ] hg, input Aerospace 2021, 8, 151 8 of 20 The numerical investigations on a single DEMO cooling channel, extracted from the complete model, were performed by means of ANSYS Fluent v17 (Canonsburg, Pennsylva- nia, USA) [27]. The solution of the governing equations, such as continuity, momentum and energy in three-dimensional form were accomplished in a steady state regime and consider- ing an NIST (National Institute of Standard and Technology) real gas model and turbulent flow with thermo-physical properties, calculated through REFPROP v7.0 database [28]. Conduction effects have been contemplated. Rough channel walls have been taken into account while k-w sst turbulence model was assumed [27,29]. A pressure-based method was selected to solve the energy and momentum equations and a second-order upwind scheme was chosen together with PISO (Pressure-Implicit with Splitting of Operators) coupling to couple pressure and velocity, respectively. Concerning the convergence criteria, residuals of velocity components and energy values were considered equal to 10 and 10 , respectively. Initialization was performed at inlet section conditions, P = 16.0 MPa in and T = 110 K for methane-cooled mode (overall mass flow rate is equal to 1.92 kg/s). For in the water-cooled mode (overall mass flow rate is equal to 4.5 kg/s and 5.0 kg/s) P = 16.0 in and 12.0 MPa (to take into account different test conditions offered by the test facility) and T = 293 K, respectively, were adopted. A NIST real gas model was applied and the in single-species flow form was selected to eventually handle both liquid and vapour phases in supercritical pressure conditions [27,28], in the case of methane. In fact, if methane is considered, it is fundamental to the transcritical operating conditions of the working fluid since both the simulation initialization and the convergence history can be influenced. According to Figure 7a,b, the considered solid materials were: (a) a copper alloy (Cu- CrZr) for the liner, chosen because of its high thermal conductivity and good mechanical properties, for both modes; and (b) Inconel or pure nickel, due their mechanical perfor- mances, for the close-out part in the case of brazed and electroplated versions, respectively. For DEMO-0A, a copper layer was deposited between the liner and the external close-out. The thermo-physical properties of all the materials were assumed to be dependent on temperature and they were calculated through a characterization campaign [14]. Simulations contemplate a half channel to limit the computational efforts; in fact, a symmetry condition is adopted (due to the geometrical, thermal and fluid-dynamic symmetry); top wall was considered adiabatic (for a conservative approach) as well as the left one to take into account half rib. The thermal load was applied on the liner bottom surface by means of specific user-defined functions (udf ). Moreover, the following boundary conditions have been assigned: inlet section: uniform velocity (at a fixed mass flow) and uniform temperature profile; outlet section: pressure outlet in order to define the static pressure at flow outlets; channel walls: velocity components equal to zero. The nominal heat flux profile, adopted for the design, is depicted in Figure 8 and was obtained through reactive CFD simulations of the combustion chamber side, considering the thrust walls at a temperature of 300 K [15]. However, this approach is conservative but useful to adopt suitable margins in the cooling jacket design; then, a weak thermal coupling was taken into account for further analyses, as suggested by [18]. In this case, the thermal load was applied by means of a hot gas side convective heat transfer coefficient, calculated by scaling the “design” heat flux (obtained at a constant value of T equal to 300 K) wg and considering the average hot gas temperature inside the chamber (T ), according aw to Equation (1). T values, given in Figure 9, are evaluated through RPA code [30] for aw four significant sections, corresponding to inlet (1), throat (2), convergent/cylindrical part interface (3) and outlet (4). Then, at a fixed position, T is calculated by means of a linear aw interpolation between the values of adjacent sections. CFD,Twg = 300K h = (1) c, hg T T aw wg Aerospace 2021, 8, x 9 of 21 Simulations contemplate a half channel to limit the computational efforts; in fact, a symmetry condition is adopted (due to the geometrical, thermal and fluid-dynamic sym- metry); top wall was considered adiabatic (for a conservative approach) as well as the left one to take into account half rib. The thermal load was applied on the liner bottom surface by means of specific user-defined functions (udf). Moreover, the following boundary con- ditions have been assigned: • inlet section: uniform velocity (at a fixed mass flow) and uniform temperature profile; • outlet section: pressure outlet in order to define the static pressure at flow outlets; • channel walls: velocity components equal to zero. The nominal heat flux profile, adopted for the design, is depicted in Figure 8 and was obtained through reactive CFD simulations of the combustion chamber side, considering the thrust walls at a temperature of 300 K [15]. However, this approach is conservative but useful to adopt suitable margins in the cooling jacket design; then, a weak thermal coupling was taken into account for further analyses, as suggested by [18]. In this case, the thermal load was applied by means of a hot gas side convective heat transfer coeffi- cient, calculated by scaling the “design” heat flux (obtained at a constant value of Twg equal to 300 K) and considering the average hot gas temperature inside the chamber (Taw), according to Equation (1). Taw values, given in Figure 9, are evaluated through RPA code [30] for four significant sections, corresponding to inlet (1), throat (2), convergent/cylin- drical part interface (3) and outlet (4). Then, at a fixed position, Taw is calculated by means of a linear interpolation between the values of adjacent sections. Aerospace 2021, 8, 151 9 of 20 CFD ,Twg =300 K (1) h = c,hg TT − () aw wg T = 2456 K T = 3543 K T = 3538 K T = 3370 K aw aw aw aw 4 3 2 1 Section CT Section NZ Section CP Outlet Inlet Figure 9. Sketch reporting the hot gas average temperature (K) values along the engine for the Figure 9. Sketch reporting the hot gas average temperature (K) values along the engine for the cylindrical part, convergent, throat and nozzle sections, evaluated through RPA code. cylindrical part, convergent, throat and nozzle sections, evaluated through RPA code. W Wi ith th rega regards rds to to the a the adopted dopted mesh, a struct mesh, a structur ured ed grid, grid, composed composed of ofabout about2.7 2.7 million million nodes, was chosen and a sketch is reported in Figure 10. A mesh sensitivity analysis, de- nodes, was chosen and a sketch is reported in Figure 10. A mesh sensitivity analysis, described scribed byby TaT ble 2 able , wa 2, was s conconducted ducted on ton he model cons the model considering idering the nom the inominal nal condit conditions ions (nom- (nominal inal input input heat f heat lux, T flux, in = 11 T 0 K, P = 110 in K, = 1P 6.0 MP = 16.0 a and MPa m = and 0.01m kg/s = 0.01 for si kg/s ngle ha for single lf channel) half . in in channel). Three structured mesh distributions (“coarse”, “fine” and “finest”), generated Three structured mesh distributions (“coarse”, “fine” and “finest”), generated by follow- by following the mesh suggestions given by Ansys Fluent user ’s guide in the case of ing the mesh suggestions given by Ansys Fluent user’s guide in the case of rough channel rough channel walls [27], were considered: they have about 1.3, 2.7 and 5.6 million nodes, walls [27], were considered: they have about 1.3, 2.7 and 5.6 million nodes, respectively. respectively. Finally, the second grid case was chosen to perform the investigations, pre- Finally, the second grid case was chosen to perform the investigations, presented in the Aerospace 2021, 8, x 10 of 21 sented in the present paper, because it ensured a good compromise between the machine present paper, because it ensured a good compromise between the machine computa- computational time and the accuracy requirements. tional time and the accuracy requirements. (a) (b) Figure 10. Numerical modelling: (a) global view of cooling channel model extracted from cooling system geometry; (b) Figure 10. Numerical modelling: (a) global view of cooling channel model extracted from cooling system geometry; (b) mesh distribution with a detail on inlet zone (axial direction) and on a transversal section (divergent zone). mesh distribution with a detail on inlet zone (axial direction) and on a transversal section (divergent zone). Table 2. Grid-independence test results. Table 2. Grid-independence test results. Outlet Fluid Bulk Liner Maximum Channel Bottom Wall ΔP Outlet Fluid Bulk Liner Maximum Channel Bottom Wall Type of Mesh Temperature Temperature Maximum Temperature DP (MPa) Type of Mesh Temperature Temperature Maximum Temperature Tb,f out (K) Tw, hg max Tw, ch max (MPa) T (K) T T b,f out w, hg max w, ch max 1—coarse 5.051 420.1 600.4 555.4 1—coarse 5.051 420.1 600.4 555.4 2—fine 5.094 420.4 610.8 562.7 2—fine 5.094 420.4 610.8 562.7 3—finest 5.096 420.5 611.5 563.4 3—finest 5.096 420.5 611.5 563.4 The numerical approach and settings were validated through results obtained in the The numerical approach and settings were validated through results obtained in the rebuilding activity of the test campaign conducted on the aforementioned MTP bread- board, de rebuilding scribed in Figure activity of the test 6, with re campaign gard conducted to the tran on scri the tica afor l beha ementioned viour of methane [ MTP breadboar 15,26]. d, described in Figure 6, with regard to the transcritical behaviour of methane [15,26]. In In fact, the test article was provided with a channel, having dimensions representative of fact, the test article was provided with a channel, having dimensions representative of ones adopted in the DEMO and this is also true for the operating conditions. Figure 11 ones adopted in the DEMO and this is also true for the operating conditions. Figure 11 reports the comparisons between the experimental data (when stationary thermal regime reports the comparisons between the experimental data (when stationary thermal regime was reached) and the numerical results for two relevant test cases: test #24 and test #26, was reached) and the numerical results for two relevant test cases: test #24 and test #26, characterized by mass flow rates equal to 20.87 g/s and 20.57 g/s, Tin = 137.1 K and 140.8 K, and Pin = 11.21 MPa and 12.89 MPa, respectively. Numerical results fitted the experi- mental data very well, as depicted by Figure 11a and Figure 11b, with regards to pressure and fluid bulk temperature axial profiles, which are compared with test data. Moreover, very low discrepancies are observed in terms of solid temperature results if the numerical temperature profile at a distance of 4 mm from the channel base is compared with the related experimental data for test #24, as reported in Figure 11b. Further details on CFD simulations, modelling, solving strategies and both numerical and experimental results are given in [26]. throat Aerospace 2021, 8, 151 10 of 20 characterized by mass flow rates equal to 20.87 g/s and 20.57 g/s, T = 137.1 K and in 140.8 K, and P = 11.21 MPa and 12.89 MPa, respectively. Numerical results fitted the in experimental data very well, as depicted by Figure 11a,b with regards to pressure and fluid bulk temperature axial profiles, which are compared with test data. Moreover, very low discrepancies are observed in terms of solid temperature results if the numerical temperature profile at a distance of 4 mm from the channel base is compared with the related experimental data for test #24, as reported in Figure 11b. Further details on CFD simulations, modelling, solving strategies and both numerical and experimental results are Aerospace 2021, 8, x 11 of 21 given in [26]. inlet flange sensor outlet flange sensor inlet flange sensor Test n.24 - Numerical Rebuilding Test n.26 - Numerical Rebuilding Test n.24 - experimental data outlet flange sensor Test n.26 - experimental data 0 0.2 0.4 0.6 0.8 1 x/L (a) center line of channel upper wall center line of channel bottom wall solid T - line at 4 mm depth CFD results at sensors locations T experimental data - 4 mm depth experimental data - T b,f cr,f Test n.24 steady state regime 0 0.2 0.4 0.6 0.8 1 x/L (b) Figure 11. MTP breadboard rebuilding of hot tests: (a) Pressure drops (test #24 and #26); and (b) Fluid temperature and Figure 11. MTP breadboard rebuilding of hot tests: (a) Pressure drops (test #24 and #26); and (b) Fluid bottom wall temperature profiles (test 24). temperature and bottom wall temperature profiles (test 24). 3. Results and Discussion In the present paper, results for numerical simulations ran at nominal conditions (Tin, f = 110 K, Pin = 16.0 MPa and overall m = 1.92 kg/s) for methane mode for both the DEMO configurations, characterized by the electrodeposition and brazing process, are presented. Moreover, since the validation of the electrodeposition process will be accomplished by means of a specific test campaign considering water as refrigerant evolving in the demon- strator cooling system, the following initial conditions are also taken into account: Tin, f = 293 K, Pin = 16.0 and 12.0 MPa and overall m = 5.0 and 4.5 kg/s. These conditions were set according to the capabilities expressed by the test bench. Temperature[K] Pressure [MPa] Aerospace 2021, 8, 151 11 of 20 3. Results and Discussion In the present paper, results for numerical simulations ran at nominal conditions (T = 110 K, P = 16.0 MPa and overall m = 1.92 kg/s) for methane mode for both the in, f in DEMO configurations, characterized by the electrodeposition and brazing process, are presented. Moreover, since the validation of the electrodeposition process will be accom- plished by means of a specific test campaign considering water as refrigerant evolving in the demonstrator cooling system, the following initial conditions are also taken into account: T = 293 K, P = 16.0 and 12.0 MPa and overall m = 5.0 and 4.5 kg/s. These in, f in conditions were set according to the capabilities expressed by the test bench. The test matrix is reported by Table 3. Table 3. Test matrix of present numerical campaign. T P m Imposed Heat Flux Manufacturing in,f in Run Fluid (K) (MPa) (kg/s) (MW/m ) Process 1 110 16.0 1.92 Methane Nominal Electrodeposition 2 110 16.0 1.92 Methane Weak Coupling Electrodeposition 3 110 16.0 1.92 Methane Nominal Brazing 4 293 16.0 5.0 Water Nominal Electrodeposition 5 293 16.0 5.0 Water Nominal Brazing 6 293 16.0 4.5 Water Nominal Electrodeposition 7 293 12.0 5.0 Water Nominal Electrodeposition 8 293 12.0 4.5 Water Nominal Electrodeposition Results are presented in terms of axial profiles for the liner and channel wall temper- ature, fluid bulk temperature, convective heat transfer coefficient and pressure drops as well as fluid significant properties. Moreover, some temperature and fluid properties’ field plots, including some slices of significant interest, are presented. Figures 12 and 13 depict the axial profiles of hot gas walls, channel bottom walls and fluid bulk temperature for water-cooled and methane-cooled modes, respectively. Both figures point out that DEMO-0A, the optimized electroplated configuration of DEMO, presents a general reduction in terms of thermal stresses with respect to the brazed version. This is mainly due to the insertion of the copper layer to overlay the liner part and the possibility of changing the close-out material from Inconel to pure nickel. For the water- cooled mode, temperature peaks in the throat region and in the cylindrical section of the chamber resulted in reductions of about 30–40 K