Mixed Convection Heat and Mass Transfer with Phase Change and Flow Reversal in Channels

Abstract

Due to its widespread applications, heat and mass transfer in an air stream with liquid film evaporation or condensation in open channels has received considerable attention in the literature. This kind of flows is present in many natural and engineering processes, such as human transpiration, desalination, film cooling, liquid film evaporator, cooling of microelectronic equipments and air conditioning. Since the original theory for flow of a mixture of vapour and a non-condensable gas by Nusselt (1916) and its extension by Minkowycz & Sparrow (1966), many theoretical and experimental studies have been published in the literature. These studies deal with different geometric configurations such as a flat plate, parallel-plate channel and rectangular or circular-section ducts. Heat and mass transfer convection over a flat plate wetted by a liquid film has been investigated by Vachon (1979). He performed an analytical and experimental study of the evaporation of a liquid film streaming along a porous flat plate into a naturally driven airflow. This author established correlations for Nusselt and Sherwood numbers in connection with a combined Grashof number. Ben Nasrallah & Arnaud (1985) investigated theoretically film evaporation in buoyancy driven airflow over a vertical plate heated with a variable heat flux. The solutions of the governing equation have been obtained by means of semi-analytical and finite difference methods. The authors present their results in term of expressions of the wall temperature and mass fraction as well as the local Nusselt and Sherwood numbers. A numerical and experimental analysis has been carried out by Tsay et al. (1990) to explore the detailed heat transfer characteristics for a falling liquid film along a vertical insulated flat plate. Free stream air temperature was set at 30°C and inlet liquid film temperature was taken equal to 30°C or 35°C. The results show that latent heat transfer connected with vaporization is the main cause for cooling of the liquid film. The authors affirm that when the inlet liquid temperature is equal to the ambient one, latent heat transfer due to the film vaporization initiates heat transfer in the film and gas flow. Aguanoun et al (1994; 1998) studied numerically the evaporation of a falling film on an inclined plate heated