BOILING HEAT TRANSFER BY NUCLEATE BOILING OF IMMISCIBLE LIQUIDS

Abstract

Among the various methods to improve the heat transfer characteristics of nucleate boiling, the present research concentrates on the selection of coolant. Most of the existing research on nucleate boiling of binary mixtures clarified the heat transfer characteristics peculiar to the miscible mixtures, while the number of studies on immiscible mixtures is very limited. Pool boiling experiments on nucleate boiling of three immiscible mixtures, FC72/water, Novec649/water, and Novec7200/water, are performed in a closed vessel at 0.1 MPa. In the experiments, the thicknesses of stratified liquid layers for both component liquids on a horizontal heating surface before the heating are varied as one of the important parameters. When the layer thickness of the more volatile component with higher density is very small, e.g., 5 mm, a new phenomenon, “intermediate burnout,” is observed. At heat flux higher than that of intermediate burnout, the surface temperature for the immiscible mixtures decreases from that for pure water, and simultaneously the critical heat flux (CHF) for the mixtures increases under the same pressure. The increase of CHF is mainly caused by the high subcooling of water, i.e., the less volatile component, resulting from the pressurization by the vapor of the more volatile component. The immiscible mixture has the potential to realize high performance heat exchange by the self-sustaining subcooling of component liquids even in closed systems.