Numerical Simulation Of Fluid Flow And Heat Transfer Characteristics Of Falling Film Evaporation Outside Horizontal Tubes

As a kind of high-efficiency and energy-saving heat transfer technology,horizontal-tube falling film evaporation technology has great application prospects in refrigeration field due to its advantages of good heat transfer performance,small heat transfer temperature difference and low refrigerant charge.Based on relevant experimental models,three-dimensional numerical models for the falling film evaporation of refrigerant R410 A outside the horizontal single tube and in the evaporator are established.The calculation model and boundary conditions are determined,and the flow and heat transfer simulation process is clarified..The falling film evaporation process of refrigerant R410 A outside the horizontal tube was simulated and analyzed.The characteristics of liquid film flow and the local heat transfer coefficient in the circumferential and axial directions of the tube were investigated.The results show that the horizontal falling film flow process can be divided into four stages: free fall stage,drop impact stage,free flow stage and complete flow stage.When the flow is stable,the liquid film thickness gradually reduced along the circumferential direction of the tube and reaches the minimum value between 100 ° and 120 °;the circumferential distribution of local heat transfer coefficient firstly reduces sharply and then tends to be stable,and decreases near the bottom of the tube due to the local liquid accumulation.The axial distribution of liquid film thickness and local heat transfer coefficient are characterized by periodic fluctuations.The effects of different parameters on the flow and heat transfer characteristics of the falling film outside the horizontal tube were simulated.The results show that the total heat transfer coefficient increases with the increase of spray density.When the Reynolds number reaches 2000,the increase of the total heat transfer coefficient becomes smaller and tends to be stable;when the spray density is larger,the increase of heat flux has a certain promotion effect on the heat transfer performance,while the effect is reversed when the spray density is small.As the evaporation temperature increases,the total heat transfer coefficient also increases accordingly.The liquid film thickness outside the tube and the heat transfer coefficient are gradually reduced as the tube diameter increases.With the rise of the liquid dispenser height,the circumferential distribution of the liquid film outside the tube shows a decreasing trend,and the total heat transfer coefficient shows an increasing trend.The change of liquid film thickness in the semi-circumference is larger than that in the lower half of the week.The increase of the density of the liquid hole will reduce the fluctuation of the axial liquid film thickness distribution,thereby increasing the total heat transfer coefficient.As the horizontal distance from the spray hole to the tube axle increases,the flow of liquid film outside the tube will have regular fluctuations,which will enhance the heat transfer to a certain extent.With the further increase of the offset distance,the liquid film outside the tube will rupture to form a "dry" area which will lead to the sharp drop of the heat transfer coefficient.The simulation explores the flow and heat transfer characteristics of the fluid inside the horizontal tube falling film evaporator.The results show that the fluid in the evaporator shell is relatively concentrated and the flow velocity is higher at the outlet near the liquid distributor and the upper and lower ends of the refrigerant,while the flow velocity in the falling film flow region is lower and the distribution is more uniform.The surface of the tube wall is more prone to the “dryout” area.The temperature of the tube wall the fluid in the tube gradually increases along the vertical direction,while the fluid temperature in the middle heat exchange tube is slightly higher than the two sides in the horizontal direction.As the refrigerant spray density increases,the high temperature region of tube walls moves toward the inlet direction and gradually decreases.The local heat transfer coefficient of the heat exchange tube surface decreases in the vertical direction.The smaller the spray density is,the more obvious the downward trend will be.The research results of this subject will provide theoretical support and reference for the research and application of horizontal-tube falling film evaporation technology.