The Motion Of Water Droplets On Fins And Heat Transfer Characteristics Of Fins Under Dehumidification Condition

The air conditioning evaporator cools the moist air during operation.When the moist air temperature is cooled below the dew point temperature,the water vapor in the moist air will condense into droplets,and the heat exchange equipment is in the dehumidifying condition.The formation process of condensation droplets on the fin surface is affected by surface tension coefficient,supersaturation,critical radius of droplets and contact angle.The growth process of droplets is affected by water vapor in wet air and mass transfer of droplets at the gas-liquid interface.The movement of droplets is affected by their own gravity,airflow drag force and wall adhesion force.The formation,growth and movement of the above droplets will affect the heat transfer efficiency and flow resistance of the heat exchanger,and then affect the overall performance of the air conditioning system.Therefore,it is very important to research the above process of condensed droplets.In this paper,the motion model,droplet formation and growth model of condensed droplets on the surface of vertical fins are established to explore the influence of droplet formation,growth and motion on heat and mass transfer.The specific work is as follows:（1）By writing the corresponding UDF program,the motion characteristics of droplets under different force conditions,different fin surface wetting characteristics,different inlet air velocities,and different fin inclination angles are analyzed.The numerical simulation results are compared with the experimental results.It is found that the larger the receding contact angle is,the easier the droplets on the fin surface fall off.The larger the cross-sectional area of the droplets and the gas-liquid interface area are,the smaller the contact area between the droplets and the fin is.The airflow drag force has a great influence on the droplet motion process.The shape of the bottom surface of the droplet is an inclined ellipse.The larger the inlet air velocities,the greater the inclination,the smaller the cross-sectional area of the droplet,the smaller the contact area between the droplet and the fin surface,and the larger the gas-liquid interface area.The larger the inclination angle of the fin,the more difficult it is for the droplet to fall off from the fin surface.The smaller the cross-sectional area of the droplet and the gas-liquid interface area,the larger the contact area between the droplet and the fin surface.（2）The model of water vapor mass transfer rate during droplet formation and growth was established respectively.Calculation of latent heat transfer based on mass transfer rate;by changing the thickness of the first layer mesh near the wall,the independence of the mesh is verified.It is found that the heat transfer factor j_hand the mass transfer factor j_mdecrease with the increase of the mesh thickness,but the change is not obvious.The numerical simulation results of heat transfer factor j_hand mass transfer factor j_mare compared with the experimental data and empirical formula calculation results in the literature.It is found that the average error between the numerical simulation results,the experimental data and empirical formula is about 10%,indicating that the numerical simulation results are in good agreement with the experimental values and empirical formulas.（3）The variation of Nusselt number Nu,air resistance coefficient f,heat transfer factor j_hand mass transfer factor j_munder different fin spacing F_p,tube row number N,inlet air velocity v_inand relative humidity RH were researched by numerical simulation.It is found that Nu decreases with the increase of F_pand N,and increases with the increase of v_inand RH.f decreases with the increase of F_pand increases with the increase of v_in.The effect of v_inon f is very significant,and RH has little effect on f.The heat and mass transfer factors j_hand j_mdecrease with the increase of F_p,N and v_in.The larger the v_inis,the slower the decrease is.RH has little effect on j_h,but has a significant effect on j_m.