Research On Experimental And Simulation Of Soil-crop-environment Hydrothermal System In Typical Double-flim Solar Greenhouse In Cold And Arid Regions

Greenhouse cultivation is the main form of agricultural production in the cold and arid areas of North China,which is a very important means of production to improve land utilization rate,reduce production cost and improve the quality of agricultural products.Solar greenhouse is a closed or semi-closed thermal system,in which the distribution of indoor temperature and humidity and other environmental factors relates closely with crop growth.At the same time,there are complex mass and heat transfer among soil,crop and environment in solar greenhouse,constructing a hydrothermal coupling system.According to previous studies,the distribution rules and characteristics of heat-moisture environmental factors will be different when indoor crops form canopy structure,and the components of hydrothermal system will change under the coupling effect.Therefore,it is of great significance to master the change mechanism of greenhouse hydrothermal system to improve the yield and quality of indoor crops and improve the efficiency of greenhouse management.Double-film solar greenhouse is a new type of solar greenhouse developed for the cold climate in northern China,which has been widely used at present.However,the simulation analysis of heat transfer rules of double-film structure,such as heat collection and energy storage,and the coupling research of hydrothermal system in double-film greenhouse are still blank.Therefore,in this study,a new double-film solar greenhouse with celery was taken as the research object,the environmental factors of crop canopy and greenhouse hydrothermal system were tested and analyzed in 2018-2019 and 2019-2020,respectively.A model of celery canopy based on porous media characteristics was established,and the feasibility of applying this model to the simulation of greenhouse hydrothermal system was also verifed.Also,a numerical model of soil-plant-environment hydrothermal system in double-film solar greenhouse was established based on the CFD technology,and the simulation analysis was carried out.In this thesis,the distribution of environmental factors of the hydrothermal system and the coupling mechanism of the components in the double-film solar greenhouse were studied.The specific work and conclusions are as follows:(1)The indoor and outdoor environmental factors in different growth stages of celery crops were tested and analyzed.It was found that the indoor environmental temperature,humidity,soil temperature and CO₂ concentration were significantly affected by the crop canopy,while the soil moisture content did not change significantly under the similar outdoor environmental conditions.With the growth of celery,the canopy increased,the porosity and permeability of canopy decreased,and the temperature and humidity difference and CO₂ concentration difference between inside and outside the canopy increased.(2)The heat and moisture transfer process among soil,crop and environment was analyzed,and the energy-balance equations were constructed under closed condition.Two-dimensional(2D)and three-dimensional(3D)CFD models of double-film solar greenhouse were established by using turbulence model,radiation model,component transport model and porous medium model.And,the calculation and selection methods of crop porous medium model parameters were proposed.(3)Combined with the canopy regional experiment,taking the middle section of greenhouse as the research object,the 2D CFD simulation of indoor hydrothermal environment was carried out.The simulation results were in good agreement with the experimental data,and the simulation method was feasible.It was found that the porous medium model could accurately represent the characteristics of crops in the simulation of greenhouse hydrothermal system.The distribution law and characteristics of temperature and humidity inside and outside 0.8m and 1.0m canopy were compared.It was found that the porous medium characteristics of crop canopy were the main factors affecting the distribution of hydrothermal environmental factors.The maximum temperature difference between the top and bottom of the canopy at 0.8m and 1.0m was 10.3?and 13.2?,respectively,and the maximum relative humidity difference was 32%and 39%.With the increase of canopy height,the difference of temperature and humidity in canopy increased.(4)A greenhouse environmental factor monitoring system based on R485communication protocol,including LG207 and G780 modules,was built.The 2D CFD model was optimized to improve the calculation speed and accuracy of the model.The relative errors of temperature and humidity values were less than 7%and 5%,and the root mean square errors were less than 0.6?and 3%.The simulated value of the model could be calculated in the monitoring system instead of the measured value.Combined with the monitoring and feedback measuring point setting function of the UDF module in Fluent,a CFD simulation analysis and prediction module for greenhouse automatic control was built.(5)According to the physical characteristics of celery,a layered porous medium model was established,and the CFD model of greenhouse soil-plant-environment hydrothermal system was optimized and numerical simulation was carried out.The simulation results were in good agreement with the experimental data,and the model was effective.The characteristics of indoor air velocity,turbulent kinetic energy,and temperature and humidity distribution in double layer film solar greenhouse without ventilation were analyzed.It was found that a hydrothermal system was formed in the canopy,which was different from the indoor environment.The hydrothermal transfer between the double-film region and the indoor environment was conducted through the inner film opening.The direct heat-moisture coupling between indoor air and soil weakened,while the hydrothermal coupling between indoor air and crop canopy increased.Different temperature and humidity distribution intervals were formed in the middle part and the two sides of the greenhouse,in which the temperature in the middle part of the greenhouse was3-4?higher than that in the two sides,and the humidity was 8%-12%.(6)Through the studies,it is found that compared with the traditional solar greenhouse,the double layer film solar greenhouse has better heat collection and storage effects,and the combined ventilation mode can provide a suitable growth environment for indoor crops,and the natural vent on the top of the outer film can effectively adjust the temperature and humidity in the greenhouse.At the same time,the optimization stimulation of the east and west walls of the double-film solar greenhouse was carried out on the established CFD model.And the optimization scheme of the greenhouse structure was proposed.In this thesis,the hydrothermal coupling mechanism of each component in the greenhouse under the dynamic balancing state is provided through experiement and stimulation research on the hydrothermal system of doubl-film solar greenhouse.And the study provided theoretical basis for structural optimization of solar greenhouse and the control of indoor environmental factors in the cold and arid areas of China.