Agriculture Reference
In-Depth Information
The discharge coefficient of the vents,
which measures the reduction of the air
flow when passing through a vent, has been
studied in Mediterranean greenhouses
(Muñoz, 1998; Pérez-Parra, 2002).
The variability in the wind conditions,
the greenhouse types, the vents (shape,
location, presence or not of screens) induced
a simplification of the joint incidence of
these effects under a global wind effect coef-
ficient (see Appendix 1).
where it is heated and its humidity increased
(Wacquant, 2000). The hottest spot is located
on the side exposed to the wind (Fig. 8.9).
8.4.5
Measuring the ventilation of
greenhouses
The most commonly used method to meas-
ure the natural ventilation in greenhouses is
the use of a tracer gas, which uses an inert
and non-reactive gas (Goedhart
et al
., 1984).
This gas (which is usually nitrous oxide) is
homogeneously distributed inside the green-
house and the evolution of its concentration
in the air over a period of time, in the exist-
ing ventilation conditions, quantified. The
ventilation rate is proportional to the decay
rate of the tracer gas content in the air.
Recently, the use of models has facili-
tated the study of ventilation. The flow
visualization techniques using scale models
(Montero and Antón, 2000a) have been used
with satisfactory results, at a low cost
(Plate 16). Recent work with scale models,
in a wind tunnel, has permitted the study of
ventilation in low-cost parral-type green-
houses (Pérez-Parra, 2002).
Another method of studying ventilation
is the use of fluid dynamics simulation pro-
grams known as CFD (Computational Fluid
Dynamics), illustrated in Plate 17. In recent
years increasing attention is being paid to
this CFD tool in greenhouse technology stud-
ies (De Pascale
et al
., 2008; Dorais, 2011).
Several models have tried to relate
the ventilation rate with the area of open
The presence of plants affects the air exchange
rate and the convective movements, depend-
ing on the density of the vegetation and the
arrangement of the crop rows.
Plants, when transpiring, cool the air and
modify its density. High ventilation with a LAI
above 2 limits the temperature gradient and
modifies the chimney effect. The vegetation
forms a screen that limits the air movement.
Therefore, the location of the crop rows in the
same direction as the dominant winds facili-
tates the air circulation and the ventilation.
In tall greenhouses, in which there is a
large air chamber between the crop and the
cover, the crop has little influence on the air
movements if it does not block the sidewall
vents.
When the greenhouse is closed, the
wind also affects the internal air movements.
At the top, the circulation is parallel and in
the same direction with that of the outside
wind, returning through the lower part
Suction
Wind
High pressure
Suction
T: +1-1.5°C higher than temperature on other side of greenhouse
RH: +5-15% higher than other side of greenhouse
Fig. 8.9.
The effect of the external wind on the air movement inside a closed greenhouse (adapted from
Wacquant, 2000).
