Measurements of scalar dissipation in a turbulent plume with planar laser-induced fluorescence of acetone

Abstract

In order to gain a better understanding of the scalar dissipation rate χ in turbulent flows and to test available models for this quantity, high-resolution two-dimensional planar laser-induced fluorescence measurements were undertaken in the mixing field formed by the axisymmetric injection of a fluorescent tracer (acetone) into a confined turbulent co-flow of air, with emphasis on the less explored early region close to the nozzle and on the spatial resolution and level of image denoising necessary for the correct measurement of χ . In the mean, the resulting plumes had Gaussian profiles and axial decay as expected from previous investigations. It was found that, with Kolmogorov lengthscale resolution and careful image processing prior to the calculation of the scalar gradients, the measured χ satisfied global conservation of scalar energy across the plume to within 20%. The estimated mean three-dimensional scalar dissipation rate was used to calculate C D (twice the timescale ratio) that was found to decrease from values higher than 10 adjacent to the nozzle, to approximately 2 at an axial distance of 2–3 nozzle diameters (corresponding to residence times of 0.1–0.2 turbulent timescales) and retaining this value further downstream. The data can assist the validation of models for 〈 χ 〉 .