Analysis of Thermally Driven Ventilation in Tunnel Greenhouses using Small Scale Models

A laboratory method for studying natural ventilation by thermal effects was applied to a singlespan tunnel greenhouse with different opening arrangements. 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 salty solution through the floor of the inverted scale model. Tests were video recorded and images digitized so that the temperature distribution in a cross-section could be obtained. Dimensional analysis and the scaling laws established relationships between variables in the model and at full scale. First, the method was verified by comparing laboratory results on temperature rise and air exchange rate with measurements from a 3 m wide tunnel greenhouse. The effect of insect-proof screens on the air exchange rate was also investigated in the laboratory and compared to fullscale results. After verification, tests were conducted on a scale model of 6m wide tunnel greenhouses. Four vent arrangements were considered: 16 and 33% of sidewall openings, 8% sidewall opening plus 10% roof opening and 16% sidewall opening plus 10% roof openings. For each configuration, expressions of the temperature 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 three types of net over the openings is given. The results show the importance of combining roof and sidewall ventilation, especially when netting of reduced permeability covers the openings.