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Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics
Journal Pre-proofs Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics He Song, Zhou ping, Liu Wei, Liu Zhi-chun PII: DOI: Reference:
S1359-4311(19)35932-0 https://doi.org/10.1016/j.applthermaleng.2019.114897 ATE 114897
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
25 August 2019 11 December 2019 31 December 2019
Please cite this article as: H. Song, Z. ping, L. Wei, L. Zhi-chun, Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114897
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© 2019 Published by Elsevier Ltd.
Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics He Song, Zhou ping, Liu Wei, Liu Zhi-chun* ( School of Energy and Engineering Power, Huazhong University of Science and Technology, Wuhan, 430074, China)
Abstract: A loop heat pipe (LHP) is one of the most efficient two-phase heat transfer devices. During its operation, a portion of the heat load applied to the evaporator is transferred to its compensation chamber (CC) through the evaporator sidewall, which is known as a heat leak, decreasing the performance of the LHP startup to a certain degree. To reduce a heat leak through the evaporator sidewall, an LHP with a composite-material evaporator was proposed. The evaporator is composed of two types of material, namely, red copper (heating surface) and 316L stainless steel (upper part), and has a reinforced rib structure on the heating surface to improve the evaporator strength. Two sintered nickel wicks are incorporated inside the evaporator. The experimental results demonstrate that the LHP with a composite-material evaporator can operate successfully within a heat load range of 10 to 140 W while heating the surface to below 80 °C at a heat sink temperature of 25 °C and 35 °C. Compared with an LHP with the same evaporator structure (but with a different in material) [17], the temperature difference between the evaporator outlet and the CC left (right) is smaller under the same heat load, indicating that the heat leak through the evaporator sidewall is reduced. Keywords: loop heat pipe, composite-material evaporator, reinforced rib structure, large thermal footprint 1. Introduction Loop heat pipes (LHPs) are two-phase heat transfer devices with a capillary pumping of the working fluid, and are promising candidates for the cooling of
*
Corresponding author: Address: 318 Power Building, 1037 Luoyu Road, Hongshan District, Wuhan 430074, China. Tel.: +86 27 87542618; Fax: +86 27 87540724; E-mail: [email protected] (Liu Zhi-chun).
electronics. An LHP system consists of an evaporator, a wick, vapor/liquid transport lines, and a condenser. The evaporator is isolated from the condenser, and the two are joined by transport lines, forming a closed loop. Thus, LHPs have the advantages of a high heat flux, heat transfer over a long distance, and a small temperature difference [1]. Chips used for a server, switcher, and router, among other components, have a large thermal footprint (area that is cooled), and at the same time operate with a high heat flux. Traditional cooling methods cannot meet the required heat dissipation, and new methods are urgently needed. Theoretical and experimental studies have shown that LHPs have broad prospects in the field of electronic cooling systems and in the thermal management of spacecraft [2, 3]. Since the first conception of an LHP in the 1970s, extensive studies have been conducted to determine the fundamental principle of its operating process [4]. For LHP system components, the evaporator is one of the most important parts determining the operating performance. With the development of LHP technology, the evaporator structure can be roughly divided into two categories: cylindrical and flat [5]. The first LHP had a cylindrical evaporator, and achieved an easy fabrication, heat transfer capability over longer distances, and a large capillary force, although they required a wide space for installation. In the miniaturization of electronic devices, a small amount of space is usually reserved for heat transfer devices, and therefore flat plate LHPs have proven to be more efficient within a narrow space. At the inception of LHP technology, flat-plate LHPs were interrupted because of the complexity of the flat evaporator, and were again stimulated through the development of high-power semiconductor electronics. In terms of electronic device packaging technology, flat plate LHPs have numerous advantages, such as a large heat transfer area and small installation space, and are more suitable for practical engineering applications. Thus, various types of flat evaporators have been developed. Numerous flat plate LHPs with methanol and ammonia as the working fluid using an opposite replenishment method have been investigated [6-14], and the thicknesses of these evaporator structures were no less than 10 mm, as limited by the dimensions of the compensation chamber (CC) and the diameter of the transport lines. To decrease the
thickness of the evaporator, the CC was designed to be laterally adjacent to the active zone of the evaporator (heating surface) [15, 16] and the evaporator thickness only depends on the installation space. In addition, some flat plate LHPs with a squared evaporator and a disk-shaped evaporator using a bimetal wall have also been proposed for practical engineering applications or for solving certain problems faced by flat evaporators [17-21]. If LHPs with a flat evaporator are to be applied in the cooling of electronic elements in the future, certain disadvantages need to be addressed, such as the small absorbing area of the wick, the deformative nature of the evaporator, and a serious heat leak from the heating surface to the CC, the second two being the most significant. The former impedes the further increase of the heating surface area, which is closely related with absorbing area of the wick. In addition, the deformation affects the contact quality between the heating surface and the electronic element, even destroying the porous wick inside the evaporator owing to an uneven force. The latter makes it extremely difficult to establish a temperature difference between the CC and the vapor grooves, which corresponds to the pressure difference circulating the working fluid inside the loop. Aiming at the problems mentioned above, a modified evaporator structure was proposed to improve its ability against a high pressure, and includes two wicks placed on two sides of the reinforced rib structure, avoiding the destruction of the heating surface. The reinforced rib structure is placed through the groove on the upper part, and the heating surface is joined with the upper part through argon arc welding. The CC is formed using two parts shared by a heating surface. This evaporator structure can be regarded as two flat evaporators laterally parallel in the LHP system. It is made of red copper (acting as the heating surface) and 316L stainless steel (upper part) to reduce heat leaking through the evaporator sidewall, and is called a composite-material evaporator. A part of the heating surface is red copper with a high conductivity coefficient of 393 W/(m·K), and the evaporator sidewall is stainless steel with a low conductivity coefficient of 16.2 W/(m·K). Under these circumstances, most of the heat load applied is transferred to the porous wick through the heating surface for liquid
evaporation. Very little heat is conductive to the CC through the evaporator sidewall. In this study, the operating characteristics of an LHP with a composite-material evaporator were systematically investigated, and the effect of the reinforced rib structure on the heat transfer capacity was also analyzed. This type of approach can provide researchers and engineers with useful guidelines for designing a flat evaporator structure, facilitating its further development. For example, the CC can be divided into multiple interconnected parts to extend the area of the heating surface, realizing the full potential of flat plate LHPs. 2. Experimental setup The structural parameters of the LHP components are shown in Table 1, and the 3D geometrical model of the composite-material evaporator is shown in Figure 1. The corresponding LHP system was designed to test its heat transfer characteristics. An image of the LHP with a composite-material evaporator is shown in Figure 2. In our experiments, the LHP system was placed in a horizontal orientation. Before charging the working fluid, the LHP system was vacuumed to a pressure of 10-4 Pa. R245fa was chosen to be the working fluid, and the charge ratio of the working fluid was calculated based on the following equations: α V total V cc V ll ε V wick 1 V cond 2
(1)
α V total V cc V ll ε V wick V cond
(2)
where α is the charge ratio, Vtotal is the total internal volume of the LHP system, Vll is the internal volume of the liquid line, Vwick is the wick volume, ε is the porosity of the wick, and Vcond is the internal volume of the condenser. For normal operation of the LHP system, the wick and liquid transport line should be filled with the working fluid, while the condenser and the CC contained two phase depending on heat load. The calculated result indicates that α ranges from 0.81 to 0.85. Because the CC has the largest volume in the LHP system, the charge ratio was determined to be 85% to allow the CC to be filled with liquid.
