Design And Optimization Of High-efficiency Condenser For Cryogenic Loop Heat Pipe

In space exploration programs,a thermal control system is required to control the temperature of the cryogenic optical system and electronic system.Mechanical cryocooler can provide large cooling capacity while ensuring the accuracy and stability of the output temperature.It is an ideal cold source for aerospace thermal control systems.The loop heat pipe is an efficient two-phase heat transfer equipment,which can transmit the cold capacity from the cryocooler to the controlled components over a long distance,and at the same time keep the electromagnetic and mechanical vibration interference from the optical system.The loop heat pipe pipelines are flexible,convenient for the layout in the spacecraft.The condenser of loop heat pipe is the core component of heat exchange with the cryocooler.Since the size of the condenser is usually much larger than the cold finger of the cryocooler,it is necessary to use a cold plate for connection,which causes additional heat loss and weight burden.Based on the problems above,this paper has carried out relevant research on the miniaturization design of the cryogenic loop heat pipe condenser.In order to achieve the same heat exchange in a smaller condenser,a compact design of the flow channel is required,and the cross-sectional size of the flow channel needs to be smaller.In this paper,the condenser is designed as a disc shape close to the size of the cold finger of the cryocooler,and the external size is Φ60 mm×10 mm.The internal flow channel draws on the tree-shaped flow channel network commonly used in the field of high heat flux heat dissipation,and is designed as side inlet and side outlet form.In the Y-shaped bifurcated channels,the number of channel stages is 4symmetrical,and the number of branches after the bifurcation is 2.In each branch level,the cross-sectional dimensions of the mother channel and the sub-channel follow a fixed ratio.The cross-sectional area of the single-branch sub-channel is smaller than that of the mother channel,but the total cross-sectional area of the two sub-channels is larger.The flow rate decreases after each bifurcation.The Y-shaped bifurcated channel has a larger heat exchange area and a smaller pressure drop than a serial channel with the same channel length and hydraulic diameter.At the same time,the temperature distribution is more uniform,which can meet the high-efficiency heat exchange of small condensers.The designed condenser can be directly coupled with the cold finger of the cryocooler,and the weight is only 1/3 of the traditional vortex tube condenser.Based on the optimized design of the disc-shaped condenser with Y-shaped bifurcated channels,a loop heat pipe machine using propylene was designed and processed,and test experiments were carried out.The experiment draws the following conclusions: when the condenser temperature is 223 K,203 K,and 183 K,the corresponding minimum thermal resistance are 0.19 K/W,0.29 K/W and 0.50 K/W,respectively.When the working temperature drops,the heat leakage from the evaporator to the compensator increases,and the thermal resistance of the loop heat pipe gradually increases.The turning point from the variable thermal resistance zone to the fixed thermal resistance zone moves backward at low temperature.At 183 K,the loop heat pipe works in the variable thermal resistance zone all the way.The analysis of the outlet temperature of the condenser indicates that as the temperature drops,the length of the two-phase condensing section becomes shorter,and the length of the condenser designed gradually changes from insufficient to redundant.Suggestions are made for the design of the condenser flow channel length.The Y-shaped bifurcated channels condenser and the U-shaped single-tube condenser with the same flow channel length(90 mm)were compared in experiment.The experimental results show that at the condenser temperature of 223 K,the loop heat pipe with Y-shaped channels condenser and U-shaped channel condenser enters the fixed thermal resistance zone at 30 W and 20 W,respectively,and its minimum thermal resistance is 0.19 K/W and 0.47 K/W,respectively.From the perspective of heat exchange and flow resistance,with the same flow channel length,the Y-shaped channels condenser has a larger heat exchange area and smaller flow resistance,which is beneficial to reduce the condensation phase transition temperature and outer loop pressure drop is lower,so as to obtain a smaller heat exchange temperature difference and thermal resistance.The loop heat pipe using the Y-shaped channels condenser has obviously higher heat exchange efficiency,and has great application prospects in the thermal control system coupled with the cryocooler.