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A new way of supercritical startup of a cryogenic loop heat pipe
International Journal of Heat and Mass Transfer 145 (2019) 118793
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
A new way of supercritical startup of a cryogenic loop heat pipe Ya-nan Zhao ⇑, Tao Yan, Jingtao Liang, Nailiang Wang Key Laboratory of Space Energy Conversion Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
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Article history: Received 19 August 2019 Received in revised form 26 September 2019 Accepted 26 September 2019 Available online 5 October 2019 Keywords: Cryogenic Loop heat pipe Heat transfer Supercritical startup Porous wick
a b s t r a c t Space applications require low power consumption, light weight and no gravity-assistant heat transfer devices. Cryogenic loop heat pipe (CLHP) is an effective heat transport device, which is able to fulfil supercritical startup with the help of a secondary evaporator or a secondary loop. However, both of the secondary evaporator and the secondary loop require additional power consumption on the CLHPs, which increase the burden of a cryocooler. This paper presents a new way of supercritical startup of a cryogenic loop heat pipe with simple structure and reliable operation performance. A flexible porous wick was introduced into the liquid line, connecting the condenser and the evaporator, to provide capillary force as the motive power for the liquid in the condenser flowing into the evaporator, which dispensed with the gravity assistance and additional power consumption. The CLHP operated at liquid-state nitrogen temperature range with nitrogen as working fluid. Investigations on the supercritical startup process and the operation process were presented in the paper. The CLHP could be cooled down from 299 K and realize supercritical startup without additional power consumption. 20 W heat loads were transported across a 0.51 m distance, and dryout was not happened all through the experiments. Furthermore, the supercritical startup process with 0.5 W heat load on the evaporator from the initial ambient temperature, and the operation process of 15 W-1 W-15 W-1 W-15 W heat load on the evaporator were also investigated. The CLHP with new way of supercritical startup performed well during the tests. The experimental results were presented and analyzed in this paper. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Advanced cryogenic integration technologies are needed urgently in many application fields such as aerospace and superconductors. A variety of advanced integration components working in the low temperature range are required to achieve lower parasitics, better vibration isolation, tighter temperature control, improved reliability, higher heat transport capability, increased transport distances, and longer life [1,2]. Cryocoolers are usually used for detectors cooling to fulfil a cryogenic temperature range, e.g. below 100 K, so that the detectors can work normally. When the cold heads of cryocoolers are directly connected to the detectors, some disadvantages will be introduced such as mechanical vibration, electromagnetic interference and layout limitation. Therefore, cryogenic heat transfer devices with high efficiency are desired as thermal links between the cryocoolers and the detectors. Cryogenic loop heat pipes (CLHPs), utilizing latent heat of working fluid in transferring heat without moving parts, are regarded as
⇑ Corresponding author at: 29 Zhongguancun East Road, 100190 Beijing, China. E-mail address: [email protected] (Y.-n. Zhao). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118793 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
highly efficient two-phase thermal management devices, and have promising application prospects in the thermal control of satellites, spacecrafts, electronics, etc. CLHPs are developed from ambient loop heat pipes (LHPs), which originated from 1972. The first LHP was created and tested by the Russian scientists Gerasimov and Maydanik from the Ural Poly technical Institute [3,4]. Compared with conventional heat pipes, in which the capillary structure is situated along the whole length, rigid capillary wicks of the LHPs with micron magnitude pore size are employed only inside the evaporator, which enable much better flexible connection and lower hydraulic resistance. As a consequence, LHPs inherit all the main advantages of conventional heat pipes. Moreover, they are less sensitive to the change of orientation in the gravity field [5]. CLHPs have a lot in common with LHPs in terms of working principle, structure, advantages and so on. Mechanical vibration, electromagnetic interference and layout limitation between cryocoolers and detectors can be solved with the help of CLHPs. However, there are some differences between CLHPs and LHPs due to the different operating temperature ranges. The most critical difference is that CLHPs have to complete a supercritical startup process before the steady state operation. The LHPs are operating within the ambient temperature range, and the working fluids
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(ammonia, water, acetone or methanol, etc.) are in the liquid state or gas-liquid two phase state. Porous wicks in the evaporators are saturated by the liquid all the time, and the LHPs can start up easily. In contrast, the working fluids in CLHPs are in the gas state in the ambient temperature range. A CLHP should be cooled down from the ambient temperature to the operating cryogenic temperature with only the condenser cooled by a cold source, and significant temperature gradient is resulted in across a large transport distance. The liquid condensed in the condenser must be transport to the evaporator of the CLHP, so as to cool down the evaporator to the cryogenic temperature range before startup process. The heat transport operation will be terminated if the temperature in the evaporator is above the critical temperature of the working fluid. At present, to overcome these limitations, there are two ways to pump liquid from a condenser to an evaporator of a CLHP and fulfil the supercritical startup without gravity assistance: one is introducing a secondary evaporator in series with the loop of the CLHP; the other is introducing a secondary loop in parallel with the main loop of the CLHP. Many researches on CLHPs based on the two supercritical startup ways have been developed recently. Researches on CLHPs with a secondary evaporator in series with the loop are summarized below. A CLHP with oxygen as the working fluid was developed and tested by Khustale. A secondary evaporator was hydraulically linked to the main condenser. It is capable of cooling down from the supercritical temperature of the working fluid with only the condenser end being cooled and the main evaporator elevated versus the condenser end by 5 cm. The secondary evaporator was loaded with 4 W for 2.5 h after the test started. Then the heat load was increased to 6 W to accelerate cooling down of the main evaporator. The CLHP operated with the evaporator temperature of about 75 K and 9 W heat loads could be transported [6–8]. Mo et al. designed a nitrogen-charged CLHP, whose secondary evaporator was actually a short pipe with axial grooves on its internal surface. The CLHP could transport 12 W heat loads under horizontal and adverse gravity orientations and up to 20 W under gravity-assisted orientation [9]. Tests were performed for the CLHP at horizontal position and with the liquid line 3.4 cm and 6.4 cm below the vapor line, respectively [10]. The effects of the reservoir volume, the primary wick pore size and the working fluid of the CLHP were also investigated [11]. To meet the challenging cryocooling requirements of large-area cryocooling, a conventional CLHP system was utilized to ‘‘spread” cooling from a small cryocooler cold finger over a large-area heat source, which was designed by Hoang et al. A capillary pump (also called secondary evaporator) was utilized to generate fluid flow in the loop. The maximum cooling capacity of the Ne-CLHP was 4.2 W over an evaporator area of 48 in2 with 2.5 W applied to the capillary pump. With helium as working fluid and with the pump power kept at 50 mW, the CLHP cooling capacity reached the limit of 100 mW [12,13]. Besides utilizing a secondary evaporator in series with the loop for supercritical startup of a CLHP, a secondary loop in parallel with the main loop of a CLHP was the major way to overcome the supercritical startup problem. Startup of the primary evaporator from a supercritical temperature could be achieved by simply applying power to the secondary evaporator. This initiated circulation of fluid within the secondary flow path. As vapor generated by the secondary evaporator passed through the condenser, liquid was pumped into the primary evaporator where it evaporated, thus cooling the primary evaporator. This cycle is continued until the primary evaporator temperature was at the cryogenic operating temperature [14–16]. Extensive studies have been carried out on this kind of CLHPs. Hoang et al. designed a H2-CLHP with a secondary capillary pump thermally strapped to the cryocooler cold finger. During
the initial cooldown of the loop, the secondary pump was activated to circulate cold fluid from the main condenser to bring the loop temperatures below the critical point. During normal operation, the secondary pump managed the parasitics by removing vapor build-up in the reservoir to the condenser for heat removal. When the secondary pump was activated with 2.5 W, a 5 W heat transport capability was realized over 2.5 m [17]. Weight saving was concentrated in the fabrication of H2-CLHP_2 [18]. The advanced CLHP technology was also utilized for across-gimbal cooling applications. Less than 5 W secondary pump power was required to cool down the primary pump from 295 K to 100 K in less than 30 min. During normal operation, less than 1 W of the secondary pump power was needed to manage the heat parasitics [19,20]. Bugby et al. also developed an across-gimbal CLHP to solve the issues of flexible thermal link in the cryogenic integration [1,2,21]. Gully et al. developed a CLHP prototype around 80 K, which featured a secondary circuit for heat leak management and cool-down of the thermal link. A 5 W heat loads was applied to the secondary evaporator during the cool down test, and a cold power of 19 W is obtained across a 0.5 m distance. For the configuration without thermal shunt, a significant secondary heat load of 4 W is necessary to get a stable thermal behavior at very low transferred cold power (<1 W) [22–24]. Zhao et al. investigated an improved CLHP prototype with high heat transfer capacity. The CLHP can reach a maximum heat transfer capacity of 41 W with a 6 K temperature difference across a 0.48 m distance [25]. In addition, the possible range of working fluid inventory for the CLHP is researched. The CLHP operated stably with the primary heat load increasing from 1 W to 45 W, while the secondary heat load was kept at 5 W all the time [26]. Bai et al. designed and experimentally investigated a miniature nitrogen-charged CLHP to push forward its future space applications. The CLHP could achieve reliable supercritical startup when the secondary heat load was no less than 3 W. The CLHP had a heat transport capacity of 12 W 0.56 m. When the primary heat load was smaller than 3 W, the secondary evaporator must be kept in operation to assist the normal operation of the primary evaporator [27–31]. Guo et al. designed an integrated system with two CLHPs for actual application in future space detecting system, which could operate at 80 K with four different working modes [32]. They also designed and experimentally investigated a neon CLHP working at 35 K, reaching a heat transport capability of 4.8 W 0.6 m. The auxiliary loop played a key role in the supercritical startup of the CLHP and a 1.5 W was needed to apply to the secondary evaporator [33–38]. Qu et al. developed a global model of a cryogenic loop heat pipe and the effects of filling pressure, parasitic heat load, auxiliary heat load, and use of bayonet on the CLHP performance were investigated [39]. A CLHP with a secondary loop could achieve supercritical startup successfully. The startup time could be reduced by increasing the secondary heat load applied to the secondary evaporator. The secondary heat load could be reduced to a lower value when the primary evaporator was primed at the cryogenic operating temperature. During the normal operation, a minimum power to the secondary evaporator was required to ensure reliable operation. The fluid circulation in the secondary flow path purged vapor bubbles out of the primary evaporator core and towards the condenser, which prevented the vapor bubbles blocking off the liquid supply to the evaporator wick and causing the primary evaporator dryout. From the above review, the CLHP is an effective and promising heat transport device, which is able to fulfil supercritical startup successfully from ambient temperature with a secondary evaporator or a secondary loop. However, both of the two ways require additional power consumption on the CLHPs during the
