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Experimental study on transient performance of the loop heat pipe with a pouring porous wick
Applied Thermal Engineering 164 (2020) 114450
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on transient performance of the loop heat pipe with a pouring porous wick
T
Song Hea, Ping Zhoua, Zhengyuan Maa, Weizhong Denga, Hao Zhanga, Zhaokun Chib, Wei Liua, ⁎ Zhichun Liua, a b
School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Wuhan Yongxinmei Green Construction New Technology Co., Ltd, Wuhan 430061, China
H I GH L IG H T S
heat pipe with a pouring porous wick for low-cost manufacturing was designed and tested. • Loop could start up from 5 W to 80 W with the evaporator surface temperature below 100 °C. • LHP • The maximum heat flux of the LHP was in the horizontal orientation 11.33 W/cm . 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Two-phase heat transfer Loop heat pipe Pouring porous wick Cooling electronics Low-cost production
The cost of manufacturing loop heat pipes (LHP) makes it difficult for engineering applications to be popularized on the ground. As for LHP components, the wick fabrication is the most difficult, important and expensive process. In this paper, we adopted a new way of the pouring porous wick. The morphology of the pouring porous wick was observed with the aid of scanning electronic micro-scope (SEM). Surface morphology analysis revealed that pore distribution was relatively uniform, and the pore size roughly distributed between 3 μm and 10 μm. At high resolution, Micro-structure of the pouring porous wick was similar to that of sintered biporous wick that distributed large pores and small pores. The selected samples’ porosity was tested by means of Archimedes drainage method, and ranged from 0.35 to 0.45. Meanwhile, the LHP with a pouring porous wick was constituted, and its transient performance was investigated at different heat sink temperatures. The experimental results demonstrated that this LHP system could start up at a heat load range from 5 W to 80 W with the evaporator surface temperature below 100 °C in the horizontal orientation. The LHP heat transfer characteristics were analyzed based on continuous operation with variable heat load.
1. Introduction Loop heat pipes (LHPs) have been regarded as one of the most promising candidates for cooling electronics in many fields, not just limited in aerospace field. They are a kind of two-phase heat transfer device using capillary force produced by porous wick inside the evaporator to circulate the working fluid without any external power [1]. Since the conception of LHPs was born, researchers have made much effort to investigate the LHP heat transfer performance for adapting to engineering applications [2,3]. Fortunately, many successful examples in aerospace fields have appeared in the past years [4,5], which demonstrated the LHP feasibility as thermal control technology. Owing to expensive cost of manufacturing the LHP system, commercial applications on the ground are seriously limited. ⁎
The LHP system mainly consists of the capillary evaporator, the condenser and the liquid/vapor transport lines. The fabrication of the capillary evaporator takes up a large share of cost. In Ref. [6,7], they proposed a common approach to manufacture the cylindrical evaporator for the LHP, which greatly reduced the cost. At the same time, lots of the LHP with a cylindrical evaporator were successfully used in LED lights on the streets, TEG (thermoelectric generator) and SWH (solar water heater). In fact, the capillary evaporator is integrated with a porous wick and a compensation chamber (CC). Although this method dramatically reduced the production cost of the LHP, the price of a set of 100 W LHP still needed more USD 20. Based on the operating principle of the LHP, porous wick works as the capillary pump to drive the circulation of the working fluid. It plays two important roles in the LHP system: providing capillary force and
Corresponding author at: 318 Power Building, 1037 Luoyu Road, Hongshan District, Wuhan 430074, China. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.applthermaleng.2019.114450 Received 18 June 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 Available online 26 September 2019 1359-4311/ © 2019 Published by Elsevier Ltd.
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preventing heat leak from the vapor grooves and liquid/vapor mutual interference between vapor grooves and the CC. Therefore, an effective capillary evaporator must have porous wick with good thermal and hydraulic performance. In the production process of capillary evaporators, an excellent approach to manufacture a porous wick should be explored. The wick shape must match the capillary evaporator well depending on different applications. The evaporator for LHPs can be roughly divided into two categories: the cylindrical evaporator and the flat evaporator [8]. Aiming at the evaporator with different structures, the corresponding porous wick can be made in the same way. Recently, the fabrication of porous wick is mainly focused on sintering from metal powder, processing from metal foam and metal mesh. Nickel powder [9], stainless steel powder [10,11], and copper powder [12,13] are used to sinter porous wick while stainless steel mesh and copper mesh can be directly tailored and pressed to the shape based on the evaporator structure. Sintered porous wick possesses small pores and can produce high capillary force while metal mesh porous wick possesses relatively large pores and can reduce hydraulic resistance. However, the two methods aforementioned are faced with an expensive cost. So a new way to manufacture porous wick must be put forward to reduce the production cost of capillary evaporators. In this paper, we have proposed a direct, easy and cheap pouring method to manufacture a porous wick. The surface morphology of pouring porous wick was analyzed with the help of SEM, and Archimedes’ method was used to measure porosity of selected samples. The LHP with a pouring porous wick was constituted to test its heat transfer performance with the evaporator surface below 100 °C. The experimental results showed that the LHP with a pouring porous wick was equipped with good startup performance and quick response to variable heat load, the same as the LHP with a sintered porous wick. From the view of the cost of manufacturing porous wick, the pouring method was much cheaper than the sintering method. Thus, the pouring porous wick will further reduce the cost of the manufacturing the LHP system under the same heat transfer performance as sintered porous wick. Otherwise, the pouring porous wick was made from cement and adhesive, and it has lower heat conduction coefficient compared to sintered porous wick from metal powder. Heat leak through the pouring porous wick becomes small, and the temperature difference between vapor grooves and the CC will be easily established, contributing to startup process.
