Improvement of Nucleate Boiling Heat Transfer Characteristics by Using Immiscible Mixtures
The increase of critical heat flux, the reduction of surface temperature, the operation at pressure larger than the atmospheric to avoid the air mixing, and the reduction of surface temperature overshoot at the boiling incipience are fundamental requirements for the development of high-performance cooling systems by the application of boiling phenomena. The present authors found the nucleate boiling of immiscible mixtures to satisfy all of the above requirements , however, its knowledge has not been clarified yet. Pool boiling experiments for five immiscible mixtures, FC72/water, Novec649/water, Novec7200/water, FC72/n-Propanol, and FC72/i-Propanol, are performed at atmospheric pressure. When the thickness of more-volatile liquid with higher density on a horizontal heating surface is very small, a new phenomenon of “intermediate burnout” accompanied by a small jump of surface temperature is observed, and the heat transfer mode is changed from the nucleate boiling of more-volatile liquid with higher density to the natural convection or nucleate boiling of less-volatile liquid of lower density. The heat transfer due to nucleate boiling of lessvolatile liquid is enhanced by the generated vapor of more-volatile component, which results in the reduction of surface temperature. Moreover, CHF is increased by high subcooling of less-volatile liquid compressed by the high vapor partial pressure of more-volatile component. No serious overshoot at the boiling incipience is observed if the more-volatile liquid with higher density is contacting the horizontal heating surface before heating. This trend is especially important for e.g. the cooling of inverters for electric vehicles with a large heat generation during the acceleration.