Free-air carbon dioxide enrichment (FACE) systems have been developed
by others to study the effect of rising atmospheric carbon dioxide con
centration on the growth of the earth's vegetation. One important desi
gn objective for a FACE system is to produce a uniform spatial and tem
poral distribution of carbon dioxide. This objective is strongly influ
enced by the flow pattern near the point of gas injection into the atm
osphere. This paper reports on a computational fluid dynamics (CFD) mo
del that has been formulated to analyze the three-dimensional mean flo
w and gaseous dispersion in a FACE system. The model gives numerical s
olutions of the Reynolds-averaged form of the conservation equations o
f mass, momentum and carbon dioxide concentration. The Reynolds-averag
ed turbulent fluxes were represented by an isotropic gradient diffusio
n k-epsilon model. The flow in the crop canopy was modelled as an isot
ropic porous medium, Numerical solutions were obtained using a finite
volume methodology. The CFD model was used to calculate the distributi
on of carbon dioxide gas concentration in a 22 m diameter FACE system
equipped with typical high velocity triple-jet gas injectors. The opti
mum height for the gas injectors was shown to be a compromise between
the improving utilization of carbon dioxide as the height was reduced
and the lowering of carbon dioxide gradients within the crop as the he
ight was increased. A good compromise between these conflicting requir
ements was obtained with the injector height slightly greater than the
crop canopy height. (C) 1997 Elsevier Science B.V.