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LHP heat transfer performance: A comparison study about sintered copper powder wick and copper mesh wick
Applied Thermal Engineering 92 (2016) 104–110
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
LHP heat transfer performance: A comparison study about sintered copper powder wick and copper mesh wick Yiwei Wang, Jiwen Cen, Fangming Jiang *, Wenjiong Cao, Jian Guo Laboratory of Advanced Energy Systems, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
H I G H L I G H T S
• • • •
We experimentally study heat transfer performance of LHPs with different wicks. The sintered wick LHP performs a little better than the mesh wick LHP. The sintered wick LHP can start up at very low heat load. The main reason may be the sintered wick is of smaller pore size than the mesh wick.
A R T I C L E
I N F O
Article history: Received 13 April 2015 Accepted 28 August 2015 Available online 9 October 2015 Keywords: Loop heat pipe Heat transfer characteristics Sintered wick Mesh wick Cooling of electronics
A B S T R A C T
Heat transfer performance of loop heat pipe (LHP) is tightly related with the wick positioned between its evaporator and compensation chamber. Experiments were carried out to investigate the effects of wick on LHP heat transfer performance. Two wicks, a sintered copper powder wick and a copper mesh wick, were considered for comparison. The former has larger porosity; its pore size spans within a wide range, but smaller than that of the latter. The measured temperature data indicate that the sintered wick LHP starts up faster and operates more stably. The overall thermal resistance of the sintered wick LHP is also slightly lower than that of the mesh wick LHP. Moreover, the sintered wick LHP is found to be able to start up with heat load as low as 5 Watts. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Loop heat pipe (LHP), relying on phase change (evaporation and condensation) of fluid to realize effective and fast heat transfer, was first created and successfully tested by two Russian scientists, Gerasiomov and Maydanik, in 1972 [1]. Due to its excellent long distance heat transfer ability, ease and flexibility of installing, LHP has been widely used in energy generation, conversion, and utilizationrelevant fields, including aerospace [2,3], electronics cooling [4,5], and solar heating [6]. LHP consists of an evaporator, a condenser, a compensation chamber, and some vapor and liquid lines. Inside the evaporator, a wick, which is a very important part of LHP, is set to separate the compensation chamber from the evaporator. The wick is essentially a porous medium of capillary pore structure. The pore configuration (including size and shape of pores, and porosity) determines its macroscopic properties or performance, such as the permeability, effective thermal conductivity, and the maximum cap-
* Corresponding author. Tel.: +86 20 87057656; fax: +86 20 87057656. E-mail address: [email protected] (F. Jiang). http://dx.doi.org/10.1016/j.applthermaleng.2015.08.109 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
illary force. A good wick requires having sufficiently large capillary force to prevent vapor from penetrating the wick and entering into the liquid line, high permeability to lower the flow resistance of liquid flow, and low thermal conductivity to reduce the through-plane heat leakage [7–9]. A single-structured wick may not meet all these requirements [10,11]. Developing high-performance wick is a hot research topic in the arena of LHP. In recent years, numerous works [7–20] on heat transfer performance of LHPs with various wicks have been published. The most popular class of LHP wicks includes screen mesh wick and sintered metal powder wick. Ren et al. [8] developed a mathematical model for heat transfer in LHP wick to study the effects of porous structure parameters. Singh et al. [9] studied the effects of wick characteristics on LHP thermal performance and found from experiments that smaller pore size, larger porosity, and higher permeability in the sintered metal wick gave better LHP heat transfer performance. Espinosa et al. [10] measured the physical properties (porosity, permeability, maximum capillary pressure and thermal conductivity) of sintered metal wicks. Celata et al. [12] investigated the thermal characteristics of a flat disk LHP with a stainless steel mesh wick. Wang et al. [13] studied the startup and steadystate operation performance of a miniature LHP with a
Y. Wang et al./Applied Thermal Engineering 92 (2016) 104–110
sintered copper wick; the LHP could start up under a 6 W heat load. Choi et al. [14] reported a miniature LHP using a sintered metal wick, which could transfer heat of 27.8 W/cm2 flux to a place with a distance of 500 mm away and maintained the evaporator temperature below 70 °C. Plastic and ceramic wicks can be viewed as a relatively new type of LHP wicks. Nagano et al. [15] and Nishikawara and Nagano [16] used PTFE as LHP wick. Owing to the low thermal conductivity of PTFE, the wick could reduce the heat leakage from the evaporator to the compensation chamber. Santos et al. [17] used ceramic as LHP wick. Wan et al. [18] used a sintered metal fiber sheet as LHP wick. With optimization design to the condenser, the LHP was able to operate under 200 W heat load and the thermal resistance was 0.05 °C/W. Biporous wicks, as another relatively new type of wick, also aroused wide interests in the relevant community. Li et al. [11] reported a couple of methods to fabricate biporous nickel wicks for LHP and determined optimal fabrication conditions. Xu et al. [7] modulated porous wick sintered on the heater wall to enhance pool boiling heat transfer. The modulated biporous wick significantly shortened the LHP startup time, and the wall temperature of evaporator was maintained at 63 °C under 200 W heat load and antigravity operation conditions. The calculated thermal resistance was 0.12 °C/W. Chen et al. [19] experimentally investigated the thermal performance of a miniature stainless-steel-ammonia LHP with biporous wick. The results demonstrated that the maximum heat load that the LHP could take was 130 W at −15 °C heat sink temperature, and the LHP thermal resistance was 0.33 °C/W. Specially, Liu et al. [20] reported a composite wick, which was of standard cylindrical design and actually consisted of two wicks: the primary wick was a sintered nickel powder wick and the secondary wick a stainless steel mesh. The working fluid of LHP was methanol. Their results showed that the LHP could start up under heat loads within a range of 20–160 W, and the evaporator temperature was kept below 85 °C when coolant temperature was −10 °C. The derived thermal resistance of LHP was within 0.46–2.28 °C/W. Sintered copper powder wick and copper mesh wick are two typical wicks used in LHPs. However no direct comparison has been made to discern which is better. The present work designs and fabricates a stainless steel/water LHP and sets up an experimental system for the study of LHP heat transfer performance. The main goal is to experimentally study the effects of wick on LHP heat transfer performance including the startup performance, temperature oscillation characteristics, and the overall thermal resistance. The LHP wicks specially chosen for conducting the comparison study are right the sintered copper powder wick and copper mesh wick.
