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Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion Applications

Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion Applications Decarbonising the energy grid through renewable energy requires a grid firming technology to harmonize supply and demand. Hydrogen-fired gas turbine power plants offer a closed loop by burning green hydrogen produced with excess power from renewable energy. Conventional dry low NOx (DLN) combustors have been optimized for strict emission limits. A higher flame temperature of hydrogen drives higher NOx emissions and faster flame speed alters the combustion behavior signifi- cantly. Micromix combustion offers potential for low NOx emissions and optimized conditions for hydrogen combustion. Many small channels, so-called airgates, accelerate the airflow followed by a jet-in-crossflow injection of hydrogen. This leads to short-diffusion flames following the principle of maximized mixing intensity and minimized mixing scales. This paper shows the challenges and the potential of an economical micromix application for an aero-derivative industrial gas turbine with a high-pressure ratio. A technology transfer based on the micromix combustion research in the ENABLEH2 project is carried out. The driving parameter for ground use adaption is an increased fuel orifice diameter from 0.3 mm to 1.0 mm to reduce cost and complexity. Increasing the fuel supply mass flow leads to larger flames and higher emissions. The impact was studied through RANS simulation and trends for key design parameters were shown. Increased velocity in the airgates leads to a higher pressure drop and reduced emissions through faster mixing. Altering the penetration depth shows potential for emission reduction without compromising on pressure loss. Two improved designs are found, and their performance is discussed. Keywords Decarbonisation · Hydrogen Combustion · Stationary Gas Turbine · Industrial Gas Turbine · Micromix · NOx · Jet-in-Crossflow · Computational Fluid Dynamics · RANS · FGM Abbreviations and Symbols Ma Mac h number in the fuel orifice fuel DLN Dry Low Nitrogen Oxide Emissions Ma Mach number in the airgate air ENABLEH2 Enabling Cryogenic Hydrogen-Based CO D Diame ter of the fuel orifice 2 fuel Free Air Transport W Airgate width gate FGM Flamelet Generated Manifold w N on-dimensional airgate width gate LES Large Eddie Simulation h N on-dimensional airgate height gate NOx Nitrogen Oxide Emissions WH Width-to-height ratio of the airgate RANS R eynolds Averaged Navier–Stokes h Non-dimensional radial spacing rs SST Shear Stress Transport w Non-dimensional lateral spacing ls J Momentum Flux Ratio hte Non-dimensional Height trailing edge Φ Equivalence ratio h Non-dimensional radial distance from pair rs to pair u Velocity of the hydrogen jet jet * Johannes Berger u Velocity of the air in the airgate air st108655@stud.uni-stuttgart.de ρ Density of the jet jet Institute of Combustion Technology for Aerospace ρ Density of the air in the airgate air Engineering, University of Stuttgart, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany Centre for Propulsion Engineering, Cranfield University, College Road, Cranfield MK43 0AL, UK Vol.:(0123456789) 1 3 240 J. Berger o N on-dimensional offset injection from by Cranfield University works to ‘mature critical technolo- inj airgate outlet gies for LH2-based propulsion’ [14]. Part of this goal is to o Non-dimensional offset trailing edge from develop a novel micromix combustor for hydrogen combus- te injection tion. The aim of this work is to investigate a technology transfer of hydrogen combustion in aero applications to sta- tionary gas turbines. The research presented in this paper 1 Introduction builds on the ongoing work and generated knowledge at Cranfield University [15– 24] to develop a micromix hydro- The global average surface temperature in 2020 was 1.02 °C gen combustor for aero applications. In this paper, RANS higher compared to the average temperature between 1950 CFD is used to model design adaptations and to assess their and 1981 [1]. An uncontrolled increase in temperature led impact on flame structure, outlet conditions, and nitrogen and presumably leads to devastating environmental and oxide (NOx) emissions. social catastrophes [2–4]. The global temperature change needs to be limited to well below 2 °C as named in the Paris Agreement [5] and this ‘would require unprecedented transi- 2 Stationary Gas Turbine Combustion tions in all aspects of society’, says the Intergovernmental Panel on Climate Change [6]. Decarbonising the energy sys- The requirements for gas turbine combustors are high com- tems and thus reducing the greenhouse effect by reducing bustion efficiency and low pressure loss, evenly distributed carbon dioxide emissions is one measure to achieve the set temperature on the outlet plane, and a minimum of emis- goals. The potential use of hydrogen as an alternative energy sions. Stationary gas turbines offer more freedom in design carrier has recently been more widely recognized and is in than their aero counterparts regarding size, weight and the process of being explored. auxiliary systems [25]. On the other hand, a longer lifespan Hydrogen can be applied to sectors in which electrifica- is desired as well as lower operating cost [10]. Combus- tion is not economically feasible and has the potential to tors initially constructed for ground application as well as firm the energy grid and harmonize the supply of renewable those derived from aero application were designed as diffu- energy with demand through green hydrogen production [7]. sion flame combustors with uncontrolled NOx emissions. This so-called power-to-gas process can form a decarbonised These diffusion flame combustors were later modified by and firmed cycle of clean power utilizing a fuel cell or a sta- adding demineralized water, steam or nitrogen as diluent tionary gas turbine for power production and potentially heat and, because of the reduced flame temperature leading to supply. A strategic positioning of the power plants can maxi- reduced NOx emissions, met emission standards and could mize the synergy effects of power supply and waste heat use thereby increase power output. Using a diluent, however, as well as the blending with renewable energy sources [8]. limits the operability and drives direct operation cost of the Carbon-based fuels have been subject to research for dec- engines and is thus not desired. Newer DLN combustors ades. Auto-ignition times, flammability limits, mixing and offer low levels of emissions of NOx through premixed, lean quenching behavior have been and are intensively studied. combustion at low flame temperatures. A variety of differ - An overview and detailed description can be found in Glass- ent DLN combustor designs for stationary gas turbines have man et al. [9]. Specifically, gas turbines have been subject to been developed [26]. Multi-Fuel premixed DLN combustors research to lower emissions and increase cycle efficiency. In offer superior operability but are challenging to control at all the past years, the development of combustors for dry and load levels regarding fuel and air levels, ambient conditions, low nitrogen oxide (DLN) emissions has been the focus [10]. and fuel composition. Introducing high hydrogen levels in By lean premixing, the flame temperature has been lowered the fuel alters the combustion behavior; flame speed, auto- and thus NOx emissions have decreased. ignition risk, thermo-acoustic risk, and flame temperature However, due to significantly different combustion behav - increase. Lower CO emissions and wider flammability lim- ior, the current technologies and models are not easily adapt- its are positive effects, but lower volumetric energy density able for pure hydrogen combustion. Engine manufacturers of requires adapted fuel supply controls and storage solutions stationary gas turbines currently modify their state-of-the-art [27]. Combustion of 100% hydrogen without diluent requires engines to a higher hydrogen content capability [11, 12]. The ‘intensive R&D activity to pave the way for such a tech- first entirely hydrogen burning DLN industrial gas turbine nology’ [28], since ‘safe and reliable operation [for pure with a power output of 1.1 MW has recently been put into hydrogen combustion] is obtained in diffusion combustors use [13]. Aero gas turbines can be potentially modified for only’ [29]. hydrogen combustion and enable flying without direct car - Burning pure hydrogen and meeting emissions require- bon emissions. The ‘Enabling Cryogenic Hydrogen-Based ments as well as ensuring economical operability requires CO Free Air Transport’ (ENABLEH2) project coordinated a new combustion technology. One of the promising 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 241 technologies for burning hydrogen in gas turbines is micro- University’s micromix system showed that a fuel orifice mix combustion. The leading principle is to maximize mix- diameter of 0.3 mm is the best compromise of applicability ing intensity and minimize the mixing scale using many and performance [20]. For this study, a diameter of 1.0 mm small high-velocity channels followed by a jet-in-crossflow was selected as a design target to allow lower-quality fuel injection of hydrogen resulting in a fast mixing, lean burn- and compromise on cost and complexity, while attempting ing, short-diffusion flame. This is limited by compromising not to trade-off with increased NOx emissions. The change on the overall pressure loss [30] and manufacturability. The in fuel mass flow by increasing the fuel orifice diameter micromix principle is shown in Fig. 1 [18]: The inner and subsequently changes the air mass flow per gate, the airgate outer vortices stabilize the flame and ensure good mixing. size, and the required number of airgates. A similar study The residence time and flame temperature are kept low to was done by Haj Ayed et al. [35] at 1 bar, showing satisfac- minimize the formation of thermal NOx but also flashback tory results for a larger fuel orifice. and auto-ignition risks are eliminated by not premixing Less restrictive size and weight limitations as well as air and fuel in the channels. Micromix has been subject to demand for multi-fuel capability and restrictive emission research at Aachen University of Applied Science working regulations led to the development of stand-alone industrial together with Kawasaki Heavy Industries [31]. NASA pub- gas turbine combustor architectures [25]. This was due to lished a paper applying lean direct injection to develop a the necessity of lean and staged combustion for higher resi- micromix-like ‘low emission hydrogen combustor’ [32]. A dence times which reduce CO emissions but also produce similar concept but with micro-premixing channels has been low flame temperatures limiting NOx emissions. In general, called ‘Multi-Tube Mixer’ by GE [33]. The micro-premixed the micromix system tends to have a larger cross section: the 'multiple-injection burner’ was researched by MHPS and airstream is split into many airgates and the airgates need to Hitachi [34]. be spaced out for flame separation leading to a higher block - In the ENABLEH2 project, micromix combustion is age ratio than conventional combustors and thus to a larger being matured to TRL 3 for high-pressure ratio aviation face area for the airgate arrangement. On the other hand, application. This project explores feasibility of liquid hydro- due to the intense and fast mixing leading to short flames, gen as an aircraft fuel and includes the fuel system as well. the combustor length decreases. Following this, the archi- The micromix development focuses on Reynolds-averaged tecture which suits micromix combustion best is the annular Navier–Stokes (RANS) and large eddie simulations (LES) combustor with an increased face area. Redirecting the flow adapted for micromix hydrogen combustion and their limita- into can combustors as in a state-of-the-art stationary dry tions [15–19], the design of the micromix geometry [20, 21], low natural gas combustor, which permit short shafts, long and thermoacoustic modeling [22–24]. residence times and lean combustion, is not necessary. Stationary gas turbines are constrained by different The single shaft gas turbine is limited to lower pressure boundary conditions, but a technology transfer still offers ratios and has a lower single-cycle efficiency. However, this synergies. In a power plant application, gas turbine com- is desirable for ground use because it leads to higher exhaust bustors face fewer challenges regarding altitude relight and temperatures and supports a supercritical steam cycle, which critical safety measures. Weight and size limitations are less leads to higher combined cycle efficiencies compared to strict. Contrary to that, lifespan, development, production a non-single shaft gas turbine. The combined cycle has a and direct operation cost are strongly restricted. Cranfield slower reaction time and is well suited for base loads and more predictable load changes. For adaptive grid firming, when quick reaction times are wanted, the aero-derivative two or three shaft stationary gas turbines stand out due to their high single-cycle efficiency and fast ramp-up time. Accordingly, for grid firming supporting renewable energies, the former industrial RB211, now SGT-A35, aero-derivative model was chosen as reference for this paper. 