Heat and Mass Transfer on Laminar Forced Film Condensation of Vapour–Gas Mixture

Abstract

Through the heat and mass transfer analyses, the equations of condensate heat and mass transfer rates are provided, where only dimensionless wall temperature gradient, interfacial vapour saturation temperature and defined condensate mass flow rate parameter are non-given conditions. The defined mass flow rate parameter depends on condensate liquid film thickness as well as the interfacial condensate liquid velocity components. Then, the laminar forced film condensation of water vapour in the presence of air on a horizontal flat plate is taken as an example, and a system of numerical results of the wall dimensionless temperature gradient and mass flow rate parameter are obtained. It is found that although decreasing the bulk vapour mass fraction (i.e. increasing the bulk gas mass fraction) causes increase of the wall dimensionless temperature gradient, it decreases the condensate mass flow rate parameter. However increasing the reference wall subcooled grade causes decrease of the wall temperature gradient and increase of the condensate mass flow rate parameter. These phenomena are closely related to the effect of the non-condensable gas on the condensation. The system of the rigorous key solutions of the wall dimensionless temperature gradient and condensate mass flow rate paramater is formulated using the simple and reliable equations for the laminar film condensation of water vapour–air mixture on a horizontal flat plate. In combination with the provided theoretical heat transfer equation, the formulated equation of the wall dimensionless temperature gradient and condensate mass flow rate parameter can be used for simple and reliable evaluation of the condensate heat transfer rate. The interfacial vapour saturation temperature, necessary for correct prediction of laminar film condensation of vapour–gas mixture, is deeply investigated here. By taking the laminar forced film condensation of water vapour–air mixture as an example, three methods are reported for evaluation of the interfacial vapour saturation temperature: (1) the numerical calculation method, (2) prediction with the formulation equation and (3) prediction by solving the condensate mass–energy transformation equation. The calculated results of the interfacial vapour saturation temperature related to different methods are well coincident. It proves that the similarity analysis method, the similarity mathematical model, the numerical calculation and treatment method of variable physical properties reported in this chapter are valid for extensive investigation of heat and mass transfer of laminar forced film condensation of vapour–gas mixture. The author believes that the analysis and calculation methods reported in this work can be conveniently extended to investigate other different types of laminar forced film condensation from vapour–gas mixture.