Measurement of instantaneous fully 3D scalar dissipation rate in a turbulent swirling flow

This paper describes the measurement methodology for quantifying the instantaneous full 3D scalar dissipation rate (SDR or χ) in order to characterize the rate of mixing. Measurements are performed in a near field of a jet-in-swirling-coflow configuration. All three components of χ are measured using a dual-plane acetone planar laser-induced fluorescence technique. To minimize noise, a Wiener filtering approach is used. The out-of-plane SDR component (χ3) is validated by assuming isotropy between axial and azimuthal components of SDR. An optimum laser-sheet separation distance (Δs) is identified by comparing the SDR components on the basis of instantaneous, mean, and probability density function data. The in-plane resolution needs to match the Batchelor scale (λB) for the central difference scheme-based SDR deduction. However, the out-of-plane resolution, Δs, requirement is different owing to the use of two-point difference based SDR and systematic biases. The optimum Δs is found to be 2.5λB. Finally, measurement guidelines are provided to assess the accuracy of 3D SDR measurements