Investigation on thermal behaviours of a methane charged cryogenic loop heat pipe

Accepted Manuscript Investigation on thermal behaviours of a methane charged cryogenic loop heat pipe

Yuandong Guo, Guiping Lin, Hongxing Zhang, Jianyin Miao PII:

S0360-5442(18)30967-8

DOI:

10.1016/j.energy.2018.05.133

Reference:

EGY 12968

To appear in:

Energy

Received Date:

02 November 2017

Accepted Date:

20 May 2018

Please cite this article as: Yuandong Guo, Guiping Lin, Hongxing Zhang, Jianyin Miao, Investigation on thermal behaviours of a methane charged cryogenic loop heat pipe, Energy (2018), doi: 10.1016/j.energy.2018.05.133

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ACCEPTED MANUSCRIPT

Investigation on thermal behaviours of a methane charged cryogenic loop heat pipe Yuandong Guo1,2,3, Guiping Lin1, Hongxing Zhang3, Jianyin Miao3,* 1 Laboratory of Fundamental Science on Ergonomics and Environmental Control, School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, PR China 2 Shenyuan Honors School, Beihang University, Beijing 100191, PR China 3 Beijing Key Laboratory of Space Thermal Control Technology, Beijing Institute of Spacecraft System Engineering, China Academy of Space Technology, Beijing 100094, PR China

Abstract: As a highly efficient cryogenic heat transfer device, cryogenic loop heat pipe (CLHP) promises great application potential in the thermal control of future space infrared detection system. In this work, a CLHP using methane as working fluid operating at 100-190K was developed, and its thermal performance including the supercritical startup, heat transport capacity under different heat sink, power cycling characteristics, temperature hysteresis phenomenon and thermal resistance variation, was experimentally investigated. Experimental results showed that the CLHP could successfully realize the supercritical startup under various auxiliary heat loads applied to secondary evaporator, reach a various heat transfer capacity under different heat sink temperature over a 0.6m distance, and manifest good response characteristics to the cycle of heat load applied to the primary evaporator. The temperature hysteresis phenomenon was detected and thermal resistance of the CLHP varied with increasing heat load applied to the primary evaporator, but not the same with that in heat load reverse motion. Keywords: methane; loop heat pipe; cryogenic; supercritical startup; temperature hysteresis

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ACCEPTED MANUSCRIPT 1

Introduction As a highly effective two-phase heat transfer technique, heat pipe can transport several orders of magnitude greater heat loads than that of highly conductive solids such as copper bar. Among all types of heat pipes, loop heat pipe(LHP) have the advantage of being able to provide reliable thermal control function over long distance and the ability to operate against gravity, which has been successfully employed in a wide sphere of application both terrestrial based[1–3] as well as space applications[4]. Cryogenic loop heat pipe(CLHP) developed from LHP focuses on the thermal cooling and control demand in low temperature, which could guarantee the infrared sensors and superconductive magnetic materials at a certain temperature range. The usual measures are using cryogenic liquid or mechanical cryocooler, but both of the two methods have some disadvantages such as liquid evaporating leakage, mechanical vibrations and space limitation. Meanwhile, these problems lead to lower efficiency or other energy problems, which are becoming more and more serious. CLHP could solve these problems to a certain extent by cooling down the sensors and magnetic materials directly or indirectly. It could separate vibrations and keep an outstanding temperature consistency and could be used widely for superconducting magnet and electronic devices for harvesting energy. From the time CLHP was put forward and test in ground surroundings, CLHPs have attracted many researchers all over the world, and extensive experimental studies have been conducted almost through all of the low temperature range such as propane for the operating temperature range of 200–240 K, oxygen for 90–140 K, nitrogen for 80–110 K, 2

ACCEPTED MANUSCRIPT neon for 30–40 K and hydrogen for 20–30 K, and helium as low as 2–4 K. Bai et al.[5] presented a review of cryogenic loop heat pipes, which highlighted the key issues for designing CLHP in terrestrial and space application together with five different types in its development. And from the working temperature range, the literature could be classified as follows. In the relatively high temperature range, Pereira et al.[6] constructed and experimentally investigated a CLHP using gravity to assist the startup process and liquid saturation for evaporator wick, whose heat transport capacity could reach 20 W and 30W with argon(90-150K) and propane(200-240K) as the working fluid respectively. Yun et al. [7]firstly reported the experimental test of an auxiliary loop CLHP with ethane as the working fluid, whose operating temperature range was 215K to 218 K. All the transport lines were plated with gold to minimize parasitic heat leak so that it could achieve a heat transport capability of 50W with auxiliary heat load of 5 W applied to the secondary evaporator. The CLHP using auxiliary method to accomplish supercritical startup process is the most potential type, which has an additional auxiliary loop composed of a secondary evaporator(EV2), secondary compensation chamber(CC2), secondary condenser(Con2) and secondary loop line(LL2) in addition to the gas reservoir. The researches of nitrogen CLHP operating in 80K~100K range has been published the most. Mo et al.[8,9] designed and experimentally investigated a nitrogen-charged CLHP with an additional secondary evaporator within condenser line path to assist the supercritical startup process. The secondary evaporator was a traditional grooved heat pipe and was attached directly to the cryogenic heat sink simulated by LN2 copper plate 3