Table 1 Structural parameters of the LHP components (unit: mm) Evaporator
Heating surface
Nickel wick
Vapor transport line Liquid transport line
Tube-in-tube condenser
Overall dimensions
80(L)×80(W) ×21(H)
Groove on the upper part
52(L)×2(W) ×12(H)
Wall thickness
3
Reinforced ribbed structure
65(L)×2(W) ×17.7(H)
Grooves on the heating surface
65(L)×2(W) ×1(H)
Compensation chamber
70(L)×61(W) ×9(H)
Overall size
65(L)×36(W) ×4.5(H)
Permeability
2.39×10-13
porosity
70%
Length
270
Inner/outer diameter
5/6
Length
280
Inner/outer diameter
5/6
Length
910
Inner/outer diameter for inner tube
5/6
Inner/outer diameter for inner tube
18/20
Annotation: L— length; W—width; H—height
Figure 1 Sectional view of the evaporator model A tube-in-tube condenser was adopted, and the circulating water was controlled at 25 °C and 35 °C. A total of 16 T-type thermocouples with an accuracy of ± 0.5 °C were used to test the characteristic point temperature, and were attached to a Keithley 2700 data acquisition system. Characteristic points along the loop are shown in Figure 2, namely, Tc1-Tc4 for the heating surface, Tc5 for the evaporator outlet, Tc7 for the condenser inlet (on the vapor line), Tc9 for the condenser outlet (on the liquid line),
Tc10 for the evaporator inlet, Tc11-Tc12 for the evaporator back (for indirectly measuring the CC temperature), Tc8 and Tc14 for the coolant temperature, and Tc16 for the ambient temperature. A rectangular block embedded with five heating rods of 100 W acts as a simulated heat source, and the applied heat load is controlled by a voltage regulator and power meter with an accuracy of ±0.1 W.
Figure 2 Photograph of the LHP system with a composite-material evaporator and the location of thermocouples on the loop 3. Description of evaporator heat transfer The distribution of heat flow for an evaporator with a reinforced rib plate is shown in Figure 3. The applied heat load Qa is roughly divided into five parts: heat for liquid vaporization Qfg, heat conduction through the evaporator sidewall Qhl,wall, heat conduction through the wick Qhl,wick, heat conduction through the reinforced rib plate Qhl, r, and heat dissipation into the ambient air Qloss. The heat absorbed by the evaporator heating surface for Qa is mainly used to evaporate the liquid, and the rest is as follows: First, the latent heat of the working fluid, Qfg, takes up the largest share of the applied heat load. Second, a heat leak to the CC, Qhl, consists of three parts, namely, Qhl,wall, Qhl,wick, and Qhl, which can be expressed through the following equation: Q hl Q hl,wall Q hl,wick Q hl,r
(3)
Third, a small part of the heat input directly exchanges with the ambient air. The CC temperature is determined based on the amount of heat leak and subcooled liquid entering it. A heat leak leads to an increase in CC temperature and thus the temperature difference between the CC and vapor grooves on the heating surface is decreased. This temperature difference corresponds to the pressure difference that
circulates the working fluid in the loop. The normal circulation of the working fluid in the loop is an indicator of a successful startup. Therefore, decreasing the heat leak contributes to the startup process. The composite-material evaporator can help reduce a heat leak to the CC through the evaporator sidewall, theoretically achieving a better startup performance.
Figure 3 Analysis of heat flow inside the composite-material evaporator 4 Results and discussions In the LHP system, the startup performance and continuous operation with a variable heat load are the two important indicators. The thermal characteristics of the LHP are influenced by numerous factors, such as the heat load, heat sink, ambient temperature, and physical properties of the working fluid. In this study, the startup performance was tested at two heat sink temperatures of 25 °C and 35 °C, and the effect of the ambient temperature was also considered. 4.1 Startup characteristic at different heat sink temperatures The startup processes from 10 to 140 W were tested with a heating surface temperature of below 85 °C. The experimental results showed that the LHP with a composite-material evaporator has a similar startup behavior as flat plate LHPs with conventional evaporators [13, 14]. Figures 4 and 5 show the startup processes with a 10 W LHP system at heat sink temperatures of 25 °C and 35 °C. Vapor was quickly generated in the evaporator, and the LHP system eventually entered into a steady state when heat load was applied to the heating surface. For a heat sink temperature of 25 °C, the startup process was clearly divided into two stages: an initial stage and a steady state stage. During the initial stage of the startup process, when
heat load was provided, it was first absorbed by the heating surface owing to the evaporator’s own thermal capacity. A small portion of the heat was transferred to the liquid for evaporation, particularly for an LHP with a composite-material evaporator at a low heat load. It took approximately 58.5 min to enter into a steady state. The heating surface maintained a higher temperature than the steady state temperature. Once the absorbing heat process of the heating surface was over, the heating surface temperature decreased rapidly owing to a liquid vaporization. The LHP system entered into a steady state stage, and the heating surface remained at 27 °C. The difference between the highest temperature and the steady state temperature was 6.1 °C during the startup process.
Figure 4 Startup process with 10 W
Figure 5 Startup process with 10 W
at heat sink temperature of 25 oC
at heat sink temperature of 35 oC
The evaporator’s own thermal capacity was an important factor during the startup process when heat load was extremely small. During the initial stage, the temperature of heating surface, evaporator inlet and outlet, and the condenser inlet and outlet increased together. However, the condenser inlet temperature was lower than the condenser outlet temperature, which is abnormal compared with the common startup process of an LHP. The heat absorbed by the evaporator resulted in liquid evaporation in the CC, and the generated vapor then entered into the condenser through the liquid transport line. Thus, a peculiar phenomenon was observed during the initial stage. When the superheat was satisfied, liquid evaporation immediately occurred in the vapor grooves. The heating surface temperature decreased rapidly, and the working fluid
circulation in the loop returned to normal. The startup process at 10 W showed an obvious temperature overshoot at a heat sink temperature of 35 °C. The superheating of the heating surface reached 41 °C at approximately 40 min. The circulation of the working fluid in the loop showed a similar trend under two different heat sink temperatures. The startup process at 10 W did not require a long platform period at a heat sink temperature of 35 °C. At a higher heat sink temperature, the time required for superheating was shorter, and the temperature between the highest temperature and the steady state temperature was smaller. With an increase in heat sink temperature, the temperature of the working fluid in the loop was also improved before the heat load was provided. Prior to reaching a steady state, more liquid was driven into the evaporator, which laid a good foundation for liquid evaporation.
