| Currently,the electricity consumptions of refrigeration and air conditioning equipment accounts for more than 20%of the total power generation of the whole society in China.In the context of the national strong promotion of carbon reduction and emission reduction,there is an urgent need for more energy-saving and efficient refrigeration and air conditioning equipment to help China achieve the goal of"carbon peaking and carbon neutrality".The efficiency loss of the finned tube heat exchangers in the refrigeration and air conditioning system is the largest,of which the airside thermal resistance usually accounts for more than90%of the total thermal resistance of heat exchangers.Improving the airside performance of the finned tube heat exchangers is of great significance to improving the energy efficiency of the refrigeration and air conditioning system.When the finned tube heat exchanger is operated under dehumidification conditions,the effect of condensate on the fin surface is more complex on its airside performance.In particular,changing the wetness of the fin surface makes the geometry of the condensate and its motion characteristics show significant differences,which leads to unpredictable changes in the airside performance of finned tube exchangers with hydrophilic coating.Therefore,this paper investigates the effect of operating conditions and structural changes on the airside performance of finned tube heat exchangers with hydrophilic coating under dehumidification conditions through a combination of experimental and numerical calculations.To clarify the heat transfer,mass transfer,and resistance characteristics of the airside of finned tube heat exchangers with hydrophilic coating under dehumidification conditions and to obtain better structural and airside performance correlations.Firstly,the airside performances of finned tube heat exchangers with hydrophilic coating ware tested by a wind tunnel test bench and compared with the airside performance of finned tube heat exchangers with uncoated.By analyzing the effects of hydrophilic coating and operating conditions on the airside performances of the finned tube heat exchangers,it was found that the airside heat transfer,mass transfer,and resistance performance of finned tube heat exchangers with hydrophilic coating was less than that of finned tube heat exchangers with uncoated,and maximum differences in friction factor,mass transfer factor and heat transfer factor of finned tube heat exchangers between the hydrophilic coating and uncoated were 53.9%,24.8%,and 16.6%,respectively.The heat transfer factor,mass transfer factor,and friction factor of finned tube heat exchangers with hydrophilic coating decreased with the decrease of air inlet temperature and relative humidity,as well as the increase of airside Reynolds number and refrigerant inlet temperature,and the mass transfer factor decreased significantly when the difference of air inlet temperature and refrigerant inlet temperature was too small.It shows that the existing findings of finned tube heat exchangers with uncoated were no longer applicable to finned tube heat exchangers with hydrophilic coating.Secondly,a numerical calculation method for the airside of finned tube heat exchangers with hydrophilic coating under dehumidification conditions was constructed based on CFD software.The Eulerian polynomial flow model-thermal phase change mass transfer model embedded was selected to carry out the numerical calculation of the airside of finned tube heat exchangers with hydrophilic coating under dehumidification conditions,and the average deviations of heat transfer factor,mass transfer factor,and friction factor were found to be6.9%,6.3%,and 10.9%,respectively,when the calculated results were compared with the experimental data.The effects of changes in four structural parameters,namely,the outer diameter of the tube,fin spacing,transverse tube spacing,and longitudinal tube spacing,on the airside performance of finned tube heat exchangers with hydrophilic coating under different operating conditions were investigated by the constructed numerical simulation method.The study showed that the operating conditions do not change the trend of the effect of structural changes on the airside performance.The maximum decreases in heat transfer,mass transfer,and friction factors were 34.3%,37.1%,and 13.8%,respectively,when the outer diameter of tube D_o of the heat exchangers was increased from 8 mm to 12 mm.The heat transfer factor and mass transfer factor decreased with the increase of fin spacing,with a decrease of 4.8%to 13.6%and 5.0%to 30.0%for different structures of the heat exchangers,respectively.The friction factor increased with the increase of fin spacing,with an increase of6.0%to 23.7%.The larger the transverse tube distance,the better the airside heat transfer characteristics,more heat transfer rate,and less pressure drop,but the resistance characteristics become worse;while increasing the longitudinal tube distance,more heat transfer,better resistance characteristics,but also makes the pressure drop slightly higher,lower heat transfer characteristics.Finally,the airside comprehensive heat exchange performance of the finned tube heat exchangers with different structures under dehumidification conditions was analyzed by the comprehensively enhanced heat exchange evaluating indicator JF factor.The airside comprehensive heat transfer performance of the finned tube heat exchangers with hydrophilic coating was found to be better with larger transverse tube spacing,smaller tube diameter,fin spacing,and longitudinal tube spacing in the studied range of structural parameter variations.Furthermore,the optimal structure of the finned tube heat exchangers with hydrophilic coating was deduced:the outer diameter of tube D_o of 8 mm,the fin spacing S_p of 1.5 mm,the transverse tube spacing P_t of 28.3 mm,and the longitudinal tube spacing P_l of 18 mm.The analysis revealed that the existing airside performance correlations for finned tube heat exchangers with uncoated had a significant deviation in predicting the airside performance of finned tube heat exchangers with hydrophilic coating,and subsequently developed an airside performance correlation for the finned tube heat exchangers with hydrophilic coating based on the existing experimentally and numerically calculated data.The correlations for the heat and mass transfer factors covered 96.6%and 97.7%of the experimental and numerically calculated data within a deviations range of±20%,and the correlations for the friction factor were able to cover 98.9%of the experimentally and numerically calculated data within a deviations range of±15%.The mean deviations of the three were 5.3%,6.6%,and 4.7%,respectively.Since the optimal structure of the finned tube heat exchangers with hydrophilic coating(Do=8 mm,Sp=1.5 mm,Pt=28.3 mm,Pl=18 mm)was deduced from the conclusion of analyzing the effect of structural parameters on the airside performance of finned tube heat exchangers,its airside performance was verified by using the developed correlations under different operating conditions,and it was found that the enhanced heat transfer factors of finned tube heat exchangers with the optimal structure all reached maximum,proving that the airside performance is optimal within the range of variation of the structural parameters studied in this paper.It is also further demonstrated that the predictive performance of the airside performance correlation equation is reliable. |
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