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supercritical startup process or even during the steady state operation. The additional power consumption significantly increases the burden of a cryocooler, which has a very limited cooling power especially at a cryogenic temperature range. Moreover, the additional component of a secondary evaporator or a secondary loop results in a more complicated structure of a CLHP. Most of all, space applications require low power consumption, light weight and no gravity-assistant heat transfer devices. This paper presents a new way of supercritical startup of a cryogenic loop heat pipe with simple structure and reliable operation performance. A flexible porous material is introduced into the liquid line to provide capillary pressure as the motive power for the liquid in the condenser flowing towards the evaporator, which dispenses with the gravity assistance and additional heat load. Investigations on the cool down process and the operation process are presented in the paper. 2. Configuration of the CLHP A CLHP requires liquid be present in the evaporator prior to startup. One crucial technical problem for a CLHP is that it should be capable of cooling down from room temperature to the operating cryogenic temperature with only the condenser end being cooled by a cryocooler. To overcome this limitation, effective measures should be adopted to transport the cryogenic fluid from the condenser to the evaporator and remove liquid line parasitic heat leak during the supercritical startup process. Capillary effect is a remarkable way to achieve this purpose. Fig. 1 presents a schematic diagram of a CLHP, which provides a new way of supercritical startup for a CLHP. A new design of a flexible wick is employed into the CLHP, which is filled into the liquid line connecting the condenser and the liquid core in the evaporator. As a result, liquid state working fluid condensed in the condenser will be pumped from the condenser to the evaporator by capillary effect of the flexible porous wick. The evaporator will be cooled down to the cryogenic temperature by the liquid and accomplish the supercritical startup process. The components of the CLHP are similar to other LHPs including an evaporator, a condenser, a liquid line, a vapor line and a hot reservoir. A cup-shaped porous wick sintered by stainless steel powder is enclosed inside the evaporator, which provides the capillary pressure for the working fluid circulating in the loop during the normal operation. The cylindrical envelop of the evaporator is made of copper with many machined grooves on the inner wall for the vapor flowing into the vapor line. The condenser is made of
Fig. 1. The schematic diagram of the CLHP.
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a cubic copper block with a few machined parallel pipelines inside for the working fluid condensation in the condenser. The flow resistance of the working fluid in this improved condenser is considerably reduced [25]. A hot reservoir is connected with the CLHP and disposed in the room temperature surroundings during the experiments. It guarantees that enough liquid-state working fluid is available in the CLHP during the operation at low temperature range and a tolerable pressure inside the CLHP at room temperature range. Particularly, a flexible wick is employed into the CLHP, as shown in Fig. 2, which is convoluted by stainless steel wire mesh, in order to eliminate the additional power consumption which is unwanted burden for the cryocoolers. The wire mesh is curled into a roll and a cavity channel is formed inside the wire mesh. The flexible wick is filled into the liquid line connecting the condenser and the liquid core in the evaporator. The curled wire mesh is connected tightly with the sintered porous wick in the evaporator. It provides the motive power by capillary effect for the liquid working fluid flowing from the condenser to the evaporator to realize supercritical startup of the CLHP and has no effect on the flexibility of the thermal connection. Compared with capillary structures in conventional heat pipes, which cover the whole length of rigid casings and work all through the heat transfer operation, the flexible structure inside the liquid line plays an important role in transporting the cryogenic liquid especially during the supercritical startup process. During the normal operation, the cryogenic liquid mainly flows in the cavity channel, which can receive a lower flow resistance. In other words, the flexible structure has little effect on the liquid flow and heat transfer during the normal operation. When the working fluid is condensed in the condenser, the liquid will be pumped into the flexible wick in the liquid line by capillary pressure and towards the porous wick in the evaporator step by step. Meanwhile, parasitic heat leak of the liquid line will be removed gradually during the supercritical startup process. The liquid flows into the sintered porous wick eventually and the evaporator will be cooled down after some time. Meanwhile, heat load can be applied on the evaporator and evaporation takes place on the interface of the sintered porous wick. The vapor generated flows into the vapor line through the grooves in the inner wall of the evaporator and flows back to the condenser. At the same time, the liquid is pumped from the flexible wick in the liquid line to the sintered porous wick in the evaporator which has a higher capillary pressure. A cryogenic loop heat pipe prototype is developed with this new structure, as shown in Fig. 3. The heat transport distance of the CLHP is 0.51 m. A flexible porous material is introduced into the liquid line to provide capillary pressure as the motive power for the liquid in the condenser flowing towards the evaporator, with no need for gravity assistance and additional power
Fig. 2. The liquid line filled with a flexible wick.
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Fig. 3. Configuration of the CLHP prototype.
consumption. The CLHP is simple in structure and the main parameters of the CLHP components are listed in Table 1.
3. Experimental setup and procedures
Fig. 4. Experimental setup of the CLHP.
Fig. 4 shows an experimental setup of the CLHP. All the experiments are performed in a vacuum chamber, which can be divided into two parts for installation of the internal components. A liquid nitrogen tank, which is filled with liquid nitrogen during the tests, is applied to the CLHP system as cold source. It is made up of a stainless steel cylinder and a copper bottom, whose configuration is similar to the cold head of a cryocooler. The condenser of the CLHP is bolted to the copper bottom of the tank and the CLHP is thus hanged up in the vacuum chamber without touching any other part. In this way, the heat leak of conduction between the CLHP and its surroundings can be reduced effectively. Furthermore, a level bracket with light mass and low heat conductivity is fixed to the CLHP to prevent the transport lines from bending because of the gravity of the evaporation part. It keeps the axes of the condenser and the evaporator in a same level. The CLHP is wrapped by a shroud (a.k.a. thermal shield, as shown in Fig. 7) which contains a helical copper pipe and multilayer insulation materials. Liquid-state nitrogen flows in the helical copper pipe to cool down the environmental temperature around the CLHP and prevent parasitic heat from outside. Platinum resistance thermometers with the maximum measurement error of ±0.1 K are distributed on all the locations of the prototype for obtaining temperature data (as shown in Fig. 4). A film electric resistance heater is employed to simulate heat source and provide heat loads during the normal operation. The heater is attached tightly to the outside surface of the evaporator. The heat loads are able to be adjusted continuously in the range of 0–20 W by altering the output voltage of a DC power. Temperature data and heat load data are recorded and collected by a data acquisition system linked to a PC. All the components are situated in a vacuum chamber during the experimental process except a hot reservoir left outside, which is responsible for pressure reduction of the CLHP at the room temperature. A pressure sensor is connected to the hot reservoir to show the pressure inside the CLHP. All the experiments are operated under vacuum conditions. The CLHP is adjusted to horizontal state or slight anti-gravity state before each experiment, to avoid the influence of gravity
assistance. First, high purity N2 is filled into the hot reservoir and CLHP up to a required pressure at room temperature. Then, liquid nitrogen is filled into the liquid nitrogen tank and the thermal shield, to cool the condenser and the surrounding of the CLHP. The gas-state working fluid is condensed in the condenser and the liquid flows through the flexible wick in the liquid line towards the evaporator with the help of capillary effect. When the evaporator is cooled down under the critical temperature of the working fluid, the CLHP is at the operating temperature range. Heat load imposed on the evaporator results in evaporation at the surface of porous wick and the generated vapor flows back to the condenser through the vapor line. The working fluid begins to cycle in the loop continuously. 4. Experimental results and discussions 4.1. Typical experiment of the CLHP Experiments were carried out to test the new way of supercritical startup of the CLHP, which employed a flexible wick into the liquid line to transport cryogenic liquid from condenser to evaporator without power consumption. In order to prevent the cryogenic liquid flowing towards the evaporator with gravityassistance, the evaporator was elevated 0–5 mm higher relative to the condenser, to keep the CLHP in a horizontal orientation or slight anti-gravity orientation before each experiment. The CLHP was filled with high purity Nitrogen as working fluid and operated at the temperature range of 77–120 K. The tests were performed with 1.28 MPa filling pressure which was filled into the CLHP at ambient temperature. The supercritical startup, normal operation and temperature hysteresis of the CLHP were researched. Typical test result (a.k.a. test 1) is presented in the following paragraph. 4.1.1. Supercritical startup Fig. 5 presents the temperature variations of measuring points distributed on the CLHP during the cool down process. All the temperature measuring points were at the temperature of 299 K in the
Table 1 Parameters of the CLHP (mm). Components
Dimensions
Components
Dimensions
Evaporator Condenser volume Condenser parallel pipes Vapor line Liquid line
(OD L)16.5 24 2296.85 mm3 (ID L n)2.5 40 5 (OD d L)6 0.5 585 (OD d L)3 0.5 473
Pore size (sintered wick) Permeability (sintered wick) Mesh number (flexible wick) Porosity (flexible wick) Hot reservoir
5 lm 5.5 10 100 89% 600 ml
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m2
Y.-n. Zhao et al. / International Journal of Heat and Mass Transfer 145 (2019) 118793
Fig. 5. Cool down process of the CLHP.