Fig. 1. Flow chart of pouring porous wick fabrication.
pore diameter and distribution gradually became clear. When the magnification was 500 times in Fig. 2(c), large pores and small pores could be found on the wick surface, which was the same as the nickel sintered wick in the Ref. [14]. This resulted from different particle diameter of cement and adhesive. It meant that the pouring method had the same result as the sintering method, and the biporous wick could be achieved by altering the particle diameter of cement and adhesive. At the magnification of 1000 times, it could be seen from Fig. 2(e) that pore diameter roughly distributed between 3 μm and 10 μm, and large pores were surrounded by small pores. Similar to sintered biporous wick, this pore distribution helped much to produce low flow resistance and at the same time high capillary force. 3. Comparisons with other types of porous wick Porous wick is one of the most components in the LHP system. There are many types of wicks used in the LHP system, including nickel wick, copper wick, stainless steel mesh wick, PTFE wick, polypropylene wick, composite wick, etc. Table 1 gave performance comparisons of LHPs with different types of wicks. For different types of porous wick, they have their own special functions, such as achieving fine pores, reducing heat leak through the wick and hydraulic resistance, etc. For the existing porous wicks, nickel wicks and copper wicks are the most common in the LHP system. Both of them possess advantages of high strength, fine pores and good compatibility with many working fluids, such as water, methanol, acetone, ammonia (except for copper wicks), and so on. However, porous wicks sintered from metal powder are generally equipped with complex production process and high cost. These shortcomings will limit LHP ground commercial applications in a certain degree. It can be seen from Table 1 that the LHP with a pouring porous wick has the same performance as LHPs with other types of porous wick. So the pouring method for porous wick has a potential to replace other methods because of simple, cheap and efficient production process.
2. The pouring porous wick manufacture The pouring porous wick was made of cement and adhesive without consuming extra energy during the fabrication, and this method showed safer and cheaper compared to the sintering method. The fabrication process of pouring porous wick was depicted in Fig. 1. The pouring method had low requirement for the mold, and could produce porous wick for LHPs in mass production. Depending on the shape of the evaporator, the mold could be accordingly designed. So the geometric parameters of the pouring porous wick were easily controlled. When de-molding, the appearance of samples was first detected. For the qualified samples, further tests were performed. We selected 20 samples from the first batch of products, and tested their geometric parameters. After multiple measurements, the diameter and thickness of the selected samples are (31 ± 0.2) mm and (3.4 ± 0.1) mm, respectively. In the previous production process, the yield rate exceeded more than 90%. By the means of Archimedes’ method, the samples’ porosity was tested, and their value ranged from 35% to 45%. Their surface morphology was observed with the help of SEM, Fig. 2 gave the morphologies at different magnifications. The disk-shaped porous wick was manufactured through the pouring method, and its external shape was similar to sintered porous wick [9]. The magnification was increased from 100 times to 4000 times,
4. Experimental setup One of the selected samples was chosen as the wick for the LHP, and the corresponding LHP system was constituted. In order to estimate its heat transfer performance, operating performance of the LHP with a pouring porous wick, including startup performance and continuous operation with variable heat load, was tested in the horizontal 2
Applied Thermal Engineering 164 (2020) 114450
S. He, et al.
Fig. 2. Surface morphologies of the pouring surface at different magnifications, (a) the pouring porous wick image; (b) ×100; (c) ×500; (d) ×1000; (e) ×3000; (f) ×4000.
orientation. Simultaneously, different heat sink temperatures were investigated, including 5 °C, 20 °C, and 45 °C. The structure parameters of this system were shown in Table 2. The test system diagram was shown in Fig. 3. The flat disk-shaped evaporator was designed, and made of brass. The tube-in-tube condenser was adopted. The water cooling unit was able to vary from 0 °C to 80 °C with an accuracy of ± 0.1 °C, which could provide wide temperature range for tests. T-type thermocouples were used to monitor the temperature change along the LHP loop. The temperature data was acquired through Keithley 2700, and served for analyzing the system heat transfer. For the aim of reducing the effect of the ambient, transport lines were wrapped by insulation materials. A cylinder with three 100 W heaters embedded worked as the heating source, which had a heating area of 7.06 cm2. The supplied power was regulated by voltage regulator and power meter with accuracy of 0.1 W. Methanol was chosen as the working fluid, and the charge ratio was determined at 65.7% based on Eqs. (1) and (2). It was assumed that the internal volume of liquid line (Vll), half of the condenser (1/2Vcond), CC (Vcc) and void of wick (εVwick) was filled with liquid during the operation.
Vtotal = Vgroove + Vvc + Vvl + Vcond + Vll + Vcc + ε ·Vwick
Table 2 Parameters of the LHP with a pouring porous wick (unit: mm). The evaporator
Active zone diameter Overall thickness
39 11.5
Compensation chamber Liquid line Vapor line Tube-in-tube condenser
I.D. × height O. D. × I. D. × length O. D. × I. D. × length O. D. × I. D. × length O. D. × I. D. × length Porosity diameter × thickness Pore size range
29 × 4 4 × 3 × 220 4 × 3 × 215 4 × 3 × 341 18 × 16 × 341 40% 32 × 3.5 3 ~ 10 μm
Porous wick
Notation: O. D.—outer diameter, I. D.—inner diameter.
α=
Vll + 1/2·Vcond + Vcc + ε ·Vwick Vtotal
(2)
where Vtotal was the internal volume of the LHP, Vgroove was the vapor groove volume, Vvc was the vapor chamber volume, Vvl was the vapor line internal volume and ε was the porosity of the wick. α was the charge ratio.
(1)
Table 1 Performance comparison of the LHPs with different types of wicks. Wick type [this paper]
Pouring wick Copper wick (mon-porous) [15] Nickel wick (mon-porous) [15] Nickel wick (bi-porous) [16] Nickel wick (bi-porous) [9] Copper fiber [17] PTFE wick [18] Polypropylene wick [19] Stainless steel mesh [20] Composite wick (copper powder and absorbent wool) [21] Metal foam wick [22] Nickel wick + secondary wick (bi-porous/stainless steel mesh) [23]
Evaporator shape
Working fluid
Maximum heat load (W)/ flux (W/cm2)
Operating temperature (°C)
Flat disk-shaped Flat disk-shaped Flat disk-shaped Cylindrical Flat disk-shaped Flat rectangular Cylindrical Flat rectangular Flat rectangular Flat disk-shaped Rectangular Flat disk-shaped
Methanol Water water ammonia Methanol Water Ammonia Methanol Acetone Water Ethanol ammonia
80/11.33 60/16 60/16 570/29.28 100/10.4 200/– 450/23.11 80 W/6.5 72/6 200 W/40 100/1.56 110 W/10.8
95 ≈96.1 ≈94.7 85 75 95.73 85 90 – 63 85 60
3
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Fig. 3. Diagram of the LHP test system and placement of thermocouples.