3 Micromix Geometry Definition The cycle assessment based on data of the SGT-A35 from Siemens Energy [36] defined the necessary fuel and air mass flows. The total size limit for the combustor was defined by evaluating the engine cross section. Agarwal et al. [20] showed a rectangular airgate which has on the opposite side Fig. 1 Micromix combustion schematic from www. enabl eh2. eu 1 3 242 J. Berger of the fuel injection an arch. This airgate shape was chosen by the single airgate fuel flow. The key airgate design param- for this paper, too. Sun et al. [17] varied the airgate shape eters are shown in Fig. 2. With the available total design and showed the beneficial effect of a pair arrangement for space for the injector plate, limiting the trailing edge height the flame structure. Placing the blunt side of the airgates (h ) to a set value to accommodate the fuel tube and aiming te towards each other as shown in Fig. 2 leads to a stronger for 6 mm in radial distance (h ) from pair to pair to avoid rs’ recirculation between the gates. This helps to stabilize and radial merging in between airgate pairs, the number of rows form the flame as well as simplifies the design due to a single and subsequently the radial and lateral spacing (h, w ) of rs ls fuel tube for both airgates. The equivalence ratio (Φ) was set an airgate were found. A reference baseline design for a pair to 0.4 for lean and stable combustion and the Momentum of airgates with a fuel orifice diameter of 0.3 mm is shown Flux Ratio (J) was set to 5, which has shown good results in on the left side of Fig. 3. Following the same design routine current simulations at Crane fi ld University. The Momentum for a fuel orifice of 1.0 mm leads to the baseline design on Flux Ratio connects the density and velocity of fuel and the right in Fig. 3. airflow. This leads to a significant impact on the penetration depth and influences recirculation and flame merging. The Momentum Flux Ratio is defined as shown in (3.1):4 Simulation Setup ⋅ u jet jet Combustion simulation is challenging because it necessi- J = (3.1) tates resolving a broad range of three-dimensional and time- ⋅ u air air dependant scales of the combustion process and flow field, The optimum fuel orifice diameter (D ) is 0.3 mm for which influence each other. The geometries were modeled fuel hydrogen aero applications. However, in stationary gas tur- in STAR-CCM + using RANS with k-ω Menter shear stress bines, a limitation of cost and the potential for blockage transport (SST), the reaction mechanism from Naik et al. due to contaminants in pipeline delivered fuel makes a fuel [37, 38] and the Zeldovich combustion model. The models orifice diameter of 1.0 mm desirable. The ratio of fuel-to- are set up to be three-dimensional, ideal gas with segre- air density is below 1/5. Combined with a Momentum Flux gated flow and fluid enthalpy, and an all y + wall treatment. Ratio of 5, this leads to a lower Mach number of air (Ma ) The chosen RANS approach limits computational power air than fuel (Ma). Ma was limited to 0.3 to avoid com- necessary but produces only time-averaged solutions. The fuel fuel pressibility effects and high-pressure losses. The fuel mass turbulent mixing of micromix combustion is expected to flow per gate was obtained through the assumption of D produce a dynamic flow field and local hot spots cannot be fuel and Ma . With the set equivalence ratio, this leads to the captured by RANS. The chosen k-ω SST model combines air airflow per gate. Keeping the Width-to-Height ratio (WH) at the positive effects of the k-ω and k-ε models: good bound- 1 as in [20] yields the airgate size (w and h ). The total ary layer and free flow resolution. The mechanism from Naik gate gate number of airgates was found by dividing the total fuel flow is applied via FGM: a reduced number of parameters are Fig. 2 Airgate Design Param- eters 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 243 Fig. 3 Baseline Micromix Com- bustor (left) and scaled version (right) to accommodate a fuel orifice of 1.0 mm. Crosswise section through airgates scaled to 2:1 solved during the simulation and through these, the combus- 22.6 bar. An isentropic efficiency of 0.82 for the compres- tion is modeled using a previously computed lookup table. sor and 0.8 for the turbine was assumed to match the given This approach reduces the stiffness of the model and leads outlet conditions. The fuel temperature was set to 300 K to to a faster convergence. mirror non-cryogenic storage on ground or pipeline outlet The baseline designs are shown in Fig. 3. In an annular conditions. combustor, the injectors will be arranged side by side in the The combustion is modeled using the flamelet-generated lateral direction to form a ring around the engine core and manifold (FGM) approach. The computational cost in this stacked row on row in radial direction. A total of three inlets approach is limited by resolving the fast chemical time scales are defined: the air inlet upstream of the pair of micromix separated from the flow based on a look-up table for the gates and two hydrogen inlets into the fuel supply tube. Sym- chemistry parameters and, by doing so, making the model metry is used for the radial surfaces and interface bound- less stiff. The species and temperatures are parametrized, ary conditions connect the lateral boundaries. The domains and the transport equations are solved for the parametrizing are discretized by two to three million cells. The polyhedral variables. Based on the table and the differential equations mesh with prismatic boundary layer ensures good resolu- for the parametrizing variables, the chemical kinetics are sta- tion of the eddies and boundary layer while optimizing the tistically embedded into the flow field. The FGM model can cell count. Within the gates, the orifice and the flame area, predict non-equilibrium effects which ‘are not negligible in the mesh has been refined. The increased mesh base size this context. Hence, the FGM model was deemed most suit- is found to not affect the solution. The overall mesh inde- able to capture the turbulence–chemistry interaction inside pendence using an intermediate design with a fuel orifice the micromix combustion chamber’ [15] and performed best diameter of 0.6  mm is shown in Fig.  4: results obtained in the investigation of Funke et al. [40]. However, the FGM with a mesh base size of 0.6 mm and 0.9 mm are within the model is not able to capture all non-equilibrium ee ff cts, such expected limited numerical deviation. as the slow chemistry of NOx formation, and ‘only trends Assessing a single cycle of the SGT-A35 based on data can be correctly predicted’ [16]. Zghal et al. [19] found that available in Gas Turbine World [39] in ambient condi- the RANS FGM model under-predicts the flame tempera - tions yielded a combustor entry temperature of 800 K and ture by averaging, and thus the NOx emissions, compared Fig. 4 Comparison of different mesh base sizes for identical domains 1 3 244 J. Berger to LES simulations. This also highlights the limitation of increased to keep the cell count constant and limiting com- absolute NOx values, but the error can be held ‘within rea- putational cost. The non-dimensional parameters are based sonable tolerances’ [19] and needs experimental data for on the respective value of Baseline 0.3 mm. The equation proper validation. The turbulent Schmidt number was set below (5.1) was adjusted and applied for each dimension- at 0.2 following López-Juárez et al. [18] to compromise on less variable: prediction of NOx emissions and flame length. The chosen gate setup trades off complexity and computational cost which w = (5.1) gate gate, Baseline 0.3 mm leads to an error in prediction in NOx values but allows cross-model comparison and trend prediction. Experimental MOMENTUM FLUX RATIO (Imposed initial value: data are needed to validate this simulation and to interpret J = 5) The Momentum Flux Ratio (J) connects the product the absolute values correctly. of density and velocity of the jet and the airstream. Alter- With this setup, the variation of design parameters was ing J leads to a change in the penetration depth of fuel and assessed regarding the flame structure and anchoring, the influences the flame position and merging tendency. Chang- thermal NOx formation, the temperature profiles, and the ing J leads to a higher or lower Ma affecting the airgate air distribution of hydroxyl (OH) radicals. The enlarged 1.0 mm size, since Ma is kept continuously at 0.3 and no density fuel fuel orifice results in larger flames and subsequently in changes occur. longer residence times and higher NOx emissions. The MACH NUMBER (Imposed initial limit: Ma = 0.3) fuel design parameters have been varied to understand the pos- Ma at a Momentum Flux Ratio of 5 is higher than Ma . fuel air sibility to mitigate negative impacts and to find an improved This leads to limiting Ma to 0.3 rather than limiting Ma . fuel air design for stationary gas turbine application. An increased Mach number of fuel and by that an increased mass flow, while maintaining the equivalence ratio of 0.4, leads to an increased Ma and air mass flow through the air 5 Micromix Design Adaption airgate. For example, the increased velocity affects the tur - bulent eddies and can wash the flame further downstream, The original design introduces turbulence to form rapidly but might also lead to critical effects such as compressibility. micromixed diffusion flames leading to low-peak temper - WIDTH-TO-HEIGHT RATIO (Initial value: WH = 1 [18]) atures and short residence times. The recirculation zones The airgate cross section consists of a rectangular base and shape and anchor the flame but should not generate hotspots a half-circle on top. The ratio between airgate width and to ensure low thermal NOx formation. Merging of the flames height (WH) without the top half-circle is set to 1. Changing can lead to larger flames with higher residence times and the ratio affects the penetration depth, the flame shape and should be suppressed by the recirculation zones. The param- its lateral and radial dimension as well as offers a wider or eters for both baseline designs are given in Table 1. They narrower lateral recirculation. It is den fi ed as shown in (5.2): have been kept mostly constant while scaling to ensure simi- lar behavior and comparability. Changing the fuel orifice gate WH = (5.2) diameter leads to larger airgates and a lower total number of gate airgates. The scaling can increase flame length, thus the CFD domain length was increased. The base size of the mesh was Table 1 Baseline Design Variable Abbreviation Baseline 0.3 mm Baseline 1.0 mm Parameters Diameter fuel orifice D 0.3 mm 1.0 mm fuel Non-dimensional airgate width and height w, h 1 3.33 gate gate Number of gates, approx. 