Abstract
The present works intend to study nucleate boiling heat transfer at high heat fluxes, which is characterized by the existence of macrolayer. Two-region equations are proposed to simulate both thermo-capillary driven flow in the liquid layer and heat conduction in the solid wall. The numerical results indicate that the thermo-capillary driven flow in the macrolayer, evaporation at the vapor-liquid interface and interaction between macrolayer and heated wall constitute a very efficient heat transfer mechanism to explain the high heat transfer coefficient of nucleate boiling heat transfer near CHF. This study also explores the flow pattern of macrolayer with a high conducting solid wall, e.g. copper, which the temperature is uniformly made at the liquid-solid interface, and the heat fluxes and the evaporation coefficient are found to have significant effect on flow pattern in the liquid layer. For a very thin wall and/or wall with a poor thermal conductivity are found to have significant effect on flow pattern in the liquid layer and the temperature distribution in the heated wall. The calculation result of water can be extended to predict the result of dielectric liquid FC-72. Furthermore, a parameter "evaporation fraction"as well as "aspect ratio"is proposed as an index to investigate the thermo-capillary driven flow system. And,, parameters the wall thickness multiplied by thermal conductivity or the wall thickness multiplied by square root of thermal conductivity may be used to evaluate the effect of wall thickness and materials. The model prediction also agrees well with the experimental data in the literature. It illustrates the reasonable numerical simulation of present study that can be used to explore the nucleate boiling heat transfer at high heat flux.