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2. Experimental aspects 2.1. Characteristics of LHP wicks Wick is one important component of LHP, which affects the LHP performance mainly from the following two aspects: (i) the pores in the wick provide pathways for the working fluid to flow back into the evaporator; (ii) the partially liquid-saturated wick produces capillary force, which prevents the steam from penetrating the wick and entering the liquid line. We consider two wicks for comparison in the present work. One is a sintered copper powder wick. Fig. 1 displays the appearance of the sintered wick. It is a thin circular flat plate of 58.5 mm diameter and 3 mm thickness. The digital photo shown in Fig. 1b was taken by a Keyence VHX-600 microscope camera. Directly from Fig. 1b, it is not difficult to get that the pore diameter is within 2.6~30 μm. To determine the porosity of the sintered wick is a relatively involved task. First, we measured the diameter (dw) and thickness (lw) of the wick by a Vernier caliper, and calculated ⎛ π dw2 lw ⎞ . Second, we weighed the wick the apparent volume Vw ⎜ = ⎝ 4 ⎟⎠ to get its mass (m) by an electronic scale. Third, we calculated the m ρ , where ρ represents the intrinsic density porosity (ε) with ε = 1 − Vw of the wick material, i.e. copper here. The porosity of this sintered wick was calculated to be 77.5%. Further, the permeability (Ks) and effective thermal conductivity (hs-eff) of the wick were calculated to be 2.1 × 10−11 m2 and 4.6 W/m/K, respectively [21]. The other is a copper mesh wick. The wick is made of a 500 PPI (pores per inch) copper mesh, and contains about 31 layers of this mesh. Fig. 2 displays the appearance of the mesh wick. It is a thin circular flat plate of 58.6 mm diameter and 3 mm thickness. The Keyence microscope digital photo shown in Fig. 2b indicates that the diameter of the copper threads is about 26.2 μm. The effective pore diameter of the mesh wick was immediately determined to be about 35 μm and the porosity (εm) was calculated to be 66.5%. The permeability (Km) and effective thermal conductivity (hm-eff) of the wick were calculated to be 1.5 × 10−11 m2 and 4.8 W/m/K, respectively [21–23]. 2.2. Experimental system As shown in Fig. 3, the experimental system consists of four subsystems: the LHP system, the heater system, the cooling system and the data acquisition system. The LHP is made of stainless steel. Choice
Fig. 1. The sintered copper wick. (a) Normal photograph. (b) An enlarged photograph.
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Fig. 2. The mesh copper wick. (a) Normal photograph. (b) An enlarged photograph.
of working fluid is very important. The working fluid must be compatible with the materials of the wick and the LHP, i.e. no chemical reaction, otherwise the wick or LHP material will dissolve in the working fluid and metal ions will deposit on the tube wall and the wick, leading to local hot spots or blockage of wick pores. In addition, it produces non-condensing gas, which gathers in and gradually blocks the condenser. The secondary distilled water has good compatibility with both stainless steel and copper [24,25] and is thus chosen as the working fluid. The inner structure of the LHP has been described in our previous paper [26], and the detailed geometrical parameters of key components of the present experimental system are tabulated in Table 1. Six K-type thermocouples (±0.3 °C inaccuracy) are used to measure temperature, and the locations of temperature monitoring points are displayed in Fig. 4. Thermocouple 1 (T1) is positioned at the evaporator wall; thermocouples 2 and 3 (T2, T3) at the inlet and middle of the vapor line, respectively; thermocouple 4 (T4) at the outlet of the vapor line or the inlet of condenser; thermocouple 5 (T5) at the outlet of the condenser or the inlet of the compensation chamber; thermocouple 6 (T6) at the ambient. The inner structure of the evaporator and compensation chamber is detailed in Fig. 5. Many mini-channels are machined in the interior of the evaporator, which instruct the hot vapor to flow into the vapor line. The dimension (2.5 mm wide and 4 mm deep) of the mini-vapor channels is large in comparison with that in similar literature, like Refs. [5,7,19]. This design probably facilitates the vapor flow in the evaporator. The wick is positioned between the evaporator and compensation chamber. Moreover, a rubber ring of 1 mm thickness is set between the compensation chamber and the wick
to reduce heat leakage from the evaporator to the compensation chamber. The upper and lower parts of the chamber are connected via a flange with special O-ring design to ensure sealing. Heating rods implanted inside the evaporator wall provide heat load to the LHP. The charging ratio of working fluid is about 60% (ratio of filled liquid volume to the total interior volume of LHP), which can ensure the normal operation of LHP [27]. The inclination angle (θ) of LHP is defined as the angle of the LHP tilting to the horizontal plane (see Fig. 4). The water stored in the water tank is 20 ± 2 °C and the water flow rate is well controlled. During experiments, the wall temperature of the evaporator is controlled below 75 °C and the temperature of the ambient is 25 ± 2 °C. 3. Results and discussion The startup characteristic of LHP reflects its reliability and stability [28]. An LHP gets startup fast, meaning rapid thermal response, which is important to heat dissipation of high power electronic equipment. We define the time duration for the heat pipe reaching a quasi-steady operation status, i.e. all the temperatures monitored (see in Fig. 4) unchanged or in a dynamic but periodic oscillation status, as its startup time. Fig. 6 illustrates the startup performance of two LHPs with different wicks under 20 W heat load. The coolant water flow rate is about 1 mL/s. Seen from Fig. 6, the LHPs with different wicks both can successfully start up. Upon heat loading, the evaporator temperature (T1) first rises; then the compensation chamber temperature (T5), the temperature at vapor line inlet (T2), the temperature at the
Fig. 3. Schematic of the experimental system.
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Table 1 Geometrical parameters of the LHP. Key components
Size (mm)
Evaporator Diameter (outer/inner) Vapor channel width Vapor channel depth Wall thickness Diameter (O/I) of heating rods Vapor line Diameter (O/I) Length Liquid line Diameter (O/I) Length Condenser Diameter (O/I) Length Compensation chamber Diameter (O/I) Wall thickness
/ 70/67 2.5 4 1.5 85/40 / 6/4 800 / 6/4 100 / 50/48 600 / 40/37 1.5
mid of vapor line (T3), and the temperature at condenser inlet (T4) rise subsequently. Due to heat leakage, T5 is seen to rise even earlier than T2. After some minutes, all the temperatures monitored get into an oscillation status, indicating successful startup. After startup, the periodic temperature oscillation at monitoring points is caused by the alternation of vapor-liquid two-phase flow status. The temperature fluctuating to high indicates more vapor is involved in the flow and fluctuating to low indicates more liquid is included in the flow. For the evaporator, a large part of volume is occupied by liquid water and it is thus seen to have the smallest temperature fluctuation amplitude. During some time period, the temperature T5 can be higher than T3 and T4, indicating flow oscillation in the LHP. Variation of the evaporator temperature (T1) is actually indicative of the working status of LHP. Once T1 reaches a steady value or fluctuates periodically, a successful startup of LHP may be achieved. Fig. 7 displays the temporal evolution of evaporator temperature (T1) for LHPs with different wicks; the LHPs were positioned horizontally (θ = 0°, Fig. 7a) or with 30° tilting angle to the hori-
Fig. 6. Measured temperatures of LHP under 20 W heat load (θ = 0°). (a) Sintered wick. (b) Mesh wick.