13,800 1200 Airgate width-to-height ratio WH 1 1 Domain length – 100 mm 140 mm Momentum flux ratio J 5 5 Mach number fuel Ma 0.3 0.3 fuel Non-dimensional offset airgate outlet to injection o 1 1 inj Non-dimensional offset injection to trailing edge o 1 1 te Airgate thickness – 5 mm 5 mm Cell count in million, approx. – 2.3 2.3 Mesh base size – 0.4 mm 0.9 mm 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 245 INJECTION OFFSET AND OFFSET TO TRAILING The residence time in those regions is rather short due to EDGE (Initial value: o = o = 1 [18]) The center of the the short flames. Contrary to that, in the Baseline 1.0 mm, inj te fuel injection outside the airgate and close to the outlet. The the hot zone recirculates to the trailing edge and forms one trailing edge is further downstream to accommodate the fuel large flame region merging both airgate streams together. tube. Both parameters are the same for both, 0.3 mm and NOx forms in the stagnating area at the trailing edge and 1.0 mm Baselines, and not scaled to limit the number of high concentrations of hydroxyl radicals lead to high NOx changed parameters. Both affect mixing and merging of the formation in the main combustion area. The large area of flames and influence the heat transfer to the trailing edge. hydrogen in a flammable range suggests that the unburnt TRAILING EDGE HEIGHT (Imposed initial value: fuel is sucked early into the recirculation zone and that the h = 1) The trailing edge height (h ) is an important param- residence time in the hot zone is long. The first entry in the te te eter: it defines how the flames of each airgate pair affect legends of Figs. 6, 7 refers to the Baseline configurations. each other through recirculation and influences the overall The NOx emissions trend including a Baseline 0.6 mm case injector plate size, but also needs to accommodate space for is shown in Fig.  6. The pressure loss shown in Fig.  7 is the fuel supply. increasing slightly with a larger airgate while maintaining ROUNDING The baseline designs are set up to have the same blockage ratio. sharp edges. This can be challenging for manufacturing and PARAMETER VARIATION The key parameters were increases the boundary layer size within the channel. Round- isolated and gradually changed to obtain trends. The impact ing the airgate inlet significantly reduces the boundary layer on NOx emissions and pressure drop is shown in Figs. 6, 7, in the airgate and decreases the pressure drop. Rounding 8, 9. Figures 5, 6 show the trendlines of parameter varied the edges of the trailing edge works as a diffuser and can in steps and Figs. 8, 9 show the impact of singular param- help pushing the flame anchoring point to a more suitable eters. Ma and J both impact the air velocity. Changing fuel position. Ma influences the necessary airflow to keep the equiva- fuel AIRGATE THICKNESS (Imposed initial thick- lence ratio constant and reducing J leads to an increase of ness = 5  mm) Lengthening the channels should affect the Ma independently of Ma . WH affects the penetration air fuel flow similarly to a rounded airgate inlet and show a more depth into the main airflow. A wider gate circulates more developed flow in the airgate. air around and later under the fuel jet. This does not change J and thus not change the airspeed and the pressure drop. Increasing the distance from airgate exit to fuel injection 6 Results (o ) has a similar effect as changing WH. The airflow flat- inj tens and widens leaving the airgate. It is flowing around the The starting point to understand the effect of scaling the fuel fuel jet and this leads to a higher penetration depth of fuel. orifice in the presented results are the ‘Baseline’ configura- By increasing the distance between fuel injection and trail- tions with a D of 0.3 mm and 1.0 mm shown in Fig. 3. The ing edge (o ), the flame gradually attaches stronger to the fuel te Baseline 0.3 mm model shows NOx emissions of 99 ppm trailing edge. Increasing h from 1 to 1.36, which then is te dry and corrected to 15% oxygen. As discussed, the absolute identical to the airgate height, leads to an increased recircu- value is not trustworthy, so all other values are given nor- lation and no change in the pressure drop. The rounding of malized by the predicted NOx emissions of 456 ppm of the the airgate inlet reduces the boundary layer and reduces the Baseline 1.0 mm model to understand the trends. The Base- pressure drop to almost 50% of Baseline 1.0 mm. Round- line 1.0 mm performance regarding NOx was worse than the ing the trailing edge affects the flow field and reduces the Baseline 0.3 mm. In this paper, the potential to mitigate this temperature directly at the trailing edge. Increasing the wall impact by adapting the key design parameters is explored. thickness from 5 to 10 mm allows the flow to develop in the This was done by changing one design parameter after the airgate and acts similar to the rounding of the airgate inlet other and comparing the results to the Baseline 1.0 mm. Two or an increased Momentum Flux Ratio, but not as strong. designs were chosen based on this variation. The r fi st design Mach number (Ma ) = [0.2, 0.3, 0.4, 0.5, 0.6] and fuel is improved to imitate the flame structure of the Baseline Momentum Flux Ratio (J) = [1, 3, 5, 7, 9] in Figs. 6, 7: The 0.3 mm model, the second model to perform well under an trends show that increasing the airgate velocity, through emissions viewpoint. either parameter, leads to a reduction of NOx emissions BASELINE CONFIGURATIONS The Baseline models but increase in pressure drop. The higher velocity stretches show a significantly different combustion behavior. In the the flame region downstream, but also narrows the hot zone top row of Fig. 5, the Baseline 0.3 mm is shown on the left resulting in a lower residence time. The zone containing and the Baseline 1.0 mm is shown on the right. The Baseline high concentrations of OH radicals can be greatly reduced. 0.3 mm design is characterized by two separated flames each The designs with decreasing air velocity show a longer resi- with a hot core in which the NOx formation rate is high. dence time and lower mixing of the hot gases downstream 1 3 246 J. Berger Fig. 5 Plots of the Mean values of Temperature, H2 Mass Fraction, OH Mass Fraction and NOx Formation Rate for the Baseline 0.3 mm, the Baseline 1.0 mm design, the design for an Improved Flame Shape (D = 1.0 mm) and the fuel design for Improved Emissions (D = 1.0 mm) from top left fuel to bottom right. The designs are shown to the same scale: Domain height and width vary due to changes in the height and width of the airgate and the height of the trailing edge combined with a prolonged formation of NOx further down- The penetration depth decreases and the mixing downstream stream. For J = 7, the trend shows a surprising behavior. In is not as efficient anymore leading to a stretched region of this case, a smaller recirculation zone forms right after the high NOx formation. None of the designs change the pres- trailing edge which is not as hot as in the other designs. This sure drop significantly. lowers the residence time in the following hot zone. The Offset from airgate to fuel injection (o ) = [1, 2, 3, 4, inj change in pressure drop for the variation of both parameters 5] in Figs. 6, 7: The trend to lower emissions is due to the follows the same trend and is not negligible. The best design altered recirculation zone. The recirculation does not trans- from an emissions standpoint (up to 65%) shows the highest port hot gas or hydroxyl back to the trailing edge. At o = 5, inj pressure drop (10%) and vice versa. the trend is turned around because the recirculation zone Width-to-Height Ratio (WH) = [0.8, 1, 1.2, 1.4, 1.6, 2.0, splits up downstream and stagnates leading to a larger hot 2.4] in Figs. 6, 7: Increasing WH and thus increasing pen- gas zone with higher residence times. This parameter varia- etration depth help to separate the NOx formation region tion does not affect the pressure drop majorly. from the trailing edge and reducing NOx for the cases up to Offset from fuel injection to trailing edge (o ) = [1, 2, 3, te a ratio of 1.6. At 2.0 and 2.4, the heat release region merges 4, 5] in Figs. 6, 7: The effect of a longer offset is inconsist- with the top and bottom interfaces leading to a stagnating ent, but on average, it lowers the emissions. The impact on trend, but a strong and short recirculation zone with better the emissions in the first step to o = 2 is minor. The flame te NOx emissions than Baseline 1.0 mm. A negative impact on gets pushed downstream and the recirculation zone slightly the emissions is shown by a higher airgate with a ratio of 0.8. stretched. The flame and NOx formation attach strongly to 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 247 Fig. 8 Relative Mean NOx emissions change by singular parameters Fig. 6 Relative NOx emission trends of selected parameters Fig. 9 Pressure loss over blockage ratio change by singular param- eters cross-sectional area and the local Momentum Flux Ratio for the designs with sharp inlets is presumably lower than the calculated value. Rounding the trailing edge in Figs. 8, 9: Rounding the trailing edge leads to the reduction of NOx formation right after the trailing edge and thus slightly better emissions. The Fig. 7 Pressure loss over blockage ratio trends of selected parameters hot gas is less attached while the pressure loss stays constant. The following recirculation zone is not visibly affected. Doubling the airgate wall thickness in Figs.  8, 9: The the trailing edge for the cases with o = 3 and o = 5. Both te te larger airgate wall thickness shows increased NOx emissions show an early formation of NOx in the recirculation to the due to a stronger recirculation to the trailing edge similar to trailing edge, but also a very high temperature close to the increasing J. The pressure drop is not affected. The thick - trailing edge. This effect gets disrupted at o = 4 by hot gas te ness is limited due to structural integrity and the necessity in a stagnating zone increasing NOx production additionally to integrate the fuel tube in the injector plate. to the NOx formation close to the trailing edge. The pres- IMPROVED FLAME SHAPE (Figs.  8, 9) The flame sure loss for these designs is almost identical to the Baseline shape of Baseline 0.3 mm is significantly different to Base- 1.0 mm design. line 1.0 mm: The flames are not separated anymore and the Trailing edge height (h ) = 1.36 in Figs. 8, 9: The stronger te formation of NOx isn’t limited to two distinct zones. By recirculation due to the increased trailing edge height combining variations in three of the previous parameters, increases the residence time and the emissions rise. Reduc- a similar flame structure and D = 1.0 mm was achieved. ing the trailing edge height is limited by the necessity for a fuel This is shown in the bottom left of Fig.  5. The Momen- fuel tube in between the gates and the structural and thermal tum Flux Ratio is set to 9, the airgate inlet is rounded and integrity of the trailing edge. the trailing edge height increased to the gate height. All of Rounding the airgate inlet in Figs. 8, 9: The rounded inlet these parameters alone increased the NOx emissions, yet the can be interpreted similarly to increasing J. The thinner combination did not just accumulate the negative impact. boundary layer reduces peak velocity and the velocity profile In fact, it shows better results than the rounded airgate inlet at the airgate outlet. Accordingly, the flow field shows less design alone. The negative impact was reduced by the effect turbulences and forms a larger hot gas recirculation zone. of the separated flames. Nevertheless, the emissions are 25% This suggests that the rounding works similar to increas- higher compared to Baseline 1.0 mm. In Figs. 10, 11 the ing J significantly. The boundary layer narrows the flow 1 3 248 J. Berger the given data, but the change in Momentum Flux Ratio affects the penetration inversed to the increase in WH. The offset from airgate exit to fuel injection was set to 3 to profit from the positive effect on the flame structure and emissions. The results are shown in the bottom right of Fig. 5. The hot zone is smaller and the H2 and OH plot even show an unex- pected tendency of flame separation. The NOx formation rate decreased in value and the area got smaller. Overall, the emissions were reduced by more than 50%. Only even higher J and thus a higher pressure drop as well as the Base- line 0.3 mm showed better results. The pressure drop is as expected increased to around 4%. The temperature gradi- ent in Fig. 10 is higher, especially considering the trailing Fig. 10 Mean temperature distribution averaged on cross sections of edge ends further downstream. The OH emissions plotted Baseline 1.0 mm, Baseline 0.3 mm, the design for an Improved Flame Shape and the design for Improved Emissions in Fig. 11 underline the positive effects by showing overall reduced OH concentrations. 