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,φ) of 0.207 for screen 1 (10 × 20 threads·cm(−2); porosity φ = 35.0%), 0.151 for screen 2 (13 × 30 threads·cm(−2); φ = 26.3%) and 0.325 for screen 3 (10 × 20 threads·cm(−2); porosity φ = 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 ΔT(io) diminishes when the inside velocity increases following the statistically significant relationship ΔT(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,φ) = [(2eμ/K(p)ρ)·(1/u(s)) + (2eY/K(p)(0.5))](−0.5) that relates the discharge coefficient of the screen C(d,φ) with the air velocity u(s). 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 ρ and viscosity μ). However, for a determined temperature of air, the pressure drop-velocity relationship can be characterised only with two parameters: ΔP = au(s)(2) + bu(s).