Airflow Resistance of Greenhouse Ventilators with and without Insect Screens

Measurements of the discharge coefficients of ventilation openings with aspect ratios between 1 and 36, and with and without flaps attached by a hinge along one side, were made using a modified fan test facility. The discharge coefficient of an opening without a flap increased with aspect ratio. For openings with a flap and aspect ratios greater than 5, the discharge coefficient also increased with aspect ratio and the flow through the opening varied with the sine of the flap angle. As the aspect ratio decreased below five, flow through the triangular ends of ventilators with flaps became increasingly significant in comparison to the flow through the front rectangular face. The discharge coefficient of a ventilator with a flap was independent of whether the flow was into or out of the opening. New correlation equations were derived for discharge coefficients of ventilation openings, and these are compared with previously published data. The test facility was also used to determine the pressure differences required to create airflows through five insect-exclusion screens comprising well-defined, regular meshes. Pressure loss coefficients for the screens were obtained from the experimental values and correlated with functions of Reynolds number (based on fibre thickness or diameter) and screen porosity. The correlation equation was shown to compare well with the majority of values derived from five publications which reported data on 27 nets, meshes and screens. The correlation enables the discharge coefficient of a screened opening to be predicted from the aspect ratio and flap angle of the opening, the fibre thickness and porosity of the screen and air speed. The results show that for a given porosity, the discharge coefficient of a screened ventilator decreases as the fibre thickness decreases and also as the air speed decreases.