Experiment And Numerical Simulation Of Liquid Film Formation In Falling Film Evaporation

Seawater and brackish water desalination provide an effective way to solve the problem of shortage of fresh water resources.The horizontal tube falling film evaporation technology has been widely used in desalination because of its advantages of smaller temperature difference heat exchange,lower evaporation temperature,higher heat transfer coefficient.The study of horizontal tube falling film evaporation technology can provide an important technical basis and theoretical basis for the design and production of more efficient horizontal tube falling film evaporator.In this paper,the flow pattern falling film wavelength with the four types of liquid distributor hole spacing under different spray density and tube shape had been experimented by using the high-speed imaging method.The distribution of the liquid film on the surface of the single hole and the three holes had been experimented by using the laser-induced fluorescence method.The results of this paper show that: when the hole spacing of the same tube is small,the transition Re number is low;The transition Re number of the drip-drip column and drip column-column of the smooth tube is smaller than that of the spiral finned tube,column-column curtain transformation Re number of two tube shapes are similar.The wavelength of the same tube decreases with increasing Re number;the wavelength increases with the increase of the hole spacing;the wavelength of the spiral finned tube is smaller than that of the smooth tube with the same Re number.Liquid film axial spread exist extremity within a certain range of Re number;the difference of the axial spreading length between the top and bottom of the liquid film decreases with the increase of the Re number in the single hole.Liquid column center distance changed periodically over time in the three holes condition,the center distance of the two liquid columns when Re = 302 is larger than that of Re = 172.The thickest film location between two liquid column moved around slightly,the rate of it appeared in the center position is 30%.The falling film flow outside the horizontal tube and the liquid film spreading regularities under the single hole and the three holes were simulated by VOF numerical method.As the contact angle increases from 10° to 60°,the axial spreading length of the water at the circumferential angles of the tube surface decreases,the circumferential angle is smaller with the shortest spreading length,the circumferential angle region of the liquid film shortest spreading length increases,and the liquid film thickness of the corresponding circumferential angle increases,namely liquid film thickness and spread length have the opposite distribution.With the increase of the inlet speed,the spreading length under the same circumferential angle increases,and the increase is smaller.The film thickness under the tube diameter of 32 mm is larger than that of tube diameter of 19 mm and 25 mm in the same of circumferential angle,however the spreading length of the liquid film under the tube diameter of 32 mm is shorter than that of the other two tube diameters.The thickness of the liquid film in the XZ plane at the angle of 90 ° under the three holes distribute,there are two crests and three troughs of the liquid film,the crest position is between two holes,the trough is located in the liquid inlet position,the liquid film has a film projections on both sides of the spread edge respectively.With the smaller Liquid inlet speed,liquid film axial spreading length is shorter,liquid film thickness fluctuation is smaller,so the difference between crest and trough is small.