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The realization of a high heat flux removal with a flat loop heat pipe at ambient temperature is of great importance for achieving thermal control within spacecraft electronic equipment. In this article, a flat bifacial evaporator loop heat pipe with two flat thermo-contact surfaces has been proposed, fabricated, and investigated. The shell of the flat evaporator was made of stainless steel, and ammonia was selected as the working fluid. The startup and operating characteristics of the single side, both sides, and switching working modes were analyzed, as well as the influence of heat consumption, orientation, wind speed, and environmental temperature. The experimental studies indicated that this novel loop heat pipe can start up and run successfully under different working modes, and that the heat dissipation capacity of a set of loop heat pipes can reach 250 W at room temperature. Based on this, a light-weight and small electronic device has been developed using a multi-functional (efficient 'heat collection-heat dissipation-structural support') flat bifacial loop heat pipe to realize a kilowatt level heat dissipation capacity. This work can provide reference for the future design of high heat flux, light-weight, and small payload for use in spacecraft. [ABSTRACT FROM AUTHOR]

Next generation space infrared sensor and detector have pressing requirement for cryogenic heat transport technology in the temperature range of 30-40 K. Cryogenic loop heat pipe (CLHP) has excellent thermal performance and particular characteristics such as high flexibility transport lines and no moving parts, thus it is regarded as an ideal thermal control solution. A neon CLHP referring to infrared point-to-point heat transfer element in future space application has been designed and experimented. And it could realize supercritical startup successfully. Experimental results show that the supercritical startup were realized successfully at cases of 1.5 W secondary evaporator power, but the startup was failed when 0.5 and 1 W heat load applied to secondary evaporator. The maximum heat transport capability of primary evaporator is between 4.5 and 5 W with proper auxiliary heat load. Before startup, even the heat sink temperature decreased to 35 K, the primary evaporator can still maintain at almost 290 K; and the primary evaporator temperature increased at once when the powers were cut off, which indicated the CLHP has a perfect function of thermal switch. The CLHP could adapt to sudden changes of the primary evaporator power, and reach a new steady-state quickly. Besides, some failure phenomena were observed during the test, which indicated that proper secondary evaporator power and heat sink temperature play important roles on the normal operation. [ABSTRACT FROM AUTHOR]

This paper describes the development and result of ground experiment and analyzes the on-orbit operation of Loop heat pipes (LHP) for the Chinese circumlunar return and reentry spacecraft. A novel LHP was developed to satisfy the thermal control requirements of Inertial measurement units (IMU). The LHP consisted of an integrated evaporator subassembly, a cambered condenser, and an electric explosion valve. Ground experiments were conducted to investigate the startup characteristics, and heat transfer and heat insulation performance of the LHPs. During a flight mission, the LHPs work as a thermal switch and perform both heat transfer and insulation functions. Before returning to the earth's atmosphere, the valves of the reentry capsule are opened to discharge working fluid and prevent heat transfer from the capsule wall to the devices. In the flight mission, the LHPs performed all the required functions, and the IMU temperatures were maintained within an allowable range. [ABSTRACT FROM AUTHOR]

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 100–190 K 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 0.6 m 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. [ABSTRACT FROM AUTHOR]

Abstract: Cryogenic loop heat pipes (CLHPs) are effective and efficient cryogenic heat transport devices suitable for many space and terrestrial applications. Due to the complexity of its structure, the startup and operation of CLHPs might be influenced by the relative position of different components; however, little work has been done on this aspect. In this work, a miniature CLHP with nitrogen as the working fluid employing an auxiliary loop to assist the supercritical startup was developed. The effect of the layout of the secondary loop line and the gas reservoir on the startup and operation of the CLHP was experimentally investigated. The experimental results show that the relative position of these components can affect both the startup and steady-state operation of the CLHP. The heat transport capacity of the CLHP can be increased considerably when the secondary loop line is connected to the middle or top of the primary compensation chamber. [Copyright &y& Elsevier]