beginning. After the system reached a required vacuum condition, liquid nitrogen was filled into the shroud from the inlet, which was connected with a 100 L cryogenic dewar. The liquid nitrogen flew along the helical copper pipe in the shroud and the temperature of the shroud (TS) dropped immediately. The environmental temperature around the CLHP was cooled down and parasitic heat from outside was prevented by the cryogenic shroud. At the same time, temperatures of the CLHP decreased slowly due to the thermal radiation from the cryogenic shroud. At about 35 min, the shroud temperature (TS) reached about 95 K and began to decrease slowly. It meant that the surrounding temperature of the CLHP achieved the operating temperature range. At this time, liquid nitrogen was filled into the liquid nitrogen tank and the temperature of the condenser (TC) which is bolted to the liquid nitrogen tank dropped rapidly. About 8 min later, the temperature of the condenser (TC) reached about 83 K and began to decrease very slowly. It indicated that gaseous nitrogen began to condense into liquid state gradually in the condenser. With the time went by, the cryogenic condensate accumulated progressively and wetted the flexible wick stretched into the condenser. Due to the capillarity effect of the porous structure, the cryogenic liquid was pumped into the liquid line through the flexible wick and the gas-liquid interface moved forward gradually. Meanwhile, the liquid line was cooled down to the cryogenic temperature by the liquid passed. At about 48 min, the temperature of liquid line 2 (TL2) dropped immediately, which suggested the liquid reached the middle location of the liquid line. After a few minutes, the temperature of vapor line 2 (TV2) was cooled down quickly and then slowly after a while, indicating that the liquid reached the middle location of the vapor line. Because the diameter of the vapor line was larger than the liquid line and condenser channel, the lowest position of the vapor line was lower than the lowest position of the condenser channel at the inlet of the condenser. When the condenser channel was full of the cryogenic liquid, the liquid near the inlet of the condenser tended to flow into the vapor line due to the gravity effect. That was the reason why some liquid flew into the vapor line and reached vapor line 2 undesirably. At about 58 min, the temperature of liquid line 3 (TL3) dropped immediately and then remained almost constant. It suggested that the liquid reached the end location of the liquid line and would flow into the evaporator soon. After a short while, the slope of the temperature curve of the evaporator increased slightly, which indicated the liquid had flown into the evaporator. The temperature of the evaporator (TE) decreased gradually with a probably constant slope. Compared with the liquid line, the evaporator had a much larger heat capacity and could not be cooled down in a short time. The temperature of the vapor line 1 (TV1) decreased following the trend of the evapo-
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rator because of their close position. At about 135 min, the temperature of the evaporator dropped rapidly to about 90 K and maintained almost constant. It indicated that the evaporator (TE) was cooled down to the operating temperature range. Meanwhile, the temperature of the vapor line 1 (TV1) decreased faster and finally maintained constant, which was cooled down by the cryogenic vapor evaporated in the evaporator. The liquid continuously flew into the evaporator and accumulated in the liquid core. At last, the porous wick of the evaporator was saturated by the liquid, which suggested that the CLHP had achieved the cool down process. Fig. 6 presents the startup process of the CLHP after achieving the cool down process. At about 222 min, the evaporator was heated with a 0.5 W heat load and the evaporator temperature (TE) rose gradually. Meanwhile, the temperatures of liquid line and vapor line (TL2, TL3, TV1 and TV2) also rose gradually. However, the temperature of liquid line 2 (TL2) dropped rapidly after a few minutes, while other temperatures continued rising slowly. Several minutes later, the temperature of liquid line 3 (TL3) dropped rapidly and rose immediately again, and the temperature of the evaporator (TE) slightly decreased at this moment. At about 240 min, with the heat load increased to 1 W, the temperatures (TL2, TL3, TV1 and TE) dropped rapidly again. Then, the temperatures of liquid line (TL2, TL3) rose slightly. After a few minutes, the heat loads were increased to 1.5 W and similar variation occurred with smaller amplitude. When the heat loads were increased to 2 W at about 262 min, all the temperatures reached steady state, which indicated that the CLHP started up successfully with this new way of flexible wick. When the heat loads applied to the evaporator were lower, the liquid continued being transported through the flexible porous wick from the condenser to the evaporator. Moreover, the parasitic heat leak had an impact on the heat transfer performance of the CLHP with a lower heat load, which resulted in higher temperatures of evaporator and transport lines. When the heat loads were increased step by step to 2 W, the liquid was pumped into the cavity channel inside the flexible wick, which could achieve a higher flow rate and overcome the parasitic heat leak effect. At the same time, the liquid line and evaporator were cooled down by the cryogenic liquid flowing in the cavity channel to lower temperatures.
4.1.2. Normal operation From the experimental result above, it indicates that the CLHP is able to achieve supercritical startup from ambient temperature and require no power consumption additionally, with this new way of introducing a flexible wick into the liquid line. Furthermore,
Fig. 6. Startup process of the CLHP.
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the heat transfer performance is the most crucial characteristics of a CLHP. The normal operation process of this CLHP after the supercritical startup process is presented in Fig. 7. The shroud temperature was maintained at about 83 K during the experiment. The heat load was increased step by step from the value of 2 W to 10 W, with 1 W increment. The temperatures of the evaporator (TE) and the condenser (TC) rose gradually to little higher values with each increase of the heat load. Meanwhile, the temperatures of liquid line (TL2, TL3) remained almost constant during this process. After that, the heat load was increased continuously from 10 W to 15 W and then increased to 20 W, and all the temperatures rose to higher values each time. The temperature difference was 10.4 K corresponding to 20 W heat loads. During the test with the heat load in the range of 2–20 W, the CLHP prototype performed stably and reliably. In this test, the maximum heat load applied to the evaporator was 20 W and dryout did not occur all through, indicating that the heat transport capacity of the CLHP prototype was much higher than 20 W. However, the heat load could not be increased larger than 20 W during the test, because of the limitation of the heater in the system. The heat transport capacity will be researched in the future. 4.1.3. Temperature hysteresis test Temperature hysteresis is a common phenomenon in loop heat pipe experiments, which refers to different operating temperatures with equal heat loads during the heat load increasing process and decreasing process. This phenomenon will lead to inconsistent results of LHPs with the same conditions, which has an adverse effect on the performance of the LHPs. Therefore, test on temperature hysteresis of the CLHP prototype was researched. Fig. 8 presents the performance of the CLHP with decreasing heat loads. The heat load was decreased from 20 W to 0.5 W step by step while all the temperatures declined gradually. When the heat load was decreased to 1 W, the temperatures of the evaporator (TE) and the condenser (TC) declined as before. However, the temperatures of the liquid line (TL2, TL3) rose up at this moment and then declined slightly. When the heat load was decreased to 0.5 W, the temperature of the condenser (TC) continued to decline as before, while the temperatures of the evaporator (TE) and transport lines (TL2, TL3) rose up much higher. Then the temperature of the liquid line 2 (TL2) declined to a certain extent. The reason for the variation of the temperatures was that the flow rate of the liquid decreased seriously in the loop when the heat load was decreased to 1 W and 0.5 W. In that case, the parasitic heat leak could not be overcome completely and the temperatures began to rise up. During the test with the heat load decreased from
Fig. 7. Performance of the CLHP.
Fig. 8. Performance of the CLHP with decreasing heat loads.
20 W to 0.5 W, the CLHP prototype reached steady state operation with each heat load. Test results of the temperature difference and thermal resistance with comparison of the forward motion and the reverse motion were presented in Figs. 9 and 10. There were only slight differences with the same value of heat load between the two processes, which demonstrated that the CLHP had an outstanding heat transfer performance in the test. With the increasing of the heat load, the thermal resistance decreased. The minimum thermal resistance was 0.52 K/W versus 20 W heat loads during this test. 4.2. Experiment starting with 0.5 W heat load at room temperature Besides the typical operation of the test 1 presented above, the test 2 was researched, with 0.5 W heat load applied to the evaporator at room temperature before the test begin, which was used for exploring supercritical startup ability of this new way in harsh conditions. The test 2 was performed in the same experimental conditions with the test 1. The supercritical startup, normal operation and power cycling of the CLHP were researched, and comparison of the test 1 and test 2 was carried out. The test result is presented in the following paragraph. 4.2.1. Cool down process Fig. 11 presents the cool down process of the test 2. High purity N2 was filled into the CLHP with 1.28 MPa filling pressure at ambi-
Fig. 9. Temperature difference comparison.
Y.-n. Zhao et al. / International Journal of Heat and Mass Transfer 145 (2019) 118793
Fig. 10. Thermal resistance comparison.
