Analysis of Natural Ventilation Rate of Multi-span Greenhouses built on Reclaimed lands using CFD

Abstract

Greenhouse area has increased continuously making possible for intensive cultivation, high quality agricultural product in South Korea. However, there is no enough space for installing large-scale greenhouse complex in South Korea, because the land area that account for 70% of South Koreas territory consist of mountain area. Reclaimed lands which are newly developed have great worth as agricultural lands because reclaimed lands can be used deliberately regardless of near terrain. Recently, Korea government made a public announced about the development plan of large-scale greenhouse complex in representative reclaimed lands. However, wind environments of reclaimed land are entirely different from those of inland. Natural ventilation studies for greenhouses built on reclaimed land should be conducted with proper wind profiles of the reclaimed land. Meanwhile, many standard documents for ventilation design did not describe quantitative design standard of natural ventilation which is commonly used. Therefore, natural ventilation rates were computed and analyzed to suggest standard for ventilation design of multi-span greenhouse built on reclaimed lands. In this study, natural ventilation rate of multi-span greenhouse was computed and analyzed according to greenhouse type (Venlo, Wide-span, 1-2W), number of spans (two, five, eight), wind speed (1.0, 2.5, 5.5 m•s-1), wind direction (90, 45, 0°), and vent opening (side vent, side-roof vent, roof vent). Computational Fluid Dynamics (CFD) simulation was used for the purpose of overcoming the limitation of field experiments, such as laborious and cost consuming, requirement of equipment for multipoint measurement, vulnerability to unstable weather condition, and so on. An analysis of the wind environment was performed to design the wind profiles of reclaimed lands in South Korea. CFD simulation models for multi-span greenhouses were designed based on a design method for a CFD simulation model, which was validated by Ha (2015). The designed wind profiles of the reclaimed lands were applied to the CFD simulation models. Two methods, mass flow rate (MFR) and tracer gas decay (TGD), were used to calculate the natural ventilation rates of multi-span greenhouses. The natural ventilation rates computed by these two methods were evaluated by comparing them with the ventilation requirements. As the result of comparing the natural ventilation rates computed by the two methods, it was judged that the tracer gas decay method evaluated the actual natural ventilation more closely than the mass flow rate method. In analyzing the overall ventilation rates, the results also showed that the natural ventilation rates were influenced considerably by wind speed and wind direction. The natural ventilation rates increased linearly as wind speed increased. As the number of spans increased, the natural ventilation rates generally decreased. It was observed that the effect of ventilation through the roof ventilators of the 1-2W type greenhouse was higher than in the Venlo and wide-span greenhouses. As a result of analyzing the homogeneity of the local ventilation rates, it was found that the homogeneity was mainly influenced by wind direction and the configuration of the ventilators except for wind direction. The results of analyzing the overall ventilation rates will be used as basic data to establish design standards for multi-span greenhouses built on reclaimed lands. Additionally, the results of analyzing the local ventilation rates are expected to be utilized for controlling the microclimate in large-scale greenhouses uniformly. The charts for expecting the natural ventilation rates will be used for designing the ventilation of greenhouses and the guidance of maintenance control. The natural ventilation rates were computed and evaluated according to various conditions as basic research to create design standards for natural ventilation. A study evaluating the effects of crops and buoyancy on natural ventilation is still required. When crops exist in a greenhouse, the air flow patterns and natural ventilation rates are completely changed according to the arrangement of the crops, variety of the crops, crop height, etcetera. Also, when wind speed is low, buoyancy-driven ventilation is more important. With additional research, it will be possible to suggest quantitative standards for designing the ventilation of greenhouses that will be practical for the managers and designers of greenhouses.