Experimental Investigation And Numerical Simulation Of The Heat And Mass Transfer In Evaporative Cooler

In industry, due to the fact that the cooling water in cooling towers directly contacts with air, cooling water can easily be polluted. Evaporative coolers because of its environmentally friendly and energy-savings, are widely used in chemical, metallurgy, etc. Based on the experimental investigation, computational analysis and CFD simulation, the heat and mass transfer and thermal performance of the evaporative coolers are presented as follows:(1) The experimental test platform was set up and the thermal performance characteristics of the oval-tube and twisted-tube evaporative coolers were studied. The impact of deluge water flow rate, air velocity and air wet bulb temperature on the process water outlet temperature, deluge water temperature, air outlet enthalpy and heat transfer coefficient were analyzed. The deluge water flow rate had little impact on the heat transfer rate when it reached the minimum flow rate. From the experimental results, correlations for the water film heat transfer coefficient and air-water mass transfer coefficient were developed.(2) By employing these assumptions and following an approach similar to Poppe and Merkel, analytical models of the evaporative cooler were derived from basic principles. Flowchart of the calculating process were showed as one and two-dimensional models. Compared with the experimental data, the predicted outlet process water temperatures showed little difference between the one and two-dimensional models, thus one-dimensional model was proposed. The distribution of heat flux along the evaporative cooler by Poppe and Merkel methods compared well with each other and was between1.01and1.06. The predicted outlet air temperature and humidity ratio by Poppe method were closer to the experimental data. Analytical model was adopted to investigate the thermal performance under supersaturated condition based on Poppe method and solved by a sectional method. Under the supersaturated condition, whether the air was supersaturated or not was determined by the program, then the program automatically selected the corresponding governing equations. The effect of degree of saturation of inlet air on the heat transfer rate and process-water outlet temperature was presented, and little difference was found between the unsaturated and supersaturated conditions if the outlet air was supersaturated. The air temperature, humidity ratio and enthalpy distributions under both unsaturated and supersaturated conditions could be predicted with the analytical model and the differences between the two conditions were also presented. The assumption that the air was unsaturated was a very useful assumption to predict the total heat transfer rate and outlet process-water temperature of the evaporative coolers if the actual air was supersaturated. Supersaturation was likely to occur at low inlet air temperature. The predicted air temperatures and humidity ratio were not very close to the two conditions if the outlet air was supersaturated.(3) The thermal and hydraulic performance of plain tube and oval tube evaporative coolers were investigated numerically. Computational fluid dynamics (CFD), ANSYS FLUENT12.1, was implemented for the numerical solution. Species transport without reactions was adopted to simulate the mass transfer from the air-deluge water interface to the air. Different turbulence models and near-wall treatments were used to assess which model fit the data better. The mass transfer Colburn factor jm, and friction factor/were presented and compared with experimental data. The Standard k-ε model with non-equibibrium wall functions was the most suitable combination for the prediction. The proposed FLUENT model was also applied to predict the mass transfer coefficient of other evaporative coolers and compared well with experimental data. The comparison between computational analysis and FLUENT simulation showed an acceptable agreement with each other.(4) The bypass flow, tube type and tube layout pattern were investigated numerically. The fraction of the bypass to total flow rate increased with the increasing of the bypass width. Thus, the fraction of the air crossing the tube bundle decreased, which resulted in the lower of the outlet air enthalpy and less heat transfer rate. It was proposed to avoid leakage if the minimum requirement of assembling was reached. Different tube types (plain and oval tube with three different axial ratio) were adopted to analyze thermal performance, which included the out air enthalpy, mass transfer Colburn factor, fraction of bypass. The results showed mass transfer Colburn factor for the oval tube was higher than that for the plain tube. The simulation of the tube layout pattern concluded the larger the tube layout angle, the higher the turbulent intensity and mass transfer.