Study On Film Flow And Heat Transfer Characteristics Of Vertical Tube In Falling Film Evaporater

Falling film evaporator is widely used in Evaporation process,because of its high heat transfer efficiency,low power consumption and compact structure.The film-forming quality and heat transfer characteristics of liquid in falling film tube directly affect the evaporation efficiency and normal operation of evaporator.In order to study the flow and heat transfer characteristics of liquid-film in vertical tube,a numerical simulation study was carried out.The falling film simulation test scheme is designed.The falling film of vertical pipe refers to the process in which the working fluid flows along the pipe wall under the action of gravity to realize film distribution.Based on the reasonable assumption of circumferential symmetry of vertical pipe,a 2-d symmetric geometric model was established.The structured grid of the model was divided by ICEM,and the experiment of grid independence was carried out.In order to study the flow and heat transfer characteristics of fluid in tube,cold film simulation and heat transfer simulation were carried out respectively.The general method of CFD simulation of falling-film evaporator is discussed.In this paper,the pressure-velocity coupling solver and the k-?turbulence equation was selected to simulate the liquid flow and heat transfer process in the vertical tube under unsteady conditions.More importantly,in the cold state simulation,water and air are used as the medium,and VOF method and geometric reconstruction technology are used to accurately capture the gas-liquid interface.The heat transfer simulation is based on the cold state simulation,using water and water vapor as the medium,adding the energy equation and evaporation phase transition equation to simulate the heat transfer and evaporation process of the liquid film.The forming and flow characteristics of cold film in tube were studied.Based on the thickness and waveform of liquid film,the effects of inlet velocity,plug-in components and pipe diameter on the stability of liquid film were studied.The results show that the fluctuation degree of liquid film and the radial velocity of liquid film increase gradually along the flow direction.The thickness and stability of liquid film increase with the increase of inlet velocity.When the annular gap is less than 2mm,the thickness and stability of the liquid film increase with the increase of the annular gap;when the annular gap is larger than 2mm,the liquid film accumulates;the best annular gap is 2mm.what is more,the improper diameter of the tube will affect the stability of the liquid film.The heat transfer and flow characteristics of liquid film in tube were studied.Based on the stability of liquid film,wall heat flux and liquid evaporation,the effects of liquid intake,wall temperature and material inlet temperature on the heat transfer and evaporation characteristics of liquid film were studied.The results show that the stability of liquid film can be improved by increasing feed rate,but the evaporation of liquid can be reduced.Increasing the wall temperature can effectively increase the evaporation and heat flux of working substance.The wall temperature should be increased properly under the condition of ensuring the stability of liquid film.In this paper,the optimum wall temperature is 423 K.Increasing material inlet temperature can increase evaporation,but reduce wall heat flux.When working fluid inlet temperature is higher than 368 K,the influence of inlet temperature on working fluid evaporation and wall heat flux is weakened.