Numerical study of a full-size free-air fumigation system

Free-air carbon dioxide enrichment (FACE) systems have been developed by others to study the effect of rising atmospheric carbon dioxide concentration on the growth of the earth's vegetation. One important design objective for a FACE system is to produce a uniform spatial and temporal distribution of carbon dioxide. This objective is strongly influenced by the flow pattern near the point of gas injection into the atmosphere. This paper reports on a computational fluid dynamics (CFD) model that has been formulated to analyze the three-dimensional mean flow and gaseous dispersion in a FACE system. The model gives numerical solutions of the Reynolds-averaged form of the conservation equations of mass, momentum and carbon dioxide concentration. The Reynolds-averaged turbulent fluxes were represented by an isotropic gradient diffusion k-epsilon model. The flow in the crop canopy was modelled as an isotropic porous medium, Numerical solutions were obtained using a finite volume methodology. The CFD model was used to calculate the distribution of carbon dioxide gas concentration in a 22 m diameter FACE system equipped with typical high velocity triple-jet gas injectors. The optimum 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 height was increased. A good compromise between these conflicting requirements was obtained with the injector height slightly greater than the crop canopy height. (C) 1997 Elsevier Science B.V.