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High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters

High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with... applied sciences Article High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters 1 1 2 , 1 Xiangdong Liu , Qing Sun , Chengbin Zhang * and Liangyu Wu School of Hydraulic, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China; xdliu_yzu@126.com (X.L.); sun_qing_yzu@163.com (Q.S.); lywu@yzu.edu.cn (L.W.) Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China * Correspondence: cbzhang@seu.edu.cn; Tel.: +86-25-8379-2483 Academic Editor: Yuyuan Zhao Received: 1 September 2016; Accepted: 20 October 2016; Published: 26 October 2016 Abstract: The oscillating heat pipe (OHP) is a new member in the family of heat pipes, and it has great potential applications in energy conservation. However, the fluid flow and heat transfer in the OHP as well as the fundamental effects of inner diameter on them have not been fully understood, which are essential to the design and optimization of the OHP in real applications. Therefore, by combining the high-speed visualization method and infrared thermal imaging technique, the fluid flow and thermal performance in the OHPs with inner diameters of 1, 2 and 3 mm are presented and analyzed. The results indicate that three fluid flow motions, including small oscillation, bulk oscillation and circulation, coexist or, respectively, exist alone with the increasing heating load under different inner diameters, with three flow patterns occurring in the OHPs, viz. bubbly flow, slug flow and annular flow. These fluid flow motions are closely correlated with the heat and mass transfer performance in the OHPs, which can be reflected by the characteristics of infrared thermal images of condensers. The decrease in the inner diameter increases the frictional flow resistance and capillary instability while restricting the nucleate boiling in OHPs, which leads to a smaller proportion of bubbly flow, a larger proportion of short slug flow, a poorer thermal performance, and easier dry-out of working fluid. In addition, when compared with the 2 mm OHP, the increasing role of gravity induces the thermosyphon effect and weakens the ‘bubble pumping’ action, which results in a little smaller and bigger thermal resistances of 3 mm OHP under small and bulk oscillation of working fluid, respectively. Keywords: oscillating heat pipe; fluid flow motion; flow pattern; thermal performance; inner diameter 1. Introduction With rapid increase in energy consumption, in order to realize the sustainable development of energy and environment, it is necessary to improve the efficiency of energy transfer, especially that of thermal energy transfer [1–5]. Note that, as one of several types of high efficiency heat transfer elements, the heat pipe has been widely used for improving the efficiency of thermal energy transfer and reducing the environmental impact during the heat exchanging process in real applications [6–9]. The oscillating heat pipe (OHP) is a new member in the family of heat pipes, which was proposed by Akachi [10,11] in 1990. Generally, OHP is fabricated by a sealed wickless capillary partially filled with the working fluid, which is arranged in an interconnected meandering manner. Compared with the conventional wick heat pipe, OHP has its own special operation mechanism due to its unique wickless structure [12]. In the OHP, owing to the sufficient small scale of the meandering capillary, working fluid Appl. Sci. 2016, 6, 321; doi:10.3390/app6110321 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 321 2 of 16 can be distributed naturally into a series of vapor-liquid slugs by surface tension. When heated in one section (i.e., evaporator) and cooled in another (i.e., condenser), OHP establishes the temperature difference inside. As a result of this temperature difference and the uneven vapor-liquid distribution, a saturation pressure difference is established between the evaporator and condenser, coupled with non-uniform pressure oscillation produced in the OHP. These make the working fluids undergo complex oscillatory motions in the OHP, which achieves the heat transfer between the hot and cold sections. Both the special configuration and operation mechanism produce several additional advantages for the OHP in contrast to the conventional heat pipe [13,14]: simple construction and low cost, operational flexibility, capability for efficient heat transfer among multiple heat sources and sinks, etc. Therefore, the OHP possesses great application prospects in the areas of waste heat recovery [15,16], solar energy utilization [17,18], thermal management of hybrid vehicle [19,20], etc. In this context, considerable efforts have been devoted to investigating the fluid flow and heat transfer in the OHP, so as to deeply understand its heat transfer mechanisms which are helpful for the design and optimization of the OHP in practical applications. Several theoretical models were proposed to investigate the oscillating motion and heat transfer of the vapor-liquid slugs in the OHP [21,22]. In addition, some researchers made efforts to enhance the thermal performance of the OHP by using the nanofluids [23,24] and mixed working fluid [25]. The optimal design of capillary channel geometry has also been conducted to successfully improve the operation stability and heat transfer of the flat-plate OHP [26–28]. Note that the previous studies indicated that the thermal performance of OHP is affected by many parameters such as geometric parameters, working fluid properties, heating load and heating modes, inclination angle, filling ratio, number of turns, and gravity load, etc. [29]. In particular, the inner geometry scale of the OHP must be small enough to form the necessary uneven distribution of vapor-liquid slugs inside, which is essential for successful operation of the OHP. Note that this uneven distribution of vapor-liquid slugs mainly depends on the ratio between the surface tension and gravity, as characterized by the Bond number: g(  ) l v Bo = D , (1) where D is the inner diameter of the OHP,  and  are the corresponding densities of liquid i l v and vapor,  is the surface tension coefficient, and g is the gravity acceleration. Accordingly, several researchers [12,30] suggested the inner diameter of the OHP should be satisfied as D  2 . (2) (  )g l v Meanwhile, it should be noted that the frictional flow resistance increases considerably with the decreasing inner diameter of the OHP, which impedes the normal oscillatory motions of working fluid in the OHP. Therefore, the optimal range of D is suggested within the following range [31]: r r 0.7  D  1.8 . (3) (  ) g (  ) g l v l v Thus, it can be concluded that the inner diameter has great influence on the fluid flow and heat transfer in the OHP, which has been further demonstrated by several available studies. Charoensawan et al. [32] experimentally investigated the thermal performance of the horizontal OHPs with D = 1.0 and 2.0 mm, which indicated the better thermal performance of the OHP with bigger D . i i In addition, via the comparison between the thermal resistance of two OHPs with different inner diameters, Yang et al. [33,34] found that the thermal performance of the OHP with D = 1.0 mm was decreased by about 10% relative to that with D = 2.0 mm. In addition, the shape of the cross section of the OHP also affects its thermal performance under the same hydraulic diameter. The effects of inner diameter on the thermal performance of OHP might be opposite for different working fluid, which is indicated by the experimental results by Rittidech et al. [35,36]. Particularly, based on an experimental test on the open-loop OHPs, Saha et al. [37] found that when compared with the OHP Appl. Sci. 2016, 6, 321 3 of 16 with D = 0.9 mm, the OHP with D = 1.5 mm demonstrated worse thermal performance, which is i i different from the conclusions by Charoensawan et al. [32] and Yang et al. [33,34]. In summary, although available research has demonstrated significant influence of the inner diameter on the thermal performance of the OHP, the real causes of this influence have not been fully revealed due to the limited insight into the fluid flow in the OHP during its operation. It is worth noting that several researchers (e.g., Tong et al. [38], Xu et al. [39], Khandekar et al. [40–42], and Borgmeyer et al. [43,44]) have proven that high-speed visualization of the operating OHP is an effective way for understanding the fluid flow in it. In addition, this previous research also pointed out that the detailed fluid flow behaviors inside are closely correlated with the thermal performance of the OHP, which are critical for clarifying the heat transfer mechanisms of the OHP. However, the available high-speed visualization experiments are mainly carried out on the OHPs with single inner diameters, with limited focus on the effects of inner diameter on the fluid flow. As a result (from the scenario described), the effects of inner diameters on the thermal performance of the OHP have not been deeply understood. Therefore, in this work, based on the high-speed visualization method and infrared thermal imaging technology, the vapor-liquid two-phase fluid flow in the OHPs with D = 1.0, 2.0, 3.0 mm is investigated and compared to reveal the fundamental effects of the inner diameter. In addition, the motions of working fluid, distributions of vapor and liquid phases as well as the temperature distribution in the condenser are clarified and analyzed, in an effort to elucidate the relationship between the motions of working fluid and the heat transfer characteristics in the OHPs. 2. Experimental Setup Figure 1 schematically illustrates the experimental setup utilized in the current study. As shown, the OHPs used here are fabricated by bending the pyrex glass capillaries into the interconnected meandering manner, forming 10 U-turns and parallel vertical tubes. The pyrex glass capillaries have external diameter of D = 6.0 mm and inner diameters D = 1.0, 2.0, 3.0 mm, respectively. As depicted in Figure 1b, the OHP with the dimensions of 400 mm  185 mm is set vertically and heated at the bottom, which includes evaporator, adiabatic section and condenser with corresponding lengths of 100 mm, 25 mm and 275 mm. Before the experiment, the OHP is baked at 100 C, evacuated to be 4.0  10 torr for 8 h by a vacuum pump, and then filled with methanol as the working fluid. The filling ratio is maintained at  = 47% in this work. For the evaporator, the Ni-Cr wire with diameter of 0.25 mm is wrapped on the outer tube wall to supply the uniform heating load for the OHP. The whole evaporator and adiabatic section are embedded into the insulation box stuffed with the aluminum silicate insulation fibers, so as to ensure the relative error of heating load Q within 4.9%. The whole experiment is conducted in an environment with a constant temperature of 15  0.5 C, and thus the condenser of OHP can be cooled by the forced convection of the surrounding air via a cooling fan. By using an NEC TH9260 infrared camera (NEC Corporation, Tokyo, Japan) and the corresponding software, the infrared thermal images of the condenser during the operation of OHP are monitored, recorded and analyzed. The infrared camera possesses operation wavelength of 8–14 m and noise equivalent temperature difference (NETD) of 0.08 C, as well as thermal image resolution of 640  480 pixels. The emissivity of the condenser surface is corrected before the experiment via comparing temperature signals measured by the infrared camera and the K-type thermal couple with a measuring error of 0.1 C. By checking the emissivity of condensation section, the deviation between the tested result of the condenser temperature and its real value via the infrared camera is less than 2.3 C. The temperature of evaporator is measured by the thermocouples (diameter of 0.25 mm, OMEGA K-type with measuring error of 0.1 C, Omega Engineering, Santa Ana, CA, USA) fixed at the bottom of the evaporator, as marked in Figure 1b. The evaporator temperature are read and recorded by an Agilent 34970A data acquisition switch unit (Agilent Technologies, Santa Clara, CA, USA) with 6.5-digit accuracy, which has a maximum relative error of 0.5%. Appl. Sci. 2016, 6, 321 4 of 16 Appl. Sci. 2016, 6, 321  4 of 16  Figure 1. Experimental apparatus: (a) schematic of experimental setup; (b) schematic of experimental  Figure 1. Experimental apparatus: (a) schematic of experimental setup; (b) schematic of experimental oscillating heat pipe; and (c) cross section geometry of A–A in inset (b).  oscillating heat pipe; and (c) cross section geometry of A–A in inset (b). 3. Results and Discussions  3. Results and Discussions 3.1. Fluid Flow Motions inside OHP  3.1. Fluid Flow Motions inside OHP As mentioned above, a deep insight into the vapor‐liquid two‐phase flow motions in the OHP  during its quasi‐steady operation is helpful for understanding the oscillating operation characteristics  As mentioned above, a deep insight into the vapor-liquid two-phase flow motions in the OHP and heat transfer mechanisms of the OHP. In this work, after the start‐up process [45], a quasi‐steady  during its quasi-steady operation is helpful for understanding the oscillating operation characteristics station is achieved in the OHPs. Based on the summary of our visualization results under the quasi‐ and heat transfer mechanisms of the OHP. In this work, after the start-up process [45], a quasi-steady steady operation of the OHPs, it can be seen from Figure 2 that the fluid flow inside the OHPs with  station is achieved in the OHPs. Based on the summary of our visualization results under the three inner diameters under quasi‐steady operation condition mainly exhibits three motions: small  quasi-steady operation of the OHPs, it can be seen from Figure 2 that the fluid flow inside the OHPs oscillation (S‐O), bulk oscillation (B‐O) and circulation (C). Since the evaporator and adiabatic section  with three inner diameters under quasi-steady operation condition mainly exhibits three motions: are embedded into the thermal insulation box in the current experiment, in order to show these three  motions  clearly,  Figure  2b  selects  the  corresponding  fluid  flow  motions  in  either  U‐turn  at  the  small oscillation (S-O), bulk oscillation (B-O) and circulation (C). Since the evaporator and adiabatic condenser  of  the  OHP  as  the  representative  for  analysis.  