Natural ventilation of a greenhouse with ridge and side openings: Sensitivity to temperature and wind effects
An air exchange rate model of a Mediterranean greenhouse with ridge and side openings combining temperature and wind effects as driving forces of the natural ventilation was developed and tested on experimental data. it is shown that both effects are dependent on the total area of openings, but the temperature effect depends also on the ratio of ridge to side openings. The wind effect is related to a global wind effect coefficient, relative to the whale building, which includes both static and turbulent transfers. As far as if concerns the combination of the temperature and wind induced fluxes, it is shown that the vectorial sum of these fluxes, gives a better fit of the experimental data than their addition. The relative importance of the thermal buoyancy and wind forces depends on the ratio between wind velocity and the root of the inside-outside temperature difference (u/root<(Delta T)over bar>). In our experimental conditions the wind effect predominates on the chimney effect when this ratio u/root<(Delta T)over bar> becomes greater than 1. The use of the reduced ventilation flux (per unit of ventilator opening area and wind speed) shows that for typical conditions of temperature difference (= 5 K) the roof and side ventilation system is muck more efficient than roof ventilation only for wind velocities smaller than 2.5 ms(-1). Finally, a simple design procedure for dimensioning the area of the vents under extreme climatic conditions is proposed and a relative nomograph is given.