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

Lithium bromide absorption chil ers have the advantages of energy saving,environmental friendliness,and using remaining heat.However,compared with vapor compression chil er,the low energy efficiency ratio is not good for the development of absorption refrigeration,so it is necessary to improve its energy efficiency ratio.The evaporator is one of the parts of heat exchange,and the refrigerant flows along the faling film of the horizontal tube in a vacuum environment.This paper mainly studies the influencing factors of the evaporation flow and heat transfer process of the lower spray density faling film from the perspective of theoretical analysis and numerical simulation,and propose a simple-process and low-cost enhanced heat exchange tubes coated metal mesh and verify its enhanced heat exchange capability.First,this paper establishes a physical model and basic control equations for the faling film evaporation outside the horizontal tube.After simplifying the assumptions,the analytical formulas of liquid film flow₁ and heat transfer are obtained:k_x=λ/δ_t=λ/（?）,and the relationship between the various parameters of the liquid film and the circumference angle,spray density,etc.alternative relation:（1）With the increase of circumferential angle,the liquid film thickness first decreases and then increases and reaches the minimum value at 90 angle.With the increase of spray density,the thickness of liquid film increases gradually.（2）With the increase of circumferential angle,the total velocity of liquid film first increases and then decreases and reaches the maximum value at 90 angle.With the increase of spray density,the total velocity of liquid film increases.（3）With the increase of circumferential angle,the thickness of thermal boundary layer increases.With the increase of spray density,the thickness of thermal boundary layer decreases.（4）With the increase of circumferential angle,the local heat transfer coefficient decreases.With the increase of spray density,the local heat transfer coefficient increases.Secondly,after verifying the validity of the model and carrying out experiments to measure the actual contact angle.After using ICEM software for the grid division,Fluent software was used to simulate the flow and heat transfer of double-row horizontal tubes in the faling film evaporation process of lithium bromide absorption refrigeration cycle with low spray density.The optimum heat transfer conditions under different spray density,liquid distribution height,pipe diameter and pipe spacing were obtained:（1）With the increase of spray density,the thickness and flow velocity of liquid film increase,and the excess temperature of liquid film decreases.The heat transfer is the best when the spray density is 0.1 kg/（m·s）.（2）With the increase of liquid distribution height,the liquid film thickness decreases,the liquid film velocity increases,and the excess temperature of liquid film does not change very much.The heat transfer is the best when the liquid distribution height is 15 mm.（3）With the increase of tube diameter,the thickness of liquid film decreases,and the velocity and excess temperature of liquid film increase.The heat transfer is the best when the tube diameter is 20 mm.（4）With the increase of tube spacing,the liquid film thickness decreases,and the liquid film velocity and excess temperature increase.The heat transfer is the best when the tube spacing is 15 mm.Finally,the faling film evaporation performance of single-row el iptical tubes with the same heat transfer area as circular tubes and enhancing heat transfer tube wrapped with a wire-mesh screen.The average excess temperature of the el iptical tube with 1.6 ratio of long axis to short axis is increased by 18.4%compared with the circular tube.The average excess temperature of the circular tubes with 0.05 mm wire diameter and 10 mesh number was 32.1%higher than that of the conventional circular tubes,which verifies that heat transfer performance of the tube can be improved by enhancing heat transfer tube wrapped with a wire-mesh screen.It does not mean that the heat transfer capacity of the tube is stronger than that of the el iptical tube under other working conditions.In addition,the results show that there is a great relationship between the mesh number and the heat transfer capability.When the mesh size is too large,the resistance of the mesh is greater than the tangential component of gravity,the liquid film will leave the wall ahead of time,and the"dry spot"area will be increased,which will worsen the heat transfer performance.When the mesh size is too smal,the flow and heat transfer are basically the same as the smooth circular tube.Compared with the conventional tube,the el iptical tube has smal er or no"dry spot"area,which is superior to the circular tube for faling film flow and heat transfer.