Study On Heat Transfer And Instability Of Loop Thermosyphon Cooling System With Microchannel Heat Exchangers

With the increasing requirements of heat dissipation for high-power closed cabinets,traditional air-cooling,liquid-cooling,semiconductor refrigeration,and mechanical cooling methods are no longer applicable due to insufficient heat dissipation capacity or excessive energy consumption.The loop thermosyphon cooling system with microchannel heat exchangers has excellent heat transfer performance and higher energy efficiency ratio,which has become the best choice for heat dissipation for high-power closed cabinets.This study designed and built an experimental platform,and also established a three-dimensional fluid-solid coupling calculation model for the flow boiling in microchannel.The research combined experiments and simulation,studied the effects of multiple factors on the flow and heat transfer characteristics of the system.In the experiments,the effects of multiple factors on the performance of the system were studied by analyzing temperature,pressure and thermal resistance distribution.The study found that the higher the heating power,the higher the system pressure and saturation temperature.The subcooling degree of the evaporator inlet was high in a large liquid filling amount,and continuously increased with the increase of heating power.When the heating power increased over the critical power,there was a significant superheat at the evaporator outlet.With the increase of heating power,the thermal resistance of the evaporator,condenser and system all decreased first and then increased,while the flow thermal resistance changed slightly.When the liquid filling amount was low,the system performed better at low heating power,and the thermal resistance risen at high heating power due to the local overheating of the evaporator.Besides,the performance decreased with the increase of the circulating air volume of the condenser,and had nothing to do with the circulating air volume of the evaporator.When the heating power increased over the critical power,a local hot spot appeared on the upper part of the left flat tube,which was always far from the outlet of the evaporator.When the liquid filling amount was extremely low and the heating power was significantly greater than the critical power,the temperature and the pressure of the system oscillated fiercely.During the oscillation period,the circulating flow rate of the working fluid gradually decreased until completely cut off.Meanwhile,the pressure and condenser temperature decreased sharply.In the simulation calculation,the effects of the flat tube positions and the heating power on the flow and heat transfer performance were studied by VOF numerical simulation method and compared with the experimental results.It was found that there were significant differences in the flow pattern distribution of different flat tube positions and heating power.The flat tube near the evaporator outlet had greater pressure drop and flow rate,so that local overheating was less likely to occur.The liquid film near the outlet of the microchannel disappeared when the heating power became higher,which led to a sharply decrease of the heat dissipation capacity of the system.The conclusions obtained from experiments and simulations in this paper were of great significance for the structural design and operation conditions selection of the loop thermosyphon cooling system with microchannel heat exchangers.