5. Experimental results and discussion
evaporation in the vapor grooves occurred easily, but heat leak to the CC also became more serious. The generated vapor needed to drive the liquid in the condenser back to the CC. Under the condition of sufficient condensing capacity, the vapor was quickly condensed at relatively low heat load. During this procedure, the front of the vapor-liquid interface moved back and forward near the inlet of the condenser. It could be seen from Fig. 4 that the condenser inlet temperature oscillations possessed the largest amplitude, further leading to other characteristic point temperature oscillations. With heat load higher than 40 W, the LHP system possessed a fast startup process and quickly entered into the steady state, as shown in Fig. 5. At relatively high heat load, heat sink temperature had weak effect on startup process. The rate of the generated vapor increased with the increase in heat load, and startup process could be quickly finished. The circulation of the working fluid was accelerated at relatively high heat load. Under this circumstance, operating temperature of the LHP system was affected by heat sink temperature. The higher heat sink temperature was, the higher the corresponding LHP operating temperature was.
A good LHP system should possess good startup process and quick response to variable heat load. During the procedure of experimental tests, we focused on startup behavior and continuous operation with variable heat load of the LHP with a pouring porous wick at different heat sink temperatures of 5 °C, 20 °C and 45 °C. 5.1. Startup tests In the horizontal orientation, startup tests were carried out at different heat sink temperatures. The experimental results showed that the LHP with a pouring porous wick could successfully start up at a heat load range from 5 W to 80 W, but accompanying with different phenomenon. In the same way, startup tests at 10 W were carried out at three heat sink temperatures. Fig. 4 gave the startup process at 10 W. The LHP startup behavior was closely associated with initial state of the working fluid in the loop. When the ambient temperature was close to the heat sink temperature, temperature oscillations showed very obvious at relatively low heat load. When heat sink temperature was much higher than the ambient temperature, startup process was accompanied with temperature overshoot, but no temperature oscillations were observed in the steady state. Prior to entering into the steady state, both the ambient temperature and heat sink temperature determined initial distribution of the working fluid in the loop. When heat sink temperature was higher than ambient temperature, two-phase section was mainly distributed in the part of the condenser and the evaporator was filled with liquid. As heat load was applied to the evaporator, the generated vapor in the vapor grooves firstly pushed the accumulated liquid in the vapor line into the condenser, and then subcooling liquid returned to the compensation chamber (CC). The liquid evaporation in the evaporator required certain superheat to overcome flow resistance of the liquid in the vapor line to the condenser. Otherwise, heat leak to the CC through the evaporator sidewall was serious, further deteriorating startup condition at relatively low heat load. Thus, temperature overshoot showed very obvious at 10 W. Once initial boiling inception occurred, the temperature of heating source immediately dropped, and the LHP system reached the steady state. When heat sink temperature was smaller than the ambient temperature, vapor grooves or the CC was two-phase state. At this time,
5.2. Operation with variable heat load In order to verify the reliability of the LHP with a pouring porous wick, operation with heat load range from 10 W to 80 W was conducted at three heat sink temperatures of 5 °C, 20 °C, and 45 °C. Under different heat sink temperatures, the LHP system had a fast response to variable heat load, and adapted well to various working conditions. During the tests of continuous operation with variable heat load, temperature oscillations and temperature overshoot were also observed. However, there existed some difference from startup tests for heat sink temperature higher than the ambient temperature. For operation with 10 W at 45 °C in Fig. 6, temperature overshoot showed obvious when the LHP started up in the cold state, but this phenomenon was not seen when heat load was decreased from 20 W to 10 W. The reason of temperature overshoot in the startup process was mentioned above. When the LHP operated continuously with heat load decreased, distribution of the working fluid in the loop had hysteresis due to inertia. Thus, Operation in the second 10 W at 45 °C showed very steady, and no temperature overshoot was observed. On one hand, the vapor line was filled with vapor. On the other hand, evaporation in the vapor grooves occurred 4
Applied Thermal Engineering 164 (2020) 114450
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Fig. 4. Startup tests with 10 W at heat sink temperature of (a) 5 °C, (b) 20 °C and (c) 45 °C.
evaporator surface increased with heat load, but both of them showed a little bit different for two cases, known as temperature hysteresis. Except for the evaporator surface temperature, other characteristic point temperature had also the same trend. With heat load increased, evaporation was gradually intensified and the circulation of the working fluid was enhanced, and vice versa. Heat transfer process was closely related with the flow of the working fluid. For continuous operation with variable heat load, evaporation behavior had taken place in the evaporator except for startup process. The flow hysteresis of the working fluid in the loop was the main factor. With heat load changed, redistribution of the working fluid occurred. It took some time for the working fluid to flow from one component to another.
prior to changing heat load from 20 W to 10 W. For tests at heat sink temperature close to the ambient temperature in Figs. 7 and 8, temperature oscillations were still observed under the condition of heat load smaller than 30 W. As long as the condenser had enough condensing capacity, the front of liquid-vapor interface moved back and forward near the inlet of the condenser. Therefore, temperature oscillations always existed at relatively low heat load under the circumstance that the condenser was not made full of use. Tests for continuous operation with variable heat load were different from startup tests. Each operation at the corresponding heat load was affected by the previous operation, further interfering with the distribution of the working fluid. Thus, some measures can be taken to achieve the LHP steady state without temperature oscillations or temperature overshoot based on the experimental results above at relatively low heat load. For example, temperature overshoot can be eliminated by the pre-operation with larger heat load, and temperature oscillations can be solved by increasing heat sink temperature.
