Phase Change

In this chapter we examine evaporation and condensation in porous media in detail and briefly review melting and solidification in porous media. The heat supply or removal causing these to occur is generally through the bounding surfaces and these surfaces can be impermeable or permeable. We begin by considering condensation and evaporation adjacent to vertical impermeable surfaces. These are the counterparts of the film condensation and evaporation in plain media. The presence of the solid matrix results in the occurrence of a two-phase flow region governed by gravity and capillarity. The study of this two-phase flow and its effect on the condensation or evaporation rate (i.e., the heat transfer rate) has begun recently. Evaporation from horizontal impermeable surfaces is considered next. Because the evaporation is mostly from thin-liquid films forming on the solid matrix (in the evaporation zone), the evaporation does not require a significant superheat. The onset of dryout, i.e., the failure of the gravity and capillarity to keep the surface wet, occurs at a critical heat flux but only small superheat is required. We examine the predictions of the critical heat flux and the treatment of the vapor-film and the two-phase regions. We also examine the case of thin porous-layer coating of horizontal surfaces and review the limited data on the porous-layer thickness dependence of the heat flow rate versus the superheat curve.