ISHS Acta Horticulturae 534: International Conference and British-Israeli Workshop on Greenhouse Techniques towards the 3rd Millennium
WIND DRIVEN LEEWARD VENTILATION IN A LARGE GREENHOUSE |
| Author: | B.J. Bailey |
| Keywords: | Ventilation, wind, air exchange rate, leeward |
| Abstract:
Ventilation rates were measured in a 3.8 ha Venlo greenhouse containing a tomato crop on a commercial nursery. Each ventilator had dimensions 2.25 m by 1.20 m and the total ventilator area was 14% of the greenhouse floor area. Carbon dioxide, obtained from the flue gases of a natural gas fired boiler, supplied at a constant rate and uniformly distributed through the greenhouse for enrichment was used as the tracer gas. The supply rate and internal concentrations at 6 positions inside the house and one position outside were measured. Data were recorded at 5-minute intervals over several weeks during July and August 1998. Ventilation rates were calculated from a greenhouse CO2 mass balance. The CO2 assimilated by the crop was estimated using a photosynthesis model, and reached a maximum value of 17% of the CO2 supply rate. The correction for fluctuations in the greenhouse CO2 concentration did not exceed 15% of the estimated ventilation rate. Records with wind speeds less than 1 m s-1 and internal to external temperature differences greater than 5°C were excluded from the analysis. The air exchange rate through leeward ventilators was found to be linearly dependent on wind speed and could be represented by a simple model involving wind speed, the effective ventilation area, the discharge coefficient and the square root of a global wind pressure coefficient. For leeward ventilation the value of 0.11 ± 0.02 obtained for the global wind pressure coefficient was in good agreement with values reported for much smaller greenhouses. The non-dimension ventilation coefficient G(a) exhibited a slightly non-linear relationship with angle of ventilator opening that was attributed to the edge effects of the ventilators which had a small aspect ratio of 1.875. Comparison of published values for G(a) showed that considerable variations in linearity and magnitude exist which cannot be attributed to the size of greenhouse or of ventilator. | |
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