Simulation and experimental study of the air distribution characteristics of large-span film air duct in greenhouses

Air handling units (AHUs) with long distance flexible film air ducts are commonly adopted in modern large-scale greenhouses for temperature controlling. To study the performance of this type of AHU, theoretical analysis and numerical simulation methods were used to discover its air supply characteristics, and a full-scale experiment was carried out to verify the correctness of the calculation model. The calculation showed that the static pressure of different cross-sections in the thin film air duct sharply decreased to 0 at the small-hole opening, where the airflow velocity increased significantly. However, this influence only exists locally and had little effect on the pressure and gas velocity fields inside the wind duct. The velocity and velocity distribution of the gas flowing out from different openings on the same section of the wind duct are basically the same. Taking the wind speed of 2.45 m/s at the inlet end of the wind duct as an example, the maximum velocity of the airflow at the opening position of the wind duct wall at 60.06 m is 5.64 m/s, located at the center of the opening and flowing outward perpendicular to the opening plane. The velocity distribution is that the center has the highest velocity, and the closer to the edge of the hole, the lower the velocity. The volum of the airflow flowing out through the openings on the wall of the wind duct along the length direction shows that the air supply remains basically constant under different inlet wind speed conditions, and the uniformity coefficient is greater than 0.92. This indicates that this type of AHU can evenly distribute airflow along the length direction, and can evenly distribute heat or cold in the large space of the greenhouse, thus providing a guarantee for maintaining a uniform temperature field in modern large greenhouses.