Piloted Diffusion Flames of Diluted Methane Near Extinction: Detailed Structure from Laser Measurements

Simultaneous laser Raman, Rayleigh and fluorescence measurements have been made in diluted turbulent methane diffusion flames. Time and space resolved results for CH₄, O₂, N₂, H₂O, CO, CO₂, H₂, OH, temperature and mixture fraction are presented as scatter plots and conditional probability density functions for conditions close to extinction. Dilution, whether by air or nitrogen, results in a flame structure with wider reaction zones, and it is shown that the reaction zone width markedly affects the development of bimodality of species and temperature pdfs around stoichiometric composition. The position of the mean stoichiometric contour, relative to the shear layer, is also found to alter the shapes of pdfs of temperature and species. Hydrogen and hydroxyl radical mass fractions are much in excess of predictions for stretched laminar flames, Dilution with air does not seem to alter the chemical composition near extinction, either at stoichiometric mixture or in the richer reactive region of the flame, and there are strong indications that a two-variable approach to the modelling of the chemical kinetics is feasible.