Study On Phase Change Heat Transfer In Microgrooovs For High Power LED Cooling

As LED light has so many advantages, such as low voltage, short reaction time, long life and no pollution, it is widely used in display and lighting system. With the power and integration increases, the cooling problem became more and more serious. Excessive junction temperature will not only make the life of LED shorter, but also will affect the peak wavelength, optical power, luminous flux, and many other performance parameters of LED. It is necessary to design an effective cooling device for LED light to control the junction temperature at a relatively low level. The main purpose of the present dissertation is to clarify the flow and heat transfer behavior in vertical rectangular capillary microgrooves theoretically and to design a cooling system for high power LED based on mircogrooves.The liquid-gas interface in the cross section of rectangular micro channels is studied theoretically. Phase-change heat transfer in the extended meniscus is mainly concentrated in the evaporation thin film region. Wayner's evaporation model is simplified and the heat flux in the thin film region is found to be a constant value approximately. A new judgement method for the contact line is proposed. The analytical expressions are presented for the length of the thin film, the contact angle and the curvature of the intrinsic meniscus, which are all influenced by the width of the micro channels, heat flux and superheat temperature.A cooling device for the high power LED is designed based on the study on the microgrooves. The performance of the refrigerant, filling rate and the inclination angle of the device is also studied experimentally and the optimum parameters are found. Compared with the conventional cooling device, the cooling device with phase change heat transfer in the microgrooves could reduce the operating tempreratuer of the LED effectively, while the weight decresed to about 15% and the volume decresed to about 10%. The cooling device with phase change heat transfer in the microgrooves is cheap and very suitable for industrialization.