Wind Tunnel Analysis of the Airflow through Insect-Proof Screens and Comparison of Their Effect When Installed in a Mediterranean Greenhouse

The present work studies the effect of three insect-proof screens with different geometrical and aerodynamic characteristics on the air velocity and temperature inside a Mediterranean multi-span greenhouse with three roof vents and without crops, divided into two independent sectors. First, the insect-proof screens were characterised geometrically by analysing digital images and testing in a low velocity wind tunnel. The wind tunnel tests gave screen discharge coefficient values of C-d,C-phi of 0.207 for screen 1 (10 x 20 threads.cm(-2); porosity phi = 35.0%), 0.151 for screen 2 (13 x 30 threads.cm(-2); phi = 26.3%) and 0.325 for screen 3 (10 x 20 threads.cm(-2); porosity phi = 36.0%), at an air velocity of 0.25 m.s(-1). Secondly, when screens were installed in the greenhouse, we observed a statistical proportionality between the discharge coefficient at the openings and the air velocity u(i) measured in the centre of the greenhouse, u(i) = 0.856 C-d + 0.062 (R-2 = 0.68 and p-value = 0.012). The inside-outside temperature difference Delta T-io diminishes when the inside velocity increases following the statistically significant relationship Delta T-io = (-135.85 + 57.88/u(i))(0.5) (R-2 = 0.85 and p-value = 0.0011). Different thread diameters and tension affects the screen thickness, and means that similar porosities may well be associated with very different aerodynamic characteristics. Screens must be characterised by a theoretical function C-d,C-phi = [(2e mu/K-p rho).(1/u(s)) + (2eY/K(p)0.5)](-0.5) that relates the discharge coefficient of the screen C-d,C-phi with the air velocity us. This relationship depends on the three parameters that define the aerodynamic behaviour of porous medium: permeability K-p, inertial factor Y and screen thickness e (and on air temperature that determine its density rho and viscosity mu). However, for a determined temperature of air, the pressure drop-velocity relationship can be characterised only with two parameters: Delta P = au(s)(2) + bu(s).