【作者】 程秀花；

【导师】 毛罕平；

【作者基本信息】 江苏大学 ， 机械设计及理论， 2011， 博士

【摘要】 温室作为一个半封闭的热力系统,其内部微气候分布是影响作物生长发育最直接的因素。通风作为温室微环境调控的主要手段,一直是设施农业研究的热点问题。由于温室类型较多,通风布局各异,地域之间气候环境差异大,使商用大型温室的应用推广受到一定限制。CFD技术作为目前国内外广泛采用的数值模拟技术,可对不同外界环境条件及通风布局下温室内的气流、温度、湿度等的分布进行预测,是进行温室结构设计和环境调控参数优化的有力工具。本文以Venlo型玻璃温室为研究对象,基于CFD数值技术,开展了对不同边界条件下温室内部微环境因子时空分布及变化机理的理论研究及试验分析,提出了温室环境调控策略。具体工作及结论如下：1、构建了3维超声风速风向仪测速系统和温湿度传感器自动测量系统；试验测量了有无、作物存在两种种植模式下温室内外气象参数；明确了太阳辐射强度、温度、湿度等之间的相互耦合关系以及室内风速变化规律。2、基于传热学和能量平衡理论,分析了温室内外各物理场之间的质热交换关系,建立了以温室覆盖层和四周围护结构、土壤、作物、室内外混合空气为主要单元的显热和潜热交换数学模型,确立了以防虫网空气动力学参数和结构参数为特征参数的计算模型。使CFD数值模型边界设置更接近温室实际物理过程。3、基于流体动力学理论,结合温室内气流湍流流动过程,建立了求解室内湿空气质能传输控制方程；采用非结构化四面体网格对计算域进行离散化处理；提出采用标准k-ε湍流模型求解温室内湿空气的湍流输运过程,近壁区气流流动采用标准壁面函数法进行处理的方法,达到计算时间短,收敛快的目的；提出采用基于Boussinesq假设和组分传输方程求解由热浮力引起的温室内的自然对流过程,解决了单独采用Boussinesq假设无法求解湿空气传输的不足；提出采用基于DO辐射模型的Solar Ray Tracing方法处理太阳辐射边界设置问题,解决了传统方法CFD模型没有现场试验数据就无法对不同地域、不同时间和云遮率情况下温室内部环境进行预测的不足,能更加真实地反映温室建筑材料对太阳光谱的选择性,为探究“温室效应”形成原因提供了依据。4、采用辐射、对流、热传导耦合计算方法对没有栽种作物的空温室内微气候进行了数值模拟,结果发现：(1)边界条件中不考虑湿度影响时模拟得到的各时刻室内平均温度与试验测试结果基本吻合,最大相对误差为11.3%,平均相对误差为7.6%,模拟精度提高5.9%。边界条件中考虑相对湿度时,温室内温度模拟值与实测值的平均相对误差为11.6%；相对湿度模拟值与实测值的平均相对误差为5.2%,精度提高了8.3%。温室内相对湿度的分布受通风过程中气流流动模式的影响,具有与温度分布类似的梯度模式,即温度高的区域相对湿度低,温度低的区域相对湿度高。温室中部作物区温度较低,相对湿度较高,整体上该区域微环境分布比较均匀一致。(2)采用西侧窗与天窗联合通风的调控方式,室外风速风向对温室内微气候分布有影响。温室通风换气率随室外风速的增大呈线性升高的趋势,二者的决定系数R2=0.9846。室外风向显著影响温室内气流流动模式,且各通风窗在通风换气过程中所起的主要作用不同。室外风向垂直屋脊,温室内形成强度不等位置不一的涡流,侧窗是主要的进风口,天窗则起到“烟囱效应”的作用。风向平行屋脊,天窗通风是温室通风换气的主要驱动力,靠近迎风侧侧窗为进风口,其余大部分侧窗区域为出风口,进口处气流流速较低,出口处流速较高。(3)自然通风面积对室内温度分布有影响。天窗开度为10。时,室内温度较天窗开度为21。时升高至少1℃；天窗开度为45。时,室温至少降低2℃,且低温范围广。(4)采用双侧窗与天窗联合通风可使温室内温度较单独采用西侧窗与天窗的调控方式最大多降2℃,相对湿度则可多降6%。采用外遮阳与湿帘-风机强制通风的调控策略降温幅度高达11℃。5、采用高速摄像及激光片光源技术对温室模型内气流流动过程进行定性分析,结果发现：天窗和侧窗在通风换气过程中所起作用与CFD数值模拟结果一致,进一步验证了所建立的CFD数值模型有效。6、提出CFD数值模拟中作物边界处理新方法。基于多孔介质渗流理论,以Darcy-Forchheimer定律为理论依据,模拟了番茄作物对气流的动量汇作用。将土壤和番茄与环境之间的显热和潜热交换设为“体积热源边界”条件,对以上复杂边界条件的模拟结果表明：温室中部测点平均温湿度的模拟值与实测值的平均相对误差分别为6.8%和7.9%。温室中部测点总气流速度模拟值与实测值平均相对误差为15.0%；y向速度分量的模拟值与实测值的平均相对误差为10.9%。晴天室内作物区平均温度较阴天时高1.6℃左右,相对湿度约低3.2%;双密度栽培作物区温度较单密度高0.7℃,相对湿度高18%。采用湿帘-风机通风系统对环境进行调控,作物区温度可控制在25℃-27℃,较自然通风调控方式下室温可降低10℃左右。以上研究结果表明采用上述数值计算理论方法能够对温室环境因子时空变化规律,作物-环境系统的相互作用机制进行有效预测。更多还原