for a fixed mass flow rate (i.e., 5.0 kg/s), as depicted by Figure 12. A little decrease in terms of mass flow rate (4.5 kg/s, i.e., 10%) causes DEMO-0A profiles to overlap with ones obtained for the brazed version at mass flow rate equal to 5.0 kg/s. Moreover, fluid bulk temperature increases from about 400 to 420 K as mass flow rate decreases; water constantly remains in liquid phase, because of high pressure conditions as confirmed by the fluid bulk profiles and maximum values of channel walls. If methane is considered as refrigerant, the thermal behaviour of the DEMO-0A and the “brazed” versions of the DEMO detach significantly from each other, as depicted by Figure 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux is imposed, reduces from about 640 to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondence with the re-attachment zone of hot gases on the combustion chamber wall) decreases by about 90 K in DEMO-0A. Moreover, it is worthy to emphasise that in that zone, methane flows as a supercritical gas. A third relative maximum, observed in the brazed configuration and located in the divergent zone, tends to disappear in DEMO- 0A, due to a reduced local thermal stratification of the fluid. In fact, all the channel walls are made of copper alloys or pure copper, including the upper wall, and this determines a more homogeneous heat distribution inside the material and, consequently, in the fluid. Moreover, from that region fluid begins to locally change its conditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temperature profile: the working Aerospace 2021, 8, 151 12 of 20 fluid, after entering in liquid conditions, reaches critical temperature (about 190 K) near the throat region. From this section, methane tends to behave like a highly compressible fluid near the hot walls of the channel, and like a liquid near the cold ones, especially in the upper part of the channel. However, moving towards the outlet section, a larger fluid fraction behaves like a vapour. In the cylindrical part, methane is completely composed of a supercritical gas since the temperature is very high and the pressure is much higher than the critical value (4.6 MPa). If the “scaled” thermal load (considering a weak coupling approach) is applied, the liner results in being less thermally stressed as expected and, Aerospace 2021, 8, x FOR PEER REVIEW 12 of 20 in fact, a reduction of about 40 K is evaluated in terms of temperature peaks, located at Aerospace 2021, 8, x 13 of 21 x/L = 0.61 and x/L = 0.13. DEMO "brazed" version - m = 5.0 kg/s If methane is considered as refrigerant, the thermal behaviour of the DEMO-0A and DEMO-0A - m = 4.5 kg/s the “brazed” versions of the DEMO detach significantly from each other, as depicted by DEMO-0A - m = 5.0 kg/s Figure 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux w,hg is imposed, reduces from about 640 to 540 K while the absolute maximum, observed at w,ch about x/L = 0.13 (in correspondence with the re-attachment zone of hot gases on the com- b,f bustion 500 chamber wall) decreases by about 90 K in DEMO-0A. Moreover, it is worthy to emphasise that in that zone, methane flows as a supercritical gas. A third relative maxi- mum, observed in the brazed configuration and located in the divergent zone, tends to disappear in DEMO-0A, due to a reduced local thermal T stratification of the fluid. In fact, w,hg all the channel walls are made of copper alloys or pure copper, including the upper wall, and this determines a more homogeneous heat distribution inside the material and, con- sequently, in the fluid. Moreover, from that region fluid begins to locally change its con- w,ch ditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temper- ature profile: the working fluid, after entering in liquid conditions, reaches critical tem- b,f perature (about 190 K) near the throat region. From this section, methane tends to behave like a highly compressible fluid near the hot walls of the channel, and like a liquid near water cooled mode the cold ones, especially in the upper part of the channel. However, moving towards the outlet section, a larger fluid fraction behaves like a vapour. In the cylindrical part, methane 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 is completely composed of a supercritical gas since the temperature is very high and the x/L pressure is much higher than the critical value (4.6 MPa). If the “scaled” thermal load Fi (consi gurederin 12. Wa g t a er- wea cooled m k coupl od in e: a g approach xial profiles ) is of app hot lied gas w , tha e l lls, cha iner re nnel bott sults in om wa being llles s as n t d flu hermal id bu ly lk Figure 12. Water-cooled mode: axial profiles of hot gas walls, channel bottom walls and fluid temperature. stressed as expected and, in fact, a reduction of about 40 K is evaluated in terms of tem- bulk temperature. perature peaks, located at x/L = 0.61 and x/L = 0.13. If methane is considered as refrigerant, the thermal behaviour of DEMO-0A and “brazed” version of DEMO detaches significantly from each other, as depicted by Figure methane-cooled mode 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux is im- posed, reduces from about 640 K to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondence of the re-attachment zone of hot gases on the combustion 600 T w,hg chamber wall) decreases of about 90 K in DEMO-0A. Moreover, it is worthy to underline that in that zone, methane flows as a supercritical gas. A third relative maximum, ob- served in the brazed configuration and located in the divergent zone, tends to disappear in DEMO-0A, due to a reduced local thermal stratification of the fluid. In fact, all the chan- nel walls are made of copper alloys or pure copper, including the upper wall, and this w,ch determines a more homogeneous heat distribution inside the material and, consequently, in the fluid. Moreover, from that region fluid begins to locally change its conditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temperature profile: b,f the working fluid, after entering in liquid conditions, reaches the critical temperature DEMO "brazed" version (about 190 K) near the throat region. From this section, methane tends to behave like a DEMO-0A highly compressible fluid near the hot walls of the channel, and like a liquid near the cold DEMO-0A scaled ones, especially in the upper part of the channel. However, moving towards the outlet w,hg sectio 100 n, a larger flu Tid fraction behaves like a vapour. In the cylindrical part, methane is w,ch b,f completely composed by a supercritical gas since temperature is very high and pressure 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 is much higher than the critical value (4.6 MPa). If the “scaled” thermal load (considering x/L a weak coupling approach) is applied, the liner results to be less thermally stressed as Figure 13. Methane-cooled mode: axial profiles of hot gas walls, channel bottom walls and fluid expected and, in fact, a reduction of about 40 K is evaluated in terms of temperature peaks, Figure 13. Methane-cooled mode: axial profiles of hot gas walls, channel bottom walls and fluid bulk temperature. located at x/L = 0.61 and x/L = 0.13. bulk temperature. Figure 14 depicts the axial profiles for average convective heat transfer both for DEMO-0A and DEMO in the brazed configuration. It is shown that values are generally higher in the case of DEMO-0A if methane is considered as refrigerant and it is particu- larly evident in the divergent part and in the cylindrical part of the cooling system. Alt- hough, in the divergent zone, the fluid is in large part in liquid form, especially near the inlet section where the heat transfer coefficient values are lower than the ones obtained in the cylindrical part, where methane behaves like a supercritical vapour. This is mainly T[K] T[K] throat throat Aerospace 2021, 8, 151 13 of 20 Figure 14 depicts the axial profiles for average convective heat transfer both for DEMO- 0A and DEMO in the brazed configuration. It is shown that values are generally higher in the case of DEMO-0A if methane is considered as refrigerant and it is particularly evident in the divergent part and in the cylindrical part of the cooling system. Although, Aerospace 2021, 8, x 14 of 21 in the divergent zone, the fluid is in large part in liquid form, especially near the inlet section where the heat transfer coefficient values are lower than the ones obtained in the cylindrical part, where methane behaves like a supercritical vapour. This is mainly due to the high velocity attained by the fluid in the last sections of the cooling jacket, while in due to the high velocity attained by the fluid in the last sections of the cooling jacket, while most of the divergent part velocity remains low because of fluid state and the channel cross in most of the divergent part velocity remains low because of fluid state and the channel section dimension. Maximum values are attained in the throat region, where the minimum cross section dimension. Maximum values are attained in the throat region, where the dimensions of passages are designed, and there are very little differences between the minimum dimensions of passages are designed, and there are very little differences be- versions. Moreover, a profile for the water-cooled DEMO-0A mode is plotted in order to tween the versions. Moreover, a profile for the water-cooled DEMO-0A mode is plotted compare data, underlining that the convective heat transfer values are larger in the case of in order to compare data, underlining that the convective heat transfer values are larger water because of its basic thermo-physical properties. in the case of water because of its basic thermo-physical properties. DEMO-0A - methane-cooled DEMO (brazed conf.) - methane-cooled DEMO-0A - water-cooled (m = 5 kg/s) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L Figure 14. Convective heat transfer axial profiles. Figure 14. Convective heat transfer axial profiles. Regar Regard ding ing st static atic pressure pressure pr profiles, plott ofiles, plotted ed for for the the DEMO-0A DEMO-0A version version in in Figur Fige ure 15 ,15, it is it clear is clethat ar that pressur pressure e drops dro in ps in the nozzle the nozzle are very are ver lowybecause low because methane behaves like methane behaves like a liquid. a The highest increase is localized in the convergent part and the cylindrical one, as pointed liquid. The highest increase is localized in the convergent part and the cylindrical one, as out in Figure 15, because density is very low and velocity tends to increase towards the pointed out in Figure 15, because density is very low and velocity tends to increase to- outlet section. As a result, large pressure drops are evaluated although methane mass wards the outlet section. As a result, large pressure drops are evaluated although methane flow rate is about 2.5 times lower than water in the water-cooled mode. In fact, in the mass flow rate is about 2.5 times lower than water in the water-cooled mode. In fact, in water-cooled mode, the refrigerant remains in liquid form throughout the system. the water-cooled mode, the refrigerant remains in liquid form throughout the system. Figure 16 depicts the temperature field for DEMO-0A, including channel walls and some slices, pointing out that maximum temperature values are attained in correspondence with throat region and re-attachment point while Figure 17 gives a comparison with DEMO brazed configuration considering some significant cross sections. It is evident that the liner part is hotter if the brazed version of DEMO is compared with DEMO-0A in all of the considered cross sections. Furthermore, for both versions, it is shown that the thermal stratification of fluid is significant: near the bottom of the channel and near the rib walls the refrigerant is very hot while moving up towards the upper part of the channel, the temperature tends to decrease as it is possible to observe the sections until the throat region). Moreover, after entering the jacket as a compressed liquid, the fluid is heated h [W/m K] av throat Aerospace 2021, 8, 151 14 of 20 by the hot gases and from the throat region exhibits temperature values higher than the critical one on average. In this region and in the first part of convergence, it is possible to observe a gas-like fluid moving near the bottom walls of the channels while a liquid-like Aerospace 2021, 8, x 15 of 21 one is present in the upper parts. Water - m = 5.0 kg/s 1.6x10 Water - m = 4.5 kg/s Methane - m = 1.92 kg/s DEMO-0A P = 16.0 and 12.0 MPa in 1.4x10 1.2x10 1.0x10 8.0x10 6.0x10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L Aerospace 2021, 8, x 16 of 21 Figure 15. DEMO-0A: axial profiles of static pressure for both methane-cooled (Pin = 16.0 MPa) and Figure 15. DEMO-0A: axial profiles of static pressure for both methane-cooled (P = 16.0 MPa) and in water-cooled versions (Pin = 16.0 MPa and 12.0 MPa). water-cooled versions (P = 16.0 MPa and 12.0 MPa). in Figure 16 depicts the temperature field for DEMO-0A, including channel walls and some slices, pointing out that maximum temperature values are attained in correspond- ence with throat region and re-attachment point while Figure 17 gives a comparison with DEMO brazed configuration considering some significant cross sections. It is evident that the liner part is hotter if the brazed version of DEMO is compared with DEMO-0A in all of the considered cross sections. Furthermore, for both versions, it is shown that the ther- mal stratification of fluid is significant: near the bottom of the channel and near the rib walls the refrigerant is very hot while moving up towards the upper part of the channel, the temperature tends to decrease as it is possible to observe the sections until the throat region). Moreover, after entering the jacket as a compressed liquid, the fluid is heated by the hot gases and from the throat region exhibits temperature values higher than the crit- ical one on average. In this region and in the first part of convergence, it is possible to observe a gas-like fluid moving near the bottom walls of the channels while a liquid-like one is present in the upper parts. Figure 16. DEMO-0A: Temperature field of channel walls, including some slices—methane-cooled mode. Figure 16. DEMO-0A: Temperature field of channel walls, including some slices—methane-cooled mode. After the convergence zone, the fluid appears to be in supercritical gas state. This is also confirmed by the density axial profiles, strictly linked to temperature ones, and by the specific heat distribution, given in Figure 18a: from the throat region, the fluid core is characterized by low values of density and high values of specific heat. In particular, this is evident in the convergence section where a large part of the fluid exhibits the highest values of specific heat while relatively low values of thermal conductivity can be detected. The pseudo-critical condition is observed at about x/L = 0.50 and in that region low ther- mal conductivity values are observed as depicted by Figure 18b. However, a deterioration mode, characterized by a sort of a thermal barrier between the fluid near to the hot wall and the cold upper part of the channel [22], is not significant for both the demonstrator versions. In fact, aforementioned profiles of the liner temperature do not exhibit this phe- nomenon and this is due to the channel dimensions (optimized by means of the design codes), relatively high value of the wall roughness and fluid pressure, sufficiently far from the critical value [23]. Pressure[Pa] throat Aerospace 2021, 8, 151 15 of 20 Aerospace 2021, 8, x 17 of 21 Aerospace 2021, 8, x 17 of 21 Figure 17. Methane-cooled mode—temperature fields plotted for some significant slices. Figure 17. Methane-cooled mode—temperature fields plotted for some significant slices. DEMO-0A After the convergence zone, the fluid appears to be in supercritical gas state. This is DEMO "brazed" version also confirmed by the density axial profiles, strictly linked to temperature ones, and by Density Specific Heat the specific heat distribution, given in Figure 18a: from the throat region, the fluid core is methane-cooled mode characterized by low values of density and high values of specific heat. In particular, this is evident in the convergence section where a large part of the fluid exhibits the highest values of specific heat while relatively low values of thermal conductivity can be detected. The pseudo-critical condition is observed at about x/L = 0.50 and in that region low thermal conductivity values are observed as depicted by Figure 18b. However, a deterioration c 4000 mode, characterized by a sort of a thermal barrier between the fluid near to the hot wall and the cold upper part of the channel [22], is not significant for both the demonstrator versions. In fact, aforementioned profiles of the liner temperature do not exhibit this phenomenon and this is due to the channel dimensions (optimized by means of the design codes), relatively high value of the wall roughness and fluid pressure, sufficiently far from the critical value [23]. Figure 17. Methane-cooled mode—temperature fields plotted for some 3000 significant slices. