Numerical Simulation And Experimental Study Of Heat And Mass Transfer Mechanisms Of Droplet Evaporation

The widespread use of mechanical refrigeration has led to an increasingly rise ofair conditioning energy consumption, and the high temperature in the hot summerweather also makes urban environment deteriorating. Promotion of evaporative coolingtechnology can help to decrease the mechanical refrigeration energy consumption andthe bad affection of the status on environment to some extent. In recent years, spraycooling technology are paid more attention as a new application form of directevaporative cooling, with advantages of energy saving, high efficiency andenvironmental friendly. Compared to conventional direct evaporative air cooling units,spray cooling needs no circulating water, gets simplification on the equipment structure,and obtains high evaporation efficiency, thus the application have broad prospect. In thispaper, numerical simulation and experimental research of direct evaporative spraycooling system was processed, aiming at the key issues of spray cooling process, thefollowing exploration are conducted.First, heat and mass transfer mechanism of spray cooling is analyzed, and built theheat and mass transfer balance equation for droplets group and air, as well as the heatand mass transfer mathematical model of single droplets. The main influencing factorsof the heat and mass transfer are analyzed through mathematical model. Mathematicalmodel is the basis of numerical simulation, provides guidance to the choice ofnumerical simulation models. For the calculation of the direct evaporative coolingprocess, both analytical method and experimental studies have great restrictions. Forcomplex mathematical models, computational fluid dynamics (CFD) provides a feasiblemethod of calculation, and numerical simulation has the advantage of being lessexpensive and can simulate more complex or ideal conditions.Based on CFD software FLUENT, the model principles and simulation methods ofspray evaporation process are described, explored from the continuous phase anddiscrete phase model, coupling between phases calculations and important parametersettings respectively, and the simulation method of direct evaporative spray coolingprocess are summed up. For heat and mass transfer problems of direct evaporativecooling process, reasonable mathematical models and numerical simulations areguidance to experimental studies and theoretical analysis, which can make up for thelack of experimental work. During the study, the evaluation parameters and method of spray cooling effect areproposed first. A spray cooling system experimental platform is build to test the coolingeffect of the spray under high temperature environment. The impact of outdoor air andwater gas ratio on the evaporation efficiency and saturation efficiency are analyzedthrough experimental results, and a corresponding simulation model of experiments isestablished, using the discrete phase model to simulate and get the temperature,humidity distribution after spray cooling, and the experimental and simulation resultsare compared.After credible results of numerical simulation obtained, more spray cooling systemwas simulated under variable condition using the simulation method, and spray coolingsystem has been optimized. Numerical simulation conditions are:1) The establishment of the air duct spray evaporative cooling systems, research ofinfluence by droplet radii, nozzle spacing, changing parameters such as wind speed andsprayed ratio on spray cooling evaporating efficiency.2) Add conditions to simulate the experiment model, the impact of outdoor airparameters and water-gas ratio on the spray cooling effect on the in-depth analysis.3) Spray cooling in the outdoor open space is simulated, the main influencingfactors on the wind velocity and flow rate on spray cooling effect in the pedestrian fixedheight areas is analyzed.For air duct spray cooling, after the optimization of the design through numericalsimulation method, the evaporation efficiency in optimal conditions can be near100%and saturation efficiency can reach90%. Unlike spray cooling process in finite area,outdoor spray cooling effect is intimately influenced by outfield wind speed, winddirection, and the scope of the wind field expand or shrink have a direct result of thecooling effect..For spray cooling in air duct system, numerical simulation method has greatsignificance in the study of spray cooling mechanism, device development andoptimization. For spray cooling in outdoor open space, numerical simulation results canbe used to predicted, and be guidance to spray cooling system design.