The Effect of Surface Motion on Forced Convection Film Boiling Heat Transfer

Abstract

The growth in demand for high-quality metallic alloys has placed greater emphasis on the predictability of cooling methods used in manufacturing processes. Several methods involve forced convection film boiling, which can occur on metallic strips or plates cooled by water jet impingement or on strips inside cooling jackets of continuous annealing processes. Since surface temperatures are typically well above the boiling point of water, a substantial portion of the surface area can involve film boiling. The strip or plate speed often exceeds the water velocities and strongly influences boundary layer development in the vapor and liquid. The purpose of this paper is to estimate the effect of plate motion on heat transfer in the film boiling regime. Conservation equations for mass, momentum, and energy have been solved by the integral method for film boiling in forced convection boundary layer flow on a flat isothermal plate in motion parallel to the flow direction. Unlike previous studies, which have shown that heat transfer is chiefly governed by the plate and subcooled liquid temperatures, heat transfer is shown to also depend on the plate velocity. For large velocities, the importance of radiation heat transfer across the vapor layer is reduced. However, when the velocities of the plate and liquid are oppositely directed and of nearly equal magnitude, radiation across the vapor layer can become significant, even at low plate temperatures.