AirFlows and Temperature Patterns induced in a Confined Greenhouse

Two-dimensional airflows and temperature distributions, corresponding to a typical single-cell Rayleigh-Benard convection pattern, were characterized in a reduced scale, mono-span, greenhouse module. The module has floor heating; it is kept airtight and the walls are insulated. It is designed to simulate free convection, generated above a plant canopy heated by solar radiation, in a real multi-span greenhouse. The temperatures were measured with thin thermocouples placed on a static grid; airflow lines were determined by a visualization technique, laser tomography, and by velocity measurements by laser Doppler anemometry. Stationary, weakly turbulent, regimes were observed for Rayleigh numbers ranging from 0·8×1010to 2·3×1010. The only stable flow pattern, which breaks the symmetry of the greenhouse, is a large convective cell which sweeps the heated floor, rises along one insulated side, cools down along the roof and descends along the opposite side. Temperature and velocity gradients are confined to borders and a large static and homogeneous air core stays at the centre without participating in heat transfer. Due to confinement, actual air velocities inside the convective cell are much larger than those that would be obtained vertically from a simple heated plate. Apparent heat exchange coefficients from the floor to the air and from the air to the roof were deduced from this experiment and compared with other values found in the literature for full-scale greenhouses.