As  shown,  when  the  small  oscillation  section are embedded into the thermal insulation box in the current experiment, in order to show appears in the OHP, the vapor‐liquid two‐phase fluid only exhibits small, local oscillations in the  these three motions clearly, Figure 2b selects the corresponding fluid flow motions in either U-turn tube, with limited mass exchange between the evaporator and condenser in each single vertical tube.  at the condenser of the OHP as the representative for analysis. As shown, when the small oscillation Correspondingly, when the bulk oscillation occurs, the working fluid begins to oscillate with large  appears in the OHP, the vapor-liquid two-phase fluid only exhibits small, local oscillations in the amplitude in the OHP, resulting in bulk mass exchange between the evaporator and condenser and  tube, with limited mass exchange between the evaporator and condenser in each single vertical tube. even among multiple U‐turns. Differing from the oscillation motion, the circulation of working fluid  is  characterized  by  the  circulation  of  working  fluid  in  the  whole  OHP  along  the  fixed  direction.  Correspondingly, when the bulk oscillation occurs, the working fluid begins to oscillate with large However, due to the non‐uniformity driving pressure distributions in the OHP, the circulation speed  amplitude in the OHP, resulting in bulk mass exchange between the evaporator and condenser and of working fluid is changed.  even among multiple U-turns. Differing from the oscillation motion, the circulation of working fluid is In order to reflect the heat transfer performance of the OHP produced by these three motions,  characterized by the circulation of working fluid in the whole OHP along the fixed direction. However, Figure 2c gives  the  typical  infrared  thermal images of  the  condenser  under  different  motions.  In  due to the non-uniformity driving pressure distributions in the OHP, the circulation speed of working addition, corresponding to the infrared thermal images in Figure 2c, Figure 3 further quantitatively  fluid is changed. represents  the  vertical  temperature  distribution  in  two  adjacent  tubes  in  a  typical  U‐turn  of  the  In order to reflect the heat transfer performance of the OHP produced by these three motions, Figure 2c gives the typical infrared thermal images of the condenser under different motions. In addition, corresponding to the infrared thermal images in Figure 2c, Figure 3 further quantitatively represents the vertical temperature distribution in two adjacent tubes in a typical U-turn of the condenser under different quasi-steady operation states. In this figure, each curve is plotted by the Appl. Sci. 2016, 6, 321 5 of 16 Appl. Sci. 2016, 6, 321  5 of 16  average temperature data of every seven horizontal pixel points along each vertical tube in the typical condenser under different quasi‐steady operation states. In this figure, each curve is plotted by the  U-turn. As indicated in Figures 2c and 3a, under small oscillation motion, the heat transfer of the average temperature data of every seven horizontal pixel points along each vertical tube in the typical  condenser mainly relied on its own heat conduction, except for the bottom regime, which is dependent U‐turn. As indicated in Figures 2c and 3a, under small oscillation motion, the heat transfer of the  on the sensible and latent heat transfer of the locally oscillating working fluid. Therefore, with the condenser  mainly  relied  on  its  own  heat  conduction,  except  for  the  bottom  regime,  which  is  exception of high temperature at the bottom of the condenser, the temperature at the other part of dependent on the sensible and latent heat transfer of the locally oscillating working fluid. Therefore,  the condenser nearly decreases linearly along the vertical direction to the top. As the bulk oscillation with the exception of high temperature at the bottom of the condenser, the temperature at the other  comes out, working fluid at the hot and cool ends of the OHP are able to exchange with each other, part of the condenser nearly decreases linearly along the vertical direction to the top. As the bulk  which obviously enhances the heat and mass transfer from the evaporator to the condenser. As a result, oscillation comes out, working fluid at the hot and cool ends of the OHP are able to exchange with  the areach ea of  other, high-temperatur   which  obvio eursly egion   enh at ance thes bottom the  heat of  and the  condenser mass  transfis er expanded, from  the  ev leading aporato to r  to an  the incr  ease condenser.  As  a  result,  the  area  of  high‐temperature  region  at  the  bottom  of  the  condenser  is  in the temperature level of the condenser (see Figures 2c and 3b). Note that, under the bulk oscillation expanded, leading to an increase in the temperature level of the condenser (see Figures 2c and 3b).  motion, due to the non-uniform heat and mass transfer strength caused by the uneven oscillation Note that, under the bulk oscillation motion, due to the non‐uniform heat and mass transfer strength  of working fluid among the OHP, every parallel tube at the condenser has different temperature caused  by  the  uneven  oscillation  of  working  fluid  among  the  OHP,  every  parallel  tube  at  the  distributions along the vertical direction. When the circulation of working fluid appears in the condenser has different temperature distributions along the vertical direction. When the circulation  OHP, the adjacent parallel vertical tubes become ‘upheaders’ and ‘downcomers’ alternatively with of working fluid appears in the OHP, the adjacent parallel vertical tubes become ‘upheaders’ and  corresponding hot and cold fluid flowing inside, resulting in alternatively high and low temperature ‘downcomers’  alternatively  with  corresponding  hot  and  cold  fluid  flowing  inside,  resulting  in  on them (see Figure 3c). alternatively high and low temperature on them (see Figure 3c).  Figure  2.  Fluid  flow  motions  in  the  oscillating  heat  pipe  (OHP)  and  corresponding  experimental  Figure 2. Fluid flow motions in the oscillating heat pipe (OHP) and corresponding experimental images images as well as infrared thermal images of the condenser: (a) schematic of fluid flow motions; (b)  as well as infrared thermal images of the condenser: (a) schematic of fluid flow motions; (b) snapshots of different fluid flow motions in a typical U-turn of condenser; and (c) infrared thermal images of the condenser. Appl. Sci. 2016, 6, 321  6 of 16  snapshots of different fluid flow motions in a typical U‐turn of condenser; and (c) infrared thermal  Appl. Sci. 2016, 6, 321 6 of 16 images of the condenser.  250 250 250 6# (a) 6# 6# (c) (b) 7# 7# 7# 200 200 200 150 150 150 100 100 100 50 50 0 0 24 36 48 60 72 24 36 48 60 72 24 36 48 60 72 T [C] T [C] T [C] Figure 3. Vertical temperature distribution of condenser under different quasi‐steady operation states  Figure 3. Vertical temperature distribution of condenser under different quasi-steady operation states corresponding to Figure 2c (6#‐7# U‐turn): (a) small oscillation (Q = 30 W); (b) big oscillation (Q = 70 W);  corresponding to Figure 2c (6#-7# U-turn): (a) small oscillation (Q = 30 W); (b) big oscillation (Q = 70 W); and (c) circulation (Q = 140 W).  and (c) circulation (Q = 140 W). Furthermore, the current experimental results also indicated that the three motions above coexist  or,  respectively,  exist  alone  with  increasing  heating  load  under  different  inner  diameters,  with  Furthermore, the current experimental results also indicated that the three motions above coexist different  coupling  characteristics,  as  shown  in  Figure  4,  where  pt  represents  the  percentage  of  or, respectively, exist alone with increasing heating load under different inner diameters, with different duration for a certain fluid flow motion with respect to the total statistical duration. It can be seen  coupling characteristics, as shown in Figure 4, where p represents the percentage of duration for a that when Q is very small, the driving pressure inside the OHP is limited, which only triggers small  certain fluid flow motion with respect to the total statistical duration. It can be seen that when Q is very oscillation of working fluids. With the increasing heating load, the driving pressure inside the OHP  small, the driving pressure inside the OHP is limited, which only triggers small oscillation of working rises to overcome the flow resistance between the evaporator and condenser or even the adjacent U‐ fluids. With the increasing heating load, the driving pressure inside the OHP rises to overcome the flow turns, which induces the bulk oscillation of working fluid. At this time, the small and bulk oscillations  resistance between the evaporator and condenser or even the adjacent U-turns, which induces the bulk appear intermittently in the OHP, and the duration of bulk oscillation is gradually increased when  oscillation the heating of working  load (i.efluid. ., the dAt riving this pressure time, the  in the small  OHP) and furt bulk her oscillations rises, as depicappear ted in (i)intermittently  and (ii) of Figure in  the 4b. By further raising the heating load, the region of bulk oscillation is expanded to multiple U‐turns,  OHP, and the duration of bulk oscillation is gradually increased when the heating load (i.e., the driving which eventually produces the circulation of working fluid (see (iii) of Figure 4b). Finally, the small  pressure in the OHP) further rises, as depicted in (i) and (ii) of Figure 4b. By further raising the heating oscillation and bulk oscillation of working fluid disappear sequentially under a large heating load,  load, the region of bulk oscillation is expanded to multiple U-turns, which eventually produces the and the pure circulation of working fluid is achieved in the OHP (see Figure 4a and (iv) of Figure 4b).  circulation of working fluid (see (iii) of Figure 4b). Finally, the small oscillation and bulk oscillation of Note that, as shown by the comparison among the flow motions of working fluid in the OHPs with  working fluid disappear sequentially under a large heating load, and the pure circulation of working different inner diameters under the same heating load (see (ii), (v) and (vi) in Figure 4b), with respect  fluid is achieved in the OHP (see Figure 4a and (iv) of Figure 4b). Note that, as shown by the comparison to the OHP with Di = 2.0 mm, OHP with Di = 1.0 mm has larger frictional flow resistance inside, and  among the flow motions of working fluid in the OHPs with different inner diameters under the same thus the fluid flow motions under small driving pressure (e.g., small heating load) appears more  heating load (see (ii), (v) and (vi) in Figure 4b), with respect to the OHP with D = 2.0 mm, OHP with easily (e.g., small oscillation in (ii), (v) and (vi) of Figure 4b). On the other hand, although the frictional  D = 1.0 flow mm  resist has anc lar e ger is lower frictional  in the OHP flow wi resistance th Di = 3.0 inside,  mm, the and  resist thus ance the offluid  capillflow ary hy motions steresis is under  higher, small   and  the  vapor‐liquid  meniscus  of  vapor  slugs is  less  rigid,  which  weakens  the  necessary ʹbubble  driving pressure (e.g., small heating load) appears more easily (e.g., small oscillation in (ii), (v) and pumpingʹ action [33,34] for the momentum transfer of working fluid inside the OHPs. Consequently,  (vi) of Figure 4b). On the other hand, although the frictional flow resistance is lower in the OHP with the percentage of oscillation motions is larger than that in the OHP with Di = 2.0 mm (e.g., small  D = 3.0 mm, the resistance of capillary hysteresis is higher, and the vapor-liquid meniscus of vapor oscillation in (ii), (v) and (vi) of Figure 4b).  slugs is less rigid, which weakens the necessary ‘bubble pumping’ action [33,34] for the momentum transfer of working fluid inside the OHPs. Consequently, the percentage of oscillation motions is larger than that in the OHP with D = 2.0 mm (e.g., small oscillation in (ii), (v) and (vi) of Figure 4b). z[mm] z[mm] z[mm] Appl. Sci. 2016, 6, 321 7 of 16 Appl. Sci. 2016, 6, 321  7 of 16  (a) S-O S-O & B-O & & S-O B-O C B-O & C Increasing heating load (b) (i) Q = 50W, D = 2mm, p (S-O)=69.9%, p (B-O)=31.1% S-O i t t B-O (ii) Q = 80W, D = 2mm, p (S-O)=28.6%, p (B-O)=72.4% S-O i t t B-O (iii) Q =110W, D = 2mm, p (S-O)=4.4%, p (B-O)=24.6%, p (C)=71.0% i t t t S-O B-O (iv) Q =140W, D = 2mm, p (B-O)=10.6%, p (C)=89.4% i t t B-O (v) Q = 80W, D = 1mm, p (S-O)=59.6%, p (B-O)=40.4% i t t S-O B-O (vi) Q =80W, D = 3mm, p (S-O)=58.4%, p (B-O)=41.6% i t t S-O B-O 0 200 400 600 800 1000 t [s] Figure 4. Variation of fluid flow motions with increasing heating load: (a) schematic of change in fluid  Figure 4. Variation of fluid flow motions with increasing heating load: (a) schematic of change in flow  motions  with  increasing  heating  load;  and  (b)  time  series  of  fluid  flow  motions  and  their  fluid flow motions with increasing heating load; and (b) time series of fluid flow motions and their corresponding  duration  fractions  under  different  heating  loads  and  inner  diameters.  