5.4. Thermal resistance analysis The LHP resistance, evaporator resistance and evaporator heat transfer coefficient are usually used to evaluate the performance of the LHP system. They are calculated with the following equations:
5.3. Characteristic point temperature analysis The characteristic point temperature analysis was based on continuous operation with variable heat load at 5 °C. Fig. 9 gave the dependence of characteristic point temperature on heat load. The solid symbol represented the case 1 with heat load increasing from 10 W to 80 W, and the hollow symbol stood for the case 2 with heat load decreasing from 80 W to 10 W. The trend of the temperature of the
RLHP =
Te − Tcond Qa
(3)
R evap =
Te − Te − outlet Qa
(4)
Qa Fe (Te − Te − outlet )
(5)
Ke = 5
Applied Thermal Engineering 164 (2020) 114450
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Fig. 5. Startup tests with 40 W at heat sink temperature of (a) 5 °C, (b) 20 °C and (c) 45 °C.
Fig. 6. Continuous operation with 10 W-80 W-10 W at 45 °C.
Fig. 7. Continuous operation with 10 W-80 W-10 W at 5 °C.
where Te is the evaporator surface temperature, Te-outlet is the evaporator outlet temperature, Tcond is the average of the condenser inlet and outlet temperature, Qa is the applied heat load, Fe is the area of heating source. It was noted that Te, Te-outlet, and Tcond were calculated based on the steady state operating temperature for every stage.
Fig. 10 gave the dependence of total thermal resistance and evaporator thermal resistance on heat load at heat sink temperature of 5 °C, 20 °C and 45 °C. For total thermal resistance, the case for 5 °C had a wide range from 0.729 W/K to 2.959 W/K, and was above the cases for 20 °C and 45 °C. The case for 45 °C was higher than the case for 20 °C. 6
Applied Thermal Engineering 164 (2020) 114450
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For evaporator thermal resistance, the case for 5 °C increased linearly with the increase in heat load while the cases for 20 °C and 45 °C maintained relatively stable. Total thermal resistance was used to evaluate heat transfer capacity from heating source to the condenser, and was related with heat sink temperature, ambient temperature, evaporator thermal resistance and condenser thermal resistance. At relatively low heat load, the liquidvapor interface moved forward and back near the condenser inlet and the condenser had low efficiency. At this moment, the condenser thermal resistance was the main factor determining total thermal resistance. It could be seen from Fig. 10 that total thermal resistance for 5 °C was largest when evaporator thermal resistance was smallest. With heat load increased, the condensing surface was gradually opened up in the condenser and the condenser efficiency was improved. Total thermal resistance for 5 °C firstly decreased and then tended to a constant value of 0.74 W/K with heat load higher than 50 W. However, total thermal resistance for 20 °C and 45 °C had no big change within the given heat load range, and maintained around 0.43 W/K and 0.35 W/K, respectively. Under the condition of sufficient condensing capacity (the condenser outlet temperature was only determined by heat sink temperature), Tcond was increased with heat sink temperature increased, but the evaporator surface temperature didn’t change a lot with heat sink temperature. Thus, total thermal resistance generally decreased with heat sink temperature under the same heat load. In the process of testing, part of the evaporator wasn’t wrapped with thermal insulation material. Owing to heat loss to the ambient from the evaporator, the evaporator surface temperature was reduced. Total thermal resistance for 45 °C was larger than that for 20 °C. From the view of evaporator thermal resistance, the case for 5 °C showed a upward trend with the increase in heat load while the cases for 20 °C and 45 °C fluctuated in a narrow range. As heat load was gradually increased, more and more pores on the porous wick surface were motivated to suck liquid and at the same time vapor film also formed on the porous wick surface. At lower heat sink temperature, liquid returning to the CC carried more cold capacity for the same working condition. Liquid was firstly heated before evaporation. Thus, evaporator thermal resistance for 5 °C was smaller than cases for 20 °C and 45 °C with heat load less than 50 W. With heat load increased further, the circulation rate of working fluid was improved, and the effect of the returning liquid on the CC became weak. The evaporator heat transfer coefficient was related with the evaporator thermal resistance and heating surface area, and was an indicator to evaluate heat transfer capacity of the evaporator. Therefore, its trend was contrary to that of the evaporator thermal resistance.
Fig. 8. Continuous operation with 30 W-80 W-10 W at 20 °C.
Fig. 9. The characteristic point temperature analysis with heat load changed at 5 °C.
Fig. 10. The dependence of thermal resistance on heat load.
Fig. 11. The dependence of evaporator heat transfer coefficient on heat load. 7
Applied Thermal Engineering 164 (2020) 114450
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Fig. 11 gave the trend of evaporator heat transfer coefficient with heat load changed. The evaporator heat transfer coefficient for 5 °C varied from 4857 W/(m2 K) to 2847 W/(m2 K). The cases for 20 °C and 45 °C fluctuated around 3293 W/(m2 K) and 3454 W/(m2 K), respectively.
[5]
[6]
6. Conclusions
[7]
In this paper, the pouring method for porous wick used in the LHP was put forward. At different resolutions, the morphology of the selected wick sample was observed with aid of SEM. The parameters of the pouring porous wick showed that pore size mainly distributed between 3 μm and 10 μm and porosity ranged from 0.35 to 0.45. These parameters were almost similar to porous wick that was made in other ways. The corresponding LHP with a pouring porous wick was fabricated, and methanol was chosen as the working fluid. The thermal performance tests demonstrated that the LHP system were equipped with good startup performance and fast response to variable heat load. At heat sink temperature of 45 °C, the maximum heat load that it could transfer was 80 W in the horizontal orientation, and the corresponding heat flux was 11.33 W/cm2.
[8] [9] [10]
[11] [12]
[13]
[14]
Declaration of Competing Interest
[15]
The authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere.