Abstract: Aiming at future space applications, a miniature cryogenic loop heat pipe (CLHP) with nitrogen as the working fluid was designed, whose condenser could provide the interface with the cold finger of cryocooler, and its operating characteristics were experimentally investigated in this work. Based on the experimental results, important conclusions below have been drawn: (1) with only 2.5W applied to the secondary evaporator, the CLHP can realize the supercritical startup, and the larger the heat load applied to the secondary evaporator, the sooner the temperature drop process of the primary evaporator; (2) when the heat load applied to the primary evaporator is no less than 3W, the primary evaporator can operate independently; whereas when it is smaller than 3W, the secondary evaporator must be kept in operation to assist the normal operation of the primary evaporator; (3) the CLHP has a heat transport capacity of 12W×0.56m, and its thermal resistance decreases with the increase of the heat load applied to the primary evaporator; (4) the CLHP has the ability to operate with a small heat load applied to the primary evaporator for a long time, and manifests good thermal control performance. [Copyright &y& Elsevier]

Abstract: Cryogenic loop heat pipes (CLHPs) are effective and efficient cryogenic heat transport devices suitable for many space applications. In this work, a miniature cryogenic loop heat pipe (CLHP) with nitrogen as the working fluid was designed and experimentally investigated. An auxiliary loop was employed to assist the supercritical startup of the primary evaporator. The operational characteristics of the CLHP and the matching characteristics of heat loads applied to the primary and secondary evaporators were investigated experimentally. The results show that the CLHP can achieve reliably the supercritical startup when the heat load applied to the secondary evaporator is no less than 3W; when the heat load applied to the primary evaporator is no less than 2.5W, the primary evaporator can operate independently, otherwise a proper selection of the heat load applied to the secondary evaporator should be considered to overcome the parasitic heat load from the ambient. The CLHP is working at the variable conductance mode and can achieve smooth operational transition subject to a large step change of the heat load applied to the primary evaporator. [Copyright &y& Elsevier]

• A neon CLHP integrated with a G-M cryocooler was developed and investigated. • Three factors were identified to cause supercritical startup failure of the CLHP. • The CLHP can realize supercritical startup with a small amount of impurity air. • Three factors were found to have detrimental effect on CLHP steady-state operation. • The CLHP can reach a heat transport capability of 4.8 W over a distance of 0.6 m. As a highly efficient cryogenic heat transfer device, cryogenic loop heat pipes (CLHP) hold significant application potential in the thermal control of future generation space infrared detection system. However, operation failure or anomaly occurs more easily for CLHPs, no matter in the supercritical startup or steady-state operation, due to very low operating temperature combined with narrow operating temperature range. In this work, a neon CLHP operating at 30–40 K was developed, and its operating characteristics were investigated experimentally mainly about the temperature fluctuation and operation failure phenomena. To be specific, the influences of charged pressure of working fluid, primary heat load and impurity air on the supercritical startup, and the effects of heat sink temperature, charged pressure of working fluid and auxiliary heat load on the steady-state operation were studied. Experimental results showed that the application of small primary and secondary heat loads may lead to a supercritical startup failure. Insufficient working fluid under relatively low charged pressure would cause temperature fluctuation of the secondary evaporator during the supercritical startup. There existed a maximum percentage of impurity gas, i.e., about 0.35%, allowing the CLHP to achieve a successful supercritical startup. The secondary evaporator would dry out if the CLHP was subject to improper heat sink temperature, charged pressure of working fluid or auxiliary heat load, due to insufficient liquid supply to the secondary evaporator wick or the capillary limit. [ABSTRACT FROM AUTHOR]

Abstract A cryogenic integrated system, consisting of a neon loop heat pipe, two pulse tube cryocoolers and other components, was designed and tested in the terrestrial environment. This paper presents the test results of the pre-flight thermal performance of the cryogenic integrated system, such as supercritical characteristics, steady-state operating characteristics and critical heat transfer capacity. The integrated system could achieve supercritical startup under auxiliary heat load of 0.4 W and obtain maximum heat transfer capacity of 1.3 W under anti-gravity attitude. The results show that both the function and the thermal performance are in concordance with the expectations, which validates the design of this cryogenic integrated system. Radiator temperature play an important role in the operation of the integrated system, where a temperature increment of 5°C would lead the supercritical startup to fail. [ABSTRACT FROM AUTHOR]