Figure 6 Startup process with 120W at heat sink temperature of 25 oC
Figure 7 Startup process with 120W at heat sink temperature of 35 oC
Figures 6 and 7 show the startup process at 120 W, and the LHP system quickly started and subsequently reached a steady state. For a large heat load, a short time was required for the absorbing heat process of the heating surface to be completed. Vapor was immediately generated in the vapor grooves and the circulation of the working fluid was sped up. Thus, the heat sink temperature had little effect on the startup process, and no temperature overshoot was observed during this time. Finally, the heating surface temperature remained at approximately 64 °C. 4.2 Effect of ambient temperature
Figure 8 Startup process with 20 W
Figure 9 Startup process with 90 W
During the experiment, the effect of the ambient temperature on the steady state was considered. Under a constant heat sink temperature, the room temperature was controlled using an air conditioner. Figures 8 and 9 show the startup process at 20 W and 90 W, respectively. When the LHP system entered a steady state, the room temperature was periodically changed with the aid of an air conditioner. It can be seen that the heating surface temperature was little affected by the room temperature. The temperatures on the left and right of the CC experienced slight oscillations at a heat load of 90 W, whereas no oscillating behavior was shown at 20 W. The back of the evaporator exchanged heat through convection. When the ambient temperature was changed, heat loss through the back of the evaporator also differed. With an increase in the heat load, both the mass flow rate of the working fluid and the heat loss through the back of the evaporator were increased. The thermal equilibrium of the CC was periodically broken by the change in ambient temperature. Thus, the temperatures on the left and right of the CC oscillated periodically. The experimental results show that the change in ambient temperature only affected the temperature of certain characteristic points, but had little influence on the steady state temperature. 4.3 Transient response to variable heat load In an engineering application, the heat load often varies depending on the outside conditions, such as the central processor unit. Thus, the response to a variable heat load was tested to identify the LHP system. The heat load was changed in steps of 10 W from 10 W to 140 W approximately every 30 min, and the thermal response curve is as
shown in Figure 10. With a heating surface temperature of below 80 °C, the LHP system can successfully shift from the previous steady state to the next state, showing good stability. When the heating surface temperature does not exceed 60 °C and 80 °C, the maximum heat load is 110 and 140 W, respectively. The present LHP can address the thermal issues of a CPU with a thermal design power (TDP) equal to 140 W when the heat sink temperature is controlled at 25 °C.
Figure 10 Continuous operation for LHP with a composite-material evaporator 4.4 Characteristic point temperature analysis Based on the continuous operation above, figure 11 gives the dependence of the characteristic point temperature on the heat load. With an increase in the heat load, the temperatures of the heating surface, evaporator inlet, and condenser inlet also increase. With a heat load of higher than 80 W, the evaporator inlet temperature gradually deviates from the outlet temperature of the condenser. For the temperature of the evaporator outlet and the condenser inlet, this phenomenon begins occurring at a heat load of 110 W. When the heat load is gradually increased, the circulation flow rate is improved. The heat exchange of the working fluid with the ambient air is intensified with the heat load. The heat exchange intensification in the liquid line is lower than that in the vapor line because of the relatively small flow rate of the liquid. Thus, the temperature difference between the evaporator inlet and the condenser outlet is smaller. It can be seen from Figure 11 that a portion of the heat is lost to the ambient air when vapor moves in the vapor line, whereas the heat is obtained when the liquid moves through the liquid line.
Figure 11 The dependence of characteristic point temperature on heat load
Figure 12 Temperature difference comparison of two evaporators
Figure 12 shows the change in temperature difference between the evaporator outlet and the back of the evaporator for the same evaporator structure with different materials. As Figure 3 indicates, the temperature of the back of the evaporator is determined by the heat conduction through the evaporator sidewall, the reinforced rib structure, and the wicks. Thus, under the same heat load, the greater the temperature difference is, the less heat leak to the CC that occurs. At a relatively low heat load, the LHP with a composite-material evaporator possesses a lower evaporator inlet temperature, and the greater the temperature difference that occurs between the evaporator outlet and the back of the evaporator when compared to an evaporator with a single-material evaporator, as described in Ref. [18]. A heat leak to the CC can be efficiently compensated, further lowering the LHP operating temperature. The temperature difference between the back of the evaporator (representing the CC) and the evaporator outlet (representing the evaporating surface of the wick) corresponds to the pressure drop. In Ref. [1], it was indicated that such a pressure drop should be equal to the sum of the pressure losses in all sections of the circulation of the working fluid except for the wick. Therefore, the greater the temperature difference is between the back of the evaporator and the evaporator outlet, the better the circulation created in the working fluid within the loop. 5 Heat transfer performance To analyze and evaluate the heat transfer capacity of the evaporator with a
reinforced rib plate and its corresponding LHP system, RLHP for the LHP total thermal resistance, Revap for the evaporator thermal resistance, and Ke for its heat transfer coefficient are calculated based on the following equations: R evap
T e T e outlet Qa
R LHP
Ke
T e T cond Qa
Qa F e (T e T e outlet )
(4) (5) (6)
where Te is the heating surface temperature (average of Tc1, Tc2, Tc3, and Tc4), Teoutlet
is the evaporator outlet temperature (Tc5), Tcond is the average temperature of the
condenser inlet (Tc7) and outlet temperature (Tc9), Qa is the heat load applied to the evaporator, and Fe is the effective heating surface area.
Figure 13 Dependence of thermal performance on heat load Figure 13 shows the dependence of the thermal characteristics on the heat load. Here, Revap indicates the relative stability except at 10 W, which is maintained at approximately 0.095 W/K. On the one hand, the thermal resistance resulting from the heating surface was smallest for red copper, which has the highest thermal conductivity among common metals, and more heat was directly transferred to the wick surface for liquid vaporization. On the other hand, the evaporator sidewall was made of stainless steel, which has a relatively low thermal conductivity, and less heat was transferred to the CC through this sidewall. Considering the two factors above, the evaporator was able to transfer heat with a small temperature difference. Thus, the thermal resistance of the evaporator remained relatively stable. However, the trend in RLHP first decreased,
and then increased with a heat load of higher than 60 W. The total thermal resistance of the LHP is related with the thermal resistance of the evaporator and the condenser and the heat exchange with the ambient air. Based on Equation (5), RLHP was underestimated because of the heat exchange with the ambient air. Thus, the thermal resistance of the condenser was the main factor increasing RLHP. With an increase in the heat load, more vapor entered into the tube-in-tube condenser for condensation. The circulating rate and temperature of the coolant was controlled by the water chiller. The condenser inlet temperature was improved, resulting in an increase in Tcond. For the composite-material evaporator, Ke was maintained at a high level except for at 10 W. In [11] and [14], the authors pointed out that there exist two modes of nucleate boiling and vapor film evaporation. At relatively low heat loads, the evaporator, CC, liquid transport line, and half of the condenser should be filled with liquid owing to their operation at a constant heat conduction depending on the temperature analysis. Thus, nucleate boiling first occurred in the evaporator, and the heat transfer coefficient gradually increased. At relatively high heat loads, liquid was dispelled from the vapor grooves and vaporization occurred on the porous wick surface. With an increase in the heat load, more pores on the porous wick were utilized for pumping the liquid, and a vapor film gradually formed on it. Thus, the heat transfer coefficient showed a decreasing tendency. For the same cooling efficiency as the condenser, the thermal resistance of the evaporator played a leading role. Therefore, RLHP showed a trend similar as Revap. The resulting uncertainty analysis was evaluated according to the calculations in [22] and [23]. The resulting uncertainty of RLHP, Revap, and Ke were 6.5%, 4.8% and 5.6%, respectively. 6 Conclusions In this study, focusing on certain disadvantages of a flat evaporator, a compositematerial evaporator was proposed for use in flat plate LHPs. At the same time, an LHP with a composite-material evaporator was designed to test its heat transfer characteristics under the conditions of a horizontal orientation and different heat sink temperatures. Some conclusions drawn from the experimental results are as follows:
(1) The proposed LHP with a composite-material evaporator showed a good startup performance and a quick response to the heat load recycle at a heat load range of 10 to 150 W. (2) The proposed evaporator could solve thermal issues with an area of 64 cm2, and achieve good temperature uniformity. (3) The composite-material evaporator can effectively reduce the heat leak through the evaporator sidewall, forming a larger temperature difference between the evaporator back and the evaporator outlet. (4) The evaporator heat transfer coefficient was maintained at a high level except at 10 W. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51776079 and 51376004), and the National Key Research and Development Program of China (Grant No. 2017YFB0603501-3).