Fluid  flow  corresponding duration fractions under different heating loads and inner diameters. Fluid flow motions motions mode index: 0: S‐O; 1: B‐O; and 2: C.  mode index: 0: S-O; 1: B-O; and 2: C. 3.2. General Flow Patterns in OHP  3.2. General Flow Patterns in OHP As shown in Figures 5–8, the above‐mentioned complex fluid flows and phase changes also lead  As shown in Figures 5–8, the above-mentioned complex fluid flows and phase changes also to the occurrences and evolutions of various flow patterns in the OHPs. Herein, three types of flow  lead to the occurrences and evolutions of various flow patterns in the OHPs. Herein, three types patterns, bubbly flow, slug flow and annular flow, are observed in the OHPs. Significantly, these flow  of flow patterns, bubbly flow, slug flow and annular flow, are observed in the OHPs. Significantly, patterns and their evolutions exhibit different characteristics under different inner diameters of the OHPs.  these flow patterns and their evolutions exhibit different characteristics under different inner diameters of the OHPs. 3.2.1. Bubbly Flow  Fluid flow motion elements indexes Appl. Sci. 2016, 6, 321 8 of 16 3.2.1. Bubbly Flow Appl. Sci. 2016, 6, 321  8 of 16  As indexed by ‘B’ in Figure 5, bubbly flow is characterized by some dispersed bubbles flowing with the continuous liquid, with the length smaller than the inner diameter of the OHP. This flow As indexed by ‘B’ in Figure 5, bubbly flow is characterized by some dispersed bubbles flowing  pattern is mainly produced by the continuous nucleate boiling in the evaporator, which usually exists with the continuous liquid, with the length smaller than the inner diameter of the OHP. This flow  in the stream flowing from the evaporator to the condenser. Due to the small size and low volume pattern is mainly produced by the continuous nucleate boiling in the evaporator, which usually exists  fraction in the of the stream dispersed  flowingbubbles,  from the the evap bubbly orator to flow  theis co easily ndenser. condensed  Due to the to small pure size liquid,  andand  lowthus  volume it b arely fraction of the dispersed bubbles, the bubbly flow is easily condensed to pure liquid, and thus it  appears in the downstream flowing from the condenser to the evaporator. Note that, when the fluid barely appears in the downstream flowing from the condenser to the evaporator. Note that, when the  mixture undergoes fast turning and large disturbance, the bubbly flow may appear for a short time at fluid mixture undergoes fast turning and large disturbance, the bubbly flow may appear for a short  the end of the long vapor slugs. Generally, bubbly flow is unstable in the OHPs due to the coalescence time at the end of the long vapor slugs. Generally, bubbly flow is unstable in the OHPs due to the  and shrinking of the dispersed bubbles. Via the comparison among the characteristics of bubbly flow coalescence and shrinking of the dispersed bubbles. Via the comparison among the characteristics of  in the OHPs with different inner diameters, it can be seen that the confined space in the OHP with bubbly flow in the OHPs with different inner diameters, it can be seen that the confined space in the  D = 1 mm restricts the nucleate boiling in the evaporator [46], resulting in the fewer dispersed bubbles OHP  with  Di  =  1  mm  restricts  the  nucleate  boiling  in  the  evaporator  [46],  resulting  in  the  fewer  in the bubbly flow than that under D = 2 mm, 3 mm. In addition, the dispersed bubbles in the OHP dispersed bubbles in the bubbly flow than that under Di = 2 mm, 3 mm. In addition, the dispersed  with D = 1 mm are hardly mixed due to the confinement, and always flow with the main stream bubbles i  in the OHP with Di = 1 mm are hardly mixed due to the confinement, and always flow with  in turn. the main stream in turn.  B S A B S A B S A D = 1mm D = 2mm D = 3mm i i i Figure 5. Flow patterns occurring in the OHPs with different diameters. Flow patterns indexes: B:  Figure 5. Flow patterns occurring in the OHPs with different diameters. Flow patterns indexes: bubbly flow; S: slug flow; A: annular flow.  B: bubbly flow; S: slug flow; A: annular flow. 3.2.2. Slug Flow  3.2.2. Slug Flow When the slug flow appears in the OHPs, a series of vapor slugs with lengths bigger than the  When the slug flow appears in the OHPs, a series of vapor slugs with lengths bigger than inner diameter flow with the main stream, as indexed by ‘S’ in Figure 5. This flow pattern is generally  the inner diameter flow with the main stream, as indexed by ‘S’ in Figure 5. This flow pattern is formed by the self‐growth and coalescence of dispersed bubbles and the breakup of very long slugs.  generally The  slug formed   flow by almost the self-g emerges rowth   unde and r  alcoalescence l  conditions  und of dispersed er  different bubbles   inner  diameters and the br bueakup t  is  most of  very common in the fluid mixture with oscillation and ‘downcomers’ under the fluid circulation in the  long slugs. The slug flow almost emerges under all conditions under different inner diameters but is OHPs. In addition, the length of slugs in the slug flow is usually changed because of the slugsʹ growth,  most common in the fluid mixture with oscillation and ‘downcomers’ under the fluid circulation in the shrinking, coalescence and breakup. It is worth noting that as the mixing of dispersed bubbles is  OHPs. In addition, the length of slugs in the slug flow is usually changed because of the slugs’ growth, resisted in the small tube, the slug flow in the OHP with Di = 1 mm is mainly formed by the self‐ shrinking, coalescence and breakup. It is worth noting that as the mixing of dispersed bubbles is growth of dispersed bubbles and breakup of very long slugs rather than the coalescence of dispersed  resisted in the small tube, the slug flow in the OHP with D = 1 mm is mainly formed by the self-growth bubbles, which is different from that under the inner diameter of 2 mm and 3 mm.  of dispersed bubbles and breakup of very long slugs rather than the coalescence of dispersed bubbles, which is different from that under the inner diameter of 2 mm and 3 mm. 3.2.3. Annular Flow  In the OHPs, when the continuous vapor flows through the center of the tubes with a liquid film  3.2.3. Annular Flow formed around the inner tube wall, the annular flow comes out, as indexed by ‘A’ in Figure 5. This  flow In the paOHPs, ttern is usu when ally the trancontinuous sformed fromvapor  the slug flows  flow,thr  and ough  usuathe lly acenter ppears in of the the ‘uphea tubesders’ with with a liquid   a large volume fraction of vapor produced by the drastic boiling in the evaporator under high heating  film formed around the inner tube wall, the annular flow comes out, as indexed by ‘A’ in Figure 5. load. In the ‘upheaders’, the high‐speed vapor induced by drastic boiling always breaks the liquid  This flow pattern is usually transformed from the slug flow, and usually appears in the ‘upheaders’ bridges  between  the  vapor  slugs  and  triggers  the  transition  from  slug  flow  to  the  annular  flow.  with a large volume fraction of vapor produced by the drastic boiling in the evaporator under high Moreover, as shown by Figure 5, smaller inner diameter induces greater effect of surface tension in  heating load. In the ‘upheaders’, the high-speed vapor induced by drastic boiling always breaks the the OHP and thus generates larger capillary instability, which leads to more irregular waves on the  liquid bridges between the vapor slugs and triggers the transition from slug flow to the annular flow. Appl. Sci. 2016, 6, 321 9 of 16 Moreover, as shown by Figure 5, smaller inner diameter induces greater effect of surface tension in the OHP and thus generates larger capillary instability, which leads to more irregular waves on the liquid film of the annular flow in the OHP with D = 1 mm. These irregular waves easily cause the break Appl. Sci. 2016, 6, 321  9 of 16  of the continuous vapor core in the annular flow via the formation of the liquid bridges, which can induce the transition from the annular flow to the slug flow. Therefore, the stability of annular flow liquid film of the annular flow in the OHP with Di = 1 mm. These irregular waves easily cause the  deteriorates with the decreasing inner diameter of the OHPs. break of the continuous vapor core in the annular flow via the formation of the liquid bridges, which  can induce the transition from the annular flow to the slug flow. Therefore, the stability of annular  3.3. Flow Pattern Evolutions in Evaporator and Condenser flow deteriorates with the decreasing inner diameter of the OHPs.  As discussed above, the general flow patterns in the OHPs with different diameters are closely 3.3. Flow Pattern Evolutions in Evaporator and Condenser  related to the motions of vapor-liquid fluid mixture in the evaporator and condenser. Therefore, As discussed above, the general flow patterns in the OHPs with different diameters are closely  deeply analyzing the behaviors of vapor-liquid fluid mixture in the evaporator and condenser is related  to  the  motions  of vapor‐liquid fluid  mixture  in  the  evaporator and  condenser.  Therefore,  beneficial for understanding the occurrences and evolutions of flow patterns in the OHPs. deeply analyzing the behaviors of vapor‐liquid fluid mixture in the evaporator and condenser is  beneficial for understanding the occurrences and evolutions of flow patterns in the OHPs.  3.3.1. Flow Pattern Evolutions in Evaporator 3.3.1. Flow Pattern Evolutions in Evaporator  In order to observe the flow patterns evolutions in the evaporator, the insulation box is opened in some experimental cases for visualization. As shown in Figure 6, when the working fluid in the In order to observe the flow patterns evolutions in the evaporator, the insulation box is opened  evaporator is heated to reach the saturation state, nucleate boiling appears, and some dispersed in some experimental cases for visualization. As shown in Figure 6, when the working fluid in the  small bubbles are produced, flowing up quickly due to the buoyancy and driving pressure difference evaporator is heated to reach the saturation state, nucleate boiling appears, and some dispersed small  bubbles  are  produced,  flowing  up  quickly  due  to  the  buoyancy  and  driving  pressure  difference  between the hot and cold ends of the OHP. Meanwhile, the dispersed bubbles grow up or coalesce into between the hot and cold ends of the OHP. Meanwhile, the dispersed bubbles grow up or coalesce  the bigger ones and even the vapor slugs, which triggers the transition from bubbly flow to the slug into the bigger ones and even the vapor slugs, which triggers the transition from bubbly flow to the  flow. It can be clearly seen from the comparison among the flow pattern evolutions in the evaporators slug  flow.  It  can  be  clearly  seen  from  the  comparison  among  the  flow  pattern  evolutions  in  the  with different inner diameters that the transition from the bubbly flow to the slug flow is mainly evaporators with different inner diameters that the transition from the bubbly flow to the slug flow  dependent on the self-growth of the dispersed bubbles in the evaporator with D = 1 mm, rather than is mainly dependent on the self‐growth of the dispersed bubbles in the evaporator with Di = 1 mm,  the coalescence among them in the evaporators with diameter of 2 mm and 3 mm. Furthermore, rather  than  the  coalescence  among  them  in  the  evaporators  with  diameter  of  2  mm  and  3  mm.  less dispersed bubbles are produced by the nucleate boiling with the decreasing inner diameter, Furthermore, less dispersed bubbles are produced by the nucleate boiling with the decreasing inner  which further explains the characteristics of bubbly flow in the OHP with D = 1 mm as discussed in diameter, which further explains the characteristics of bubbly flow in the OH i P with Di = 1 mm as  the above discusse section. d in the above section.  Figure 6. Evolution of flow pattern in the evaporators of the OHPs with different inner diameters (Q  Figure 6. Evolution of flow pattern in the evaporators of the OHPs with different inner diameters ~ 100 W).  (Q ~100 W). 3.3.2. Flow Pattern Evolutions in the Condenser  3.3.2. Flow Pattern Evolutions in the Condenser As depicted in Figure 7, the visualization results indicate that the condensation of vapor in the  As depicted in Figure 7, the visualization results indicate that the condensation of vapor in the condenser is the major cause of the breakup of continuous vapor core in the OHPs. It can be seen that,  condenser is the major cause of the breakup of continuous vapor core in the OHPs. It can be seen because  of  the  continuous  condensation  of  vapor,  the  vapor  bonds  emerge  at  several  locations  connecting the non‐broken long vapor slug or the continuous vapor core of the annular flow (see long  Appl. Sci. 2016, 6, 321 10 of 16 that, because of the continuous condensation of vapor, the vapor bonds emerge at several locations connecting the non-broken long vapor slug or the continuous vapor core of the annular flow (see long Appl. Sci. 2016, 6, 321  10 of 16  slugs A and B in Figure 7a and slug A in Figure 7b). As the condensation further proceeds, the long slugs A and B in Figure 7a and slug A′ in Figure 7b). As the condensation further proceeds, the long  vapor slugs are broken up into several shorter ones via the breakup of vapor bonds, and the produced vapor slugs are broken up into several shorter ones via the breakup of vapor bonds, and the produced  shorter ones continue shrinking (see slugs C and D produced by slug A as well as slugs E and F shorter ones continue shrinking (see slugs C and D produced by slug A as well as slugs E and F  0 0 0 0 produced by slug B in Figure 7a, slug B and bubbles C , D produced by slug A in Figure 7b). produced by slug B in Figure 7a, slug B′ and bubbles C′, D′ produced by slug A′ in Figure 7b). Based  Based on the comparison among the breakup of long vapor slugs in the OHPs with different inner on the comparison among the breakup of long vapor slugs in the OHPs with different inner diameters,  diameters, it is indicated that the reduction in the inner diameter of OHP results in the increasing role it is indicated that the reduction in the inner diameter of OHP results in the increasing role of surface  of surface tension, which enhances the capillary instability and thus induces more irregular fluctuation tension, which enhances the capillary instability and thus induces more irregular fluctuation on the  on the vapor-liquid interface. Therefore, as indicated in Figure 7a, there are more collapses on long vapor‐liquid interface. Therefore, as indicated in Figure 7a, there are more collapses on long slugs in  the OHP with Di = 1 mm, forming more short slugs than that under Di = 2 mm, 3 mm.  slugs in the OHP with D = 1 mm, forming more short slugs than that under D = 2 mm, 3 mm. i i Figure 7. Evolution of flow pattern in the vertical tube at the condenser of the OHPs with different  Figure 7. Evolution of flow pattern in the vertical tube at the condenser of the OHPs with different inner diameters: (a) snapshots of flow pattern evolution under Di = 1mm; (b) snapshots of flow pattern  inner diameters: (a) snapshots of flow pattern evolution under D = 1mm; (b) snapshots of flow pattern evolution under Di = 2mm; and (c) snapshots of flow pattern evolution under Di = 3mm.  evolution under D = 2mm; and (c) snapshots of flow pattern evolution under D = 3mm. i i Appl. Sci. 2016, 6, 321 11 of 16 Appl. Sci. 2016, 6, 321  11 of 16  Additionally, to further clarify the influence of inner diameter on the vapor-liquid two-phase Additionally, to further clarify the influence of inner diameter on the vapor‐liquid two‐phase  distributions, Figure 