Study On Microclimate And Ventilation Of Plastic-film Covered Sunlight Greenhouse Based On CFD

Plastic-film covered sunlight greenhouse technology is an indispensable part of modern agriculture, as well as an important protective cultivation method. Integrated with drip irrigation under soil covering plastic mulsh, modern polyhouse cultivation provides indoor microclimate favorable for crop growing with the minimum irrigation water requirement, greatly improving production. According to related research, the simplest polyhouse without any complemental conditioning devices provides better production than open field. Since the beginning of the21st century, the emphasis of polyhouse and greenhouse technology is no longer a simple pursuit of quanitity, but on improving crop quality, especially with high added value, under the premise of better production.The most notable aspect of microclimate in a plastic-film covered sunlight greenhouse is its heterogeneity. Large temperature gradient reportedly resulted up to5degrees celsius difference of two different areas in a same greenhouse. This heterogeneity not only leads to non-uniform in crop quality and production, but also be the reason for pest and disease. As a matter of fact, hot areas give rise to pest infestation, while at the cold areas relative humidity is relatively higher which causes fungi. Extensively in-depth studies on plastic-film covered sunlight greenhouse microclimate and the coupled interactions among environmental factors, as well as regulation method of this microclimate, are essential prerequiseites of solving the long-term plastic-film covered sunlight greenhouse microclimate heterogeneity problem. For this reason, a comprehensive mechanism microclimate model developed basing itself on basic physics laws is presented in this dissertation. Through Computational Fluid Dynamics (CFD) simulations, plastic-film covered sunlight greenhouse microclimate, its ventilation process and crop transpiration under this circumstances are throughly studied, summarising as follows:In chapter1, the mechanism of generating plastic-film covered sunlight greenhouse microclimate is introduced, as is the domestic and abroad research progress. Mathematic methodology used in our research is thoroughly discussed. Content in this chapter provides theoretical foundation for following studies.In chapter2, basing on fractal geometry theory and methods, two statistical fractal dimensions are introduced for the purpose of giving quantitative description of the microstructure within crop canopy. Using graphics processing and pattern recognition for2-D digital photo images of the crop, specific values of these parameters are acquired. Satisfactory analysis result shows consistency with hypothesis, porous structure of canopy thus revealed to be of fractal characteristics. This work improves methodology for detailed mathematic modeling of crop canopy structure and distributing density.In chapter3, an indoor microclimate model for plastic-film covered sunlight greenhouse is developed basing on basic fluid dynamics law of Navier-Stokes equation to give better description of climate heterogeneity and interactions among environmental factors. Two sub-models are included. One of which is A radiation sub-model added to describe the coupling of convective transfers and radiative exchanges at the cover and the roof, instead of using the usual coupling approach based on energy balance. The other is a fractal permeability sub-model innovatively adopted in the modelling of the crop canopy. Later study suggests that this deliberately developed nonlinear mechanism system model, along with its relatively steadier and quicker3-D CFD simulation, can be served as a useful tool in macroclimate research and polyhouse design investigating.In chapter4, a microclimate study is conducted using CFD simulation of our theoretical model for a typical plastic-film covered sunlight greenhouse (with a sector shape vertical cross-section) popularly used in central China, Compared the numerical results with measured experimental data, the model is proved to be successful; which then (with its simulations) is used to explore the microclimate variable distributions in the plastic-film covered sunlight greenhouse. It shows that different airflow pattern, temperature and humidity profiles from those in a sawtooth Mediterranean-type greenhouse can be found.In chapter5, the wind pressure coefficient and discharge coefficient of a same type plastic-film covered sunlight greenhouse are determined through3-D CFD simulation. Combining the popular ventilation formula and fractal permeability model which we have used in our microclimate study, our method provides a concise way in calculating ventilation rate. The result shows that this method gives ventilation characteristics, coefficients rather quick and easy under steady-state hypothesis. It is also found this Chinese plastic-film covered sunlight greenhouse provides good ventilation and constant indoor airflow direction. The calculating process is time-economical and simple thus can be in favor of robotizing using personal computer.In chapter6, crop transpiration is determind by ventilation rate calculated using the formula regarding wind pressure coefficient and discharge coefficient aquired both by CFD simulation. Checked by comparing with measured experimental data, the calculated transpiration rate is proved to be effective, thus validates the method itself. The best corresponding result comes in noon. This work not only gives good reference result for studying transpiration rate but also shows the mentioned method effective, reasonable and its results realizable and valuable for future robotized regulation system of a plastic-film covered sunlight greenhouse.In chapter7, issue by issue discussion is devoted for further studies on plastic-film covered sunlight greenhouse climate and its interdisciplinary expansion.Our study indicates the mechanism model based on fluid dynamics, radiative transfer equation and fractal theory valid. CFD simulation of this model gives qualitatively and quantatively results about microclimate and ventilation of plastic-film covered sunlight greenhouse. The results acquired provide guidance in daily plastic-film covered sunlight greenhouse management. The model itself serves as foundation for further studies on soil-plant-environment-coutinuum system and water saving effect of plastic-film covered sunlight greenhouse cultivation.