Numerical Investigation On Condensation With And Without Non-condensable Gas

Condensation is one of the most important heat transfer processes in many engineering systems, such as heat pipe, passive containment cooling system (PCCS), seawater desalination by dewvaporation process, natural gas condensing technology and many other energy conversion systems. The study on condensation is very significant for enhancing heat transfer, thermal safety, heat recovery, and so on.In this paper, the numerical investigation is conducted on condensation of pure vapor and air-vapor mixture by Fluent. The volume of fluid (VOF) and the species transport model are applied to simulate the two phase flow and vapor transport in gas mixture, respectively. In Chapter1, the background of condensation, the mechanic of condensation and an extensive literature review of condensation and evaporation are presented in detail. In Chapter2, the model of two phase flow with condensation is introduced. A modified phase change model, derived from basic equations related to the kinetic gas theory, is proposed and executed by user defined function in Fluent (UDF) to simulate condensation. According to many references related to physical properties of mixed gas, combined with the operation conditions in this paper, the gas state equation is selected to calculate density of gas mixture. The other physical parameters are calculated by empirical or polynomial fitting formula, and set in Fluent by UDF.Chapter3is mainly about numerical investigation on surface condensation and bubble condensation of pure water vapor. The model is firstly verified based on three classical cases. The predicted results show that the mean film thickness increases and the heat transfer performance become worse with decrease of gravity. A high value of surface tension or contact angle, leads to an enhancement of heat transfer on the plate with large amplitude waves. The bubble deformation and terminal velocity during the condensation process is related to M, Eo and Re, respectively. For a larger bubble, the bubble deformation during condensation and rising process is more complicated, and the condensation acceleration exists while the bubble is broken into small fragments. It also can be found that the bubble lifetime in condensation is nearly proportional to the bubble diameter.Chapter4is mainly about numerical simulation on water vapor condensation with non-condensable air upon a vertical plate and in a horizontal pipe. The predicted results show that a small amount of non-condensable gas did not show significant effect on the convective condensation of water vapor. However, the heat transfer coefficient of convective condensation was shrinking obviously with increasing the concentration of air. And the heat transfer coefficient was about8%higher than that of ignoring liquid film. The heat transfer coefficient of convective condensation is improved in a lower extent than pure water vapor convection by increasing inlet velocity of gases mixture. Besides, for condensation in a horizontal pipe, condensation contributes to89%-91%of the total heat transfer, while pure convection contributes to9%-11%. The suction effect caused by condensation enhances the pure convective heat transfer about100%-200%.