Study On Heat Transfer Characteristics Of Ammonia Low Temperature Flat Loop Heat Pipe Based On Capillary Wick Aperture Optimization

Since the beginning of the 21st century,the rapid development of rockets,satellites and space stations in the space field leads to increasingly frequent human activities related to the exploration and utilization of the outer space.In the orbit of a satellite in space,high vacuum,low temperature,micro-gravity,solar and planetary radiation and other factors will strongly impact the spacecraft heat dissipation process.To meet the harsh space working environment and strict heat dissipation requirements,it is necessary to develop an efficient thermal control system,characterizing as high heat dissipation power,low thermal resistance,anti-gravity operation and variable attitude operation.As a two-phase dissipative heat device,ammonia low temperature flat loop heat pipe can be more suitable to operate in the complex space environment than other loop heat pipes.Specifically,the flat loop heat pipe can fit the heating surface,improving the contact with the heat source for uniform heat transmission.From the perspective of field coordination,the angle between the velocity gradient and temperature gradient of the flat loop heat pipe is smaller than that of the cylindrical loop heat pipe,resulting in a preferable heat transfer characteristics.In order to develop an ammonia low temperature flat loop heat pipe with good performance,the optimization of capillary wick aperture is studied in this paper.The internal pore size structure of the capillary wick is improved by adopting an optimal proportion of pore-forming agent,and the surface pore size structure of the capillary wick is enhanced by using surface treatment.An experimental investigation is conducted to explore the sensitivity between the proportion of pore-forming agent and several key parameters,including capillary wick suction characteristics,porosity,permeability,thermal conductivity,pore size characteristics.The results show that the porosity and permeability of the capillary core increase with the proportion of pore-forming agent,while a reverse trend is observed for the thermal conductivity.Besides,the modified surface pore structure of capillary wick(special texture,directional,high surface rough)can further enhance the capillary core of suction limit and improve its comprehensive performance(high pumping speed,high porosity,high permeability,low coefficient of thermal conductivity).Based on the capillary wick aperture optimization,a flat loop heat pipe system is developed based on innovating evaporator design,capillary wick design and condenser design.The loop heat pipe is perfused through the self-designed high-precision ammonia working medium perfusion experiment platform.An overall performance test system for the loop heat pipe is also established,consisting of a heating module,a data acquisition module,and an angle adjustment module.Considering that the ammonia low temperature flat loop heat pipe is in a state of variable power,attitude and temperature when running in the universe,the heat transfer test is carried out from the start-up characteristics,operation characteristics,variable power operation to evaluate the operation characteristics of ammonia low temperature flat loop heat pipe in the real process.Furthermore,the heat sink temperature,the gas-liquid interface angle in the evaporator,the relative height of the condenser and the evaporator and other aspects of the heat transfer characteristics of the loop heat pipe are studied.The experimental results show that the minimum start-up power of the loop heat pipe is 20W,and the thermal resistance decreases with the increase of power.When the power load is taken as 120W,the thermal resistance reaches the minimum of 0.77℃/W,with the ultimate power growing to 130W.The temperature fluctuation can be observed at a heat sink temperature of-10℃,which,however,disappears with the increase of heating power.With the rise of heat sink temperature,the overall operating temperature of the loop heat pipe rises.To be specific,when the heat sink temperature is 40℃,the loop heat pipe has high stability,but it is not conducive to the heat dissipation of the system.When the compensation chamber is located above the evaporation,the loop heat pipe will operate in reverse;When the compensation chamber is located below the evaporator,the overall operating temperature of the loop heat pipe increases with the evaporator angle;When the loop heat pipe condenser is located above the evaporator,the operating temperature of the loop heat pipe decreases with the increase of the angle.This phenomenon has a threshold of 60°.If the threshold is exceeded,the influence of increasing the angle on the loop heat pipe will be slight.