| The radiant heating terminal has been used more and more for its advantages of low energy consumption and high thermal comfort.However,the device has not been widely used because of the risk of condensation in which has limited its cooling capacity,it can meet the requirements of heating,cooling,and dehumidification of buildings throughout the year,which is very suitable for near-zero energy consumption buildings.However,when using the numerical simulation method to study the performance improvement of the equipment,it is found that there is still a lack of a simulation model for the dehumidification capacity of the equipment under moist conditions in summer.In the subsequent literature research,it is found that the existing condensation models are mostly the forced convection condensation process of pure steam or mixed gas with a small amount of non-condensable gas,there are few studies on the natural convection condensation process of moist air under normal temperature and pressure,and in the directly related studies,not only the form of the condensation wall is different from that in this paper,but also the quantitative characterization of the condensation rate is rare.And there is no directly available numerical simulation model.Whether the model is suitable for this paper remains to be verified.Therefore,the purpose of this paper is to analyze the natural convection condensation process of moist air on the vertical cold tube surface under normal temperature and pressure,deduce the mathematical model of condensation rate and realize the simulation calculation of condensation rate,improve the numerical simulation calculation model of this device,and provide a reference for such terminal or similar physical models.First,this paper analyzes and establishes the mathematical model of condensation rate of natural convection condensation heat transfer of wet air on a low-temperature vertical tube surface from three perspectives: mass transfer differential equation,convective mass transfer theory and developed semi-empirical formula,and obtains the semi-empirical formula of condensation rate which is convenient for engineering application by fitting the experimental test data,and the average relative error of condensation rate compared with the experimental test results of this paper is 8.1 %.Secondly,this paper builds an experimental bench to test the condensation rate of moist air in natural convection air conditioning terminal device,which is used to verify the semiempirical criterion model of condensation rate obtained in the previous research and also to analyze the factors affecting the condensation rate of moist air on the vertical tube surface.The experimental results show that under the experimental conditions of this paper,the wall subcooling,the temperature,and relative humidity of the moist air are the causes of the natural convection condensation rate,with the wall subcooling being the main influencing factor.In the moist air temperature remains unchanged,the wall subcooling degree is large,the increase in the relative humidity of moist air will make the trend of condensation rate accelerated more obvious;in the wall subcooling degree and air relative humidity remains unchanged,the increase in the temperature of moist air will also make the condensation rate of water-vapor accelerated.Then,to improve the numerical simulation calculation model under moist conditions for this kind of natural heat transfer air conditioning terminal device,this paper compares and analyzes the two evaporation-condensation models provided by Fluent,and find that there are evaporation and condensation coefficients in the model that need to be repeatedly adjusted according to experimental data,and there is no relevant research literature to give reference values.Therefore,the UDF framework for the condensation rate is designed under simplified conditions through the UDF function provided by the software,and the semi-empirical equations for the condensation rate derived in this paper are compiled and mounted to establish a numerical simulation model for the natural convection condensation of moist air on a vertical tube surface.Finally,applying the simulation model established in this paper,the moist air parameters of 24 ℃ & 80 % and the tube surface temperature of 14.3 ℃ and 15.8 ℃ in the experimental test conditions are selected to simulate the condensation rate of moist air in the single-tube domain of the tube natural heat exchange air conditioning terminal and compared with the experimental test results,the relative error of the condensation rate of the simulation calculation results is about 5 %,compared with the experimental test results.On this basis,numerical simulations are performed for the full-size terminal under moist conditions,and the results show that the relative errors of condensation rate,indoor air temperature,and relative humidity in the main area are 5.5 %,1.7 ℃,and-1.0 %,respectively,which further verifies the accuracy of this simulation model.The indoor airflow field created by the model is also calculated and briefly analyzed to provide a reference for the subsequent research on the performance improvement of these terminals and the analysis of the indoor air thermal environment created by them. |
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