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 DEMO-0A x/L DEMO "brazed" version (a) Density Specific Heat methane-cooled mode c 4000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L (a) Figure 18. Cont. Density [kg/m ] Density [kg/m ] throat throat Specific Heat [Jkg/K] Specific Heat [Jkg/K] Aerospace 2021, 8, 151 16 of 20 Aerospace 2021, 8, x 18 of 21 1.6E-04 0.25 DEMO-0A DEMO "brazed" version Thermal Conductivity 1.4E-04 Viscosity 0.2 methane-cooled mode 1.2E-04 1.0E-04 0.15 8.0E-05 0.1 6.0E-05 4.0E-05 0.05 2.0E-05 0.0E+00 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L (b) Figure 18. Methane-cooled mode, axial profiles: (a) average density and specific heat; (b) thermal conductivity and vis- Figure 18. Methane-cooled mode, axial profiles: (a) average density and specific heat; (b) thermal cosity. conductivity and viscosity. 4. Conclusions 4. Conclusions In this paper, the thermal and fluid-dynamic analyses, supporting the design of In this paper, the thermal and fluid-dynamic analyses, supporting the design of the the cooling system of the 30-kN thrust class HYPROB LO /LCH demonstrator, were cooling system of the 30-kN thrust class HYPROB LOX/LCH4 demonstrator, were dis- discussed. In particular, the final demonstrator cooling jacket configuration, based on cussed. In particular, the final demonstrator cooling jacket configuration, based on elec- electroplating manufacturing process was analysed by means of 3-D CFD simulations troplating manufacturing process was analysed by means of 3-D CFD simulations and and compared with the behaviour of the original brazed configuration. An NIST real compared with the behaviour of the original brazed configuration. An NIST real gas gas model was adopted to describe the behaviour of evolving fluids and, in particular, of model was adopted to describe the behaviour of evolving fluids and, in particular, of the the transcritical conditions experimented by methane flowing in the cooling system. The transcritical conditions experimented by methane flowing in the cooling system. The se- selected arrangement was discussed by means of results, given in terms of temperature, lected arrangement was discussed by means of results, given in terms of temperature, fluid bulk temperature and pressure profiles. Simulations supported the optimization fluid bulk temperature and pressure profiles. Simulations supported the optimization of of the design of the cooling jacket, realized through electrodeposition: in fact, the most the design of the cooling jacket, realized through electrodeposition: in fact, the most ther- thermally stressed zones, such as the throat region and the re-attachment point zone, mally stressed zones, such as the throat region and the re-attachment point zone, were were identified. identified. The adoption of the electrodeposition process, instead of the brazing one, led to The adoption of the electrodeposition process, instead of the brazing one, led to sig- significant thermal benefits all over the cooling jacket. In fact, in the case of nominal nificant thermal benefits all over the cooling jacket. In fact, in the case of nominal operat- operating conditions and if methane cooled versions are considered, a decrease of about ing conditions and if methane cooled versions are considered, a decrease of about 100 K 100 K is observed both in the throat region (x/L = 0.61) and at the end of the cylindrical is observed both in the throat region (x/L = 0.61) and at the end of the cylindrical part of part of the chamber (x/L = 0.13), as shown by Figure 13. In fact, convective heat transfer the chamber (x/L = 0.13), as shown by Figure 13. In fact, convective heat transfer values values are generally higher in the case of DEMO-0A and the differences with the brazed are generally higher in the case of DEMO-0A and the differences with the brazed config- configuration are significant in the divergent part (t times higher at maximum) and in uration are significant in the divergent part (t times higher at maximum) and in the cylin- the cylindrical part of the cooling system (1.2 times on average), as can be observed in drical part of the cooling system (1.2 times on average), as can be observed in Figure 14. Figure 14. Moreover, a different behaviour is observed in the divergent zone, where a Moreover, a different behaviour is observed in the divergent zone, where a thermal relax- thermal relaxation of the liner is reported, since at x/L = 0.70, a difference of about 180 K is ation of the liner is reported, since at x/L = 0.70, a difference of about 180 K is evaluated evaluated comparing DEMO-0A results with the brazed configuration ones. This is due to comparing DEMO-0A results with the brazed configuration ones. This is due to the re- the reduced thermal stratification of fluid in the case of the electroplated demonstrator, as duced thermal stratification of fluid in the case of the electroplated demonstrator, as also also evident from the temperature field given for the cross-section at x/L = 0.70 in Figure 17. evident from the temperature field given for the cross-section at x/L = 0.70 in Figure 17. In In addition, the water-cooled mode was considered since the first firing campaign will be addition, the water-cooled mode was considered since the first firing campaign will be conducted considering water as refrigerant to accomplish the acceptance tests of DEMO-0A conducted considering water as refrigerant to accomplish the acceptance tests of DEMO- and to validate the adopted technological process. In the water-cooled mode, temperature 0A and to validate the adopted technological process. In the water-cooled mode, temper- peaks in the throat region and in the cylindrical section of the chamber decreases by about ature peaks in the throat region and in the cylindrical section of the chamber decreases by Viscosity [Pa s] throat Thermal Conductivity [W/mK] Aerospace 2021, 8, 151 17 of 20 30–40 K for a fixed mass flow rate in the electroplated version of demonstrator, as reported in Figure 12. Results about the most significant thermo-physical properties have been presented to describe the transcritical behaviour of methane inside the cooling jacket. Furthermore, temperature fields in Figure 17 have been discussed to underline the thermal stratification occurring inside the channel and the thermal response of the liner. Methane, injected as a compressed liquid, tends to be in those conditions until half divergent, then the fluid layers near the bottom part of the channel tends to behave like a vapour. In the throat region the fluid bulk temperature reaches critical values (about 190 K on average) and all the thermo-physical properties dramatically change in the near-critical conditions as depicted by Figure 18 while the pseudo-critical conditions are attained in the convergent part (at x/L = 0.50). No peaks or temperature irregularities are observed in that zone; thus, no evident thermal deterioration phenomena are significant since pressure is two times the critical values. In the cylindrical part of the chamber, the fluid is fully composed of a supercritical vapour. The present activity was propaedeutic to perform thermo-structural simulations, nec- essary to estimate the lifecycle of the thrust chamber assembly, whose manufacturing has been recently completed and is ready to withstand the firing test campaign. The adoption of the electroplating process, instead of the brazing one, to join the liner and the close-out has provided evident benefits in terms of the thermal response of the demonstrator (besides the other advantages linked to the process repeatability and the absence of heat treatments). It is expected that the objective of 5 firing tests with a duration of 30 s (considering a safety margin of 4) will be fulfilled. After completing the test activity, experimental results will be extracted to run further simulations and accomplish a wide numerical rebuilding activity. Author Contributions: For the present research paper, authors declare as following: Conceptualiza- tion, D.R., F.B. and M.F.; methodology, D.R., F.B. and M.F.; software, D.R.; validation, D.R. and F.B.; formal analysis, D.R., F.B. and M.F.; investigation, D.R., F.B. and M.F.; resources, D.R. and F.B.; data curation, D.R. and F.B.; writing—original draft preparation, D.R., F.B. and M.F.; visualization, D.R.; supervision, F.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was performed in the framework of the HYPROB program, funded by the Italian Ministry of University and Research. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data presented in this study are available on request from the corre- sponding author. Data are not publicly available due to CIRA policy on intellectual copyright. Acknowledgments: This work was performed in the framework of the HYPROB Program, financed by the Italian Ministry of University and Research. The authors would like to thank our colleagues, Daniele Cardillo, Pasquale Natale and Michele Ferraiuolo, whose efforts were thoroughly appreciated, as well as Andrea Ceracchi, Luca Manni and CECOM personnel for their professionalism and competences, proved throughout the cooperation activities. Conflicts of Interest: The authors declare no conflict of interest. Aerospace 2021, 8, 151 18 of 20 Nomenclature b Cooling channel width: m BB Breadboard CFD Computational Fluid-Dynamic –1 –1 c Specific heat, J kg K CP Cylindrical part of the combustion chamber CT Convergent section d Diameter, m DEMO Demonstrator FSBB Full Scale BreadBoard h Channel height of DEMO, m –2 –1 h Convective heat transfer coefficient, W m K HS Heat Sink HYPROB Hydrocarbon PROpulsion test Bench I Impulse, s 2 –2 k Turbulence kinetic energy, m s L Length of test articles, m LCH Liquid methane LO Liquid oxygen LRE Liquid rocket engine –1 m Mass flow rate, kg s MTP Methane thermal properties NIST National Institute of Standard and Technology NZ Nozzle section O/F Mixture ratio (oxidizer mass/fuel mass), - P Pressure, Pa –2 q Input heat flux, W m SSBB SubScale BreadBoard sst shear stress transport T Temperature, K w Rib width, m x, y, z Spatial coordinates, m Greek symbols –1 –1 Thermal conductivity, W m K m Viscosity, Pa s r Density, kg m –1 w Specific dissipation rate of turbulence kinetic energy, s Subscripts av average aw adiabatic wall b bulk cc combustion chamber ch channel cr critical cu copper f fluid h hydraulic hg hot gas side in inlet l liner ni nickel out outlet pc pseudo-critical s static sp specific t throat w wall Aerospace 2021, 8, 151 19 of 20 References 1. 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Numerical Investigation on the Thermal Behaviour of a LOx/LCH4 Demonstrator Cooling System

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aerospace Article Numerical Investigation on the Thermal Behaviour of a LOx/LCH Demonstrator Cooling System Daniele Ricci * , Francesco Battista and Manrico Fragiacomo CIRA, Centro Italiano Ricerche Aerospaziali, Via Maiorise, 81043 Capua, Italy; f.battista@cira.it (F.B.); m.fragiacomo@cira.it (M.F.) * Correspondence: d.ricci@cira.it; Tel.: +39-0823-623096; Fax: +39-0823-623100 Abstract: Reliability of liquid rocket engines is strictly connected with the successful operation of cooling jackets, able to sustain the impressive operative conditions in terms of huge thermal and mechanical loads, generated in thrust chambers. Cryogenic fuels, like methane or hydrogen, are often used as coolants and they may behave as transcritical fluids flowing in the jackets: after injection in a liquid state, a phase pseudo-change occurs along the chamber because of the heat released by combustion gases and coolants exiting as a vapour. Thus, in the development of such subsystems, important issues are focused on numerical methodologies adopted to simulate the fluid thermal behaviour inside the jackets, design procedures as well as manufacturing and technological process topics. The present paper includes the numerical thermal analyses regarding the cooling jacket belonging to the liquid oxygen/liquid methane demonstrator, realized in the framework of the HYPROB (HYdrocarbon PROpulsion test Bench) program. Numerical results considering the nominal operating conditions of cooling jackets in the methane-fuelled mode and the water-fed Citation: Ricci, D.; Battista, F.; one are included in the case of the application of electrodeposition process for manufacturing. A Fragiacomo, M. Numerical comparison with a similar cooling jacket, realized through the conventional brazing process, is Investigation on the Thermal addressed to underline the benefits of the application of electrodeposition technology. Behaviour of a LOx/LCH Demonstrator Cooling System. Keywords: liquid rocket engine; numerical analyses; thermal control; cooling jacket design; regener- Aerospace 2021, 8, 151. ative cooling; methane transcritical behaviour; electrodeposition technology; brazing process https://doi.org/10.3390/ aerospace8060151 Academic Editor: 1. Introduction Konstantinos Kontis In the last few years, an increasing interest has arisen in the utilization of the LO /CH X 4 Received: 14 April 2021 propellant combination for space propulsion applications as testified by the efforts spent by Accepted: 24 May 2021 several academic and research institutions, international agencies and private companies. Published: 27 May 2021 The utilization of the LO /CH combination for space propulsion applications provides X 4 many advantages, as indicated by several authors [1–3]: Publisher’s Note: MDPI stays neutral high specific impulse; with regard to jurisdictional claims in thrust-to-weight ratio performances; published maps and institutional affil- good cooling capability; iations. engine reusability and throttlability; fewer storage, handling and insulation concerns; reduced pollution impact on ground, atmosphere and space; compatibility with ISRU (in situ resource utilization) purposes for lunar/Martian missions. Copyright: © 2021 by the authors. These capabilities result in a large number of applications and missions enabled Licensee MDPI, Basel, Switzerland. by methane-based propulsion systems, from in-space systems (landing or descent ve- This article is an open access article hicles, service modules, etc.) to space launchers (main stages or upper stages). In fact, distributed under the terms and oxygen/methane couple represents a potential candidate to substitute hypergolic and conditions of the Creative Commons solid propellants in the future. Thus, its versatility makes methane a good candidate Attribution (CC BY) license (https:// for several applications, from in-space propulsion