Startup characteristics of a multi-evaporator cryogenic loop heat pipe

Abstract

With the rapid development of space exploration technology, the traditional point-to-point heat transfer methods cannot fulfill the heat dissipation requirements of multi- and large-array detectors. Loop heat pipe is an effective and efficient two-phase heat transfer device that has played a significant role in spacecraft for the thermal control system. In addition, it has high heat transfer efficiency, long heat transfer distance, heat switch characteristics, and pipeline flexibility. However, higher requirements are proposed for space thermal control. We need to design and handle more complicated detector structures, heat dissipation from scattered distribution, and heat dissipation of large area array. In this work, a heterotypic loop heat pipe has been investigated to solve the heat dissipation issue of a multi-point distributed heat source. A multi-evaporator loop heat pipe (MeLHP) in low temperature is designed and manufactured, via connecting three evaporators in parallel with gas coupling method. The pipelines are arranged asymmetrically as required. The working temperature is set to 170 K while ethane has been used as a working fluid. A series of experiments are conducted to study the start-up characteristics of the prototype. Different heating methods and various charging ratios are compared with a single evaporator loop heat pipe, where the MeLHP prototype is explored variously in the process of cooling and start-up. Initially, a start-up experiment of a single evaporator loop heat pipe is carried out. As we know, the loop heat pipe (LHP) research is relatively mature, so all the conditions of LHP are adopted for the startup characteristics of MeLHP’s. It is found that every single evaporator of MeLHP has similar performance to that of LHP. Later, we employ different experimental conditions on the evaporators such as various charging ratios and different heating methods. The temperature fluctuation and heat sharing characteristics are compressively analyzed from the comparison of temperature curves at different working conditions. Finally, the experimental process is verified from the nice comparison of single evaporator temperature change and cooling rate of each case. In addition, we observe that evaporators in each loop of MeLHP are highly uniform compared to that of a single evaporator loop heat pipe, during the start-up process. So, MeLHP can start under the condition of single evaporator heating and multiple evaporators heating as well. When a single evaporator heating process takes place in MeLHP, the unheated evaporators share the heat via the influence of gas coupling from the heated evaporators. So, this process simultaneously starts three evaporators; the initial state of working fluid would influence the temperature of each evaporator during cooling, while the heat exchange of gas coupling effectively inhibits the effect of evaporators’ temperature. The structure of gas coupling effectively shares the heating load among all evaporators. Different charging ratios affect the start-up process of MeLHP. Compared to the charging ratio of LHP (0.6), the results of MeLHP are consistent with LHP at a charging ratio of 0.7. So, the fluid charging ratio of MeLHP should be appropriately increased to obtain an equivalent performance to that of a single loop heat pipe. Our experiments have verified the start-up feasibility of a multi-evaporator loop heat pipe with parallel gas coupling, revealed the law of cooling, and the start-up process, which is highly significant for the promotion and application of MeLHP.