Fig. 4. Schematic of the LHP system and the locations of thermocouples.
Fig. 5. Inner structure of evaporator and compensation chamber.
zontal plane (θ = 30°, Fig. 7b); three different heat loads were considered. For a given LHP, the startup time decreases with the increase of heat load; the temperature oscillation is effectively restrained if heat load is high. These are due mainly to a larger amount of vapor that is included in the fluid flow when heat load is increased. Seen from Fig. 7, the startup time of the LHP with sintered wick is generally shorter than that with mesh wick. This may be due mainly to the smaller pores in the sintered wick (refer to Figs. 1 and 2), leading to larger capillary force to prevent vapor from entering into the compensation chamber. At steady operation, the evaporator temperature (T1) of the LHP with sintered wick is about 1 °C lower than that of the LHP with mesh wick, which means the heat transfer performance of the former is slightly better. Increasing the inclination angle (θ) facilitates liquid to flow back to the compensation chamber. Therefore, heat transfer performance for the cases related with Fig. 7b is generally better than those related with Fig. 7a. The startup time is shorter and the steady operation temperature is slightly lower. Three main types of temperature oscillation of LHP were summarized in Refs. [29,30]. The first shows ultra-high oscillation
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Fig. 8. Overall thermal resistance of LHPs with different wicks.
Tc′ is approximated by the arithmetic average of T4 and T5 as
Tc ′ =
Fig. 7. Comparison of T1 evolution between LHPs with different wicks. (a) θ = 0°; (b) θ = 30°.
T4 + T5 2
(2)
The calculated thermal resistance values for the two LHPs with different wicks are plotted in Fig. 8. The thermal resistance of LHP decreases with the increase of heat load and increasing the LHP inclination angle (θ) leads to smaller thermal resistance, in good agreement with the qualitative observations from Figs. 6 and 7. As evidenced by Fig. 8, the thermal resistance of LHP with the sintered wick is slightly lower than that of LHP with the mesh wick under the same operating conditions, according well with the qualitative observations from Figs. 6 and 7. It is worth pointing out that the measuring inaccuracy of temperature is ±0.3 °C and the maximum relative uncertainty of thermal resistance R is estimated to be 2.7% [32]. The coolant water flow rate in the condenser may affect the heat transfer performance of LHP. Fig. 9 presents the calculated thermal resistance as a function of heat load for both the two LHPs with two different coolant water flow rates, 1 mL/s and 10 mL/s. The thermal
frequency (the period is a few seconds or less) with low oscillation amplitude (less than 1 °C), which occurs due mainly to the intimely coming back of working fluid to the evaporator. The second has high oscillation frequency (the period is longer and may reach several minutes) with low oscillation amplitude (less than 1 °C), which is caused by the vapor moving near to the inlet or outlet of the condenser. The third has low oscillation frequency (the period may be a few minutes or even a few hours) with high oscillation amplitude (a few °C or more than 10 °C), which occurs when the LHP is imposed with a low heat load or the condenser possesses a high cooling capacity. Inspecting Fig. 8, we classify the observed temperature oscillations to the second (50 W and 100 W cases) and third (10 W case) types. To quantitatively evaluate the heat transfer performance of LHP at steady operation, we calculate the overall thermal resistance, which is defined as [31]
RLHP =
T1 − Tc ′ ΔT = Q UI
(1)
where Tc′ denotes the average temperature of the LHP condensation section and Q represents the heat load. In the present study
Fig. 9. Overall thermal resistance of LHPs under different flow rates of cooling water (θ = 0°).
Y. Wang et al./Applied Thermal Engineering 92 (2016) 104–110
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required to carefully consider all the above-mentioned factors when selecting between the sintered copper wick and the copper mesh wick for practical applications of LHPs. Acknowledgements Financial supports received from the Guangdong Science and Technology Project (2012A080304002), the Zhuhai Science and Technology Project (2012D0501990019), the Guangzhou Science and Technology Project (2013J4300001), and the CAS “100 Talents” Program (FJ) are gratefully acknowledged. Nomenclature
Fig. 10. Measured temperatures of LHP with sintered wick under 5 W heat load. (θ = 0°).
resistance of LHP increases with the flow rate of coolant water and decreases with the heat load, qualitatively in agreement with the results obtained in existing literature like Refs. [33,34]. Seen from Fig. 9 also, the LHP with the sintered wick shows slightly better heat transfer performance. To further test the performance of LHPs with different wicks, we tried to start up the LHPs under a small heat load, 5 W. The LHP with sintered wick was found to start up successfully, whereas the LHP with mesh wick failed. The measured temperatures for the LHP with sintered wick are shown in Fig. 10. Although the temperatures are seen to intensely fluctuate, they can reach a quasi-steady state, indicating successful startup. The startup time is about 50 minutes. In Ref. [20], the LHP had a similar design with the LHP studied in the present work, except the wick that the LHP in Ref. [20] used was a composite-structured wick, manufacturing of which is relatively more complicated, and for the LHP in present study, the condenser was put nearer to the compensation chamber, which is deemed to be able to effectively retrain the temperature oscillations though may prolong the LHP startup time. The LHP in Ref. [20] failed to start up at 10 W heat load. 4. Concluding remarks Heat transfer performance of stainless steel-water LHPs with different wicks was studied experimentally. Two wicks, a sintered copper powder wick and a copper mesh wick, were considered for comparison. The sintered wick LHP performs better than the mesh wick LHP in terms of the minimum startup heat load, startup time and overall thermal resistance. Under the same working conditions (i.e. the same heat load and the same tilting angle), the sintered wick LHP can start up faster, and the overall thermal resistance is slightly lower. Under a heat load as low as 5 W, the sintered wick LHP can start up successfully. Analyzing the microscopic pore structures of the two wicks finds that the sintered wick is of smaller pores, which induce larger capillary force to prevent vapor from entering into the liquid line. This may be the major reason, leading to the difference of heat transfer performance between the two LHPs. Stacking multi-layers of mesh wick is not an easy procedure while the sintering operation of metal wick is relatively more mature; the cost of screen mesh wick is generally lower than the sintered wick. Although the sintered copper wick LHP shows slightly more superior thermal performance than the copper mesh wick LHP, it is
