A CFD MODEL TO STUDY ABOVE ROOF SHADE AND ON ROOF SHADE OF GREENHOUSES
Shading, ventilation and crop transpiration play a major role on the cooling of a greenhouse. Shading, ventilation and crop transpiration are closely linked, and a change in any of them produces a change in the other two. Previous research works have studied the relationship between these three variables under a number of external weather conditions. While most of the previous studies are based on analytical models of the greenhouse climate, this paper presents a numerical model (CFD model) to study the effect of shading on the greenhouse climate and transpiration rate. Two shading methods were considered and compared: above roof shade consisting of a porous screen above the greenhouse structure and on roof shade consisting of a porous screen directly placed on the greenhouse cover. Especial attention was paid to the implementation of the transpiration model into the CFD model. Crop transpiration was modelled by using a simplification of the Penman-Monteith equation. For this purpose, the radiation absorbed by each cell of the computational domain was calculated by using absorption models and extinction coefficients available in literature. Stomata resistance corrections for extreme high temperatures and low VPD were incorporated to the transpiration model. 3D simulations of a three span arched roof greenhouse were done to study the distribution of humidity, temperature and air speed of above roof shade and on roof shade greenhouses versus unshaded greenhouses. Comparisons were made for leaf area indexes ranging from 0 to 4 and for different ventilation rates. Comparing both shade methods results showed that the above roof shade increased slightly the ventilation rate, but it produced minor changes in the greenhouse temperature and humidity distributions. In agreement with previous studies, shading generated a stronger reduction in temperature for the less ventilated greenhouses as well as for low transpiration rates.
Piscia, D., Montero, J.I., Melé , M., Flores, J., Perez-Parra , J. and Baeza, E.J. (2012). A CFD MODEL TO STUDY ABOVE ROOF SHADE AND ON ROOF SHADE OF GREENHOUSES. Acta Hortic. 952, 133-139
climate, multi-tunnel, transpiration, temperature, CFD