systems (service modules, landing or creativecommons.org/licenses/by/ descent vehicles, and ascent stages) to accessing to space (first stages of launchers or upper 4.0/). Aerospace 2021, 8, 151. https://doi.org/10.3390/aerospace8060151 https://www.mdpi.com/journal/aerospace Aerospace 2021, 8, x 2 of 21 Aerospace 2021, 8, 151 2 of 20 applications, from in-space propulsion systems (service modules, landing or descent ve- hicles, and ascent stages) to accessing to space (first stages of launchers or upper stages) [3]). Some important programs have recently been launched in Europe like the LYRA pro- stages) [3]. Some important programs have recently been launched in Europe like the ject (ASI/Italian Space Agency-Avio) [4], which provided the first impulse to the develop- LYRA project (ASI/Italian Space Agency-Avio) [4], which provided the first impulse to ment of future generation of VEGA upper stage engines, currently on-going due to AVIO the development of future generation of VEGA upper stage engines, currently on-going efforts [5]; Prometheus engine (100-t-thrust) project, led by CNES (Centre National due to AVIO efforts [5]; Prometheus engine (100-t-thrust) project, led by CNES (Centre d'Etudes Spatiales)/Airbus-Safran Launcher (France), is inserted in ESA FLPP Neo Pro- National d’Etudes Spatiales)/Airbus-Safran Launcher (France), is inserted in ESA FLPP gram (European Space Agency Future Launchers Preparatory Programme) and is devoted Neo Program (European Space Agency Future Launchers Preparatory Programme) and is to developing the future launcher family after Ariane 6 [6]. In the Russian Federation, devoted to developing the future launcher family after Ariane 6 [6]. In the Russian Federa- several projects, like Energomash, KBKhM (KB KhimMash/Volga staged combustion de- tion, several projects, like Energomash, KBKhM (KB KhimMash/Volga staged combustion rived engine with up to 10 t of thrust), KBKhA (KB KhimAvtomatika) and Starsem (So- derived engine with up to 10 t of thrust), KBKhA (KB KhimAvtomatika) and Starsem yuz), are historically active with this kind of issue and involved in several studies [7]. (Soyuz), are historically active with this kind of issue and involved in several studies [7]. Japan has reached high levels of readiness [8] through the consolidated cooperation be- Japan has reached high levels of readiness [8] through the consolidated cooperation be- tween JAXA (Japan Aerospace Exploration Agency) and IHI. In the United States, a lot of tween JAXA (Japan Aerospace Exploration Agency) and IHI. In the United States, a lot players are involved in several projects, oriented to different applications. It is worth re- of players are involved in several projects, oriented to different applications. It is worth membering t remembering he SpaceX the SpaceX Raptor Project [9], Bl Raptor Project ue-O [9],rig Blue-Origin in (240-t-thr (240-t-thr ust BE-4 ust engine BE-4 , based on engine, based staged combustion cycle) [10] and NASA, developing a 20-kN pressure-fed engine, in- on staged combustion cycle) [10] and NASA, developing a 20-kN pressure-fed engine, tended for the Morpheus lunar lander (Armadillo Aerospace, Mesquite, Texas, USA) [11]. intended for the Morpheus lunar lander (Armadillo Aerospace, Mesquite, TX, USA) [11]. Besides the aforementione Besides the aforementioned d initiatives, in It initiatives, aly there is a strong in Italy there is a str interest in chem ong interest in chemical ical space propu space pr lsopulsion ion issues issues and in t and he l iniq the uid liquid oxygen/ oxygen/liquid liquid methane c methane ombinat combination. ion. In fact In , tfact, he the Italian Ministry of University and Research is funding a specific program devoted to the Italian Ministry of University and Research is funding a specific program devoted to the consoli consolidation dation of current of curr teent chnologi technologies, es, methodologie methodologies s and m and anu manufacturing facturing capabilit capabilities ies as as well as the development of future propulsion systems. The program is named HYPROB well as the development of future propulsion systems. The program is named HYPROB (HYdrocarbon PROpulsion test Bench) and has been assigned to the Italian Aerospace (HYdrocarbon PROpulsion test Bench) and has been assigned to the Italian Aerospace Research Centre (CIRA) [12]. Among the different goals, the most significant objective Research Centre (CIRA) [12]. Among the different goals, the most significant objective is is the design, realization and testing of a LO /LCH demonstrator (DEMO), capable the design, realization and testing of a LOX/LCH4 dem Xonstrator (DE 4 MO), capable of 30 kN of 30 kN of thrust. An incremental approach strategy has been adopted to enlarge the of thrust. An incremental approach strategy has been adopted to enlarge the comprehen- comprehension of critical physical aspects through the design, manufacturing and testing of sion of critical physical aspects through the design, manufacturing and testing of specific specific test articles. Basic activities like the design and experiments on injectors or material test articles. Basic activities like the design and experiments on injectors or material char- characterizations have been accomplished. However, central issues were represented by the acterizations have been accomplished. However, central issues were represented by the experimental campaigns on igniters and subscale mono-injector engines (to investigate the experimental campaigns on igniters and subscale mono-injector engines (to investigate comprehension of supercritical combustion, heat release, injection and mixing phenomena, the comprehension of supercritical combustion, heat release, injection and mixing phe- etc.) as well the experimental studies on methane transcritical behaviour as depicted by nomena, etc.) as well the experimental studies on methane transcritical behaviour as de- Figure 1 [13]. picted by Figure 1 [13]. Figure 1. Logical steps of LOX/CH4 demonstrator (DEMO) development. Figure 1. Logical steps of LO /CH demonstrator (DEMO) development. X 4 The final object is represented by a 30-kN-class-thrust demonstrator. The baseline The final object is represented by a 30-kN-class-thrust demonstrator. The baseline thrust chamber assembly concept is depicted in Figure 2: it includes an igniter, an injector thrust chamber assembly concept is depicted in Figure 2: it includes an igniter, an injec- tor head with 18 injectors and a regenerative cooling jacket with 96 axial channels and inlet/outlet manifolds. In Table 1 the main parameters are detailed. Aerospace 2021, 8, x 3 of 21 Aerospace 2021, 8, 151 3 of 20 head with 18 injectors and a regenerative cooling jacket with 96 axial channels and in- let/outlet manifolds. In Table 1 the main parameters are detailed. 1—igniter; 2—injector head; 3—outlet manifold with collector; 4—thrust chamber including the cooling jacket; 5—inlet fuel collector and distributor. Figure 2. DEMO thrust chamber assembly view with the main components. Figure 2. DEMO thrust chamber assembly view with the main components. Table 1. HYPROB (HYdrocarbon PROpulsion test Bench) DEMO main performance parameters. Table 1. HYPROB (HYdrocarbon PROpulsion test Bench) DEMO main performance parameters. O/FO/F 3.4 3.4 PPCC 5. 5.55 MPaMPa CC Reaction efficiency 0.98 Isp 286 s Reaction efficiency 0.98 I 286 s sp Thrust 30 kN A /A 4.0 Thrust 30 kN A cc cc/At t 4.0 Based on a counter-flow architecture, methane enters the channels in the nozzle re- Based on a counter-flow architecture, methane enters the channels in the nozzle gion in liquid phase, is heated by the combustion gases along the chamber and, then, in- region in liquid phase, is heated by the combustion gases along the chamber and, then, jected into the combustion chamber as a supercritical gas through the injection head. The injected into the combustion chamber as a supercritical gas through the injection head. cooling jacket represents the most critical component: it is composed of an inner liner, The cooling jacket represents the most critical component: it is composed of an inner made of high thermal conductive materials (generally copper alloys), and by a close-out liner, made of high thermal conductive materials (generally copper alloys), and by a structure, made up of robust alloys, like Inconel or nickel, and consists 96 axial channels, close-out structure, made up of robust alloys, like Inconel or nickel, and consists 96 axial surrounding the combustion chamber. The design activities have been internally con- channels, surrounding the combustion chamber. The design activities have been internally ducted by means of in-house codes, as reported by [14], and supported by CFD analyses conducted by means of in-house codes, as reported by [14], and supported by CFD analyses (to characterize the thermal and fluid-dynamic behaviour of the cooling jacket) and (to characterize the thermal and fluid-dynamic behaviour of the cooling jacket) and thermo- thermo-structural verifications. In the process of developing the final DEMO version, a structural verifications. In the process of developing the final DEMO version, a firing test firing test campaign is needed in water-cooled mode to characterize the cooling jacket campaign is needed in water-cooled mode to characterize the cooling jacket behaviour and to behavio accomplish ur and t qualification o accompliof sh the qual innovative ification ofmanufacturing the innovative m method anufact (i.e., uring electr met oplating) hod (i.e., selected electroplating to join ) selected the copper to j liner oin the copp with the nickel er liner wi close-out. th the ni In fact, ckel CIRA, close- after out. In accomplishing fact, CIRA, aafter accomplishin specific technological g a spec activity ific techno , decided logical ac to substitute tivity, de the cided to planned subuse stitute the planned use of a conventional pr of a conventional proc ocess, based on brazing ess, of based on non-homogeneous brazing of non-h components, omogeneous with galvanic components, deposition with gal- of copper and nickel layers for the realization of the cooling jacket. In fact, large difficulties vanic deposition of copper and nickel layers for the realization of the cooling jacket. In were encountered in the repeatability of the brazing process for chambers with tens of fact, large difficulties were encountered in the repeatability of the brazing process for cooling channels working in tough conditions: they have to withstand pressure values up chambers with tens of cooling channels working in tough conditions: they have to with- to 16.0 MPa, are required to keep methane liquid as long as possible and huge thermal stand pressure values up to 16.0 MPa, are required to keep methane liquid as long as pos- loads (fluxes up to tens of MW/m ); walls should be as thin as possible for efficient thermal sible and huge thermal loads (fluxes up to tens of MW/m ); walls should be as thin as exchange and weight reduction purposes, as depicted in Figure 3a. According to the new possible for efficient thermal exchange and weight reduction purposes, as depicted in Fig- process, grooves in the liner part are milled and then overlaid with copper and nickel [15]: ure 3a. According to the new process, grooves in the liner part are milled and then over- in this way, channels are generated by a combination of two special galvanic depositions of laid with copper and nickel [15]: in this way, channels are generated by a combination of pure copper and nickel, as depicted by Figure 3b. two special galvanic depositions of pure copper and nickel, as depicted by Figure 3b. Aerospace 2021, 8, 151 4 of 20 Aerospace 2021, 8, x 4 of 21 Aerospace 2021, 8, x 4 of 21 (a) (b) (a) (b) Figure 3. Example of a typical liquid rocket engine (LRE) cooling jacket: (a) milled channels brazed onto the external close- Figure 3. Example of a typical liquid rocket engine (LRE) cooling jacket: (a) milled channels brazed onto the external close- Figure 3. Example of a typical liquid rocket engine (LRE) cooling jacket: (a) milled channels brazed onto the external out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. close-out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. The advantages are summarized in: (a) brazing and welding free process since cop- The advantages are summarized in: (a) brazing and welding free process since cop- The advantages are summarized in: (a) brazing and welding free process since copper per and alloys could be applied without thermal stresses and deterioration of base mate- per and alloys could be applied without thermal stresses and deterioration of base mate- and alloys could be applied without thermal stresses and deterioration of base materials rials and (b) high reliability because the process repeatability can be controlled, improving rials and (b) high reliability because the process repeatability can be controlled, improving and (b) high reliability because the process repeatability can be controlled, improving mechanical resistance, thermal and electrical conductivity of the deposited copper. Thus, memechanical chanical resist resistance, ance, ther ther mamal l and and elect electrical rical con conductivity ductivity of the of the deposited copp deposited copper er. Th . us, Thus, the use of this advanced process has been motivated by the possibility of avoiding any the use of this advanced process has been motivated by the possibility of avoiding any the use of this advanced process has been motivated by the possibility of avoiding any deterioration of base materials, and by a high level of repeatability and reliability [16]. deterioration of base materials, and by a high level of repeatability and reliability [16]. deterioration of base materials, and by a high level of repeatability and reliability [16]. However, this choice led to a plan to optimize activity on the cooling system arrangement However, this choi However, this choice ce led to a pla led to a plan n to optimi to optimize ze actactivity ivity on the coolin on the cooling g system system arrarrangement angement to adapt the brazed configuration to the electroplated one. At this point, the DEMO-0A to a to da adapt pt the bra the brazed zed confi configuration guration to the el to theectropla electroplated ted one. At thi one. At this s poi point, nt, the DEMO-0 the DEMO-0A A manufacturing phase has been completed and the product has been accepted by means of manufacturing phase has been completed and the product has been accepted by means of manufacturing phase has been completed and the product has been accepted by means of leak and proof test (Figure 4). The next step will be the integration with the injector head, leak and proof test (Figure 4). The next step will be the integration with the injector head, leak and proof test (Figure 4). The next step will be the integration with the injector head, alr already eady v validated alidated in in a a p pr reviou evious s f firing iring c campaign, ampaign, a and nd the the igniter igniter in in order order to to sta start rt the the final final already validated in a previous firing campaign, and the igniter in order to start the final test test a activity ctivity. . test activity. Figure 4. Pictures of DEMO-0A in the manufacturing phase and after the completion of realization phase [17]. Figure 4. Pictures of DEMO-0A in the manufacturing phase and after the completion of realization phase [17]. Figure 4. Pictures of DEMO-0A in the manufacturing phase and after the completion of realization phase [17]. Concerning the description of the cooling jacket thermal behaviour, in a regeneratively Concerning the description of the cooling jacket thermal behaviour, in a regenera- Concerning the description of the cooling jacket thermal behaviour, in a regenera- cooled LRE, the coolant is represented by the propellant. Moreover, in the case of