Fig. 11. Cool down process of the CLHP with 0.5 W heat load.
ent temperature. All the temperature measuring points were at the temperature of 282 K in the beginning. The vacuum chamber was evacuated to a required vacuum condition before the experiment startup. At first, a 0.5 W heat load was applied to the evaporator, which led to a rapid temperature increase of the evaporator. After that, liquid nitrogen was filled into the shroud from a 100 L cryogenic dewar. The liquid nitrogen flew along the helical copper pipe in the shroud and the temperature of the shroud (TS) dropped immediately, which resulted in the decrease of the environmental temperature around the CLHP. In this case, the parasitic heat from outside was prevented by the cryogenic shroud. After a short while, temperatures of the CLHP decreased slowly due to the thermal radiation from the cryogenic shroud. At about 20 min, the temperature of the evaporator (TE) stopped rising up and began to decrease slowly. A few minutes, the shroud temperature (TS) reached a cryogenic temperature range under 100 K and began to decrease slowly, which meant that the surrounding temperature of the CLHP had achieved the operating temperature range. At this time, liquid nitrogen was filled into the liquid nitrogen tank and the temperature of the condenser (TC) contacting with the liquid nitrogen tank dropped rapidly. At about 40 min, the temperature of the condenser (TC) reached about 81 K and began to decrease very slowly. At this moment, gaseous nitrogen began to condense into liquid state gradually in the condenser and the cryogenic condensate accumulated progressively. The flexible wick stretched into the condenser was wetted by the cryogenic liquid, and the liquid was pumped into the liquid line through the flexible
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wick due to the capillarity effect of the porous structure. Meanwhile, the gas-liquid interface moved forward gradually and the liquid line was cooled down to the cryogenic temperature by the liquid passed. After several minutes, the temperature of liquid line 2 (TL2), the temperature of vapor line 2 (TV2) and the temperature of liquid line 3 (TL3) dropped immediately and then remained almost constant after that moment, which was similar to the cool down process of test 1. From then on, the cryogenic liquid reached the evaporator and began to cool it down. The temperature of the evaporator (TE) decreased slowly and the slope of the temperature curve of the evaporator also decreased gradually, which was caused by the heat load continuously applied to the evaporator. After quite a long time and at about 360 min, the temperature the evaporator dropped rapidly to about 90 K and finally maintained constant, which indicated that the liquid began to accumulate in the liquid core of the evaporator. At the same time, the temperature of the vapor line 1 (TV1) dropped rapidly following the evaporator. Finally, the porous wick was saturated by the liquid and the CLHP achieved the cool down successfully with 0.5 W heat load applied to the evaporator all along the process.
4.2.2. Startup and normal operation Fig. 12 presents the startup process and the normal operation of the CLHP. Initially, the heat load applied to the evaporator was increased from 0.5 W to 1 W at about 390 min, and then the evaporator temperature (TE) and the transport line temperatures (TL2, TL3, TV1 and TV2) declined quickly after a short while. The reason was that the cryogenic liquid was pumped into the cavity channel of the liquid line and flew towards the evaporator, which were similar to the startup process of test 1. Meanwhile, the condenser temperature (TC) rose slightly and then declined again, suggesting that some vapor suddenly flew into the condenser and then condensed to liquid. The temperatures could maintain steady state and indicated a successful startup of the CLHP in this condition. The experimental result suggested that the CLHP was able to achieve supercritical startup even with 0.5 W heat load applied to the evaporator from ambient temperature. The heat transfer performance was also researched after startup. The heat load was increased step by step from the value of 1 W to 5 W, with 1 W increment. The temperatures of the evaporator (TE) and the condenser (TC) rose gradually to little higher values with each increase of the heat load. Meanwhile, the temperatures of liquid line (TL2, TL3) remained almost constant during this process. Then, the heat load was increased to 7 W, 10 W and 15 W step by step, and the temperatures rose slightly with the increased heat load. The tem-
Fig. 12. Result of supercritical startup and normal heat transfer.
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Fig. 15. Result of power cycling. Fig. 13. Comparison of the temperature difference.
Fig. 16. Thermal resistance variation with power cycling.
Fig. 14. Comparison of the thermal resistance.
perature difference was 11.4 K corresponding to 15 W heat loads. The CLHP performed stably and reliably during this experiment. Both of the test 1 and test 2 could achieve stable performance during the experiments. Fig. 13 presents the comparison of the temperature differences between test 1 and test 2. Obviously, the temperature differences of the test 2 were a little larger than test 1. When a 0.5 W heat load was applied to the evaporator during the supercritical startup of the test 2, the CLHP was cooled down and achieved steady state with a higher saturation temperature. However, the condenser temperature was the same in the test 1 and test 2, so the temperature differences of the test 2 were larger than test 1. Fig. 14 presents the comparison of the thermal resistances between test 1 and test 2. The same descending trends of variation of the thermal resistances were achieved during the tests. With the increase of the heat loads, the thermal resistances of the CLHP decreased gradually, and the differences of the thermal resistances between the test 1 and test 2 became smaller and smaller. 4.3. Power cycling In some practical applications, the heat loads imposed on the CLHP were changed severely during the heat transfer process, which might lead to bad performance or even dryout of the CLHP. Therefore, power cycling test was carried out to examine the performance of the CLHP with severe conditions.
Fig. 15 presents results of a power cycling test conducted with severe 15 W/1 W/15 W/1 W/15 W step-ups/downs cycling. The temperature of the evaporator and the condenser could achieve new steady equilibrium quickly with periodical variation of heat loads. Fig. 16 presents the thermal resistance variations versus the heat load, indicating consistent results with the same heat load. As a result, the CLHP prototype responded extremely well during the power cycling test. 5. Conclusions In this paper, a new way of supercritical startup of a cryogenic loop heat pipe with a flexible porous wick being introduced into the liquid line was presented, which achieved a much simpler structure compared with conventional CLHPs. The flexible porous wick connected the condenser and the evaporator, and transported the cryogenic liquid from the condenser to the evaporator by capillary force. The CLHP operated at liquid-state nitrogen temperature range with nitrogen as working fluid. Experimental investigations on the supercritical startup process and the operation process were presented in the paper, and important conclusions were reached and summarized as follows: &
The CLHP could achieve supercritical startup successfully from the initial ambient temperature of 299 K, without additional power consumption. With a 0.5 W heat load applied to the evaporator, the CLHP could start up successfully.
Y.-n. Zhao et al. / International Journal of Heat and Mass Transfer 145 (2019) 118793 &
&
&
&
The CLHP could transport 20 W heat loads across a 0.51 m distance, and dryout did not happen all through the experiments. With the heat load range of 2–20 W, the CLHP prototype performed stably and reliably. With the increase of the heat load, the thermal resistance decreased. The minimum thermal resistance was 0.52 K/W versus 20 W heat loads. With the comparison of temperature difference and thermal resistance during the forward motion and the reverse motion, temperature hysteresis phenomenon was not obvious in the test. With 0.5 W heat load being applied to the evaporator from the initial ambient temperature, the CLHP could also achieve supercritical startup successfully and operate stably, indicating that the CLHP could achieve supercritical startup even in the harsh conditions. During the power cycling test with 15 W-1 W-15 W-1 W-15 W heat load applied on the evaporator, the temperatures of the evaporator and the condenser could achieve new steady equilibrium quickly with periodical variation of heat loads, which demonstrated that the CLHP performed reliably with this new structure.
Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, ‘‘A new way of supercritical startup of a cryogenic loop heat pipe”. Acknowledgements The work was supported by the National Natural Science Foundation of China (No. 51606207) and Youth Innovation Promotion Association, CAS (No. 2018036). The technical support offered by the staff and colleagues is greatly appreciated. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijheatmasstransfer.2019.118793. References [1] D. Bugby, B. Marland, C. Stouffer, et al., Across-gimbal and miniaturized cryogenic loop heat pipes, Space Technol. Int. Forum-STAIF 2003, pp. 218–226. [2] D. Bugby, B. Marland, C. Stouffer, et al., Development of advanced tools forcryogenic integration, Adv. Cryogenic Eng.: Trans. Cryogenic Eng. Conf. 49 (2004) 1914–1922. [3] Heat Pipe, USSR Inventors Certificate 449213, 1974. [4] Yu.F. Gerasimov, Yu.F. Maydanik, G.T. Shchogolev, et al., Low-temperature heat pipes with separate channels for vapor and liquid, Eng.-Phys. J. 28 (6) (1975) 957–960 (in Russian). [5] Y.F. Maydanik, Loop heat pipes, Appl. Therm. Eng. 25 (2005) 635–657. [6] D. Khrustalev, Cryogenic loop heat pipes as flexible thermal links for cryocoolers, Cambridge, MA, USA, in: Proceedings of the 12th International Cryocooler Conference, June 18-20, 2002, pp. 709–716. [7] D. Khrustalev, Test data for a cryogenic loop heat pipe operating in the temperature range from 65K to 140K, in: Presentation at the International Two-Phase Thermal Control Technology Workshop, September 24–26, Mitcheville, MD, 2002.
