| With the promotion of cleaner agricultural production,solar greenhouses have developed significantly in China.However,due to its own structure,the solar greenhouses have insufficient ability to regulate the thermal and humidity,so it needs auxiliary facilities to regulate the indoor thermal and humidity environment.Traditional regulation methods consume a lot of energy,increase carbon emissions and generate environmental pollution.Solar greenhouses use renewable energy to regulate the thermal and humid environment,which saves energy and protects the environment,reducing environmental pollution.Soil-air heat exchanger is a kind of thermal and humidity regulation method which uses shallow geothermal energy.It can heat the air in the heat exchange tube in winter and cool it in summer.Applying it to the control of thermal and humid environment in solar greenhouse has good use effect and economic benefit.Therefore,in this paper,the soil-air heat exchanger in the solar greenhouse is taken as the research object,and carries on the experiment and the simulation research to the air temperature and humidity change in the heat exchanger tube.An experimental platform was built in a solar greenhouse in Xiaodian District of Taiyuan city to test the operation of soil-air heat exchanger in summer and winter.The experimental results showed that:(1)In a solar greenhouse,the latent heat exchange quantity of the soil-air heat exchanger cannot be ignored.The total heat exchange quantity should be used to comprehensively consider the heat transfer performance of the soil-air heat exchanger.(2)During the summer operation period,when the length of the heat exchange tube was the same,the smaller the air velocity in the tube,the lower the air outlet temperature,and the smaller the fluctuation.When the length of the tube was 17.2m,the air velocity in the tube was 2 m·s-1,4 m·s-1,6 m·s-1,and the average temperature drop during operation was 8.78 ℃,7.84℃ and 6.26℃,respectively.With the increase of tube length,the temperature drop of unit tube length decreases gradually.It was uneconomical to increase the tube length to obtain lower outlet air temperature.The longer the tube length,the greater the air velocity in the tube,and the greater the heat exchange capacity of the tube.The greater the air velocity in the tube,the less effective the tube length is on the heat transfer gain of the soil-air heat exchanger.The larger the air velocity,the greater the heat exchange capacity,but the air velocity gradually increased,and the increase in total heat exchange capacity became less obvious.It was suggested that the soil-air heat exchanger should be connected in parallel with short tubes.When the air velocity was the same and tube length used was the same,the air velocity of each heat exchange tube was smaller and the heat exchange would be more sufficient.(3)During the operation of soil-air heat exchanger in winter,the variation of air temperature and humidity in the tube was similar to that in summer.The longer the tube was,the greater the air temperature rose in the tube,and the greater the heat exchange quantity of the heat exchange tube.Due to the accumulation of water in the heat exchange tube,the air in the heat exchange tube absorbed the water in the heat exchange process,and the general trend of the the air humidity ratio in the tube increased along the tube length.As the temperature increases,the relative humidity of the air in the tube decreases along the tube length.The trend of heat exchange quantity in the heat exchange tube was that the heat exchange quantity increases gradually with the tube length.A three-dimensional numerical simulation model containing 0.5m thick soil and considering the occurrence of air condensation in the tube was established by Fluent software.The simulation results were compared with experimental data to verify the accuracy of the model.The response surface method was used to design the experimental scheme,analyzed the simulation results,and established a mathematical model which can predict the temperature,humidity and heat transfer of the air at the outlet of the heat exchange tube.The response surface method was used to design the experimental scheme and analyze the simulation results.A regression equation was established to predict the temperature and humidity of the air at the outlet of the heat exchanger tube and the heat transfer capacity.Based on the established regression model,the effects of the six parameters of inlet air temperatureTin,inlet air relative humidityφin,tube diameter D,tube length L,air velocity v,and soil temperature Ts on the heat transfer performance of the soil-air heat exchanger were further analyzed.The results show:(1)Soil-air heat exchanger was suitable for high temperature and humidity environment such as solar greenhouse.(2)The relative humidity of the inlet air was positively correlated with difference in air humidity ratio at the inlet and outlet of the heat exchange tube dio and negatively correlated with difference in air temperature at the inlet and outlet of the heat exchange tube Tio,but has little effect on Tio.Inlet air temperature Tin and tube length L were positively correlated with Tio and dio,and dio increased exponentially with the inlet air temperature Tin.Tio and dio showed logarithmic growth with tube length.Air velocity and soil temperature had negative correlation with Tio and dio,and the effect of air velocity on Tio was gradually reduced,and the rate of Tio reduction was gradually reduced.(3)Except that the soil temperature Ts was negatively related to the cooling capacity,and inlet air relative humidity φin had a small negative effect on the sensible cooling capacity,other parameters were positively related to the cooling capacity.The cooling capacity increased exponentially with the increase of the inlet air temperature Tin,and increased logarithmically with the increase of the tube length L and air velocity v,and the growth rate gradually decreased.The tube diameter D and soil temperature Ts was almost linearly related to the cooling capacity. |
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