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 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.