8 compares the distribution of dimensionless bubble/slug length (L/D ) at the distributions, Figure 8 compares the distribution of dimensionless bubble/slug length (L/Di) at the  condensers of the OHPs with different D under S-O and B-O motions of working fluid at Q = 90 W, condensers of the OHPs with different Di under S‐O and B‐O motions of working fluid at Q = 90 W,  where L is the real length of bubble/slug (see Figure 5). Herein, the percentage of bubble/slug length, where L is the real length of bubble/slug (see Figure 5). Herein, the percentage of bubble/slug length,  P , is introduced to quantitatively represent the length distribution of bubbles/slugs Pi, is introduced to quantitatively represent the length distribution of bubbles/slugs  P =  100%, (4) j   P   100% (4)  where n is the bubble/slug number within a certain range of dimensionless size, and n is the total where nj is the bubble/slug number within a certain range of dimensionless size, and nt is the total  number of bubbles/slugs in a statistic duration of 10 s. As shown, the proportion of dispersed bubble number of bubbles/slugs in a statistic duration of 10 s. As shown, the proportion of dispersed bubble  (L/D < 1) increases as the tube diameter increases, implying that the nucleate boiling at the evaporator (L/Di < 1) increases as the tube diameter increases, implying that the nucleate boiling at the evaporator  with bigger inner diameter produces more dispersed bubbles flowing into the condenser and thus with bigger inner diameter produces more dispersed bubbles flowing into the condenser and thus  increases their proportion there. Compared with dispersed bubbles, as the inner diameter increases, increases their proportion there. Compared with dispersed bubbles, as the inner diameter increases,  the proportion of short slugs (1 L/D < 10) experiences a decrease. As mentioned above, this is mainly the proportion of short slugs (1 ≤ L/Di < 10) experiences a decrease. As mentioned above, this is mainly  attributed to the drop of capillary instability in the OHP, which reduces the probability of breakup from attributed to the drop of capillary instability in the OHP, which reduces the probability of breakup  the long slugs to the shorter ones. Moreover, this decreasing capillary instability with increasing inner from the long slugs to the shorter ones. Moreover, this decreasing capillary instability with increasing  diameter also induces the larger proportion of long slugs (100  L/D ) under D = 2 mm than that at inner diameter also induces the larger proportion of long slugs (100 ≤ L/Di) under Di = 2 mm than that  i i D = 1 mm. On the other hand, enlarging the inner diameter of the OHP also amplifies the absolute at Di = 1 mm. On the other hand, enlarging the inner diameter of the OHP also amplifies the absolute  distance between vapor slugs, which decreases the probability of coalescences among vapor slugs  distance between vapor slugs, which decreases the probability of coalescences among vapor slugs into into long vapor slugs, resulting in smaller proportion of long slugs (100 ≤ L/Di) under Di = 3 mm than  long vapor slugs, resulting in smaller proportion of long slugs (100  L/D ) under D = 3 mm than i i that at Di = 2 mm.  that at D = 2 mm. (a) D = 1mm, Q = 90W, S-O & B-O (b) D = 2mm, Q = 90W, S-O & B-O (c) D = 3mm, Q = 90W, S-O & B-O 110 100 L / D 1<L/D 1L/D <10 10L/D <40 i i i 40L/D <70 70L/D <100 100L/D i i Figure Figure 8. Distribution   8.  Distribution of bubbles/slugs of  bubbles/slugsle  leng ngth th at at the the condenser condenser  in in  the the  OHPs OHPs  with with  diffe differ rent ent  inner inner   diameters under quasi‐steady S‐O and B‐O fluid motion: (a) Di = 1mm under Q = 90W; (b) Di = 2mm  diameters under quasi-steady S-O and B-O fluid motion: (a) D = 1mm under Q = 90W; (b) D = 2mm i i under Q = 90W; and (c) Di = 3mm under Q = 90W.  under Q = 90W; and (c) D = 3mm under Q = 90W. P [%] P [%] P [%] j j Appl. Sci. 2016, 6, 321 12 of 16 3.4. Thermal Performance To clarify the correlations among heating load, fluid flow motions and thermal performance of the OHPs and further reveal the corresponding fundamental effects of inner diameter, the variation in the overall thermal resistance R versus heating load under different inner diameters is presented in Figure 9, where R is defined as R = T T /Q. (5) e c In Equation (5), T is the average temperature of evaporator calculated as the mean temperature of T ~T as shown in Figure 1b, and T is the average temperature of condenser computed by 1 5 c averaging the temperature values of all pixel points on the infrared thermal images of the condenser. In addition, in order to represent the effect of inner diameter on the thermal performance of the OHP, the dimensionless inner diameter of the OHP, D *, is defined as D D i i D = p = p (6) /gD /g(  ) l v where D is the inner diameter of the OHP, g is the gravity acceleration, and D and s are the difference of density between liquid and vapor and surface tension coefficient at (T + T )/2, respectively. e-r c-r Herein, T and T are the reference temperatures of the evaporator and condenser, which are e-r c-r correspondingly defined as the highest operating temperature of the evaporator T = 123.8 C and e-r ambient temperature of the condenser T = 15 C. Actually, D * is another expression form of the c-r i Bond number, which can characterize the ratio between the surface tension and gravity. As indicated in Figure 9, under the small oscillation of working fluid, the heat exchanged between the hot and cold ends of the OHP mainly relies on the heat conduction of working fluid and tube body, resulting in a slight decrease of R with the increasing heating load. When the bulk oscillation of working fluid occurs, the heat and mass transfer in the OHPs are significantly improved, leading to the most apparent decrease in R versus the increasing heating load (e.g., the changing of R within Q = 60–120 W under D * = 1.31 (D = 2 mm) in Figure 9). After the appearance of working fluid circulation, the capabilities i i of heat and mass transfer in the OHPs are further enhanced to be a steady level due to a certain amount of filled working fluid, when the thermal resistance of the OHPs gradually drops to be a steady value (e.g., the changing of R within Q = 110–200 W under D * = 1.31 (D = 2 mm) in Figure 9). Note that i i the dry-out of working fluid will emerge in the evaporator, represented by the sudden growing-up of the thermal resistance (e.g., Q = 160 W under D * = 0.65 (D = 1 mm) and Q = 220 W under D * = 1.99 i i i (D = 2 mm) in Figure 9). Herein, from the comparison among the changes of the thermal resistance and fluid flow motions versus heating load under different inner diameters of the OHPs, it can be seen that only the oscillation motions of working fluid occur in the major heating loads range (Q  110 W) under D * = 0.65 (D = 1 mm), due to the large frictional flow resistance. Furthermore, even under the same mode of working fluid motions, the large frictional flow resistance under D * = 0.65 (D = 1 mm) also reduces i i the heat and mass transfer rate into the OHP. Therefore, the OHP with D * = 0.65 (D = 1 mm) has the i i highest thermal resistance relative to those under D * = 1.31 (D = 2 mm) and D * = 1.99 (D = 3 mm). i i i i In addition, owing to the hardest supplement of working fluid for the nucleate boiling in the evaporator under the least amount of filled working fluid, the dry-out of working fluid occurs most easily in the OHP with D * = 0.65 (D = 1 mm). Compared with the OHP with D * = 1.31 (D = 2 mm), the gravity i i i i plays a more important role in the OHP with D * = 1.99 (D = 3 mm), producing the thermosyphon i i effect under the small oscillation of working fluid which facilitates the backflow of working fluid into the evaporator and enhances the heat and mass transfer [34]. Accordingly, at the small heating load (Q  40 W), thermal resistance under D * = 1.99 (D = 3 mm) is a little smaller than that under i i D * = 1.31 (D = 2 mm). However, under the bulk oscillation depending on the momentum exchange of i i working fluid, the thermal resistance under D * = 1.99 (D = 3 mm) turns out to be slightly larger than i i that under D * = 1.31 (D = 2 mm) (50 W Q  100 W). It must be attributed to the decrease of rigidity i i Appl. Sci. 2016, 6, 321 13 of 16 on the vapor-liquid meniscus of vapor slugs, which weakens the necessary ‘bubble pumping’ effect for the exchange of working fluid in the OHPs and reduces the efficiency of heat and mass transfer. Appl. Sci. 2016, 6, 321  13 of 16  2.4 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0 20 40 60 80 100 120 140 160 180 200 220 Q [W] Green symbols: D *= 0.65(D = 1mm); S-O B-O&C i i Red symbols: D *= 1.31(D = 2mm); S-O&B-O C i i Blue symbols: D *= 1.99(D = 3mm); S-O&B-O&C dry-out i i Figure 9. Variation of thermal resistance and fluid flow motions versus increasing heating load under  Figure 9. Variation of thermal resistance and fluid flow motions versus increasing heating load under different inner diameters of the OHPs.  different inner diameters of the OHPs. 4. Conclusions Herein,  from  the  comparison  among  the  changes  of  the  thermal  resistance  and  fluid  flow  motions versus heating load under different inner diameters of the OHPs, it can be seen that only the  By combining the high-speed visualization method and infrared thermal imaging technology, oscillation motions of working fluid occur in the major heating loads range (Q ≤ 110 W) under Di* =  the vapor-liquid two-phase fluid flow in the OHPs with different inner diameters are observed and 0.65 (Di = 1 mm), due to the large frictional flow resistance. Furthermore, even under the same mode  presented. The motions of working fluid, distributions of vapor and liquid phases, as well as the of working fluid motions, the large frictional flow resistance under Di* = 0.65 (Di = 1 mm) also reduces  temperature distribution in the condenser under different diameters, are compared and analyzed to the heat and mass transfer rate into the OHP. Therefore, the OHP with Di* = 0.65 (Di = 1 mm) has the  elucidate the fundamental effects of the inner diameter on the fluid flow as well as heat and mass highest thermal resistance relative to those under Di* = 1.31 (Di = 2 mm) and Di* = 1.99 (Di = 3 mm). In  transfer in the OHPs. In addition, the relationship between the motions of working fluid and heat addition, owing to the hardest supplement of working fluid for the nucleate boiling in the evaporator  transfer characteristics in the OHPs is discussed and clarified. Accordingly, several conclusions are under the least amount of filled working fluid, the dry‐out of working fluid occurs most easily in the  drawn as follows: OHP with Di* = 0.65 (Di = 1 mm). Compared with the OHP with Di* = 1.31 (Di = 2 mm), the gravity  (1) The major fluid flow motions inside OHP under quasi-steady operation condition are small plays a more important role in the OHP with Di* = 1.99 (Di = 3 mm), producing the thermosyphon  oscillation, bulk oscillation and circulation, which are closely correlated with the heat and mass transfer effect under the small oscillation of working fluid which facilitates the backflow of working fluid into  performance in the OHPs, which can be reflected by the characteristics of infrared thermal images the evaporator and enhances the heat and mass transfer [34]. Accordingly, at the small heating load  of the condenser. These three fluid flow motions coexist or correspondingly exist alone with the (Q ≤ 40 W), thermal resistance under Di* = 1.99 (Di = 3 mm) is a little smaller than that under Di* = 1.31  increasing heating load under different inner diameters. (Di = 2 mm). However, under the bulk oscillation depending on the momentum exchange of working  (2) The general flow patterns in OHP include bubbly flow, slug flow and annular flow. fluid, the thermal resistance under Di* = 1.99 (Di = 3 mm) turns out to be slightly larger than that  The occurrence and transition of the flow patterns are mainly dependent on the phase-change under Di* = 1.31 (Di = 2 mm) (50 W≤ Q ≤ 100 W). It must be attributed to the decrease of rigidity on  phenomena in the evaporator and condenser of the OHPs. In particular, nucleate boiling in the the vapor‐liquid meniscus of vapor slugs, which weakens the necessary ʹbubble pumpingʹ effect for  evaporator is restricted by decreasing the inner diameter of the OHP, which leads to the smallest the exchange of working fluid in the OHPs and reduces the efficiency of heat and mass transfer.  proportion of bubbly flow in the 1 mm OHP with respect to the other two. In addition, the decrease in inner diameter of the OHP can enhance the effects of surface tension and thus enlarge capillary 4. Conclusions  instability on the vapor-liquid interface, which produces more collapses on the vapor core and long By combining the high‐speed visualization method and infrared thermal imaging technology,  vapor slugs, forming more short slugs in the OHP with D = 1 mm than that under D = 2 and 3 mm. i i the vapor‐liquid two‐phase fluid flow in the OHPs with different inner diameters are observed and  (3) Due to the increasing driving pressure difference in the OHPs with growing heating load, presented. The motions of working fluid, distributions of vapor and liquid phases, as well as the  the major fluid flow motions are changed from small oscillation to bulk ones and finally reach the temperature distribution in the condenser under different diameters, are compared and analyzed to  circulation, which triggers a decrease in the overall thermal resistance of the OHPs. Particularly, elucidate the fundamental effects of the inner diameter on the fluid flow as well as heat and mass  transfer in the OHPs. In addition, the relationship between the motions of working fluid and heat  transfer characteristics in the OHPs is discussed and clarified. Accordingly, several conclusions are  drawn as follows:  R [W/C] Appl. Sci. 2016, 6, 321 14 of 16 the occurrence of bulk oscillation of working fluid causes the most apparent decrease in the thermal resistance versus the increasing heating load, whereas the thermal resistance of the OHPs drops to become a steady value when the working fluid circulation appears. Owing to the large frictional flow resistance and small amount of filled working fluid under the small diameter, the OHP with an inner diameter of 1 mm possesses the largest thermal resistance and undergoes the dry-out of working fluid most easily. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters

