Film boiling on a vertical high-temperature surface: focusing on melt jet-water interactions

Abstract

This paper addresses film boiling on a vertical high-temperature surface, such as that of a melt jet in water. The modelling of the film boiling process is based on the vapor-film-unit theory, which couples a two-phase boundary layer analysis with an instability analysis of the vapor–liquid interface. The vapor-film-unit model is extended to include thermal radiation and forced coolant convection, by numerical integration of the boundary layer energy equations. The model predictions are compared with experimental heat transfer data and applied to analyse film boiling characteristics under a flow of subcooled water. A forced convection-induced change of the film boiling process is indicated, leading to a particularly thin vapor film. The preconditions estimated for a jet-water interaction mode with a very thin vapor film are found to fall reasonably close to experimentally observed conditions for a fast jet disintegration. Jet breakup regimes with saturated film boiling (continuous coolant liquid phase) or a vapor chimney (dispersed coolant liquid phase) around the jet are also considered. For such two-phase coolant conditions, the jet breakup is studied by addressing not just the detailed vapor film conditions, but also the influence of vapor bubbles and larger void regions. With this multiphase flow perspective, the film boiling predictions play a key role in determining the existence of a significant vapor film, and if there is one, in estimating the vapor-film-unit behaviour.