A Study On The Heat Transfer Of Condensation In The Humidified Air

In the condensation heat transfer process, the presence of noncondensable gas greatly deteriorates the heat transfer efficiency. The former studies only dealt with the problems with little noncondensable gas; but in fact, the situation with a great deal of noncondensable gas is often encountered. This work deals with the condensation heat transfer in the conditions that the concentration of noncondensable gas reaches 50~90%, which is the basic research part of dewvaporation technology for seawater desalination. It is a subject with challenge to study the effect of noncondensable gas on condensation heat transfer and look for the effective approaches to improve the heat transfer efficiency. In this article, the main influencing factors on condensation heat transfer include air flow rate, coolant flow rate, coolant inlet temperature and mixed gas velocity. By adjusting the ratio and rate between dry air and steam and altering the feed rate and temperature, we investigated the effect of various manipulations on condensation heat transfer process. In order to gain our ends, we set up appropriate experimental equipment and data collection system. The process of condensation heat transfer in the presence of noncondensable gases is briefly stated as following. As condensation takes place outside the condenser tube, a condensate film forms. Noncondensable gas, unable to pass into the water, accumulates at the liquid-vapor interface, forming a gas-vapor diffusion layer through which the steam must pass by diffusion and convection, to be condensed. Meanwhile, there exists a thermal boundary layer on the wall surface of cool side. When various manipulations disturb the thermal boundary layer inside or the condensate film and gas-vapor diffusion layer outside, the heat transfer effect will be better or be worse off. Among various factors, the concentration of noncondensable is the most important. High coolant temperature avails increasing heat transfer efficiency, but in view of condensate production, high coolant temperature is unfavourable. The increasing of coolant flow rate benefits the increasing of inside convective heat transfer coefficients, but it has no effect on outer ones, so it is helpless to increasing overall heat transfer coefficients. As the component of inlet mixture is fixed, the higher mixture velocity is, the bigger overall heat transfer coefficients are. The heat transfer coefficients of liquid side predicted by the Dittus-Boelter correlation are compared with experimental values. The comparison result proves the adopted computation method is reasonable. We study the laws of condensation heat transfer with noncondensable gases just to seek the methods of intensifying heat transfer. From the investigation, several paths to intensify heat transfer can be applied, such as reducing noncondensable gas concentration, higher coolant temperature and increasing mixture velocity. Otherwise, droplet falling condensate, coarse condensation surface and intermittent liquid feed are also in favor of heat transfer.