【Abstract】 The greenhouse microclimate distributions are key to crops growth because the greenhouse looks like a closed heat system. In agriculture engineering, the researches focus on the effects of the ventilation on microclimates. However, the popularization of big commercial greenhouses is confined because of the lack of datum which is from the different greenhouse and the ventilation configuration and climates difference. The CFD numerical technique is widely used to predict the airflow and temperature and relative humidity distributions inside greenhouse for the different boundary conditions and ventilation configurations. So, it has been a huge tool for greenhouse design and the parameters optimization in microclimates adjustments and controls. In this paper, the greenhouse microclimates temporospatial distributions were studied for the different boundary conditions based on the CFD technique in the Venlo-type glasshouse. And the field measurements were developed to verify the CFD models. At last, the strategy was suggested for microclimates control. The main results were generalized as follows:1、The automatic measurement systems were constructed for 3-D wind speed and direction and the temperature-humidity. The climatic factors were measured for the empety greenhouse and for the greenhouse with crops, respectively. The coupling relations were found among the solar radiation intension and the temperature and the relative humidity. But the airflow speed and direction were changeable inside greenhouse.The above datum were used for the boundary conditions for the CFD simulations.2、Based on the heat transfer and energy balance theories, the matter and energy exchangement were analysis for the all physical fields inside and outside of greenhouse. And the sensitive heat and latent heat transfer mathematic models were established according to the energy balance among the greenhouse cover and the enclosing structure and the soil and crops and mixed air. Meanwhile, the mathematic model that was characterized on the base of the air dynamic parameters and the construction parameters was established for the insect screen. The boundary conditions for the CFD simulations were closer to the actual physical process.3、Based on the fluid dynamics theory, the turbulent transfer characters were analysis and the control equations were established for the humid airflow inside greenhouse. The standardκ-εturbulent model was used to solve the airflow transferring and the standard wall-function method was advanced for the fluid transfer near the wall. Based on the Boussinesq hypothesis and species transport equation, the method was projected to solve the free convection that was caused by the buoyancy effect. The problem was resolved that humid couldn’t be simulated only based on the Boussinesq hypothesis. The Solar Ray Tracing method in DO radiation model was firstly used to solve effect of the solar radiation on the microclimates. As a result, it was easier to predict the micrclimates without measuring datum for the different regions and the time and the sunshine factors, and for the material, the solar spectrum selectivity could be simulated in this model.it was helpful to understand the greenhouse effect.4、A couple computational method for radiation, convection and heat transfer was used to simulate the microclimate distributions inside empety greenhouse. The results were shown as follows:(1) when the computation did not include the relative humidity, the average simulated temperature values were agreed with the measured values for different times inside greenhouse. The max relative error was 11.3% and the average relative error was 7.6%. The accuracy was 5.9% higher. When the relative humidity was taken into the boundary condition account, the average relative error between the simulated and measured temperature values was 11.6%. However, the average relative error was 5.2% between the simulated and measured humidity values. The accuracy was 8.3% higher. The relative humidity distribution pattern was similarity to the corresponding temperature. Namely, the humidity was lower in the warmer zone and it was higher in the cooler zone. A homogeneous temperature and humidity fields were observed at the crops level where was characterized by the higher temperature and the lower humidity.(2) When west side-vent and roof-vent were open, the inside microclimates were effected by the outside wind speed and direction. A linear relationship between ventilation rates and the outside wind speeds was founded, and the decided coefficient R2=0.9846. Outside wind direction had a significant effect on inside airflow patterns and the vents played different roles for greenhouse ventilation. When outside wind direction was normal to the ridge, the vortexes with different intensity were founded inside greenhouse. The side-vent was the inlet and the buoyancy effect that was caused by the roof-vents was unobvious. When outside wind direction was parallel to the ridge, the roof-vents ventilation was dominating over the side-vent for the air exchanged. The part near the windward side-vent was inlet and the others were outlet. The airflow speed was lower for inlet than for outlet.(3) The vent area had an effect on inside temperature distributions. When the roof-vent was opened for 10°, the inside temperature was 1℃lower than it was 21°. However, when it was 45°, the temperature was 2℃higher than it was 21°and the low temperature zone was wide.(4) When the east-western side-vents and the roof-vents associated ventilation should be used, it was 2℃lower for the inside temperature and it was 6% higher for the relative humidity than that for the west-side-vent and roof-vent associated ventilation. When the regulating strategy that outside shading-screens and pad-fans system was used, the temperature was lowered 11℃.5、The qualitative analysis was done for airflow pattern inside scale-greenhouse by means of the high-speed photography and the laser sheet light technology. The results showed that the roof-vent and the side-vent played a similary act with CFD results. The CFD numerical models were validating against the results from the high-speed photography.6、Anew crop boundary method was put forwarded in CFD numerical simulations. On above assumption that tomato crops were the isotropic porous medium, the "momentum sink" that was caused when air flew through the tomato crops was simulated based on the Darcy-Forchheimer law. The sensitive and latent heat exchange among the soil and tomato crops and microclimates was well-set for "volume heat source" boundary. The simulation results showed the average relative error between the simulated and the measured temperature values were 6.8% and it was 7.9% for the simulated and the measured humidity values. The average relative error between the simulated and the measured total airflow velocity values was 15.0%, and the average relative error was 10.9% for the y component. The average temperature in crop zone was warmer about 1.6℃and the humidity was 3.2% lower in clear day than those in cloudy day. The solar radiation had an effect on the temperature and relative humidity distribution. the temperature was warmer 0.7℃and the humidity was 18% higher for double plants than for single plants. The temperature in crop zone was 25℃-27℃for the pad-fan system and it was about 10℃lower than natural ventilation.更多还原