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Loop heat pipe (LHP) promises great application potential in electronics cooling. However, the selection of environmentally friendly working fluid remains a big issue, especially in commercial applications. Although R134a is a common refrigerant, the R134a-charged LHP is limited by small heat transfer capacity (HTC). In this work, a LHP with R134a as the working fluid is designed and fabricated. In order to enhance its HTC, the structure optimization has been made based on the theoretical analysis of flow resistance where the smaller wick thickness and larger diameter of the vapor and condenser lines are adopted. Experimental results show that the R134a LHP can reach a high HTC up to 250 W at the horizontal orientation with the condenser cooled by air forced convection. With an adverse elevation of 5 cm, the HTC is reduced to about 230 W. In addition, an extensive experimental study on its startup characteristics with the heat loads ranging from 5 to 30 W is conducted. The influence of the initial liquid/vapor distribution in the evaporator on the startup performance is investigated. This work proposes an effective method to enhance the HTC of the R134a LHP, which contributes greatly to its structure optimization and future commercial applications.

Flat-plate loop heat pipe (FLHP) is a passive two-phase heat transfer device. Comparing with traditional LHP with a cylindrical evaporator, it can be directly connected to a flat heat source without the employment of a saddle, which can effectively reduce the system thermal resistance and enhance the temperature uniformity. In this work, a stainless steel-ammonia FLHP was developed, and extensive experiments have been conducted to investigate its startup characteristics with the evaporator in the horizontal and vertical positions. Experimental results show that the FLHP exhibits excellent startup performance. It can successfully start up at a small heat load as low as 2 W with no obvious temperature overshoot. In a wide power range of 5–35 W, the FLHP generally starts up in only one situation, much simpler than the startup of a LHP with a cylindrical evaporator. For this rectangular evaporator, the heat leak from the evaporator to the compensation chamber (CC) becomes very small. As a result, the vapor can easily exit the condenser in most cases in the power range of 5–35 W, leading to a 100% utilization efficiency of the condenser and the resultant satisfactory thermal performance of the FLHP. In addition, the startup performance and the system thermal resistance of the FLHP are insensitive to the evaporator orientation, promising great application potential in future electronics cooling.

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.

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.

Cryogenic loop heat pipes (CLHP) hold significant potential for applications in the thermal control of advanced, space based infrared exploration systems. In the current investigation, a neon CLHP operating in the 30–40 K temperature range was developed and evaluated. The thermal performance characteristics, included assessment of the supercritical startup, matching characteristics of heat loads applied to the primary and secondary evaporators, power cycling, heat transfer limit and thermal resistance variations, all of which were explored and evaluated experimentally. The longitudinal temperature and pressure variations were measured simultaneously at several locations along the loop. The experimental results indicated that the CLHP could successfully achieve the supercritical startup with an input power of 1.5 W applied to the secondary evaporator, and reach a heat transfer limit of approximately 4.0 W over a 0.6 m distance. The CLHP exhibited good response characteristics to step changes in the heat load applied to the primary evaporator, resulting in smooth temperature transitions in the primary evaporator. The thermal resistance of the CLHP was found to decrease monotonically with increasing heat load applied to the primary evaporator, with values as low as 0.4 K/W.

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.

Cryogenic system is important for the infrared detectors, which can increase the ratio of signal-to-noise at low temperature. Long lifetime, high efficiency mechanical cryocoolers and two-phase heat pipes can reduce the energy consumption of the spacecraft and ensure a low weight. A co-designing cryogenic system consisting of two pulse tube cryocoolers (PTCs) and two neon cryogenic loop heat pipes (CLHPs) operating at temperature around 35 K is designed, manufactured, and tested in the present work. A novel integrated multifunctional heat exchanger is proposed to couple the heat exchange between the cold head of PTC and the condenser of CLHP. To our best knowledge, it is the first time to couple these two technologies directly for space application. The thermal design and efficiency analysis of this cryogenic system are presented, which optimize both the structural layout and insulation to reduce the heat load from surroundings. The thermal performance of the PTCs is ground tested and the co-designing cryogenic system is integrated in a vacuum chamber. The no-load temperatures of the PTCs are 25.3 K and 25.7 K, respectively, while both of the cooling capacities are over 2 W@35 K with the input power of 250 W AC. Meanwhile, the thermal performance of supercritical startup and steady-state operation are tested, and the results are discussed and analyzed.

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.

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.

Deployable radiator is a radiation cooling device which is folded next to side wall of spacecraft while launching, and can expand itself after the spacecraft is on track. It can effectively improve the cooling capacity of the spacecraft. In this paper, the current research and development situations of the deployable radiator were studied and analyzed. Depending on this, design and simulation of the deployable radiator based on the loop heat pipe (LHP) was completed, and the experiment was verified in the vacuum environment. The result showed that the thermal experimental results and the thermal analysis data were in good accordance under hot case as well as cold case. Moreover, the temperature oscillations at the shell of the LHP evaporator and the outlet of saturated vapor and the condenser pipeline were found in the experiment. The phenomenon of temperature oscillations was eliminated experimentally by three measures, which included increasing the input heat load, heating the compensation chamber to control the temperature and cooling the compensation chamber.

Dual compensation chamber loop heat pipe (DCCLHP) is an advanced two-phase heat transfer device, which features an orientation-free operation in the terrestrial surroundings and promises great application potential in the aircraft thermal management. In this work, a DCCLHP with an extended bayonet tube was designed and fabricated, aiming to improve its startup performance especially at a small heat load. The startup characteristics of the DCCLHP in five different orientations was experimentally investigated and theoretically analyzed. According to the experimental results, some important conclusions have been drawn: 1) The DCCLHP with an extended bayonet tube could achieve successful startup under the heat load ranging from 10 to 100 W in the horizontal orientation, and no startup failure occurred. 2) The DCCLHP could start up successfully even at a small heat load of 10 W from -90° to +90° tilt angle in the terrestrial surroundings. 3) Different startup phenomena such as the unique change of the temperature overshoot were observed in the experiments. 4) Operating instability such as the temperature oscillation occurred during the startup process in the favorable directions.