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Liang, et al., Investigation on optimal working fluid inventory of a cryogenic loop heat pipe, Int. J. Heat Mass Transfer 66 (2013) 334–337. [27] L. Bai, G. Lin, H. Zhang, et al., Operating characteristics of a miniature cryogenic loop heat pipe, Int. J. Heat Mass Transf. 55 (2012) 8093–8099. [28] L. Bai, G. Lin, H. Zhang, et al., Experimental study of a nitrogen-charged cryogenic loop heat pipe, Cryogenics 52 (2012) 557–563. [29] C. Du, L. Bai, G. Lin, et al., Determination of charged pressure of working fluid and its effect on the operation of a miniature CLHP, Int. J. Heat Mass Transfer 63 (2013) 454–462. [30] L. Bai, G. Lin, H. Zhang, et al., Effect of component layout on the operation of a miniature cryogenic loop heat pipe, Int. J. Heat Mass Transf. 60 (2013) 61–68. [31] L. Bai, L. Zhang, G. Lin, et al., Development of cryogenic loop heat pipes: a review and comparative analysis, Appl. Therm. Eng. 89 (2015) 180–191. [32] Y. Guo, G. Lin, L. 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Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
A new way of supercritical startup of a cryogenic loop heat pipe Ya-nan Zhao ⇑, Tao Yan, Jingtao Liang, Nailiang Wang Key Laboratory of Space Energy Conversion Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 19 August 2019 Received in revised form 26 September 2019 Accepted 26 September 2019 Available online 5 October 2019 Keywords: Cryogenic Loop heat pipe Heat transfer Supercritical startup Porous wick
a b s t r a c t Space applications require low power consumption, light weight and no gravity-assistant heat transfer devices. Cryogenic loop heat pipe (CLHP) is an effective heat transport device, which is able to fulfil supercritical startup with the help of a secondary evaporator or a secondary loop. However, both of the secondary evaporator and the secondary loop require additional power consumption on the CLHPs, which increase the burden of a cryocooler. This paper presents a new way of supercritical startup of a cryogenic loop heat pipe with simple structure and reliable operation performance. A flexible porous wick was introduced into the liquid line, connecting the condenser and the evaporator, to provide capillary force as the motive power for the liquid in the condenser flowing into the evaporator, which dispensed with the gravity assistance and additional power consumption. The CLHP operated at liquid-state nitrogen temperature range with nitrogen as working fluid. Investigations on the supercritical startup process and the operation process were presented in the paper. The CLHP could be cooled down from 299 K and realize supercritical startup without additional power consumption. 20 W heat loads were transported across a 0.51 m distance, and dryout was not happened all through the experiments. Furthermore, the supercritical startup process with 0.5 W heat load on the evaporator from the initial ambient temperature, and the operation process of 15 W-1 W-15 W-1 W-15 W heat load on the evaporator were also investigated. The CLHP with new way of supercritical startup performed well during the tests. The experimental results were presented and analyzed in this paper. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Advanced cryogenic integration technologies are needed urgently in many application fields such as aerospace and superconductors. A variety of advanced integration components working in the low temperature range are required to achieve lower parasitics, better vibration isolation, tighter temperature control, improved reliability, higher heat transport capability, increased transport distances, and longer life [1,2]. Cryocoolers are usually used for detectors cooling to fulfil a cryogenic temperature range, e.g. below 100 K, so that the detectors can work normally. When the cold heads of cryocoolers are directly connected to the detectors, some disadvantages will be introduced such as mechanical vibration, electromagnetic interference and layout limitation. Therefore, cryogenic heat transfer devices with high efficiency are desired as thermal links between the cryocoolers and the detectors. Cryogenic loop heat pipes (CLHPs), utilizing latent heat of working fluid in transferring heat without moving parts, are regarded as
⇑ Corresponding author at: 29 Zhongguancun East Road, 100190 Beijing, China. E-mail address: [email protected] (Y.-n. Zhao). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118793 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
highly efficient two-phase thermal management devices, and have promising application prospects in the thermal control of satellites, spacecrafts, electronics, etc. CLHPs are developed from ambient loop heat pipes (LHPs), which originated from 1972. The first LHP was created and tested by the Russian scientists Gerasimov and Maydanik from the Ural Poly technical Institute [3,4]. Compared with conventional heat pipes, in which the capillary structure is situated along the whole length, rigid capillary wicks of the LHPs with micron magnitude pore size are employed only inside the evaporator, which enable much better flexible connection and lower hydraulic resistance. As a consequence, LHPs inherit all the main advantages of conventional heat pipes. Moreover, they are less sensitive to the change of orientation in the gravity field [5]. CLHPs have a lot in common with LHPs in terms of working principle, structure, advantages and so on. Mechanical vibration, electromagnetic interference and layout limitation between cryocoolers and detectors can be solved with the help of CLHPs. However, there are some differences between CLHPs and LHPs due to the different operating temperature ranges. The most critical difference is that CLHPs have to complete a supercritical startup process before the steady state operation. The LHPs are operating within the ambient temperature range, and the working fluids
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(ammonia, water, acetone or methanol, etc.) are in the liquid state or gas-liquid two phase state. Porous wicks in the evaporators are saturated by the liquid all the time, and the LHPs can start up easily. In contrast, the working fluids in CLHPs are in the gas state in the ambient temperature range. A CLHP should be cooled down from the ambient temperature to the operating cryogenic temperature with only the condenser cooled by a cold source, and significant temperature gradient is resulted in across a large transport distance. The liquid condensed in the condenser must be transport to the evaporator of the CLHP, so as to cool down the evaporator to the cryogenic temperature range before startup process. The heat transport operation will be terminated if the temperature in the evaporator is above the critical temperature of the working fluid. At present, to overcome these limitations, there are two ways to pump liquid from a condenser to an evaporator of a CLHP and fulfil the supercritical startup without gravity assistance: one is introducing a secondary evaporator in series with the loop of the CLHP; the other is introducing a secondary loop in parallel with the main loop of the CLHP. Many researches on CLHPs based on the two supercritical startup ways have been developed recently. Researches on CLHPs with a secondary evaporator in series with the loop are summarized below. A CLHP with oxygen as the working fluid was developed and tested by Khustale. A secondary evaporator was hydraulically linked to the main condenser. It is capable of cooling down from the supercritical temperature of the working fluid with only the condenser end being cooled and the main evaporator elevated versus the condenser end by 5 cm. The secondary evaporator was loaded with 4 W for 2.5 h after the test started. Then the heat load was increased to 6 W to accelerate cooling down of the main evaporator. The CLHP operated with the evaporator temperature of about 75 K and 9 W heat loads could be transported [6–8]. Mo et al. designed a nitrogen-charged CLHP, whose secondary evaporator was actually a short pipe with axial grooves on its internal surface. The CLHP could transport 12 W heat loads under horizontal and adverse gravity orientations and up to 20 W under gravity-assisted orientation [9]. Tests were performed for the CLHP at horizontal position and with the liquid line 3.4 cm and 6.4 cm below the vapor line, respectively [10]. The effects of the reservoir volume, the primary wick pore size and the working fluid of the CLHP were also investigated [11]. To meet the challenging cryocooling requirements of large-area cryocooling, a conventional CLHP system was utilized to ‘‘spread” cooling from a small cryocooler cold finger over a large-area heat source, which was designed by Hoang et al. A capillary pump (also called secondary evaporator) was utilized to generate fluid flow in the loop. The maximum cooling capacity of the Ne-CLHP was 4.2 W over an evaporator area of 48 in2 with 2.5 W applied to the capillary pump. With helium as working fluid and with the pump power kept at 50 mW, the CLHP cooling capacity reached the limit of 100 mW [12,13]. Besides utilizing a secondary evaporator in series with the loop for supercritical startup of a CLHP, a secondary loop in parallel with the main loop of a CLHP was the major way to overcome the supercritical startup problem. Startup of the primary evaporator from a supercritical temperature could be achieved by simply applying power to the secondary evaporator. This initiated circulation of fluid within the secondary flow path. As vapor generated by the secondary evaporator passed through the condenser, liquid was pumped into the primary evaporator where it evaporated, thus cooling the primary evaporator. This cycle is continued until the primary evaporator temperature was at the cryogenic operating temperature [14–16]. Extensive studies have been carried out on this kind of CLHPs. Hoang et al. designed a H2-CLHP with a secondary capillary pump thermally strapped to the cryocooler cold finger. During
the initial cooldown of the loop, the secondary pump was activated to circulate cold fluid from the main condenser to bring the loop temperatures below the critical point. During normal operation, the secondary pump managed the parasitics by removing vapor build-up in the reservoir to the condenser for heat removal. When the secondary pump was activated with 2.5 W, a 5 W heat transport capability was realized over 2.5 m [17]. Weight saving was concentrated in the fabrication of H2-CLHP_2 [18]. The advanced CLHP technology was also utilized for across-gimbal cooling applications. Less than 5 W secondary pump power was required to cool down the primary pump from 295 K to 100 K in less than 30 min. During normal operation, less than 1 W of the secondary pump power was needed to manage the heat parasitics [19,20]. Bugby et al. also developed an across-gimbal CLHP to solve the issues of flexible thermal link in the cryogenic integration [1,2,21]. Gully et al. developed a CLHP prototype around 80 K, which featured a secondary circuit for heat leak management and cool-down of the thermal link. A 5 W heat loads was applied to the secondary evaporator during the cool down test, and a cold power of 19 W is obtained across a 0.5 m distance. For the configuration without thermal shunt, a significant secondary heat load of 4 W is necessary to get a stable thermal behavior at very low transferred cold power (<1 W) [22–24]. Zhao et al. investigated an improved CLHP prototype with high heat transfer capacity. The CLHP can reach a maximum heat transfer capacity of 41 W with a 6 K temperature difference across a 0.48 m distance [25]. In addition, the possible range of working fluid inventory for the CLHP is researched. The CLHP operated stably with the primary heat load increasing from 1 W to 45 W, while the secondary heat load was kept at 5 W all the time [26]. Bai et al. designed and experimentally investigated a miniature nitrogen-charged CLHP to push forward its future space applications. The CLHP could achieve reliable supercritical startup when the secondary heat load was no less than 3 W. The CLHP had a heat transport capacity of 12 W 0.56 m. When the primary heat load was smaller than 3 W, the secondary evaporator must be kept in operation to assist the normal operation of the primary evaporator [27–31]. Guo et al. designed an integrated system with two CLHPs for actual application in future space detecting system, which could operate at 80 K with four different working modes [32]. They also designed and experimentally investigated a neon CLHP working at 35 K, reaching a heat transport capability of 4.8 W 0.6 m. The auxiliary loop played a key role in the supercritical startup of the CLHP and a 1.5 W was needed to apply to the secondary evaporator [33–38]. Qu et al. developed a global model of a cryogenic loop heat pipe and the effects of filling pressure, parasitic heat load, auxiliary heat load, and use of bayonet on the CLHP performance were investigated [39]. A CLHP with a secondary loop could achieve supercritical startup successfully. The startup time could be reduced by increasing the secondary heat load applied to the secondary evaporator. The secondary heat load could be reduced to a lower value when the primary evaporator was primed at the cryogenic operating temperature. During the normal operation, a minimum power to the secondary evaporator was required to ensure reliable operation. The fluid circulation in the secondary flow path purged vapor bubbles out of the primary evaporator core and towards the condenser, which prevented the vapor bubbles blocking off the liquid supply to the evaporator wick and causing the primary evaporator dryout. From the above review, the CLHP is an effective and promising heat transport device, which is able to fulfil supercritical startup successfully from ambient temperature with a secondary evaporator or a secondary loop. However, both of the two ways require additional power consumption on the CLHPs during the
