BUOYANCY DRIVEN VENTILATION IN TROPICAL GREENHOUSES

Natural ventilation by thermal effects can be studied using laboratory scale models. For this purpose, a 1:10 scale model of a tunnel greenhouse with sidewall and roof openings, a type of greenhouse used in some tropical countries like Malaysia, was immersed in a water tank. A solution of salty water and black dye was injected to produce density differences that stimulated the buoyancy force due to the heating of greenhouse air. The flow was filmed using a video camera and images were processed. Similitude analysis allowed the calculation of the temperature increments associated with relative pixel intensity increments. Also, the ventilation rate for each experiment was calculated from scaling laws.

Four ventilator configurations were investigated: 1) Sidewall openings having a surface equal to 16% of the floor area; 2) 33% sidewall openings referred to the floor area; 3) 8% side vents and 10% roof vents; and 4) 16% side vents plus 10% roof vents. The temperature increase for each greenhouse is given as a function of the sensible heat transferred to the greenhouse. Results showed that sidewall ventilation with only a 16% opening surface does not provide enough ventilation. The importance of the roof vent on the air exchange process was clearly demonstrated, especially when insect-proof screens cover the openings.

Download Adobe Acrobat Reader (free software to read PDF files)