Spontaneous Raman-Rayleigh measurements of temperature and species concentrations have been made in turbulent nonpremixed flames of methane close to extinction. For each data point, five different values of reactedness are calculated from the measured temperature and the mass fractions of CH4, O2, H2O and CO2. These data are then sampled with respect to five ranges of mixture fraction and the conditional single and joint probability density functions (PDFs) of reactedness calculated from the various reactive scalars are presented for each range. Conditional means and root mean squares (rms) of fluctuations of reactedness, temperature, and species mass fractions are also presented. It is found that for mixture fraction ranges lying below the rich reaction limit, ξ < ξR (ξR = 0.087 for methane) combustion is generally bimodal and fluid samples are either fully burned or unburned. For ranges with ξ > ξR, the majority of mixtures are partially reacted and the conditional PDFs have centered distributions. The correlation between the conditional PDFs of various reactive scalars indicate that more than a single-step reaction mechanism is required to adequately describe the chemistry. Close to extinction, the flame was kept alight by fully burned pockets of fluid that provide a source of reignition for mixtures with slower mixing rates. The flame blows off when the frequency of occurrence of these pockets becomes small.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology
- Physics and Astronomy(all)