Porous Wick In An Open Evaporator Study On Evaporation Heat Transfer Performance Of Composite

With the vigorous development of information technology,the improvement of electronic equipment towards miniaturization and high performance leads to a sharp increase in power consumption per unit area.Heat dissipation has become a difficult problem to be solved.Loop heat pipe is a kind of device that use the phase change of working liquid to realize efficient heat transfer,it has the advantages of good flexibility,high stability and long-distance transmission.Porous wick is the key component of loop heat pipe,the research on phase change characteristics and heat transfer mechanism of porous wick is of great significance to further enhance the performance of loop heat pipe.Under the guidance of orthogonal experimental research,further experiments on evaporation heat transfer characteristics were carried out.When the proportion and size of the pore former are certain,the proportion of spherical-dendritic metal powder and material selection are changed.Four kinds of composite porous wicks were prepared by sintering method.After the characterization of physical parameters,it is applied to an open evaporator for the visualization experiment of evaporation heat transfer,and the flow heat transfer characteristics and operation mechanism are explored.Considering the possible temperature overshoot caused by the influence of bubbles in the compensation chamber,the experiment of different height design of the compensation chamber is carried out to study the heat transfer characteristics,so as to further clarify the influence of external parameters on the operation of the system.The main contents and results are as follows:(1)In the experiment,spherical-dendritic metal powders were used to prepare composite porous wick with stable structure by adding pore former.The calculated porosity is 64.9%-66.3% under different material and powder ratios.Through the combination of experiment and theoretical model,it is concluded that the effective thermal conductivity increases with the decrease of porosity,ranging from4.81W/(m·K)to 6.34W/(m·K).(2)From the evaporation mass-temperature curves of four porous wicks,it is found that the system has two types of start-up: stable and overshoot.Repeated experiments on the same porous wick show that the start-up process are different under the same heat load,which may occur either smooth or overshoot start-up.(3)There are four types of temperature overshoot in the same porous wick under different heat loads:(1)Temperature overshoot during start-up;(2)Temperature overshoot in stable stage;(3)Intermittent temperature overshoot;(4)Continuous temperature overshoot.In the progress of evaporation heat transfer,the different pressure differences between the inside and outside of the vapor channel causes the asymmetric vopor-liquid distribution on the upper surface of the porous wick.The redistribution of vopor-liquid makes the different intensity of evaporation and boiling in the vopor channel,as a result,the temperature of each point on the upper surface of the evaporator will not remain at the same level.The experiments of adding water under different heat loads show that the addition of water in the stable stage may not lead to temperature overshoot.(4)The evaporator wall temperature increases approximately linearly with the heat load.Under the same heat load of the four porous wicks,the maximum temperature difference on the wall appears between No.1 and No.2,reaching 7.5 ℃.By comparing the evaporator wall temperature and evaporating rate,it is found that lower evaporator wall temperature does not mean higher surface evaporating rate due to the influence of heat leak.The critical heat loads of the four porous wicks are 370 W,500W,120 W and 490 W respectively.And the wall temperature will not be stable if the critical heat load is exceeded.(5)The evaporator wall temperature shows good thermal response characteristics in the continuous operating experiment of variable heat loads.In the open evaporator system,the temperature hysteresis phenomenon of evaporation heat transfer is also found as in loop heat pipe.When the heat load increases or decreases to the same value,it will lead to different operating temperatures.And the wall temperature in decreasing the heat load is higher than that increasing the heat load.(6)The experimental results of evaporation heat transfer with different compensation chamber heights show that the temperature overshoot will still occur due to the existence of bubbles,and the start-up at the height of 12 mm compensation chamber is least affected by bubbles.In the range of no exceeding critical heat load,the wall temperature with the 12 mm height of compensation chamber is the highest under the same heat load,and the maximum temperature difference reaches 13.2℃compared with 8mm height compensation chamber.In the range of 120W-340 W,the evaporator wall temperature at the 8mm height of compensation chamber is the lowest.It is found that the surface evaporating rates of the three heights have little difference under the same heat load.The critical heat load of porous wick with compensation height of 4mm,8mm and 12 mm is 500 W,400W and 420 W,respectively.