ACCEPTED MANUSCRIPT together with condenser lines. Hoang et al. firstly developed a proof-of-concept CLHP with nitrogen and hydrogen as the working fluids. Hoang and O’Connell[10] designed, fabricated and tested a nitrogen-charged CLHP, which could realize the supercritical startup process and operate at the temperature range of 80–100 K. The CLHP displayed a good performance in power cycling characteristics, heat transport capacity and could reach a transport limit of 5Wwith a transport distance of 4.3m. Gully et al. [11,12] designed and experimentally investigated a nitrogen-charged CLHP. Experimental results were analyzed and discussed both in the supercritical startup and in steady state conditions, and a maximum heat transport capacity of 19Wwith a limited temperature difference (5 K) over a large distance (0.5 m). In order to enlarge the heat transfer capacity of CLHP, Zhao et al. [13,14]introduced a parallel condenser adopted to reduce the flow resistance in the condenser and increase its cooling capability. Experimental results confirmed that the nitrogen CLHP could achieve a significantly enhanced heat transport capacity up to 41W and a limited temperature difference of 6 K across a 0.48m transport distance. Bai et al. [15–17] designed and experimentally investigated a miniature nitrogen CLHP, where the thermal performance such as supercritical startup, effects of component layout and charging pressure were studied extensively. And testing results showed that the miniature CLHP using LN2 as heat sink could realize supercritical startup and steady state operating test. To solve the problems of flexible thermal link for future cryogenic integration, Bugby et al. [18,19]developed three CLHPs: an across-gimbal CLHP, a short transport length miniaturized CLHP and a long transport length miniaturized CLHP. The across-gimbal 4

ACCEPTED MANUSCRIPT CLHP was designed with nitrogen as the working fluid with a heat transport capacity of 20W, in which the coils were designed to sustain at least 500 thousand cycles in the lifetime. In Ref.[20,21] D. Bugby introduced six advanced cryogenic thermal management devices/subsystems developed by Swales Aerospace for ground/space-based applications of interest to NASA, DoD and some of them were designed with redundancy for thermal control in actual space application. In the redundancy design process, R.G. Ross[22] presented an analysis of the reliability advantages and disadvantages of a variety of cryocooler redundancy options, and determined a double cryocooler together with double cryogenic switches scheme based on their total reliability, mass, and power impact at the cryogenic system level. In order to enhance the operation reliability and realize a long-life operation, Guo et al. [23] also designed and tested a redundancy system of two nitrogen charged CLHPs operating at 80K~100K for space application, which had four working modes. In the temperature range below 80K, Khrustalev et al.[24,25] experimentally investigated a CLHP using oxygen as working fluid for flexible thermal linking with cryocoolers. The CLHP owned an additional secondary evaporator, secondary CC and secondary condenser in addition to the gas reservoir to accelerate supercritical startup process and could sustain at 75 K and 100 K when the shroud temperature was maintained at 170 K and 290 K, respectively. However, the researches in oxygen CLHP has been reported fewer than that of nitrogen CLHP. While temperature went deeper, in Ref. [18,19] Bugby et al. developed another two CLHPs: a short transport length miniaturized CLHP and a long transport length 5

ACCEPTED MANUSCRIPT miniaturized CLHP. Both of them utilized neon as the working fluid operating in 30K~40 K, and the short one was as small as an adult hand while the long one was 250cm. Guo et al.[26,27,28] designed and experimentally investigated the supercritical startup characteristics and heat transfer capability of a neon CLHP working at 35K, and the effect of auxiliary heat load and charging pressure was discussed detailly. Hoang[29] experimentally studied a hydrogen-charged CLHP operating at the temperature range of 20–30 K, which could obtain a maximum heat transport capability of 5W over 2.5m. In order to be conducted for future space applications, the CLHP was optimized to minimize its mass and volume and optimized testing results were published in Ref.[30]. From the literature review of CLHP working between 20K and 250K, there was a most vacancy in 150K temperature range and only a couple of papers have mentioned the related research. While the CLHP operates in 150K temperature range between 100K and 200K, there exist four kinds of working fluid available to choose from as shown in Fig. 1. The Dunbar Number that is calculated according to Eq. 1 by surface tension, density, latent heat and viscosity coefficient represents the thermal-physical properties potential to be working fluid.

Du 

v h fg v

Eq.1

where h fg and  are the evaporative latent heat and surface tension of the working fluid,

respectively;  v and v are the density and viscosity coefficient of the vapor working fluid, respectively.

From Fig.1 the argon CLHP could operate between 90K and 150K but get the best performance at 120K and the krypton CLHP could operate between 120K and 200K 6

ACCEPTED MANUSCRIPT getting the best performance at 165K. Similarly, the ethane CLHP could work through a wide temperature range from 100K to 300K, whose best operating temperature was 250K. Meanwhile, with the temperature going down, the expenses for system containing cryocooler, parasitic heat load controlling measures, system complexity would increase exponentially. Therefore, there is a balance between cryogenic system and expenses.

