Etude des transferts de chaleur et de masse lors de la condensation en film liquide.

Abstract

In this work, we present an analysis of two phase flow and heat transfer during laminar film condensation of vapor-gas mixtures inside vertical tubes with porous walls. The mathematical model developed is based on the conservation equations of mass, momentum and energy in both phases and vapor diffusion in the mixture. The flow in the saturated porous coating is described by the Darcy-Brinkman-Forchheimer model. The liquid film thickness is calculated by an iterative procedure applied to the mass balance equation at the liquid-mixture interface. Transfer equations are then discretized using an implicit finite difference method. Analytical methods corresponding to the end of condensation have been also developed. Results presented include radial profiles of velocity, temperature, vapor concentration, and axial evolutions of film thickness, mass flow rate and heat flux from the inlet until the end of condensation. A parametric study is then conducted to determine the influence of operating conditions on the condensation process. We consider successively the following parameters: Reynolds number, temperature, relative humidity, pressure of the mixture at the inlet of the tube, inlet-to-wall temperature difference, type of non-condensable gas, distance between walls of coaxial cylinders and external heat transfer coefficient in the case of non isothermal wall. The effects of main properties of the porous medium such as thickness, effective thermal conductivity and permeability are also highlighted. Finally, a comparative study of various saturated refrigerants-air mixtures in a vertical tube was conducted to determine the evolutions of the rate of condensation and heat flux for each mixture.