ABSTRACT
Because these processes involve a fluid medium, we generally classify them as convection processes. Beyond all of the parameters that we previously examined in convection, we must now add the variables of surface tension, cr, ambient pressure, P\ and either the enthalpy or latent heat of evaporation, hfg', the latent heat of solidification, hfs\ or the latent heat of sublimation, hsg. The latent heat is the amount of heat required to convert a unit mass of a substance from one phase to another phase. There is also a difference in density between the liquid phase and the vapor phase that induces a buoyancy force, Ap = Apl - pv, when bubbles are present. Also, we will note that heat transfer during phase change does not always occur with a change in temperature of the media. In fact, a very large rate of heat transfer can be achieved with very little change in temperature. This is one of the attractions of phase change heat transfer. Furthermore, in contrast to natural or forced convection, increasing the AT may result in a decrease in the heat transfer coefficient. Because of the number of variables, there are no accurate general equations or correlations to use. Of the usable equations, most have an empirical value that changes with the surface characteristics and must be evaluated by experimentation. The accuracy of these correlations without experimental verification may be ±50%.