Fig. 1 Variation of Dunbar Parameter of working fluid between 100K and 300K.

In Ref.[20,21], a methane cryogenic diode heat pipe (CDHP) thermal switching system containing three groups of cryocooler and thermal switch was designed, manufactured and tested. And the methane CDHP operating at 100K worked as a cryogenic thermal switch between each cryocooler and sensor with an ON conductance of at least 1 W/K and an OFF resistance of at least 2000 K/W. J. Cepeda[31] designed a 7

ACCEPTED MANUSCRIPT methane cryogenic heat pipe with a liquid trap for on-off actuation for the Space Interferometer Mission Lite (SIM Lite) pre-Phase test. The cryogenic heat pipe got a transport capacity of 15 W across a 1.5 m span coupled with a cold radiator at 160K in space and a LN2 cold plate in ground test. It is worth pointing out that the LHP literature above are all about evaporator with inverted meniscus, which is the classical and has become mainstream in cylindrical LHP. Nonetheless, there is some literature of non-inverted meniscus evaporator in the loop heat pipes, which could be used in miniature loop heat pipe(MLHP) [32][33]. This structural style could be used in relatively small sizes such as PC components, allowing to avoid the inverted meniscus principle. In Ref [33], copper with high thermal conductivity was used for evaporator case and capillary structure to obtain a relatively small thermal resistance. As to the comparison of the advantage and disadvantage between inverted meniscus and non-inverted meniscus, they could not be determined dogmatically. The non-inverted LHP could be used in MLHP with sufficient liquid supply to the evaporator, which may start under relatively small heat load with less axial heat leak. The inverted LHP, as the popular type, are usually used in long distance heat transfer, with wick of relatively small pore diameter (1-2um). Meanwhile, it is usually more expensive and widely used than the former. Due to the lower surface tension and latent heat load of cryogenic working fluid, the capillary wick with smaller radius used in inverted structure is more beneficial. In conclusion, extensive experimental investigation has been conducted on CLHPs through almost each temperature range, which contributes to a better understanding of the working principle and operating performance. However, as discussed above, most of the 8

ACCEPTED MANUSCRIPT researches focused on the temperature range under 120K or higher than 200K. There were few reports about the temperature range of 150K usually using methane as working fluid. And another research significance is that the methane CLHP could be used for pre-cooling and thermal insulation of energy storage equipment. The two-phase temperature range of methane is between 91K and 191K with different thermal physical properties, thus new phenomenon on methane CLHP would be discovered. For instance, temperature hysteresis is always studied in ambient loop heat pipe, and it has not been discussed in CLHP. Therefore, temperature hysteresis would be analyzed as for its relatively large heat transport capacity compared to neon CLHP. In addition, the supercritical startup characteristics under different auxiliary heat load and charging pressure and steady state characteristics would be discussed.

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Experimental system design 2.1 Design of the CLHP prototype The cryogenic loop heat pipe was manufactured in China Academy of Space Technology(CAST) where there is a mature production line of loop heat pipe. The evaporator containing a porous wick is connected with compensation chamber (CC), which is the most complex component of CLHP. As shown in Fig. 2 the structures of the evaporator and compensation chamber that could supply enough liquid into the evaporator continuously are demonstrated. Moreover, the liquid core and liquid conduit, together with vapor groove where vapor is generated are shown in the figure. In this experiment, the wick was made sintered nickel powder.

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Fig. 2 The structure of the evaporator together with compensation chamber.

The design of the condenser is shown in Fig. 3 and it has two spiral condensers made of copper, which are called primary and secondary condenser. The condenser line(Con) is brazed into the spiral channel manufactured on the copper condenser pillar. Meanwhile, the secondary compensation chamber(CC2) and the secondary evaporator(EV2) are connected and brazed to the top of the copper condenser pillar. The condenser copper pillar is processed into a sleeve so that it could be connected with the cylindrical cryocooler cold finger with high thermal conductivity grease conveniently.

Fig. 3 The structure of the condenser and connection between secondary evaporator. 10

ACCEPTED MANUSCRIPT Based on the design of the cryogenic loop heat pipe, it could transmit the heat from primary evaporator to condenser for a long distance and they are just connected through flexible transport lines. Heat of the infrared sensors could be absorbed by primary evaporator and cause the evaporation of internal working fluid (methane). The vapor generated in evaporator flows into the condenser through vapor line and condenses into liquid with latent heat transferred to the pulse tube cryocooler. In this experiment, the thermal load is simulated by the film electrical heater and the DC power source to simulate the heat load with different input watts. Fig. 4 shows the stainless steel cryogenic loop heat pipe used in the experiment which was conducted in CAST. And the parameters of the cryogenic loop heat pipe are shown in Table 1.

