Experimental And Numerical Study Of The Heat And Mass Transfers In Small-scale Evaporative Air Cooler

Evaporative air cooler(EAC)is widely used in refrigeration,petrochemical and other fields due to its advantages of energy and water saving,and its research has also attracted the attention of many researchers.Presently,with regard to the heat and mass transfer process,profound and accepted theory has not been built because the unawareness of interaction between the water and air outside the tubes.Based on the small scale EAC,the heat and mass transfers between the spray water and air outside the tubes were studied experimentally and numerically in this paper.Under the 25 sets operating conditions of spray density and air face velocity(air velocity at the maximum section in EAC),the parameters such as temperature of spray water and air,air humidity and pressure on the upper and lower position of tube bundle were measured,and the relationships between spray density,air face velocity and heat transfer capacity,heat transfer coefficient etc.were studied.Using VOF model and SST ?-? turbulence model,the water-air mass transfer and shear force were considered,the spray water flow pattern and three-dimensional distributions of film thickness,the wall-film heat transfer coefficient(heat transfer coefficient between wall and water film)and water-air heat transfer coefficient and water-air mass transfer coefficient were numerically investigated.Besides,the effects of the spray density and air face velocity on thermo-hydraulic behaviors were systematically analyzed.The experimental results show that: with the increase of air face velocity and spray density,the heat transfer capacity,water-air heat transfer coefficient,water-air mass transfer coefficient and air pressure drop increase,while the outlet air temperature and humidity decrease;there is an optimal value of spray density under small air face velocity conditions(?1.6m/s),for example,the maximum value of heat transfer appears at the spray density of 0.0354 kg/(m?s)when the air face velocity is 1.0m/s;compared to the influence of spray density,the air face velocity has greater effect on the heat and mass transfers;the empirical correlations show that the water-air heat transfer coefficient and mass transfer coefficient are proportional to the Nth(positive number)power of spray water and air Reynolds number,and the power of air Reynolds number is greater,which also suggests that the influence of air face velocity is larger than that of spray density.The numerical simulation results show that: the outlet air humidity difference between simulation values and experimental values is less than 10%,which demonstrates that the numerical model in this study has high accuracy;under complete wetting condition of tube wall surface,the film thickness decreases first and then increases along the circumferential direction,and decreases slightly along the axial direction;the water film of first-row tube is thinnest at the angle of 60 degree and 120 degree,while that of second-row tube is thinnest at angle of 90 degrees;when the air face velocity increases,the water film thins between the angles of 135~165 degrees,but thickens between the angles 15~120 degrees;with the increase of circumferential angle,the wall-film heat transfer coefficient,water-air heat and mass transfer coefficient increase first and then decrease,while the wall-film heat transfer coefficient increases slightly at 170~180 degrees;when increasing the axial position(z value),the wall-film heat transfer coefficient and water-air heat transfer coefficient decrease,while the water-air mass transfer coefficient increases.