Dynamic Response of a Liquid-Vapor Interface During Flow Film Boiling From a Sphere

Abstract

Film boiling is the mode if boiling during which the hot surface is separated from the vaporizing liquid by a nearly continuous film vapor. Film boiling is usually considered a very undesirable boiling regime since it is a relatively quiet and inefficient mode of heat transfer, particularly as compared to nucleate boiling. It is customary to analyze the two-phase flow regime of laminar flow film boiling by assuming the two-phase flow regime of laminar flow film boiling by assuming an idealized vapor film flow characterized by a smooth liquid-vapor interface. However, during stable flow film boiling, the wavy nature of the liquid-vapor interface and its role in local heat and mass transport have been largely ignored. The vapor interface is rarely stationary. Interfacial waves may substantially augment the heat transfer rates throughout the layer. The present analysis treats stagnation point flow film boiling on a sphere immersed in a subcooled liquid. The effect of system parameters on the dynamic behavior of the liquid-vapor interface as well as the response to step changes in the temperature and velocity fields are investigated. The study corrects and extends the theoretical analysis previously developed by Sheppard and Bradfield (1972). The new theoretical model notmore » only allows for the influence of liquid velocity, vapor superheat, and liquid subcooling on the wavy nature of the liquid-vapor interface, but it also sheds some light on the response of the system of perturbations in heat flux and superheat and into the conditions leading toward instances of liquid-solid contact during stable flow film boiling.« less