Fig. 4 The photo of the stainless steel cryogenic loop heat pipe.

Table 1 Basic parameters of the tested methane CLHP.

Components

Parameter

Dimensions

Material

Casing OD/ID ×length of /mm

13×11×50

Stainless steel

Wick OD/ID ×length/mm

11/4×40

Nickel

Primary evaporator

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Casing OD/ID ×length/mm

13/11×35

Stainless steel

Wick OD/ID ×length/mm

11/4×37

Nickel

Liquid line OD/ID ×length/mm

3/2×600

Stainless steel

Condenser line OD/ID ×length/mm

2/1×700

Stainless steel

Vapor line OD/ID ×length/mm

3/2×700

Stainless steel

Liquid line OD/ID ×length/mm

3/2×600

Stainless steel

Condenser line OD/ID ×length/mm

2/1×260

Stainless steel

Vapor line OD/ID ×length/mm

3/2×30

Stainless steel

Liquid line OD/ID ×length/mm

2/1×700

Stainless steel

Porosity

55.0%

——

Maximum capillary radius/μm

0.5

——

Volume/ml

537

Stainless steel

Secondary evaporator

Main loop

Auxiliary loop

Wick

Gas reservoir

2.2 Experimental system for the test As for the working temperature of cryogenic range which is much lower than ambient temperature, the experiment is always conducted in a vacuum chamber in order to minimize the effect of the ambient environment. And the experimental rig was composed of six parts including vacuum system, heat sink system, heat source system, water circulation system, temperature measurement & data acquisition system as well as cryogenic loop heat pipe. CLHPs should be tested in a vacuum chamber, which prevents convective heat exchange. As a protection against radiation heat input, the heat pipe, fluid lines and cold wall should all be covered with multiple insulation layers (MIL) of 15 layers. 12

ACCEPTED MANUSCRIPT If the heat pipe is mounted such that the mounting points are all at the same temperature, it can be assumed that all heat put on the evaporator will be transported by the heat pipe as there will be no heat path to the environment. A pulse tube cryocooler was adopted to simulate the cryogenic heat sink, the cold finger of which was connected directly to the primary and secondary condensers of the CLHP. Two thin-film electric resistance heaters were employed to provide heat loads to the primary and secondary evaporators, respectively, which were attached tightly to the outer surfaces of the evaporators. The heat loads can be adjusted continuously by altering the output voltage of the DC power, and the maximum uncertainty of the heat load was within ±5.0%. Meanwhile, the system pressure and temperature of characteristic points were measured and recorded during the working process of CLHP. And there are a temperature acquisition module and a pressure module which are both connected to a computer. The temperature change of the CLHP circuit feature point can be displayed in real time and the data is stored with a time interval of 3s. In order to reduce the influence of gravity, all the components of the cryogenic loop heat pipe were set on a horizontal plane during the experimental process.

Fig. 5 Schematic view of the experimental system. 13

ACCEPTED MANUSCRIPT Fig. 5 schematically shows the methane-charged CLHP investigated in this work, which was composed of a main loop and an auxiliary loop together with a gas reservoir. The auxiliary loop was used to realize the supercritical startup, making the CLHPs applicable in both terrestrial and space surroundings in spite that it led to a certain complexity to the system structure of the CLHP. As shown in Fig. 6, the temperature variations of some characteristic points along the loop were monitored by ten type T thermocouples with a measurement uncertainty of ±0.5 K. Meanwhile, a pressure transducer with a measurement uncertainty of about ±5.0% was placed at the outlet of the gas reservoir to monitor the pressure variation in the system.

Fig. 6 Schematic view of the methane CLHP and thermocouple locations.

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Experimental results and analysis 3.1 Supercritical startup 3.1.1 Typical supercritical startup process of CLHP The cryogenic loop heat pipe works in the low temperature range and the internal 14

ACCEPTED MANUSCRIPT working fluid is in a supercritical state on ambient temperature. The supercritical startup of CLHP refers to the process from the room temperature state to a complete cooling down state with a smooth running through a series of measures. A successful realization of the supercritical startup process is one of the important prerequisites for a steady operation for the CLHP. Therefore, it is one of the important indexes during the experimental study of CLHP. At the same time, different structural parameters and working parameters are also important for the supercritical startup. As mentioned above, the supercritical startup process is generally carried out by the gravity assisting method and the capillary pump method. And this prototype used the capillary auxiliary loop to realize the supercritical startup. As shown in Fig. 7, the variation of temperature measurement points and system pressure during the supercritical startup process is demonstrated, and the initial system charging pressure was 1.455MPa. Since the secondary evaporator and secondary compensation chamber (CC2) were fixed to the condenser copper pillar by brazing connection, the temperature of condenser T5 (Con) and secondary evaporator T2 (EV2) dropped rapidly when the pulse tube cryocooler was opened. And the thermal resistance between the condenser line and copper pillar is lower than that of EV2, therefore, the figure showed that T5(Con) had a higher cooling rate than T2(EV2). However, the primary evaporator was connected with the condenser through long stainless pipe whose inner diameter was only 1mm, where the too long transmission distance makes it difficult to transfer heat to primary evaporator. Therefore, the temperature of primary evaporator still kept at room temperature with the secondary evaporator and condenser cooling down. 15