Neon-charged cryogenic loop heat pipe (CLHP) can realize efficient cryogenic heat transport in the temperature range of 30–40 K, and promises great application potential in the thermal control of future space infrared exploration system. In this work, extensive experimental studies on the supercritical startup and heat transport capability of a neon-charged CLHP integrated with a G-M cryocooler were carried out, where the effects of the auxiliary heat load applied to the secondary evaporator and charged pressure of the working fluid were investigated. Experimental results showed that the CLHP could successfully realize the supercritical startup with an auxiliary heat load of 1.5 W, and there existed an optimum auxiliary heat load and charged pressure of the working fluid respectively, to achieve the maximum temperature drop rate of the primary evaporator during the supercritical startup. The CLHP could reach a maximum heat transport capability of 4.5 W over a distance of 0.6 m corresponding to the optimum charged pressure of the working fluid; however, the heat transport capability decreased with the increase of the auxiliary heat load. Furthermore, the inherent mechanisms responsible for the phenomena observed in the experiments were analyzed and discussed, to provide a better understanding from the theoretical view. [ABSTRACT FROM AUTHOR]

Cryogenic loop heat pipe (CLHP) is one of the key components in the future space infrared exploration system, which enables effective and efficient cryogenic heat transport over a long distance with a flexible thermal link. To realize reliable and long life operation, a CLHP-based thermal control system with redundancy design was proposed in this work, where two nitrogen-charged CLHPs were employed to provide cryocooling at 80–100 K. This study focused on the supercritical startup of the CLHPs with redundancy design, and an extensive experimental study under four possible working modes was conducted. Experimental results showed that with 2.5 W applied to the secondary evaporator, each CLHP could realize the supercritical startup successfully in the normal working mode; however, the required time differed a lot because the difference in the transport line diameter significantly affected the cryocooling capacity to the primary evaporator. In the backup conversion mode, instant switch of the two primary evaporators may cause an operation failure, and an auxiliary operation of the secondary evaporator in advance was necessary to make the primary liquid line filled with liquid. In the malfunction conversion mode, the simulated infrared detector had to be first shut down, but the time needed for the backup CLHP to realize the supercritical startup became obviously shorter than that in the normal working mode, because the primary evaporator of the backup CLHP was always in a cryogenic state. In the dual operation mode, the two CLHPs could realize the supercritical startup simultaneously, but a temperature oscillation phenomenon was observed, which can be eliminated by increasing the heat load applied to the secondary evaporator. [ABSTRACT FROM AUTHOR]

The porous wick in a heat pipe determines the working fluid capillary replenishment ability and heat transfer process. To study the evaporation characteristics of meshed wicks fabricated with wires of micron-scale, a mathematical model based on the augmented Young-Laplace equation and molecular kinetic theory to predict the evaporation interface profile is developed, and the Marangoni effect and slip velocity in the boundary are considered. The evaporation interface can be divided into the thin-film region and intrinsic meniscus region according to the ratio of capillary pressure and disjoining pressure. A new solution method considering the constraints at the intersection of the two regions to obtain the interface profile is proposed and validated. Based on the two-dimensional results, the area and heat transfer rates of the two regions formed in the meshed wicks are calculated by integrating along the four side wires. Then, the evaporation characteristics of the thin-film region, including the pressure and thermal resistance distribution, are studied. Finally, the influence of the Marangoni effect, superheat, wire spacing, interface pressure difference and slip velocity on the interface profile and heat transfer of these two regions are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Non-condensable gas (NCG) is one of the main causes affecting the thermal performance and lifespan of the two-phase heat transfer devices. In this work, the effect of NCG on the operating performance of an ammonia–stainless steel loop heat pipe (LHP) was experimentally investigated. Nitrogen was selected as NCG, and the steady-state operating characteristics of the LHP with NCG was systematically studied for different NCG inventories, heat loads applied to the evaporator, heat load cycles and heat sink temperatures. Experimental results reveal that NCG elevates the operating temperature of the evaporator, and the more NCG exists, the higher the operating temperature of the evaporator and the larger the heat load range corresponding to the variable conductance mode. Meanwhile, the effect of NCG is notable for a smaller heat load or lower heat sink temperature. The increase of the operating temperature attributes to that NCG breaks the original energy/pressure balance of the components of the LHP, which leads to further change of heat and mass transfer as well as the redistribution of the working fluid in the loop. With NCG in the LHP, the temperature hysteresis is a common phenomenon for different heat load cycles. In addition, unique temperature oscillation was observed in the experiments, and it may be associated with the frequent repeated processes of the generation/collapse of the bubbles in the evaporator core. [Copyright &y& Elsevier]

