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An exact understanding of the mechanisms of greenhouse air exchange can be used to better control air temperature, CO2 concentration and to lower excessive humidity. Up to about a decade ago a simplified approach of the perfectly stirred enclosure was usually used in understanding greenhouse microclimate. Over the last decade the concern for higher crop quality and the desire to preserve the environment acted as driving forces in the determination of the distributed microclimate within greenhouses. A comparison between CFD (computational fluid dynamics), experimental and model results with regard to the determination of ventilation rate of a naturally ventilated small mono-span greenhouse is presented. The greenhouse was ventilated by two longitudinal side openings one on the windward wall and the other on the leeward wall and rose plants were grown in it. In the experiments the ventilation rates were estimated from the decay rate of a tracer gas. In the CFD simulations it was estimated from the decay rate of a virtual tracer gas and from calculated airflow rates through the openings. In the model, ventilation rates were calculated from an equation that relates flow rate through the vents to the pressure drop on them, using local wind pressure coefficients for walls of low-rise buildings. The agreement among all methods appears to be good up to a wind speed of 4 m s-1. At higher wind velocities the values deduced from the decay of tracer gas (both in the experiments and CFD simulations) were much lower than those obtained from the other techniques. It is anticipated that the lower ventilation rates at high wind speeds are associated with non-perfect mixing of the tracer gas in the greenhouse volume.

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A Comparison among Three Methods for Estimating Ventilation Rate of a Greenhouse
A Comparison among Three Methods for Estimating Ventilation Rate of a Greenhouse

An exact understanding of the mechanisms of greenhouse air exchange can be used to better control air temperature, CO2 concentration and to lower excessive humidity. Up to about a decade ago a simplified approach of the perfectly stirred enclosure was usually used in understanding greenhouse microclimate. Over the last decade the concern for higher crop quality and the desire to preserve the environment acted as driving forces in the determination of the distributed microclimate within greenhouses. A comparison between CFD (computational fluid dynamics), experimental and model results with regard to the determination of ventilation rate of a naturally ventilated small mono-span greenhouse is presented. The greenhouse was ventilated by two longitudinal side openings one on the windward wall and the other on the leeward wall and rose plants were grown in it. In the experiments the ventilation rates were estimated from the decay rate of a tracer gas. In the CFD simulations it was estimated from the decay rate of a virtual tracer gas and from calculated airflow rates through the openings. In the model, ventilation rates were calculated from an equation that relates flow rate through the vents to the pressure drop on them, using local wind pressure coefficients for walls of low-rise buildings. The agreement among all methods appears to be good up to a wind speed of 4 m s-1. At higher wind velocities the values deduced from the decay of tracer gas (both in the experiments and CFD simulations) were much lower than those obtained from the other techniques. It is anticipated that the lower ventilation rates at high wind speeds are associated with non-perfect mixing of the tracer gas in the greenhouse volume.

Scientific Publication
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