Theoretical And Experimental Study On The Effects Of The Figures Of Merit And Inventories Of Working Fluids On The Performance Of Loop Heat Pipes In 180?250 K

The loop heat pipe is a passive vapor-liquid two-phase heat transfer device,the numerous advantages of which like high heat transfer efficiency,long-distance and flexible heat transfer ability have made it widely used in the supporting thermal control systems of the space optical detection subsystems.With the increasing variety of the target characteristics,the operating temperature range of the corresponding optical system has become gradually wider,and the working fluids that can be selected for the loop heat pipe in the target temperature range have also increased accordingly.The improper selection or charging of the working fluid may restrict the heat transfer performance of the loop heat pipe,making it insufficient for engineering applications.Therefore,it is crucial to study the effect of the working fluids on the thermal performance of the loop heat pipes.Based on the analysis of the background demand and the investigation results,this paper mainly studies the influence of the working fluids on the performance of the loop heat pipes from the aspects of the thermophysical properties and inventories of the working fluids.Among them,the key thermophysical parameters that affect the flow and heat transfer of the working fluid are usually condensed in the figure of merit.Therefore,the research on the thermophysical properties is mainly conducted by studying the effect of the figures of merit of working fluids on the heat transfer capacity and efficiency of the loop heat pipes.The research on the influence of the working fluid inventory is implemented by the numerical simulation of the steady-state operation of the loop heat pipe,the experimental study on the heat transfer performance of the loop heat pipes with different inventories,and the visualization study on the compensation chamber in the target low temperature region.In order to achieve the research goal,ethylene,ethane and propylene were selected as the working fluid candidates in the target temperature range of 180 ? 250 K according to the basic requirements and the investigation results.Various figures of merit of the working fluids were compared,and the distribution of the working fluid pressure drop in each components of the loop heat pipe was analyzed.The results show that the dominant pressure drop of the working fluid is that through the porous wick and the vapor line.In the derivation of the Dunbar parameter,the selection of the dominant pressure drop of the working fluid is improper,and the simplified calculation of the pressure drop in the derivation of the Mishkinis criterion is unreasonable.Based on the comparative analysis and the combining of different contact angles,the figure of merit of the working fluid to predict the heat transfer capacity of the loop heat pipe was updated.In addition,the analysis on the figures of merit of the working fluid to predict the heat transfer efficiency of the loop heat pipe shows that the predicted results of the Joung parameter and the dP/dT value of the working fluid at saturation state are the same,and the Joung parameter and dP/dT value of ethylene is higher than that of ethane and propylene,and both parameters increase with the rising temperature.The analysis and simulation on the effect of the working fluid inventory show that the inventory can directly affect the liquid level inside the compensation chamber,thus affecting the distribution of the working fluid in the wick core and the mode of the heat leak between the evaporator and the compensation chamber,and finally affecting the heat transfer efficiency of the loop heat pipe.When the liquid level in the compensation chamber is not high enough to cover the wick core,the heat leakage from the evaporator to the compensation chamber is significantly high due to the phase change heat transfer,and the heat transfer efficiency of the loop heat pipe is therefore restricted.To carry out the corresponding experimental verification of the above theoretical analysis,three loop heat pipe prototypes(LHP-1,LHP-2,LHP-3)were designed and fabricated,a working fluid charging system was built,and a sealing method based on the quartz compensation chamber,O-rings with alloy spring,and the flanges was designed.A visualization experimental system based on a high-speed camera and a thermal vacuum experimental platform was built.The corresponding results show that the accuracy of the charging system is better than 0.1 g;the connection structure of the quartz compensation chamber and the stainless steel flanges can withstand the pressure of 1.5 M Pa and the temperature of the liquid nitrogen without damage,which can meet the requirements of the visualization experiment.The experimental results of the effect of the working fluid inventory on the heat transfer performance of LHP-1 indicate that both too high and too low of the inventory will cause the start-up failure of the prototype,but the specific reasons for the failure are different.The start-up time and heat transfer temperature difference of LHP-1 decreases and then increases with the rising inventory,but the effect of the working fluid inventory within a moderate range on the heat transfer capacity of LHP-1 is not obvious.The liquid level in the compensation chamber and the measured evaporator temperature in the visualization experiment of LHP-3 both show good agreement with the calculated results from the steady-state operation model of the loop heat pipe.The visualization experiment also showed that the working fluid will condensate and flow on the outer surface of the bayonet when the liquid level in the compensation chamber is lower than the bayonet,and the phase change heat transfer would cause great heat leakage from the evaporator to the compensation chamber,thereby restricting the heat transfer efficiency of the loop heat pipe.The heat leakage from the evaporator to the compensation chamber calculated by the velocity of the condensed liquid drop on the outer surface of the bayonet shows good agreement with that calculated by the model.The loop heat pipe exhibits the optimal heat transfer efficiency when the liquid level in the compensation chamber is between the bayonet and the top of the wick core.The experimental results of the heat transfer capacity and efficiency of LHP-1 and LHP-2 under different heat loads and heat sink temperatures show that the magnitude and variation trends of the heat transfer limits are more consistent with the prediction of the updated figures of merit,and the experimental values of the heat transfer limits are close to the updated figures of merit for different contact angles at different temperatures.Therefore,the accuracy of the updated figure of merit to predict the heat transfer capacity of the loop heat pipe is better than that of the Dunbar parameter and Mishkinis criterion.The thermal resistance of the ethylene loop heat pipe is lower than that of the loop heat pipe loaded with ethane and propylene,and the thermal resistances decrease with the rising operating temperature.All experimental results are in good agreement with the prediction of both the Joung parameter and the dP/dT value of the working fluid at steady-state.The pressure drop of the working fluid in the vapor line is an important factor affecting the heat transfer efficiency of the loop heat pipe.The?P/(dP/dT)value of working fluids inside the loop heat pipe can be utilized to predict the magnitude and variation trends of the evaporator heat transfer coefficient.In addition,the value can also predict the evaporator heat transfer coefficient and the corresponding overall heat transfer coefficient of different loop heat pipes charged with the same working fluid.