7 Limitation The presented work is exploring micromix combustion for a high-pressure ratio stationary gas turbine at a base load of more than 30 MW. The inlet conditions are 800 K and 22.6  bar. The high pressure and temperature as well as energy density are expected to lead to higher NOx emissions underlining the challenges of hydrogen combustion. There are no experimental data for the micromix design at these conditions to compare to and thus, in combination with the restrictions of RANS simulations, the significance of abso- lute values is limited. The simulation itself does not capture Fig. 11 Mean hydroxyl mass fraction rise on cross sections of Base- time-dependant effects and a further analysis and verification line 1.0  mm, Baseline 0.3  mm, the design for an Improved Flame Shape and the design for Improved Emissions using LES is advisable. Nevertheless, this study focuses on trends which can be predicted by RANS simulations. Not only is the simulation limited by the specific numeri- temperature and OH mass fraction is shown. The reason for cal setup, but more so the simulation of hydrogen in general the worse emissions is due to the reduced temperature gradi- is challenging due to the altered combustion behavior [41]. ent and high hydroxyl concentration downstream because of Deeply studied and validated combustion mechanism are the longer flames. The hot gas zone is accordingly long, the available for hydrocarbon fuels, but the experimental study residence time is high and the peak temperature above the regarding flame speed and ignition time delay of Kéromnès peak temperature of the other designs. However, this design et al. showed over a wide pressure and equivalence ratio has the lowest pressure drop of all shown designs. range, how ‘reactivity of the syngas mixtures was found to IMPROVED EMISSIONS (Figs. 8, 9) A combination of be governed by hydrogen chemistry for CO concentrations variation in three parameters was also used to find a design lower than 50% in the fuel mixture’ [42]. Accordingly, the with low emissions and a fuel orifice of 1.0 mm. The great- reaction mechanisms need to be adapted [43]. The mecha- est single influence on emissions is the air velocity. The nism by Naik et al. in combination with the FGM applied in change of Momentum Flux Ratio does not increase the fuel this paper is recommended by Zghal et al. [19], who stud- velocity and was chosen to avoid negative effects due to too ied it using RANS and LES simulations adapting complex high fuel stream velocities. A compromise between pressure chemistry, the thickened flame method and the flamelet- drop and emissions performance was made and J was set to generated manifold approach. Furthermore, the assumption 3, which previously led to a pressure drop of 4%. A design of Schmidt, Lewis and Prandtl number equal to 1 for hydro- with J increased to 7 to reduce emissions is possible, but carbon fuels needs to be altered according to [18] to properly the effect is not as significant as a lowered Momentum Flux represent hydrogen combustion within the presented mod- Ratio. WH is set to 2. This might seem exaggerated from eling approach. The proposed values are studied comparing 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 249 LES and RANS simulations and applied in the presented drop in combustors is in general around 2% to 3%. This study. One of the affected aspects is the increased diffusiv - design offers potential to further reduce the NOx emissions ity of hydrogen, which needs to be properly replicated, but while increasing the pressure drop from below 1% to a still applied micromix combustion lacks data at high pressure acceptable level. to confirm or revoke the model assumptions. Comparable The Improved Emissions design compromised pressure experimental data from Cranfield University’s test rig fit - loss and performance to reduce the emissions by more than ted for hydrogen micromix combustion are expected within 50%. The hot gas region and the NOx formation volume this year. were reduced by increased mixing and penetration depth as The experimental data available at atmospheric condi- well as reduced residence time. Especially the significance tions from Haj Ajed et al. show for micromix combustion of hydroxyl for the formation of NOx is visible in Fig. 11. NOx emissions slightly above 2 ppm (measured) and NO Altering the Width-to-Height ratio leads to a higher penetra- emission slightly below 2 ppm (calculated) with a hydrogen tion depth. An increased offset from airgate to fuel injection orifice of 1.0 mm and an equivalence ratio of 0.4 [35]. In has a smaller but similar ee ff ct. This implies that the penetra - the numerical design exploration, the authors were able to tion depth needs to be increased for a given Momentum Flux reduce the NO emissions to as little as 0.28 ppm at 1 bar Ratio in a scaled micromix setup. A Width-to-Height ratio and 560 K inlet conditions. Also, at atmospheric conditions, exceeding the aero counterpart seems beneficial. Funke et al. showed in a similar experimental setup that the The equivalence ratio in this study was kept constant at a ‘investigated combustor module exceeds 99.4% combustion value of 0.4. The engine cycle assessment shows potential to efficiency for hydrogen contents of 80–100% in the fuel mix- further decrease the equivalence ratio by redistributing com- ture and shows NOx emissions less than 4 ppm corrected bustion and cooling air. This might lead to shorter flames to 15 Vol.% O ’ [44]. These values show the potential of and better emissions behavior due to leaner combustion. An micromix combustion to achieve low NOx emissions. analysis of the flame length showed the close coupling of flame length and NOx emissions. This study helps to understand the challenges and limi- 8 Conclusion tations of scaling a micromix injector for ground use. Sta- tionary hydrogen gas turbines have the potential to firm the The results for a hydrogen industrial gas turbine micromix energy grid by interacting with renewable energy sources. airgate pair with a scaled fuel oric fi e from 0.3 mm to 1.0 mm Potentially, fuel of lesser quality is used on ground com- have been presented in this paper. The key parameters have pared to hydrogen for aviation and the manufacturing cost been varied and two designs were found: an Improved Flame restrictions on ground favor using a larger fuel orifice. By Shape design mimicking the flame shape of the 0.3  mm varying the key design parameters, the negative impact of Baseline and an Improved Emissions design. According to the increased fuel orifice could be mitigated. Increasing the the results, the scaled Micromix injector can be improved fuel orifice leads to significant challenges to control NOx by: and imitating the flame structure of a smaller micromix burner did not yield satisfying results. The strong correla- o A narrow and short flame with a high temperature gradi- tion between pressure loss and emissions was highlighted. ent contributing to low residence times. Even though the negative impacts were mitigated, all scaled o A limited recirculation not sucking hot gas upstream to designs led to higher NOx emissions. the trailing edge which would lead to NOx formation at Acknowledgements This research complements the research of Cran- the trailing edge. field University in the ENABLEH2 project which received funding o High airstream velocity increasing turbulence and from the European Union’s Horizon 2020 research and innovation pro- decreasing residence time. This leads in general to an gramme under grant agreement N 769241. The author would like to increased pressure drop. thank Dr. David Abbott and Dr. Xiaoxiao Sun for their support of this project and acknowledges gratefully the cooperation of Cranfield Uni - o Limiting the concentration of hydroxyl both in value and versity’s Centre for Propulsion Engineering, and Stuttgart University volume. and the German Aerospace Centre’s Institute of Combustion Technol- o A sufficient penetration depth through, for example, a ogy, which facilitated this work. larger than one Width-to-Height ratio leading to more Funding Open Access funding enabled and organized by Projekt air flowing around the fuel jet. DEAL. The Author was funded by the ERASMUS + European Union student exchange program for the time of his placement at Cranfield The Improved Flame Shape design was able to yield University (01.10.2020–31.05.2021). This research complements the results better than expected due to the separated flames. research of Cranfield University in the ENABLEH2 project which received funding from the European Union’s Horizon 2020 research The emissions were 25% above the emissions of Baseline and innovation programme under grant agreement N 769241. 1.0 mm, but the pressure drop was reduced. The pressure 1 3 250 J. Berger Data availability Using no material but such available in the public 13. Japan claims world first: DLN gas turbine combustors verified domain was a requirement for this research and applies to this paper. on 100% hydrogen. Modern Power Systems. https:// www. moder n pow e rsy s t ems. com/ featu r es/ featu r ejap an- claims- wor ld- firs t- dln-g as-turbi ne- combu s tors-v eri fied-on- 100- h ydrog en-81850 77/ Code availability Computational fluid dynamics: Siemens Star (2020). Accessed 3 Mar 2021 CCM + 2020.2 (15.04.008). Other: Microsoft Word and Excel. 14. Sethi, V.: ENABLing cryogEnic Hydrogen based CO2 free air transport (ENABLEH2). h t t p s : // w w w . e n a b l e h 2 . e u (2018). Declarations Accessed 15 Feb 2021 15. Ben Abdallah, R., Sethi, V., Gauthier, P., Rolt, A.M., Abbott, Conflicts of interest The author holds currently no other position than D.: A detailed analytical study of hydrogen reaction in a novel student at Stuttgart and Cranfield University. An Non-disclosure agree- micromix combustion system. Proc. ASME Turbo. Expo. (2018). ment for the ENABLEH2 project was signed, but does not apply to this https:// doi. org/ 10. 1115/ GT2018- 76586 publication. 16. Babazzi, G., Gauthier, P., Agarwal, P., McClure, J., Sethi, V.: NOX emissions predictions for a hydrogen micromix combustion sys- tem. Proc. ASME Turbo. Expo. (2019). https:// doi. org/ 10. 1115/ Open Access This article is licensed under a Creative Commons Attri- GT2019- 90532 bution 4.0 International License, which permits use, sharing, adapta- 17. Sun, X., Agarwal, P., Carbonara, F., Abbott, D., Gauthier, P., tion, distribution and reproduction in any medium or format, as long Sethi, B.: Numerical investigation into the impact of injector geo- as you give appropriate credit to the original author(s) and the source, metrical design parameters on hydrogen micromix combustion provide a link to the Creative Commons licence, and indicate if changes characteristics. Proc. ASME Turbo. Expo. (2020). https://doi. or g/ were made. The images or other third party material in this article are 10. 1115/ GT2020- 16084 included in the article's Creative Commons licence, unless indicated 18. López-Juárez, M., Sun, X., Sethi, B., Gauthier, P., Abbott, D.: otherwise in a credit line to the material. If material is not included in Characterising hydrogen micromix flames: combustion model the article's Creative Commons licence and your intended use is not calibration and evaluation. Proc. ASME Turbo. Expo. 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Ansys Inc., Canons- jurisdictional claims in published maps and institutional affiliations. burg. https:// suppo r t. ansy s. com/ Ansy s Cus t o merP o r t al/ en_ us/ Downl oads/ Curre nt+ Relea se (2014). Accessed 20 Mar 2021 38. Naik, C., Puduppakkam, K., Meeks, E.: An improved core reac- tion mechanism for saturated C0–C4 fuels. J. Eng. Gas Turbines Power (2012). https:// doi. org/ 10. 1115/1. 40043 88 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aerotecnica Missili & Spazio Springer Journals

Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion Applications

Aerotecnica Missili & Spazio , Volume 100 (3) – Sep 1, 2021

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Springer Journals
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0365-7442
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10.1007/s42496-021-00091-5
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Abstract

Decarbonising the energy grid through renewable energy requires a grid firming technology to harmonize supply and demand. Hydrogen-fired gas turbine power plants offer a closed loop by burning green hydrogen produced with excess power from renewable energy. Conventional dry low NOx (DLN) combustors have been optimized for strict emission limits. A higher flame temperature of hydrogen drives higher NOx emissions and faster flame speed alters the combustion behavior signifi- cantly. Micromix combustion offers potential for low NOx emissions and optimized conditions for hydrogen combustion. Many small channels, so-called airgates, accelerate the airflow followed by a jet-in-crossflow injection of hydrogen. This leads to short-diffusion flames following the principle of maximized mixing intensity and minimized mixing scales. This paper shows the challenges and the potential of an economical micromix application for an aero-derivative industrial gas turbine with a high-pressure ratio. A technology transfer based on the micromix combustion research in the ENABLEH2 project is carried out. The driving parameter for ground use adaption is an increased fuel orifice diameter from 0.3 mm to 1.0 mm to reduce cost and complexity. Increasing the fuel supply mass flow leads to larger flames and higher emissions. The impact was studied through RANS simulation and trends for key design parameters were shown. Increased velocity in the airgates leads to a higher pressure drop and reduced emissions through faster mixing. Altering the penetration depth shows potential for emission reduction without compromising on pressure loss. Two improved designs are found, and their performance is discussed. Keywords Decarbonisation · Hydrogen Combustion · Stationary Gas Turbine · Industrial Gas Turbine · Micromix · NOx · Jet-in-Crossflow · Computational Fluid Dynamics · RANS · FGM Abbreviations and Symbols Ma Mac h number in the fuel orifice fuel DLN Dry Low Nitrogen Oxide Emissions Ma Mach number in the airgate air ENABLEH2 Enabling Cryogenic Hydrogen-Based CO D Diame ter of the fuel orifice 2 fuel Free Air Transport W Airgate width gate FGM Flamelet Generated Manifold w N on-dimensional airgate width gate LES Large Eddie Simulation h N on-dimensional airgate height gate NOx Nitrogen Oxide Emissions WH Width-to-height ratio of the airgate RANS R eynolds Averaged Navier–Stokes h Non-dimensional radial spacing rs SST Shear Stress Transport w Non-dimensional lateral spacing ls J Momentum Flux Ratio hte Non-dimensional Height trailing edge Φ Equivalence ratio h Non-dimensional radial distance from pair rs to pair u Velocity of the hydrogen jet jet * Johannes Berger u Velocity of the air in the airgate air st108655@stud.uni-stuttgart.de ρ Density of the jet jet Institute of Combustion Technology for Aerospace ρ Density of the air in the airgate air Engineering, University of Stuttgart, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany Centre for Propulsion Engineering, Cranfield University, College Road, Cranfield MK43 0AL, UK Vol.:(0123456789) 1 3 240 J. Berger o N on-dimensional offset injection from by Cranfield University works to ‘mature critical technolo- inj airgate outlet gies for LH2-based propulsion’ [14]. Part of this goal is to o Non-dimensional offset trailing edge from develop a novel micromix combustor for hydrogen combus- te injection tion. The aim of this work is to investigate a technology transfer of hydrogen combustion in aero applications to sta- tionary gas turbines. The research presented in this paper 1 Introduction builds on the ongoing work and generated knowledge at Cranfield University [15– 24] to develop a micromix hydro- The global average surface temperature in 2020 was 1.02 °C gen combustor for aero applications. In this paper, RANS higher compared to the average temperature between 1950 CFD is used to model design adaptations and to assess their and 1981 [1]. An uncontrolled increase in temperature led impact on flame structure, outlet conditions, and nitrogen and presumably leads to devastating environmental and oxide (NOx) emissions. social catastrophes [2–4]. The global temperature change needs to be limited to well below 2 °C as named in the Paris Agreement [5] and this ‘would require unprecedented transi- 2 Stationary Gas Turbine Combustion tions in all aspects of society’, says the Intergovernmental Panel on Climate Change [6]. Decarbonising the energy sys- The requirements for gas turbine combustors are high com- tems and thus reducing the greenhouse effect by reducing bustion efficiency and low pressure loss, evenly distributed carbon dioxide emissions is one measure to achieve the set temperature on the outlet plane, and a minimum of emis- goals. The potential use of hydrogen as an alternative energy sions. Stationary gas turbines offer more freedom in design carrier has recently been more widely recognized and is in than their aero counterparts regarding size, weight and the process of being explored. auxiliary systems [25]. On the other hand, a longer lifespan Hydrogen can be applied to sectors in which electrifica- is desired as well as lower operating cost [10]. Combus- tion is not economically feasible and has the potential to tors initially constructed for ground application as well as firm the energy grid and harmonize the supply of renewable those derived from aero application were designed as diffu- energy with demand through green hydrogen production [7]. sion flame combustors with uncontrolled NOx emissions. This so-called power-to-gas process can form a decarbonised These diffusion flame combustors were later modified by and firmed cycle of clean power utilizing a fuel cell or a sta- adding demineralized water, steam or nitrogen as diluent tionary gas turbine for power production and potentially heat and, because of the reduced flame temperature leading to supply. A strategic positioning of the power plants can maxi- reduced NOx emissions, met emission standards and could mize the synergy effects of power supply and waste heat use thereby increase power output. Using a diluent, however, as well as the blending with renewable energy sources [8]. limits the operability and drives direct operation cost of the Carbon-based fuels have been subject to research for dec- engines and is thus not desired. Newer DLN combustors ades. Auto-ignition times, flammability limits, mixing and offer low levels of emissions of NOx through premixed, lean quenching behavior have been and are intensively studied. combustion at low flame temperatures. A variety of differ - An overview and detailed description can be found in Glass- ent DLN combustor designs for stationary gas turbines have man et al. [9]. Specifically, gas turbines have been subject to been developed [26]. Multi-Fuel premixed DLN combustors research to lower emissions and increase cycle efficiency. In offer superior operability but are challenging to control at all the past years, the development of combustors for dry and load levels regarding fuel and air levels, ambient conditions, low nitrogen oxide (DLN) emissions has been the focus [10]. and fuel composition. Introducing high hydrogen levels in By lean premixing, the flame temperature has been lowered the fuel alters the combustion behavior; flame speed, auto- and thus NOx emissions have decreased. ignition risk, thermo-acoustic risk, and flame temperature However, due to significantly different combustion behav - increase. Lower CO emissions and wider flammability lim- ior, the current technologies and models are not easily adapt- its are positive effects, but lower volumetric energy density able for pure hydrogen combustion. Engine manufacturers of requires adapted fuel supply controls and storage solutions stationary gas turbines currently modify their state-of-the-art [27]. Combustion of 100% hydrogen without diluent requires engines to a higher hydrogen content capability [11, 12]. The ‘intensive R&D activity to pave the way for such a tech- first entirely hydrogen burning DLN industrial gas turbine nology’ [28], since ‘safe and reliable operation [for pure with a power output of 1.1 MW has recently been put into hydrogen combustion] is obtained in diffusion combustors use [13]. Aero gas turbines can be potentially modified for only’ [29]. hydrogen combustion and enable flying without direct car - Burning pure hydrogen and meeting emissions require- bon emissions. The ‘Enabling Cryogenic Hydrogen-Based ments as well as ensuring economical operability requires CO Free Air Transport’ (ENABLEH2) project coordinated a new combustion technology. One of the promising 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 241 technologies for burning hydrogen in gas turbines is micro- University’s micromix system showed that a fuel orifice mix combustion. The leading principle is to maximize mix- diameter of 0.3 mm is the best compromise of applicability ing intensity and minimize the mixing scale using many and performance [20]. For this study, a diameter of 1.0 mm small high-velocity channels followed by a jet-in-crossflow was selected as a design target to allow lower-quality fuel injection of hydrogen resulting in a fast mixing, lean burn- and compromise on cost and complexity, while attempting ing, short-diffusion flame. This is limited by compromising not to trade-off with increased NOx emissions. The change on the overall pressure loss [30] and manufacturability. The in fuel mass flow by increasing the fuel orifice diameter micromix principle is shown in Fig. 1 [18]: The inner and subsequently changes the air mass flow per gate, the airgate outer vortices stabilize the flame and ensure good mixing. size, and the required number of airgates. A similar study The residence time and flame temperature are kept low to was done by Haj Ayed et al. [35] at 1 bar, showing satisfac- minimize the formation of thermal NOx but also flashback tory results for a larger fuel orifice. and auto-ignition risks are eliminated by not premixing Less restrictive size and weight limitations as well as air and fuel in the channels. Micromix has been subject to demand for multi-fuel capability and restrictive emission research at Aachen University of Applied Science working regulations led to the development of stand-alone industrial together with Kawasaki Heavy Industries [31]. NASA pub- gas turbine combustor architectures [25]. This was due to lished a paper applying lean direct injection to develop a the necessity of lean and staged combustion for higher resi- micromix-like ‘low emission hydrogen combustor’ [32]. A dence times which reduce CO emissions but also produce similar concept but with micro-premixing channels has been low flame temperatures limiting NOx emissions. In general, called ‘Multi-Tube Mixer’ by GE [33]. The micro-premixed the micromix system tends to have a larger cross section: the 'multiple-injection burner’ was researched by MHPS and airstream is split into many airgates and the airgates need to Hitachi [34]. be spaced out for flame separation leading to a higher block - In the ENABLEH2 project, micromix combustion is age ratio than conventional combustors and thus to a larger being matured to TRL 3 for high-pressure ratio aviation face area for the airgate arrangement. On the other hand, application. This project explores feasibility of liquid