Study On Visualization And Simulation Of Loop Heat Pipe Evaporator And Reservoir

With the development of aerospace exploration and electronic information industry,the traditional heat exchangers have been difficult to meet the temperature control requirements of high heat flux and high temperature uniformity.As a high efficiency heat transfer device,loop heat pipe(LHP)achieves heat transfer through the phase change of working fluid flow.It has a wide application prospect in the field of space thermal control and high heat flux electronic products heat dissipation because of its advantages such as strong heat transfer capacity,flexible connection,long transmission distance,no moving parts,etc.As the core components of LHP,the flow and heat exchange of working fluid in the evaporator and reservoir will directly affect the operation of the LHP system,and the heat load and filling ratio of working fluid will directly affect the distribution and flow state of working fluid in evaporator and reservoir.Based on the analysis of background requirements and literature research results,this paper studies the influence of working fluid state in the reservoir and evaporator on the start-up,stable operation and heat transfer performance of LHP from the aspects of working fluid filling ratio and heat load application mode.Firstly,the coupling structure of evaporator and reservoir was designed,which is sealed by O-ring and flange connection.Based on high-speed camera and normal temperature experimental platform,the visualization experiment system of loop heat pipe evaporator and liquid reservoir was built.Then,a new environment-friendly refrigerant R245 fa with high heat transfer performance was selected as the working fluid through the quality factor.According to the design conditions,the pressure drop of the prototype under various working conditions was checked through the pressure drop model of the LHP,so as to ensure the normal operation of the prototype.Visualization experiments were conducted to study the effects of thermal load application mode(bottom heating,top heating and double-sided heating),thermal load size(10W-50 W thermal load)and working fluid filling ratio(50%-80%)on the operation state of LHP.Finally,based on the visual experiment,a mathematical model of evaporator and reservoir coupling structure was established.The model includes multiple calculation domains,and the gravity factor is considered.In this model,darcy permeation and capillary suction are fully considered in the wick region,and capillary force is applied according to the phase gradient in the gas-liquid transition region to simulate the capillary effect.The establishment of the evaporator and reservoir simulation model provides a reference value for the design of LHP.The following conclusions are drawn through experiments and simulations: The way of applying heat load directly affects the heat transfer process to the evaporator,and then affects the starting process of the heat pipe.The heat transfer performance of LHP during stable operation is affected by the heating method and the heat load.With the increase of the heat load,the heat transfer performance of the LHP system improves continuously.When the heat load is lower,the heat transfer performance of the single side heating mode is better than that of the double side heating mode,while the heat load is higher,the heat transfer performance of the double side heating mode is better.With the increase of heat load,the gas-liquid interface of the working fluid in the central channel of the evaporator increases,and the nucleation quantity increases gradually.The height of the gas-liquid interface and the nucleation quantity under different heating methods affect the heat leakage of the evaporator to the reservoir,and then Affect the performance of the loop heat pipe.The filling ratio of working fluid directly affects the distribution of working fluid in LHP,thus affecting the start-up process of LHP.Obvious temperature fluctuations appear at the inlet of the condenser at a lower filling ratio.As the filling ratio of the working fluid increases,the temperature fluctuations disappear,and the start-up speed of the LHP becomes faster.The heat transfer performance of the LHP system changes V-shaped with the liquid filling ratio,and there is an optimal liquid filling ratio.The best liquid filling ratio of the experimental prototype is about 70%,and the minimum heat transfer resistance is 0.52 K/W.The simulation results show that the flow and heat transfer process of working fluid in evaporator and reservoir are affected by various heat transfer mechanisms such as the magnitude of the heat load,the flow rate and temperature of the return liquid,and the heat leakage from the evaporator to the reservoir.At the same time,the bayonet structure has a great influence on the flow and heat transfer process of working fluid in evaporator and reservoir.The bayonet structure should be used in loop hot hook design.