h I K Q R Ti(i = 1,2,3,4,5,6) U
Effective thermal conductivity [W/(mK)] Current [mA] Permeability [m2] Heat load [W] Thermal resistance [K/W] Monitoring temperature during experiment [K] Voltage [V]
Greek symbols ε θ
Porosity Inclination angle [°]
Subscripts c eff l m s w
Condensation section Effective thermal conductivity Liquid Mesh; mass Sintered Wick
Abbreviations O I
Outer Inner
References [1] Y.F. Maydaink, Review: loop heat pipe, Appl. Therm. Eng. 25 (2005) 635–657. [2] G. Wang, D. Mishkinis, D. Nikanpour, Capillary heat loop technology: space applications and recent Canadian activities, Appl. Therm. Eng. 28 (2008) 284–303. [3] R.R. Riehl, T. Dutra, Development of an experimental loop heat pipe for application in future space missions, Appl. Therm. Eng. 25 (2005) 101–112. [4] V.G. Pastukhov, Y.F. Maydanik, Low-noise cooling system for PC on the base of loop heat pipes, Appl. Therm. Eng. 27 (2007) 894–901. [5] J. Li, F. Lin, D.M. Wang, W.K. Tian, A loop-heat-pipe heat sink with parallel condensers for high-power integrated LED chips, Appl. Therm. Eng. 56 (2013) 18–26. [6] Z.Y. Wang, X.D. Zhao, Analytical study of the heat transfer limits of a novel loop heat pipe system, Int. J. Energy Res. 35 (2011) 404–414. [7] J.L. Xu, X.B. Ji, W.L. Yang, Z.W. Zhao, Modulated porous wick evaporator for loop heat pipes: experiment, Int. J. Heat Mass Transf. 72 (2014) 163–176. [8] C. Ren, Q.S. Wu, M.B. Hu, Heat transfer in loop heat pipe’s wick: effect of porous structure parameters, J. Thermophys. Heat Transf. 21 (2007) 702–711. [9] R. Singh, A. Akbarzadeh, M. Mochizuki, Effect of wick characteristics on the thermal performance of the miniature loop heat pipe, J. Heat Transfer 131 (2009) 82601–82611. [10] F.A.D. Espinosa, T.B. Peters, J.G. Brisson, Effect of fabrication parameters on the thermophysical properties of sintered wicks for heat pipe applications, Int. J. Heat Mass Transf. 55 (2012) 7471–7486. [11] H. Li, Z.C. Liu, B.B. Chen, W. Liu, C. Li, J.G. Yang, Development of biporous wicks for flat-plate loop heat pipe, Exp. Therm. Fluid Sci. 37 (2012) 91–97. [12] G.P. Celata, M. Cumo, M. Furrer, Experimental tests of a stainless steel loop heat pipe with flat evaporator, Exp. Therm. Fluid Sci. 34 (2010) 866–878. [13] S.F. Wang, W.B. Zhang, X.F. Zhang, J.J. Chen, Study on start-up characteristics of loop heat pipe under low-power, Int. J. Heat Mass Transf. 54 (2011) 1002– 1007. [14] J. Choi, W. Sano, W.J. Zhang, Y. Yuan, Y. Lee, D.A. Borca-Tasciuc, Experimental investigation on sintered porous wicks for miniature loop heat pipe applications, Exp. Therm. Fluid Sci. 51 (2013) 271–278.
110
Y. Wang et al./Applied Thermal Engineering 92 (2016) 104–110
[15] H. Nagano, F. Fukuyoshi, H. Ogawa, H. Nagai, Development of an experimental small loop heat pipe with polytetrafluoroethylene wicks, J. Thermophys. Heat Transf. 25 (2011) 547–552. [16] M. Nishikawara, H. Nagano, Parametric experiments on a miniature loop heat pipe with PTFE wicks, Int. J. Therm. Sci. 85 (2014) 29–39. [17] P.H.D. Santos, E. Bazzo, S. Becker, R. Kulenovic, R. Mertz, Development of LHPs with ceramic wick, Appl. Therm. Eng. 30 (2010) 1784–1789. [18] Z.P. Wan, X.W. Wang, Y. Tang, Condenser design optimization and operation characteristics of a novel miniature loop heat pipe, Energy Convers. Manag. 64 (2012) 35–42. [19] B.B. Chen, W. Liu, Z.C. Liu, Experimental investigation of loop heat pipe with flat evaporator using biporous wick, Appl. Therm. Eng. 42 (2012) 34– 40. [20] Z.C. Liu, H. Li, B.B. Chen, Operational characteristics of flat type loop heat pipe with biporous wick, Int. J. Therm. Sci. 58 (2012) 180–185. [21] A.S. Demidov, E.S. Yatsenko, Investigation of heat and mass transfer in the evaporation zone of a heat pipe operating by the “inverted meniscus” principle, Int. J. Heat Mass Transf. 37 (1994) 2155–2163. [22] S.W. Chi, Heat Pipe Theory and Practice, McGraw-Hill, New York, 1976. [23] M. Kaviany, Principles of Heat Transfer in Porous Media, Springer Verlag, New York, 1995. [24] Y.F. Maydanik, S. Vershinin, M. Chernysheva, S. Yushakova, Investigation of a compact copper-water loop heap pipe with a flat evaporator, Appl. Therm. Eng. 31 (2011) 3533–3541. [25] X.H. Nguyen, B.H. Sung, J. Choi, S.R. Ryoo, H.S. Ko, C. Kim, Study on heat transfer performance for loop heat pipe with circular flat evaporator, Int. J. Heat Mass Transf. 55 (2012) 1304–1315.
[26] Y.W. Wang, J.W. Cen, F.M. Jiang, An experimental study on the performance of a loop heat pipe, Exp. Heat Transf. 28 (2015) 1–8. [27] L. Zhang, J.Y. Xu, H. Xu, Effect of inventory on the heat performance of copperwater loop heat pipe, Exp. Therm. Fluid Sci. 44 (2013) 875–882. [28] H.X. Zhang, G.P. Lin, T. Ding, W. Yao, X.G. Shao, R.G. Sudakov, et al., Investigation of startup behavior of a loop heat pipe, J. Thermophys. Heat Transf. 19 (2005) 509–518. [29] J. Ku, High frequency low amplitude temperature oscillations in loop heat pipe operation, SAE 33rd International Conference on Environmental Science (ICES), Vancouver, Canada, July 7–10, 2003, Paper No. 2003-01-2387. [30] J. Ku, J. Rodriguez, Low frequency high amplitude temperature oscillations in loop heat pipe operation, SAE 33rd International Conference on Environmental Science (ICES), Vancouver, Canada, July 7–10, 2003, Paper No. 2003-01-2388. [31] E. Bartuli, S. Vershinin, Y. Maydanik, Visual and instrumental investigations of a copper-water loop heat pipe, Int. J. Heat Mass Transf. 61 (2013) 35–40. [32] J. Guo, Y.X. Yan, W. Liu, F.M. Jiang, A.W. Fan, Effects of upwind area of tube inserts on heat transfer and flow resistance characteristics of turbulent flow, Exp. Therm. Fluid Sci. 48 (2013) 147–155. [33] Y. Chen, M. Groll, R. Mertz, Y.F. Maydanik, S.V. Vershinin, Steady-state and transient performance of a miniature loop heat pipe, Int. J. Therm. Sci. 45 (2006) 1084–1090. [34] S. Becker, S. Vershinin, V. Sartre, E. Laurien, J. Bonjour, Y.F. Maydaink, Steady state operation of a copper-water LHP with a flat-oval evaporator, Appl. Therm. Eng. 31 (2011) 686–695.
