Abstract
The present work analyses the natural ventilation of a multi-span greenhouse with one roof vent and two side vents by means of sonic anemometry. Opening the roof vent to windward, one side vent to leeward, and the other side vents to windward (this last vent obstructed by another greenhouse), causes opposing thermal GT (m3 s−1) and wind effects Gw (m3 s−1), as outside air entering the greenhouse through the roof vent circulates downward, contrary to natural convection due to the thermal effect. In our case, the ventilation rate RM (h−1) in a naturally ventilated greenhouse fits a second order polynomial with wind velocity uo (RM = 0.37 uo2 + 0.03 uo + 0.75; R2 = 0.99). The opposing wind and thermal effects mean that ventilation models based on Bernoulli’s equation must be modified in order to add or subtract their effects accordingly—Model 1, in which the flow is driven by the sum of two independent pressure fields GM1=GT2±Gw2, or Model 2, in which the flow is driven by the sum of two independent fluxes GM2=GT±Gw. A linear relationship has been obtained, which allows us to estimate the discharge coefficient of the side vents (CdVS) and roof vent (CdWR) as a function of uo [CdVS = 0.028 uo + 0.028 (R2 = 0.92); CdWR = 0.036 uo + 0.040 (R2 = 0.96)]. The wind effect coefficient Cw was determined by applying models M1 and M2 proved not to remain constant for the different experiments, but varied according to the ratio uo/∆Tio0.5 or δ [CwM1 = exp(-2.693 + 1.160/δ) (R2 = 0.94); CwM2 = exp(−2.128 + 1.264/δ) (R2 = 0.98)].
Highlights
Natural ventilation is perhaps the main means of climate control in greenhouses [1,2], in regions such as the Mediterranean, where new technologies have not been widely incorporated [3].During most of the year, good management of natural ventilation may prove sufficient to maintain suitable levels of temperature, humidity, and CO2 concentration inside the greenhouse [1]
The roof vent was windward, the windward side vent was protected from the wind by another greenhouse located in close proximity, and the leeward side vent was free of obstacles
Given the particular situation of the greenhouse and the location of the vents, in conditions of natural ventilation with prevailing southwest wind (SW) or “Poniente winds”, the eolic and thermal effects oppose each other. The former causes air to enter through the windward roof vent and to leave through the leeward and windward side vents
Summary
Natural ventilation is perhaps the main means of climate control in greenhouses [1,2], in regions such as the Mediterranean, where new technologies have not been widely incorporated [3].During most of the year, good management of natural ventilation may prove sufficient to maintain suitable levels of temperature, humidity, and CO2 concentration inside the greenhouse [1]. Natural ventilation is perhaps the main means of climate control in greenhouses [1,2], in regions such as the Mediterranean, where new technologies have not been widely incorporated [3]. It is important to have both quantitative and qualitative knowledge of how natural ventilation functions in greenhouses in order to correctly design and use the vents [4]. The earliest studies on the circulation of air in greenhouses date back to the mid-20th century [5,6]. Numerous methods have been used: scale models, tracer gas method, CFD (Computational Fluid Dynamics) simulations, and direct measurements in the greenhouse using many types of sensors
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