cryogenic tively cooled LRE, the coolant is represented by the propellant. Moreover, in the case of tively cooled LRE, the coolant is represented by the propellant. Moreover, in the case of fluids like methane or hydrogen, the propellant behaviour in terms of phase and working cryogenic fluids like methane or hydrogen, the propellant behaviour in terms of phase cryogenic fluids like methane or hydrogen, the propellant behaviour in terms of phase conditions evolves rapidly in the cooling jacket. Thus, fluid is injected into the cooling and working conditions evolves rapidly in the cooling jacket. Thus, fluid is injected into and working conditions evolves rapidly in the cooling jacket. Thus, fluid is injected into system in liquid state (characterized by values of pressure and temperature higher and the cooling system in liquid state (characterized by values of pressure and temperature the cooling system in liquid state (characterized by values of pressure and temperature lower than critical values, respectively) and generally undergoes a “pseudo-phase change” higher and lower than critical values, respectively) and generally undergoes a “pseudo- higher and lower than critical values, respectively) and generally undergoes a “pseudo- from liquid-like state to a vapour-like one, as carried out by [18,19]. Thermo-physical phase change” from liquid-like state to a vapour-like one, as carried out by [18,19]. phase change” from liquid-like state to a vapour-like one, as carried out by [18,19]. properties change quickly around the near critical zone (T = 190.56 K and P = 4.59 MPa cr cr Thermo-physical properties change quickly around the near critical zone (Tcr = 190.56 K Thermo-physical properties change quickly around the near critical zone (Tcr = 190.56 K for methane), as reported in Figure 5a, but gradually if subcritical phase change phenomena and Pcr = 4.59 MPa for methane), as reported in Figure 5a, but gradually if subcritical phase and Pcr = 4.59 MPa for methane), as reported in Figure 5a, but gradually if subcritical phase are compared [20–22]. As aforementioned, methane thermo-physical properties inside change phenomena are compared [20–22]. As aforementioned, methane thermo-physical change phenomena are compared [20–22]. As aforementioned, methane thermo-physical the LRE channels may change rapidly and a heat transfer deterioration phenomenon properties inside the LRE channels may change rapidly and a heat transfer deterioration properties inside the LRE channels may change rapidly and a heat transfer deterioration may be observed from to the axial coordinate where the specific heat at constant pressure phenomenon may be observed from to the axial coordinate where the specific heat at con- phenomenon may be observed from to the axial coordinate where the specific heat at con- exhibits the peak (pseudo-critical conditions, T > T ) [23–25]. Pseudo-critical temperature pc cr stant pressure exhibits the peak (pseudo-critical conditions, Tpc > Tcr) [23–25]. Pseudo-crit- stant pressure exhibits the peak (pseudo-critical conditions, Tpc > Tcr) [23–25]. Pseudo-crit- increases as pressure increases while the specific heat peak value reduces, up to vanishing ical temperature increases as pressure increases while the specific heat peak value re- ical temperature increases as pressure increases while the specific heat peak value re- at very high values of pressure, as depicted by Figure 5b. Designers generally pay a lot of duces, up to vanishing at very high values of pressure, as depicted by Figure 5b. Designers duces, up to vanishing at very high values of pressure, as depicted by Figure 5b. Designers attention to properly conceiving the cooling passages to avoid the aforementioned critical generally pay a lot of attention to properly conceiving the cooling passages to avoid the generally pay a lot of attention to properly conceiving the cooling passages to avoid the phenomenon. In fact, due to the thermal stratification and disadvantageous physical aforementioned critical phenomenon. In fact, due to the thermal stratification and disad- aforementioned critical phenomenon. In fact, due to the thermal stratification and disad- conditions, low convective heat transfer coefficient values can be detected because a thin vantageous physical conditions, low convective heat transfer coefficient values can be de- vantageous physical conditions, low convective heat transfer coefficient values can be de- layer (behaving as a vapour and having low thermal conductivity) may divide the heated tected because a thin layer (behaving as a vapour and having low thermal conductivity) tected because a thin layer (behaving as a vapour and having low thermal conductivity) Aerospace 2021, 8, 151 5 of 20 Aerospace 2021, 8, x 5 of 21 wall from the core, composed of liquid-like layers. In the view of developing robust may divide the heated wall from the core, composed of liquid-like layers. In the view of design solutions for cooling jackets, risks, linked to the heat transfer deterioration on- developing robust design solutions for cooling jackets, risks, linked to the heat transfer set, can be reduced either by increasing the coolant pressure or by increasing the surface deterioration on-set, can be reduced either by increasing the coolant pressure or by in- roughness [24]. creasing the surface roughness [24]. ρ (density) [kg/m ] ρ [kg/m ] 18.0 16.0 Supercritical vapour 14 0 . 350 12.0 300 10.0 8.0 6.0 4.0 Critical Point 2.0 1 100 150 200 250 300 350 400 450 T[K] (a) (b) Figure 5. Methane thermo-physical properties, as a function of temperature and pressure: (a) density 3 Figure 5. Methane thermo-physical properties, as a function of temperature and pressure: (a) density (kg/m ); (b) specific (kg/m ); (b) specific heat (J/kg K). heat (J/kg K). Given this background, the coolant flow analysis and the deep comprehension of fluid Given this background, the coolant flow analysis and the deep comprehension of transcritical behaviour represent central points in the design activity of LO /LCH rocket X 4 fluid transcritical behaviour represent central points in the design activity of LOX/LCH4 engines: the prediction of surface temperature and heat flux from the combustion gases to rocket engines: the prediction of surface temperature and heat flux from the combustion the engine wall is directly dependent on heat transfer capabilities of the coolant [25]. More- gases to the engine wall is directly dependent on heat transfer capabilities of the coolant over, classical semi-empirical correlations for the evaluation of heat transfer coefficients do [25]. Moreover, classical semi-empirical correlations for the evaluation of heat transfer co- not work in the deteriorated mode and relatively high values of roughness, which may efficients do not work in the deteriorated mode and relatively high values of roughness, P [MPa] Aerospace 2021, 8, 151 6 of 20 Aerospace 2021, 8, x 6 of 21 occur since the typical dimension of channels (in the order of some mm) also represent which may occur since the typical dimension of channels (in the order of some mm) also represent challenging chaconditions llenging cond from itions a numerical from a numeric point of al poi view nt. of view Thus, an . Th ac us, curate an acc investigation urate inves- about configurations of rocket-engine-like cooling channels was needed before designing tigation about configurations of rocket-engine-like cooling channels was needed before the DEMO jacket. For this reason, a specific test article, named MTP-BB (Methane Thermal designing the DEMO jacket. For this reason, a specific test article, named MTP-BB (Me- Properties Breadboard) and shown by Figure 6, has been designed by CIRA and tested at thane Thermal Properties Breadboard) and shown by Figure 6, has been designed by different conditions [26]. The breadboard, made of a copper alloy, was provided with a CIRA and tested at different conditions [26]. The breadboard, made of a copper alloy, was single rectangular channel on the top, connected to the facility supplies through mechanical provided with a single rectangular channel on the top, connected to the facility supplies flanges, where temperature and pressure sensors were accommodated. Liquid methane through mechanical flanges, where temperature and pressure sensors were accommo- was injected in the rectangular passage (mass flow rate ranging from 0.01 to 0.06 kg/s, dated. Liquid methane was injected in the rectangular passage (mass flow rate ranging T ranging from 120 to 140 K and P in the range 6.0–16.0 MPa) and gradually heated in in from 0.01 to 0.06 kg/s, Tin ranging from 120 to 140 K and Pin in the range 6.0–16.0 MPa) and by means of the electrical cartridges, placed on the bottom part, to reach transcritical gradually heated by means of the electrical cartridges, placed on the bottom part, to reach conditions before exiting. Several internal thermocouples were placed inside the body transcritical conditions before exiting. Several internal thermocouples were placed inside to collect temperature data at different axial positions and at a distance from the channel the body to collect temperature data at different axial positions and at a distance from the bottom up to 4 mm. channel bottom up to 4 mm. Figure 6. MTP breadboard: sketch of the test article. Figure 6. MTP breadboard: sketch of the test article. The collected experimental data were useful to conduct a rebuilding activity in order The collected experimental data were useful to conduct a rebuilding activity in order to set the thermal numerical models and support the design of the DEMO cooling jacket. to set the thermal numerical models and support the design of the DEMO cooling jacket. In fact, the channel located on the top of the MTP breadboard has similar dimensions In fact, the channel located on the top of the MTP breadboard has similar dimensions with with respect to the DEMO cooling jacket; as well as, input heat flux (imposed from the respect to the DEMO cooling jacket; as well as, input heat flux (imposed from the base- basement), inlet conditions and mass flow rates are in the typical range of DEMO operating ment), inlet conditions and mass flow rates are in the typical range of DEMO operating conditions. Details on the experimental rebuilding and validation activity are reported conditions. Details on the experimental rebuilding and validation activity are reported in in [26]. [26]. In this paper, both methane-cooled and water-cooled modes of the DEMO cooling In this paper, both methane-cooled and water-cooled modes of the DEMO cooling system have been analysed through a 3-D CFD model regarding a single channel. The system have been analysed through a 3-D CFD model regarding a single channel. The validation of the numerical procedure has been accomplished indirectly through the exper- validation of the numerical procedure has been accomplished indirectly through the ex- imental results obtained in the MTP breadboard campaign for methane while waiting for perimental results obtained in the MTP breadboard campaign for methane while waiting future firing tests. However, the present investigation allows describing of the transcritical for future firing tests. However, the present investigation allows describing of the tran- behaviour in an LRE typical cooling jacket, on one hand, and to compare the response of such scritical behaviour in an LRE typical cooling jacket, on one hand, and to compare the re- systems, manufactured by means of different technological process, on the other one. In fact, sponse of such systems, manufactured by means of different technological process, on the comparisons between the brazed configuration of the demonstrator and the electroplated other one. In fact, comparisons between the brazed configuration of the demonstrator and one, named DEMO-0A, are carried out to underline the achieved improvements under the electroplated one, named DEMO-0A, are carried out to underline the achieved im- the thermal point of view. CFD results were adopted as input for the thermo-structural provements under the thermal point of view. CFD results were adopted as input for the simulations, needed to evaluate the lifecycle of the thrust chamber assembly. thermo-structural simulations, needed to evaluate the lifecycle of the thrust chamber as- sembly. 2. Materials and Methods The final demonstrator architecture provides a regeneratively cooled ground engine, 2. Materials and Methods which implements a typical counter-flow architecture. The cooling jacket is characterized The final demonstrator architecture provides a regeneratively cooled ground engine, by several narrow axial channels, defined in the bottom part by a copper alloy liner, covered which implements a typical counter-flow architecture. The cooling jacket is characterized with electrodeposited layers of pure copper and pure nickel, or by a brazed nickel part, by several narrow axial channels, defined in the bottom part by a copper alloy liner, cov- which represent the close-out part. ered with electrodeposited layers of pure copper and pure nickel, or by a brazed nickel An in-house design tool, based on typical correlations, was developed to design the part, which represent the close-out part. cooling jacket in terms of channel dimension, liner and rib thickness [14]. The adopted An in-house design tool, based on typical correlations, was developed to design the strategy considers a constant number of channels and a constant value for the rib width cooling jacket in terms of channel dimension, liner and rib thickness [14]. The adopted (w) while a variable value of the rib height (h) has been considered, according to Figure 7a. strategy considers a constant number of channels and a constant value for the rib width In this way, an optimization of the cooling performances was achieved, taking into account Symmetry Symmetry Aerospace 2021, 8, x 7 of 21 Aerospace 2021, 8, 151 7 of 20 (w) while a variable value of the rib height (h) has been considered, according to Figure 7a. In this way, an optimization of the cooling performances was achieved, taking into some account significant some sign sections, ificant se such ctions, s as the uch a nozzle s the nozz (NZ), thr le (oat NZ), t (CT) hro and at (C cylindrical T) and cylin part dric(CP), al part (CP), highlighted in Figure 8. Furthermore, Figure 8 depicts the dimensionless profile of highlighted in Figure 8. Furthermore, Figure 8 depicts the dimensionless profile of the thr th ust e th chamber rust cham , the bevariations r, the variat ofion cooling s of cooling ch channel hydraulic annel hydr diameter aulic diam (d et ) er along (dh) a the long the axial axis axiand al axis the and applied the ap heat plie flux d he pr atofile flux p (defined rofile (d as ef “nominal” ined as “nom in the inal “Results ” in the “ and ResDiscussion” ults and Dis- section), representing the design profile. It has been calculated through reactive simulations cussion” section), representing the design profile. It has been calculated through reactive inside the combustion chamber [15]. simulations inside the combustion chamber [15]. Considering L, the overall thrust chamber length, as the reference length, the geometric Considering L, the overall thrust chamber length, as the reference length, the geo- parameters are equal to: metric parameters are equal to: channel height (h/L), ranging from 0.0018 to 0.0061; • channel height (h/L), ranging from 0.0018 to 0.0061; channel width (b/L), ranging from 0.0019 to 0.0107; • channel width (b/L), ranging from 0.0019 to 0.0107; rib width (w/L) = 0.0032; • rib width (w/L) = 0.0032; liner thickness (h /L) = 0.0020; • liner thickness (h1/L) = 0.0020; copper layer height (h /L) = 0.0023; cu • copper layer height (hcu/L) = 0.0023; nickel layer height (h /L) = 0.0034. ni • nickel layer height (hni/L) = 0.0034. In the case of the DEMO brazed version, the copper layer and nickel close-out are In the case of the DEMO brazed version, the copper layer and nickel close-out are substituted by a unique Inconel part, joined with the liner rib through the brazing process. substituted by a unique Inconel part, joined with the liner rib through the brazing process. Close-out Close-out Copper Layer Fluid Fluid Liner Liner Thermal input Thermal input (a) (b) Figure 7. Sketch of the model concerning: (a) geometry description of the DEMO-0A cooling jacket—cross-section; (b) Figure 7. Sketch of the model concerning: (a) geometry description of the DEMO-0A cooling jacket—cross-section; (b) Aerospace 2021, 8, x 8 of 21 materials and boundary conditions for electroplated (left) and brazed (right) DEMO versions. materials and boundary conditions for electroplated (left) and brazed (right) DEMO versions. 