4. Introduction to Evaporative Heat Transfer

Evaporation refers to the change of phase occurring at the air or gas interface formed with a liquid medium. The discussion here is restricted to evaporation of a water body. The quantity of importance is the evaporation rate, or, the evaporative mass flux. In a continuum formulation, it is determined by requiring continuity of temperature and pressure at the interface, followed by visualizing a layer of air saturated with water vapor just above the water body. The gradient in humidity in the gas phase sets up mass fluxes of water vapor and hence decides the evaporative mass flux. Additional phenomena such as cooling of the water body and fluid convection will define the overall transport process. This description is referred to as the quasi-equilibrium model. In contrast, the non-equilibrium model, based on the kinetic theory of gases postulates the appearance of a Knudsen layer at the air-water interface. The extent of jump in temperature across the Knudsen layer can significantly affect the evaporation rate. These two models are described at length in this chapter.