References [1] Maydanik Y F. Review: loop heat pipes. Applied Thermal Engineering, 25 (2005): 635-657 [2] Swanson T D, Birur G C. NASA thermal control technologies for robotic spacecraft. Applied Thermal Engineering, 23(2003): 1055–1065 [3] Ku J, Ottenstein L, Douglas D. Technology overview of a multi-evaporator miniature loop heat pipe for spacecraft applications. Journal of Spacecraft and Rockets, 2012, 6(49): 999-1007. [4] Amrit A, Abhijit A A, Jasvanth S V, et al. Loop heat pipes: A review of fundamentals, operation, and design. Heat Transfer Engineering, 2012, 33(4-5): 387405 [5] Maydanik Y F, Chernysheva M A, Pastukhov V G. Review: Loop heat pipes with flat evaporators. Applied Thermal Engineering, 67(2014): 294-307 [6] Li H, Liu Z C, Chen B B, et al. Development of biporous wicks for flat-plate loop heat pipe. Experimental Thermal and Fluid Science, 37 (2012): 91-97
[7] Liu Z C, Li H, Chen B B, et al. Operational characteristics of flat type loop heat pipe with biporous wick. International Journal of Thermal Sciences, 58 (2012):180-185 [8] Liu Z C, Wang D D, Jiang C, et al. Experimental study on loop heat pipe with twowick flat evaporator. International Journal of Thermal Sciences, 94 (2015): 9-17 [9]Chen B B, Liu Z C, Liu W, et al. Operational characteristics of two biporous wicks used in loop heat pipe with flat evaporator. International Journal of Heat and Mass Transfer, 55 (2012): 2204-2207 [10] Chen B B, Liu W, Liu Z C, et al. Experimental investigation of loop heat pipe with flat evaporator using biporous wick. Applied Thermal Engineering, 42 (2012): 34-40 [11] Wang D D, Liu Z C, Shen J, et al. Experimental study of the loop heat pipe with a flat disk-shaped evaporator. Experimental Thermal and Fluid Science, 57 (2014): 157164 [12] Wang D D, Liu Z C, He S, et al. Operational characteristics of a loop heat pipe with a flat evaporator and two primary biporous wicks. International Journal of Heat and Mass Transfer, 89 (2015): 33-41 [13] He S, Liu Z C, Zhao J, et al. Experimental study of an ammonia loop heat pipe with a flat plate evaporator. International Journal of Heat and Mass Transfer, 102 (2016): 1050-1055 [14] He S, Liu Z C, Wang D D, et al. Investigation of the flat disk-shaped LHP with a shared compensation chamber. Applied Thermal Engineering, 104 (2016):139-145 [15] Becker S, Vershinin S, Sartre V, et al. Steady state operation of a copper-water LHP with a flat-oval evaporator. Applied Thermal Engineering, 31 (2011): 686-695 [16] Tian W, He S, Liu Z C, et al. Experimental Investigation of a Miniature Loop Heat Pipe with Eccentric Evaporator for Cooling Electronics. Applied Thermal Engineering, 159(2019)113982 [17] Liu Z C, Gai D X, Li H, et al. Investigation of impact of different working fluids on the operational characteristics of miniature LHP with flat evaporator. Applied Thermal Engineering, 31 (2011): 3387-3392 [18] He S, Zhao J, Liu Z C, et al. Experimental investigation of loop heat pipe with a large squared evaporator for cooling electronics. Applied Thermal Engineering, 144
(2018): 383-391 [19] Li J, Lin F, Wang D M, et al. A loop-heat-pipe heat sink with parallel condensers for high-power integrated LED chips. Applied Thermal Engineering, 56 (2013): 18-26 [20] Li J, Wang D M, “Bud” Peterson G P. A compact loop heat pipe with flat square evaporator for high power chip cooling. IEEE Trans. Compon. Packag. Manu. Technol, 2011, 1 (4): 519-527 [21 Maydanik Y F, Vershinin S V, Chernysheva M A, et al. Experimental Study of an Ammonia Loop Heat Pipe with a Flat Disk-shaped Evaporator Using a Bimetal Wall. Applied Thermal Engineering, 126(2017): 643-652 [22] Coleman H W, Steele W G. Experimentation, Validation, and Uncertainty Analysis for Engineers. Third edition [23] Robert J. Describing the uncertainties in the experimental results. Experimental Thermal Fluid Science, 1 (1988):3–17
Graphical abstract: A loop heat pipe (LHP) with a composite-material evaporator has been proposed to overcome the disadvantages of heat leak through the evaporator sidewall and easy deformation of the flat evaporator. The evaporator was composed of two materials: red copper (heating surface) and 316L stainless steel (upper part), and it had features that a reinforced rib structure was designed on the heating surface to improve the strength of the evaporator. The two sintered nickel wicks were incorporated inside the evaporator. In the horizontal orientation, startup performance and response to variable heat load of the proposed LHP were tested with the heating block temperature below 80 oC. The experimental results showed that the LHP with a composite evaporator could operate stably at heat load range from 10 W to 140 W at heat sink temperature of 25 oC and 35 oC,
and had better startup performance. Figure 1 gave the temperature difference
between the evaporator outlet and the evaporator back. With the increase in heat load, the LHP with a composite-material evaporator had a increasingly larger temperature difference than that of the LHP with a single-material evaporator. Compared to the LHP with a single-material evaporator, heat leak to the CC through the evaporator sidewall was effectively reduced.
Figure 1 The temperature difference between the evaporator outlet and the evaporator back for the evaporator with the same structure
Highlights： (1) A composite-material evaporator has been proposed for the flat type loop heat pipes. (2) The LHP with a composite-material evaporator was studied for reducing heat leak through the evaporator sidewall. (3) The proposed LHPs could work at heat load range from 10W to 140W with the heating surface temperature below 80 oC. (4) A way to design the flat evaporator has been put forward for further facilitating its development.
