The Research Of Liquid Film Behavior In Vertical Wall Falling Film Evaporation Process

At present,three types of condensers are mainly used in industrial applications:air-cooled,water-cooled and evaporative condensers.The evaporative condenser simplifies the cooling water circulation system of the water-cooled condenser.At the same time,the high airflow velocity required by air-cooled condenser is obviously reduced in the evaporative condenser.Therefore,in oil,chemical,beer,food and other industries in refrigeration,low-temperature processing,refrigeration and other fields,more and more use of the advantages of water saving,energy saving and compact structure of evaporative condenser.An important condition for the normal operation of the evaporative condenser is the uniform film distribution on the heat transfer surface.Under the action of the same heat load in the evaporation process,insufficient liquid flow will cause the liquid film thickness to be too thin,resulting in the rupture of the liquid film,local "dry spots" appear on the outer surface of the tube,and the heat transfer capacity will be reduced.And the liquid flow rate is too large to make the liquid film is too thick,the heat transfer tube will be due to insufficient heat transfer deterioration.In this paper,the theoretical model of vertical plate falling film evaporation is established,and the heat transfer process is divided into four parts: convection heat transfer between heat transfer wall and water film,convection heat transfer between heat transfer wall and air,mass transfer and heat transfer between water film and air,and heat absorbed by water film due to temperature rise.The effects of air flow rate,cooling water flow rate and liquid film coverage on the heat transfer performance were calculated theoretically,and the accuracy of the calculated results was verified by experiments.According to the difference of reflectance of liquid film and solid surface to infrared light emission,the coverage of liquid film on solid surface is captured by infrared thermography.The effects of different surface structure,mass flow rate,wind speed and heat flux on the film coverage and the heat transfer performance were studied.The experiment shows that the spread of liquid film on ribbed and hydrophilic plate is better than that onordinary copper plate,the better rib structure can keep the liquid film coverage above 0.7,and the average liquid film coverage is increased by at least 5 times.At the same time,the phenomenon of liquid film shrinkage by heat was observed on the copper plates,and the ribs of the ribbed plate played a role in inhibiting shrinkage.With the increase of mass flow rate of liquid film,the spread of liquid film on copper plate is better,the coverage of liquid film increases,and the increase of wind speed and heat flux decreases.From the perspective of its heat transfer performance,the improvement of wind speed can enhance heat and mass transfer and effectively improve the heat transfer performance,while the improvement of mass flow rate of liquid film is mainly to enhance heat transfer by removing heat from the liquid film temperature rise.In the experiment,the temperature rise and heat absorption of the liquid film of hydrophilic plate and vertical ribbed plate are small,and the wall temperature is relatively low.However,the temperature rise and heat absorption of the liquid film of triangular ribbed are large,and the cooling of the wall surface mainly depends on the heat absorption of the liquid film during the evaporation process of the vertical wall falling film.Combined with the spreading and cooling behavior of the liquid film during the evaporation process of the vertical falling film,the hydrophilic coating and the surface treatment of the processed rectangular rib can make the copper plate have better heat transfer performance under the condition of better liquid film coverage.