Theoretical Modeling of Rapid Surface Vaporization with Back Pressure

Abstract

In this paper, a theoretical model is developed for rapid surface vaporization into a surrounding ambient atmosphere. The primary emphasis is on metallic surfaces. Power input levels are taken to be low enough so that thermodynamic equilibrium can be assumed before phase change to gas. At high evaporation rates there will be a narrow Knudsen layer region in the gaseous flow just outside the phase interface in which translational nonequilibrium prevails. The modeling treats this layer as a gasdynamic discontinuity and approximate jump conditions are derived. The flow Mach number just outside the Knudsen layer is dictated by the state well away from the surface. Its determination is discussed both for a simple model of transient flow induced by a laser pulse and for the general case of time-varying power input. Examples assume the metallic surface is aluminum and the surrounding air pressure ranges from 1 atm to hard vacuum.