基于大跨度拱棚湍流结构演化的棚型优化

    Optimizing large-span arch greenhouse based on turbulent structure evolution

    • 摘要: 大跨度拱棚在园艺设施中应用日益广泛,跨度、高度增加会加大棚内空间,但风温性能与时空演化更为复杂。为探明大跨度拱棚湍流结构演化规律并进行棚型优化,该研究以生产中常用的20 m跨度宜机化拱棚为主要对象,建立融合设施结构与番茄植株气动特征的多场耦合仿真分析模型,并开展示范工程应用。结果表明:大跨度拱棚存在湍流现象,断面涡流多呈双旋涡特征;棚的脊高越低,温度、气流分布均匀性越差,通风效率虽较高但换气率较低;风温分布均匀性会伴随拱棚高度增加变优,但太高时因热压通风演化为主通风方式,风压通风效能会明显下降;针对20 m大跨度拱棚,脊高6.0~6.5 m时构型较优,兼顾经济性、安全性等因素脊高取6.0 m更优。通过示范工程验证了模型的可靠性,为同类园艺设施结构构型优化提供了高效可靠手段。

       

      Abstract: Large-span arch greenhouse has gained widespread application in fruit and vegetable production in recent years, due to the convenient facility operation, high land use efficiency, suitability for multiple coverings, and external heat preservation. However, internal turbulence can often occur in the expansion of the span and height of the large-span arch greenhouse. The resulting uniformity of the greenhouse environment can then affect the efficient production of greenhouse crops. In this study, a multi-field coupled computational fluid dynamics (CFD) model was constructed to integrate the facility structure and tomato plant aerodynamics, according to agricultural industry demand. The research object was selected as the 20 m span large-span arch greenhouse in Laiwu district, Jinan City, Shandong province (36.14° N, 117.28° E), China. A systematic investigation was carried out to explore the influence of the height of the arch greenhouse on the spatial and temporal distribution and evolution of the turbulent structure inside the large-span arch greenhouse. An analysis was also made to clarify the relationship between the wind and the temperature distribution, turbulence intensity, summer ventilation, and cooling efficiency, and the height of the arch greenhouse. Seven kinds of large-span arch greenhouse were established with 4.5, 5.0, 5.5, 6.0, 6.5, 7.0 and 7.5 m heights. The results show that there was turbulent behavior in the large-span arch greenhouses, indicating the mostly double vortex in the section. The air-flow entered the greenhouse via the bottom vent on the windward side, then ascended along the lower side of the greenhouse film, and finally descended towards the roof area. There was a tendency for the stabilization in the position of the inflection point, as the ridge height increased beyond 6.0 m. Therefore, the better uniformity of air flow and temperature distribution were, the higher the uniformity of the greenhouse was. The large-span arch greenhouse with a height of 6.0 m shared the highest average flow velocity at the crop canopy, which was in the appropriate range and relatively high. The average temperatures were also relatively low, which were 0.404 ℃ and 0.026 ℃ lower than those of the arch greenhouse with a height of 5.5 and 6.5 m, respectively. The turbulence intensity of the large-span arch greenhouse with a height of 4.5-5.5 m was significantly higher than that with a height of more than 6.0 m. There was a small difference, after the ridge height exceeded 6.0 m. Once the height of the greenhouse was 4.5 m, the ventilation efficiency was 2.24. After that, the efficiency of ventilation decreased with the increase in the height of the greenhouse. There was a relatively high value when the ridge height reached 6.0 and 6.5 m. The ridge height of 7.0 m corresponded to the lowest ventilation efficiency of 1.60. The large span greenhouse with a 4.5 m ridge height exhibited the higher ventilation efficiency, but the lower air change rate. The large span greenhouse with a 7.5 m ridge height shared the lower air change rate and ventilation efficiency. The outstanding ventilation and cooling performance of the large-span arch greenhouse in summer was determined by many factors. In a 20 m long-span arch greenhouse with shoulder, a better structure was achieved with the ridge height of 6.0-6.5 m and the ridge height of 6.0 m, in order to better consider the factors of economy and safety. The finding can provide a strong reference to optimize a similar greenhouse configuration in the construction of the large-span arch greenhouse.

       

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