hydro- due to the intense and fast mixing leading to short flames, gen as an aircraft fuel and includes the fuel system as well. the combustor length decreases. Following this, the archi- The micromix development focuses on Reynolds-averaged tecture which suits micromix combustion best is the annular Navier–Stokes (RANS) and large eddie simulations (LES) combustor with an increased face area. Redirecting the flow adapted for micromix hydrogen combustion and their limita- into can combustors as in a state-of-the-art stationary dry tions [15–19], the design of the micromix geometry [20, 21], low natural gas combustor, which permit short shafts, long and thermoacoustic modeling [22–24]. residence times and lean combustion, is not necessary. Stationary gas turbines are constrained by different The single shaft gas turbine is limited to lower pressure boundary conditions, but a technology transfer still offers ratios and has a lower single-cycle efficiency. However, this synergies. In a power plant application, gas turbine com- is desirable for ground use because it leads to higher exhaust bustors face fewer challenges regarding altitude relight and temperatures and supports a supercritical steam cycle, which critical safety measures. Weight and size limitations are less leads to higher combined cycle efficiencies compared to strict. Contrary to that, lifespan, development, production a non-single shaft gas turbine. The combined cycle has a and direct operation cost are strongly restricted. Cranfield slower reaction time and is well suited for base loads and more predictable load changes. For adaptive grid firming, when quick reaction times are wanted, the aero-derivative two or three shaft stationary gas turbines stand out due to their high single-cycle efficiency and fast ramp-up time. Accordingly, for grid firming supporting renewable energies, the former industrial RB211, now SGT-A35, aero-derivative model was chosen as reference for this paper. 3 Micromix Geometry Definition The cycle assessment based on data of the SGT-A35 from Siemens Energy [36] defined the necessary fuel and air mass flows. The total size limit for the combustor was defined by evaluating the engine cross section. Agarwal et al. [20] showed a rectangular airgate which has on the opposite side Fig. 1 Micromix combustion schematic from www. enabl eh2. eu 1 3 242 J. Berger of the fuel injection an arch. This airgate shape was chosen by the single airgate fuel flow. The key airgate design param- for this paper, too. Sun et al. [17] varied the airgate shape eters are shown in Fig. 2. With the available total design and showed the beneficial effect of a pair arrangement for space for the injector plate, limiting the trailing edge height the flame structure. Placing the blunt side of the airgates (h ) to a set value to accommodate the fuel tube and aiming te towards each other as shown in Fig. 2 leads to a stronger for 6 mm in radial distance (h ) from pair to pair to avoid rs’ recirculation between the gates. This helps to stabilize and radial merging in between airgate pairs, the number of rows form the flame as well as simplifies the design due to a single and subsequently the radial and lateral spacing (h, w ) of rs ls fuel tube for both airgates. The equivalence ratio (Φ) was set an airgate were found. A reference baseline design for a pair to 0.4 for lean and stable combustion and the Momentum of airgates with a fuel orifice diameter of 0.3 mm is shown Flux Ratio (J) was set to 5, which has shown good results in on the left side of Fig. 3. Following the same design routine current simulations at Crane fi ld University. The Momentum for a fuel orifice of 1.0 mm leads to the baseline design on Flux Ratio connects the density and velocity of fuel and the right in Fig. 3. airflow. This leads to a significant impact on the penetration depth and influences recirculation and flame merging. The Momentum Flux Ratio is defined as shown in (3.1):4 Simulation Setup ⋅ u jet jet Combustion simulation is challenging because it necessi- J = (3.1) tates resolving a broad range of three-dimensional and time- ⋅ u air air dependant scales of the combustion process and flow field, The optimum fuel orifice diameter (D ) is 0.3 mm for which influence each other. The geometries were modeled fuel hydrogen aero applications. However, in stationary gas tur- in STAR-CCM + using RANS with k-ω Menter shear stress bines, a limitation of cost and the potential for blockage transport (SST), the reaction mechanism from Naik et al. due to contaminants in pipeline delivered fuel makes a fuel [37, 38] and the Zeldovich combustion model. The models orifice diameter of 1.0 mm desirable. The ratio of fuel-to- are set up to be three-dimensional, ideal gas with segre- air density is below 1/5. Combined with a Momentum Flux gated flow and fluid enthalpy, and an all y + wall treatment. Ratio of 5, this leads to a lower Mach number of air (Ma ) The chosen RANS approach limits computational power air than fuel (Ma). Ma was limited to 0.3 to avoid com- necessary but produces only time-averaged solutions. The fuel fuel pressibility effects and high-pressure losses. The fuel mass turbulent mixing of micromix combustion is expected to flow per gate was obtained through the assumption of D produce a dynamic flow field and local hot spots cannot be fuel and Ma . With the set equivalence ratio, this leads to the captured by RANS. The chosen k-ω SST model combines air airflow per gate. Keeping the Width-to-Height ratio (WH) at the positive effects of the k-ω and k-ε models: good bound- 1 as in [20] yields the airgate size (w and h ). The total ary layer and free flow resolution. The mechanism from Naik gate gate number of airgates was found by dividing the total fuel flow is applied via FGM: a reduced number of parameters are Fig. 2 Airgate Design Param- eters 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 243 Fig. 3 Baseline Micromix Com- bustor (left) and scaled version (right) to accommodate a fuel orifice of 1.0 mm. Crosswise section through airgates scaled to 2:1 solved during the simulation and through these, the combus- 22.6 bar. An isentropic efficiency of 0.82 for the compres- tion is modeled using a previously computed lookup table. sor and 0.8 for the turbine was assumed to match the given This approach reduces the stiffness of the model and leads outlet conditions. The fuel temperature was set to 300 K to to a faster convergence. mirror non-cryogenic storage on ground or pipeline outlet The baseline designs are shown in Fig. 3. In an annular conditions. combustor, the injectors will be arranged side by side in the The combustion is modeled using the flamelet-generated lateral direction to form a ring around the engine core and manifold (FGM) approach. The computational cost in this stacked row on row in radial direction. A total of three inlets approach is limited by resolving the fast chemical time scales are defined: the air inlet upstream of the pair of micromix separated from the flow based on a look-up table for the gates and two hydrogen inlets into the fuel supply tube. Sym- chemistry parameters and, by doing so, making the model metry is used for the radial surfaces and interface bound- less stiff. The species and temperatures are parametrized, ary conditions connect the lateral boundaries. The domains and the transport equations are solved for the parametrizing are discretized by two to three million cells. The polyhedral variables. Based on the table and the differential equations mesh with prismatic boundary layer ensures good resolu- for the parametrizing variables, the chemical kinetics are sta- tion of the eddies and boundary layer while optimizing the tistically embedded into the flow field. The FGM model can cell count. Within the gates, the orifice and the flame area, predict non-equilibrium effects which ‘are not negligible in the mesh has been refined. The increased mesh base size this context. Hence, the FGM model was deemed most suit- is found to not affect the solution. The overall mesh inde- able to capture the turbulence–chemistry interaction inside pendence using an intermediate design with a fuel orifice the micromix combustion chamber’ [15] and performed best diameter of 0.6  mm is shown in Fig.  4: results obtained in the investigation of Funke et al. [40]. However, the FGM with a mesh base size of 0.6 mm and 0.9 mm are within the model is not able to capture all non-equilibrium ee ff cts, such expected limited numerical deviation. as the slow chemistry of NOx formation, and ‘only trends Assessing a single cycle of the SGT-A35 based on data can be correctly predicted’ [16]. Zghal et al. [19] found that available in Gas Turbine World [39] in ambient condi- the RANS FGM model under-predicts the flame tempera - tions yielded a combustor entry temperature of 800 K and ture by averaging, and thus the NOx emissions, compared Fig. 4 Comparison of different mesh base sizes for identical domains 1 3 244 J. Berger to LES simulations. This also highlights the limitation of increased to keep the cell count constant and limiting com- absolute NOx values, but the error can be held ‘within rea- putational cost. The non-dimensional parameters are based sonable tolerances’ [19] and needs experimental data for on the respective value of Baseline 0.3 mm. The equation proper validation. The turbulent Schmidt number was set below (5.1) was adjusted and applied for each dimension- at 0.2 following López-Juárez et al. [18] to compromise on less variable: prediction of NOx emissions and flame length. The chosen gate setup trades off complexity and computational cost which w = (5.1) gate gate, Baseline 0.3 mm leads to an error in prediction in NOx values but allows cross-model comparison and trend prediction. Experimental MOMENTUM FLUX RATIO (Imposed initial value: data are needed to validate this simulation and to interpret J = 5) The Momentum Flux Ratio (J) connects the product the absolute values correctly. of density and velocity of the jet and the airstream. Alter- With this setup, the variation of design parameters was ing J leads to a change in the penetration depth of fuel and assessed regarding the flame structure and anchoring, the influences the flame position and merging tendency. Chang- thermal NOx formation, the temperature profiles, and the ing J leads to a higher or lower Ma affecting the airgate air distribution of hydroxyl (OH) radicals. The enlarged 1.0 mm size, since Ma is kept continuously at 0.3 and no density fuel fuel orifice results in larger flames and subsequently in changes occur. longer residence times and higher NOx emissions. The MACH NUMBER (Imposed initial limit: Ma = 0.3) fuel design parameters have been varied to understand the pos- Ma at a Momentum Flux Ratio of 5 is higher than Ma . fuel air sibility to mitigate negative impacts and to find an improved This leads to limiting Ma to 0.3 rather than limiting Ma . fuel air design for stationary gas turbine application. An increased Mach number of fuel and by that an increased mass flow, while maintaining the equivalence ratio of 0.4, leads to an increased Ma and air mass flow through the air 5 Micromix Design Adaption airgate. For example, the increased velocity affects the tur - bulent eddies and can wash the flame further downstream, The original design introduces turbulence to form rapidly but might also lead to critical effects such as compressibility. micromixed diffusion flames leading to low-peak temper - WIDTH-TO-HEIGHT RATIO (Initial value: WH = 1 [18]) atures and short residence times. The recirculation zones The airgate cross section consists of a rectangular base and shape and anchor the flame but should not generate hotspots a half-circle on top. The ratio between airgate width and to ensure low thermal NOx formation. Merging of the flames height (WH) without the top half-circle is set to 1. Changing can lead to larger flames with higher residence times and the ratio affects the penetration depth, the flame shape and should be suppressed by the recirculation zones. The param- its lateral and radial dimension as well as offers a wider or eters for both baseline designs are given in Table 1. They narrower lateral recirculation. It is den fi ed as shown in (5.2): have been kept mostly constant while scaling to ensure simi- lar behavior and comparability. Changing the fuel orifice gate WH = (5.2) diameter leads to larger airgates and a lower total number of gate airgates. The scaling can increase flame length, thus the CFD domain length was increased. The base size of the mesh was Table 1 Baseline Design Variable Abbreviation Baseline 0.3 mm Baseline 1.0 mm Parameters Diameter fuel orifice D 0.3 mm 1.0 mm fuel Non-dimensional airgate width and height w, h 1 3.33 gate gate Number of gates, approx. 