The use of screens to reduce insect entry into greenhouses has become a common practice in many countries. The screens act as a mechanical barrier that prevents migratory insects from reaching the plants, and thus reduce the incidence of direct crop damage and of insect-transmitted virus diseases. As a consequence, the need for pesticide application is significantly reduced and consumers can buy healthier products. However, the exclusion of very small insects requires installation of fine mesh screens which impede ventilation and thus increase temperature and humidity within the greenhouse which may negatively affect crop growth and yield. To reduce the effect of screens on temperature and humidity there is a need to keep the screen resistance to airflow as low as possible. There are basically three levels in which one can operate: (i) micro: changes at the screen pore/thread level; (ii) meso: changes in the effective size of the enclosure openings (e.g. concertina shaped screens) and (iii) macro: changes in whole structure geometry. The first two are aimed at reducing the screen resistance to airflow while the third is aimed at changing the pressure distribution on the structure and hence the airflow within and around it. To explore the effect of modifications in the micro and meso scales on airflow through screens, computational fluid dynamics (CFD) simulations were done. It is shown that changes in both the micro and meso scales can significantly improve ventilation. The results indicate that manufacturing screens with threads that have an aerodynamic shape may under some conditions improve ventilation by 30-50%. Furthermore, folding a fine mesh screen into a concertina shape can reduce the resistance of the screen to airflow. A screen, with a typical 60 degrees fold, allowed a higher airflow (by about 37%) in comparison to a flat screen under similar pressure drops across the screens.
展开▼