Conflict of interest There are no conflicts of interest. This work is not submitted to other journals at the same time, and all the co-authors agree to submit this paper to Applied Thermal Engineering.
S1359-4311(19)35932-0 https://doi.org/10.1016/j.applthermaleng.2019.114897 ATE 114897
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
25 August 2019 11 December 2019 31 December 2019
Please cite this article as: H. Song, Z. ping, L. Wei, L. Zhi-chun, Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114897
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Ltd.
Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics He Song, Zhou ping, Liu Wei, Liu Zhi-chun* ( School of Energy and Engineering Power, Huazhong University of Science and Technology, Wuhan, 430074, China)
Abstract: A loop heat pipe (LHP) is one of the most efficient two-phase heat transfer devices. During its operation, a portion of the heat load applied to the evaporator is transferred to its compensation chamber (CC) through the evaporator sidewall, which is known as a heat leak, decreasing the performance of the LHP startup to a certain degree. To reduce a heat leak through the evaporator sidewall, an LHP with a composite-material evaporator was proposed. The evaporator is composed of two types of material, namely, red copper (heating surface) and 316L stainless steel (upper part), and has a reinforced rib structure on the heating surface to improve the evaporator strength. Two sintered nickel wicks are incorporated inside the evaporator. The experimental results demonstrate that the LHP with a composite-material evaporator can operate successfully within a heat load range of 10 to 140 W while heating the surface to below 80 °C at a heat sink temperature of 25 °C and 35 °C. Compared with an LHP with the same evaporator structure (but with a different in material) [17], the temperature difference between the evaporator outlet and the CC left (right) is smaller under the same heat load, indicating that the heat leak through the evaporator sidewall is reduced. Keywords: loop heat pipe, composite-material evaporator, reinforced rib structure, large thermal footprint 1. Introduction Loop heat pipes (LHPs) are two-phase heat transfer devices with a capillary pumping of the working fluid, and are promising candidates for the cooling of
*
Corresponding author: Address: 318 Power Building, 1037 Luoyu Road, Hongshan District, Wuhan 430074, China. Tel.: +86 27 87542618; Fax: +86 27 87540724; E-mail: [email protected] (Liu Zhi-chun).
electronics. An LHP system consists of an evaporator, a wick, vapor/liquid transport lines, and a condenser. The evaporator is isolated from the condenser, and the two are joined by transport lines, forming a closed loop. Thus, LHPs have the advantages of a high heat flux, heat transfer over a long distance, and a small temperature difference [1]. Chips used for a server, switcher, and router, among other components, have a large thermal footprint (area that is cooled), and at the same time operate with a high heat flux. Traditional cooling methods cannot meet the required heat dissipation, and new methods are urgently needed. Theoretical and experimental studies have shown that LHPs have broad prospects in the field of electronic cooling systems and in the thermal management of spacecraft [2, 3]. Since the first conception of an LHP in the 1970s, extensive studies have been conducted to determine the fundamental principle of its operating process [4]. For LHP system components, the evaporator is one of the most important parts determining the operating performance. With the development of LHP technology, the evaporator structure can be roughly divided into two categories: cylindrical and flat [5]. The first LHP had a cylindrical evaporator, and achieved an easy fabrication, heat transfer capability over longer distances, and a large capillary force, although they required a wide space for installation. In the miniaturization of electronic devices, a small amount of space is usually reserved for heat transfer devices, and therefore flat plate LHPs have proven to be more efficient within a narrow space. At the inception of LHP technology, flat-plate LHPs were interrupted because of the complexity of the flat evaporator, and were again stimulated through the development of high-power semiconductor electronics. In terms of electronic device packaging technology, flat plate LHPs have numerous advantages, such as a large heat transfer area and small installation space, and are more suitable for practical engineering applications. Thus, various types of flat evaporators have been developed. Numerous flat plate LHPs with methanol and ammonia as the working fluid using an opposite replenishment method have been investigated [6-14], and the thicknesses of these evaporator structures were no less than 10 mm, as limited by the dimensions of the compensation chamber (CC) and the diameter of the transport lines. To decrease the
thickness of the evaporator, the CC was designed to be laterally adjacent to the active zone of the evaporator (heating surface) [15, 16] and the evaporator thickness only depends on the installation space. In addition, some flat plate LHPs with a squared evaporator and a disk-shaped evaporator using a bimetal wall have also been proposed for practical engineering applications or for solving certain problems faced by flat evaporators [17-21]. If LHPs with a flat evaporator are to be applied in the cooling of electronic elements in the future, certain disadvantages need to be addressed, such as the small absorbing area of the wick, the deformative nature of the evaporator, and a serious heat leak from the heating surface to the CC, the second two being the most significant. The former impedes the further increase of the heating surface area, which is closely related with absorbing area of the wick. In addition, the deformation affects the contact quality between the heating surface and the electronic element, even destroying the porous wick inside the evaporator owing to an uneven force. The latter makes it extremely difficult to establish a temperature difference between the CC and the vapor grooves, which corresponds to the pressure difference circulating the working fluid inside the loop. Aiming at the problems mentioned above, a modified evaporator structure was proposed to improve its ability against a high pressure, and includes two wicks placed on two sides of the reinforced rib structure, avoiding the destruction of the heating surface. The reinforced rib structure is placed through the groove on the upper part, and the heating surface is joined with the upper part through argon arc welding. The CC is formed using two parts shared by a heating surface. This evaporator structure can be regarded as two flat evaporators laterally parallel in the LHP system. It is made of red copper (acting as the heating surface) and 316L stainless steel (upper part) to reduce heat leaking through the evaporator sidewall, and is called a composite-material evaporator. A part of the heating surface is red copper with a high conductivity coefficient of 393 W/(m·K), and the evaporator sidewall is stainless steel with a low conductivity coefficient of 16.2 W/(m·K). Under these circumstances, most of the heat load applied is transferred to the porous wick through the heating surface for liquid
evaporation. Very little heat is conductive to the CC through the evaporator sidewall. In this study, the operating characteristics of an LHP with a composite-material evaporator were systematically investigated, and the effect of the reinforced rib structure on the heat transfer capacity was also analyzed. This type of approach can provide researchers and engineers with useful guidelines for designing a flat evaporator structure, facilitating its further development. For example, the CC can be divided into multiple interconnected parts to extend the area of the heating surface, realizing the full potential of flat plate LHPs. 2. Experimental setup The structural parameters of the LHP components are shown in Table 1, and the 3D geometrical model of the composite-material evaporator is shown in Figure 1. The corresponding LHP system was designed to test its heat transfer characteristics. An image of the LHP with a composite-material evaporator is shown in Figure 2. In our experiments, the LHP system was placed in a horizontal orientation. Before charging the working fluid, the LHP system was vacuumed to a pressure of 10-4 Pa. R245fa was chosen to be the working fluid, and the charge ratio of the working fluid was calculated based on the following equations: α V total V cc V ll ε V wick 1 V cond 2
(1)
α V total V cc V ll ε V wick V cond
(2)
where α is the charge ratio, Vtotal is the total internal volume of the LHP system, Vll is the internal volume of the liquid line, Vwick is the wick volume, ε is the porosity of the wick, and Vcond is the internal volume of the condenser. For normal operation of the LHP system, the wick and liquid transport line should be filled with the working fluid, while the condenser and the CC contained two phase depending on heat load. The calculated result indicates that α ranges from 0.81 to 0.85. Because the CC has the largest volume in the LHP system, the charge ratio was determined to be 85% to allow the CC to be filled with liquid.
