Modelling of Air Exchange Rate in a Greenhouse Equipped with Continuous Roof Vents

Abstract

Several models of air exchange in a greenhouse are derived considering the two main driving forces of natural ventilation (stack effect and wind effect) and compared with experimental results presented in a previous paper. The coupling mechanisms between the "stack" and "wind" effects have been studied and the nature of the wind effect is discussed with reference to the turbulent and average hypothesis already presented in the literature.Two main model parameters were identified by fitting models to experimental values, namely, a coefficient of pressure drop through the vents, Al, whose value is in accord with the other values found in the literature for greenhouse vents or other types of opening, and a coefficient of wind effect, Cw, which integrates both steady and turbulent aspects of wind action.The models are presented and the magnitude of the main driving forces discussed in relation to climatic parameters and characteristics of the ventilation systems. A linear dependence is found between the surface of the vent opening and the wind speed on one hand and the ventilation rate on the other. Nevertheless, analysis of ventilation rate versus wind speed for different classes of wind shows that the wind dependence is not exactly linear because the slope of the linear regression of the ventilation flux versus the wind speed decreases slightly with increasing wind speed. Finally, it is shown that in our experimental conditions, characterized by roof vents all facing the same direction, the wind is the main driving force of ventilation and the air exchange rate seems mainly dependent on the wind turbulence.