Characterising the effects of geometry on the natural convection heat transfer in closed even-span gable-roof greenhouses

Abstract

Over the years, researchers have attempted to reduce the heat loss that arises from the natural convection in confined even-span gable-roof greenhouses due to the temperature difference between the floor and the walls (particularly at night and during winter). However, the effect that the geometry of these greenhouses has on this natural convection does not appear to have been systematically investigated.To address this, this study examined the influence of aspect ratio and roof pitch of greenhouses on the natural convection inside them. In this vein, a numerical model was developed using computational fluid dynamics (CFD) and validated against experimental data. It was found that decreasing the aspect ratio and roof pitch typically reduced the natural convection heat transfer, with changes of up to 25% for aspect ratios and 15% for roof pitch angles being observed. This suggests that a low-profile greenhouse with a low roof pitch offered the best opportunity for reducing heat loss. That said, under some conditions a multicellular flow regime can occur, and this leads to an increase in the heat transferred from the greenhouse. Hence, the choice of aspect ratio and roof pitch angle require careful consideration.Finally, a parametric relationship was developed to assist greenhouse designers determine their heat loss for a range of greenhouse geometries. Designers of greenhouses are encouraged to take advantage of the proposed relationship to develop model-based control strategies that minimize supplementary heating and deliver energy efficient greenhouses into the future.