Numerical Study On Condensate Formation Characteristics Of Heat Transfer Channels In Wet Conditions

With the energy shortage caused by the rapid development of the global economy,the excessive energy consumption and the resulting environmental problems are the severe situation facing the current society.The high energy consumption caused by China's high population growth rate and social development speed is worthy of our attention,especially the high energy consumption of buildings and equipment with a large proportion cannot be ignored.Among them,refrigeration and air-conditioning products exacerbate energy waste due to their consumables in production,power consumption during operation,and overcapacity and low utilization rate of capacity caused during use.The heat exchangers is the main energy-consuming equipment of air-conditioning system,its performance plays a key role in air-conditioning system.At present,the air side of the heat exchanger used in the air conditioning system in the building generally has a moisture evolution phenomenon,and the air and the surface of the heat exchanger fin simultaneously perform heat exchange and mass transfer under the condition of moisture deposition,then the liquid has a complex effect on the heat transfer and resistance characteristics in the heat exchange channel.Therefore,in order to further investigate the influence of the condensate in the heat transfer channel of the fin-and-tube heat exchanger on the overall heat and mass transfer process under the wet condition,In this paper,the heat and mass transfer characteristics of the air-side of a compact fin-and-tube heat exchanger commonly used in air-conditioning products under wet conditions are numerically studied.?1?Establish a numerical calculation model of simplified plane finned tube and wavy finned tube heat exchanger,calculation of two different structures of finned tube models in three-dimensional two-phase two-component unsteady state using Fluent software.The simulated calculation conditions are:inlet humid air flow rates of 1.0m/s,2.0m/s,3.0m/s,4.0m/s,and relative humidity of 50%,60%,70%,80%.The simulation results are compared with the experimental results of the heat transfer and resistance characteristics of the fin-and-tube heat exchanger under wet conditions,and the degree of coincidence is relatively high.?2?Comparative analysis of the flow field distribution of the plane finned tube and the wavy finned tube model under wet conditions and the condensation characteristics of the fin surface.The results show that the heat transfer effect of the wet air around the cold water pipe wall is stronger than that under the dry condition;the increase of the number of pipe rows has little effect on the flow field distribution in the upstream part;the fin structure has a significant effect on the temperature field,velocity field and concentration field of the finned tube model.The diameter of the condensate on the surface of the fins of the two finned tube models decrease with the increase of the inlet air flow rate,and the number increases with the increase of the inlet relative humidity,which is consistent with the experimental results.?3?By investigating the distribution of the surface of the fin-and-tube heat exchanger fins,it can be found that the total moisture content of the fin surface increases with the increase of the inlet relative humidity,and decreases with the increase of the inlet air flow rate.The maximum increment of the wavy finned tube model at different speeds is 2.03 times that of the plane finned tube model.The average mass fraction of water vapor in the inlet and outlet wet air increases with the increase of inlet relative humidity and air flow rate.The maximum value of the plane finned tube model is 1.16 times that of the wavy finned tube model.The average volume fraction of the condensate on the surface of the fin increased sharply with time and then began to fluctuate upward and upward,When u_a,in=4.0m/s,RH=50%,RH=60%,RH=70%,RH=80%,the average volume fraction of condensate reaches 0.0001541 at different times.?4?Comparing plane finned tubes and wavy fins when inlet air flow rate is 1.0 m/s,2.0m/s,3.0 m/s,4.0 m/s,relative humidity is 50%,60%,70%,80%Air-side heat and mass transfer characteristics of the tube model,it is found that the total heat transfer,heat and mass transfer coefficient and average Nu of the two finned tube models increase with the increase of the inlet air velocity,the drag coefficient,fin efficiency,heat transfer factor j_h and mass transfer factor j_m decrease with increasing inlet air velocity;the total heat transfer,total heat transfer coefficient and average Nu increase with the increase of the relative humidity of the inlet,while the fin efficiency decreases with the increase of the relative humidity of the inlet;the sensible heat transfer,sensible heat transfer coefficient,mass transfer coefficient,drag coefficient,heat transfer factor j_h and mass transfer factor j_m are all insensitive to changes in inlet relative humidity.The parameters of the wavy finned tube model are almost higher than the values of the plane finned tube model under the same working conditions.The integrated heat transfer factor JF value of the wavy finned tube model is 1.1854 at u_a,in=4.0m/s and RH=60%,indicating that the enhanced heat transfer effect is best under this condition.?5?According to the simulation results of the plane finned tube model under different tube rows,it can be found that the number of tubes rows at high flow rate has a significant effect on the total heat transfer of the finned tube model;the heat transfer coefficient and mass transfer coefficient decrease with the increase of the number of rows of tubes;the heat transfer factor j_h decreases slightly with the increase of the number of rows of tubes.When u_a,in=1.0m/s,compared with the finned-tube heat exchanger model of row=4,the heat transfer factor j_h of row=1 increases by 6.06%,while the mass transfer factor j_m increases 241%.