EFFECT OF CEILING HEIGHT ON THE NATURAL VENTILATION OF AN 'INFINITE' SCREENHOUSE: MODEL PREDICTIONS
Ventilation is essential to maintaining proper temperature and humidity levels in greenhouses. In small greenhouses both roof and wall openings contribute to the exchange of air, and increasing the height of such structures contributes to both forced and free ventilation processes. In particular, a higher wall reduces the resistance to flow through the structure, thus improving cross ventilation. As the area of individual greenhouses expands over the years, the contribution of ventilation through the walls diminishes and most of the exchange of heat and vapor is taking place through the roof. Here we report on a study of the expected effect of roof height of a large screenhouse on the ventilation rate. On the one hand we use a one-dimensional (vertical) computational model, based on the mixing-length concept, which at this preliminary stage covers only forced turbulent transfer. On the other hand we present preliminary measurements in two screenhouses of different heights. Both computation and measurement indicate that under windy noon-time conditions, increasing roof height by 2 m is expected to increase the indoors temperature by about 1K. The explanation for the increase in temperature is that when the exchange process is mostly by vertical turbulent transport, adding an air layer between the canopy and the roof increases the resistance to transfer and thus increases the temperature and humidity levels in the canopy layer.
Seginer, I. and Teitel, M. (2014). EFFECT OF CEILING HEIGHT ON THE NATURAL VENTILATION OF AN 'INFINITE' SCREENHOUSE: MODEL PREDICTIONS. Acta Hortic. 1037, 337-344
screenhouse, roof height, ventilation, one-dimensional profiles, mixing- length, energy balance