Computational Fluid Dynamics (CFD) Study of Large Scale Screenhouses

In areas with hot climates the need for increasing greenhouse ventilation to improve climate control has led to the substitution of plastic covers by a porous screen characterized by the number of threads per unit length (crosswise and lengthwise). These structures are commonly called screen or shadehouses. The placement of a very thin screen reduces the entrance of pests and diseases and also creates a drop in pressure of the air passing through the screen, so reducing air exchange inside and outside the sreenhouse. While greenhouse ventilation has been an active research subject, ventilation studies on screenhouses are still emerging despite the increasing importance of the screenhouses. In this study, the CFD technique was used to simulate the performance of a horizontal flat-roof screen covered greenhouse 245 m long, 110 m wide and 4 m high. Two types of mesh (16.1 x 10.2, 22.5 x 11 threads cm(-2)) were simulated, in order to provide information relating the effect of screen porosity to internal climate uniformity. Five external wind speeds ranging from 1 to 5 m s(-1) were considered, with the wind direction along the greenhouse length. Simulations showed strong reduction in air speed caused by the sidewall screen perpendicular to the incoming air. Part of the entering flow exited through the roof area closest to the windward side wall, with a reverse flow and circulation cell observed near the windward side. When there was no crop or when it was in its earliest stages, the ratio of internal to external wind speed increased slowly along the greenhouse length, mainly due to exchange of air through the roof. Simulations with a fully developed crop considered as a porous media showed some differences in the air pattern, with the air much lower than without crop. Comparisons of screens of different porosities considered in this study did not show any relevant difference on the air flow in the central zones of the shade house. Simulations of the temperature field showed a difference of less than 3 degrees K between the hottest and coolest areas.