NATURAL VENTILATION BY THERMAL EFFECT IN A ONE-HALF SCALE-MODEL MONO-SPAN GREENHOUSE

Airflow and temperature patterns induced by natural convection were ch aracterized in a 1/2 scale model mono-span greenhouse model simulating the absorption of solar radiation at the floor surface. Temperatures were studied by means of thermocouples and airflows by means of hot wi re anemometry. The main features of the temperature and flow patterns were observed in permanent regime for Grashof numbers based on the Del ta T between inside and outside and on the width of the opening, rangi ng from 10(6) to 10(7). With a single sided vent situated on the roof inflows at the lower part of the opening were always observed, and a s ingle airflow loop which followed the walls, the floor and the roof of the greenhouse before escaping through the upper part of the vent ope ning. With a two sided vent opening, a single airflow loop always prev ailed with a large recirculation rate, some outside air coming in thro ugh the lower parts of the openings, compensated by air exiting throug h the upper parts of the same openings. In all situations, thermal gra dients extended over thinner layers (5-6 cm) than velocity gradients ( about 50 cm). In these conditions, the vertical temperature profile wa s characterized by a strong temperature drop just above the heated soi l surface which represents about 70% of the total temperature differen ce between the soil surface and the outside. Values for the ventilatio n rates were deduced by expressing the thermal balance of the greenhou se and compared with those deduced from Bernoulli's theorem. More gene rally the measured flow patterns were compared with those reported in the literature for ventilation and heat transfers in closed greenhouse s. The similitude of the inside patterns observed in each case suggest s that both the exchanges between inside and outside the greenhouse an d between the warm soil and the roof can be considered as driving forc es influencing the flow.