Research On Enhancing Heat And Mass Transfer Performance In Falling-film Evaporation Outside Horizontal Tubes In Lithium Bromide Absorption Refrigeration System

Lithium bromide absorption refrigeration system has an advantage on energy-saving and environmental protection,and it can also use the waste heat of production.In recent years,the performance study of Lithium bromide absorption refrigeration system is very extensive.Evaporator is one of the lithium bromide absorption refrigeration refrigeration devices,it's heat transfer and mass transfer efficiency is one of the important factors of the Lithium bromide absorption refrigeration system performance.The evaporator in the lithium bromide absorption chillers is in the form of falling film.Under the vacuum environment,the aqueous solution flows in falling film out of the horizontal tube which contains the vapor-liquid two-phase flow.The flow process is accompanied by complicated heat transfer and mass transfer.From the point of view of the heat transfer and mass transfer of the falling film evaporation of the horizontal pipe,the heat transfer model and the heat transfer mechanism and the factors influencing the heat transfer of the falling film are studied.In this paper,the influence of transverse convection on the falling film evaporation is analyzed,and the mathematical model including the influence of lateral convection is established.Based on the solution of the basic control equation,the law of changes under the influence of transverse convection in the liquid film thickness ?,the liquid film velocity u along the tube wall,the transverse flow velocity v and the total velocity u with the influence of the contact angle ?,spray density ? and dimensionless dimension ? of the y direction of y direction distance are studied.The thickness of the film increases first and then decreases with the increase of the contact angle,and reaches the minimum value when the contact angle is 90°.The average value increases with the increase of the spray density.The flow velocity of the liquid film along the pipe wall increases first and then decreases with the increase of the contact angle,and reaches the maximum value when the contact angle is 90°.The average value increases with the increase of the spray density and the dimensionless dimension of the y direction.The flow velocity of the liquid film increases first and then increases with the increase of the contact angle,and reaches the minimum value when the contact angle is 90°.The average value increases with the increase of the spray density and the dimensionless dimension of the y direction.The change of the total velocity of the liquid film is the same as that of the liquid film velocity u along the tube wall.The numerical value of U is influenced by the lateral flow velocity v of the liquid film.At the same time,this paper proposes a method to simplify the basic control equations by using dimensionless quantity,and obtain the dimensionless temperature and dimensionless mass fraction of heat and mass transfer process.In this paper,the numerical simulation software Fluent and its pre-processing software Gambit and post-processing software CFD-Post are used to simulate and analyze the flow,mass transfer and heat transfer of the basic physical model(Horizontal tube falling film evaporation flow model with the spray density ? is 0.1 kg /(m · s),the diameter ? is 10 mm,the spray height h is 10 mm,the tube pitch d is 10 mm,and the time t is 0.32s)established in this paper.The physical model was meshed with the pre-processing software Gambit,and the simulation results were processed and plotted by the post-processing software CFD-Post,and the data was processed using the origin software.The effects of different spray density,different diameter,different spray height and different pipe spacing on flow,mass transfer and heat transfer were simulated and analyzed respectively.Before the simulation,the VOF multiphase flow model and the User Defined Function(UDF)are selected for the multiphase flow problem studied in this paper.The segregated solver is used to solve the control equations,the physical parameters of the solution are set and the working pressure is 870 Pa.The UDF file of the two-phase heat transfer and mass transfer based on the evaporation principle is embedded in the model.The PISO algorithm is used to solve the iteration.The results obtained after the simulation are as follows.(1)Select the different spray density for the base model,with the increase of spray density,the film thickness increases,the liquid film velocity increases,the liquid film heat transfer and mass transfer effect decreases,but the liquid film stability is reduced,resulting in liquid film heat transfer and mass transfer effect first increased and then decreased.This paper simulates the best heat and mass transfer effect when the spray density is 0.1kg /(m·s).(2)Select the different diameter for the base model,with the increase of the diameter,the film thickness decreases,the liquid film velocity increases,but the liquid film stability decreases,resulting in liquid film heat transfer and mass transfer effect first increased and then decreased.This paper simulates the best heat and mass transfer effect when the diameter is 16 mm.(3)Select the different pipe spacing for the base model,with the increase of the spacing of the tube,the thickness of the liquid film decreases,the liquid film velocity increases,the heat transfer and mass transfer effect of the liquid film increase first and then decrease.This paper simulates the best heat transfer and mass transfer when the height of the cloth is 10 mm.(4)Select the different tube spacing for the base model,with the increase of the spacing of the tube,the thickness of the liquid film decreases,the liquid film velocity increases,the heat transfer and mass transfer effect of the liquid film increase first and then decrease.This paper simulates the best heat and mass transfer effect when the tube spacing is 10 mm.(5)As a conclusion,the heat transfer and mass transfer of the liquid film are coupled,and the change trend is the same.In the condition of liquid film stable spreading,the thinner the thickness of the liquid film is,and the lower the speed is,the better the heat and mass transfer effection are.