Velocity/Mixture Fraction Statistics of Round, Self-Preserving, Buoyant Turbulent Plumes

An experimental study of the structure of round buoyant turbulent plumes was carried out, limited to conditions in the self-preserving portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still and unstratified air environment. Velocity/mixture-fraction statistics, and other higher-order turbulence quantities, were measured using laser velocimetry and laser-induced fluorescence. Similar to earlier observations of these plumes, self-preserving behavior of all properties was observed for the present test range, which involved streamwise distances of 87–151 source diameters and 12–43 Morton length scales from the source. Streamwise turbulent fluxes of mass and momentum exhibited countergradient diffusion near the edge of the flow, although the much more significant radial fluxes of these properties satisfied gradient diffusion in the normal manner. The turbulent Prandtl/Schmidt number, the ratio of time scales characterizing velocity and mixture function fluctuations and the coefficient of the radial gradient diffusion approximation for Reynolds stress, all exhibited significant variations across the flow rather than remaining constant as prescribed by simple turbulence models. Fourth moments of velocity and velocity/mixture fraction fluctuations generally satisfied the quasi-Gaussian approximation. Consideration of budgets of turbulence quantities provided information about kinetic energy and scalar variance dissipation rates, and also indicated that the source of large mixture fraction fluctuations near the axis of these flows involves interactions between large streamwise turbulent mass fluxes and the rapid decay of mean mixture fractions in the streamwise direction.