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

摘要

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 Δ T io diminishes when the inside velocity increases following the statistically significant relationship Δ 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,φ = [(2 eμ / K p ρ )·(1/ u s ) + (2 eY/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: Δ P = au s 2 + bu s .