A SIMULATION OF NATURAL VENTILATION FOR A MULTI-SPAN GREENHOUSE

With this method, the air change in each compartment of a multi-span greenhouse as well as the average air change over all compartments can be simulated. The simulation has been made using a computer program for greenhouses with continuous side and roof ventilators under various weather conditions.

The simulated results show that when the wind speed is lower than 2m·s^-1, the number of air changes is mainly dependent upon the inside-outside temperature difference, and is much less dependent upon the wind speed and the number of spans. On the contrary, when the wind speed is higher than 2m·s^-1, the number of air changes is approximately proportional to the wind speed and is dependent upon the number of spans, and is almost independent of the inside-outside temperature difference.

The number of air changes in the windward span of a multi-span greenhouse is entirely different from that of the leeward span when the wind speed is higher than 2m·s^-1, because in the windward span a large amount of fresh air flows in through the windward sidewall. In the leeward span, the fresh air flows in only through the windward part of the roof ventilators.

The effects of design factors such as the dimension and position of the side or roof ventilators, the angle of ventilator opening, the number of spans etc. on the number of air changes can be quantitatively estimated with this simulation technique. The method, therefore, seems to be very useful as a tool for greenhouse ventilation design, although the present model is only a rough approximation.

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