13,800 1200 Airgate width-to-height ratio WH 1 1 Domain length – 100 mm 140 mm Momentum flux ratio J 5 5 Mach number fuel Ma 0.3 0.3 fuel Non-dimensional offset airgate outlet to injection o 1 1 inj Non-dimensional offset injection to trailing edge o 1 1 te Airgate thickness – 5 mm 5 mm Cell count in million, approx. – 2.3 2.3 Mesh base size – 0.4 mm 0.9 mm 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 245 INJECTION OFFSET AND OFFSET TO TRAILING The residence time in those regions is rather short due to EDGE (Initial value: o = o = 1 [18]) The center of the the short flames. Contrary to that, in the Baseline 1.0 mm, inj te fuel injection outside the airgate and close to the outlet. The the hot zone recirculates to the trailing edge and forms one trailing edge is further downstream to accommodate the fuel large flame region merging both airgate streams together. tube. Both parameters are the same for both, 0.3 mm and NOx forms in the stagnating area at the trailing edge and 1.0 mm Baselines, and not scaled to limit the number of high concentrations of hydroxyl radicals lead to high NOx changed parameters. Both affect mixing and merging of the formation in the main combustion area. The large area of flames and influence the heat transfer to the trailing edge. hydrogen in a flammable range suggests that the unburnt TRAILING EDGE HEIGHT (Imposed initial value: fuel is sucked early into the recirculation zone and that the h = 1) The trailing edge height (h ) is an important param- residence time in the hot zone is long. The first entry in the te te eter: it defines how the flames of each airgate pair affect legends of Figs. 6, 7 refers to the Baseline configurations. each other through recirculation and influences the overall The NOx emissions trend including a Baseline 0.6 mm case injector plate size, but also needs to accommodate space for is shown in Fig.  6. The pressure loss shown in Fig.  7 is the fuel supply. increasing slightly with a larger airgate while maintaining ROUNDING The baseline designs are set up to have the same blockage ratio. sharp edges. This can be challenging for manufacturing and PARAMETER VARIATION The key parameters were increases the boundary layer size within the channel. Round- isolated and gradually changed to obtain trends. The impact ing the airgate inlet significantly reduces the boundary layer on NOx emissions and pressure drop is shown in Figs. 6, 7, in the airgate and decreases the pressure drop. Rounding 8, 9. Figures 5, 6 show the trendlines of parameter varied the edges of the trailing edge works as a diffuser and can in steps and Figs. 8, 9 show the impact of singular param- help pushing the flame anchoring point to a more suitable eters. Ma and J both impact the air velocity. Changing fuel position. Ma influences the necessary airflow to keep the equiva- fuel AIRGATE THICKNESS (Imposed initial thick- lence ratio constant and reducing J leads to an increase of ness = 5  mm) Lengthening the channels should affect the Ma independently of Ma . WH affects the penetration air fuel flow similarly to a rounded airgate inlet and show a more depth into the main airflow. A wider gate circulates more developed flow in the airgate. air around and later under the fuel jet. This does not change J and thus not change the airspeed and the pressure drop. Increasing the distance from airgate exit to fuel injection 6 Results (o ) has a similar effect as changing WH. The airflow flat- inj tens and widens leaving the airgate. It is flowing around the The starting point to understand the effect of scaling the fuel fuel jet and this leads to a higher penetration depth of fuel. orifice in the presented results are the ‘Baseline’ configura- By increasing the distance between fuel injection and trail- tions with a D of 0.3 mm and 1.0 mm shown in Fig. 3. The ing edge (o ), the flame gradually attaches stronger to the fuel te Baseline 0.3 mm model shows NOx emissions of 99 ppm trailing edge. Increasing h from 1 to 1.36, which then is te dry and corrected to 15% oxygen. As discussed, the absolute identical to the airgate height, leads to an increased recircu- value is not trustworthy, so all other values are given nor- lation and no change in the pressure drop. The rounding of malized by the predicted NOx emissions of 456 ppm of the the airgate inlet reduces the boundary layer and reduces the Baseline 1.0 mm model to understand the trends. The Base- pressure drop to almost 50% of Baseline 1.0 mm. Round- line 1.0 mm performance regarding NOx was worse than the ing the trailing edge affects the flow field and reduces the Baseline 0.3 mm. In this paper, the potential to mitigate this temperature directly at the trailing edge. Increasing the wall impact by adapting the key design parameters is explored. thickness from 5 to 10 mm allows the flow to develop in the This was done by changing one design parameter after the airgate and acts similar to the rounding of the airgate inlet other and comparing the results to the Baseline 1.0 mm. Two or an increased Momentum Flux Ratio, but not as strong. designs were chosen based on this variation. The r fi st design Mach number (Ma ) = [0.2, 0.3, 0.4, 0.5, 0.6] and fuel is improved to imitate the flame structure of the Baseline Momentum Flux Ratio (J) = [1, 3, 5, 7, 9] in Figs. 6, 7: The 0.3 mm model, the second model to perform well under an trends show that increasing the airgate velocity, through emissions viewpoint. either parameter, leads to a reduction of NOx emissions BASELINE CONFIGURATIONS The Baseline models but increase in pressure drop. The higher velocity stretches show a significantly different combustion behavior. In the the flame region downstream, but also narrows the hot zone top row of Fig. 5, the Baseline 0.3 mm is shown on the left resulting in a lower residence time. The zone containing and the Baseline 1.0 mm is shown on the right. The Baseline high concentrations of OH radicals can be greatly reduced. 0.3 mm design is characterized by two separated flames each The designs with decreasing air velocity show a longer resi- with a hot core in which the NOx formation rate is high. dence time and lower mixing of the hot gases downstream 1 3 246 J. Berger Fig. 5 Plots of the Mean values of Temperature, H2 Mass Fraction, OH Mass Fraction and NOx Formation Rate for the Baseline 0.3 mm, the Baseline 1.0 mm design, the design for an Improved Flame Shape (D = 1.0 mm) and the fuel design for Improved Emissions (D = 1.0 mm) from top left fuel to bottom right. The designs are shown to the same scale: Domain height and width vary due to changes in the height and width of the airgate and the height of the trailing edge combined with a prolonged formation of NOx further down- The penetration depth decreases and the mixing downstream stream. For J = 7, the trend shows a surprising behavior. In is not as efficient anymore leading to a stretched region of this case, a smaller recirculation zone forms right after the high NOx formation. None of the designs change the pres- trailing edge which is not as hot as in the other designs. This sure drop significantly. lowers the residence time in the following hot zone. The Offset from airgate to fuel injection (o ) = [1, 2, 3, 4, inj change in pressure drop for the variation of both parameters 5] in Figs. 6, 7: The trend to lower emissions is due to the follows the same trend and is not negligible. The best design altered recirculation zone. The recirculation does not trans- from an emissions standpoint (up to 65%) shows the highest port hot gas or hydroxyl back to the trailing edge. At o = 5, inj pressure drop (10%) and vice versa. the trend is turned around because the recirculation zone Width-to-Height Ratio (WH) = [0.8, 1, 1.2, 1.4, 1.6, 2.0, splits up downstream and stagnates leading to a larger hot 2.4] in Figs. 6, 7: Increasing WH and thus increasing pen- gas zone with higher residence times. This parameter varia- etration depth help to separate the NOx formation region tion does not affect the pressure drop majorly. from the trailing edge and reducing NOx for the cases up to Offset from fuel injection to trailing edge (o ) = [1, 2, 3, te a ratio of 1.6. At 2.0 and 2.4, the heat release region merges 4, 5] in Figs. 6, 7: The effect of a longer offset is inconsist- with the top and bottom interfaces leading to a stagnating ent, but on average, it lowers the emissions. The impact on trend, but a strong and short recirculation zone with better the emissions in the first step to o = 2 is minor. The flame te NOx emissions than Baseline 1.0 mm. A negative impact on gets pushed downstream and the recirculation zone slightly the emissions is shown by a higher airgate with a ratio of 0.8. stretched. The flame and NOx formation attach strongly to 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 247 Fig. 8 Relative Mean NOx emissions change by singular parameters Fig. 6 Relative NOx emission trends of selected parameters Fig. 9 Pressure loss over blockage ratio change by singular param- eters cross-sectional area and the local Momentum Flux Ratio for the designs with sharp inlets is presumably lower than the calculated value. Rounding the trailing edge in Figs. 8, 9: Rounding the trailing edge leads to the reduction of NOx formation right after the trailing edge and thus slightly better emissions. The Fig. 7 Pressure loss over blockage ratio trends of selected parameters hot gas is less attached while the pressure loss stays constant. The following recirculation zone is not visibly affected. Doubling the airgate wall thickness in Figs.  8, 9: The the trailing edge for the cases with o = 3 and o = 5. Both te te larger airgate wall thickness shows increased NOx emissions show an early formation of NOx in the recirculation to the due to a stronger recirculation to the trailing edge similar to trailing edge, but also a very high temperature close to the increasing J. The pressure drop is not affected. The thick - trailing edge. This effect gets disrupted at o = 4 by hot gas te ness is limited due to structural integrity and the necessity in a stagnating zone increasing NOx production additionally to integrate the fuel tube in the injector plate. to the NOx formation close to the trailing edge. The pres- IMPROVED FLAME SHAPE (Figs.  8, 9) The flame sure loss for these designs is almost identical to the Baseline shape of Baseline 0.3 mm is significantly different to Base- 1.0 mm design. line 1.0 mm: The flames are not separated anymore and the Trailing edge height (h ) = 1.36 in Figs. 8, 9: The stronger te formation of NOx isn’t limited to two distinct zones. By recirculation due to the increased trailing edge height combining variations in three of the previous parameters, increases the residence time and the emissions rise. Reduc- a similar flame structure and D = 1.0 mm was achieved. ing the trailing edge height is limited by the necessity for a fuel This is shown in the bottom left of Fig.  5. The Momen- fuel tube in between the gates and the structural and thermal tum Flux Ratio is set to 9, the airgate inlet is rounded and integrity of the trailing edge. the trailing edge height increased to the gate height. All of Rounding the airgate inlet in Figs. 8, 9: The rounded inlet these parameters alone increased the NOx emissions, yet the can be interpreted similarly to increasing J. The thinner combination did not just accumulate the negative impact. boundary layer reduces peak velocity and the velocity profile In fact, it shows better results than the rounded airgate inlet at the airgate outlet. Accordingly, the flow field shows less design alone. The negative impact was reduced by the effect turbulences and forms a larger hot gas recirculation zone. of the separated flames. Nevertheless, the emissions are 25% This suggests that the rounding works similar to increas- higher compared to Baseline 1.0 mm. In Figs. 10, 11 the ing J significantly. The boundary layer narrows the flow 1 3 248 J. Berger the given data, but the change in Momentum Flux Ratio affects the penetration inversed to the increase in WH. The offset from airgate exit to fuel injection was set to 3 to profit from the positive effect on the flame structure and emissions. The results are shown in the bottom right of Fig. 5. The hot zone is smaller and the H2 and OH plot even show an unex- pected tendency of flame separation. The NOx formation rate decreased in value and the area got smaller. Overall, the emissions were reduced by more than 50%. Only even higher J and thus a higher pressure drop as well as the Base- line 0.3 mm showed better results. The pressure drop is as expected increased to around 4%. The temperature gradi- ent in Fig. 10 is higher, especially considering the trailing Fig. 10 Mean temperature distribution averaged on cross sections of edge ends further downstream. The OH emissions plotted Baseline 1.0 mm, Baseline 0.3 mm, the design for an Improved Flame Shape and the design for Improved Emissions in Fig. 11 underline the positive effects by showing overall reduced OH concentrations. 