Experimental And Numerical Study On Precise Spray Ventilation Cooling And Humidity Control In Greenhouse

The summer temperature in East China is high,and when the greenhouse only adopts natural ventilation,the indoor temperature of the greenhouse greatly exceeds the maximum temperature that crops can grow.Crop production needs to control the greenhouse environment,and the production system needs to run smoothly within the temperature range of normal plant growth and development.Therefore,during the summer,the greenhouse needs to take cooling measures to create the best temperature and humidity environment suitable for crop growth to meet the needs of crops twice a year.In this paper,a continuous thin-film greenhouse equipped with a precision spray ventilation cooling and humidity control system in Shanghai was used as the research object,and the full-day cooling test of summer greenhouse was carried out in Zhuanghang Comprehensive Experimental Base of Shanghai Academy of Agricultural Sciences from July to August 2022,and an adjacent greenhouse with the same structure without cooling equipment was used as the control group to analyze the cooling effect of the cooling system under different meteorological conditions,and the automatic control strategy of the spray system and ventilation equipment was corrected and optimized.Then,according to the field test data,a precise spray ventilation cooling model of continuous film greenhouse was established,and the influence of nozzle layout height,fan layout height and ventilation mode on the indoor temperature field and humidity field of greenhouse was studied.Subsequently,based on the basic knowledge of computational fluid dynamics and convective heat transfer,the standard Realizable k-ε model,DO radiation model,component transport model and discrete phase model were selected as the greenhouse model simulation methods,and the boundary conditions of the model were set according to the outdoor environmental parameters,indoor air temperature and humidity parameters,indoor soil temperature and heat transfer coefficient of the envelope obtained by the field test.In order to verify the validity of the model calculation results,the measured temperature and humidity data of each measurement point in the greenhouse were compared with their simulation data,and the results showed that the simulated temperature value was slightly higher than the measured temperature value,and the average relative error was2.9%.The simulated humidity value is slightly lower than the measured humidity value,with an average relative error of 6.8%.The results of the CFD model are consistent with the actual situation,and can be used as an effective tool to design and optimize the structure of the spray combined mechanical air supply system.Finally,the effects of nozzle layout height,fan layout height and ventilation mode on the indoor temperature field and humidity field of greenhouse are studied,and the average temperature of the greenhouse horizontal plane,the vertical gradient of the average temperature and the variance of the temperature data at each point on the same horizontal plane are used as the analysis indicators of greenhouse cooling effect,vertical temperature uniformity and horizontal temperature uniformity,and the simulation effects of different schemes are compared,and the hardware layout structure of the precision spray ventilation cooling and humidity control system is optimized.The results show that: a)When the fan power is the same,the ventilation rate of the greenhouse with positive pressure ventilation is higher than that of the greenhouse with negative pressure ventilation.When positive pressure ventilation is used,the average temperature of the greenhouse is lower and the temperature uniformity in the vertical direction of the greenhouse is better,and when negative pressure ventilation is adopted,the temperature uniformity of each horizontal plane of the greenhouse is better.b)When positive pressure ventilation,the average temperature of the greenhouse increases with the increase of the height of the nozzle arrangement.When the fan arrangement height is 0.5m,the average greenhouse temperature decreases with the increase of the nozzle layout height,and when the fan arrangement height is 1.5m,the average greenhouse temperature increases with the increase of the nozzle arrangement height.The temperature uniformity of each level of the greenhouse is highly dependent on the arrangement of the nozzles.c)Increasing the height of the fan arrangement can effectively improve the cooling effect of the bottom area of the greenhouse.When negative pressure ventilation is adopted,the height change of fan arrangement has little effect on the temperature uniformity of each horizontal plane of the greenhouse,and when positive pressure ventilation is adopted,the closer the distance from the air inlet,the worse the temperature uniformity of the horizontal plane.Temperature uniformity in the vertical direction of the greenhouse is less dependent on the fan arrangement.In general,when the nozzle arrangement height is 2.5m,the fan arrangement height is 1.5m,and the ventilation method is positive pressure ventilation,the operation effect of precise spray ventilation temperature and humidity control is the best.