Numerical Simulation On Flow And Double-phase Coupled Heat Transfer Of Horizontal-tube Falling Film Evaporation

The falling film evaporators are widely applied in sea water desalination and waste water treatment due to high heat transfer coefficients specially when the mass flow rates, temperature differences and heat exchange area are comparatively small. The liquid film uniformity and the double-phase coupled heat transfer process, especially phase transformation inside and outside tube coupled with flow-tube correlatives, are the crucial problem in falling film evaporation. So it lays the foundation for the optimization design of the falling film evaporator that investigating the film thickness and heat transfer coefficient to get the mechanism of the double-phase coupled heat transfer and improve the heat transfer efficiency. The following general conclusions are as following:(1) The simulation results of liquid film flow are as follows. The CFD model of the falling-film pattern outside horizontal tube was built in the paper. The film thickness distribution characteristics of along the circumferential angle of tube are investigated by simulating with different spray density and tube diameter parameters and spray height. It illustrated that the film thickness increases with rising spray density and the thickest film formed on the top area of tube while the thinnest film appears at a circular angle of about 105°.The liquid film thickness decreases slowly with the increasing of tube size. Meanwhile, the film thickness decreases with rising spray height, and the reduction of the film thickness decreases with the increasing spray height.(2) The simulation results of the double-phase coupled heat transfer process are as follows. The method of element sliced out of tube was presented and the continuous double phase transformation was represented by the discrete mass fraction of steam in the tube. The model of the double-phase coupled heat transfer of the horizontal-tube falling film evaporation was built up to simulate the process of coupling heat-transfer process inside and outside tube. The Volume of Fluid (VOF) method was applied to investigate the influence of the spray density on the distributions of the film thickness and the circumferential and axial heat transfer coefficient of horizontal-tube. The computation results are as follows. ?The dry spot area can be observed and no liquid film covering at the bottom of the tube, and the area increases with the increase in the spray density. ?The velocity of liquid film increases with the increasing circumferential angle, then decreases slowly. The film velocity increases gradually in the direction of increasing film thickness. ? The temperature of tube wall reaches its minimum value where the steam mass fraction is higher in the tube inlet section. With the steam inside tube condensed, the wall temperature increases then reaches stability. ? In the circumferential direction of the horizontal-tube, the wall temperature increases with increasing the circumferential angle. The external film heat transfer coefficient of circumferential horizontal tube gradually decreased. ? In the axial direction the overall heat transfer coefficient was mainly impacted by the internal film heat transfer coefficient, which was improved with the increasing of water vapor condensation and not sensitive to the spray density.(3) In the horizontal tube bundles of multi-rows, the increasing of tube rows makes the increasing of temperature and velocity of liquid film and the decreasing of film thickness and wall heat flux and external heat transfer coefficient in the circumferential direction of the horizontal-tube. And in the axial direction the external heat transfer coefficient decreases with the increasing tube rows.