Table 1 Structural parameters of the LHP components (unit: mm) Evaporator
Heating surface
Nickel wick
Vapor transport line Liquid transport line
Tube-in-tube condenser
Overall dimensions
80(L)×80(W) ×21(H)
Groove on the upper part
52(L)×2(W) ×12(H)
Wall thickness
3
Reinforced ribbed structure
65(L)×2(W) ×17.7(H)
Grooves on the heating surface
65(L)×2(W) ×1(H)
Compensation chamber
70(L)×61(W) ×9(H)
Overall size
65(L)×36(W) ×4.5(H)
Permeability
2.39×10-13
porosity
70%
Length
270
Inner/outer diameter
5/6
Length
280
Inner/outer diameter
5/6
Length
910
Inner/outer diameter for inner tube
5/6
Inner/outer diameter for inner tube
18/20
Annotation: L— length; W—width; H—height
Figure 1 Sectional view of the evaporator model A tube-in-tube condenser was adopted, and the circulating water was controlled at 25 °C and 35 °C. A total of 16 T-type thermocouples with an accuracy of ± 0.5 °C were used to test the characteristic point temperature, and were attached to a Keithley 2700 data acquisition system. Characteristic points along the loop are shown in Figure 2, namely, Tc1-Tc4 for the heating surface, Tc5 for the evaporator outlet, Tc7 for the condenser inlet (on the vapor line), Tc9 for the condenser outlet (on the liquid line),
Tc10 for the evaporator inlet, Tc11-Tc12 for the evaporator back (for indirectly measuring the CC temperature), Tc8 and Tc14 for the coolant temperature, and Tc16 for the ambient temperature. A rectangular block embedded with five heating rods of 100 W acts as a simulated heat source, and the applied heat load is controlled by a voltage regulator and power meter with an accuracy of ±0.1 W.
Figure 2 Photograph of the LHP system with a composite-material evaporator and the location of thermocouples on the loop 3. Description of evaporator heat transfer The distribution of heat flow for an evaporator with a reinforced rib plate is shown in Figure 3. The applied heat load Qa is roughly divided into five parts: heat for liquid vaporization Qfg, heat conduction through the evaporator sidewall Qhl,wall, heat conduction through the wick Qhl,wick, heat conduction through the reinforced rib plate Qhl, r, and heat dissipation into the ambient air Qloss. The heat absorbed by the evaporator heating surface for Qa is mainly used to evaporate the liquid, and the rest is as follows: First, the latent heat of the working fluid, Qfg, takes up the largest share of the applied heat load. Second, a heat leak to the CC, Qhl, consists of three parts, namely, Qhl,wall, Qhl,wick, and Qhl, which can be expressed through the following equation: Q hl Q hl,wall Q hl,wick Q hl,r
(3)
Third, a small part of the heat input directly exchanges with the ambient air. The CC temperature is determined based on the amount of heat leak and subcooled liquid entering it. A heat leak leads to an increase in CC temperature and thus the temperature difference between the CC and vapor grooves on the heating surface is decreased. This temperature difference corresponds to the pressure difference that
circulates the working fluid in the loop. The normal circulation of the working fluid in the loop is an indicator of a successful startup. Therefore, decreasing the heat leak contributes to the startup process. The composite-material evaporator can help reduce a heat leak to the CC through the evaporator sidewall, theoretically achieving a better startup performance.
Figure 3 Analysis of heat flow inside the composite-material evaporator 4 Results and discussions In the LHP system, the startup performance and continuous operation with a variable heat load are the two important indicators. The thermal characteristics of the LHP are influenced by numerous factors, such as the heat load, heat sink, ambient temperature, and physical properties of the working fluid. In this study, the startup performance was tested at two heat sink temperatures of 25 °C and 35 °C, and the effect of the ambient temperature was also considered. 4.1 Startup characteristic at different heat sink temperatures The startup processes from 10 to 140 W were tested with a heating surface temperature of below 85 °C. The experimental results showed that the LHP with a composite-material evaporator has a similar startup behavior as flat plate LHPs with conventional evaporators [13, 14]. Figures 4 and 5 show the startup processes with a 10 W LHP system at heat sink temperatures of 25 °C and 35 °C. Vapor was quickly generated in the evaporator, and the LHP system eventually entered into a steady state when heat load was applied to the heating surface. For a heat sink temperature of 25 °C, the startup process was clearly divided into two stages: an initial stage and a steady state stage. During the initial stage of the startup process, when
heat load was provided, it was first absorbed by the heating surface owing to the evaporator’s own thermal capacity. A small portion of the heat was transferred to the liquid for evaporation, particularly for an LHP with a composite-material evaporator at a low heat load. It took approximately 58.5 min to enter into a steady state. The heating surface maintained a higher temperature than the steady state temperature. Once the absorbing heat process of the heating surface was over, the heating surface temperature decreased rapidly owing to a liquid vaporization. The LHP system entered into a steady state stage, and the heating surface remained at 27 °C. The difference between the highest temperature and the steady state temperature was 6.1 °C during the startup process.
Figure 4 Startup process with 10 W
Figure 5 Startup process with 10 W
at heat sink temperature of 25 oC
at heat sink temperature of 35 oC
The evaporator’s own thermal capacity was an important factor during the startup process when heat load was extremely small. During the initial stage, the temperature of heating surface, evaporator inlet and outlet, and the condenser inlet and outlet increased together. However, the condenser inlet temperature was lower than the condenser outlet temperature, which is abnormal compared with the common startup process of an LHP. The heat absorbed by the evaporator resulted in liquid evaporation in the CC, and the generated vapor then entered into the condenser through the liquid transport line. Thus, a peculiar phenomenon was observed during the initial stage. When the superheat was satisfied, liquid evaporation immediately occurred in the vapor grooves. The heating surface temperature decreased rapidly, and the working fluid
circulation in the loop returned to normal. The startup process at 10 W showed an obvious temperature overshoot at a heat sink temperature of 35 °C. The superheating of the heating surface reached 41 °C at approximately 40 min. The circulation of the working fluid in the loop showed a similar trend under two different heat sink temperatures. The startup process at 10 W did not require a long platform period at a heat sink temperature of 35 °C. At a higher heat sink temperature, the time required for superheating was shorter, and the temperature between the highest temperature and the steady state temperature was smaller. With an increase in heat sink temperature, the temperature of the working fluid in the loop was also improved before the heat load was provided. Prior to reaching a steady state, more liquid was driven into the evaporator, which laid a good foundation for liquid evaporation.
