CFD Numerical Simulation And Optimization Research Of Environment In A Modern Greenhouse

The greenhouse environment is the most direct impact on crops growth because of the greenhouse looks like a semi-closed system. The CFD (Computational Fluid Dynamics) technique is widely used to analysis the airflow and the distributions of the inner air temperature and the relative humidity under the different external conditions and ventilation layout, in addition, it can also be associated with CAD to optimization design. So, it has been a powerful tool for greenhouse physical structure design and the parameters optimization in environment regulations and controls. In this artical, the inner micro-environment was studied based on CFD numerical simulation software and fields tests in a Dutch Venlo-type modern glasshouse, and the different heating methods of greenhouse in winter. The main contents and conclusions were generalized as follows:(1)In order to study the effect of the fog cooling system in the rainy season in greenhouse, a cooling test was designed to quantitative analysis the actual cooling effect in Nanjing for which was also in the hot and high humidity weather conditions. After spraying for2min, the average cooling rate of the temperature was3.2℃and the increase of the relative humidity was13.1%. The test proved that the fog cooling system also had a certain cooling ability in southern area in summer, however, the cooling effect of the fog system was only20%because of the higher relative humidity in greenhouse.(2)The greenhouse environment heat transfer mechanism is studied theoretically, and based on the theories of heat transfer and energy balance, the exchange of matter and energy was detailed analysis for the each influencing factor inside and outside the greenhouse. The mathematic models were established according by the main influencing factors of the energy balance among the greenhouse covering layer and the indoor crops and the mixed air of indoor and outdoor. By the way, the mathematic model that was established by the characteristic parameters of the greenhouse structure, so it could be able to use the actual greenhouse testing data to set the boundary conditions of the CFD model.(3)The3-D model was established by the CFD software in the same size with the test glass greenhouse. The airflow transfer was simulated by the standard k-ε turbulence model, the standard wall function method was used to dealt with the near-wall region, DO radiation model and the porous medium was used to described the cucumber crops planted in the greenhouse. The boundary conditions of the CFD model were given by the actual measurement data. The simulated results showed that:the maximum relative error between the simulated and measured air temperature of the measuring points was10.2%and the average relative error was3.6%, by the way, the maximum relative error and the average relative error of the air relative humidity were9.2%and5.0%, so this CFD model was effective and feasible.The paper provide a optimization design for the physical structure of greenhouse, used the CFD software to compared the environment before and after optimization, the results showed that the inner air were more uniformly mixed and the temperature and the humidity distribution was more uniform, more conducive for crops growth and development.(4)The DPM model was used to join in the CFD numerical simulation based on the continuous phase, the results showed that the average relative errors of the temperature and the relative humidity in greenhouse were4.9%and5.7%, the CFD model was proved to be feasible and effective, along with the porous model and the DPM model.The CFD model was used to analysis the distributions of the inner air temperature and the relative humidity. The temperature across crop was lower, and the temperature stratification was also obvious. The temperature was dropped rapidly because of the heat took away by droplets evaporation after spraying for1min, especially of the temperature bellow the nozzles. With the influence of solar radiation and air convection, the inner temperature was rising, but due to the distribution of droplets was rare around maintenance structure area, the temperature raised faster than the temperature in the middle of the greenhouse.(5)The paper used the CFD method to simulate and analysis three different heating models for greenhouse. The results showed that the circular profile heating system basic met the goal of greenhouse, indoor temperature field was to the requirements of the plants growth at the lowest temperature, but the temperature was lower in the cultivated layer, it was worse for plants in greenhouse; Light tube heater heating system was a double heater distribution, this made the temperature ranged very small in a receive timely, because of this, the province operation cost was much more little; The operation cost of the air heater heating system was higher than the hot water heating system, however, its one-time investment was smaller, easier installation, so it was mainly used in areas which winter heating time was short, especially suitable for small area pavilions greenhouse.