Contents lists available at ScienceDirect
Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
LHP heat transfer performance: A comparison study about sintered copper powder wick and copper mesh wick Yiwei Wang, Jiwen Cen, Fangming Jiang *, Wenjiong Cao, Jian Guo Laboratory of Advanced Energy Systems, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
H I G H L I G H T S
• • • •
We experimentally study heat transfer performance of LHPs with different wicks. The sintered wick LHP performs a little better than the mesh wick LHP. The sintered wick LHP can start up at very low heat load. The main reason may be the sintered wick is of smaller pore size than the mesh wick.
A R T I C L E
I N F O
Article history: Received 13 April 2015 Accepted 28 August 2015 Available online 9 October 2015 Keywords: Loop heat pipe Heat transfer characteristics Sintered wick Mesh wick Cooling of electronics
A B S T R A C T
Heat transfer performance of loop heat pipe (LHP) is tightly related with the wick positioned between its evaporator and compensation chamber. Experiments were carried out to investigate the effects of wick on LHP heat transfer performance. Two wicks, a sintered copper powder wick and a copper mesh wick, were considered for comparison. The former has larger porosity; its pore size spans within a wide range, but smaller than that of the latter. The measured temperature data indicate that the sintered wick LHP starts up faster and operates more stably. The overall thermal resistance of the sintered wick LHP is also slightly lower than that of the mesh wick LHP. Moreover, the sintered wick LHP is found to be able to start up with heat load as low as 5 Watts. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Loop heat pipe (LHP), relying on phase change (evaporation and condensation) of fluid to realize effective and fast heat transfer, was first created and successfully tested by two Russian scientists, Gerasiomov and Maydanik, in 1972 [1]. Due to its excellent long distance heat transfer ability, ease and flexibility of installing, LHP has been widely used in energy generation, conversion, and utilizationrelevant fields, including aerospace [2,3], electronics cooling [4,5], and solar heating [6]. LHP consists of an evaporator, a condenser, a compensation chamber, and some vapor and liquid lines. Inside the evaporator, a wick, which is a very important part of LHP, is set to separate the compensation chamber from the evaporator. The wick is essentially a porous medium of capillary pore structure. The pore configuration (including size and shape of pores, and porosity) determines its macroscopic properties or performance, such as the permeability, effective thermal conductivity, and the maximum cap-
* Corresponding author. Tel.: +86 20 87057656; fax: +86 20 87057656. E-mail address: [email protected] (F. Jiang). http://dx.doi.org/10.1016/j.applthermaleng.2015.08.109 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
illary force. A good wick requires having sufficiently large capillary force to prevent vapor from penetrating the wick and entering into the liquid line, high permeability to lower the flow resistance of liquid flow, and low thermal conductivity to reduce the through-plane heat leakage [7–9]. A single-structured wick may not meet all these requirements [10,11]. Developing high-performance wick is a hot research topic in the arena of LHP. In recent years, numerous works [7–20] on heat transfer performance of LHPs with various wicks have been published. The most popular class of LHP wicks includes screen mesh wick and sintered metal powder wick. Ren et al. [8] developed a mathematical model for heat transfer in LHP wick to study the effects of porous structure parameters. Singh et al. [9] studied the effects of wick characteristics on LHP thermal performance and found from experiments that smaller pore size, larger porosity, and higher permeability in the sintered metal wick gave better LHP heat transfer performance. Espinosa et al. [10] measured the physical properties (porosity, permeability, maximum capillary pressure and thermal conductivity) of sintered metal wicks. Celata et al. [12] investigated the thermal characteristics of a flat disk LHP with a stainless steel mesh wick. Wang et al. [13] studied the startup and steadystate operation performance of a miniature LHP with a
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sintered copper wick; the LHP could start up under a 6 W heat load. Choi et al. [14] reported a miniature LHP using a sintered metal wick, which could transfer heat of 27.8 W/cm2 flux to a place with a distance of 500 mm away and maintained the evaporator temperature below 70 °C. Plastic and ceramic wicks can be viewed as a relatively new type of LHP wicks. Nagano et al. [15] and Nishikawara and Nagano [16] used PTFE as LHP wick. Owing to the low thermal conductivity of PTFE, the wick could reduce the heat leakage from the evaporator to the compensation chamber. Santos et al. [17] used ceramic as LHP wick. Wan et al. [18] used a sintered metal fiber sheet as LHP wick. With optimization design to the condenser, the LHP was able to operate under 200 W heat load and the thermal resistance was 0.05 °C/W. Biporous wicks, as another relatively new type of wick, also aroused wide interests in the relevant community. Li et al. [11] reported a couple of methods to fabricate biporous nickel wicks for LHP and determined optimal fabrication conditions. Xu et al. [7] modulated porous wick sintered on the heater wall to enhance pool boiling heat transfer. The modulated biporous wick significantly shortened the LHP startup time, and the wall temperature of evaporator was maintained at 63 °C under 200 W heat load and antigravity operation conditions. The calculated thermal resistance was 0.12 °C/W. Chen et al. [19] experimentally investigated the thermal performance of a miniature stainless-steel-ammonia LHP with biporous wick. The results demonstrated that the maximum heat load that the LHP could take was 130 W at −15 °C heat sink temperature, and the LHP thermal resistance was 0.33 °C/W. Specially, Liu et al. [20] reported a composite wick, which was of standard cylindrical design and actually consisted of two wicks: the primary wick was a sintered nickel powder wick and the secondary wick a stainless steel mesh. The working fluid of LHP was methanol. Their results showed that the LHP could start up under heat loads within a range of 20–160 W, and the evaporator temperature was kept below 85 °C when coolant temperature was −10 °C. The derived thermal resistance of LHP was within 0.46–2.28 °C/W. Sintered copper powder wick and copper mesh wick are two typical wicks used in LHPs. However no direct comparison has been made to discern which is better. The present work designs and fabricates a stainless steel/water LHP and sets up an experimental system for the study of LHP heat transfer performance. The main goal is to experimentally study the effects of wick on LHP heat transfer performance including the startup performance, temperature oscillation characteristics, and the overall thermal resistance. The LHP wicks specially chosen for conducting the comparison study are right the sintered copper powder wick and copper mesh wick.