Figure 6 Startup process with 120W at heat sink temperature of 25 oC
Figure 7 Startup process with 120W at heat sink temperature of 35 oC
Figures 6 and 7 show the startup process at 120 W, and the LHP system quickly started and subsequently reached a steady state. For a large heat load, a short time was required for the absorbing heat process of the heating surface to be completed. Vapor was immediately generated in the vapor grooves and the circulation of the working fluid was sped up. Thus, the heat sink temperature had little effect on the startup process, and no temperature overshoot was observed during this time. Finally, the heating surface temperature remained at approximately 64 °C. 4.2 Effect of ambient temperature
Figure 8 Startup process with 20 W
Figure 9 Startup process with 90 W
During the experiment, the effect of the ambient temperature on the steady state was considered. Under a constant heat sink temperature, the room temperature was controlled using an air conditioner. Figures 8 and 9 show the startup process at 20 W and 90 W, respectively. When the LHP system entered a steady state, the room temperature was periodically changed with the aid of an air conditioner. It can be seen that the heating surface temperature was little affected by the room temperature. The temperatures on the left and right of the CC experienced slight oscillations at a heat load of 90 W, whereas no oscillating behavior was shown at 20 W. The back of the evaporator exchanged heat through convection. When the ambient temperature was changed, heat loss through the back of the evaporator also differed. With an increase in the heat load, both the mass flow rate of the working fluid and the heat loss through the back of the evaporator were increased. The thermal equilibrium of the CC was periodically broken by the change in ambient temperature. Thus, the temperatures on the left and right of the CC oscillated periodically. The experimental results show that the change in ambient temperature only affected the temperature of certain characteristic points, but had little influence on the steady state temperature. 4.3 Transient response to variable heat load In an engineering application, the heat load often varies depending on the outside conditions, such as the central processor unit. Thus, the response to a variable heat load was tested to identify the LHP system. The heat load was changed in steps of 10 W from 10 W to 140 W approximately every 30 min, and the thermal response curve is as
shown in Figure 10. With a heating surface temperature of below 80 °C, the LHP system can successfully shift from the previous steady state to the next state, showing good stability. When the heating surface temperature does not exceed 60 °C and 80 °C, the maximum heat load is 110 and 140 W, respectively. The present LHP can address the thermal issues of a CPU with a thermal design power (TDP) equal to 140 W when the heat sink temperature is controlled at 25 °C.
Figure 10 Continuous operation for LHP with a composite-material evaporator 4.4 Characteristic point temperature analysis Based on the continuous operation above, figure 11 gives the dependence of the characteristic point temperature on the heat load. With an increase in the heat load, the temperatures of the heating surface, evaporator inlet, and condenser inlet also increase. With a heat load of higher than 80 W, the evaporator inlet temperature gradually deviates from the outlet temperature of the condenser. For the temperature of the evaporator outlet and the condenser inlet, this phenomenon begins occurring at a heat load of 110 W. When the heat load is gradually increased, the circulation flow rate is improved. The heat exchange of the working fluid with the ambient air is intensified with the heat load. The heat exchange intensification in the liquid line is lower than that in the vapor line because of the relatively small flow rate of the liquid. Thus, the temperature difference between the evaporator inlet and the condenser outlet is smaller. It can be seen from Figure 11 that a portion of the heat is lost to the ambient air when vapor moves in the vapor line, whereas the heat is obtained when the liquid moves through the liquid line.
Figure 11 The dependence of characteristic point temperature on heat load
Figure 12 Temperature difference comparison of two evaporators
Figure 12 shows the change in temperature difference between the evaporator outlet and the back of the evaporator for the same evaporator structure with different materials. As Figure 3 indicates, the temperature of the back of the evaporator is determined by the heat conduction through the evaporator sidewall, the reinforced rib structure, and the wicks. Thus, under the same heat load, the greater the temperature difference is, the less heat leak to the CC that occurs. At a relatively low heat load, the LHP with a composite-material evaporator possesses a lower evaporator inlet temperature, and the greater the temperature difference that occurs between the evaporator outlet and the back of the evaporator when compared to an evaporator with a single-material evaporator, as described in Ref. [18]. A heat leak to the CC can be efficiently compensated, further lowering the LHP operating temperature. The temperature difference between the back of the evaporator (representing the CC) and the evaporator outlet (representing the evaporating surface of the wick) corresponds to the pressure drop. In Ref. [1], it was indicated that such a pressure drop should be equal to the sum of the pressure losses in all sections of the circulation of the working fluid except for the wick. Therefore, the greater the temperature difference is between the back of the evaporator and the evaporator outlet, the better the circulation created in the working fluid within the loop. 5 Heat transfer performance To analyze and evaluate the heat transfer capacity of the evaporator with a
reinforced rib plate and its corresponding LHP system, RLHP for the LHP total thermal resistance, Revap for the evaporator thermal resistance, and Ke for its heat transfer coefficient are calculated based on the following equations: R evap
T e T e outlet Qa
R LHP
Ke
T e T cond Qa
Qa F e (T e T e outlet )
(4) (5) (6)
where Te is the heating surface temperature (average of Tc1, Tc2, Tc3, and Tc4), Teoutlet
is the evaporator outlet temperature (Tc5), Tcond is the average temperature of the
condenser inlet (Tc7) and outlet temperature (Tc9), Qa is the heat load applied to the evaporator, and Fe is the effective heating surface area.
Figure 13 Dependence of thermal performance on heat load Figure 13 shows the dependence of the thermal characteristics on the heat load. Here, Revap indicates the relative stability except at 10 W, which is maintained at approximately 0.095 W/K. On the one hand, the thermal resistance resulting from the heating surface was smallest for red copper, which has the highest thermal conductivity among common metals, and more heat was directly transferred to the wick surface for liquid vaporization. On the other hand, the evaporator sidewall was made of stainless steel, which has a relatively low thermal conductivity, and less heat was transferred to the CC through this sidewall. Considering the two factors above, the evaporator was able to transfer heat with a small temperature difference. Thus, the thermal resistance of the evaporator remained relatively stable. However, the trend in RLHP first decreased,
and then increased with a heat load of higher than 60 W. The total thermal resistance of the LHP is related with the thermal resistance of the evaporator and the condenser and the heat exchange with the ambient air. Based on Equation (5), RLHP was underestimated because of the heat exchange with the ambient air. Thus, the thermal resistance of the condenser was the main factor increasing RLHP. With an increase in the heat load, more vapor entered into the tube-in-tube condenser for condensation. The circulating rate and temperature of the coolant was controlled by the water chiller. The condenser inlet temperature was improved, resulting in an increase in Tcond. For the composite-material evaporator, Ke was maintained at a high level except for at 10 W. In [11] and [14], the authors pointed out that there exist two modes of nucleate boiling and vapor film evaporation. At relatively low heat loads, the evaporator, CC, liquid transport line, and half of the condenser should be filled with liquid owing to their operation at a constant heat conduction depending on the temperature analysis. Thus, nucleate boiling first occurred in the evaporator, and the heat transfer coefficient gradually increased. At relatively high heat loads, liquid was dispelled from the vapor grooves and vaporization occurred on the porous wick surface. With an increase in the heat load, more pores on the porous wick were utilized for pumping the liquid, and a vapor film gradually formed on it. Thus, the heat transfer coefficient showed a decreasing tendency. For the same cooling efficiency as the condenser, the thermal resistance of the evaporator played a leading role. Therefore, RLHP showed a trend similar as Revap. The resulting uncertainty analysis was evaluated according to the calculations in [22] and [23]. The resulting uncertainty of RLHP, Revap, and Ke were 6.5%, 4.8% and 5.6%, respectively. 6 Conclusions In this study, focusing on certain disadvantages of a flat evaporator, a compositematerial evaporator was proposed for use in flat plate LHPs. At the same time, an LHP with a composite-material evaporator was designed to test its heat transfer characteristics under the conditions of a horizontal orientation and different heat sink temperatures. Some conclusions drawn from the experimental results are as follows:
(1) The proposed LHP with a composite-material evaporator showed a good startup performance and a quick response to the heat load recycle at a heat load range of 10 to 150 W. (2) The proposed evaporator could solve thermal issues with an area of 64 cm2, and achieve good temperature uniformity. (3) The composite-material evaporator can effectively reduce the heat leak through the evaporator sidewall, forming a larger temperature difference between the evaporator back and the evaporator outlet. (4) The evaporator heat transfer coefficient was maintained at a high level except at 10 W. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51776079 and 51376004), and the National Key Research and Development Program of China (Grant No. 2017YFB0603501-3).