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applied sciences Article High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters 1 1 2 , 1 Xiangdong Liu , Qing Sun , Chengbin Zhang * and Liangyu Wu School of Hydraulic, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China; xdliu_yzu@126.com (X.L.); sun_qing_yzu@163.com (Q.S.); lywu@yzu.edu.cn (L.W.) Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China * Correspondence: cbzhang@seu.edu.cn; Tel.: +86-25-8379-2483 Academic Editor: Yuyuan Zhao Received: 1 September 2016; Accepted: 20 October 2016; Published: 26 October 2016 Abstract: The oscillating heat pipe (OHP) is a new member in the family of heat pipes, and it has great potential applications in energy conservation. However, the fluid flow and heat transfer in the OHP as well as the fundamental effects of inner diameter on them have not been fully understood, which are essential to the design and optimization of the OHP in real applications. Therefore, by combining the high-speed visualization method and infrared thermal imaging technique, the fluid flow and thermal performance in the OHPs with inner diameters of 1, 2 and 3 mm are presented and analyzed. The results indicate that three fluid flow motions, including small oscillation, bulk oscillation and circulation, coexist or, respectively, exist alone with the increasing heating load under different inner diameters, with three flow patterns occurring in the OHPs, viz. bubbly flow, slug flow and annular flow. These fluid flow motions are closely correlated with the heat and mass transfer performance in the OHPs, which can be reflected by the characteristics of infrared thermal images of condensers. The decrease in the inner diameter increases the frictional flow resistance and capillary instability while restricting the nucleate boiling in OHPs, which leads to a smaller proportion of bubbly flow, a larger proportion of short slug flow, a poorer thermal performance, and easier dry-out of working fluid. In addition, when compared with the 2 mm OHP, the increasing role of gravity induces the thermosyphon effect and weakens the ‘bubble pumping’ action, which results in a little smaller and bigger thermal resistances of 3 mm OHP under small and bulk oscillation of working fluid, respectively. Keywords: oscillating heat pipe; fluid flow motion; flow pattern; thermal performance; inner diameter 1. Introduction With rapid increase in energy consumption, in order to realize the sustainable development of energy and environment, it is necessary to improve the efficiency of energy transfer, especially that of thermal energy transfer [1–5]. Note that, as one of several types of high efficiency heat transfer elements, the heat pipe has been widely used for improving the efficiency of thermal energy transfer and reducing the environmental impact during the heat exchanging process in real applications [6–9]. The oscillating heat pipe (OHP) is a new member in the family of heat pipes, which was proposed by Akachi [10,11] in 1990. Generally, OHP is fabricated by a sealed wickless capillary partially filled with the working fluid, which is arranged in an interconnected meandering manner. Compared with the conventional wick heat pipe, OHP has its own special operation mechanism due to its unique wickless structure [12]. In the OHP, owing to the sufficient small scale of the meandering capillary, working fluid Appl. Sci. 2016, 6, 321; doi:10.3390/app6110321 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 321 2 of 16 can be distributed naturally into a series of vapor-liquid slugs by surface tension. When heated in one section (i.e., evaporator) and cooled in another (i.e., condenser), OHP establishes the temperature difference inside. As a result of this temperature difference and the uneven vapor-liquid distribution, a saturation pressure difference is established between the evaporator and condenser, coupled with non-uniform pressure oscillation produced in the OHP. These make the working fluids undergo complex oscillatory motions in the OHP, which achieves the heat transfer between the hot and cold sections. Both the special configuration and operation mechanism produce several additional advantages for the OHP in contrast to the conventional heat pipe [13,14]: simple construction and low cost, operational flexibility, capability for efficient heat transfer among multiple heat sources and sinks, etc. Therefore, the OHP possesses great application prospects in the areas of waste heat recovery [15,16], solar energy utilization [17,18], thermal management of hybrid vehicle [19,20], etc. In this context, considerable efforts have been devoted to investigating the fluid flow and heat transfer in the OHP, so as to deeply understand its heat transfer mechanisms which are helpful for the design and optimization of the OHP in practical applications. Several theoretical models were proposed to investigate the oscillating motion and heat transfer of the vapor-liquid slugs in the OHP [21,22]. In addition, some researchers made efforts to enhance the thermal performance of the OHP by using the nanofluids [23,24] and mixed working fluid [25]. The optimal design of capillary channel geometry has also been conducted to successfully improve the operation stability and heat transfer of the flat-plate OHP [26–28]. Note that the previous studies indicated that the thermal performance of OHP is affected by many parameters such as geometric parameters, working fluid properties, heating load and heating modes, inclination angle, filling ratio, number of turns, and gravity load, etc. [29]. In particular, the inner geometry scale of the OHP must be small enough to form the necessary uneven distribution of vapor-liquid slugs inside, which is essential for successful operation of the OHP. Note that this uneven distribution of vapor-liquid slugs mainly depends on the ratio between the surface tension and gravity, as characterized by the Bond number: g(  ) l v Bo = D , (1) where D is the inner diameter of the OHP,  and  are the corresponding densities of liquid i l v and vapor,  is the surface tension coefficient, and g is the gravity acceleration. Accordingly, several researchers [12,30] suggested the inner diameter of the OHP should be satisfied as D  2 . (2) (  )g l v Meanwhile, it should be noted that the frictional flow resistance increases considerably with the decreasing inner diameter of the OHP, which impedes the normal oscillatory motions of working fluid in the OHP. Therefore, the optimal range of D is suggested within the following range [31]: r r 0.7  D  1.8 . (3) (  ) g (  ) g l v l v Thus, it can be concluded that the inner diameter has great influence on the fluid flow and heat transfer in the OHP, which has been further demonstrated by several available studies. Charoensawan et al. [32] experimentally investigated the thermal performance of the horizontal OHPs with D = 1.0 and 2.0 mm, which indicated the better thermal performance of the OHP with bigger D . i i In addition, via the comparison between the thermal resistance of two OHPs with different inner diameters, Yang et al. [33,34] found that the thermal performance of the OHP with D = 1.0 mm was decreased by about 10% relative to that with D = 2.0 mm. In addition, the shape of the cross section of the OHP also affects its thermal performance under the same hydraulic diameter. The effects of inner diameter on the thermal performance of OHP might be opposite for different working fluid, which is indicated by the experimental results by Rittidech et al. [35,36]. Particularly, based on an experimental test on the open-loop OHPs, Saha et al. [37] found that when compared with the OHP Appl. Sci. 2016, 6, 321 3 of 16 with D = 0.9 mm, the OHP with D = 1.5 mm demonstrated worse thermal performance, which is i i different from the conclusions by Charoensawan et al. [32] and Yang et al. [33,34]. In summary, although available research has demonstrated significant influence of the inner diameter on the thermal performance of the OHP, the real causes of this influence have not been fully revealed due to the limited insight into the fluid flow in the OHP during its operation. It is worth noting that several researchers (e.g., Tong et al. [38], Xu et al. [39], Khandekar et al. [40–42], and Borgmeyer et al. [43,44]) have proven that high-speed visualization of the operating OHP is an effective way for understanding the fluid flow in it. In addition, this previous research also pointed out that the detailed fluid flow behaviors inside are closely correlated with the thermal performance of the OHP, which are critical for clarifying the heat transfer mechanisms of the OHP. However, the available high-speed visualization experiments are mainly carried out on the OHPs with single inner diameters, with limited focus on the effects of inner diameter on the fluid flow. As a result (from the scenario described), the effects of inner diameters on the thermal performance of the OHP have not been deeply understood. Therefore, in this work, based on the high-speed visualization method and infrared thermal imaging technology, the vapor-liquid two-phase fluid flow in the OHPs with D = 1.0, 2.0, 3.0 mm is investigated and compared to reveal the fundamental effects of the inner diameter. In addition, the motions of working fluid, distributions of vapor and liquid phases as well as the temperature distribution in the condenser are clarified and analyzed, in an effort to elucidate the relationship between the motions of working fluid and the heat transfer characteristics in the OHPs. 2. Experimental Setup Figure 1 schematically illustrates the experimental setup utilized in the current study. As shown, the OHPs used here are fabricated by bending the pyrex glass capillaries into the interconnected meandering manner, forming 10 U-turns and parallel vertical tubes. The pyrex glass capillaries have external diameter of D = 6.0 mm and inner diameters D = 1.0, 2.0, 3.0 mm, respectively. As depicted in Figure 1b, the OHP with the dimensions of 400 mm  185 mm is set vertically and heated at the bottom, which includes evaporator, adiabatic section and condenser with corresponding lengths of 100 mm, 25 mm and 275 mm. Before the experiment, the OHP is baked at 100 C, evacuated to be 4.0  10 torr for 8 h by a vacuum pump, and then filled with methanol as the working fluid. The filling ratio is maintained at  = 47% in this work. For the evaporator, the Ni-Cr wire with diameter of 0.25 mm is wrapped on the outer tube wall to supply the uniform heating load for the OHP. The whole evaporator and adiabatic section are embedded into the insulation box stuffed with the aluminum silicate insulation fibers, so as to ensure the relative error of heating load Q within 4.9%. The whole experiment is conducted in an environment with a constant temperature of 15  0.5 C, and thus the condenser of OHP can be cooled by the forced convection of the surrounding air via a cooling fan. By using an NEC TH9260 infrared camera (NEC Corporation, Tokyo, Japan) and the corresponding software, the infrared thermal images of the condenser during the operation of OHP are monitored, recorded and analyzed. The infrared camera possesses operation wavelength of 8–14 m and noise equivalent temperature difference (NETD) of 0.08 C, as well as thermal image resolution of 640  480 pixels. The emissivity of the condenser surface is corrected before the experiment via comparing temperature signals measured by the infrared camera and the K-type thermal couple with a measuring error of 0.1 C. By checking the emissivity of condensation section, the deviation between the tested result of the condenser temperature and its real value via the infrared camera is less than 2.3 C. The temperature of evaporator is measured by the thermocouples (diameter of 0.25 mm, OMEGA K-type with measuring error of 0.1 C, Omega Engineering, Santa Ana, CA, USA) fixed at the bottom of the evaporator, as marked in Figure 1b. The evaporator temperature are read and recorded by an Agilent 34970A data acquisition switch unit (Agilent Technologies, Santa Clara, CA, USA) with 6.5-digit accuracy, which has a maximum relative error of 0.5%. Appl. Sci. 2016, 6, 321 4 of 16 Appl. Sci. 2016, 6, 321  4 of 16  Figure 1. Experimental apparatus: (a) schematic of experimental setup; (b) schematic of experimental  Figure 1. Experimental apparatus: (a) schematic of experimental setup; (b) schematic of experimental oscillating heat pipe; and (c) cross section geometry of A–A in inset (b).  oscillating heat pipe; and (c) cross section geometry of A–A in inset (b). 3. Results and Discussions  3. Results and Discussions 3.1. Fluid Flow Motions inside OHP  3.1. Fluid Flow Motions inside OHP As mentioned above, a deep insight into the vapor‐liquid two‐phase flow motions in the OHP  during its quasi‐steady operation is helpful for understanding the oscillating operation characteristics  As mentioned above, a deep insight into the vapor-liquid two-phase flow motions in the OHP and heat transfer mechanisms of the OHP. In this work, after the start‐up process [45], a quasi‐steady  during its quasi-steady operation is helpful for understanding the oscillating operation characteristics station is achieved in the OHPs. Based on the summary of our visualization results under the quasi‐ and heat transfer mechanisms of the OHP. In this work, after the start-up process [45], a quasi-steady steady operation of the OHPs, it can be seen from Figure 2 that the fluid flow inside the OHPs with  station is achieved in the OHPs. Based on the summary of our visualization results under the three inner diameters under quasi‐steady operation condition mainly exhibits three motions: small  quasi-steady operation of the OHPs, it can be seen from Figure 2 that the fluid flow inside the OHPs oscillation (S‐O), bulk oscillation (B‐O) and circulation (C). Since the evaporator and adiabatic section  with three inner diameters under quasi-steady operation condition mainly exhibits three motions: are embedded into the thermal insulation box in the current experiment, in order to show these three  motions  clearly,  Figure  2b  selects  the  corresponding  fluid  flow  motions  in  either  U‐turn  at  the  small oscillation (S-O), bulk oscillation (B-O) and circulation (C). Since the evaporator and adiabatic condenser  of  the  OHP  as  the  representative  for  analysis.  