Study On Heat And Mass Transfer Mechanisms Of Evaporative Cooling Based On Two-dimensional CFD Model And One-dimensional Large-volume Flow Model

Compared to the conventional mechanical vapor compression refrigeration system the refrigeration system using evaporative cooling technology has the advantages of energy saving,economy and environmental protection,and is one of the important approach to improve production efficiency and human thermal comfort.However,after reviewing the origin and development of the evaporative cooling technology,it is found that there are still some issues needed to be solved in the development process of the technology in recent years.These issuses are as follows:(1)ignoring the real "natural-formed" boundary condition in the evaporative cooling heat and mass transfer processes,and simply utilizing the Nusselt number under constant surface temperature or constant heat flux to obtain the heat transfer coefficient,resulting in inaccurate calculation results;(2)in the existing literature,the calculation of the average heat and mass transfer coefficients based on arithmetical or logarithmic mean temperature difference method and numerical integration method is not appropriate in the evaporative cooling process.The first method may result in potential inaccuracy as it simply emphasizes on the temperature difference between the inlet and outlet values than taking into account the whole heat and mass processes.The second method can lead to the appearance of infinite local heat transfer coefficient in the wet channel under certain conditions and thus the integration process will pose a significant challenge;(3)parameters affecting the heat and mass transfer characteristics in evaporative cooler are not fully and comprehensively analyzed;(4)the study on data induction and summary is not sufficient,and there are more qualitative analyses and less quantitative empirical relations.In view of the above issues in the current evaporative cooling technology,this research mainly includes three parts: study on heat and mass transfer mechanism of direct evaporative cooler,study on heat and mass transfer mechanism of traditional indirect evaporative cooler and study on heat and mass transfer mechanism of dew point indirect evaporative cooler.In this dissertation,the main work contents include:Firstly,based on the conservation law of energy,continuity and momentum,a two-dimensional CFD model is developed to describe the heat and mass transfer processes in the direct evaporative cooler,the traditional indirect evaporative cooler and the dew point indirect evaporative cooler.To ensure the accuracy of the numerical simulation,analysis of grid independence has been carried out to determine the optimal grid size to adopt.Numerical simulations are conducted for a least favorable case with different grid arrangements.The models of the direct evaporative cooler and the traditional indirect evaporative cooler are validated with pubilshed experimental results,and the dew point indirect evaporative cooler model is validated by constructing test bench.A comparative study on the performance of counter-flow dew point evaporative coolers with two different configurations(type A and type B)has been conducted.Comparatively,type B configuration has a higher cooling effectiveness and lower product temperature than type A configuration;Secondly,for the three types of evaporative coolers,a similarity analysis is presented to deduce the two-dimensional CFD model equations into dimensionless forms with the purpose of obtaining the minimum set of grouped dimensionless factors affecting the average Nusselt and Sherwood numbers.Based on the two-dimensional CFD model,the heat transfer and mass transfer processes are simulated under different conditions.Isograms of dimensionless temperature and normalized mass fraction of water vapor in the simulated regions are presented for different typical cases.It can be found that the internal temperature distribution is relatively complex due to the unique temperature boundary condition of gas-liquid interface.Distributions of local Nusselt and Sherwood numbers are also carried out.Defects of the numerical integration method empolyed in evaporative cooling are analysed.Thirdly,a one-dimensional large-volume flow model was established for three types of evaporative coolers.The one-dimensional large-volume flow model is simple and practical.Its governing equations are a set of one-dimensional differential equations.This dissertation innovatively proposed a new approach that the one-dimensional model is utilized to analyze the average heat and mass transfer coefficienst(average Nusselt and Sherwood numbers)based on the two-dimensional CFD model outlet results,and thus the one-dimensional model can retrieve the two-dimensional model results accurately.Finally,based on the proposed new method,the effects of dimensionless parameters on the average Nusselt and Sherwood numbers for the three types of evaporative coolers are studied,and the variation patterns of the average Nusselt and Sherwood numbers are analyzed.On this basis,the data calculated are fitted by the least square method,and the correlations of the average Nusselt and Sherwood numbers these evaporative coolers are developed.The contributions of this study are as follows:(1)the real boundary condition of "natural formation" on the wet surface of the cooler is considered,that is,the whole heat and mass transfer processes in evaporative cooling are considered,thus providing a theoretical basis for accuratly calculating the average heat and mass transfer coefficient;(2)based on the similarity analysis theory,three types of the evaporative coolers are comprehensively studied;(3)the two-dimensional CFD model is combined with the one-dimensional large-volume flow model to calculate the average Nusselt and Sherwood numbers,overcoming the accuracy defect by employing the traditional methods,thus providing a practical tool to calculate the average heat and mass transfer coefficient;(4)the correlations of the average Nusselt and Sherwood number are comprehensivly summarized.The series of correlations have a wide range of applicable conditions,small errors and simple composition,which provide accurate data support for the application in practical engineering.