ACCEPTED MANUSCRIPT After about 30 minutes later, the outlet temperature of the secondary evaporator T3(EV2-o) and secondary evaporator T2(EV2) decreased abruptly and the system pressure began to drop. It could be judged that liquid began to condensate in secondary evaporator (EV2), secondary compensation chamber (CC2) and the two condensers. Due to the big difference between density of vapor and liquid, there was a large pressure decrease when working fluid condensates. The secondary evaporator and condenser temperature gradually stabilized at 155K, while at 36th minute the primary evaporator temperature T9 (EV1) began to decrease with the secondary evaporator outlet temperature T3 (EV2-o) decreasing slowly. The main reason was that with liquid appearing in secondary compensation chamber (CC2) and secondary evaporator the gas in gas reservoir could supplement through two paths respectively. The first path is through the vapor line and capillary wick into CC2 and the second one is through condenser line, liquid line, primary evaporator/CC1, secondary liquid line and secondary condenser line into CC2 and secondary evaporator(EV2). However, the flow resistance of the gas through the capillary wick is much greater than that through other pipelines, which means the gas entered into CC2/EV2 mainly through the second path. As the gas entered the primary condenser, the supercooled liquid in it was pushed into main evaporator/main compensation chamber through liquid line causing the main evaporator temperature T9 (EV1) to decrease. At 42th minute the auxiliary load of 1 W was applied to secondary evaporator for supercritical startup. It could be detected T2(EV2), T5(Con) and system pressure(Psys) stayed stable and T9(EV1) together with T10(EV1-o) was continuously reduced. In order to accelerate the cooling down of primary evaporator, the auxiliary heat load was adjusted 16

ACCEPTED MANUSCRIPT to 2W at 130 min. When T9(EV1) and T10(EV1-o) reduced to 156K at the working temperature, the main heat load of 7W was applied to primary evaporator and 0W to secondary evaporator to finish the supercritical startup process.

Fig. 7 A typical supercritical startup process (Pch = 1.455MPa).

3.1.2 The effect of charged pressure to supercritical startup Fig. 8 showed the supercritical startup process of CLHP with the initial charging pressure of 1.713MPa. Comparing with the Fig. 7 the system pressure reduced to 1.4MPa when the condenser cooling down and while the former system pressure was about 1.2MPa with initial pressure of 1.455Mpa. After the secondary evaporator stabilized, the primary evaporator began to reduce gradually with the auxiliary load of 1W being applied. However, as time went on, the condenser decreased gradually and the cooling rate of the main evaporator became slower synchronously. The main reason was that improper temperature controlling measures for cryocooler led to a high output of refrigerating capacity, thus a big super-cooling degree appeared in CC2, EV2 and condenser. The auxiliary load need to offset the supercooling capacity first and then the remnant heat load 17

ACCEPTED MANUSCRIPT could be used for liquid evaporation, which would result in a smaller mass flow rate out of the secondary evaporator. A smaller flow rate in the loop bring less supercooled liquid from condenser to primary evaporator and made it decrease slower and slower. Then the refrigerating capacity output of cryocooler together with the supercooling degree of liquid in condenser and secondary evaporator was reduced by increasing the temperature control power on cold finger. The working fluid started to evaporate under auxiliary heat load and the system pressure rose suddenly, which indicated a continuous cycle. And the step reduction phenomenon of T3(EV2-o) also confirmed the above mentioned small flow rate assumption. The cooling rate of primary evaporator grew to decrease again during the following 20 minutes although T2(EV2) and T5(Con) stayed in a stable state. The main reason was that parasitic heat load from ambient become larger and larger with temperature of CLHP decreasing. Therefore, the auxiliary heat load applied to secondary evaporator was increased to 2W in order to guarantee the primary evaporator cooling process continue. Then it could be concluded that a proper supercooling degree of liquid in EV2 and Con was the first prerequisite for the liquid to vapor phase change in secondary evaporator under certain auxiliary heat load. And as T9(EV1) went down, a larger auxiliary heat load should be applied to guarantee the same cooling rate of primary evaporator till the end of supercritical startup process. Compared with the above Fig. 7 with smaller charging pressure, the total time of condenser and secondary evaporator cooling maintained about 40min in spite of a higher initial charging pressure. However, the steady state of system pressure at the end of secondary evaporator cooling down was relatively higher, which 18

ACCEPTED MANUSCRIPT means that the phase change temperature within secondary evaporator increased.

Fig. 8 Supercritical startup process (Pch = 1.713MPa).