As  shown,  when  the  small  oscillation  section are embedded into the thermal insulation box in the current experiment, in order to show appears in the OHP, the vapor‐liquid two‐phase fluid only exhibits small, local oscillations in the  these three motions clearly, Figure 2b selects the corresponding fluid flow motions in either U-turn tube, with limited mass exchange between the evaporator and condenser in each single vertical tube.  at the condenser of the OHP as the representative for analysis. As shown, when the small oscillation Correspondingly, when the bulk oscillation occurs, the working fluid begins to oscillate with large  appears in the OHP, the vapor-liquid two-phase fluid only exhibits small, local oscillations in the amplitude in the OHP, resulting in bulk mass exchange between the evaporator and condenser and  tube, with limited mass exchange between the evaporator and condenser in each single vertical tube. even among multiple U‐turns. Differing from the oscillation motion, the circulation of working fluid  is  characterized  by  the  circulation  of  working  fluid  in  the  whole  OHP  along  the  fixed  direction.  Correspondingly, when the bulk oscillation occurs, the working fluid begins to oscillate with large However, due to the non‐uniformity driving pressure distributions in the OHP, the circulation speed  amplitude in the OHP, resulting in bulk mass exchange between the evaporator and condenser and of working fluid is changed.  even among multiple U-turns. Differing from the oscillation motion, the circulation of working fluid is In order to reflect the heat transfer performance of the OHP produced by these three motions,  characterized by the circulation of working fluid in the whole OHP along the fixed direction. However, Figure 2c gives  the  typical  infrared  thermal images of  the  condenser  under  different  motions.  In  due to the non-uniformity driving pressure distributions in the OHP, the circulation speed of working addition, corresponding to the infrared thermal images in Figure 2c, Figure 3 further quantitatively  fluid is changed. represents  the  vertical  temperature  distribution  in  two  adjacent  tubes  in  a  typical  U‐turn  of  the  In order to reflect the heat transfer performance of the OHP produced by these three motions, Figure 2c gives the typical infrared thermal images of the condenser under different motions. In addition, corresponding to the infrared thermal images in Figure 2c, Figure 3 further quantitatively represents the vertical temperature distribution in two adjacent tubes in a typical U-turn of the condenser under different quasi-steady operation states. In this figure, each curve is plotted by the Appl. Sci. 2016, 6, 321 5 of 16 Appl. Sci. 2016, 6, 321  5 of 16  average temperature data of every seven horizontal pixel points along each vertical tube in the typical condenser under different quasi‐steady operation states. In this figure, each curve is plotted by the  U-turn. As indicated in Figures 2c and 3a, under small oscillation motion, the heat transfer of the average temperature data of every seven horizontal pixel points along each vertical tube in the typical  condenser mainly relied on its own heat conduction, except for the bottom regime, which is dependent U‐turn. As indicated in Figures 2c and 3a, under small oscillation motion, the heat transfer of the  on the sensible and latent heat transfer of the locally oscillating working fluid. Therefore, with the condenser  mainly  relied  on  its  own  heat  conduction,  except  for  the  bottom  regime,  which  is  exception of high temperature at the bottom of the condenser, the temperature at the other part of dependent on the sensible and latent heat transfer of the locally oscillating working fluid. Therefore,  the condenser nearly decreases linearly along the vertical direction to the top. As the bulk oscillation with the exception of high temperature at the bottom of the condenser, the temperature at the other  comes out, working fluid at the hot and cool ends of the OHP are able to exchange with each other, part of the condenser nearly decreases linearly along the vertical direction to the top. As the bulk  which obviously enhances the heat and mass transfer from the evaporator to the condenser. As a result, oscillation comes out, working fluid at the hot and cool ends of the OHP are able to exchange with  the areach ea of  other, high-temperatur   which  obvio eursly egion   enh at ance thes bottom the  heat of  and the  condenser mass  transfis er expanded, from  the  ev leading aporato to r  to an  the incr  ease condenser.  As  a  result,  the  area  of  high‐temperature  region  at  the  bottom  of  the  condenser  is  in the temperature level of the condenser (see Figures 2c and 3b). Note that, under the bulk oscillation expanded, leading to an increase in the temperature level of the condenser (see Figures 2c and 3b).  motion, due to the non-uniform heat and mass transfer strength caused by the uneven oscillation Note that, under the bulk oscillation motion, due to the non‐uniform heat and mass transfer strength  of working fluid among the OHP, every parallel tube at the condenser has different temperature caused  by  the  uneven  oscillation  of  working  fluid  among  the  OHP,  every  parallel  tube  at  the  distributions along the vertical direction. When the circulation of working fluid appears in the condenser has different temperature distributions along the vertical direction. When the circulation  OHP, the adjacent parallel vertical tubes become ‘upheaders’ and ‘downcomers’ alternatively with of working fluid appears in the OHP, the adjacent parallel vertical tubes become ‘upheaders’ and  corresponding hot and cold fluid flowing inside, resulting in alternatively high and low temperature ‘downcomers’  alternatively  with  corresponding  hot  and  cold  fluid  flowing  inside,  resulting  in  on them (see Figure 3c). alternatively high and low temperature on them (see Figure 3c).  Figure  2.  Fluid  flow  motions  in  the  oscillating  heat  pipe  (OHP)  and  corresponding  experimental  Figure 2. Fluid flow motions in the oscillating heat pipe (OHP) and corresponding experimental images images as well as infrared thermal images of the condenser: (a) schematic of fluid flow motions; (b)  as well as infrared thermal images of the condenser: (a) schematic of fluid flow motions; (b) snapshots of different fluid flow motions in a typical U-turn of condenser; and (c) infrared thermal images of the condenser. Appl. Sci. 2016, 6, 321  6 of 16  snapshots of different fluid flow motions in a typical U‐turn of condenser; and (c) infrared thermal  Appl. Sci. 2016, 6, 321 6 of 16 images of the condenser.  250 250 250 6# (a) 6# 6# (c) (b) 7# 7# 7# 200 200 200 150 150 150 100 100 100 50 50 0 0 24 36 48 60 72 24 36 48 60 72 24 36 48 60 72 T [C] T [C] T [C] Figure 3. Vertical temperature distribution of condenser under different quasi‐steady operation states  Figure 3. Vertical temperature distribution of condenser under different quasi-steady operation states corresponding to Figure 2c (6#‐7# U‐turn): (a) small oscillation (Q = 30 W); (b) big oscillation (Q = 70 W);  corresponding to Figure 2c (6#-7# U-turn): (a) small oscillation (Q = 30 W); (b) big oscillation (Q = 70 W); and (c) circulation (Q = 140 W).  and (c) circulation (Q = 140 W). Furthermore, the current experimental results also indicated that the three motions above coexist  or,  respectively,  exist  alone  with  increasing  heating  load  under  different  inner  diameters,  with  Furthermore, the current experimental results also indicated that the three motions above coexist different  coupling  characteristics,  as  shown  in  Figure  4,  where  pt  represents  the  percentage  of  or, respectively, exist alone with increasing heating load under different inner diameters, with different duration for a certain fluid flow motion with respect to the total statistical duration. It can be seen  coupling characteristics, as shown in Figure 4, where p represents the percentage of duration for a that when Q is very small, the driving pressure inside the OHP is limited, which only triggers small  certain fluid flow motion with respect to the total statistical duration. It can be seen that when Q is very oscillation of working fluids. With the increasing heating load, the driving pressure inside the OHP  small, the driving pressure inside the OHP is limited, which only triggers small oscillation of working rises to overcome the flow resistance between the evaporator and condenser or even the adjacent U‐ fluids. With the increasing heating load, the driving pressure inside the OHP rises to overcome the flow turns, which induces the bulk oscillation of working fluid. At this time, the small and bulk oscillations  resistance between the evaporator and condenser or even the adjacent U-turns, which induces the bulk appear intermittently in the OHP, and the duration of bulk oscillation is gradually increased when  oscillation the heating of working  load (i.efluid. ., the dAt riving this pressure time, the  in the small  OHP) and furt bulk her oscillations rises, as depicappear ted in (i)intermittently  and (ii) of Figure in  the 4b. By further raising the heating load, the region of bulk oscillation is expanded to multiple U‐turns,  OHP, and the duration of bulk oscillation is gradually increased when the heating load (i.e., the driving which eventually produces the circulation of working fluid (see (iii) of Figure 4b). Finally, the small  pressure in the OHP) further rises, as depicted in (i) and (ii) of Figure 4b. By further raising the heating oscillation and bulk oscillation of working fluid disappear sequentially under a large heating load,  load, the region of bulk oscillation is expanded to multiple U-turns, which eventually produces the and the pure circulation of working fluid is achieved in the OHP (see Figure 4a and (iv) of Figure 4b).  circulation of working fluid (see (iii) of Figure 4b). Finally, the small oscillation and bulk oscillation of Note that, as shown by the comparison among the flow motions of working fluid in the OHPs with  working fluid disappear sequentially under a large heating load, and the pure circulation of working different inner diameters under the same heating load (see (ii), (v) and (vi) in Figure 4b), with respect  fluid is achieved in the OHP (see Figure 4a and (iv) of Figure 4b). Note that, as shown by the comparison to the OHP with Di = 2.0 mm, OHP with Di = 1.0 mm has larger frictional flow resistance inside, and  among the flow motions of working fluid in the OHPs with different inner diameters under the same thus the fluid flow motions under small driving pressure (e.g., small heating load) appears more  heating load (see (ii), (v) and (vi) in Figure 4b), with respect to the OHP with D = 2.0 mm, OHP with easily (e.g., small oscillation in (ii), (v) and (vi) of Figure 4b). On the other hand, although the frictional  D = 1.0 flow mm  resist has anc lar e ger is lower frictional  in the OHP flow wi resistance th Di = 3.0 inside,  mm, the and  resist thus ance the offluid  capillflow ary hy motions steresis is under  higher, small   and  the  vapor‐liquid  meniscus  of  vapor  slugs is  less  rigid,  which  weakens  the  necessary ʹbubble  driving pressure (e.g., small heating load) appears more easily (e.g., small oscillation in (ii), (v) and pumpingʹ action [33,34] for the momentum transfer of working fluid inside the OHPs. Consequently,  (vi) of Figure 4b). On the other hand, although the frictional flow resistance is lower in the OHP with the percentage of oscillation motions is larger than that in the OHP with Di = 2.0 mm (e.g., small  D = 3.0 mm, the resistance of capillary hysteresis is higher, and the vapor-liquid meniscus of vapor oscillation in (ii), (v) and (vi) of Figure 4b).  slugs is less rigid, which weakens the necessary ‘bubble pumping’ action [33,34] for the momentum transfer of working fluid inside the OHPs. Consequently, the percentage of oscillation motions is larger than that in the OHP with D = 2.0 mm (e.g., small oscillation in (ii), (v) and (vi) of Figure 4b). z[mm] z[mm] z[mm] Appl. Sci. 2016, 6, 321 7 of 16 Appl. Sci. 2016, 6, 321  7 of 16  (a) S-O S-O & B-O & & S-O B-O C B-O & C Increasing heating load (b) (i) Q = 50W, D = 2mm, p (S-O)=69.9%, p (B-O)=31.1% S-O i t t B-O (ii) Q = 80W, D = 2mm, p (S-O)=28.6%, p (B-O)=72.4% S-O i t t B-O (iii) Q =110W, D = 2mm, p (S-O)=4.4%, p (B-O)=24.6%, p (C)=71.0% i t t t S-O B-O (iv) Q =140W, D = 2mm, p (B-O)=10.6%, p (C)=89.4% i t t B-O (v) Q = 80W, D = 1mm, p (S-O)=59.6%, p (B-O)=40.4% i t t S-O B-O (vi) Q =80W, D = 3mm, p (S-O)=58.4%, p (B-O)=41.6% i t t S-O B-O 0 200 400 600 800 1000 t [s] Figure 4. Variation of fluid flow motions with increasing heating load: (a) schematic of change in fluid  Figure 4. Variation of fluid flow motions with increasing heating load: (a) schematic of change in flow  motions  with  increasing  heating  load;  and  (b)  time  series  of  fluid  flow  motions  and  their  fluid flow motions with increasing heating load; and (b) time series of fluid flow motions and their corresponding  duration  fractions  under  different  heating  loads  and  inner  diameters.  