0.12 thrust chamber profile Inlet q (reactive CFD results) hg,input 0.1 Section NZ 0.08 Section CP 0.06 Outlet 0.04 0.02 Section CT 0 0.2 0.4 0.6 0.8 1 x/L Figure 8. DEMO assembly details on chamber profile, hydraulic diameter of cooling channels and Figure 8. DEMO assembly details on chamber profile, hydraulic diameter of cooling channels and input heat flux profile. input heat flux profile. The numerical investigations on a single DEMO cooling channel, extracted from the complete model, were performed by means of ANSYS Fluent v17 (Canonsburg, Pennsyl- vania, USA) [27]. The solution of the governing equations, such as continuity, momentum and energy in three-dimensional form were accomplished in a steady state regime and considering an NIST (National Institute of Standard and Technology) real gas model and turbulent flow with thermo-physical properties, calculated through REFPROP v7.0 data- base [28]. Conduction effects have been contemplated. Rough channel walls have been taken into account while k-ω sst turbulence model was assumed [27,29]. A pressure-based method was selected to solve the energy and momentum equations and a second-order upwind scheme was chosen together with PISO (Pressure-Implicit with Splitting of Oper- ators) coupling to couple pressure and velocity, respectively. Concerning the convergence −6 criteria, residuals of velocity components and energy values were considered equal to 10 −9 and 10 , respectively. Initialization was performed at inlet section conditions, Pin = 16.0 MPa and Tin = 110 K for methane-cooled mode (overall mass flow rate is equal to 1.92 kg/s). For the water-cooled mode (overall mass flow rate is equal to 4.5 kg/s and 5.0 kg/s) Pin = 16.0 and 12.0 MPa (to take into account different test conditions offered by the test facility) and Tin = 293 K, respectively, were adopted. A NIST real gas model was applied and the single-species flow form was selected to eventually handle both liquid and vapour phases in supercritical pressure conditions [27,28], in the case of methane. In fact, if me- thane is considered, it is fundamental to the transcritical operating conditions of the work- ing fluid since both the simulation initialization and the convergence history can be influ- enced. According to Figure 7a,b, the considered solid materials were: (a) a copper alloy (Cu- CrZr) for the liner, chosen because of its high thermal conductivity and good mechanical properties, for both modes; and (b) Inconel or pure nickel, due their mechanical perfor- mances, for the close-out part in the case of brazed and electroplated versions, respec- tively. For DEMO-0A, a copper layer was deposited between the liner and the external close-out. The thermo-physical properties of all the materials were assumed to be depend- ent on temperature and they were calculated through a characterization campaign [14]. y/L, d [m] throat q [MW/m ] hg, input Aerospace 2021, 8, 151 8 of 20 The numerical investigations on a single DEMO cooling channel, extracted from the complete model, were performed by means of ANSYS Fluent v17 (Canonsburg, Pennsylva- nia, USA) [27]. The solution of the governing equations, such as continuity, momentum and energy in three-dimensional form were accomplished in a steady state regime and consider- ing an NIST (National Institute of Standard and Technology) real gas model and turbulent flow with thermo-physical properties, calculated through REFPROP v7.0 database [28]. Conduction effects have been contemplated. Rough channel walls have been taken into account while k-w sst turbulence model was assumed [27,29]. A pressure-based method was selected to solve the energy and momentum equations and a second-order upwind scheme was chosen together with PISO (Pressure-Implicit with Splitting of Operators) coupling to couple pressure and velocity, respectively. Concerning the convergence criteria, residuals of velocity components and energy values were considered equal to 10 and 10 , respectively. Initialization was performed at inlet section conditions, P = 16.0 MPa in and T = 110 K for methane-cooled mode (overall mass flow rate is equal to 1.92 kg/s). For in the water-cooled mode (overall mass flow rate is equal to 4.5 kg/s and 5.0 kg/s) P = 16.0 in and 12.0 MPa (to take into account different test conditions offered by the test facility) and T = 293 K, respectively, were adopted. A NIST real gas model was applied and the in single-species flow form was selected to eventually handle both liquid and vapour phases in supercritical pressure conditions [27,28], in the case of methane. In fact, if methane is considered, it is fundamental to the transcritical operating conditions of the working fluid since both the simulation initialization and the convergence history can be influenced. According to Figure 7a,b, the considered solid materials were: (a) a copper alloy (Cu- CrZr) for the liner, chosen because of its high thermal conductivity and good mechanical properties, for both modes; and (b) Inconel or pure nickel, due their mechanical perfor- mances, for the close-out part in the case of brazed and electroplated versions, respectively. For DEMO-0A, a copper layer was deposited between the liner and the external close-out. The thermo-physical properties of all the materials were assumed to be dependent on temperature and they were calculated through a characterization campaign [14]. Simulations contemplate a half channel to limit the computational efforts; in fact, a symmetry condition is adopted (due to the geometrical, thermal and fluid-dynamic symmetry); top wall was considered adiabatic (for a conservative approach) as well as the left one to take into account half rib. The thermal load was applied on the liner bottom surface by means of specific user-defined functions (udf ). Moreover, the following boundary conditions have been assigned: inlet section: uniform velocity (at a fixed mass flow) and uniform temperature profile; outlet section: pressure outlet in order to define the static pressure at flow outlets; channel walls: velocity components equal to zero. The nominal heat flux profile, adopted for the design, is depicted in Figure 8 and was obtained through reactive CFD simulations of the combustion chamber side, considering the thrust walls at a temperature of 300 K [15]. However, this approach is conservative but useful to adopt suitable margins in the cooling jacket design; then, a weak thermal coupling was taken into account for further analyses, as suggested by [18]. In this case, the thermal load was applied by means of a hot gas side convective heat transfer coefficient, calculated by scaling the “design” heat flux (obtained at a constant value of T equal to 300 K) wg and considering the average hot gas temperature inside the chamber (T ), according aw to Equation (1). T values, given in Figure 9, are evaluated through RPA code [30] for aw four significant sections, corresponding to inlet (1), throat (2), convergent/cylindrical part interface (3) and outlet (4). Then, at a fixed position, T is calculated by means of a linear aw interpolation between the values of adjacent sections. CFD,Twg = 300K h = (1) c, hg T T aw wg Aerospace 2021, 8, x 9 of 21 Simulations contemplate a half channel to limit the computational efforts; in fact, a symmetry condition is adopted (due to the geometrical, thermal and fluid-dynamic sym- metry); top wall was considered adiabatic (for a conservative approach) as well as the left one to take into account half rib. The thermal load was applied on the liner bottom surface by means of specific user-defined functions (udf). Moreover, the following boundary con- ditions have been assigned: • inlet section: uniform velocity (at a fixed mass flow) and uniform temperature profile; • outlet section: pressure outlet in order to define the static pressure at flow outlets; • channel walls: velocity components equal to zero. The nominal heat flux profile, adopted for the design, is depicted in Figure 8 and was obtained through reactive CFD simulations of the combustion chamber side, considering the thrust walls at a temperature of 300 K [15]. However, this approach is conservative but useful to adopt suitable margins in the cooling jacket design; then, a weak thermal coupling was taken into account for further analyses, as suggested by [18]. In this case, the thermal load was applied by means of a hot gas side convective heat transfer coeffi- cient, calculated by scaling the “design” heat flux (obtained at a constant value of Twg equal to 300 K) and considering the average hot gas temperature inside the chamber (Taw), according to Equation (1). Taw values, given in Figure 9, are evaluated through RPA code [30] for four significant sections, corresponding to inlet (1), throat (2), convergent/cylin- drical part interface (3) and outlet (4). Then, at a fixed position, Taw is calculated by means of a linear interpolation between the values of adjacent sections. Aerospace 2021, 8, 151 9 of 20 CFD ,Twg =300 K (1) h = c,hg TT − () aw wg T = 2456 K T = 3543 K T = 3538 K T = 3370 K aw aw aw aw 4 3 2 1 Section CT Section NZ Section CP Outlet Inlet Figure 9. Sketch reporting the hot gas average temperature (K) values along the engine for the Figure 9. Sketch reporting the hot gas average temperature (K) values along the engine for the cylindrical part, convergent, throat and nozzle sections, evaluated through RPA code. cylindrical part, convergent, throat and nozzle sections, evaluated through RPA code. W Wi ith th rega regards rds to to the a the adopted dopted mesh, a struct mesh, a structur ured ed grid, grid, composed composed of ofabout about2.7 2.7 million million nodes, was chosen and a sketch is reported in Figure 10. A mesh sensitivity analysis, de- nodes, was chosen and a sketch is reported in Figure 10. A mesh sensitivity analysis, described scribed byby TaT ble 2 able , wa 2, was s conconducted ducted on ton he model cons the model considering idering the nom the inominal nal condit conditions ions (nom- (nominal inal input input heat f heat lux, T flux, in = 11 T 0 K, P = 110 in K, = 1P 6.0 MP = 16.0 a and MPa m = and 0.01m kg/s = 0.01 for si kg/s ngle ha for single lf channel) half . in in channel). Three structured mesh distributions (“coarse”, “fine” and “finest”), generated Three structured mesh distributions (“coarse”, “fine” and “finest”), generated by follow- by following the mesh suggestions given by Ansys Fluent user ’s guide in the case of ing the mesh suggestions given by Ansys Fluent user’s guide in the case of rough channel rough channel walls [27], were considered: they have about 1.3, 2.7 and 5.6 million nodes, walls [27], were considered: they have about 1.3, 2.7 and 5.6 million nodes, respectively. respectively. Finally, the second grid case was chosen to perform the investigations, pre- Finally, the second grid case was chosen to perform the investigations, presented in the Aerospace 2021, 8, x 10 of 21 sented in the present paper, because it ensured a good compromise between the machine present paper, because it ensured a good compromise between the machine computa- computational time and the accuracy requirements. tional time and the accuracy requirements. (a) (b) Figure 10. Numerical modelling: (a) global view of cooling channel model extracted from cooling system geometry; (b) Figure 10. Numerical modelling: (a) global view of cooling channel model extracted from cooling system geometry; (b) mesh distribution with a detail on inlet zone (axial direction) and on a transversal section (divergent zone). mesh distribution with a detail on inlet zone (axial direction) and on a transversal section (divergent zone). Table 2. Grid-independence test results. Table 2. Grid-independence test results. Outlet Fluid Bulk Liner Maximum Channel Bottom Wall ΔP Outlet Fluid Bulk Liner Maximum Channel Bottom Wall Type of Mesh Temperature Temperature Maximum Temperature DP (MPa) Type of Mesh Temperature Temperature Maximum Temperature Tb,f out (K) Tw, hg max Tw, ch max (MPa) T (K) T T b,f out w, hg max w, ch max 1—coarse 5.051 420.1 600.4 555.4 1—coarse 5.051 420.1 600.4 555.4 2—fine 5.094 420.4 610.8 562.7 2—fine 5.094 420.4 610.8 562.7 3—finest 5.096 420.5 611.5 563.4 3—finest 5.096 420.5 611.5 563.4 The numerical approach and settings were validated through results obtained in the The numerical approach and settings were validated through results obtained in the rebuilding activity of the test campaign conducted on the aforementioned MTP bread- board, de rebuilding scribed in Figure activity of the test 6, with re campaign gard conducted to the tran on scri the tica afor l beha ementioned viour of methane [ MTP breadboar 15,26]. d, described in Figure 6, with regard to the transcritical behaviour of methane [15,26]. In In fact, the test article was provided with a channel, having dimensions representative of fact, the test article was provided with a channel, having dimensions representative of ones adopted in the DEMO and this is also true for the operating conditions. Figure 11 ones adopted in the DEMO and this is also true for the operating conditions. Figure 11 reports the comparisons between the experimental data (when stationary thermal regime reports the comparisons between the experimental data (when stationary thermal regime was reached) and the numerical results for two relevant test cases: test #24 and test #26, was reached) and the numerical results for two relevant test cases: test #24 and test #26, characterized by mass flow rates equal to 20.87 g/s and 20.57 g/s, Tin = 137.1 K and 140.8 K, and Pin = 11.21 MPa and 12.89 MPa, respectively. Numerical results fitted the experi- mental data very well, as depicted by Figure 11a and Figure 11b, with regards to pressure and fluid bulk temperature axial profiles, which are compared with test data. Moreover, very low discrepancies are observed in terms of solid temperature results if the numerical temperature profile at a distance of 4 mm from the channel base is compared with the related experimental data for test #24, as reported in Figure 11b. Further details on CFD simulations, modelling, solving strategies and both numerical and experimental results are given in [26]. throat Aerospace 2021, 8, 151 10 of 20 characterized by mass flow rates equal to 20.87 g/s and 20.57 g/s, T = 137.1 K and in 140.8 K, and P = 11.21 MPa and 12.89 MPa, respectively. Numerical results fitted the in experimental data very well, as depicted by Figure 11a,b with regards to pressure and fluid bulk temperature axial profiles, which are compared with test data. Moreover, very low discrepancies are observed in terms of solid temperature results if the numerical temperature profile at a distance of 4 mm from the channel base is compared with the related experimental data for test #24, as reported in Figure 11b. Further details on CFD simulations, modelling, solving strategies and both numerical and experimental results are Aerospace 2021, 8, x 11 of 21 given in [26]. inlet flange sensor outlet flange sensor inlet flange sensor Test n.24 - Numerical Rebuilding Test n.26 - Numerical Rebuilding Test n.24 - experimental data outlet flange sensor Test n.26 - experimental data 0 0.2 0.4 0.6 0.8 1 x/L (a) center line of channel upper wall center line of channel bottom wall solid T - line at 4 mm depth CFD results at sensors locations T experimental data - 4 mm depth experimental data - T b,f cr,f Test n.24 steady state regime 0 0.2 0.4 0.6 0.8 1 x/L (b) Figure 11. MTP breadboard rebuilding of hot tests: (a) Pressure drops (test #24 and #26); and (b) Fluid temperature and Figure 11. MTP breadboard rebuilding of hot tests: (a) Pressure drops (test #24 and #26); and (b) Fluid bottom wall temperature profiles (test 24). temperature and bottom wall temperature profiles (test 24). 