7 Limitation The presented work is exploring micromix combustion for a high-pressure ratio stationary gas turbine at a base load of more than 30 MW. The inlet conditions are 800 K and 22.6  bar. The high pressure and temperature as well as energy density are expected to lead to higher NOx emissions underlining the challenges of hydrogen combustion. There are no experimental data for the micromix design at these conditions to compare to and thus, in combination with the restrictions of RANS simulations, the significance of abso- lute values is limited. The simulation itself does not capture Fig. 11 Mean hydroxyl mass fraction rise on cross sections of Base- time-dependant effects and a further analysis and verification line 1.0  mm, Baseline 0.3  mm, the design for an Improved Flame Shape and the design for Improved Emissions using LES is advisable. Nevertheless, this study focuses on trends which can be predicted by RANS simulations. Not only is the simulation limited by the specific numeri- temperature and OH mass fraction is shown. The reason for cal setup, but more so the simulation of hydrogen in general the worse emissions is due to the reduced temperature gradi- is challenging due to the altered combustion behavior [41]. ent and high hydroxyl concentration downstream because of Deeply studied and validated combustion mechanism are the longer flames. The hot gas zone is accordingly long, the available for hydrocarbon fuels, but the experimental study residence time is high and the peak temperature above the regarding flame speed and ignition time delay of Kéromnès peak temperature of the other designs. However, this design et al. showed over a wide pressure and equivalence ratio has the lowest pressure drop of all shown designs. range, how ‘reactivity of the syngas mixtures was found to IMPROVED EMISSIONS (Figs. 8, 9) A combination of be governed by hydrogen chemistry for CO concentrations variation in three parameters was also used to find a design lower than 50% in the fuel mixture’ [42]. Accordingly, the with low emissions and a fuel orifice of 1.0 mm. The great- reaction mechanisms need to be adapted [43]. The mecha- est single influence on emissions is the air velocity. The nism by Naik et al. in combination with the FGM applied in change of Momentum Flux Ratio does not increase the fuel this paper is recommended by Zghal et al. [19], who stud- velocity and was chosen to avoid negative effects due to too ied it using RANS and LES simulations adapting complex high fuel stream velocities. A compromise between pressure chemistry, the thickened flame method and the flamelet- drop and emissions performance was made and J was set to generated manifold approach. Furthermore, the assumption 3, which previously led to a pressure drop of 4%. A design of Schmidt, Lewis and Prandtl number equal to 1 for hydro- with J increased to 7 to reduce emissions is possible, but carbon fuels needs to be altered according to [18] to properly the effect is not as significant as a lowered Momentum Flux represent hydrogen combustion within the presented mod- Ratio. WH is set to 2. This might seem exaggerated from eling approach. The proposed values are studied comparing 1 3 Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion… 249 LES and RANS simulations and applied in the presented drop in combustors is in general around 2% to 3%. This study. One of the affected aspects is the increased diffusiv - design offers potential to further reduce the NOx emissions ity of hydrogen, which needs to be properly replicated, but while increasing the pressure drop from below 1% to a still applied micromix combustion lacks data at high pressure acceptable level. to confirm or revoke the model assumptions. Comparable The Improved Emissions design compromised pressure experimental data from Cranfield University’s test rig fit - loss and performance to reduce the emissions by more than ted for hydrogen micromix combustion are expected within 50%. The hot gas region and the NOx formation volume this year. were reduced by increased mixing and penetration depth as The experimental data available at atmospheric condi- well as reduced residence time. Especially the significance tions from Haj Ajed et al. show for micromix combustion of hydroxyl for the formation of NOx is visible in Fig. 11. NOx emissions slightly above 2 ppm (measured) and NO Altering the Width-to-Height ratio leads to a higher penetra- emission slightly below 2 ppm (calculated) with a hydrogen tion depth. An increased offset from airgate to fuel injection orifice of 1.0 mm and an equivalence ratio of 0.4 [35]. In has a smaller but similar ee ff ct. This implies that the penetra - the numerical design exploration, the authors were able to tion depth needs to be increased for a given Momentum Flux reduce the NO emissions to as little as 0.28 ppm at 1 bar Ratio in a scaled micromix setup. A Width-to-Height ratio and 560 K inlet conditions. Also, at atmospheric conditions, exceeding the aero counterpart seems beneficial. Funke et al. showed in a similar experimental setup that the The equivalence ratio in this study was kept constant at a ‘investigated combustor module exceeds 99.4% combustion value of 0.4. The engine cycle assessment shows potential to efficiency for hydrogen contents of 80–100% in the fuel mix- further decrease the equivalence ratio by redistributing com- ture and shows NOx emissions less than 4 ppm corrected bustion and cooling air. This might lead to shorter flames to 15 Vol.% O ’ [44]. These values show the potential of and better emissions behavior due to leaner combustion. An micromix combustion to achieve low NOx emissions. analysis of the flame length showed the close coupling of flame length and NOx emissions. This study helps to understand the challenges and limi- 8 Conclusion tations of scaling a micromix injector for ground use. Sta- tionary hydrogen gas turbines have the potential to firm the The results for a hydrogen industrial gas turbine micromix energy grid by interacting with renewable energy sources. airgate pair with a scaled fuel oric fi e from 0.3 mm to 1.0 mm Potentially, fuel of lesser quality is used on ground com- have been presented in this paper. The key parameters have pared to hydrogen for aviation and the manufacturing cost been varied and two designs were found: an Improved Flame restrictions on ground favor using a larger fuel orifice. By Shape design mimicking the flame shape of the 0.3  mm varying the key design parameters, the negative impact of Baseline and an Improved Emissions design. According to the increased fuel orifice could be mitigated. Increasing the the results, the scaled Micromix injector can be improved fuel orifice leads to significant challenges to control NOx by: and imitating the flame structure of a smaller micromix burner did not yield satisfying results. The strong correla- o A narrow and short flame with a high temperature gradi- tion between pressure loss and emissions was highlighted. ent contributing to low residence times. Even though the negative impacts were mitigated, all scaled o A limited recirculation not sucking hot gas upstream to designs led to higher NOx emissions. the trailing edge which would lead to NOx formation at Acknowledgements This research complements the research of Cran- the trailing edge. field University in the ENABLEH2 project which received funding o High airstream velocity increasing turbulence and from the European Union’s Horizon 2020 research and innovation pro- decreasing residence time. This leads in general to an gramme under grant agreement N 769241. The author would like to increased pressure drop. thank Dr. David Abbott and Dr. Xiaoxiao Sun for their support of this project and acknowledges gratefully the cooperation of Cranfield Uni - o Limiting the concentration of hydroxyl both in value and versity’s Centre for Propulsion Engineering, and Stuttgart University volume. and the German Aerospace Centre’s Institute of Combustion Technol- o A sufficient penetration depth through, for example, a ogy, which facilitated this work. larger than one Width-to-Height ratio leading to more Funding Open Access funding enabled and organized by Projekt air flowing around the fuel jet. DEAL. The Author was funded by the ERASMUS + European Union student exchange program for the time of his placement at Cranfield The Improved Flame Shape design was able to yield University (01.10.2020–31.05.2021). This research complements the results better than expected due to the separated flames. research of Cranfield University in the ENABLEH2 project which received funding from the European Union’s Horizon 2020 research The emissions were 25% above the emissions of Baseline and innovation programme under grant agreement N 769241. 1.0 mm, but the pressure drop was reduced. The pressure 1 3 250 J. Berger Data availability Using no material but such available in the public 13. Japan claims world first: DLN gas turbine combustors verified domain was a requirement for this research and applies to this paper. on 100% hydrogen. Modern Power Systems. https:// www. moder n pow e rsy s t ems. com/ featu r es/ featu r ejap an- claims- wor ld- firs t- dln-g as-turbi ne- combu s tors-v eri fied-on- 100- h ydrog en-81850 77/ Code availability Computational fluid dynamics: Siemens Star (2020). Accessed 3 Mar 2021 CCM + 2020.2 (15.04.008). Other: Microsoft Word and Excel. 14. Sethi, V.: ENABLing cryogEnic Hydrogen based CO2 free air transport (ENABLEH2). h t t p s : // w w w . e n a b l e h 2 . e u (2018). Declarations Accessed 15 Feb 2021 15. Ben Abdallah, R., Sethi, V., Gauthier, P., Rolt, A.M., Abbott, Conflicts of interest The author holds currently no other position than D.: A detailed analytical study of hydrogen reaction in a novel student at Stuttgart and Cranfield University. An Non-disclosure agree- micromix combustion system. Proc. ASME Turbo. Expo. (2018). ment for the ENABLEH2 project was signed, but does not apply to this https:// doi. org/ 10. 1115/ GT2018- 76586 publication. 16. Babazzi, G., Gauthier, P., Agarwal, P., McClure, J., Sethi, V.: NOX emissions predictions for a hydrogen micromix combustion sys- tem. Proc. ASME Turbo. Expo. (2019). https:// doi. org/ 10. 1115/ Open Access This article is licensed under a Creative Commons Attri- GT2019- 90532 bution 4.0 International License, which permits use, sharing, adapta- 17. 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Journal

Aerotecnica Missili & SpazioSpringer Journals

Published: Sep 1, 2021

Keywords: Decarbonisation; Hydrogen Combustion; Stationary Gas Turbine; Industrial Gas Turbine; Micromix; NOx; Jet-in-Crossflow; Computational Fluid Dynamics; RANS; FGM

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