Study On Heat Transfer Characteristics Of Loop Heat Pipe With A Flat Evaporator

Nowadays, with the development of miniaturization and integration, electronic devices arebecoming more and more compact, intelligent and powerful. During the working process ofelectronics, heat is generated and may get accumulated inside the processing units while thereis no an effective heat dissipating method. The electronic system would fail to work when thesystem temperature exceed the maximum operating temperature due to heat accumulation.Therefore, the thermal management becomes an challenge for high power electronic systems,which are widely used in information industry and aerospace technology.Loop heat pipe (LHP) is a new type heat pipe technology which has good potential appliedto electronics cooling owing to its advantages including anti-gravity capability, excellent heattransfer capability, long distance heat transmission and flexible structure design. LHP with flatevaporator is an improved design in comparison with traditional cylindrical evaporator design.An flat contact surface between the LHP and the heat sources can provide quite low contactresistance. Generally, the thickness of the flat evaporator is less than10mm. Such small size isbeneficial for LHP to be integrated into the narrow enclosures of electronics system. Inaddition, according to field synergy principle, LHP with flat evaporator has better thermalperformance as the included angle between its temperature gradient and velocity gradient ofmass flow is smaller.In this work, the heat transfer characteristics of loop heat pipe with flat evaporator wasinvestigated via experimental study and theory research. The LHP developed in this work wasin miniature size and of copper-water type. The thickness of the flat evaporator was8mm.And the LHP is able to be applied to electronics cooling. In the experimental study, thetemperature oscillation phenomenon during the start-up and operating process and the effectof condenser location and operating orientation were investigated and got careful analysis.According to these research, a mathematical model was built to simulate the start-up processof the LHP. The results of calculation agreed to the experimental data quite well. And thegeometry optimization is possible for practical application of LHP technology.The main conclusions in this work are listed as follows:(1) The temperature oscillation may occur during the start-up and operating process of theloop. The insufficient capillary force inside the porous wick and the variation of two-phasedistribution in the compensation chamber are the two main reasons.(2) For specific heat load, the loop operated more stably with higher heat sink temperature,however, which is not beneficial for high heat load. (3) Condenser location has a significant impact on the start-up characteristics of the loop.When the condenser is located near the evaporator outlet, the start-up process is quite smooth.However, the start-up time would increase dramatically and the unstable phenomenon, such astemperature oscillation and overshoot occur when the condenser is moved to thecompensation chamber inlet.(4) At low heat load, the effect of operating orientation is more evident than the condenserlocation. However, at high heat load, the gravity effect is weak and condenser locationbecomes the main factor determining the operating temperature of the loop.(5) A mathematical model for the LHP with flat evaporator was built to simulate thestart-up process in the condition of different condenser location in horizontal orientation. Themodel is able to predict the saturated temperature of the vapor and compensation chambertemperature.