M. Mermier

The mechanisms involved in the natural ventilation of greenhouses

Abstract Although natural ventilation is one of the major mechanisms that controls the greenhouse climate, our understanding of the underlying 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 or heat and water balance techniques; • • 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 effect resulting from the combination of both mean wind-related and stack effects 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 and an outflow at the windward half. The mean flow of sensible heat is estimated between 55% and 80% of the total flux so that the turbulent flow does not exceed 45% of the total. Local estimations 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 is demonstrated.

Tomato leaf boundary layer climate: implications for microbiological whitefly control in greenhouses

The efficiency of mycoinsecticides as a means of whitefly control was examined under greenhouse conditions, focusing in particular on the high humidity levels favouring infection of the whitefly larvae infesting the under leaf surface. The theory of the moisture transfers occurring within the leaf boundary layer under laminar conditions is outlined and a model for the air temperature and humidity distribution is developed. Measurements were made of the humidity of the air in the boundary layer on the underside of leaves of a tomato crop grown in a full scale greenhouse. A considerable increase in relative humidity was detected 5 mm from the underside of the leaves, particularly during day-time when crop transpiration reached its maximum. These measurements were compared with the results of the model based on boundary layer theory and it was established that the data recorded were accurately predicted by the model during day-time from both the qualitative and quantitative points of view, but they were underestimated at night. The implications of these findings were examined and discussed in the context of the microbiological control of whiteflies under the conditions of confinement pertaining in various greenhouses and similar cultural systems. It is suggested that the concept of the microclimate pertaining in the leaf boundary layer developed here could be applied to the biological control of insects and microbial pathogens, in both protected and open crop growth systems.

Air flow and associated sensible heat exchanges in a naturally ventilated greenhouse

Abstract In order to investigate the three-dimensional nature of the air flow within the greenhouse, a three-dimensional sonic anemometer was used for the direct measurement of air and heat exchange inside the greenhouse and through the vent of a twin-span greenhouse equipped with a continuous roof vent at the gutter. Measurements were carried out both in the vent opening itself and in the greenhouse at the height of the ventilator. Wind blowing parallel to the greenhouse ridge gave rise to an inflow at the leeward part of the ventilator. The influx then follows a spiral trajectory guided by the internal surface of the walls and the shape of the roof before exiting at the upwind end. A large portion of the greenhouse situated between the centre and the upwind wall is ventilated less efficiently than the remainder of the greenhouse creating a large volume of higher temperature air that extends between the centre of the greenhouse and the windward wall. It is shown that the mean and turbulent components of the sensible heat flux through the vent amount to 58% and 42% of the total exchange between the greenhouse and its environment. Comparison with previous measurements demonstrates a large dependence of the wind-driven ventilation efficiency on wind speed. This confirms that other ventilation mechanisms, such as the stack effect, become important when the wind speed is low. These measurements are in a very good accord with flow patterns simulated by computational fluid dynamics techniques.

INFLUENCE OF AN INSECT SCREEN ON GREENHOUSE VENTILATION

The influence of an insect screen on ventilation rate was experimentally investigated in a multispan glass–covered greenhouse equipped with a continuous roof vent, located at the University of Thessaly near Volos in the coastal area of eastern Greece. Microclimate variables as well as the ventilation rate were measured during summer. Two measuring techniques were used for the determination of ventilation rate: (1) the decay rate tracer gas technique, using N2O as tracer gas, and (2) the water vapor balance technique. The influence of the insect screen on ventilation rate was studied using a wind–related coefficient identified by fitting a simple linear model to the experimental values. The two measuring techniques gave similar results, but the water vapor balance technique provided a better fit to the experimental data. The wind–related coefficient significantly decreased when an insect screen covered the vent. Finally, the influence of the insect screen on the discharge coefficient was investigated. The discharge coefficient was correlated to the aerodynamic properties of the screen using porous media flow analysis.

THE IMPACT OF INSECT SCREENS AND VENTILATION OPENINGS ON THE GREENHOUSE MICROCLIMATE

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.

Wetness Sensors for Detecting Condensation on Leaves and Exudates from Leaf Pruning Wounds

Abstract The presence of free water at the surface of crops is a necessary condition for infestation by most of fungal pathogens and especially by grey mould ( Botrytis cinerea ) for greenhouse crops. The requirements for studying the presence of water at the surface of a greenhouse crops were analysed and two sensors are described. which were developed to meet these requirements. The first sensor is a leaf wetness sensor that can be mount directly on leaves. The second sensor is able to detect the exudates from leave pruning wounds and is fixed on tomato stems. Experimental results are presented and analysed and the opportunity to use one of the two sensors are discussed with respect to the conditions of production and the possibilities to control the greenhouse climate.