Air flow and associated sensible heat exchanges in a naturally ventilated greenhouse

In order to investigate the three-dimensional nature of the air flow within the greenhouse, a three-dimensional sonic anemometer was used for the direct measurement of air and heat exchange inside the greenhouse and through the vent of a twin-span greenhouse equipped with a continuous roof vent at the gutter. Measurements were carried out both in the vent opening itself and in the greenhouse at the height of the ventilator. Wind blowing parallel to the greenhouse ridge gave rise to an inflow at the leeward part of the ventilator. The influx then follows a spiral trajectory guided by the internal surface of the walls and the shape of the roof before exiting at the upwind end. A large portion of the greenhouse situated between the centre and the upwind wall is ventilated less efficiently than the remainder of the greenhouse creating a large volume of higher temperature air that extends between the centre of the greenhouse and the windward wall. It is shown that the mean and turbulent components of the sensible heat flux through the vent amount to 58% and 42% of the total exchange between the greenhouse and its environment. Comparison with previous measurements demonstrates a large dependence of the wind-driven ventilation efficiency on wind speed. This confirms that other ventilation mechanisms, such as the stack effect, become important when the wind speed is low. These measurements are in a very good accord with flow patterns simulated by computational fluid dynamics techniques. (C) 1997 Elsevier Science B.V.