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supercritical startup process or even during the steady state operation. The additional power consumption significantly increases the burden of a cryocooler, which has a very limited cooling power especially at a cryogenic temperature range. Moreover, the additional component of a secondary evaporator or a secondary loop results in a more complicated structure of a CLHP. Most of all, space applications require low power consumption, light weight and no gravity-assistant heat transfer devices. This paper presents a new way of supercritical startup of a cryogenic loop heat pipe with simple structure and reliable operation performance. A flexible porous material is introduced into the liquid line to provide capillary pressure as the motive power for the liquid in the condenser flowing towards the evaporator, which dispenses with the gravity assistance and additional heat load. Investigations on the cool down process and the operation process are presented in the paper. 2. Configuration of the CLHP A CLHP requires liquid be present in the evaporator prior to startup. One crucial technical problem for a CLHP is that it should be capable of cooling down from room temperature to the operating cryogenic temperature with only the condenser end being cooled by a cryocooler. To overcome this limitation, effective measures should be adopted to transport the cryogenic fluid from the condenser to the evaporator and remove liquid line parasitic heat leak during the supercritical startup process. Capillary effect is a remarkable way to achieve this purpose. Fig. 1 presents a schematic diagram of a CLHP, which provides a new way of supercritical startup for a CLHP. A new design of a flexible wick is employed into the CLHP, which is filled into the liquid line connecting the condenser and the liquid core in the evaporator. As a result, liquid state working fluid condensed in the condenser will be pumped from the condenser to the evaporator by capillary effect of the flexible porous wick. The evaporator will be cooled down to the cryogenic temperature by the liquid and accomplish the supercritical startup process. The components of the CLHP are similar to other LHPs including an evaporator, a condenser, a liquid line, a vapor line and a hot reservoir. A cup-shaped porous wick sintered by stainless steel powder is enclosed inside the evaporator, which provides the capillary pressure for the working fluid circulating in the loop during the normal operation. The cylindrical envelop of the evaporator is made of copper with many machined grooves on the inner wall for the vapor flowing into the vapor line. The condenser is made of
Fig. 1. The schematic diagram of the CLHP.
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a cubic copper block with a few machined parallel pipelines inside for the working fluid condensation in the condenser. The flow resistance of the working fluid in this improved condenser is considerably reduced [25]. A hot reservoir is connected with the CLHP and disposed in the room temperature surroundings during the experiments. It guarantees that enough liquid-state working fluid is available in the CLHP during the operation at low temperature range and a tolerable pressure inside the CLHP at room temperature range. Particularly, a flexible wick is employed into the CLHP, as shown in Fig. 2, which is convoluted by stainless steel wire mesh, in order to eliminate the additional power consumption which is unwanted burden for the cryocoolers. The wire mesh is curled into a roll and a cavity channel is formed inside the wire mesh. The flexible wick is filled into the liquid line connecting the condenser and the liquid core in the evaporator. The curled wire mesh is connected tightly with the sintered porous wick in the evaporator. It provides the motive power by capillary effect for the liquid working fluid flowing from the condenser to the evaporator to realize supercritical startup of the CLHP and has no effect on the flexibility of the thermal connection. Compared with capillary structures in conventional heat pipes, which cover the whole length of rigid casings and work all through the heat transfer operation, the flexible structure inside the liquid line plays an important role in transporting the cryogenic liquid especially during the supercritical startup process. During the normal operation, the cryogenic liquid mainly flows in the cavity channel, which can receive a lower flow resistance. In other words, the flexible structure has little effect on the liquid flow and heat transfer during the normal operation. When the working fluid is condensed in the condenser, the liquid will be pumped into the flexible wick in the liquid line by capillary pressure and towards the porous wick in the evaporator step by step. Meanwhile, parasitic heat leak of the liquid line will be removed gradually during the supercritical startup process. The liquid flows into the sintered porous wick eventually and the evaporator will be cooled down after some time. Meanwhile, heat load can be applied on the evaporator and evaporation takes place on the interface of the sintered porous wick. The vapor generated flows into the vapor line through the grooves in the inner wall of the evaporator and flows back to the condenser. At the same time, the liquid is pumped from the flexible wick in the liquid line to the sintered porous wick in the evaporator which has a higher capillary pressure. A cryogenic loop heat pipe prototype is developed with this new structure, as shown in Fig. 3. The heat transport distance of the CLHP is 0.51 m. A flexible porous material is introduced into the liquid line to provide capillary pressure as the motive power for the liquid in the condenser flowing towards the evaporator, with no need for gravity assistance and additional power
Fig. 2. The liquid line filled with a flexible wick.
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Fig. 3. Configuration of the CLHP prototype.
consumption. The CLHP is simple in structure and the main parameters of the CLHP components are listed in Table 1.
3. Experimental setup and procedures
Fig. 4. Experimental setup of the CLHP.
Fig. 4 shows an experimental setup of the CLHP. All the experiments are performed in a vacuum chamber, which can be divided into two parts for installation of the internal components. A liquid nitrogen tank, which is filled with liquid nitrogen during the tests, is applied to the CLHP system as cold source. It is made up of a stainless steel cylinder and a copper bottom, whose configuration is similar to the cold head of a cryocooler. The condenser of the CLHP is bolted to the copper bottom of the tank and the CLHP is thus hanged up in the vacuum chamber without touching any other part. In this way, the heat leak of conduction between the CLHP and its surroundings can be reduced effectively. Furthermore, a level bracket with light mass and low heat conductivity is fixed to the CLHP to prevent the transport lines from bending because of the gravity of the evaporation part. It keeps the axes of the condenser and the evaporator in a same level. The CLHP is wrapped by a shroud (a.k.a. thermal shield, as shown in Fig. 7) which contains a helical copper pipe and multilayer insulation materials. Liquid-state nitrogen flows in the helical copper pipe to cool down the environmental temperature around the CLHP and prevent parasitic heat from outside. Platinum resistance thermometers with the maximum measurement error of ±0.1 K are distributed on all the locations of the prototype for obtaining temperature data (as shown in Fig. 4). A film electric resistance heater is employed to simulate heat source and provide heat loads during the normal operation. The heater is attached tightly to the outside surface of the evaporator. The heat loads are able to be adjusted continuously in the range of 0–20 W by altering the output voltage of a DC power. Temperature data and heat load data are recorded and collected by a data acquisition system linked to a PC. All the components are situated in a vacuum chamber during the experimental process except a hot reservoir left outside, which is responsible for pressure reduction of the CLHP at the room temperature. A pressure sensor is connected to the hot reservoir to show the pressure inside the CLHP. All the experiments are operated under vacuum conditions. The CLHP is adjusted to horizontal state or slight anti-gravity state before each experiment, to avoid the influence of gravity
assistance. First, high purity N2 is filled into the hot reservoir and CLHP up to a required pressure at room temperature. Then, liquid nitrogen is filled into the liquid nitrogen tank and the thermal shield, to cool the condenser and the surrounding of the CLHP. The gas-state working fluid is condensed in the condenser and the liquid flows through the flexible wick in the liquid line towards the evaporator with the help of capillary effect. When the evaporator is cooled down under the critical temperature of the working fluid, the CLHP is at the operating temperature range. Heat load imposed on the evaporator results in evaporation at the surface of porous wick and the generated vapor flows back to the condenser through the vapor line. The working fluid begins to cycle in the loop continuously. 4. Experimental results and discussions 4.1. Typical experiment of the CLHP Experiments were carried out to test the new way of supercritical startup of the CLHP, which employed a flexible wick into the liquid line to transport cryogenic liquid from condenser to evaporator without power consumption. In order to prevent the cryogenic liquid flowing towards the evaporator with gravityassistance, the evaporator was elevated 0–5 mm higher relative to the condenser, to keep the CLHP in a horizontal orientation or slight anti-gravity orientation before each experiment. The CLHP was filled with high purity Nitrogen as working fluid and operated at the temperature range of 77–120 K. The tests were performed with 1.28 MPa filling pressure which was filled into the CLHP at ambient temperature. The supercritical startup, normal operation and temperature hysteresis of the CLHP were researched. Typical test result (a.k.a. test 1) is presented in the following paragraph. 4.1.1. Supercritical startup Fig. 5 presents the temperature variations of measuring points distributed on the CLHP during the cool down process. All the temperature measuring points were at the temperature of 299 K in the
Table 1 Parameters of the CLHP (mm). Components
Dimensions
Components
Dimensions
Evaporator Condenser volume Condenser parallel pipes Vapor line Liquid line
(OD L)16.5 24 2296.85 mm3 (ID L n)2.5 40 5 (OD d L)6 0.5 585 (OD d L)3 0.5 473
Pore size (sintered wick) Permeability (sintered wick) Mesh number (flexible wick) Porosity (flexible wick) Hot reservoir
5 lm 5.5 10 100 89% 600 ml
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m2
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Fig. 5. Cool down process of the CLHP.
