Numerical Simulation On Flow And Heat Transfer Characteristics Of Falling Film Evaporation Outside A Horizontal Tube

The process of horizontal tube falling film evaporation, including internal condens ation and external falling film evaporation, is widely used in the industries of chemica1engineering, refrigeration system, sea-water desalination, etc due to its high efficient heat transfer of small heat transfer temperature difference and small flow density. Hori zontal tube falling film evaporation is a complex process, which couples the effect of phase change, film wave as well as the interaction between vapor and liquid phases. Thus the research on its characteristics of flow and heat transfer is of great significa nee and potential prospect.The VOF model based on finite volume method was used to simulate the micro behavior of heat and mass transfer in the horizontal-tube film. The distributions of vel ocity, temperature and volume fraction were achieved and the effects of flow density, tube pitch, tube diameter, shear stress and evaporating temperature on film thickness a nd heat transfer were analyzed. The micro-mechanism of heat and mass transfer is ex plored.The simulation results of liquid film flow are as follows:the thickness of liquid fil m outside a tube presents a trend of increasing after decreasing firstly on the circumferentia1angle of the tube wall. The film velocity shows an opposite trend. Film thickness obtai ns the minimum and film velocity approaches the maximum at the circumferential angl e of about120°to135°. The film thickness and film wave increases with increasing flow density and decreasing tube diameter. The effects of tube pitch and distributor he ight on film thickness are mainly on the upper half tube and little on the bottom half tube. The film thickness decreases with an increase in distributor height. The co-curr ent shear plays the role of thinning film thickness and enhancing liquid film flow, an d the countercurrent shear has an inverse effect. The positions where the minimum fil m thickness and maximum film velocity are lagged due to the co-current shear while those are advanced due to the countercurrent shear. The film velocity and film wave i nwrease and film thickness decreases with an increase in film temperature.Simulation results of external film heat transfer are:compared with merely consideri ng conduction process, the simulated heat transfer coefficients weigh bigger, which sho ws there exits convective process that can not be ignored. The heat transfer coefficient s decreases with an increase in flow density and a decrease in tube pitch and evapora ting temperature. As the variation of surface tension of seawater with evaporating tem peratures differs from that of pure water, the increase rate of heat transfer coefficients of seawater is less than that of water. The heat transfer correlations based on simulat ion results agree well with the previous experimental ones with an error less than10%...