THERMOCAPILLARY DRIVEN FLOW IN THE MACROLAYER FOR NUCLEATE BOILING AT HIGH HEAT FLUXES.

Abstract

Nucleate boiling heat transfer at high heat fluxes is characterized by the existence of a macrolayer between the heating surface and hovering bubble with vapor stems penetrating the layer nourishing vapor to the growing bubble. Heat transfer from the heating surface results in temperature variations along the stem–liquid and bubble–liquid interfaces which lead to the surface tension gradients along these interfaces. As a result, thermocapillary driven flow may be induced in the macrolayer. This paper develops a heat and mass transfer model in the macrolayer with consideration of surface tension gradient and evaporation at the vapor–liquid interfaces. The mathematical model thus developed is then solved numerically by a finite difference scheme. It is concluded that the thermocapillary driven flow in conjunction with the evaporation at the stem-liquid and bubble–liquid interfaces is the major heat transfer mechanism for nucleate boiling at high heat fluxes. Several circulating vortexes are generated in the macrolayer. The flow carries energy from the heating surface to the stem–liquid interface as well as the bubble–liquid interface, where the liquid is cooled by evaporation and subsequently flows back to the heating surfaces. The average wall superheat at a given heat flux predicted by the model is well below that evaluated by the pure conduction model and agrees reasonably well with experimental data. It is also found that the major part (about 98%) of the energy from the heating surface is transported by evaporation at the bubble–liquid interface and only a small percentage (about 2%) is by the evaporation at the stem–liquid interface.