Boiling Heat Transfer Characteristics of Immiscible Liquid Mixtures

Cooling technology becomes more important because of the increase in heat generation density of semiconductor chips. Boiling and two-phase flow is expected as one of the most effective methods to cool the electronic devices because of its superior heat transfer characteristics. Because the boiling heat transfer depends on the selection of coolant, the development of liquid mixtures and their components optimized for the individual cooling systems becomes very important. Most of the existing researches on nucleate boiling of binary mixtures clarify the heat transfer characteristics peculiar to the miscible ones, while the studies for immiscible mixtures were very limited. Pool boiling experiments on nucleate boiling of immiscible mixture FC72/Water are performed in a closed vessel at 0.1MPa. The experimental data is acquired under the steady-state conditions at heat flux ranging from those for the boiling incipience to CHF or to the highest possible value under the restriction of the available cooling. Before the experiments, the height of liquid layers on the horizontal heating surface is kept at a constant value of 100mm, and the thickness of FC72 layer above the heating surface is varied at three different values of 5mm, 10mm and 50mm. A new phenomena of “intermediate burnout” due to the evaporation of FC72 is observed provided that the layer thickness of FC72 is not large. At heat flux higher than that for intermediate burnout, the immiscible mixture decreases the surface temperature from that for pure water and simultaneously increases CHF. The immiscible mixture has a potential to realize high performance heat exchange systems by the self-sustaining subcooling of components under the constant system pressure. NOMENCLATURE cp isobalic specific heat, J/kg K H height, m hfg latent heat of vaporization, J/kg P pressure, N/m 2 q heat flux, W/m 2 R gas constant, J/kg K T temperature, C or K x mole fraction of liquid, y mole fraction of vapor, Greek symbols  heat transfer coefficient, W/mK ΔT surface superheat, K  density, kg/m Subscripts 1 more volatile component 2 less volatile component CHF critical heat flux g gas l liquid sat saturation sub subcooled b at boiling point of pure liquid INTRODUCTION In the recent development of electronic devices, the heat generation density from electric semiconductors is growing. On the other hand, the electronic devices are often composed of Si semiconductors with a low upper limit of operating temperature, because SiC or GaN semiconductors with toughness against higher temperature are still under development. So, it requires the improvement of the existing cooling system if high heat generation density is concerned. Liquid cooling systems using the latent heat during boiling heat transfer have more advantages than the conventional ones using air or single-phase liquid. By the application of boiling, the cooling at high heat flux becomes possible minimizing the temperature difference between the cooling surface and the coolant because of inherent high heat transfer coefficient. The cooling systems using boiling heat transfer, however, have an