Laminar mixed convection of humid air in a vertical channel with evaporation or condensation at the wall

Abstract

The effects of simultaneous heat and mass transfer on downward laminar flow of humid air in a vertical channel with isothermal wet walls have been studied numerically using an elliptical formulation. The entering air is always warmer than the channel walls but its absolute humidity can be higher or lower than the one corresponding to the wall temperature. Thus, cooling of the air stream is accompanied by either condensation or evaporation. The axial evolutions of the interfacial transverse velocity, of the average air temperature and average vapour mass fraction, of the sensible and latent Nusselt numbers, of the friction factor as well as that of the Sherwood number are presented and analysed for different inlet air conditions. The effects of the buoyancy forces on the hydrodynamic field are very important while their influence on the average air temperature and average mass fraction is small. Even though sensible heat is always transferred from the air to the walls, the latter must be heated when evaporation is important. If evaporation is weak, or if condensation occurs, the walls must be cooled.