[16] [17]
Acknowledgement [18]
This work is supported by the National Natural Science Foundation of China (Grant No. 51776079 & 51376004), and the National Key Research and Development Program of China (Grant No. 2017YFB0603501-3).
[19]
[20]
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on transient performance of the loop heat pipe with a pouring porous wick
T
Song Hea, Ping Zhoua, Zhengyuan Maa, Weizhong Denga, Hao Zhanga, Zhaokun Chib, Wei Liua, ⁎ Zhichun Liua, a b
School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Wuhan Yongxinmei Green Construction New Technology Co., Ltd, Wuhan 430061, China
H I GH L IG H T S
heat pipe with a pouring porous wick for low-cost manufacturing was designed and tested. • Loop could start up from 5 W to 80 W with the evaporator surface temperature below 100 °C. • LHP • The maximum heat flux of the LHP was in the horizontal orientation 11.33 W/cm . 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Two-phase heat transfer Loop heat pipe Pouring porous wick Cooling electronics Low-cost production
The cost of manufacturing loop heat pipes (LHP) makes it difficult for engineering applications to be popularized on the ground. As for LHP components, the wick fabrication is the most difficult, important and expensive process. In this paper, we adopted a new way of the pouring porous wick. The morphology of the pouring porous wick was observed with the aid of scanning electronic micro-scope (SEM). Surface morphology analysis revealed that pore distribution was relatively uniform, and the pore size roughly distributed between 3 μm and 10 μm. At high resolution, Micro-structure of the pouring porous wick was similar to that of sintered biporous wick that distributed large pores and small pores. The selected samples’ porosity was tested by means of Archimedes drainage method, and ranged from 0.35 to 0.45. Meanwhile, the LHP with a pouring porous wick was constituted, and its transient performance was investigated at different heat sink temperatures. The experimental results demonstrated that this LHP system could start up at a heat load range from 5 W to 80 W with the evaporator surface temperature below 100 °C in the horizontal orientation. The LHP heat transfer characteristics were analyzed based on continuous operation with variable heat load.
1. Introduction Loop heat pipes (LHPs) have been regarded as one of the most promising candidates for cooling electronics in many fields, not just limited in aerospace field. They are a kind of two-phase heat transfer device using capillary force produced by porous wick inside the evaporator to circulate the working fluid without any external power [1]. Since the conception of LHPs was born, researchers have made much effort to investigate the LHP heat transfer performance for adapting to engineering applications [2,3]. Fortunately, many successful examples in aerospace fields have appeared in the past years [4,5], which demonstrated the LHP feasibility as thermal control technology. Owing to expensive cost of manufacturing the LHP system, commercial applications on the ground are seriously limited. ⁎
The LHP system mainly consists of the capillary evaporator, the condenser and the liquid/vapor transport lines. The fabrication of the capillary evaporator takes up a large share of cost. In Ref. [6,7], they proposed a common approach to manufacture the cylindrical evaporator for the LHP, which greatly reduced the cost. At the same time, lots of the LHP with a cylindrical evaporator were successfully used in LED lights on the streets, TEG (thermoelectric generator) and SWH (solar water heater). In fact, the capillary evaporator is integrated with a porous wick and a compensation chamber (CC). Although this method dramatically reduced the production cost of the LHP, the price of a set of 100 W LHP still needed more USD 20. Based on the operating principle of the LHP, porous wick works as the capillary pump to drive the circulation of the working fluid. It plays two important roles in the LHP system: providing capillary force and
Corresponding author at: 318 Power Building, 1037 Luoyu Road, Hongshan District, Wuhan 430074, China. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.applthermaleng.2019.114450 Received 18 June 2019; Received in revised form 23 September 2019; Accepted 25 September 2019 Available online 26 September 2019 1359-4311/ © 2019 Published by Elsevier Ltd.
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preventing heat leak from the vapor grooves and liquid/vapor mutual interference between vapor grooves and the CC. Therefore, an effective capillary evaporator must have porous wick with good thermal and hydraulic performance. In the production process of capillary evaporators, an excellent approach to manufacture a porous wick should be explored. The wick shape must match the capillary evaporator well depending on different applications. The evaporator for LHPs can be roughly divided into two categories: the cylindrical evaporator and the flat evaporator [8]. Aiming at the evaporator with different structures, the corresponding porous wick can be made in the same way. Recently, the fabrication of porous wick is mainly focused on sintering from metal powder, processing from metal foam and metal mesh. Nickel powder [9], stainless steel powder [10,11], and copper powder [12,13] are used to sinter porous wick while stainless steel mesh and copper mesh can be directly tailored and pressed to the shape based on the evaporator structure. Sintered porous wick possesses small pores and can produce high capillary force while metal mesh porous wick possesses relatively large pores and can reduce hydraulic resistance. However, the two methods aforementioned are faced with an expensive cost. So a new way to manufacture porous wick must be put forward to reduce the production cost of capillary evaporators. In this paper, we have proposed a direct, easy and cheap pouring method to manufacture a porous wick. The surface morphology of pouring porous wick was analyzed with the help of SEM, and Archimedes’ method was used to measure porosity of selected samples. The LHP with a pouring porous wick was constituted to test its heat transfer performance with the evaporator surface below 100 °C. The experimental results showed that the LHP with a pouring porous wick was equipped with good startup performance and quick response to variable heat load, the same as the LHP with a sintered porous wick. From the view of the cost of manufacturing porous wick, the pouring method was much cheaper than the sintering method. Thus, the pouring porous wick will further reduce the cost of the manufacturing the LHP system under the same heat transfer performance as sintered porous wick. Otherwise, the pouring porous wick was made from cement and adhesive, and it has lower heat conduction coefficient compared to sintered porous wick from metal powder. Heat leak through the pouring porous wick becomes small, and the temperature difference between vapor grooves and the CC will be easily established, contributing to startup process.
