Improving the efficiency of insect screens in greenhouses
A CFD code was used in order to study the influence of different types of insect screens on greenhouse microclimate and natural ventilation performance. The validity of the code was checked against experimental data. The studied greenhouse was a round arch type with vertical side walls, covered with polyethylene and equipped with two side roll-up vents; it contained a tomato crop. Air velocity, temperature and humidity measurements were carried out at 62 positions equally distributed inside the greenhouse, by means of a 3-D sonic anemometer and a fast response humidity sensor, while in parallel, outside climate variables were also recorded. The commercial CFD code Fluent was used as a basis, where the required external source code for the various sub-models and boundary conditions was embodied. The corresponding transport equations were solved numerically by the finite volume method, using a two dimensional structured mesh and the crop was modeled as a porous medium. Eight parametric studies were carried out in order to investigate the effect of different screens on greenhouse microclimate and ventilation rate. The simulations were carried for a wind direction perpendicular to the ventilation openings in order to reduce the numerical model to two dimensions. Good agreement was found between the experimental and numerical results. Increasing the screen porosity reduced ventilation rate and increased greenhouse air temperature. Apart from screen porosity, the width of screen yarns seems to be an important factor in ventilation reduction.