3.1.3 The effect of auxiliary heat load to supercritical startup As shown in Fig. 9, the supercritical startup temperature variations of methane CLHP with charging pressure of 2.54MPa were shown. The first stage that T5(Con) and T2(EV2) decreased was similar with the process that of Fig. 7 and Fig. 8. However, system pressure dropped from 2.5MPa down to 2MPa rapidly and then stayed stable when working fluid condensed into liquid in secondary evaporator, secondary CC and condenser at the 20th minute. At 32min an auxiliary heat load of 1W was applied but the temperature of primary evaporator stayed unchanged. That was because system pressure stayed at 2.01MPa whose corresponding saturation pressure was 167K, while the temperature of secondary evaporator wall T2(EV2) was 160K and T5(Con) was 140K. Therefore, the auxiliary load had to offset the supercooling capacity with degree of 6K to reach the saturation temperature first and then overcome the latent heat to produce vapor if the liquid evaporated at the interface of secondary evaporator. Since the output power of the 19

ACCEPTED MANUSCRIPT cryocooler was constant and higher than the total heat load consisted of auxiliary heat load and parasitic heat load, supercooling power caused the condenser temperature T5(Con) to continue to decrease and therefore vapor could not be generated under auxiliary heat load of 1W. Therefore，the auxiliary heat load was changed to 2W and T2(EV2) increased slightly with no vapor produced. It was similar with the case of 3W that T2(EV2) and T5(Con) elevated slowly but the primary evaporator temperature T9(EV1) did not decrease indicating no liquid into primary reservoir and primary evaporator. After the auxiliary heat load of 4W being applied secondary evaporator, T9(EV1) began to decline at system pressure of 1.967MPa corresponding to the saturation temperature of 165K, which meant the cycle of working fluid in CHLP. In the following test, the primary evaporator continued to cool down with auxiliary heat load of 4W. In order to prevent secondary evaporator being dried off under such high load, the auxiliary heat load was reduced to 2W. However, the cooling rate of primary evaporator reduced and a supercooling degree began to appear in T5(Con) with time going on. The reason for the analysis was that the output power of the refrigerator was constant and greater than 2W and a supercooling degree appeared in CC2 together with EV2. Therefore, one part of auxiliary heat load should offset the supercooling capacity first, which was similar with the case in Fig. 9. At 210th minute, the system pressure(Psys), T9(EV1) and T10(EV1-o) dropped indicating liquid methane appeared in primary evaporator(EV1) and primary compensation chamber(CC1) and began to fulfill the porous wick in primary evaporator casing under auxiliary heat load of 3W. Then the supercritical 20

ACCEPTED MANUSCRIPT startup process was completed under primary heat load of 1W and auxiliary heat load of 3W respectively and all the temperature was stabilized at about 165K. It could be concluded that the auxiliary heat load is very necessary for supercritical startup and an improper auxiliary heat load such as too small is unfavorable for the process especially under a relatively high charging pressure.

Fig. 9 Supercritical startup process (Pch = 2.540MPa).

3.2 Steady operation characteristics of the methane CLHP Steady-state operating characteristic is another important feature of CLHP. It refers to the characteristics that CLHP displays after supercritical startup process including heat transport capacity, power cycling response and temperature hysteresis characteristics.

3.2.1 The effect of heat sink temperature on heat transport capacity Steady-state heat transport capability test is an important indicator to evaluate the working performance of cryogenic loop heat pipe which represents the maximum heat transfer capacity of CLHP and heat transport capacity differs under various working parameters such as different charging pressure, auxiliary heat load and heat sink 21

ACCEPTED MANUSCRIPT temperature[27]. In this paper, the effect of heat sink temperature on heat transport capacity was studied experimentally. As shown in Fig. 10 and Fig. 11, the operating curves of the methane CLHP under different heat sink temperature with charging pressure of 1.341MPa were illustrated. The heat transport capacity test was tested under heat sink temperature of 143K and 147K respectively, and the system pressure(Psys) was stabilized at about 0.75MPa and 1.0MPa through the whole test process. Fig. 10 shows heat transport capacity test under heat sink temperature of 143K, where the auxiliary heat load applied to secondary evaporator was 0W. With the main heat load applied to primary evaporator increasing from 6W to 8W gradually, the system pressure and temperature could maintain at a constant level 0.78MPa and 143 K respectively throughout the operation process although T3(EV2-o) had a low frequency fluctuation. The main reason was that auxiliary heat load of 0W made the capillary force in secondary evaporator invalid to guarantee the working fluid cycling. Therefore, the secondary evaporator would be heated by parasitic heat load from the ambient surroundings and T3(EV2-o) rose up with temperature fluctuations. When CLHP operated stably with main heat load of 8W for 20min, the primary evaporator temperature T9(EV1) rose up sharply together with a decrease in T5(Con) indicating that the heat transport capacity of this methane CLHP was about 8W under the heat sink temperature of 143K.

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Fig. 10 Heat transport capability test (Pch = 1.341MPa, Tsink=143K).

