Wind driven ventilation of a mono-span greenhouse with a rose crop and continuous screened side vents and its effect on flow patterns and microclimate (original) (raw)
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Wind-driven natural ventilation of greenhouses with vegetation
A large eddy simulation (LES) model was used to examine the wind-driven cross ventilation of gable-roof greenhouses containing vegetation. The obstruction of air flow by vegetation was described by a porous drag model in the numerical model, and the simulation results were validated using wind tunnel experiments. The numerical model was then utilised to inspect the influences of vegetation and greenhouse length (in the wind direction) on the ventilation rate. The results revealed that the diminishing effects of the vegetation, insect screen and internal friction on the ventilation rate can all be quantified by a physical-based resistance model. The driving force (the difference between windward and leeward pressures) of long, multi-span greenhouses was found to be less than that of a short, single-span greenhouse leading to a lower ventilation rate. The resistance factor of the vegetation and the insect screen depends on their porosity, while the resistance factor of the internal friction increased as the greenhouse length increased. In addition, the internal friction of multi-span greenhouses should be considered when the length of the greenhouse was greater than six times the greenhouse height.
Biosystems Engineering, 2008
The effect of wind direction, relative to a multi-span naturally ventilated greenhouse, on the airflow patterns and air temperature distribution inside the house and at the openings is investigated in the present study, in a greenhouse with vertical roof openings. Experiments in a full-scale greenhouse, CFD (Computational Fluid Dynamics) full-scale simulations and wind tunnel tests on a small-scale model were carried out. The results showed a significant effect of the wind direction on the flow patterns both inside the house and at the roof openings. Furthermore, wind direction significantly affected the ventilation rate and the air and crop temperature distributions. A reasonable qualitative agreement was achieved between the experiments, numerical simulations and wind tunnel tests with respect to flow patterns through the openings. Quantitatively, the numerically predicted ventilation rates are in reasonable agreement with estimates of ventilation rates obtained by a model given in the literature. However, numerically predicted air velocities at the greenhouse openings differ from the measured values and possible reasons for the differences are highlighted and discussed.
Scientia Agricola, 2013
Natural ventilation is the most important method of climate control in Mediterranean greenhouses. In this study, the microclimate and air flow inside a Mediterranean greenhouse were evaluated by means of sonic anemometry. Experiments were carried out in conditions of moderate wind (≈ 4.0 m s -1 ), and at low wind speed (≈ 1.8 m s -1 ) the natural ventilation of the greenhouse was supplemented by two horizontal air flow fans. The greenhouse is equipped with a single roof vent opening to the windward side and two side vents, the windward one being blocked by another greenhouse close to it, while the leeward one is free of obstacles. When no fans are used, air enters through the roof vent and exits through both side vents, thus flowing contrary to the thermal effect which causes hot air to rise and impairing the natural ventilation of the greenhouse. Using fans inside the greenhouse helps the air to circulate and mix, giving rise to a more homogeneous inside temperature and increasing the average value of normalized air velocity by 365 %. These fans also increase the average values of kinetic turbulence energy inside the greenhouse by 550 % compared to conditions of natural ventilation. As the fans are placed 4 m away from the side vents, their effect on the entrance of outside air is insufficient and they do not help to reduce the inside temperature on hot days with little wind. It is therefore recommended to place the fans closer to the side vents to allow an additional increase of the air exchange rate of greenhouses.
1997
The objective of this work was to experimentally investigate the influence of vent type (side, roof, or both) and of anti-aphid insect screens on airflow, air temperature, and air vapor pressure deficit distribution in a round arch, mono-span greenhouse with vertical side walls. The greenhouse was equipped with two side roll-up vents and a flap roof vent. A tomato crop planted in double rows was cultivated inside the greenhouse. The three components of air velocity were measured by a 3-D sonic anemometer, and the air temperature and relative humidity were simultaneously recorded at several positions inside the greenhouse. Concerning the effect of insect screens, it was found that the mean value of the normalized air velocity was 58% lower in the greenhouse with insect screens on the side vent openings than in the case of a greenhouse without screens. Furthermore, the spatial heterogeneity of the microclimate variables was reduced with screens in the vent openings. When the ventilation was provided by side openings only, the air velocity inside the greenhouse was characterized by a strong air current near the greenhouse ground and low air velocity near the roof; when the ventilation was provided by roof vents, a circulating current prevailing at the center of the greenhouse was observed. The combined use of roof and side openings increased air velocity and decreased air temperature inside the greenhouse but also increased the spatial heterogeneity of the greenhouse microclimate compared to the cases with side or roof vents only. The most homogeneous climate conditions were achieved with the use of roof openings only. The results of this study provide a better understanding of the plant environment behavior under different vent configurations and a high-resolution database for validating on-going efforts with computer simulations.
Agronomy, 2019
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 t...