Fig. 1. Flow chart of pouring porous wick fabrication.
pore diameter and distribution gradually became clear. When the magnification was 500 times in Fig. 2(c), large pores and small pores could be found on the wick surface, which was the same as the nickel sintered wick in the Ref. [14]. This resulted from different particle diameter of cement and adhesive. It meant that the pouring method had the same result as the sintering method, and the biporous wick could be achieved by altering the particle diameter of cement and adhesive. At the magnification of 1000 times, it could be seen from Fig. 2(e) that pore diameter roughly distributed between 3 μm and 10 μm, and large pores were surrounded by small pores. Similar to sintered biporous wick, this pore distribution helped much to produce low flow resistance and at the same time high capillary force. 3. Comparisons with other types of porous wick Porous wick is one of the most components in the LHP system. There are many types of wicks used in the LHP system, including nickel wick, copper wick, stainless steel mesh wick, PTFE wick, polypropylene wick, composite wick, etc. Table 1 gave performance comparisons of LHPs with different types of wicks. For different types of porous wick, they have their own special functions, such as achieving fine pores, reducing heat leak through the wick and hydraulic resistance, etc. For the existing porous wicks, nickel wicks and copper wicks are the most common in the LHP system. Both of them possess advantages of high strength, fine pores and good compatibility with many working fluids, such as water, methanol, acetone, ammonia (except for copper wicks), and so on. However, porous wicks sintered from metal powder are generally equipped with complex production process and high cost. These shortcomings will limit LHP ground commercial applications in a certain degree. It can be seen from Table 1 that the LHP with a pouring porous wick has the same performance as LHPs with other types of porous wick. So the pouring method for porous wick has a potential to replace other methods because of simple, cheap and efficient production process.
2. The pouring porous wick manufacture The pouring porous wick was made of cement and adhesive without consuming extra energy during the fabrication, and this method showed safer and cheaper compared to the sintering method. The fabrication process of pouring porous wick was depicted in Fig. 1. The pouring method had low requirement for the mold, and could produce porous wick for LHPs in mass production. Depending on the shape of the evaporator, the mold could be accordingly designed. So the geometric parameters of the pouring porous wick were easily controlled. When de-molding, the appearance of samples was first detected. For the qualified samples, further tests were performed. We selected 20 samples from the first batch of products, and tested their geometric parameters. After multiple measurements, the diameter and thickness of the selected samples are (31 ± 0.2) mm and (3.4 ± 0.1) mm, respectively. In the previous production process, the yield rate exceeded more than 90%. By the means of Archimedes’ method, the samples’ porosity was tested, and their value ranged from 35% to 45%. Their surface morphology was observed with the help of SEM, Fig. 2 gave the morphologies at different magnifications. The disk-shaped porous wick was manufactured through the pouring method, and its external shape was similar to sintered porous wick [9]. The magnification was increased from 100 times to 4000 times,
4. Experimental setup One of the selected samples was chosen as the wick for the LHP, and the corresponding LHP system was constituted. In order to estimate its heat transfer performance, operating performance of the LHP with a pouring porous wick, including startup performance and continuous operation with variable heat load, was tested in the horizontal 2
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Fig. 2. Surface morphologies of the pouring surface at different magnifications, (a) the pouring porous wick image; (b) ×100; (c) ×500; (d) ×1000; (e) ×3000; (f) ×4000.
orientation. Simultaneously, different heat sink temperatures were investigated, including 5 °C, 20 °C, and 45 °C. The structure parameters of this system were shown in Table 2. The test system diagram was shown in Fig. 3. The flat disk-shaped evaporator was designed, and made of brass. The tube-in-tube condenser was adopted. The water cooling unit was able to vary from 0 °C to 80 °C with an accuracy of ± 0.1 °C, which could provide wide temperature range for tests. T-type thermocouples were used to monitor the temperature change along the LHP loop. The temperature data was acquired through Keithley 2700, and served for analyzing the system heat transfer. For the aim of reducing the effect of the ambient, transport lines were wrapped by insulation materials. A cylinder with three 100 W heaters embedded worked as the heating source, which had a heating area of 7.06 cm2. The supplied power was regulated by voltage regulator and power meter with accuracy of 0.1 W. Methanol was chosen as the working fluid, and the charge ratio was determined at 65.7% based on Eqs. (1) and (2). It was assumed that the internal volume of liquid line (Vll), half of the condenser (1/2Vcond), CC (Vcc) and void of wick (εVwick) was filled with liquid during the operation.
Vtotal = Vgroove + Vvc + Vvl + Vcond + Vll + Vcc + ε ·Vwick
Table 2 Parameters of the LHP with a pouring porous wick (unit: mm). The evaporator
Active zone diameter Overall thickness
39 11.5
Compensation chamber Liquid line Vapor line Tube-in-tube condenser
I.D. × height O. D. × I. D. × length O. D. × I. D. × length O. D. × I. D. × length O. D. × I. D. × length Porosity diameter × thickness Pore size range
29 × 4 4 × 3 × 220 4 × 3 × 215 4 × 3 × 341 18 × 16 × 341 40% 32 × 3.5 3 ~ 10 μm
Porous wick
Notation: O. D.—outer diameter, I. D.—inner diameter.
α=
Vll + 1/2·Vcond + Vcc + ε ·Vwick Vtotal
(2)
where Vtotal was the internal volume of the LHP, Vgroove was the vapor groove volume, Vvc was the vapor chamber volume, Vvl was the vapor line internal volume and ε was the porosity of the wick. α was the charge ratio.
(1)
Table 1 Performance comparison of the LHPs with different types of wicks. Wick type [this paper]
Pouring wick Copper wick (mon-porous) [15] Nickel wick (mon-porous) [15] Nickel wick (bi-porous) [16] Nickel wick (bi-porous) [9] Copper fiber [17] PTFE wick [18] Polypropylene wick [19] Stainless steel mesh [20] Composite wick (copper powder and absorbent wool) [21] Metal foam wick [22] Nickel wick + secondary wick (bi-porous/stainless steel mesh) [23]
Evaporator shape
Working fluid
Maximum heat load (W)/ flux (W/cm2)
Operating temperature (°C)
Flat disk-shaped Flat disk-shaped Flat disk-shaped Cylindrical Flat disk-shaped Flat rectangular Cylindrical Flat rectangular Flat rectangular Flat disk-shaped Rectangular Flat disk-shaped
Methanol Water water ammonia Methanol Water Ammonia Methanol Acetone Water Ethanol ammonia
80/11.33 60/16 60/16 570/29.28 100/10.4 200/– 450/23.11 80 W/6.5 72/6 200 W/40 100/1.56 110 W/10.8
95 ≈96.1 ≈94.7 85 75 95.73 85 90 – 63 85 60
3
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Fig. 3. Diagram of the LHP test system and placement of thermocouples.