3. Results and Discussion In the present paper, results for numerical simulations ran at nominal conditions (Tin, f = 110 K, Pin = 16.0 MPa and overall m = 1.92 kg/s) for methane mode for both the DEMO configurations, characterized by the electrodeposition and brazing process, are presented. Moreover, since the validation of the electrodeposition process will be accomplished by means of a specific test campaign considering water as refrigerant evolving in the demon- strator cooling system, the following initial conditions are also taken into account: Tin, f = 293 K, Pin = 16.0 and 12.0 MPa and overall m = 5.0 and 4.5 kg/s. These conditions were set according to the capabilities expressed by the test bench. Temperature[K] Pressure [MPa] Aerospace 2021, 8, 151 11 of 20 3. Results and Discussion In the present paper, results for numerical simulations ran at nominal conditions (T = 110 K, P = 16.0 MPa and overall m = 1.92 kg/s) for methane mode for both the in, f in DEMO configurations, characterized by the electrodeposition and brazing process, are presented. Moreover, since the validation of the electrodeposition process will be accom- plished by means of a specific test campaign considering water as refrigerant evolving in the demonstrator cooling system, the following initial conditions are also taken into account: T = 293 K, P = 16.0 and 12.0 MPa and overall m = 5.0 and 4.5 kg/s. These in, f in conditions were set according to the capabilities expressed by the test bench. The test matrix is reported by Table 3. Table 3. Test matrix of present numerical campaign. T P m Imposed Heat Flux Manufacturing in,f in Run Fluid (K) (MPa) (kg/s) (MW/m ) Process 1 110 16.0 1.92 Methane Nominal Electrodeposition 2 110 16.0 1.92 Methane Weak Coupling Electrodeposition 3 110 16.0 1.92 Methane Nominal Brazing 4 293 16.0 5.0 Water Nominal Electrodeposition 5 293 16.0 5.0 Water Nominal Brazing 6 293 16.0 4.5 Water Nominal Electrodeposition 7 293 12.0 5.0 Water Nominal Electrodeposition 8 293 12.0 4.5 Water Nominal Electrodeposition Results are presented in terms of axial profiles for the liner and channel wall temper- ature, fluid bulk temperature, convective heat transfer coefficient and pressure drops as well as fluid significant properties. Moreover, some temperature and fluid properties’ field plots, including some slices of significant interest, are presented. Figures 12 and 13 depict the axial profiles of hot gas walls, channel bottom walls and fluid bulk temperature for water-cooled and methane-cooled modes, respectively. Both figures point out that DEMO-0A, the optimized electroplated configuration of DEMO, presents a general reduction in terms of thermal stresses with respect to the brazed version. This is mainly due to the insertion of the copper layer to overlay the liner part and the possibility of changing the close-out material from Inconel to pure nickel. For the water- cooled mode, temperature peaks in the throat region and in the cylindrical section of the chamber resulted in reductions of about 30–40 K for a fixed mass flow rate (i.e., 5.0 kg/s), as depicted by Figure 12. A little decrease in terms of mass flow rate (4.5 kg/s, i.e., 10%) causes DEMO-0A profiles to overlap with ones obtained for the brazed version at mass flow rate equal to 5.0 kg/s. Moreover, fluid bulk temperature increases from about 400 to 420 K as mass flow rate decreases; water constantly remains in liquid phase, because of high pressure conditions as confirmed by the fluid bulk profiles and maximum values of channel walls. If methane is considered as refrigerant, the thermal behaviour of the DEMO-0A and the “brazed” versions of the DEMO detach significantly from each other, as depicted by Figure 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux is imposed, reduces from about 640 to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondence with the re-attachment zone of hot gases on the combustion chamber wall) decreases by about 90 K in DEMO-0A. Moreover, it is worthy to emphasise that in that zone, methane flows as a supercritical gas. A third relative maximum, observed in the brazed configuration and located in the divergent zone, tends to disappear in DEMO- 0A, due to a reduced local thermal stratification of the fluid. In fact, all the channel walls are made of copper alloys or pure copper, including the upper wall, and this determines a more homogeneous heat distribution inside the material and, consequently, in the fluid. Moreover, from that region fluid begins to locally change its conditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temperature profile: the working Aerospace 2021, 8, 151 12 of 20 fluid, after entering in liquid conditions, reaches critical temperature (about 190 K) near the throat region. From this section, methane tends to behave like a highly compressible fluid near the hot walls of the channel, and like a liquid near the cold ones, especially in the upper part of the channel. However, moving towards the outlet section, a larger fluid fraction behaves like a vapour. In the cylindrical part, methane is completely composed of a supercritical gas since the temperature is very high and the pressure is much higher than the critical value (4.6 MPa). If the “scaled” thermal load (considering a weak coupling approach) is applied, the liner results in being less thermally stressed as expected and, Aerospace 2021, 8, x FOR PEER REVIEW 12 of 20 in fact, a reduction of about 40 K is evaluated in terms of temperature peaks, located at Aerospace 2021, 8, x 13 of 21 x/L = 0.61 and x/L = 0.13. DEMO "brazed" version - m = 5.0 kg/s If methane is considered as refrigerant, the thermal behaviour of the DEMO-0A and DEMO-0A - m = 4.5 kg/s the “brazed” versions of the DEMO detach significantly from each other, as depicted by DEMO-0A - m = 5.0 kg/s Figure 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux w,hg is imposed, reduces from about 640 to 540 K while the absolute maximum, observed at w,ch about x/L = 0.13 (in correspondence with the re-attachment zone of hot gases on the com- b,f bustion 500 chamber wall) decreases by about 90 K in DEMO-0A. Moreover, it is worthy to emphasise that in that zone, methane flows as a supercritical gas. A third relative maxi- mum, observed in the brazed configuration and located in the divergent zone, tends to disappear in DEMO-0A, due to a reduced local thermal T stratification of the fluid. In fact, w,hg all the channel walls are made of copper alloys or pure copper, including the upper wall, and this determines a more homogeneous heat distribution inside the material and, con- sequently, in the fluid. Moreover, from that region fluid begins to locally change its con- w,ch ditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temper- ature profile: the working fluid, after entering in liquid conditions, reaches critical tem- b,f perature (about 190 K) near the throat region. From this section, methane tends to behave like a highly compressible fluid near the hot walls of the channel, and like a liquid near water cooled mode the cold ones, especially in the upper part of the channel. However, moving towards the outlet section, a larger fluid fraction behaves like a vapour. In the cylindrical part, methane 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 is completely composed of a supercritical gas since the temperature is very high and the x/L pressure is much higher than the critical value (4.6 MPa). If the “scaled” thermal load Fi (consi gurederin 12. Wa g t a er- wea cooled m k coupl od in e: a g approach xial profiles ) is of app hot lied gas w , tha e l lls, cha iner re nnel bott sults in om wa being llles s as n t d flu hermal id bu ly lk Figure 12. Water-cooled mode: axial profiles of hot gas walls, channel bottom walls and fluid temperature. stressed as expected and, in fact, a reduction of about 40 K is evaluated in terms of tem- bulk temperature. perature peaks, located at x/L = 0.61 and x/L = 0.13. If methane is considered as refrigerant, the thermal behaviour of DEMO-0A and “brazed” version of DEMO detaches significantly from each other, as depicted by Figure methane-cooled mode 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux is im- posed, reduces from about 640 K to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondence of the re-attachment zone of hot gases on the combustion 600 T w,hg chamber wall) decreases of about 90 K in DEMO-0A. Moreover, it is worthy to underline that in that zone, methane flows as a supercritical gas. A third relative maximum, ob- served in the brazed configuration and located in the divergent zone, tends to disappear in DEMO-0A, due to a reduced local thermal stratification of the fluid. In fact, all the chan- nel walls are made of copper alloys or pure copper, including the upper wall, and this w,ch determines a more homogeneous heat distribution inside the material and, consequently, in the fluid. Moreover, from that region fluid begins to locally change its conditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temperature profile: b,f the working fluid, after entering in liquid conditions, reaches the critical temperature DEMO "brazed" version (about 190 K) near the throat region. From this section, methane tends to behave like a DEMO-0A highly compressible fluid near the hot walls of the channel, and like a liquid near the cold DEMO-0A scaled ones, especially in the upper part of the channel. However, moving towards the outlet w,hg sectio 100 n, a larger flu Tid fraction behaves like a vapour. In the cylindrical part, methane is w,ch b,f completely composed by a supercritical gas since temperature is very high and pressure 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 is much higher than the critical value (4.6 MPa). If the “scaled” thermal load (considering x/L a weak coupling approach) is applied, the liner results to be less thermally stressed as Figure 13. Methane-cooled mode: axial profiles of hot gas walls, channel bottom walls and fluid expected and, in fact, a reduction of about 40 K is evaluated in terms of temperature peaks, Figure 13. Methane-cooled mode: axial profiles of hot gas walls, channel bottom walls and fluid bulk temperature. located at x/L = 0.61 and x/L = 0.13. bulk temperature. Figure 14 depicts the axial profiles for average convective heat transfer both for DEMO-0A and DEMO in the brazed configuration. It is shown that values are generally higher in the case of DEMO-0A if methane is considered as refrigerant and it is particu- larly evident in the divergent part and in the cylindrical part of the cooling system. Alt- hough, in the divergent zone, the fluid is in large part in liquid form, especially near the inlet section where the heat transfer coefficient values are lower than the ones obtained in the cylindrical part, where methane behaves like a supercritical vapour. This is mainly T[K] T[K] throat throat Aerospace 2021, 8, 151 13 of 20 Figure 14 depicts the axial profiles for average convective heat transfer both for DEMO- 0A and DEMO in the brazed configuration. It is shown that values are generally higher in the case of DEMO-0A if methane is considered as refrigerant and it is particularly evident in the divergent part and in the cylindrical part of the cooling system. Although, Aerospace 2021, 8, x 14 of 21 in the divergent zone, the fluid is in large part in liquid form, especially near the inlet section where the heat transfer coefficient values are lower than the ones obtained in the cylindrical part, where methane behaves like a supercritical vapour. This is mainly due to the high velocity attained by the fluid in the last sections of the cooling jacket, while in due to the high velocity attained by the fluid in the last sections of the cooling jacket, while most of the divergent part velocity remains low because of fluid state and the channel cross in most of the divergent part velocity remains low because of fluid state and the channel section dimension. Maximum values are attained in the throat region, where the minimum cross section dimension. Maximum values are attained in the throat region, where the dimensions of passages are designed, and there are very little differences between the minimum dimensions of passages are designed, and there are very little differences be- versions. Moreover, a profile for the water-cooled DEMO-0A mode is plotted in order to tween the versions. Moreover, a profile for the water-cooled DEMO-0A mode is plotted compare data, underlining that the convective heat transfer values are larger in the case of in order to compare data, underlining that the convective heat transfer values are larger water because of its basic thermo-physical properties. in the case of water because of its basic thermo-physical properties. DEMO-0A - methane-cooled DEMO (brazed conf.) - methane-cooled DEMO-0A - water-cooled (m = 5 kg/s) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L Figure 14. Convective heat transfer axial profiles. Figure 14. Convective heat transfer axial profiles. Regar Regard ding ing st static atic pressure pressure pr profiles, plott ofiles, plotted ed for for the the DEMO-0A DEMO-0A version version in in Figur Fige ure 15 ,15, it is it clear is clethat ar that pressur pressure e drops dro in ps in the nozzle the nozzle are very are ver lowybecause low because methane behaves like methane behaves like a liquid. a The highest increase is localized in the convergent part and the cylindrical one, as pointed liquid. The highest increase is localized in the convergent part and the cylindrical one, as out in Figure 15, because density is very low and velocity tends to increase towards the pointed out in Figure 15, because density is very low and velocity tends to increase to- outlet section. As a result, large pressure drops are evaluated although methane mass wards the outlet section. As a result, large pressure drops are evaluated although methane flow rate is about 2.5 times lower than water in the water-cooled mode. In fact, in the mass flow rate is about 2.5 times lower than water in the water-cooled mode. In fact, in water-cooled mode, the refrigerant remains in liquid form throughout the system. the water-cooled mode, the refrigerant remains in liquid form throughout the system. Figure 16 depicts the temperature field for DEMO-0A, including channel walls and some slices, pointing out that maximum temperature values are attained in correspondence with throat region and re-attachment point while Figure 17 gives a comparison with DEMO brazed configuration considering some significant cross sections. It is evident that the liner part is hotter if the brazed version of DEMO is compared with DEMO-0A in all of the considered cross sections. Furthermore, for both versions, it is shown that the thermal stratification of fluid is significant: near the bottom of the channel and near the rib walls the refrigerant is very hot while moving up towards the upper part of the channel, the temperature tends to decrease as it is possible to observe the sections until the throat region). Moreover, after entering the jacket as a compressed liquid, the fluid is heated h [W/m K] av throat Aerospace 2021, 8, 151 14 of 20 by the hot gases and from the throat region exhibits temperature values higher than the critical one