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2. Experimental aspects 2.1. Characteristics of LHP wicks Wick is one important component of LHP, which affects the LHP performance mainly from the following two aspects: (i) the pores in the wick provide pathways for the working fluid to flow back into the evaporator; (ii) the partially liquid-saturated wick produces capillary force, which prevents the steam from penetrating the wick and entering the liquid line. We consider two wicks for comparison in the present work. One is a sintered copper powder wick. Fig. 1 displays the appearance of the sintered wick. It is a thin circular flat plate of 58.5 mm diameter and 3 mm thickness. The digital photo shown in Fig. 1b was taken by a Keyence VHX-600 microscope camera. Directly from Fig. 1b, it is not difficult to get that the pore diameter is within 2.6~30 μm. To determine the porosity of the sintered wick is a relatively involved task. First, we measured the diameter (dw) and thickness (lw) of the wick by a Vernier caliper, and calculated ⎛ π dw2 lw ⎞ . Second, we weighed the wick the apparent volume Vw ⎜ = ⎝ 4 ⎟⎠ to get its mass (m) by an electronic scale. Third, we calculated the m ρ , where ρ represents the intrinsic density porosity (ε) with ε = 1 − Vw of the wick material, i.e. copper here. The porosity of this sintered wick was calculated to be 77.5%. Further, the permeability (Ks) and effective thermal conductivity (hs-eff) of the wick were calculated to be 2.1 × 10−11 m2 and 4.6 W/m/K, respectively [21]. The other is a copper mesh wick. The wick is made of a 500 PPI (pores per inch) copper mesh, and contains about 31 layers of this mesh. Fig. 2 displays the appearance of the mesh wick. It is a thin circular flat plate of 58.6 mm diameter and 3 mm thickness. The Keyence microscope digital photo shown in Fig. 2b indicates that the diameter of the copper threads is about 26.2 μm. The effective pore diameter of the mesh wick was immediately determined to be about 35 μm and the porosity (εm) was calculated to be 66.5%. The permeability (Km) and effective thermal conductivity (hm-eff) of the wick were calculated to be 1.5 × 10−11 m2 and 4.8 W/m/K, respectively [21–23]. 2.2. Experimental system As shown in Fig. 3, the experimental system consists of four subsystems: the LHP system, the heater system, the cooling system and the data acquisition system. The LHP is made of stainless steel. Choice
Fig. 1. The sintered copper wick. (a) Normal photograph. (b) An enlarged photograph.
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Fig. 2. The mesh copper wick. (a) Normal photograph. (b) An enlarged photograph.
of working fluid is very important. The working fluid must be compatible with the materials of the wick and the LHP, i.e. no chemical reaction, otherwise the wick or LHP material will dissolve in the working fluid and metal ions will deposit on the tube wall and the wick, leading to local hot spots or blockage of wick pores. In addition, it produces non-condensing gas, which gathers in and gradually blocks the condenser. The secondary distilled water has good compatibility with both stainless steel and copper [24,25] and is thus chosen as the working fluid. The inner structure of the LHP has been described in our previous paper [26], and the detailed geometrical parameters of key components of the present experimental system are tabulated in Table 1. Six K-type thermocouples (±0.3 °C inaccuracy) are used to measure temperature, and the locations of temperature monitoring points are displayed in Fig. 4. Thermocouple 1 (T1) is positioned at the evaporator wall; thermocouples 2 and 3 (T2, T3) at the inlet and middle of the vapor line, respectively; thermocouple 4 (T4) at the outlet of the vapor line or the inlet of condenser; thermocouple 5 (T5) at the outlet of the condenser or the inlet of the compensation chamber; thermocouple 6 (T6) at the ambient. The inner structure of the evaporator and compensation chamber is detailed in Fig. 5. Many mini-channels are machined in the interior of the evaporator, which instruct the hot vapor to flow into the vapor line. The dimension (2.5 mm wide and 4 mm deep) of the mini-vapor channels is large in comparison with that in similar literature, like Refs. [5,7,19]. This design probably facilitates the vapor flow in the evaporator. The wick is positioned between the evaporator and compensation chamber. Moreover, a rubber ring of 1 mm thickness is set between the compensation chamber and the wick
to reduce heat leakage from the evaporator to the compensation chamber. The upper and lower parts of the chamber are connected via a flange with special O-ring design to ensure sealing. Heating rods implanted inside the evaporator wall provide heat load to the LHP. The charging ratio of working fluid is about 60% (ratio of filled liquid volume to the total interior volume of LHP), which can ensure the normal operation of LHP [27]. The inclination angle (θ) of LHP is defined as the angle of the LHP tilting to the horizontal plane (see Fig. 4). The water stored in the water tank is 20 ± 2 °C and the water flow rate is well controlled. During experiments, the wall temperature of the evaporator is controlled below 75 °C and the temperature of the ambient is 25 ± 2 °C. 3. Results and discussion The startup characteristic of LHP reflects its reliability and stability [28]. An LHP gets startup fast, meaning rapid thermal response, which is important to heat dissipation of high power electronic equipment. We define the time duration for the heat pipe reaching a quasi-steady operation status, i.e. all the temperatures monitored (see in Fig. 4) unchanged or in a dynamic but periodic oscillation status, as its startup time. Fig. 6 illustrates the startup performance of two LHPs with different wicks under 20 W heat load. The coolant water flow rate is about 1 mL/s. Seen from Fig. 6, the LHPs with different wicks both can successfully start up. Upon heat loading, the evaporator temperature (T1) first rises; then the compensation chamber temperature (T5), the temperature at vapor line inlet (T2), the temperature at the
Fig. 3. Schematic of the experimental system.
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Table 1 Geometrical parameters of the LHP. Key components
Size (mm)
Evaporator Diameter (outer/inner) Vapor channel width Vapor channel depth Wall thickness Diameter (O/I) of heating rods Vapor line Diameter (O/I) Length Liquid line Diameter (O/I) Length Condenser Diameter (O/I) Length Compensation chamber Diameter (O/I) Wall thickness
/ 70/67 2.5 4 1.5 85/40 / 6/4 800 / 6/4 100 / 50/48 600 / 40/37 1.5
mid of vapor line (T3), and the temperature at condenser inlet (T4) rise subsequently. Due to heat leakage, T5 is seen to rise even earlier than T2. After some minutes, all the temperatures monitored get into an oscillation status, indicating successful startup. After startup, the periodic temperature oscillation at monitoring points is caused by the alternation of vapor-liquid two-phase flow status. The temperature fluctuating to high indicates more vapor is involved in the flow and fluctuating to low indicates more liquid is included in the flow. For the evaporator, a large part of volume is occupied by liquid water and it is thus seen to have the smallest temperature fluctuation amplitude. During some time period, the temperature T5 can be higher than T3 and T4, indicating flow oscillation in the LHP. Variation of the evaporator temperature (T1) is actually indicative of the working status of LHP. Once T1 reaches a steady value or fluctuates periodically, a successful startup of LHP may be achieved. Fig. 7 displays the temporal evolution of evaporator temperature (T1) for LHPs with different wicks; the LHPs were positioned horizontally (θ = 0°, Fig. 7a) or with 30° tilting angle to the hori-
Fig. 6. Measured temperatures of LHP under 20 W heat load (θ = 0°). (a) Sintered wick. (b) Mesh wick.
Fig. 4. Schematic of the LHP system and the locations of thermocouples.
