Study on Forced-Convection Film-Boiling Heat Transfer. Effects of Liquid Flow on a State of Vapor Film Flow.

Abstract

Focusing on the effects of liquid velocity and subcooling, forced-convection film boiling heat transfer was investigated experimentally and analytically. In the experiments, a long-vertical cylinder was adopted as the test section and R-113 liquid was used. The measured heat transfer coefficients, which were higher than those predicated with the two-phase boundary layer theory even in a low liquid velocity region, showed a different trend from the values calculated by Dittus-Boelter type correlations based upon the turbulent flow of a liquid or vapor phase and were not dependent upon the distance from the leading edge. The characteristics were quite similar to those of pool film boiling. The vapor-film-unit model developed for natural-convection film boiling was extended and applied to the present conditions. In the model, the Kelvin-Helmholtz instability wavelength was used to determine the length scale of the unit. The model provided good agreement with the experimental results. It was also pointed out that if the viscous sublayer in the turbulent boundary layer, which was formed at the upstream of the heated section in the bulk liquid flow, was much thicker than the vapor film and the boundary layer formed in the liquid along the vapor film, the turbulence of the bulk fiow had little effect on the film boiling heat transfer.