NUMERICAL ANALYSIS OF BUOYANCY DRIVEN NATURAL VENTILATION IN MULTI-SPAN TYPE GREENHOUSES

Computational fluid dynamics (CFD) analysis was used to study the best configuration of roof and side vents under buoyancy driven natural ventilation, which represents the most unfavorable situation for greenhouse cooling. A CFD model was validated and then used to compare ventilation with roof vents and combined roof and sidewall vents. The effects of the distance between opposing sidewall vents and the presence of simple roof vents were investigated and quantified. Combining roof and sidewall vents gave a ventilation rate per unit ground area that was 2 times higher than with roof vents alone in a 20 span greenhouse with a distance of 152 m between the sidewalls. In a 3 span greenhouse with 22.8 m between the sidewalls but with the same roof vent area per unit ground area, 7 times more ventilation was obtained with combined ventilation compared with only roof ventilation. These results prove that, with buoyancy driven ventilation, the contribution of the sidewall vents is important even for quite large greenhouses, but more critical for greenhouses with lower number of spans. It was concluded that to maximize ventilation when wind speeds are low, combined roof and sidewall ventilation should be used and that large greenhouses should be relatively narrow with a maximum distance between opposite sidewall vents of 50–60 m.