Numerical Analysis On Condensation And Convection Heat Transfer Characteristics Of Gas-vapor Mixture

Vapor and non-condensable gas mixture widely exists in the process of Passive Containment Cooling System (PCCS), Seawater Desalination by Dewvaporation Process, Natural Gas Condensing Technology and many other industrial processes. The non-condensable gas contains air, carbon dioxide and dust particles, etc. In this paper the condensation and convection heat transfer characteristics of the gas-vapor mixture was simulated by virtue of component transportation model in Fluent and VOF model. The influence of physical property changes, condensation conditions and the gas-liquid interface capturing accuracy were fully considered.According to papers, design manuals, and charts related to physical parameters of mixed gas, combined with the operation conditions in this paper, the gas state equation was selected for dry air, water vapor and gas mixture. The dynamic viscosity and diffusivity of dry air and water, latent heat of vaporization vapor, and saturation partial pressure of vapor can be get using polynomial fitting.On condition that the condensate was ignored, only the component transportation model was used. Appropriate UFD was substituted in the model to simulate the condensation and convection process of gas mixture on condensation column surface. The simulation result of average heat transfer coefficient agrees well with the experiment. The error was less than15%. So the numerical model is correct and reliable. The change of temperature field, pressure field, velocity field, component field of gas mixture was showed as well as local and global local heat transfer characteristics by means of simulating the condensation and convection heat transfer process in circular tube. It shows that condensation can produce mass sink which induces small normal speed perpendicular to the wall. Mass transfer in laminar flow depends on static diffusion, and mass transfer in turbulent flow depends on both static diffusion and convective diffusion. The average convection heat transfer coefficient increases with the increment of entrance velocity of gas mixture and entrance mass fraction of vapor, as well as the average convection heat transfer coefficient. The ratio of local convection heat transfer coefficient to the total heat transfer coefficient increases along the flow direction, which always lower than0.5. So condensation heat transfer cannot be ignored, and it’s necessary to recovery.Taking the condensate into consideration, both component transportation model and VOF model was used to simulate the condensation and convection process of gas mixture outside smooth circular tube. Liquid film was formed during the vapor condensation, which formed liquid drop under the action of wall adhesion force and surface tension. The droplets grown up flowed downward along the wall under gravity and gas-liquid interface shear force, meanwhile they gathered together Liquid film waving exists in the droplets gathering process It is showed that the volatility of convection and condensation heat transfer coefficient decrease by less than5%with the formation of component boundary and the increment of liquid film thickness.The UFD in this paper can be applied to two-dimensional and three-dimensional rectangular coordinates, as well as two-dimensional cylindrical coordinates. The relative merits of different models were judged. It shows that the gas mixture component transportation model is available for complex physical model, while the simple and small-size physical model can be simulated well when gas mixture component transportation model and gas-water two-phase flow VOF model used simultaneously.