5. Experimental results and discussion
evaporation in the vapor grooves occurred easily, but heat leak to the CC also became more serious. The generated vapor needed to drive the liquid in the condenser back to the CC. Under the condition of sufficient condensing capacity, the vapor was quickly condensed at relatively low heat load. During this procedure, the front of the vapor-liquid interface moved back and forward near the inlet of the condenser. It could be seen from Fig. 4 that the condenser inlet temperature oscillations possessed the largest amplitude, further leading to other characteristic point temperature oscillations. With heat load higher than 40 W, the LHP system possessed a fast startup process and quickly entered into the steady state, as shown in Fig. 5. At relatively high heat load, heat sink temperature had weak effect on startup process. The rate of the generated vapor increased with the increase in heat load, and startup process could be quickly finished. The circulation of the working fluid was accelerated at relatively high heat load. Under this circumstance, operating temperature of the LHP system was affected by heat sink temperature. The higher heat sink temperature was, the higher the corresponding LHP operating temperature was.
A good LHP system should possess good startup process and quick response to variable heat load. During the procedure of experimental tests, we focused on startup behavior and continuous operation with variable heat load of the LHP with a pouring porous wick at different heat sink temperatures of 5 °C, 20 °C and 45 °C. 5.1. Startup tests In the horizontal orientation, startup tests were carried out at different heat sink temperatures. The experimental results showed that the LHP with a pouring porous wick could successfully start up at a heat load range from 5 W to 80 W, but accompanying with different phenomenon. In the same way, startup tests at 10 W were carried out at three heat sink temperatures. Fig. 4 gave the startup process at 10 W. The LHP startup behavior was closely associated with initial state of the working fluid in the loop. When the ambient temperature was close to the heat sink temperature, temperature oscillations showed very obvious at relatively low heat load. When heat sink temperature was much higher than the ambient temperature, startup process was accompanied with temperature overshoot, but no temperature oscillations were observed in the steady state. Prior to entering into the steady state, both the ambient temperature and heat sink temperature determined initial distribution of the working fluid in the loop. When heat sink temperature was higher than ambient temperature, two-phase section was mainly distributed in the part of the condenser and the evaporator was filled with liquid. As heat load was applied to the evaporator, the generated vapor in the vapor grooves firstly pushed the accumulated liquid in the vapor line into the condenser, and then subcooling liquid returned to the compensation chamber (CC). The liquid evaporation in the evaporator required certain superheat to overcome flow resistance of the liquid in the vapor line to the condenser. Otherwise, heat leak to the CC through the evaporator sidewall was serious, further deteriorating startup condition at relatively low heat load. Thus, temperature overshoot showed very obvious at 10 W. Once initial boiling inception occurred, the temperature of heating source immediately dropped, and the LHP system reached the steady state. When heat sink temperature was smaller than the ambient temperature, vapor grooves or the CC was two-phase state. At this time,
5.2. Operation with variable heat load In order to verify the reliability of the LHP with a pouring porous wick, operation with heat load range from 10 W to 80 W was conducted at three heat sink temperatures of 5 °C, 20 °C, and 45 °C. Under different heat sink temperatures, the LHP system had a fast response to variable heat load, and adapted well to various working conditions. During the tests of continuous operation with variable heat load, temperature oscillations and temperature overshoot were also observed. However, there existed some difference from startup tests for heat sink temperature higher than the ambient temperature. For operation with 10 W at 45 °C in Fig. 6, temperature overshoot showed obvious when the LHP started up in the cold state, but this phenomenon was not seen when heat load was decreased from 20 W to 10 W. The reason of temperature overshoot in the startup process was mentioned above. When the LHP operated continuously with heat load decreased, distribution of the working fluid in the loop had hysteresis due to inertia. Thus, Operation in the second 10 W at 45 °C showed very steady, and no temperature overshoot was observed. On one hand, the vapor line was filled with vapor. On the other hand, evaporation in the vapor grooves occurred 4
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Fig. 4. Startup tests with 10 W at heat sink temperature of (a) 5 °C, (b) 20 °C and (c) 45 °C.
evaporator surface increased with heat load, but both of them showed a little bit different for two cases, known as temperature hysteresis. Except for the evaporator surface temperature, other characteristic point temperature had also the same trend. With heat load increased, evaporation was gradually intensified and the circulation of the working fluid was enhanced, and vice versa. Heat transfer process was closely related with the flow of the working fluid. For continuous operation with variable heat load, evaporation behavior had taken place in the evaporator except for startup process. The flow hysteresis of the working fluid in the loop was the main factor. With heat load changed, redistribution of the working fluid occurred. It took some time for the working fluid to flow from one component to another.
prior to changing heat load from 20 W to 10 W. For tests at heat sink temperature close to the ambient temperature in Figs. 7 and 8, temperature oscillations were still observed under the condition of heat load smaller than 30 W. As long as the condenser had enough condensing capacity, the front of liquid-vapor interface moved back and forward near the inlet of the condenser. Therefore, temperature oscillations always existed at relatively low heat load under the circumstance that the condenser was not made full of use. Tests for continuous operation with variable heat load were different from startup tests. Each operation at the corresponding heat load was affected by the previous operation, further interfering with the distribution of the working fluid. Thus, some measures can be taken to achieve the LHP steady state without temperature oscillations or temperature overshoot based on the experimental results above at relatively low heat load. For example, temperature overshoot can be eliminated by the pre-operation with larger heat load, and temperature oscillations can be solved by increasing heat sink temperature.
