Numerical prediction of forced convection film boiling heat transfer from a sphere

Abstract

Forced convection film boiling over a sphere is modeled by applying the laminar boundary-layer approximation for both the vapor and liquid flows. In the vapor momentum equation, a buoyancy term is included, which has often been neglected in past analyses. The solution is obtained numerically to the point of flow separation. From the analysis of forced convection film boiling over a sphere submerged in water, it is shown that the vapor film thickness is in the submillimeter range, and as the subcooling of liquid increases, the film thickness becomes thinner, down to the order of ten microns. It is also observed that the buoyance force may not be neglected in the liquid velocity range considered in this study (up to 7 m s −1). The heat transfer results of the present model agree qualitatively with available experimental data, although quantitatively the model generally underestimates the data. Using the results of the present analysis, an improved correlation for the convective heat transfer to the bulk liquid in subcooled film boiling of a sphere in water is proposed for use in predicting the vapor generation rate.