Fig. 5. Inner structure of evaporator and compensation chamber.
zontal plane (θ = 30°, Fig. 7b); three different heat loads were considered. For a given LHP, the startup time decreases with the increase of heat load; the temperature oscillation is effectively restrained if heat load is high. These are due mainly to a larger amount of vapor that is included in the fluid flow when heat load is increased. Seen from Fig. 7, the startup time of the LHP with sintered wick is generally shorter than that with mesh wick. This may be due mainly to the smaller pores in the sintered wick (refer to Figs. 1 and 2), leading to larger capillary force to prevent vapor from entering into the compensation chamber. At steady operation, the evaporator temperature (T1) of the LHP with sintered wick is about 1 °C lower than that of the LHP with mesh wick, which means the heat transfer performance of the former is slightly better. Increasing the inclination angle (θ) facilitates liquid to flow back to the compensation chamber. Therefore, heat transfer performance for the cases related with Fig. 7b is generally better than those related with Fig. 7a. The startup time is shorter and the steady operation temperature is slightly lower. Three main types of temperature oscillation of LHP were summarized in Refs. [29,30]. The first shows ultra-high oscillation
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Fig. 8. Overall thermal resistance of LHPs with different wicks.
Tc′ is approximated by the arithmetic average of T4 and T5 as
Tc ′ =
Fig. 7. Comparison of T1 evolution between LHPs with different wicks. (a) θ = 0°; (b) θ = 30°.
T4 + T5 2
(2)
The calculated thermal resistance values for the two LHPs with different wicks are plotted in Fig. 8. The thermal resistance of LHP decreases with the increase of heat load and increasing the LHP inclination angle (θ) leads to smaller thermal resistance, in good agreement with the qualitative observations from Figs. 6 and 7. As evidenced by Fig. 8, the thermal resistance of LHP with the sintered wick is slightly lower than that of LHP with the mesh wick under the same operating conditions, according well with the qualitative observations from Figs. 6 and 7. It is worth pointing out that the measuring inaccuracy of temperature is ±0.3 °C and the maximum relative uncertainty of thermal resistance R is estimated to be 2.7% [32]. The coolant water flow rate in the condenser may affect the heat transfer performance of LHP. Fig. 9 presents the calculated thermal resistance as a function of heat load for both the two LHPs with two different coolant water flow rates, 1 mL/s and 10 mL/s. The thermal
frequency (the period is a few seconds or less) with low oscillation amplitude (less than 1 °C), which occurs due mainly to the intimely coming back of working fluid to the evaporator. The second has high oscillation frequency (the period is longer and may reach several minutes) with low oscillation amplitude (less than 1 °C), which is caused by the vapor moving near to the inlet or outlet of the condenser. The third has low oscillation frequency (the period may be a few minutes or even a few hours) with high oscillation amplitude (a few °C or more than 10 °C), which occurs when the LHP is imposed with a low heat load or the condenser possesses a high cooling capacity. Inspecting Fig. 8, we classify the observed temperature oscillations to the second (50 W and 100 W cases) and third (10 W case) types. To quantitatively evaluate the heat transfer performance of LHP at steady operation, we calculate the overall thermal resistance, which is defined as [31]
RLHP =
T1 − Tc ′ ΔT = Q UI
(1)
where Tc′ denotes the average temperature of the LHP condensation section and Q represents the heat load. In the present study
Fig. 9. Overall thermal resistance of LHPs under different flow rates of cooling water (θ = 0°).
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required to carefully consider all the above-mentioned factors when selecting between the sintered copper wick and the copper mesh wick for practical applications of LHPs. Acknowledgements Financial supports received from the Guangdong Science and Technology Project (2012A080304002), the Zhuhai Science and Technology Project (2012D0501990019), the Guangzhou Science and Technology Project (2013J4300001), and the CAS “100 Talents” Program (FJ) are gratefully acknowledged. Nomenclature
Fig. 10. Measured temperatures of LHP with sintered wick under 5 W heat load. (θ = 0°).
resistance of LHP increases with the flow rate of coolant water and decreases with the heat load, qualitatively in agreement with the results obtained in existing literature like Refs. [33,34]. Seen from Fig. 9 also, the LHP with the sintered wick shows slightly better heat transfer performance. To further test the performance of LHPs with different wicks, we tried to start up the LHPs under a small heat load, 5 W. The LHP with sintered wick was found to start up successfully, whereas the LHP with mesh wick failed. The measured temperatures for the LHP with sintered wick are shown in Fig. 10. Although the temperatures are seen to intensely fluctuate, they can reach a quasi-steady state, indicating successful startup. The startup time is about 50 minutes. In Ref. [20], the LHP had a similar design with the LHP studied in the present work, except the wick that the LHP in Ref. [20] used was a composite-structured wick, manufacturing of which is relatively more complicated, and for the LHP in present study, the condenser was put nearer to the compensation chamber, which is deemed to be able to effectively retrain the temperature oscillations though may prolong the LHP startup time. The LHP in Ref. [20] failed to start up at 10 W heat load. 4. Concluding remarks Heat transfer performance of stainless steel-water LHPs with different wicks was studied experimentally. Two wicks, a sintered copper powder wick and a copper mesh wick, were considered for comparison. The sintered wick LHP performs better than the mesh wick LHP in terms of the minimum startup heat load, startup time and overall thermal resistance. Under the same working conditions (i.e. the same heat load and the same tilting angle), the sintered wick LHP can start up faster, and the overall thermal resistance is slightly lower. Under a heat load as low as 5 W, the sintered wick LHP can start up successfully. Analyzing the microscopic pore structures of the two wicks finds that the sintered wick is of smaller pores, which induce larger capillary force to prevent vapor from entering into the liquid line. This may be the major reason, leading to the difference of heat transfer performance between the two LHPs. Stacking multi-layers of mesh wick is not an easy procedure while the sintering operation of metal wick is relatively more mature; the cost of screen mesh wick is generally lower than the sintered wick. Although the sintered copper wick LHP shows slightly more superior thermal performance than the copper mesh wick LHP, it is