Fluid  flow  corresponding duration fractions under different heating loads and inner diameters. Fluid flow motions motions mode index: 0: S‐O; 1: B‐O; and 2: C.  mode index: 0: S-O; 1: B-O; and 2: C. 3.2. General Flow Patterns in OHP  3.2. General Flow Patterns in OHP As shown in Figures 5–8, the above‐mentioned complex fluid flows and phase changes also lead  As shown in Figures 5–8, the above-mentioned complex fluid flows and phase changes also to the occurrences and evolutions of various flow patterns in the OHPs. Herein, three types of flow  lead to the occurrences and evolutions of various flow patterns in the OHPs. Herein, three types patterns, bubbly flow, slug flow and annular flow, are observed in the OHPs. Significantly, these flow  of flow patterns, bubbly flow, slug flow and annular flow, are observed in the OHPs. Significantly, patterns and their evolutions exhibit different characteristics under different inner diameters of the OHPs.  these flow patterns and their evolutions exhibit different characteristics under different inner diameters of the OHPs. 3.2.1. Bubbly Flow  Fluid flow motion elements indexes Appl. Sci. 2016, 6, 321 8 of 16 3.2.1. Bubbly Flow Appl. Sci. 2016, 6, 321  8 of 16  As indexed by ‘B’ in Figure 5, bubbly flow is characterized by some dispersed bubbles flowing with the continuous liquid, with the length smaller than the inner diameter of the OHP. This flow As indexed by ‘B’ in Figure 5, bubbly flow is characterized by some dispersed bubbles flowing  pattern is mainly produced by the continuous nucleate boiling in the evaporator, which usually exists with the continuous liquid, with the length smaller than the inner diameter of the OHP. This flow  in the stream flowing from the evaporator to the condenser. Due to the small size and low volume pattern is mainly produced by the continuous nucleate boiling in the evaporator, which usually exists  fraction in the of the stream dispersed  flowingbubbles,  from the the evap bubbly orator to flow  theis co easily ndenser. condensed  Due to the to small pure size liquid,  andand  lowthus  volume it b arely fraction of the dispersed bubbles, the bubbly flow is easily condensed to pure liquid, and thus it  appears in the downstream flowing from the condenser to the evaporator. Note that, when the fluid barely appears in the downstream flowing from the condenser to the evaporator. Note that, when the  mixture undergoes fast turning and large disturbance, the bubbly flow may appear for a short time at fluid mixture undergoes fast turning and large disturbance, the bubbly flow may appear for a short  the end of the long vapor slugs. Generally, bubbly flow is unstable in the OHPs due to the coalescence time at the end of the long vapor slugs. Generally, bubbly flow is unstable in the OHPs due to the  and shrinking of the dispersed bubbles. Via the comparison among the characteristics of bubbly flow coalescence and shrinking of the dispersed bubbles. Via the comparison among the characteristics of  in the OHPs with different inner diameters, it can be seen that the confined space in the OHP with bubbly flow in the OHPs with different inner diameters, it can be seen that the confined space in the  D = 1 mm restricts the nucleate boiling in the evaporator [46], resulting in the fewer dispersed bubbles OHP  with  Di  =  1  mm  restricts  the  nucleate  boiling  in  the  evaporator  [46],  resulting  in  the  fewer  in the bubbly flow than that under D = 2 mm, 3 mm. In addition, the dispersed bubbles in the OHP dispersed bubbles in the bubbly flow than that under Di = 2 mm, 3 mm. In addition, the dispersed  with D = 1 mm are hardly mixed due to the confinement, and always flow with the main stream bubbles i  in the OHP with Di = 1 mm are hardly mixed due to the confinement, and always flow with  in turn. the main stream in turn.  B S A B S A B S A D = 1mm D = 2mm D = 3mm i i i Figure 5. Flow patterns occurring in the OHPs with different diameters. Flow patterns indexes: B:  Figure 5. Flow patterns occurring in the OHPs with different diameters. Flow patterns indexes: bubbly flow; S: slug flow; A: annular flow.  B: bubbly flow; S: slug flow; A: annular flow. 3.2.2. Slug Flow  3.2.2. Slug Flow When the slug flow appears in the OHPs, a series of vapor slugs with lengths bigger than the  When the slug flow appears in the OHPs, a series of vapor slugs with lengths bigger than inner diameter flow with the main stream, as indexed by ‘S’ in Figure 5. This flow pattern is generally  the inner diameter flow with the main stream, as indexed by ‘S’ in Figure 5. This flow pattern is formed by the self‐growth and coalescence of dispersed bubbles and the breakup of very long slugs.  generally The  slug formed   flow by almost the self-g emerges rowth   unde and r  alcoalescence l  conditions  und of dispersed er  different bubbles   inner  diameters and the br bueakup t  is  most of  very common in the fluid mixture with oscillation and ‘downcomers’ under the fluid circulation in the  long slugs. The slug flow almost emerges under all conditions under different inner diameters but is OHPs. In addition, the length of slugs in the slug flow is usually changed because of the slugsʹ growth,  most common in the fluid mixture with oscillation and ‘downcomers’ under the fluid circulation in the shrinking, coalescence and breakup. It is worth noting that as the mixing of dispersed bubbles is  OHPs. In addition, the length of slugs in the slug flow is usually changed because of the slugs’ growth, resisted in the small tube, the slug flow in the OHP with Di = 1 mm is mainly formed by the self‐ shrinking, coalescence and breakup. It is worth noting that as the mixing of dispersed bubbles is growth of dispersed bubbles and breakup of very long slugs rather than the coalescence of dispersed  resisted in the small tube, the slug flow in the OHP with D = 1 mm is mainly formed by the self-growth bubbles, which is different from that under the inner diameter of 2 mm and 3 mm.  of dispersed bubbles and breakup of very long slugs rather than the coalescence of dispersed bubbles, which is different from that under the inner diameter of 2 mm and 3 mm. 3.2.3. Annular Flow  In the OHPs, when the continuous vapor flows through the center of the tubes with a liquid film  3.2.3. Annular Flow formed around the inner tube wall, the annular flow comes out, as indexed by ‘A’ in Figure 5. This  flow In the paOHPs, ttern is usu when ally the trancontinuous sformed fromvapor  the slug flows  flow,thr  and ough  usuathe lly acenter ppears in of the the ‘uphea tubesders’ with with a liquid   a large volume fraction of vapor produced by the drastic boiling in the evaporator under high heating  film formed around the inner tube wall, the annular flow comes out, as indexed by ‘A’ in Figure 5. load. In the ‘upheaders’, the high‐speed vapor induced by drastic boiling always breaks the liquid  This flow pattern is usually transformed from the slug flow, and usually appears in the ‘upheaders’ bridges  between  the  vapor  slugs  and  triggers  the  transition  from  slug  flow  to  the  annular  flow.  with a large volume fraction of vapor produced by the drastic boiling in the evaporator under high Moreover, as shown by Figure 5, smaller inner diameter induces greater effect of surface tension in  heating load. In the ‘upheaders’, the high-speed vapor induced by drastic boiling always breaks the the OHP and thus generates larger capillary instability, which leads to more irregular waves on the  liquid bridges between the vapor slugs and triggers the transition from slug flow to the annular flow. Appl. Sci. 2016, 6, 321 9 of 16 Moreover, as shown by Figure 5, smaller inner diameter induces greater effect of surface tension in the OHP and thus generates larger capillary instability, which leads to more irregular waves on the liquid film of the annular flow in the OHP with D = 1 mm. These irregular waves easily cause the break Appl. Sci. 2016, 6, 321  9 of 16  of the continuous vapor core in the annular flow via the formation of the liquid bridges, which can induce the transition from the annular flow to the slug flow. Therefore, the stability of annular flow liquid film of the annular flow in the OHP with Di = 1 mm. These irregular waves easily cause the  deteriorates with the decreasing inner diameter of the OHPs. break of the continuous vapor core in the annular flow via the formation of the liquid bridges, which  can induce the transition from the annular flow to the slug flow. Therefore, the stability of annular  3.3. Flow Pattern Evolutions in Evaporator and Condenser flow deteriorates with the decreasing inner diameter of the OHPs.  As discussed above, the general flow patterns in the OHPs with different diameters are closely 3.3. Flow Pattern Evolutions in Evaporator and Condenser  related to the motions of vapor-liquid fluid mixture in the evaporator and condenser. Therefore, As discussed above, the general flow patterns in the OHPs with different diameters are closely  deeply analyzing the behaviors of vapor-liquid fluid mixture in the evaporator and condenser is related  to  the  motions  of vapor‐liquid fluid  mixture  in  the  evaporator and  condenser.  Therefore,  beneficial for understanding the occurrences and evolutions of flow patterns in the OHPs. deeply analyzing the behaviors of vapor‐liquid fluid mixture in the evaporator and condenser is  beneficial for understanding the occurrences and evolutions of flow patterns in the OHPs.  3.3.1. Flow Pattern Evolutions in Evaporator 3.3.1. Flow Pattern Evolutions in Evaporator  In order to observe the flow patterns evolutions in the evaporator, the insulation box is opened in some experimental cases for visualization. As shown in Figure 6, when the working fluid in the In order to observe the flow patterns evolutions in the evaporator, the insulation box is opened  evaporator is heated to reach the saturation state, nucleate boiling appears, and some dispersed in some experimental cases for visualization. As shown in Figure 6, when the working fluid in the  small bubbles are produced, flowing up quickly due to the buoyancy and driving pressure difference evaporator is heated to reach the saturation state, nucleate boiling appears, and some dispersed small  bubbles  are  produced,  flowing  up  quickly  due  to  the  buoyancy  and  driving  pressure  difference  between the hot and cold ends of the OHP. Meanwhile, the dispersed bubbles grow up or coalesce into between the hot and cold ends of the OHP. Meanwhile, the dispersed bubbles grow up or coalesce  the bigger ones and even the vapor slugs, which triggers the transition from bubbly flow to the slug into the bigger ones and even the vapor slugs, which triggers the transition from bubbly flow to the  flow. It can be clearly seen from the comparison among the flow pattern evolutions in the evaporators slug  flow.  It  can  be  clearly  seen  from  the  comparison  among  the  flow  pattern  evolutions  in  the  with different inner diameters that the transition from the bubbly flow to the slug flow is mainly evaporators with different inner diameters that the transition from the bubbly flow to the slug flow  dependent on the self-growth of the dispersed bubbles in the evaporator with D = 1 mm, rather than is mainly dependent on the self‐growth of the dispersed bubbles in the evaporator with Di = 1 mm,  the coalescence among them in the evaporators with diameter of 2 mm and 3 mm. Furthermore, rather  than  the  coalescence  among  them  in  the  evaporators  with  diameter  of  2  mm  and  3  mm.  less dispersed bubbles are produced by the nucleate boiling with the decreasing inner diameter, Furthermore, less dispersed bubbles are produced by the nucleate boiling with the decreasing inner  which further explains the characteristics of bubbly flow in the OHP with D = 1 mm as discussed in diameter, which further explains the characteristics of bubbly flow in the OH i P with Di = 1 mm as  the above discusse section. d in the above section.  Figure 6. Evolution of flow pattern in the evaporators of the OHPs with different inner diameters (Q  Figure 6. Evolution of flow pattern in the evaporators of the OHPs with different inner diameters ~ 100 W).  (Q ~100 W). 3.3.2. Flow Pattern Evolutions in the Condenser  3.3.2. Flow Pattern Evolutions in the Condenser As depicted in Figure 7, the visualization results indicate that the condensation of vapor in the  As depicted in Figure 7, the visualization results indicate that the condensation of vapor in the condenser is the major cause of the breakup of continuous vapor core in the OHPs. It can be seen that,  condenser is the major cause of the breakup of continuous vapor core in the OHPs. It can be seen because  of  the  continuous  condensation  of  vapor,  the  vapor  bonds  emerge  at  several  locations  connecting the non‐broken long vapor slug or the continuous vapor core of the annular flow (see long  Appl. Sci. 2016, 6, 321 10 of 16 that, because of the continuous condensation of vapor, the vapor bonds emerge at several locations connecting the non-broken long vapor slug or the continuous vapor core of the annular flow (see long Appl. Sci. 2016, 6, 321  10 of 16  slugs A and B in Figure 7a and slug A in Figure 7b). As the condensation further proceeds, the long slugs A and B in Figure 7a and slug A′ in Figure 7b). As the condensation further proceeds, the long  vapor slugs are broken up into several shorter ones via the breakup of vapor bonds, and the produced vapor slugs are broken up into several shorter ones via the breakup of vapor bonds, and the produced  shorter ones continue shrinking (see slugs C and D produced by slug A as well as slugs E and F shorter ones continue shrinking (see slugs C and D produced by slug A as well as slugs E and F  0 0 0 0 produced by slug B in Figure 7a, slug B and bubbles C , D produced by slug A in Figure 7b). produced by slug B in Figure 7a, slug B′ and bubbles C′, D′ produced by slug A′ in Figure 7b). Based  Based on the comparison among the breakup of long vapor slugs in the OHPs with different inner on the comparison among the breakup of long vapor slugs in the OHPs with different inner diameters,  diameters, it is indicated that the reduction in the inner diameter of OHP results in the increasing role it is indicated that the reduction in the inner diameter of OHP results in the increasing role of surface  of surface tension, which enhances the capillary instability and thus induces more irregular fluctuation tension, which enhances the capillary instability and thus induces more irregular fluctuation on the  on the vapor-liquid interface. Therefore, as indicated in Figure 7a, there are more collapses on long vapor‐liquid interface. Therefore, as indicated in Figure 7a, there are more collapses on long slugs in  the OHP with Di = 1 mm, forming more short slugs than that under Di = 2 mm, 3 mm.  slugs in the OHP with D = 1 mm, forming more short slugs than that under D = 2 mm, 3 mm. i i Figure 7. Evolution of flow pattern in the vertical tube at the condenser of the OHPs with different  Figure 7. Evolution of flow pattern in the vertical tube at the condenser of the OHPs with different inner diameters: (a) snapshots of flow pattern evolution under Di = 1mm; (b) snapshots of flow pattern  inner diameters: (a) snapshots of flow pattern evolution under D = 1mm; (b) snapshots of flow pattern evolution under Di = 2mm; and (c) snapshots of flow pattern evolution under Di = 3mm.  evolution under D = 2mm; and (c) snapshots of flow pattern evolution under D = 3mm. i i Appl. Sci. 2016, 6, 321 11 of 16 Appl. Sci. 2016, 6, 321  11 of 16  Additionally, to further clarify the influence of inner diameter on the vapor-liquid two-phase Additionally, to further clarify the influence of inner diameter on the vapor‐liquid two‐phase  distributions, Figure 8 compares the distribution of dimensionless bubble/slug length (L/D ) at the distributions, Figure 