The Manufacture And Experimental Study Of The Pump-assisted Loop Heat Pipe System

The pump-assisted loop heat pipe is a novel phase-change heat dissipation device which combines the conception of active and passive heat transfer technology.The system solves the problems that are caused by the limited capillary force in a traditional loop heat pipe,such as the short heat tranfer distance,the low critical heat flux,the oscillation of temperature and so on.It has a large potential of application in the field of high heat flux electronics cooling and spacecraft thermal control.For the purpose of further raising the heat transfer capacity and extending the sphere of application,this thesis studies the thermal performance of a pump-assisted loop heat pipe with new working fluid and new minipump,then explores the characteristics of a pump-assisted loop heat pipe with double evaporators and testifies the ability to handle a multiple heat sources problem.As an important driving source of a pump-assisted LHP,the peculiarity of micropump has a large effect on the performace of the loop.In order to reduce the system’s weight and increase the medium’s flow rate,a self-designed impeller pump is used in our system.Meanwhile,ammonia is chosen as the working fluid to avoid the occurrence of cavitation and get a better character of the loop.In consideration of the compatibility between working fluid and material,stainless steel is used to fabricate the whole loop.The results shows that there’s only the thin-film boiling mode existing during the operating process of the system.The loop displays a prompt response to dynamic heat input without temperature oscillation.The loop can dissipate a maximum heat load of 370 W(36.35W/cm~2)with the heat transfer distance of 2320 mm when the heat source temperature is below 80°C.The preferable thermophysical properties of ammonia such as higher saturation pressure and larger evaporative latent heat as well as lower boiling temperature make it perform better in the pump-assisted LHP than methanol.Moreover,in order to prove the availability of the pump-assisted LHP when applicated in a distributed heat transfer scenario,this thesis manufactures a new loop with double evaporators,methanol and a gear pump.The start-up process and variable heat loads operation under different heat load distribution are the key aspects to be researched.The experiments reveal the pattern and effectiveness of the interaction between two evaporators.There are two heat exchange modes in the evaporator under different working conditions:the single phase convection mode and the stable liquid evaporation mode.The evaporative temperature of two evaporators always keeps the same value because of the pressure transmitting through the shared vapor line.The total heat input in the loop sets the pressures and temperatures for the entire system.Specific evaporator body temperatures are also affected by individual heat input to either evaporator.It is also found in many tests that,while one evaporator is flooded,the other can still maintain boiling.The temperature overshoot is an inherent phenomenon of a dual-evaporators system with methanol as the working fluid.This thesis studies the performance of the pump-assisted loop heat pipe in many aspects both theoretically and experimentally.The characteristics of ammonia system and double evaporators system are investigated in detail.The results of this thesis has a guiding significance to the improvement and extension of this novel heat transfer device.