基于CFD模型的大跨度温室自然通风热环境模拟

大跨度温室作为一种新型南北走向的钢骨架覆膜温室,解决了传统日光温室土地利用率低、空间狭小的问题。为了研究在自然通风条件下大跨度温室的温度和气流场的分布规律,以及不同室外风速条件下通风口开度对大跨度温室温度和气流场的影响,利用计算流体力学(computational fluid dynamics,CFD)软件构建三维稳态大跨度温室模型,模拟自然通风条件下大跨度温室内的温度场和气流场,并采集典型晴天下通风口开启50%时大跨度温室内13个测点的温度,将各测点的测量值与模拟值进行比较,最后利用已验证模型模拟分析通风口开度(25%、50%、75%、100%)在不同室外风速(1、2、3、4 m·s~(-1))条件下的大跨度温室温度和气流场。验证结果表明:模型模拟值与实测值的绝对误差在0.2~2.8℃,均方根误差为1.6℃,最大相对误差为9.9%,平均相对误差为4.1%,表明模拟值与实测值吻合良好。模拟结果显示,温室顶部温度高,底部温度低;室外冷空气从西侧通风口进入,温室内西侧温度低于东侧;温室内平均风速从南到北逐渐减小;温室中部风速明显小于东西两侧。大跨度温室上通风口及侧通风口全开时,温室内温度分布较均匀。温室通风口开度一定时,温室内通风率与室外风速呈显著线性正相关。考虑温室内温度及风速对作物的影响,以降温为主要目的时,建议通风口开度取75%~100%,若室外风速大于3m·s-1且室内温度能满足作物生长,则建议通风口开度75%。

Ventilation Simulation in a Large-scale Greenhouse Based on CFD

To solve the problem that the inner available space of the traditional Chinese solar greenhouse is usually small, a new-type large-scale greenhouse which was tunnel type and had a wide span with steel frame and south-north orientation was designed. The distribution of airflow and temperature patterns, the effect of vent openings under different outdoor wind speed conditions on airflow and temperature patterns in a naturally ventilated large-scale greenhouse were studied. Firstly, simulation model of the airflow and temperature patterns in a naturally ventilated large-scale greenhouse was established by means of three-dimensional computational fluid dynamics (CFD). Secondly, the model was validated via the comparison with the field experimental results at the same locations where 13 temperature sensors were installed under the typical sunny day when the vent opening degree was 50%. The comparison between simulations and measurements showed that the absolute error was within 2.8℃, the square error was within 1.6℃, the maximum relative error was less than 9.9% and the average relative error was around 4.1%. An agreement existed between simulated and experimental results. Finally, the model which was validated was used to study the effect of vent opening degree (25%, 50%, 75% and 100%) under different outdoor wind speed (1, 2, 3, 4m·s-1) conditions on airflow and temperature patterns. The results showed that, the average temperature of the top of the greenhouse was higher than the bottom of the greenhouse, and the colder air outside went into the greenhouse from the west side vent, so the average temperature of the west of the greenhouse was lower than the east of the greenhouse. From south to north, the average airflow rates decreased in the greenhouse. Because of the west and east vents, the average air velocity in the center of greenhouse was lower than the side. When both top and side vents full opened, the airflow in greenhouse was relatively low. Temperature distribution was uniform in the large-scale greenhouse when the vent opening degree was 100%. The outdoor wind speed had a significant positive correlation with the ventilation rate when vent opening degree was kept constant. For the purpose of cooling, the optimum vent opening degree was 75%-100%. If the temperature of the greenhouse was suitable for crop growth and the outdoor wind speed was faster than about 3m·s-1, the optimum vent opening degree should be less than 75%.