Analysis of thermally driven ventilation in tunnel greenhouses using smallscale models

A laboratory method for studying natural ventilation by thermal effects was applied to a single-span tunnel greenhouse with different opening arrangem ents. One-tenth scale models of the tunnel greenhouse were made of Perspex and immersed in a water tank. The buoyancy flux due to solar heating of the floor in real greenhouses was simulated in the laboratory by adding a sall y solution through the floor of the inverted scale model. Tests were video recorded and images digitized so that the temperature distribution in a cro ss-section could be obtained. Dimensional analysis and the scaling laws est ablished relationships between variables in the model and at full scale. First, the method was verified by comparing laboratory results on temperatu re rise and air exchange rate with measurements from a 3 m wide tunnel gree nhouse. The effect of insect-proof screens on the air exchange rate was als o investigated in the laboratory and compared to full-scale results. After verification, tests were conducted on a scale model of 6 m wide tunnel gree nhouses. Four vent arrangements were considered: 16 and 33% of sidewall ope nings, 8% sidewall opening plus 10% roof opening and 16% sidewall opening p lus 10% roof openings. For each configuration, expressions of the temperatu re rise as a function of the sensible heat given to the greenhouse air are presented. Also, the air exchange rate of all configurations studied with t hree types of net over the openings is given. The results show the importan ce of combining roof and sidewall ventilation, especially when netting of r educed permeability covers the openings. (C) 2001 Silsoe Research Institute .