h I K Q R Ti(i = 1,2,3,4,5,6) U
Effective thermal conductivity [W/(mK)] Current [mA] Permeability [m2] Heat load [W] Thermal resistance [K/W] Monitoring temperature during experiment [K] Voltage [V]
Greek symbols ε θ
Porosity Inclination angle [°]
Subscripts c eff l m s w
Condensation section Effective thermal conductivity Liquid Mesh; mass Sintered Wick
Abbreviations O I
Outer Inner
References [1] Y.F. Maydaink, Review: loop heat pipe, Appl. Therm. Eng. 25 (2005) 635–657. [2] G. Wang, D. Mishkinis, D. Nikanpour, Capillary heat loop technology: space applications and recent Canadian activities, Appl. Therm. Eng. 28 (2008) 284–303. [3] R.R. Riehl, T. Dutra, Development of an experimental loop heat pipe for application in future space missions, Appl. Therm. Eng. 25 (2005) 101–112. [4] V.G. Pastukhov, Y.F. Maydanik, Low-noise cooling system for PC on the base of loop heat pipes, Appl. Therm. Eng. 27 (2007) 894–901. [5] J. Li, F. Lin, D.M. Wang, W.K. Tian, A loop-heat-pipe heat sink with parallel condensers for high-power integrated LED chips, Appl. Therm. Eng. 56 (2013) 18–26. [6] Z.Y. Wang, X.D. Zhao, Analytical study of the heat transfer limits of a novel loop heat pipe system, Int. J. Energy Res. 35 (2011) 404–414. [7] J.L. Xu, X.B. Ji, W.L. Yang, Z.W. Zhao, Modulated porous wick evaporator for loop heat pipes: experiment, Int. J. Heat Mass Transf. 72 (2014) 163–176. [8] C. Ren, Q.S. Wu, M.B. Hu, Heat transfer in loop heat pipe’s wick: effect of porous structure parameters, J. Thermophys. Heat Transf. 21 (2007) 702–711. [9] R. Singh, A. Akbarzadeh, M. Mochizuki, Effect of wick characteristics on the thermal performance of the miniature loop heat pipe, J. Heat Transfer 131 (2009) 82601–82611. [10] F.A.D. Espinosa, T.B. Peters, J.G. Brisson, Effect of fabrication parameters on the thermophysical properties of sintered wicks for heat pipe applications, Int. J. Heat Mass Transf. 55 (2012) 7471–7486. [11] H. Li, Z.C. Liu, B.B. Chen, W. Liu, C. Li, J.G. Yang, Development of biporous wicks for flat-plate loop heat pipe, Exp. Therm. Fluid Sci. 37 (2012) 91–97. [12] G.P. Celata, M. Cumo, M. Furrer, Experimental tests of a stainless steel loop heat pipe with flat evaporator, Exp. Therm. Fluid Sci. 34 (2010) 866–878. [13] S.F. Wang, W.B. Zhang, X.F. Zhang, J.J. Chen, Study on start-up characteristics of loop heat pipe under low-power, Int. J. Heat Mass Transf. 54 (2011) 1002– 1007. [14] J. Choi, W. Sano, W.J. Zhang, Y. Yuan, Y. Lee, D.A. Borca-Tasciuc, Experimental investigation on sintered porous wicks for miniature loop heat pipe applications, Exp. Therm. Fluid Sci. 51 (2013) 271–278.
110
Y. Wang et al./Applied Thermal Engineering 92 (2016) 104–110
[15] H. Nagano, F. Fukuyoshi, H. Ogawa, H. Nagai, Development of an experimental small loop heat pipe with polytetrafluoroethylene wicks, J. Thermophys. Heat Transf. 25 (2011) 547–552. [16] M. Nishikawara, H. Nagano, Parametric experiments on a miniature loop heat pipe with PTFE wicks, Int. J. Therm. Sci. 85 (2014) 29–39. [17] P.H.D. Santos, E. Bazzo, S. Becker, R. Kulenovic, R. Mertz, Development of LHPs with ceramic wick, Appl. Therm. Eng. 30 (2010) 1784–1789. [18] Z.P. Wan, X.W. Wang, Y. Tang, Condenser design optimization and operation characteristics of a novel miniature loop heat pipe, Energy Convers. Manag. 64 (2012) 35–42. [19] B.B. Chen, W. Liu, Z.C. Liu, Experimental investigation of loop heat pipe with flat evaporator using biporous wick, Appl. Therm. Eng. 42 (2012) 34– 40. [20] Z.C. Liu, H. Li, B.B. Chen, Operational characteristics of flat type loop heat pipe with biporous wick, Int. J. Therm. Sci. 58 (2012) 180–185. [21] A.S. Demidov, E.S. Yatsenko, Investigation of heat and mass transfer in the evaporation zone of a heat pipe operating by the “inverted meniscus” principle, Int. J. Heat Mass Transf. 37 (1994) 2155–2163. [22] S.W. Chi, Heat Pipe Theory and Practice, McGraw-Hill, New York, 1976. [23] M. Kaviany, Principles of Heat Transfer in Porous Media, Springer Verlag, New York, 1995. [24] Y.F. Maydanik, S. Vershinin, M. Chernysheva, S. Yushakova, Investigation of a compact copper-water loop heap pipe with a flat evaporator, Appl. Therm. Eng. 31 (2011) 3533–3541. [25] X.H. Nguyen, B.H. Sung, J. Choi, S.R. Ryoo, H.S. Ko, C. Kim, Study on heat transfer performance for loop heat pipe with circular flat evaporator, Int. J. Heat Mass Transf. 55 (2012) 1304–1315.
[26] Y.W. Wang, J.W. Cen, F.M. Jiang, An experimental study on the performance of a loop heat pipe, Exp. Heat Transf. 28 (2015) 1–8. [27] L. Zhang, J.Y. Xu, H. Xu, Effect of inventory on the heat performance of copperwater loop heat pipe, Exp. Therm. Fluid Sci. 44 (2013) 875–882. [28] H.X. Zhang, G.P. Lin, T. Ding, W. Yao, X.G. Shao, R.G. Sudakov, et al., Investigation of startup behavior of a loop heat pipe, J. Thermophys. Heat Transf. 19 (2005) 509–518. [29] J. Ku, High frequency low amplitude temperature oscillations in loop heat pipe operation, SAE 33rd International Conference on Environmental Science (ICES), Vancouver, Canada, July 7–10, 2003, Paper No. 2003-01-2387. [30] J. Ku, J. Rodriguez, Low frequency high amplitude temperature oscillations in loop heat pipe operation, SAE 33rd International Conference on Environmental Science (ICES), Vancouver, Canada, July 7–10, 2003, Paper No. 2003-01-2388. [31] E. Bartuli, S. Vershinin, Y. Maydanik, Visual and instrumental investigations of a copper-water loop heat pipe, Int. J. Heat Mass Transf. 61 (2013) 35–40. [32] J. Guo, Y.X. Yan, W. Liu, F.M. Jiang, A.W. Fan, Effects of upwind area of tube inserts on heat transfer and flow resistance characteristics of turbulent flow, Exp. Therm. Fluid Sci. 48 (2013) 147–155. [33] Y. Chen, M. Groll, R. Mertz, Y.F. Maydanik, S.V. Vershinin, Steady-state and transient performance of a miniature loop heat pipe, Int. J. Therm. Sci. 45 (2006) 1084–1090. [34] S. Becker, S. Vershinin, V. Sartre, E. Laurien, J. Bonjour, Y.F. Maydaink, Steady state operation of a copper-water LHP with a flat-oval evaporator, Appl. Therm. Eng. 31 (2011) 686–695.