8 compares the distribution of dimensionless bubble/slug length (L/Di) at the  condensers of the OHPs with different D under S-O and B-O motions of working fluid at Q = 90 W, condensers of the OHPs with different Di under S‐O and B‐O motions of working fluid at Q = 90 W,  where L is the real length of bubble/slug (see Figure 5). Herein, the percentage of bubble/slug length, where L is the real length of bubble/slug (see Figure 5). Herein, the percentage of bubble/slug length,  P , is introduced to quantitatively represent the length distribution of bubbles/slugs Pi, is introduced to quantitatively represent the length distribution of bubbles/slugs  P =  100%, (4) j   P   100% (4)  where n is the bubble/slug number within a certain range of dimensionless size, and n is the total where nj is the bubble/slug number within a certain range of dimensionless size, and nt is the total  number of bubbles/slugs in a statistic duration of 10 s. As shown, the proportion of dispersed bubble number of bubbles/slugs in a statistic duration of 10 s. As shown, the proportion of dispersed bubble  (L/D < 1) increases as the tube diameter increases, implying that the nucleate boiling at the evaporator (L/Di < 1) increases as the tube diameter increases, implying that the nucleate boiling at the evaporator  with bigger inner diameter produces more dispersed bubbles flowing into the condenser and thus with bigger inner diameter produces more dispersed bubbles flowing into the condenser and thus  increases their proportion there. Compared with dispersed bubbles, as the inner diameter increases, increases their proportion there. Compared with dispersed bubbles, as the inner diameter increases,  the proportion of short slugs (1 L/D < 10) experiences a decrease. As mentioned above, this is mainly the proportion of short slugs (1 ≤ L/Di < 10) experiences a decrease. As mentioned above, this is mainly  attributed to the drop of capillary instability in the OHP, which reduces the probability of breakup from attributed to the drop of capillary instability in the OHP, which reduces the probability of breakup  the long slugs to the shorter ones. Moreover, this decreasing capillary instability with increasing inner from the long slugs to the shorter ones. Moreover, this decreasing capillary instability with increasing  diameter also induces the larger proportion of long slugs (100  L/D ) under D = 2 mm than that at inner diameter also induces the larger proportion of long slugs (100 ≤ L/Di) under Di = 2 mm than that  i i D = 1 mm. On the other hand, enlarging the inner diameter of the OHP also amplifies the absolute at Di = 1 mm. On the other hand, enlarging the inner diameter of the OHP also amplifies the absolute  distance between vapor slugs, which decreases the probability of coalescences among vapor slugs  distance between vapor slugs, which decreases the probability of coalescences among vapor slugs into into long vapor slugs, resulting in smaller proportion of long slugs (100 ≤ L/Di) under Di = 3 mm than  long vapor slugs, resulting in smaller proportion of long slugs (100  L/D ) under D = 3 mm than i i that at Di = 2 mm.  that at D = 2 mm. (a) D = 1mm, Q = 90W, S-O & B-O (b) D = 2mm, Q = 90W, S-O & B-O (c) D = 3mm, Q = 90W, S-O & B-O 110 100 L / D 1<L/D 1L/D <10 10L/D <40 i i i 40L/D <70 70L/D <100 100L/D i i Figure Figure 8. Distribution   8.  Distribution of bubbles/slugs of  bubbles/slugsle  leng ngth th at at the the condenser condenser  in in  the the  OHPs OHPs  with with  diffe differ rent ent  inner inner   diameters under quasi‐steady S‐O and B‐O fluid motion: (a) Di = 1mm under Q = 90W; (b) Di = 2mm  diameters under quasi-steady S-O and B-O fluid motion: (a) D = 1mm under Q = 90W; (b) D = 2mm i i under Q = 90W; and (c) Di = 3mm under Q = 90W.  under Q = 90W; and (c) D = 3mm under Q = 90W. P [%] P [%] P [%] j j Appl. Sci. 2016, 6, 321 12 of 16 3.4. Thermal Performance To clarify the correlations among heating load, fluid flow motions and thermal performance of the OHPs and further reveal the corresponding fundamental effects of inner diameter, the variation in the overall thermal resistance R versus heating load under different inner diameters is presented in Figure 9, where R is defined as R = T T /Q. (5) e c In Equation (5), T is the average temperature of evaporator calculated as the mean temperature of T ~T as shown in Figure 1b, and T is the average temperature of condenser computed by 1 5 c averaging the temperature values of all pixel points on the infrared thermal images of the condenser. In addition, in order to represent the effect of inner diameter on the thermal performance of the OHP, the dimensionless inner diameter of the OHP, D *, is defined as D D i i D = p = p (6) /gD /g(  ) l v where D is the inner diameter of the OHP, g is the gravity acceleration, and D and s are the difference of density between liquid and vapor and surface tension coefficient at (T + T )/2, respectively. e-r c-r Herein, T and T are the reference temperatures of the evaporator and condenser, which are e-r c-r correspondingly defined as the highest operating temperature of the evaporator T = 123.8 C and e-r ambient temperature of the condenser T = 15 C. Actually, D * is another expression form of the c-r i Bond number, which can characterize the ratio between the surface tension and gravity. As indicated in Figure 9, under the small oscillation of working fluid, the heat exchanged between the hot and cold ends of the OHP mainly relies on the heat conduction of working fluid and tube body, resulting in a slight decrease of R with the increasing heating load. When the bulk oscillation of working fluid occurs, the heat and mass transfer in the OHPs are significantly improved, leading to the most apparent decrease in R versus the increasing heating load (e.g., the changing of R within Q = 60–120 W under D * = 1.31 (D = 2 mm) in Figure 9). After the appearance of working fluid circulation, the capabilities i i of heat and mass transfer in the OHPs are further enhanced to be a steady level due to a certain amount of filled working fluid, when the thermal resistance of the OHPs gradually drops to be a steady value (e.g., the changing of R within Q = 110–200 W under D * = 1.31 (D = 2 mm) in Figure 9). Note that i i the dry-out of working fluid will emerge in the evaporator, represented by the sudden growing-up of the thermal resistance (e.g., Q = 160 W under D * = 0.65 (D = 1 mm) and Q = 220 W under D * = 1.99 i i i (D = 2 mm) in Figure 9). Herein, from the comparison among the changes of the thermal resistance and fluid flow motions versus heating load under different inner diameters of the OHPs, it can be seen that only the oscillation motions of working fluid occur in the major heating loads range (Q  110 W) under D * = 0.65 (D = 1 mm), due to the large frictional flow resistance. Furthermore, even under the same mode of working fluid motions, the large frictional flow resistance under D * = 0.65 (D = 1 mm) also reduces i i the heat and mass transfer rate into the OHP. Therefore, the OHP with D * = 0.65 (D = 1 mm) has the i i highest thermal resistance relative to those under D * = 1.31 (D = 2 mm) and D * = 1.99 (D = 3 mm). i i i i In addition, owing to the hardest supplement of working fluid for the nucleate boiling in the evaporator under the least amount of filled working fluid, the dry-out of working fluid occurs most easily in the OHP with D * = 0.65 (D = 1 mm). Compared with the OHP with D * = 1.31 (D = 2 mm), the gravity i i i i plays a more important role in the OHP with D * = 1.99 (D = 3 mm), producing the thermosyphon i i effect under the small oscillation of working fluid which facilitates the backflow of working fluid into the evaporator and enhances the heat and mass transfer [34]. Accordingly, at the small heating load (Q  40 W), thermal resistance under D * = 1.99 (D = 3 mm) is a little smaller than that under i i D * = 1.31 (D = 2 mm). However, under the bulk oscillation depending on the momentum exchange of i i working fluid, the thermal resistance under D * = 1.99 (D = 3 mm) turns out to be slightly larger than i i that under D * = 1.31 (D = 2 mm) (50 W Q  100 W). It must be attributed to the decrease of rigidity i i Appl. Sci. 2016, 6, 321 13 of 16 on the vapor-liquid meniscus of vapor slugs, which weakens the necessary ‘bubble pumping’ effect for the exchange of working fluid in the OHPs and reduces the efficiency of heat and mass transfer. Appl. Sci. 2016, 6, 321  13 of 16  2.4 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0 20 40 60 80 100 120 140 160 180 200 220 Q [W] Green symbols: D *= 0.65(D = 1mm); S-O B-O&C i i Red symbols: D *= 1.31(D = 2mm); S-O&B-O C i i Blue symbols: D *= 1.99(D = 3mm); S-O&B-O&C dry-out i i Figure 9. Variation of thermal resistance and fluid flow motions versus increasing heating load under  Figure 9. Variation of thermal resistance and fluid flow motions versus increasing heating load under different inner diameters of the OHPs.  different inner diameters of the OHPs. 4. Conclusions Herein,  from  the  comparison  among  the  changes  of  the  thermal  resistance  and  fluid  flow  motions versus heating load under different inner diameters of the OHPs, it can be seen that only the  By combining the high-speed visualization method and infrared thermal imaging technology, oscillation motions of working fluid occur in the major heating loads range (Q ≤ 110 W) under Di* =  the vapor-liquid two-phase fluid flow in the OHPs with different inner diameters are observed and 0.65 (Di = 1 mm), due to the large frictional flow resistance. Furthermore, even under the same mode  presented. The motions of working fluid, distributions of vapor and liquid phases, as well as the of working fluid motions, the large frictional flow resistance under Di* = 0.65 (Di = 1 mm) also reduces  temperature distribution in the condenser under different diameters, are compared and analyzed to the heat and mass transfer rate into the OHP. Therefore, the OHP with Di* = 0.65 (Di = 1 mm) has the  elucidate the fundamental effects of the inner diameter on the fluid flow as well as heat and mass highest thermal resistance relative to those under Di* = 1.31 (Di = 2 mm) and Di* = 1.99 (Di = 3 mm). In  transfer in the OHPs. In addition, the relationship between the motions of working fluid and heat addition, owing to the hardest supplement of working fluid for the nucleate boiling in the evaporator  transfer characteristics in the OHPs is discussed and clarified. Accordingly, several conclusions are under the least amount of filled working fluid, the dry‐out of working fluid occurs most easily in the  drawn as follows: OHP with Di* = 0.65 (Di = 1 mm). Compared with the OHP with Di* = 1.31 (Di = 2 mm), the gravity  (1) The major fluid flow motions inside OHP under quasi-steady operation condition are small plays a more important role in the OHP with Di* = 1.99 (Di = 3 mm), producing the thermosyphon  oscillation, bulk oscillation and circulation, which are closely correlated with the heat and mass transfer effect under the small oscillation of working fluid which facilitates the backflow of working fluid into  performance in the OHPs, which can be reflected by the characteristics of infrared thermal images the evaporator and enhances the heat and mass transfer [34]. Accordingly, at the small heating load  of the condenser. These three fluid flow motions coexist or correspondingly exist alone with the (Q ≤ 40 W), thermal resistance under Di* = 1.99 (Di = 3 mm) is a little smaller than that under Di* = 1.31  increasing heating load under different inner diameters. (Di = 2 mm). However, under the bulk oscillation depending on the momentum exchange of working  (2) The general flow patterns in OHP include bubbly flow, slug flow and annular flow. fluid, the thermal resistance under Di* = 1.99 (Di = 3 mm) turns out to be slightly larger than that  The occurrence and transition of the flow patterns are mainly dependent on the phase-change under Di* = 1.31 (Di = 2 mm) (50 W≤ Q ≤ 100 W). It must be attributed to the decrease of rigidity on  phenomena in the evaporator and condenser of the OHPs. In particular, nucleate boiling in the the vapor‐liquid meniscus of vapor slugs, which weakens the necessary ʹbubble pumpingʹ effect for  evaporator is restricted by decreasing the inner diameter of the OHP, which leads to the smallest the exchange of working fluid in the OHPs and reduces the efficiency of heat and mass transfer.  proportion of bubbly flow in the 1 mm OHP with respect to the other two. In addition, the decrease in inner diameter of the OHP can enhance the effects of surface tension and thus enlarge capillary 4. Conclusions  instability on the vapor-liquid interface, which produces more collapses on the vapor core and long By combining the high‐speed visualization method and infrared thermal imaging technology,  vapor slugs, forming more short slugs in the OHP with D = 1 mm than that under D = 2 and 3 mm. i i the vapor‐liquid two‐phase fluid flow in the OHPs with different inner diameters are observed and  (3) Due to the increasing driving pressure difference in the OHPs with growing heating load, presented. The motions of working fluid, distributions of vapor and liquid phases, as well as the  the major fluid flow motions are changed from small oscillation to bulk ones and finally reach the temperature distribution in the condenser under different diameters, are compared and analyzed to  circulation, which triggers a decrease in the overall thermal resistance of the OHPs. Particularly, elucidate the fundamental effects of the inner diameter on the fluid flow as well as heat and mass  transfer in the OHPs. In addition, the relationship between the motions of working fluid and heat  transfer characteristics in the OHPs is discussed and clarified. Accordingly, several conclusions are  drawn as follows:  R [W/C] Appl. Sci. 2016, 6, 321 14 of 16 the occurrence of bulk oscillation of working fluid causes the most apparent decrease in the thermal resistance versus the increasing heating load, whereas the thermal resistance of the OHPs drops to become a steady value when the working fluid circulation appears. Owing to the large frictional flow resistance and small amount of filled working fluid under the small diameter, the OHP with an inner diameter of 1 mm possesses the largest thermal resistance and undergoes the dry-out of working fluid most easily. 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Published: Oct 26, 2016

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