Turbulent nonpremixed flames of methane near extinction: Probability density functions

Space- and time-resolved measurements of major species concentrations and temperature have been made using the Raman-Rayleigh scattering technique in the blue (visibly soot free) regions of turbulent nonpremixed flames of methane close to extinction. The data are presented in this paper in the form of single variate probability density functions (pdfs) for the mixture fraction, temperature, and the mass fractions of CH₄, O₂, H₂O, H₂, CO₂, and CO. Representative instantaneous scatter plots and joint pdfs are also shown. When the mixing rates are low, the data show mostly fully burnt mixtures indicating that the chemistry is relatively fast. As the flame approaches extinction, most local mixtures become either partially burnt or simply mixed. The joint pdfs shown bimodality for mixture fractions less than ∼.1 and centered distributions for richer mixtures. When close to extinction, fully burnt pockets of fluid are still encountered and these may be responsible for keeping the flame alight. Questions relating to the local, instantaneous flame structure near extinction are discussed in light of existing theoretical models.