THE ESTIMATED SCALAR DISSIPATION RATE IN GAS-PHASE TURBULENT JETS

An estimate for the scalar dissipation rate in the turbulent concentration field formed between the nozzle fluid from a round, gas-phase, momentum-driven, turbulent jet and the entrained reservoir fluid is studied. The results are based on the square of the concentration time derivative from high-resolution, single-point measurements made in the self-similar far field of the jet at Reynolds numbers of 5000, 16 000, and 40 000. Results for the temporal behavior, mean value, and statistical properties of the estimated scalar dissipation rate are reported. Peaks in the instantaneous value of the estimated scalar dissipation rate are found to produce a significant, but not dominant, contribution to the average dissipation rate on the jet centerline. The occurrence statistics of these peaks are determined to be other than Poisson distributed. The mean width of the peaks is found to physically correspond to the Taylor scale associated with scalar dissipation, and depend on the inverse square root of the jet Reynolds number. The average estimated scalar dissipation rate is found to be independent of Reynolds number, and self-similar in a manner that is consistent with similarity laws for the turbulent jet. The probability density function (pdf) of the estimated scalar dissipation rate is also found to be self-similar. The compiled distributions, which are based on only one squared field gradient, deviate significantly from lognormality at small values. This result is shown to not preclude the lognormality of the distribution of the true scalar dissipation rate.