Compared with Fig. 10, the initial charging pressure of Fig. 11 was the same while the steady-state system pressure and temperature were higher than the former case. When the main heat load was increased from 5W to 6W for only 5 minutes, the primary evaporator dried off and T9 (EV1) rose up rapidly to the heat transport limit. The main reason was that the system pressure was relatively higher than that of Fig. 10 and the mass in gas reservoir was more, resulting less working fluid in the circulation to evaporate. Therefore, the primary evaporator could not be fulfilled with liquid methane and would be dried out under a relatively lower heat load.

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Fig. 11 Heat transport capability test (Pch = 1.341MPa, Tsink=147K).

3.2.2 Power cycling characteristics The temperature and pressure curves for power cycling response characteristics of methane CLHP is illustrated with an initial charging pressure of 1.110Mpa in Fig. 12. Due to the effect of auxiliary heat load on supercritical startup process and steady-state operating characteristic, various auxiliary heat loads would also affect the power cycling characteristics differently. Insufficient charging pressure will cause the operating temperature dropping and the phenomenon of liquid suppling shortage to primary evaporator(EV1). Therefore, the auxiliary heat load was set as 1.5W-1W-0.5W during the power cycling test although the methane CLHP prototype could operate stably without auxiliary heat load. In the test the main heat load was cycled between 1W and 9W under various auxiliary heat load. At first the system pressure(Psys) and T9(EV1) were stabilized at 0.276MPa and 128.6K with auxiliary and primary heat load of 1.5W and 9W respectively. The temperature T10 (EV1-o) began to rise when the main load was reduced to 1W mainly due to parasitic heat load from ambient under a lower mass flow rate in the 24

ACCEPTED MANUSCRIPT circulation; and it was also further validated from the other side when the main heat load increased from 1W to 9W in the following test. From the picture, it can be seen that the operating temperature of methane CLHP went down by 10K gradually under auxiliary heat load of 1W and 0.5W while system pressure did not differ obviously. And the fluctuation on T3(EV2-o) and T10(EV1-o) became more and more fierce. It was mainly because the temperature delay while changing the heat load under a low auxiliary heat load and the thermal-physical properties of methane in low temperature range such as density and saturation pressure. The saturation pressure and density corresponding to saturation temperature is shown in Fig. 13, where the saturating pressure varies little while temperature is below 130K and the saturation pressures are 0.235MPa and 0.286MPa corresponding to 123K and 126K. In addition, the liquid density changes evidently with temperature decreasing which meant less liquid in CC1 and EV1 with a larger gas friction. This would be one of the reasons for fluctuation on T3(EV2-o) and T10(EV1-o) and another reason was similar to the former explanation in above part that lower auxiliary heat load could not offset the effect of parasitic heat load from the ambient and caused the fluctuations. Consequently, a proper auxiliary load would not only be helpful for the supercritical startup process but also the power cycling characteristic and a small auxiliary heat load would cause fluctuations on the methane CLHP prototype.

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Fig. 12 Power cycling response characteristics (Pch =1.110MPa).

Fig. 13 Thermal-physical properties differences of methane depending on temperature.

3.2.3 Temperature hysteresis phenomenon Hysteresis phenomenon introduced by S. Vershinin in Ref. [34] is always talked and investigated in ambient loop heat pipe. It refers that the operating temperature curve would differ with the heat load rising or decreasing. This phenomenon was studied in methane CLHP in Fig. 14. As the primary heat load rose up from 2W to 10W with auxiliary heat 26

ACCEPTED MANUSCRIPT load of 1W, the operating temperature presented an upright-hook shape which consisted of variable conductance mode(VCM) and constant conductance mode (CCM). As the heat sink temperature could not be controlled in an exactly certain state, the heat sink temperature varied when heat load was changed although temperature control measures had already been taken to cryocooler cold finger. Therefore, the temperature difference between primary evaporator and condenser was calculated to represent the operating temperature state dependent on primary heat load changing. The temperature difference decreased with heat load rising when the primary heat load was smaller than 4W, indicating the VCM stage of the methane CLHP. Then the temperature difference was elevated with primary heat load rising in the constant conductance mode(CCM). However, it was a distinct level on the temperature difference when primary heat load reversed from 10W to 2W. Firstly, the difference was larger than the forward motion at each certain heat load. Secondly, the operating temperature curve did not equip a conventional upright-hook shape but a fluctuating curve in spite of a large or small primary heat load.

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Fig. 14 Temperature hysteresis phenomenon of methane CLHP.

Fig. 15 shows the thermal resistance variations of methane CLHP during the temperature hysteresis test with the same auxiliary heat load of 1W. Thermal resistance of the CLHP is defined as follows: R

Tw, pe  Tcf

（1）

Q pe

In equation (1), R is the thermal resistance of the CLHP; Tw, pe is the temperature of the primary evaporator wall, and Tcf is the temperature of the cold finger of the pulse tube cryocooler. This method to calculate thermal resistance is conservative to cover the heat interface resistance between condenser and cold finger. From Fig. 15 the thermal resistance was reduced while heat load was smaller than 4W, which meant the conductance varied to be larger in VCM. And the conductance stayed 28

ACCEPTED MANUSCRIPT nearly unchangeable in CCM, which has been explained in former part. Different from the forward motion, the thermal resistance in reverse motion was reduced almost from 2W to 10W consistent with the temperature difference curve illustrated in Fig. 15. And the main reason for the temperature hysteresis phenomenon might be as follows. On the one hand, the methane CLHP system could not get a new steady state while the primary heat load was changed from a large one to a small one due to the system heat capacity. One the other hand, there would be larger gas zone in primary evaporator(EV1) and condenser(Con) while the primary heat load was relatively large, which would be pushed into the primary compensation chamber(CC1) when primary heat load was changed to a small one suddenly. The gas into CC1 would affect the liquid/gas interface and heat leak from evaporator to CC1, which would impair the conductance of CLHP and result a larger thermal resistance.