beginning. After the system reached a required vacuum condition, liquid nitrogen was filled into the shroud from the inlet, which was connected with a 100 L cryogenic dewar. The liquid nitrogen flew along the helical copper pipe in the shroud and the temperature of the shroud (TS) dropped immediately. The environmental temperature around the CLHP was cooled down and parasitic heat from outside was prevented by the cryogenic shroud. At the same time, temperatures of the CLHP decreased slowly due to the thermal radiation from the cryogenic shroud. At about 35 min, the shroud temperature (TS) reached about 95 K and began to decrease slowly. It meant that the surrounding temperature of the CLHP achieved the operating temperature range. At this time, liquid nitrogen was filled into the liquid nitrogen tank and the temperature of the condenser (TC) which is bolted to the liquid nitrogen tank dropped rapidly. About 8 min later, the temperature of the condenser (TC) reached about 83 K and began to decrease very slowly. It indicated that gaseous nitrogen began to condense into liquid state gradually in the condenser. With the time went by, the cryogenic condensate accumulated progressively and wetted the flexible wick stretched into the condenser. Due to the capillarity effect of the porous structure, the cryogenic liquid was pumped into the liquid line through the flexible wick and the gas-liquid interface moved forward gradually. Meanwhile, the liquid line was cooled down to the cryogenic temperature by the liquid passed. At about 48 min, the temperature of liquid line 2 (TL2) dropped immediately, which suggested the liquid reached the middle location of the liquid line. After a few minutes, the temperature of vapor line 2 (TV2) was cooled down quickly and then slowly after a while, indicating that the liquid reached the middle location of the vapor line. Because the diameter of the vapor line was larger than the liquid line and condenser channel, the lowest position of the vapor line was lower than the lowest position of the condenser channel at the inlet of the condenser. When the condenser channel was full of the cryogenic liquid, the liquid near the inlet of the condenser tended to flow into the vapor line due to the gravity effect. That was the reason why some liquid flew into the vapor line and reached vapor line 2 undesirably. At about 58 min, the temperature of liquid line 3 (TL3) dropped immediately and then remained almost constant. It suggested that the liquid reached the end location of the liquid line and would flow into the evaporator soon. After a short while, the slope of the temperature curve of the evaporator increased slightly, which indicated the liquid had flown into the evaporator. The temperature of the evaporator (TE) decreased gradually with a probably constant slope. Compared with the liquid line, the evaporator had a much larger heat capacity and could not be cooled down in a short time. The temperature of the vapor line 1 (TV1) decreased following the trend of the evapo-
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rator because of their close position. At about 135 min, the temperature of the evaporator dropped rapidly to about 90 K and maintained almost constant. It indicated that the evaporator (TE) was cooled down to the operating temperature range. Meanwhile, the temperature of the vapor line 1 (TV1) decreased faster and finally maintained constant, which was cooled down by the cryogenic vapor evaporated in the evaporator. The liquid continuously flew into the evaporator and accumulated in the liquid core. At last, the porous wick of the evaporator was saturated by the liquid, which suggested that the CLHP had achieved the cool down process. Fig. 6 presents the startup process of the CLHP after achieving the cool down process. At about 222 min, the evaporator was heated with a 0.5 W heat load and the evaporator temperature (TE) rose gradually. Meanwhile, the temperatures of liquid line and vapor line (TL2, TL3, TV1 and TV2) also rose gradually. However, the temperature of liquid line 2 (TL2) dropped rapidly after a few minutes, while other temperatures continued rising slowly. Several minutes later, the temperature of liquid line 3 (TL3) dropped rapidly and rose immediately again, and the temperature of the evaporator (TE) slightly decreased at this moment. At about 240 min, with the heat load increased to 1 W, the temperatures (TL2, TL3, TV1 and TE) dropped rapidly again. Then, the temperatures of liquid line (TL2, TL3) rose slightly. After a few minutes, the heat loads were increased to 1.5 W and similar variation occurred with smaller amplitude. When the heat loads were increased to 2 W at about 262 min, all the temperatures reached steady state, which indicated that the CLHP started up successfully with this new way of flexible wick. When the heat loads applied to the evaporator were lower, the liquid continued being transported through the flexible porous wick from the condenser to the evaporator. Moreover, the parasitic heat leak had an impact on the heat transfer performance of the CLHP with a lower heat load, which resulted in higher temperatures of evaporator and transport lines. When the heat loads were increased step by step to 2 W, the liquid was pumped into the cavity channel inside the flexible wick, which could achieve a higher flow rate and overcome the parasitic heat leak effect. At the same time, the liquid line and evaporator were cooled down by the cryogenic liquid flowing in the cavity channel to lower temperatures.
4.1.2. Normal operation From the experimental result above, it indicates that the CLHP is able to achieve supercritical startup from ambient temperature and require no power consumption additionally, with this new way of introducing a flexible wick into the liquid line. Furthermore,
Fig. 6. Startup process of the CLHP.
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the heat transfer performance is the most crucial characteristics of a CLHP. The normal operation process of this CLHP after the supercritical startup process is presented in Fig. 7. The shroud temperature was maintained at about 83 K during the experiment. The heat load was increased step by step from the value of 2 W to 10 W, with 1 W increment. The temperatures of the evaporator (TE) and the condenser (TC) rose gradually to little higher values with each increase of the heat load. Meanwhile, the temperatures of liquid line (TL2, TL3) remained almost constant during this process. After that, the heat load was increased continuously from 10 W to 15 W and then increased to 20 W, and all the temperatures rose to higher values each time. The temperature difference was 10.4 K corresponding to 20 W heat loads. During the test with the heat load in the range of 2–20 W, the CLHP prototype performed stably and reliably. In this test, the maximum heat load applied to the evaporator was 20 W and dryout did not occur all through, indicating that the heat transport capacity of the CLHP prototype was much higher than 20 W. However, the heat load could not be increased larger than 20 W during the test, because of the limitation of the heater in the system. The heat transport capacity will be researched in the future. 4.1.3. Temperature hysteresis test Temperature hysteresis is a common phenomenon in loop heat pipe experiments, which refers to different operating temperatures with equal heat loads during the heat load increasing process and decreasing process. This phenomenon will lead to inconsistent results of LHPs with the same conditions, which has an adverse effect on the performance of the LHPs. Therefore, test on temperature hysteresis of the CLHP prototype was researched. Fig. 8 presents the performance of the CLHP with decreasing heat loads. The heat load was decreased from 20 W to 0.5 W step by step while all the temperatures declined gradually. When the heat load was decreased to 1 W, the temperatures of the evaporator (TE) and the condenser (TC) declined as before. However, the temperatures of the liquid line (TL2, TL3) rose up at this moment and then declined slightly. When the heat load was decreased to 0.5 W, the temperature of the condenser (TC) continued to decline as before, while the temperatures of the evaporator (TE) and transport lines (TL2, TL3) rose up much higher. Then the temperature of the liquid line 2 (TL2) declined to a certain extent. The reason for the variation of the temperatures was that the flow rate of the liquid decreased seriously in the loop when the heat load was decreased to 1 W and 0.5 W. In that case, the parasitic heat leak could not be overcome completely and the temperatures began to rise up. During the test with the heat load decreased from
Fig. 7. Performance of the CLHP.
Fig. 8. Performance of the CLHP with decreasing heat loads.
20 W to 0.5 W, the CLHP prototype reached steady state operation with each heat load. Test results of the temperature difference and thermal resistance with comparison of the forward motion and the reverse motion were presented in Figs. 9 and 10. There were only slight differences with the same value of heat load between the two processes, which demonstrated that the CLHP had an outstanding heat transfer performance in the test. With the increasing of the heat load, the thermal resistance decreased. The minimum thermal resistance was 0.52 K/W versus 20 W heat loads during this test. 4.2. Experiment starting with 0.5 W heat load at room temperature Besides the typical operation of the test 1 presented above, the test 2 was researched, with 0.5 W heat load applied to the evaporator at room temperature before the test begin, which was used for exploring supercritical startup ability of this new way in harsh conditions. The test 2 was performed in the same experimental conditions with the test 1. The supercritical startup, normal operation and power cycling of the CLHP were researched, and comparison of the test 1 and test 2 was carried out. The test result is presented in the following paragraph. 4.2.1. Cool down process Fig. 11 presents the cool down process of the test 2. High purity N2 was filled into the CLHP with 1.28 MPa filling pressure at ambi-
Fig. 9. Temperature difference comparison.
