Abstract
Time series measurements of gas density have been obtained in lean (=0.65–0.8) premixed turbulent flames stabilized on a Bunsen-type burner (U0=2.3, 3.5 m/s). Two Eulerian scalar timescales were obtained from spectral analyses of these time series. The first, T^, characterizes the flame front wrinkling process, describable by random telegraph signal statistics, and the second is a small scalar timescale, τD. This was shown to be due to the transitions of the flame fronts as they pass through the probe volume. The ratio between these two Eulerian scales τD/T^=0.4 in the present flames, is a measure of the scale range of scalar fluctuations and is potentially an interesting factor at high turbulent Reynolds numbers when broadening of the preheat zone by turbulent mixing affects this ratio directly by broadening the “dissipation range” in the spectrum. A further timescale, a direct measure of the flame front transit time, was also determined from these data. Estimates of the local mean scalar dissipation were obtained from this data using flamelet assumptions and an interior distribution of the scalar, tested by fitting to numerical simulations of laminar flames, for the local scalar gradients. A simple expression was derived which related the mean scalar disspation to the burning fraction of the scalar probability density function, γ, and a chemical time characteristic of the flame front. It was found that the dissipation rate is affected more by the local scalar gradients than by γ. A dissipation time scale was deduced from this rate and was found to be proportional to the ratio of the scalar integral timescale and the flame transit time.
Published Version
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