Fig. 15 Thermal resistance variations of methane CLHP during temperature hysteresis test. 29

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Conclusion In this work, an extensive experimental study on the transient and steady characteristics of a methane-charged CLHP integrated with a pulse tube cryocooler was conducted, where the effects of the auxiliary heat load and charged pressure of the working fluid on supercritical startup process were investigated and analyzed. The effect of heat sink temperature on heat transport capacity, power cycling characteristics and temperature hysteresis phenomenon were studied experimentally. Based on the experimental results, main conclusions below can be drawn: 1) With an auxiliary heat load of 2W applied to the secondary evaporator the methane-charged CLHP could realize the supercritical startup successfully; however, when the auxiliary heat load was 1.0 W, it failed due to the effect of parasitic heat load from the ambient surroundings. 2) During the supercritical startup process, some measures to control the heat sink temperature should be taken as a supercooling degree in Con and EV2 would consume one part of auxiliary heat load and lead the startup to failure. 3) The methane CLHP is not sensitive to the charged pressure of working fluid, and it operates with a relatively large range due to its thermal physical properties of large two-phase temperature range. 4) The heat sink temperature would affect the heat transport capacity of methane CLHP, especially with a relatively small charging pressure. 5) The methane CLHP has good power cycling characteristics while primary heat load was changed from relatively large to a small one, and a proper auxiliary heat 30

ACCEPTED MANUSCRIPT load together with temperature control measure would be helpful to its operation. 6) Temperature hysteresis phenomenon is detected in this methane CLHP, which is distinct with that of ambient LHP and the reason for it has been discussed mechanically.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51576010 and 51306009) and National Science Foundation Research (Nos.613322). This work is also supported by the Academic Excellence Foundation of BUAA for PhD Students.

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ACCEPTED MANUSCRIPT Nomenclature Pch

charged pressure of the working fluid （MPa）

Psys

pressure in the system （MPa）

Qpe

heat load applied to the primary evaporator

Qse

heat load applied to the secondary evaporator

（W） （W）

thermal resistance （K/W）

R Tw,pe

temperature of the primary evaporator wall （K）

Tcf

temperature of the cold finger of the cryocooler （K）

Du

Dunbar Number of working fluid

h fg

evaporative latent heat



surface tension

v

vapor density of working fluid

v

viscosity coefficient of the vapor working fluid

Abbreviations CLHP

cryogenic loop heat pipe

LHP

loop heat pipe

CAST

China Academy of Space Technology

LN2

liquid nitrogen

CC2

secondary compensation chamber

Con

primary condenser 35

ACCEPTED MANUSCRIPT Con-o

outlet of the primary condenser

EV1

primary evaporator

EV1-o

outlet of the primary evaporator

EV2

secondary evaporator

EV2-o

outlet of the secondary evaporator

LL2

secondary loop line

VCM

variable conductance mode

CCM

constant conductance mode

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Table captions Table 1 Basic parameters of the tested methane CLHP.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1 Variation of Dunbar Parameter of working fluid between 100K and 300K. Fig. 2 The structure of the evaporator together with compensation chamber. Fig. 3 The structure of the condenser and connection between secondary evaporator. Fig. 4 The photo of the stainless steel cryogenic loop heat pipe. Fig. 5 Schematic view of the experimental system. Fig. 6 Schematic view of the experimental system. Fig. 7 A typical supercritical startup process (Pch = 1.455MPa). Fig. 8 Supercritical startup process (Pch = 1.713MPa). Fig. 9 Supercritical startup process (Pch = 2.540MPa). Fig. 10 Heat transport capability test (Pch = 1.341MPa, Tsink=143K). Fig. 11 Heat transport capability test (Pch = 1.341MPa, Tsink=147K). Fig. 12 Power cycling response characteristics (Pch =1.110MPa). Fig. 13 Thermal-physical properties differences of methane depending on temperature. Fig. 14 Temperature hysteresis phenomenon of methane CLHP. Fig. 15 Thermal resistance variations of methane CLHP during temperature hysteresis test.

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Highlights 

A methane CLHP integrated with a pulse tube cryocooler has been developed.



Heat sink temperature could affect the heat transport capacity of methane CLHP.



The CLHP can operate with a relatively large range from 120K to 170K.



Temperature hysteresis phenomenon is found in this methane CLHP.