5.4. Thermal resistance analysis The LHP resistance, evaporator resistance and evaporator heat transfer coefficient are usually used to evaluate the performance of the LHP system. They are calculated with the following equations:
5.3. Characteristic point temperature analysis The characteristic point temperature analysis was based on continuous operation with variable heat load at 5 °C. Fig. 9 gave the dependence of characteristic point temperature on heat load. The solid symbol represented the case 1 with heat load increasing from 10 W to 80 W, and the hollow symbol stood for the case 2 with heat load decreasing from 80 W to 10 W. The trend of the temperature of the
RLHP =
Te − Tcond Qa
(3)
R evap =
Te − Te − outlet Qa
(4)
Qa Fe (Te − Te − outlet )
(5)
Ke = 5
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Fig. 5. Startup tests with 40 W at heat sink temperature of (a) 5 °C, (b) 20 °C and (c) 45 °C.
Fig. 6. Continuous operation with 10 W-80 W-10 W at 45 °C.
Fig. 7. Continuous operation with 10 W-80 W-10 W at 5 °C.
where Te is the evaporator surface temperature, Te-outlet is the evaporator outlet temperature, Tcond is the average of the condenser inlet and outlet temperature, Qa is the applied heat load, Fe is the area of heating source. It was noted that Te, Te-outlet, and Tcond were calculated based on the steady state operating temperature for every stage.
Fig. 10 gave the dependence of total thermal resistance and evaporator thermal resistance on heat load at heat sink temperature of 5 °C, 20 °C and 45 °C. For total thermal resistance, the case for 5 °C had a wide range from 0.729 W/K to 2.959 W/K, and was above the cases for 20 °C and 45 °C. The case for 45 °C was higher than the case for 20 °C. 6
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For evaporator thermal resistance, the case for 5 °C increased linearly with the increase in heat load while the cases for 20 °C and 45 °C maintained relatively stable. Total thermal resistance was used to evaluate heat transfer capacity from heating source to the condenser, and was related with heat sink temperature, ambient temperature, evaporator thermal resistance and condenser thermal resistance. At relatively low heat load, the liquidvapor interface moved forward and back near the condenser inlet and the condenser had low efficiency. At this moment, the condenser thermal resistance was the main factor determining total thermal resistance. It could be seen from Fig. 10 that total thermal resistance for 5 °C was largest when evaporator thermal resistance was smallest. With heat load increased, the condensing surface was gradually opened up in the condenser and the condenser efficiency was improved. Total thermal resistance for 5 °C firstly decreased and then tended to a constant value of 0.74 W/K with heat load higher than 50 W. However, total thermal resistance for 20 °C and 45 °C had no big change within the given heat load range, and maintained around 0.43 W/K and 0.35 W/K, respectively. Under the condition of sufficient condensing capacity (the condenser outlet temperature was only determined by heat sink temperature), Tcond was increased with heat sink temperature increased, but the evaporator surface temperature didn’t change a lot with heat sink temperature. Thus, total thermal resistance generally decreased with heat sink temperature under the same heat load. In the process of testing, part of the evaporator wasn’t wrapped with thermal insulation material. Owing to heat loss to the ambient from the evaporator, the evaporator surface temperature was reduced. Total thermal resistance for 45 °C was larger than that for 20 °C. From the view of evaporator thermal resistance, the case for 5 °C showed a upward trend with the increase in heat load while the cases for 20 °C and 45 °C fluctuated in a narrow range. As heat load was gradually increased, more and more pores on the porous wick surface were motivated to suck liquid and at the same time vapor film also formed on the porous wick surface. At lower heat sink temperature, liquid returning to the CC carried more cold capacity for the same working condition. Liquid was firstly heated before evaporation. Thus, evaporator thermal resistance for 5 °C was smaller than cases for 20 °C and 45 °C with heat load less than 50 W. With heat load increased further, the circulation rate of working fluid was improved, and the effect of the returning liquid on the CC became weak. The evaporator heat transfer coefficient was related with the evaporator thermal resistance and heating surface area, and was an indicator to evaluate heat transfer capacity of the evaporator. Therefore, its trend was contrary to that of the evaporator thermal resistance.
Fig. 8. Continuous operation with 30 W-80 W-10 W at 20 °C.
Fig. 9. The characteristic point temperature analysis with heat load changed at 5 °C.
Fig. 10. The dependence of thermal resistance on heat load.
Fig. 11. The dependence of evaporator heat transfer coefficient on heat load. 7
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Fig. 11 gave the trend of evaporator heat transfer coefficient with heat load changed. The evaporator heat transfer coefficient for 5 °C varied from 4857 W/(m2 K) to 2847 W/(m2 K). The cases for 20 °C and 45 °C fluctuated around 3293 W/(m2 K) and 3454 W/(m2 K), respectively.
[5]
[6]
6. Conclusions
[7]
In this paper, the pouring method for porous wick used in the LHP was put forward. At different resolutions, the morphology of the selected wick sample was observed with aid of SEM. The parameters of the pouring porous wick showed that pore size mainly distributed between 3 μm and 10 μm and porosity ranged from 0.35 to 0.45. These parameters were almost similar to porous wick that was made in other ways. The corresponding LHP with a pouring porous wick was fabricated, and methanol was chosen as the working fluid. The thermal performance tests demonstrated that the LHP system were equipped with good startup performance and fast response to variable heat load. At heat sink temperature of 45 °C, the maximum heat load that it could transfer was 80 W in the horizontal orientation, and the corresponding heat flux was 11.33 W/cm2.
[8] [9] [10]
[11] [12]
[13]
[14]
Declaration of Competing Interest
[15]
The authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere.
[16] [17]
Acknowledgement [18]
This work is supported by the National Natural Science Foundation of China (Grant No. 51776079 & 51376004), and the National Key Research and Development Program of China (Grant No. 2017YFB0603501-3).
[19]
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