Predication And CFD Modeling For Greenhouse Microclimates Temporospatial Distributions

The greenhouse microclimate distributions are key to crops growth because the greenhouse looks like a closed heat system. In agriculture engineering, the researches focus on the effects of the ventilation on microclimates. However, the popularization of big commercial greenhouses is confined because of the lack of datum which is from the different greenhouse and the ventilation configuration and climates difference. The CFD numerical technique is widely used to predict the airflow and temperature and relative humidity distributions inside greenhouse for the different boundary conditions and ventilation configurations. So, it has been a huge tool for greenhouse design and the parameters optimization in microclimates adjustments and controls. In this paper, the greenhouse microclimates temporospatial distributions were studied for the different boundary conditions based on the CFD technique in the Venlo-type glasshouse. And the field measurements were developed to verify the CFD models. At last, the strategy was suggested for microclimates control. The main results were generalized as follows:1,The automatic measurement systems were constructed for 3-D wind speed and direction and the temperature-humidity. The climatic factors were measured for the empety greenhouse and for the greenhouse with crops, respectively. The coupling relations were found among the solar radiation intension and the temperature and the relative humidity. But the airflow speed and direction were changeable inside greenhouse.The above datum were used for the boundary conditions for the CFD simulations.2,Based on the heat transfer and energy balance theories, the matter and energy exchangement were analysis for the all physical fields inside and outside of greenhouse. And the sensitive heat and latent heat transfer mathematic models were established according to the energy balance among the greenhouse cover and the enclosing structure and the soil and crops and mixed air. Meanwhile, the mathematic model that was characterized on the base of the air dynamic parameters and the construction parameters was established for the insect screen. The boundary conditions for the CFD simulations were closer to the actual physical process.3,Based on the fluid dynamics theory, the turbulent transfer characters were analysis and the control equations were established for the humid airflow inside greenhouse. The standardκ-εturbulent model was used to solve the airflow transferring and the standard wall-function method was advanced for the fluid transfer near the wall. Based on the Boussinesq hypothesis and species transport equation, the method was projected to solve the free convection that was caused by the buoyancy effect. The problem was resolved that humid couldn't be simulated only based on the Boussinesq hypothesis. The Solar Ray Tracing method in DO radiation model was firstly used to solve effect of the solar radiation on the microclimates. As a result, it was easier to predict the micrclimates without measuring datum for the different regions and the time and the sunshine factors, and for the material, the solar spectrum selectivity could be simulated in this model.it was helpful to understand the greenhouse effect.4,A couple computational method for radiation, convection and heat transfer was used to simulate the microclimate distributions inside empety greenhouse. The results were shown as follows:(1) when the computation did not include the relative humidity, the average simulated temperature values were agreed with the measured values for different times inside greenhouse. The max relative error was 11.3% and the average relative error was 7.6%. The accuracy was 5.9% higher. When the relative humidity was taken into the boundary condition account, the average relative error between the simulated and measured temperature values was 11.6%. However, the average relative error was 5.2% between the simulated and measured humidity values. The accuracy was 8.3% higher. The relative humidity distribution pattern was similarity to the corresponding temperature. Namely, the humidity was lower in the warmer zone and it was higher in the cooler zone. A homogeneous temperature and humidity fields were observed at the crops level where was characterized by the higher temperature and the lower humidity.(2) When west side-vent and roof-vent were open, the inside microclimates were effected by the outside wind speed and direction. A linear relationship between ventilation rates and the outside wind speeds was founded, and the decided coefficient R2=0.9846. Outside wind direction had a significant effect on inside airflow patterns and the vents played different roles for greenhouse ventilation. When outside wind direction was normal to the ridge, the vortexes with different intensity were founded inside greenhouse. The side-vent was the inlet and the buoyancy effect that was caused by the roof-vents was unobvious. When outside wind direction was parallel to the ridge, the roof-vents ventilation was dominating over the side-vent for the air exchanged. The part near the windward side-vent was inlet and the others were outlet. The airflow speed was lower for inlet than for outlet.(3) The vent area had an effect on inside temperature distributions. When the roof-vent was opened for 10°, the inside temperature was 1℃lower than it was 21°. However, when it was 45°, the temperature was 2℃higher than it was 21°and the low temperature zone was wide.(4) When the east-western side-vents and the roof-vents associated ventilation should be used, it was 2℃lower for the inside temperature and it was 6% higher for the relative humidity than that for the west-side-vent and roof-vent associated ventilation. When the regulating strategy that outside shading-screens and pad-fans system was used, the temperature was lowered 11℃.5,The qualitative analysis was done for airflow pattern inside scale-greenhouse by means of the high-speed photography and the laser sheet light technology. The results showed that the roof-vent and the side-vent played a similary act with CFD results. The CFD numerical models were validating against the results from the high-speed photography.6,Anew crop boundary method was put forwarded in CFD numerical simulations. On above assumption that tomato crops were the isotropic porous medium, the "momentum sink" that was caused when air flew through the tomato crops was simulated based on the Darcy-Forchheimer law. The sensitive and latent heat exchange among the soil and tomato crops and microclimates was well-set for "volume heat source" boundary. The simulation results showed the average relative error between the simulated and the measured temperature values were 6.8% and it was 7.9% for the simulated and the measured humidity values. The average relative error between the simulated and the measured total airflow velocity values was 15.0%, and the average relative error was 10.9% for the y component. The average temperature in crop zone was warmer about 1.6℃and the humidity was 3.2% lower in clear day than those in cloudy day. The solar radiation had an effect on the temperature and relative humidity distribution. the temperature was warmer 0.7℃and the humidity was 18% higher for double plants than for single plants. The temperature in crop zone was 25℃-27℃for the pad-fan system and it was about 10℃lower than natural ventilation.