Abstract
The aim of this work is to study and quantify the air mass flow exchanged between inside and outside of the greenhouse, in order to determine the ventilation openings layout and the design effect on greenhouse airflow and microclimate distribution. The study was conducted over a 945 m2 multi-chapels arched greenhouse with a polyethylene cover and has thirteen crop rows oriented from north to south; the greenhouse was equipped with side wall and roof vents openings. A simulation was performed using different arrangements and configurations of ventilation openings with the same wind direction. Numerical simulation has been adopted in three dimensions (CFD), using the Fluent computer code which relies on the resolution of the Navier-Stokes equations. These equations were solved in the presence of the turbulence model (k - e) and the Boussinesq model equation adopted to incorporate buoyancy forces. The effects of solar and atmospheric radiation were included by solving the radiative transfer equation (RTE), using Discrete Ordinate (DO) model. The effects of the roof openings, the presence of anti-insect screens and crops orientation were investigated and quantified. In a 3-span greenhouse with an anti-aphid in-sect screen in the vent openings, combining roof and sidewall vents gave a ventilation rate per unit opening area that was 1.4 times more than with only side vents. In the latter case, the difference of temperature between the inside and the outside of the greenhouse was greater than 3°C. Numerical simulations with an anti-insect screen having a porosity of 56% showed that the air exchange rate with combined ventilation was reduced by 48%. Finally, the paper focused on the effect of vent arrangement on the efficiency of the ventilation and the distribution of the microclimate inside the greenhouse. Results showed that computed ventilation rates varied from 53.43 to 70.95 kg/s, whereas temperature differences varied from 7.15°C to 10.14°C. This study also showed that other characteristics such as climate heterogeneity must be investigated in order to define the best ventilation configuration.
Highlights
Natural ventilation is considered to be one of the most important factors in the greenhouse environment [1] because it directly affects the greenhouse air temperature, humidity and CO2 concentration
Numerical simulation has been adopted in three dimensions (CFD), using the Fluent computer code which relies on the resolution of the Navier-Stokes equations
Numerical simulations with an anti-insect screen having a porosity of 56% showed that the air exchange rate with combined ventilation was reduced by 48%
Summary
Natural ventilation is considered to be one of the most important factors in the greenhouse environment [1] because it directly affects the greenhouse air temperature, humidity and CO2 concentration. The Mediterranean region which is characterized by a large radiating flow, such efficient air conditioning is crucial to reduce the temperature of the greenhouse air and eliminate excess humidity. Natural ventilation is used by the vast majority of growers in the Mediterranean region because it is the best economical way to regulate microclimate of greenhouses. The openings are arranged, continuously or alternatively, along the side walls and/or the roof (Venlo-type greenhouses). In some cases, the greenhouse may have only holes in the roof (parral type) or discontinuous openings obtained by separating the plastic cover on either side of a tunnel greenhouse
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