Study on High-Performanced Cooling System by Using Phase-Change Heat Transfer in Capillary Microgrooves
|School||Graduate School,Chinese Academy of Sciences|
|Keywords||rectangular capillary microgroove phase-change heat transfer liquid fill ratio thermal resistance pressure drop|
As a new technique in the field of microchannel phase-change heat transfer, micro capillary grooved heat sink can drive fluid flow by capillary force and creat enhanced evaporation heat transfer conditions by promoting the formation of an extended meniscus in the three-phase contact-line region of a microgroove. It has been used to achieve the heat exchanges with high heat transfer coefficients and high heat flux.The main purpose of the present dissertation is to clarify wetting (dryout), flow and phase-change heat transfer behavior and mechanisms in vertical rectangular capillary microgrooves experimentally and theoretically. Consequently, high-performanced micro cooling systems by using phase-change heat transfer in capillary microgrooves are designed to meet the stringent cooling demands of high power lasers, micro electronic devices and high-performanced computer chips.Firstly, the wetting (dryout) characteristics, evaporation and boiling scenes as well as bubble dynamic behavior in microgrooves are observed by using a high-speed camera with the maximum speed of 30000 frames per second, a CCD camera, and a microscope. The wetting and phase-change heat transfer characteristics in microgrooves are investigated experimentally. The experimental results show that the bubble dynamic behavior has a positive influence on advancing wetted height and enhancing phase-change heat transfer. Two phase-change heat transfer mechanisms are advanced for the enhanced heat transfer process in the microgrooves. One of them is pure evaporation heat transfer model in the extended thin film region near the triple-phase contact line of vapor-liquid-solid at lower input heat flux. The other one is combined heat transfer model of evaporating heat transfer in the extended thin film region near the triple-phase contact line of vapor-liquid-solid and boiling heat transfer in the intrinsic meniscus region at higher input heat flux. Geometric parameters of microgrooves have a remarkable effect on wetting (dryout) and phase-change heat transfer in the microgrooves. According to experimental results, a theoretical study on liquid wetting characteristics in vertical rectangular capillary microgrooves is carriedout. The proposed theoretical model for liquid wetting characteristics in the microgrooves could predict the meniscus dryout point well. And, quantificationally, it could explain the mutual influence between wetting behavior and phase-change heat transfer.Following the research on the heat sink unit of vertical rectangular capillary microgrooves, two high-performanced micro cooling systems by using phase-change heat transfer in capillary microgrooves are designed and the corresponding performances are studied. One of them, a natural convection micro cooling system by using a capillary microgroove evaporator is paid attention specially. Experimental and theoretical studies on the characteristics of thermal resistance, pressure drop and heat transfer in the cooling system are conducted. Experimental results indicate that the liquid fill ratio has a significant influence on thermal resistance and heat transfer in the cooling system. Increasing system’s cooling capacity at higher input power lies on decreasing the thermal resistance between the outer surfaces of the condenser and ambient environment. Based on experiments, this work presents a steady state physical and mathematical model for the thermodynamics and heat transfer inside the cooling system. A theoretical study on pressure drop and heat transfer characteristics inside this cooling system is conducted to explore the effects of different parameters on the performance of this cooling system and to predict the system’s maximum cooling capacity. The results suggest that the maximum pressure head loss inside system is due to vapor flow friction and local resistance along the vapor tube of the system. How to reduce the pressure head loss should be taken precedence over all other pressure head loss in system design. Vapor tube’s geometric parameter and heat transportation distance have a significant influence on the system’s maximum cooling capacity.The present natural convection micro cooling system by using a capillary microgroove evaporator has a strong heat dissipation capacity. Its best cooling performance at the surface temperature of heat source below 90 °C reaches 1.68xlO6W/m2 and the maximum heat transportation capacity is 131.8W. The novel kind of cooling system is suitable for remote cooling of those electronic parts with micro size, high power and thermal sensitivity.