日光温室热压风压耦合自然通风流量的模拟

摘要

Natural ventilation is a key measure for greenhouse environment adjustment, and ventilation flux calculation involves the discharge coefficient and wind effect coefficient. So, it is necessary to analyze the air volume flux over different vent types and the corresponding coefficients. The effects mechanism of the coupling between the stack and wind effects on ventilation flux was analyzed, and a model of air exchange in a solar greenhouse was established considering two main driving forces of ventilation. The tracer gas technique using carbon dioxide was used in a scaled greenhouse (scale rateis 1:5) to identify the ventilation flux. From measurements of volumetric flow rates, climate parameters and opening width, the model parameters, the discharge coefficient and the wind effect coefficient were identified by fitting the experimental data to the model using multi-linear regression. The results showed that the ventilation flux could be calculated by the following equations, 0.5G=0.81S⋅(H⋅ΔT/T)+0.078S⋅u,0.5G=0.63S⋅(H⋅ΔT/T )+0.067S⋅uandG=0.46S⋅(H⋅ΔT/T)0.5+0.058S⋅u, where S and H was opening area and opening width;ΔT was the temperature difference between inside and outside; T and u was the outside air temperature and velocity. The identified values of the discharge coefficient were 0.78, 0.6, and 0.44, and the wind effect coefficients were 0.78, 0.6, 0.44 when the vent opening widths were 3cm, 5cm and 7cm, respectively (equal to the opening widths in an actual greenhouse 15cm, 25cm and 35cm). It was shown that the ventilation flux due to the wind effect over total ventilation flux was over 50% when the wind velocity exceeds 1.5m·s-1 and that could be over 70% when the wind velocity exceeds 2.5m·s-1, which indicated that the temperature or buoyancy effect could be neglected.%通风是温室环境调节的重要手段，通风流量计算涉及流量系数与风压体型系数，因此有必要定量分析不同通风模式下的通风流量及对应的系数，为通风调节提供理论依据。本文分析了热压风压耦合作用对通风流量的影响机理，构建了通风流量与热压风压作用关系的数理模型；采用CO2气体示踪法测试日光温室模型(按1：5的比例缩小)在不同通风口宽度条件下的通风流量，将试验测得的通风流量、空气温度、风速和通风口宽度等参数代入模型，对模拟值与实测值进行多元线性拟合，得出拟合度最高的流量系数与风压体型系数。结果表明：当温室模型通风口宽度为3、5和7cm（相当于实际温室通风口宽度为15、25、35cm）时，热压风压耦合作用的通风流量可按0.5G=0.81S⋅(H⋅ΔT/T )+0.078S⋅u、0.5G=0.63S⋅(H⋅ΔT/T )+0.067S⋅u和0.5G=0.46S⋅(H⋅ΔT/T)+0.058S⋅u分别计算，式中S、H、ΔT、T、u分别为通风口面积、宽度、室内外温差、室外温度和风速；相应的流量系数分别为0.78、0.60和0.44，风压体型系数分别为0.04、0.05和0.07；在总通风流量中，当室外风速高于1.5m·s-1时，风压通风流量所占总通风流量的比例均高于50%，风压通风占主导作用；当室外风速大于2.5m·s-1时，风压形成的通风流量所占比例均大于70%，说明此条件下可忽略温度即热压的影响。