Effect of ventilator configuration on wind-driven ventilation in a crop protection structure for the tropics

A laboratory method for studying wind-driven natural ventilation was applie d to a single-span greenhouse with side-wall and roof ventilation openings, a design common to crop protection structures used in tropical countries. A 1:15 scale model replica of the greenhouse, made from clear Perspex, was immersed in a flume tank filled with water. A steady mean flow generated by the flume simulated wind flow past the model. The flow inside the model gr eenhouse was examined for four different ventilator configurations and vent ilation flow rates compared in each case. Neutrally-buoyant particles seede d in the flow allowed for clear flow visualization and these particles were tracked using a digital image analysis system from which detailed velocity measurements were obtained. Flow patterns observed in the scale model agre e qualitatively well with full-scale observations. Predictions of the flow at full scale inferred from measurements of pressure and flow speed made at the side-wall openings of the model differed by less than 10% and 28%, res pectively, from measurements made in the full-size greenhouse. For all vent ilator configurations using side-wall openings an incoming jet-like flow cr ossed the greenhouse and established a circulating flow. The greenhouse wit h roof ventilation only had the lowest ventilation rate and the maximum rat e was achieved with combined side-wall and roof openings. The results of th ese studies are detailed and design recommendations to improve the ventilat ion of tropical greenhouses are suggested. (C) 2001 Silsoe Research Institu te.