Y.-n. Zhao et al. / International Journal of Heat and Mass Transfer 145 (2019) 118793
Fig. 10. Thermal resistance comparison.
Fig. 11. Cool down process of the CLHP with 0.5 W heat load.
ent temperature. All the temperature measuring points were at the temperature of 282 K in the beginning. The vacuum chamber was evacuated to a required vacuum condition before the experiment startup. At first, a 0.5 W heat load was applied to the evaporator, which led to a rapid temperature increase of the evaporator. After that, liquid nitrogen was filled into the shroud from a 100 L cryogenic dewar. The liquid nitrogen flew along the helical copper pipe in the shroud and the temperature of the shroud (TS) dropped immediately, which resulted in the decrease of the environmental temperature around the CLHP. In this case, the parasitic heat from outside was prevented by the cryogenic shroud. After a short while, temperatures of the CLHP decreased slowly due to the thermal radiation from the cryogenic shroud. At about 20 min, the temperature of the evaporator (TE) stopped rising up and began to decrease slowly. A few minutes, the shroud temperature (TS) reached a cryogenic temperature range under 100 K and began to decrease slowly, which meant that the surrounding temperature of the CLHP had achieved the operating temperature range. At this time, liquid nitrogen was filled into the liquid nitrogen tank and the temperature of the condenser (TC) contacting with the liquid nitrogen tank dropped rapidly. At about 40 min, the temperature of the condenser (TC) reached about 81 K and began to decrease very slowly. At this moment, gaseous nitrogen began to condense into liquid state gradually in the condenser and the cryogenic condensate accumulated progressively. The flexible wick stretched into the condenser was wetted by the cryogenic liquid, and the liquid was pumped into the liquid line through the flexible
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wick due to the capillarity effect of the porous structure. Meanwhile, the gas-liquid interface moved forward gradually and the liquid line was cooled down to the cryogenic temperature by the liquid passed. After several minutes, the temperature of liquid line 2 (TL2), the temperature of vapor line 2 (TV2) and the temperature of liquid line 3 (TL3) dropped immediately and then remained almost constant after that moment, which was similar to the cool down process of test 1. From then on, the cryogenic liquid reached the evaporator and began to cool it down. The temperature of the evaporator (TE) decreased slowly and the slope of the temperature curve of the evaporator also decreased gradually, which was caused by the heat load continuously applied to the evaporator. After quite a long time and at about 360 min, the temperature the evaporator dropped rapidly to about 90 K and finally maintained constant, which indicated that the liquid began to accumulate in the liquid core of the evaporator. At the same time, the temperature of the vapor line 1 (TV1) dropped rapidly following the evaporator. Finally, the porous wick was saturated by the liquid and the CLHP achieved the cool down successfully with 0.5 W heat load applied to the evaporator all along the process.
4.2.2. Startup and normal operation Fig. 12 presents the startup process and the normal operation of the CLHP. Initially, the heat load applied to the evaporator was increased from 0.5 W to 1 W at about 390 min, and then the evaporator temperature (TE) and the transport line temperatures (TL2, TL3, TV1 and TV2) declined quickly after a short while. The reason was that the cryogenic liquid was pumped into the cavity channel of the liquid line and flew towards the evaporator, which were similar to the startup process of test 1. Meanwhile, the condenser temperature (TC) rose slightly and then declined again, suggesting that some vapor suddenly flew into the condenser and then condensed to liquid. The temperatures could maintain steady state and indicated a successful startup of the CLHP in this condition. The experimental result suggested that the CLHP was able to achieve supercritical startup even with 0.5 W heat load applied to the evaporator from ambient temperature. The heat transfer performance was also researched after startup. The heat load was increased step by step from the value of 1 W to 5 W, with 1 W increment. The temperatures of the evaporator (TE) and the condenser (TC) rose gradually to little higher values with each increase of the heat load. Meanwhile, the temperatures of liquid line (TL2, TL3) remained almost constant during this process. Then, the heat load was increased to 7 W, 10 W and 15 W step by step, and the temperatures rose slightly with the increased heat load. The tem-
Fig. 12. Result of supercritical startup and normal heat transfer.
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Fig. 15. Result of power cycling. Fig. 13. Comparison of the temperature difference.
Fig. 16. Thermal resistance variation with power cycling.
Fig. 14. Comparison of the thermal resistance.
perature difference was 11.4 K corresponding to 15 W heat loads. The CLHP performed stably and reliably during this experiment. Both of the test 1 and test 2 could achieve stable performance during the experiments. Fig. 13 presents the comparison of the temperature differences between test 1 and test 2. Obviously, the temperature differences of the test 2 were a little larger than test 1. When a 0.5 W heat load was applied to the evaporator during the supercritical startup of the test 2, the CLHP was cooled down and achieved steady state with a higher saturation temperature. However, the condenser temperature was the same in the test 1 and test 2, so the temperature differences of the test 2 were larger than test 1. Fig. 14 presents the comparison of the thermal resistances between test 1 and test 2. The same descending trends of variation of the thermal resistances were achieved during the tests. With the increase of the heat loads, the thermal resistances of the CLHP decreased gradually, and the differences of the thermal resistances between the test 1 and test 2 became smaller and smaller. 4.3. Power cycling In some practical applications, the heat loads imposed on the CLHP were changed severely during the heat transfer process, which might lead to bad performance or even dryout of the CLHP. Therefore, power cycling test was carried out to examine the performance of the CLHP with severe conditions.
Fig. 15 presents results of a power cycling test conducted with severe 15 W/1 W/15 W/1 W/15 W step-ups/downs cycling. The temperature of the evaporator and the condenser could achieve new steady equilibrium quickly with periodical variation of heat loads. Fig. 16 presents the thermal resistance variations versus the heat load, indicating consistent results with the same heat load. As a result, the CLHP prototype responded extremely well during the power cycling test. 5. Conclusions In this paper, a new way of supercritical startup of a cryogenic loop heat pipe with a flexible porous wick being introduced into the liquid line was presented, which achieved a much simpler structure compared with conventional CLHPs. The flexible porous wick connected the condenser and the evaporator, and transported the cryogenic liquid from the condenser to the evaporator by capillary force. The CLHP operated at liquid-state nitrogen temperature range with nitrogen as working fluid. Experimental investigations on the supercritical startup process and the operation process were presented in the paper, and important conclusions were reached and summarized as follows: &
The CLHP could achieve supercritical startup successfully from the initial ambient temperature of 299 K, without additional power consumption. With a 0.5 W heat load applied to the evaporator, the CLHP could start up successfully.
Y.-n. Zhao et al. / International Journal of Heat and Mass Transfer 145 (2019) 118793 &
&
&
&
The CLHP could transport 20 W heat loads across a 0.51 m distance, and dryout did not happen all through the experiments. With the heat load range of 2–20 W, the CLHP prototype performed stably and reliably. With the increase of the heat load, the thermal resistance decreased. The minimum thermal resistance was 0.52 K/W versus 20 W heat loads. With the comparison of temperature difference and thermal resistance during the forward motion and the reverse motion, temperature hysteresis phenomenon was not obvious in the test. With 0.5 W heat load being applied to the evaporator from the initial ambient temperature, the CLHP could also achieve supercritical startup successfully and operate stably, indicating that the CLHP could achieve supercritical startup even in the harsh conditions. During the power cycling test with 15 W-1 W-15 W-1 W-15 W heat load applied on the evaporator, the temperatures of the evaporator and the condenser could achieve new steady equilibrium quickly with periodical variation of heat loads, which demonstrated that the CLHP performed reliably with this new structure.
Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, ‘‘A new way of supercritical startup of a cryogenic loop heat pipe”. Acknowledgements The work was supported by the National Natural Science Foundation of China (No. 51606207) and Youth Innovation Promotion Association, CAS (No. 2018036). The technical support offered by the staff and colleagues is greatly appreciated. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijheatmasstransfer.2019.118793. References [1] D. Bugby, B. Marland, C. Stouffer, et al., Across-gimbal and miniaturized cryogenic loop heat pipes, Space Technol. Int. Forum-STAIF 2003, pp. 218–226. [2] D. Bugby, B. Marland, C. Stouffer, et al., Development of advanced tools forcryogenic integration, Adv. Cryogenic Eng.: Trans. Cryogenic Eng. Conf. 49 (2004) 1914–1922. [3] Heat Pipe, USSR Inventors Certificate 449213, 1974. [4] Yu.F. Gerasimov, Yu.F. Maydanik, G.T. Shchogolev, et al., Low-temperature heat pipes with separate channels for vapor and liquid, Eng.-Phys. J. 28 (6) (1975) 957–960 (in Russian). [5] Y.F. Maydanik, Loop heat pipes, Appl. Therm. Eng. 25 (2005) 635–657. [6] D. Khrustalev, Cryogenic loop heat pipes as flexible thermal links for cryocoolers, Cambridge, MA, USA, in: Proceedings of the 12th International Cryocooler Conference, June 18-20, 2002, pp. 709–716. [7] D. Khrustalev, Test data for a cryogenic loop heat pipe operating in the temperature range from 65K to 140K, in: Presentation at the International Two-Phase Thermal Control Technology Workshop, September 24–26, Mitcheville, MD, 2002.
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