References [1] Maydanik Y F. Review: loop heat pipes. Applied Thermal Engineering, 25 (2005): 635-657 [2] Swanson T D, Birur G C. NASA thermal control technologies for robotic spacecraft. Applied Thermal Engineering, 23(2003): 1055–1065 [3] Ku J, Ottenstein L, Douglas D. Technology overview of a multi-evaporator miniature loop heat pipe for spacecraft applications. Journal of Spacecraft and Rockets, 2012, 6(49): 999-1007. [4] Amrit A, Abhijit A A, Jasvanth S V, et al. Loop heat pipes: A review of fundamentals, operation, and design. Heat Transfer Engineering, 2012, 33(4-5): 387405 [5] Maydanik Y F, Chernysheva M A, Pastukhov V G. Review: Loop heat pipes with flat evaporators. Applied Thermal Engineering, 67(2014): 294-307 [6] Li H, Liu Z C, Chen B B, et al. Development of biporous wicks for flat-plate loop heat pipe. Experimental Thermal and Fluid Science, 37 (2012): 91-97
[7] Liu Z C, Li H, Chen B B, et al. Operational characteristics of flat type loop heat pipe with biporous wick. International Journal of Thermal Sciences, 58 (2012):180-185 [8] Liu Z C, Wang D D, Jiang C, et al. Experimental study on loop heat pipe with twowick flat evaporator. International Journal of Thermal Sciences, 94 (2015): 9-17 [9]Chen B B, Liu Z C, Liu W, et al. Operational characteristics of two biporous wicks used in loop heat pipe with flat evaporator. International Journal of Heat and Mass Transfer, 55 (2012): 2204-2207 [10] Chen B B, Liu W, Liu Z C, et al. Experimental investigation of loop heat pipe with flat evaporator using biporous wick. Applied Thermal Engineering, 42 (2012): 34-40 [11] Wang D D, Liu Z C, Shen J, et al. Experimental study of the loop heat pipe with a flat disk-shaped evaporator. Experimental Thermal and Fluid Science, 57 (2014): 157164 [12] Wang D D, Liu Z C, He S, et al. Operational characteristics of a loop heat pipe with a flat evaporator and two primary biporous wicks. International Journal of Heat and Mass Transfer, 89 (2015): 33-41 [13] He S, Liu Z C, Zhao J, et al. Experimental study of an ammonia loop heat pipe with a flat plate evaporator. International Journal of Heat and Mass Transfer, 102 (2016): 1050-1055 [14] He S, Liu Z C, Wang D D, et al. Investigation of the flat disk-shaped LHP with a shared compensation chamber. Applied Thermal Engineering, 104 (2016):139-145 [15] Becker S, Vershinin S, Sartre V, et al. Steady state operation of a copper-water LHP with a flat-oval evaporator. Applied Thermal Engineering, 31 (2011): 686-695 [16] Tian W, He S, Liu Z C, et al. Experimental Investigation of a Miniature Loop Heat Pipe with Eccentric Evaporator for Cooling Electronics. Applied Thermal Engineering, 159(2019)113982 [17] Liu Z C, Gai D X, Li H, et al. Investigation of impact of different working fluids on the operational characteristics of miniature LHP with flat evaporator. Applied Thermal Engineering, 31 (2011): 3387-3392 [18] He S, Zhao J, Liu Z C, et al. Experimental investigation of loop heat pipe with a large squared evaporator for cooling electronics. Applied Thermal Engineering, 144
(2018): 383-391 [19] Li J, Lin F, Wang D M, et al. A loop-heat-pipe heat sink with parallel condensers for high-power integrated LED chips. Applied Thermal Engineering, 56 (2013): 18-26 [20] Li J, Wang D M, “Bud” Peterson G P. A compact loop heat pipe with flat square evaporator for high power chip cooling. IEEE Trans. Compon. Packag. Manu. Technol, 2011, 1 (4): 519-527 [21 Maydanik Y F, Vershinin S V, Chernysheva M A, et al. Experimental Study of an Ammonia Loop Heat Pipe with a Flat Disk-shaped Evaporator Using a Bimetal Wall. Applied Thermal Engineering, 126(2017): 643-652 [22] Coleman H W, Steele W G. Experimentation, Validation, and Uncertainty Analysis for Engineers. Third edition [23] Robert J. Describing the uncertainties in the experimental results. Experimental Thermal Fluid Science, 1 (1988):3–17
Graphical abstract: A loop heat pipe (LHP) with a composite-material evaporator has been proposed to overcome the disadvantages of heat leak through the evaporator sidewall and easy deformation of the flat evaporator. The evaporator was composed of two materials: red copper (heating surface) and 316L stainless steel (upper part), and it had features that a reinforced rib structure was designed on the heating surface to improve the strength of the evaporator. The two sintered nickel wicks were incorporated inside the evaporator. In the horizontal orientation, startup performance and response to variable heat load of the proposed LHP were tested with the heating block temperature below 80 oC. The experimental results showed that the LHP with a composite evaporator could operate stably at heat load range from 10 W to 140 W at heat sink temperature of 25 oC and 35 oC,
and had better startup performance. Figure 1 gave the temperature difference
between the evaporator outlet and the evaporator back. With the increase in heat load, the LHP with a composite-material evaporator had a increasingly larger temperature difference than that of the LHP with a single-material evaporator. Compared to the LHP with a single-material evaporator, heat leak to the CC through the evaporator sidewall was effectively reduced.
Figure 1 The temperature difference between the evaporator outlet and the evaporator back for the evaporator with the same structure
Highlights： (1) A composite-material evaporator has been proposed for the flat type loop heat pipes. (2) The LHP with a composite-material evaporator was studied for reducing heat leak through the evaporator sidewall. (3) The proposed LHPs could work at heat load range from 10W to 140W with the heating surface temperature below 80 oC. (4) A way to design the flat evaporator has been put forward for further facilitating its development.
Conflict of interest There are no conflicts of interest. This work is not submitted to other journals at the same time, and all the co-authors agree to submit this paper to Applied Thermal Engineering.