Research On Heat And Mass Transfer Characteristics And Structure Optimization Of Fin-and-tube Heat Exchangers With Vortex Generators In Wet Conditions

Under the rigid growth trend of energy demand,effectively reducing total energy consumption and improving energy efficiency have become one of the important ways to promote the sustainable development of the national economy.Fin-and-tube heat exchangers are widely used in the fields of air conditioning,refrigeration,petrochemical,transportation and metallurgy due to their advantages of compact structure,easy manufacturing,convenient operation and convenient maintenance.During the actual operation of the air conditioning system,there is a general phenomenon of dehumidification on the air side of the heat exchanger.The precipitation process and movement characteristics of condensed droplets directly affect the heat and mass transfer characteristics and resistance performance of the heat exchanger,and could cause heat exchanger corrosion and bring a series of health issues.In this paper,the delta-wing vortex generators are used to strengthen the heat exchange of the copper staggered round tube fin-and-tube heat exchanger,and the focus is on a series of studies on the heat and mass transfer characteristics and structural optimization of the round tube fin-and-tube heat exchanger with vortex generators under wet conditions.Firstly,structured grid technology is used to establish a numerical heat transfer model for the round tube fin-and-tube heat exchanger with different wing height and attack angle structural parameters of the vortex generator,and the independence of grid systems are verified in this paper.The maximum relative errors of the average Nusselt number and the resistance coefficient verified by the simulation results compared with the experiments are16.15%and 9.3%under dry conditions,and the average errors are 15.35%and 10.55%under wet conditions,respectively.Secondly,the heat and mass transfer and resistance characteristics of the round tube fin-and-tube heat exchanger with delta-wing vortex generators are studied by using the constructed heat transfer model under wet conditions.The results show that under the conditions of the inlet air flow velocity u_in=1.0m/s?4.0m/s and the inlet air relative humidity RH_in=50%?80%,taking the vortex generator of the wing height H=2.00mm and the attack angle?=45°as an example,the average temperature of fins increase with the increase of air relative humidity and flow velocity by 0.63%?0.95%and 0.09%?4.07%under non-constant temperature fluid-solid coupling conditions,but the growth rates decrease;The average Nusselt number and the flow resistance coefficient under wet conditions are 59.07%and 1.52times the average increase over dry conditions,which show that wet conditions are conducive to heat and mass transfer characteristics but are not conducive to resistance characteristics;The vortex generator make the total fin efficiency,the average Nusselt number,the flow resistance coefficient,the amount of moisture evolution,and the enhanced heat and mass transfer factor large by average 2.00%,25.45%,51.70%,50.79%,and 9.61%,which show that the vortex generators are beneficial to heat and mass transfer characteristics and dehumidification characteristics but are not conducive to resistance characteristics,it has a significant strengthening effect on comprehensive heat and mass transfer performance,and it is recommended 2m/s is the better inlet air velocity under the high humidity conditions of RH_in?70%.Thirdly,the dehumidification characteristics of the round tube fin-and-tube heat exchanger with vortex generators are analyzed in wet conditions.The results show that the location of condensed droplets on the fin surface are mainly from the rear rows of tubes to the forward rows and expand away from the center of the circle,and the location of condensed droplets on the leeward side of the round tube are significantly more than the windward side;The condensed droplets downstream of vortex generators on the Fin?and Fin?surface are mainly distributed on the outer boundary of the vortex,and the shapes of the boundaries are striped and ellipsoid shape;The amount and size of the condensed droplets decrease with the increase of the inlet air flow rate,and increase with the increase of the relative humidity of the inlet air,but the increase in the size of the condensed droplets is small;The integral number and mass of condensed droplets on the wall of fins and tubes show the hourly characteristic“logarithmic increase?oscillating increase?dynamic stability”under different inlet air relative humidity and flow velocity,and the amount of condensed droplets on the surface of Fin?is higher than the cross section of the surface of Fin?and the air-middle interface.Fourthly,the structure of the round tube fin-and-tube heat exchanger with vortex generators is optimized by changing wing heights?1.50mm,1.75mm,2.00mm?and attack angles?25°,35°,45°?under the most unfavorable wet conditions.The results show that the average Nusselt number and the flow resistance coefficient increase with the increase of the wing height and the attack angle,which show that increasing the wing height and the attack angle will conducive to heat and mass transfer characteristics but not conducive to resistance characteristics;The amount of moisture evolution increase as the wing height increases,but increase first and then decrease as the attack angle increases;Based on the most unfavorable wet conditions of inlet air flow velocity uin=1.0m/s and inlet air relative humidity RHin=80%,the enhanced heat transfer factor when the wing height H=1.75mm increase by 0.11%and0.89%when H=1.50mm and H=2.00mm,and when the attack angle?=35°increased by0.44%and 2.38%when?=25°and?=45°;Based on the comprehensive heat and mass transfer performance,the optimal structural parameter of vortex generators of the round-tube and fin-and-tube heat exchangers is determined as the wing height H=1.75mm?H/T_p=0.875?and the attack angle?=35°.