Convective heat and mass transfer with evaporation of a falling film in a cavity

Abstract

The heat and mass transfer involved in the evaporation of a water falling film in a closed rectangular cavity of geometric form factor equal to 10 is studied numerically and experimentally. The wall which supports the liquid film is heated by a constant heat flux. The vapor thus formed is condensed on the opposite wall maintained at a constant and uniform temperature. The study objective is a better understanding of the evaporation phenomenon in order to improve the yield. A numerical model has been built from the conservation equations in the gas and liquid phases. The main characteristic of the present study is the way of treating the transfer in the liquid film. A method based on the mass and heat balances has been developed where the balances are obtained by integration of the mass and energy conservation equations over small height increments. This method allows us to avoid the explicit solution of the momentum and energy equations which is more difficult because of the presence of a fabric material placed on the heated wall to stabilize the liquid film. The obtained results allow us to describe the thermodynamic state of the heated film by means of the liquid temperature and evaporation flow rate. The heated film presents two zones: a heating zone located near the inlet of the cavity and an evaporation zone which covers the rest of the wetted surface. The extent of this effective surface of evaporation has been studied with respect to the operating parameters: on the one hand, the heat flux and the temperature of the condensation wall that in general depend on the climatic conditions, on the other hand, the water feed temperature and flow rate that can be varied by the user and act directly on the liquid film. The influence of the two latter parameters on the exchanges at the liquid–gas interface has been characterized in terms of local Sherwood and Nusselt numbers.