CHARACTERISATION OF THE MECHANISMS INVOLVED IN THE NATURAL VENTILATION OF GREENHOUSES

ISHS Acta Horticulturae 406: II IFAC/ISHS Workshop :Mathematical & Control Applications in Agriculture & Horticulture

CHARACTERISATION OF THE MECHANISMS INVOLVED IN THE NATURAL VENTILATION OF GREENHOUSES

Authors:

T. Boulard, J.F. Meneses, M. Mermier, G. Papadakis

Abstract:
Although natural ventilation is one of the major mechanisms controlling the greenhouse climate, our understanding of the processes remains insufficient to allow accurate prediction of the rates of such exchanges. This paper deals with the physical mechanisms involved in natural ventilation of a greenhouse equipped with continuous lateral windows, and uses the following experimental procedures:

air exchange rate measurements, using tracer gas;
direct determination of the air and heat flows through an opening, using an eddy correlation system, comprising a sonic anemometer and a fine wire thermocouple;
measurements of mean and turbulent pressure differences at ground level between inside and outside.

The methods employed allow the prediction of greenhouse air exchange rates as well as the characterization of its components: a steady state effect resulting from the combination of both mean wind-related and a turbulent effect linked to wind speed fluctuations.

Local estimations of total, mean and turbulent flows are provided: a wind parallel to the greenhouse axis produces an inflow at the leeward half of ventilator opening and an outflow at the windward half. The mean flow of sensible heat is estimated to be between 65% and 80% of the total flux whereas the turbulent flow does not exceed 35% of the total.

Measurements of total, mean and turbulent flows are compared with air exchange rate measurements using the decay rate method and a good agreement between both approaches demonstrated.

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