Effects of vent configuration and wind regime on the microclimate in the tunnel greenhouse

Abstract

In eastern China, weak ventilation prevails during hot days in summer. Greenhouse microclimate is greatly influenced by the external extreme climate with high-intensity solar radiation, high temperature and low wind speed. To date, few studies investigate how vent configuration and wind regime influence greenhouse climate under very low wind speed (<0.5 m-s−1) in summer. In this paper, the aim of this work is to investigate effects of vent configuration and wind regime on the microclimate pattern in the tunnel greenhouse under the weather conditions of high temperature and low wind speed in summer. For this purpose, a three-dimensional (3-D) steady-state computational fluid dynamics model (CFD model) was constructed for a tunnel greenhouse cultivated with water spinach (Ipomoea aquatica, ‘Qing Geng’) in eastern China. It is based on the resolution of the Navier-Stokes equations with a RNG k-e closure and the Boussinesq assumption. Solar and atmospheric radiations were included by solving the RTE and distinguishing short [0.4–2.4 μm] and long wavelength [2.4–180 μm] contributions. The model was first validated by comparing CFD simulations with the experimental temperatures. The experimental and simulated air temperatures show a good agreement with a difference of less than 5%, which falls within the acceptable range. It is then employed to investigate the effect of vent configuration on the microclimate pattern in the greenhouse. Simulations show that, vent configuration affects the airflow pattern strongly and consequently results in very different temperature patterns in the greenhouse. Meanwhile, the assessment of the natural ventilation performance highlights that the roof plus side opening is suitable for summer ventilation, since it provides the highest ventilation rate, the minimum temperature difference between indoor and outdoor, and a relatively homogeneous indoor climate. Finally, the systematic investigations of the microclimate characteristic in the greenhouse with buoyancy ventilation show that, wind regime has a significant impact on the airflow and temperature patterns in the greenhouse, which considerably affects the greenhouse cooling performance and the indoor climate homogeneity. As the outside wind speed is continuously increased, the wind-driven convection plays a more and more important influence on the indoor climate. Greenhouse microclimate is dominantly influenced by buoyancy-driven convection. The combination of roof and side opening can improve the greenhouse cooling performance remarkably and reduce the air temperature in the crop canopy effectively with zero wind speed. The roof ventilator has an important contribution to promote an efficient air exchange between indoor and outdoor when only buoyancy ventilation occurs.