Abstract: In this paper, a miniature cryogenic loop heat pipe (CLHP) with nitrogen as the working fluid was designed, and the principle and calculation method to determine the charged pressure of working fluid were proposed and experimentally validated. Meanwhile, the effect of reduced charged pressure of working fluid on the operation of the CLHP such as the supercritical startup, matching characteristics of heat loads applied to the primary and secondary evaporators and heat transport capacity was experimentally investigated. Experimental results reveal that with an appropriate charged pressure of working fluid, the CLHP can reliably realize the supercritical startup, the primary evaporator can operate independently when the heat load applied to the primary evaporator is larger than a certain threshold value, and it achieves a heat transport capacity of 12W. The effect of reduced charged pressure of working fluid on the supercritical startup and heat transport capacity of the CLHP is not salient. However, it has significant effect on the matching characteristics of heat loads applied to the primary and secondary evaporators, and the secondary evaporator must be kept in operation to assist the normal operation of the primary evaporator even though the heat load applied to the primary evaporator is comparatively large. [Copyright &y& Elsevier]

• Operating performance of DCCLHP is studied experimentally under acceleration field. • The effect of thermoelectric cooler (TEC) on operating performance are analyzed. • The effect of acceleration magnitudes on operating performance of LHP is discussed. • The effect of acceleration directions on operating performance is concluded. This paper presents an experimental investigation on a novel dual compensation chamber loop heat pipe (DCCLHP) integrated with thermoelectric cooler (TEC) for improving the overall performance under various acceleration conditions. The operating characteristics of the DCCLHP with and without the assistance of TEC for configurations A and B are analyzed systematically under different acceleration magnitudes and heat loads. The heat load on the evaporator ranges from 25 W to 300 W and the power of the TEC is 10 W. The acceleration magnitude ranges from 5 g to 13 g. Experimental results show that: (i) the application of TEC is proved to improve the operating performance for the heat load not exceeding 150 W under configuration A and 100 W under configuration B; (ii) under configuration B, the increase of the acceleration normally leads to the increase of the operating temperature and the decrease of the thermal conductance. While it shows no obvious effect on the operating performance under configuration A for most cases. The maximum operating temperature and thermal conductance is 56.7 °C at 150 W under 13 g for configuration B, while 20.7 W/K at 150 W under 5 g for configuration A; (iii) temperature fluctuation occurs at 100 W under both 5 g and 7 g for configuration B, whereas there is no fluctuation under configuration A. The acceleration effect can change the vapor-liquid distribution in the loop and the external loop pressure drop, which will further change the operating performance along with the TEC effect. This research can provide valuable insight into the influence of TEC on DCCLHP under different acceleration field, which is of great significance for its practical application in mechanical and aerospace engineering. [ABSTRACT FROM AUTHOR]

l Operating behavior of a DCCLHP subjected to assisted acceleration force is studied. l Operating performance at both loading modes and in terrestrial gravity is analyzed. l Operating temperature profile shows ''/-shape'' oblique line at large accelerations. l Operating mechanism under acceleration force assisted condition is discussed. l Both operating modes driven by capillary force and centrifugal force are proposed. In this article, a combined experimental and theoretical study has been conducted to investigate the operating characteristics of a dual compensation chamber loop heat pipe (DCCLHP) with ammonia as the working fluid under acceleration force conditions. The DCCLHP with nickel wick consists of a cylindrical evaporator and dual compensation chambers at both ends of evaporator. In the current study, a new DCCLHP test rig is set up which can provide the acceleration up to 11 g. Two types of loading mode are utilized for applying heat load prior to acceleration. The heat load ranges from 25 W to 300 W. Comparisons of operating performance of the DCCLHP are carried out under both gravity and acceleration conditions. A novel acceleration force assisted concept is proposed to address the observed operating behavior. Experimental results show that: (i) the acceleration effect with the proposed orientation can improve the operating performance of the DCCLHP which may operate at the centrifugal force driven mode and capillary-centrifugal force co-driven mode. The operating temperature profile at different heat loads shows "/-shape" oblique line with the increase of the acceleration; (ii) the transition heat load from centrifugal force driven mode to capillary-centrifugal force co-driven mode changes with the variation of acceleration magnitude at both loading modes; (iii) the acceleration effect on the operating temperature is remarkably significant as the heat load is less than 100 W. The operating temperature under acceleration conditions is apparently lower than that under terrestrial gravity; (iv) the coupling change of the loop pressure, vapor-liquid distribution, two-phase flow and heat transfer caused by acceleration effect leads to the unique operating performance of the DCCLHP. [ABSTRACT FROM AUTHOR]