on average. In this region and in the first part of convergence, it is possible to observe a gas-like fluid moving near the bottom walls of the channels while a liquid-like Aerospace 2021, 8, x 15 of 21 one is present in the upper parts. Water - m = 5.0 kg/s 1.6x10 Water - m = 4.5 kg/s Methane - m = 1.92 kg/s DEMO-0A P = 16.0 and 12.0 MPa in 1.4x10 1.2x10 1.0x10 8.0x10 6.0x10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L Aerospace 2021, 8, x 16 of 21 Figure 15. DEMO-0A: axial profiles of static pressure for both methane-cooled (Pin = 16.0 MPa) and Figure 15. DEMO-0A: axial profiles of static pressure for both methane-cooled (P = 16.0 MPa) and in water-cooled versions (Pin = 16.0 MPa and 12.0 MPa). water-cooled versions (P = 16.0 MPa and 12.0 MPa). in Figure 16 depicts the temperature field for DEMO-0A, including channel walls and some slices, pointing out that maximum temperature values are attained in correspond- ence with throat region and re-attachment point while Figure 17 gives a comparison with DEMO brazed configuration considering some significant cross sections. It is evident that the liner part is hotter if the brazed version of DEMO is compared with DEMO-0A in all of the considered cross sections. Furthermore, for both versions, it is shown that the ther- mal stratification of fluid is significant: near the bottom of the channel and near the rib walls the refrigerant is very hot while moving up towards the upper part of the channel, the temperature tends to decrease as it is possible to observe the sections until the throat region). Moreover, after entering the jacket as a compressed liquid, the fluid is heated by the hot gases and from the throat region exhibits temperature values higher than the crit- ical one on average. In this region and in the first part of convergence, it is possible to observe a gas-like fluid moving near the bottom walls of the channels while a liquid-like one is present in the upper parts. Figure 16. DEMO-0A: Temperature field of channel walls, including some slices—methane-cooled mode. Figure 16. DEMO-0A: Temperature field of channel walls, including some slices—methane-cooled mode. After the convergence zone, the fluid appears to be in supercritical gas state. This is also confirmed by the density axial profiles, strictly linked to temperature ones, and by the specific heat distribution, given in Figure 18a: from the throat region, the fluid core is characterized by low values of density and high values of specific heat. In particular, this is evident in the convergence section where a large part of the fluid exhibits the highest values of specific heat while relatively low values of thermal conductivity can be detected. The pseudo-critical condition is observed at about x/L = 0.50 and in that region low ther- mal conductivity values are observed as depicted by Figure 18b. However, a deterioration mode, characterized by a sort of a thermal barrier between the fluid near to the hot wall and the cold upper part of the channel [22], is not significant for both the demonstrator versions. In fact, aforementioned profiles of the liner temperature do not exhibit this phe- nomenon and this is due to the channel dimensions (optimized by means of the design codes), relatively high value of the wall roughness and fluid pressure, sufficiently far from the critical value [23]. Pressure[Pa] throat Aerospace 2021, 8, 151 15 of 20 Aerospace 2021, 8, x 17 of 21 Aerospace 2021, 8, x 17 of 21 Figure 17. Methane-cooled mode—temperature fields plotted for some significant slices. Figure 17. Methane-cooled mode—temperature fields plotted for some significant slices. DEMO-0A After the convergence zone, the fluid appears to be in supercritical gas state. This is DEMO "brazed" version also confirmed by the density axial profiles, strictly linked to temperature ones, and by Density Specific Heat the specific heat distribution, given in Figure 18a: from the throat region, the fluid core is methane-cooled mode characterized by low values of density and high values of specific heat. In particular, this is evident in the convergence section where a large part of the fluid exhibits the highest values of specific heat while relatively low values of thermal conductivity can be detected. The pseudo-critical condition is observed at about x/L = 0.50 and in that region low thermal conductivity values are observed as depicted by Figure 18b. However, a deterioration c 4000 mode, characterized by a sort of a thermal barrier between the fluid near to the hot wall and the cold upper part of the channel [22], is not significant for both the demonstrator versions. In fact, aforementioned profiles of the liner temperature do not exhibit this phenomenon and this is due to the channel dimensions (optimized by means of the design codes), relatively high value of the wall roughness and fluid pressure, sufficiently far from the critical value [23]. Figure 17. Methane-cooled mode—temperature fields plotted for some 3000 significant slices. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 DEMO-0A x/L DEMO "brazed" version (a) Density Specific Heat methane-cooled mode c 4000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L (a) Figure 18. Cont. Density [kg/m ] Density [kg/m ] throat throat Specific Heat [Jkg/K] Specific Heat [Jkg/K] Aerospace 2021, 8, 151 16 of 20 Aerospace 2021, 8, x 18 of 21 1.6E-04 0.25 DEMO-0A DEMO "brazed" version Thermal Conductivity 1.4E-04 Viscosity 0.2 methane-cooled mode 1.2E-04 1.0E-04 0.15 8.0E-05 0.1 6.0E-05 4.0E-05 0.05 2.0E-05 0.0E+00 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L (b) Figure 18. Methane-cooled mode, axial profiles: (a) average density and specific heat; (b) thermal conductivity and vis- Figure 18. Methane-cooled mode, axial profiles: (a) average density and specific heat; (b) thermal cosity. conductivity and viscosity. 4. Conclusions 4. Conclusions In this paper, the thermal and fluid-dynamic analyses, supporting the design of In this paper, the thermal and fluid-dynamic analyses, supporting the design of the the cooling system of the 30-kN thrust class HYPROB LO /LCH demonstrator, were cooling system of the 30-kN thrust class HYPROB LOX/LCH4 demonstrator, were dis- discussed. In particular, the final demonstrator cooling jacket configuration, based on cussed. In particular, the final demonstrator cooling jacket configuration, based on elec- electroplating manufacturing process was analysed by means of 3-D CFD simulations troplating manufacturing process was analysed by means of 3-D CFD simulations and and compared with the behaviour of the original brazed configuration. An NIST real compared with the behaviour of the original brazed configuration. An NIST real gas gas model was adopted to describe the behaviour of evolving fluids and, in particular, of model was adopted to describe the behaviour of evolving fluids and, in particular, of the the transcritical conditions experimented by methane flowing in the cooling system. The transcritical conditions experimented by methane flowing in the cooling system. The se- selected arrangement was discussed by means of results, given in terms of temperature, lected arrangement was discussed by means of results, given in terms of temperature, fluid bulk temperature and pressure profiles. Simulations supported the optimization fluid bulk temperature and pressure profiles. Simulations supported the optimization of of the design of the cooling jacket, realized through electrodeposition: in fact, the most the design of the cooling jacket, realized through electrodeposition: in fact, the most ther- thermally stressed zones, such as the throat region and the re-attachment point zone, mally stressed zones, such as the throat region and the re-attachment point zone, were were identified. identified. The adoption of the electrodeposition process, instead of the brazing one, led to The adoption of the electrodeposition process, instead of the brazing one, led to sig- significant thermal benefits all over the cooling jacket. In fact, in the case of nominal nificant thermal benefits all over the cooling jacket. In fact, in the case of nominal operat- operating conditions and if methane cooled versions are considered, a decrease of about ing conditions and if methane cooled versions are considered, a decrease of about 100 K 100 K is observed both in the throat region (x/L = 0.61) and at the end of the cylindrical is observed both in the throat region (x/L = 0.61) and at the end of the cylindrical part of part of the chamber (x/L = 0.13), as shown by Figure 13. In fact, convective heat transfer the chamber (x/L = 0.13), as shown by Figure 13. In fact, convective heat transfer values values are generally higher in the case of DEMO-0A and the differences with the brazed are generally higher in the case of DEMO-0A and the differences with the brazed config- configuration are significant in the divergent part (t times higher at maximum) and in uration are significant in the divergent part (t times higher at maximum) and in the cylin- the cylindrical part of the cooling system (1.2 times on average), as can be observed in drical part of the cooling system (1.2 times on average), as can be observed in Figure 14. Figure 14. Moreover, a different behaviour is observed in the divergent zone, where a Moreover, a different behaviour is observed in the divergent zone, where a thermal relax- thermal relaxation of the liner is reported, since at x/L = 0.70, a difference of about 180 K is ation of the liner is reported, since at x/L = 0.70, a difference of about 180 K is evaluated evaluated comparing DEMO-0A results with the brazed configuration ones. This is due to comparing DEMO-0A results with the brazed configuration ones. This is due to the re- the reduced thermal stratification of fluid in the case of the electroplated demonstrator, as duced thermal stratification of fluid in the case of the electroplated demonstrator, as also also evident from the temperature field given for the cross-section at x/L = 0.70 in Figure 17. evident from the temperature field given for the cross-section at x/L = 0.70 in Figure 17. In In addition, the water-cooled mode was considered since the first firing campaign will be addition, the water-cooled mode was considered since the first firing campaign will be conducted considering water as refrigerant to accomplish the acceptance tests of DEMO-0A conducted considering water as refrigerant to accomplish the acceptance tests of DEMO- and to validate the adopted technological process. In the water-cooled mode, temperature 0A and to validate the adopted technological process. In the water-cooled mode, temper- peaks in the throat region and in the cylindrical section of the chamber decreases by about ature peaks in the throat region and in the cylindrical section of the chamber decreases by Viscosity [Pa s] throat Thermal Conductivity [W/mK] Aerospace 2021, 8, 151 17 of 20 30–40 K for a fixed mass flow rate in the electroplated version of demonstrator, as reported in Figure 12. Results about the most significant thermo-physical properties have been presented to describe the transcritical behaviour of methane inside the cooling jacket. Furthermore, temperature fields in Figure 17 have been discussed to underline the thermal stratification occurring inside the channel and the thermal response of the liner. Methane, injected as a compressed liquid, tends to be in those conditions until half divergent, then the fluid layers near the bottom part of the channel tends to behave like a vapour. In the throat region the fluid bulk temperature reaches critical values (about 190 K on average) and all the thermo-physical properties dramatically change in the near-critical conditions as depicted by Figure 18 while the pseudo-critical conditions are attained in the convergent part (at x/L = 0.50). No peaks or temperature irregularities are observed in that zone; thus, no evident thermal deterioration phenomena are significant since pressure is two times the critical values. In the cylindrical part of the chamber, the fluid is fully composed of a supercritical vapour. The present activity was propaedeutic to perform thermo-structural simulations, nec- essary to estimate the lifecycle of the thrust chamber assembly, whose manufacturing has been recently completed and is ready to withstand the firing test campaign. The adoption of the electroplating process, instead of the brazing one, to join the liner and the close-out has provided evident benefits in terms of the thermal response of the demonstrator (besides the other advantages linked to the process repeatability and the absence of heat treatments). It is expected that the objective of 5 firing tests with a duration of 30 s (considering a safety margin of 4) will be fulfilled. After completing the test activity, experimental results will be extracted to run further simulations and accomplish a wide numerical rebuilding activity. Author Contributions: For the present research paper, authors declare as following: Conceptualiza- tion, D.R., F.B. and M.F.; methodology, D.R., F.B. and M.F.; software, D.R.; validation, D.R. and F.B.; formal analysis, D.R., F.B. and M.F.; investigation, D.R., F.B. and M.F.; resources, D.R. and F.B.; data curation, D.R. and F.B.; writing—original draft preparation, D.R., F.B. and M.F.; visualization, D.R.; supervision, F.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was performed in the framework of the HYPROB program, funded by the Italian Ministry of University and Research. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data presented in this study are available on request from the corre- sponding author. Data are not publicly available due to CIRA policy on intellectual copyright. Acknowledgments: This work was performed in the framework of the HYPROB Program, financed by the Italian Ministry of University and Research. The authors would like to thank our colleagues, Daniele Cardillo, Pasquale Natale and Michele Ferraiuolo, whose efforts were thoroughly appreciated, as well as Andrea Ceracchi, Luca Manni and CECOM personnel for their professionalism and competences, proved throughout the cooperation activities. Conflicts of Interest: The authors declare no conflict of interest. Aerospace 2021, 8, 151 18 of 20 Nomenclature b Cooling channel width: m BB Breadboard CFD Computational Fluid-Dynamic –1 –1 c Specific heat, J kg K CP Cylindrical part of the combustion chamber CT Convergent section d Diameter, m DEMO Demonstrator FSBB Full Scale BreadBoard h Channel height of DEMO, m –2 –1 h Convective heat transfer coefficient, W m K HS Heat Sink HYPROB Hydrocarbon PROpulsion test Bench I Impulse, s 2 –2 k Turbulence kinetic energy, m s L Length of test articles, m LCH Liquid methane LO Liquid oxygen LRE Liquid rocket engine –1 m Mass flow rate, kg s MTP Methane thermal properties NIST National Institute of Standard and Technology NZ Nozzle section O/F Mixture ratio (oxidizer mass/fuel mass), - P Pressure, Pa –2 q Input heat flux, W m SSBB SubScale BreadBoard sst shear stress transport T Temperature, K w Rib width, m x, y, z Spatial coordinates, m Greek symbols –1 –1 Thermal conductivity, W m K m Viscosity, Pa s r Density, kg m –1 w Specific dissipation rate of turbulence kinetic energy, s Subscripts av average aw adiabatic wall b bulk cc combustion chamber ch channel cr critical cu copper f fluid h hydraulic hg hot gas side in inlet l liner ni nickel out outlet pc pseudo-critical s static sp specific t throat w wall Aerospace 2021, 8, 151 19 of 20 References 1. 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Journal

AerospaceMultidisciplinary Digital Publishing Institute

Published: May 27, 2021

Keywords: liquid rocket engine; numerical analyses; thermal control; cooling jacket design; regenerative cooling; methane transcritical behaviour; electrodeposition technology; brazing process

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