Scalar dissipation rate statistics in turbulent flows using Planar Laser Induced Fluorescence measurements

A shearless mixing layer with isotropic decaying turbulence has been examined with a turbulent axisymmetric opposed-jet flow and a plume in pipe flow. The study has been carried out experimentally with Planar Laser Induced Fluorescence of acetone (used as a tracer to measure the passive scalar ξ) to study the conditional statistics of the scalar dissipation χ. For all flows examined, it has been found that: (i) the PDFs of the conditional χ show a consistent small deviation from log-normality at high values of ξ; (ii) the ratio F=〈χ′2|η〉1/2/〈χ|η〉 is independent of η the sample space variable for ξ), within statistical error; (iii) F increases from zero (at the injector in the plume experiment) to a value between 1 and 1.4 downstream and at later times; (iv) the rise of F to this value occurs over a few Kolmogorov timescales; and (v) the long-time value of F is close between the two experiments and seems to increase with the turbulent Reynolds number. The development of conditional scalar dissipation fluctuations is being discussed by comparing the “early” and “late” mixing regions in the plume and the counterflow. The agreement between the PLIF measurements, which can only resolve two-dimensional fields, suggests that the present conclusions are not limited by resolution or the lack of measuring the third dimension of the scalar gradient. The results are useful for turbulent reacting flow models such as advanced flamelet, CMC, and transported PDF closures.