Piloted diffusion flames of nitrogen-diluted methane near extinction: OH measurements

The chemical structure of piloted turbulent jet diffusion flames of nitrogen-diluted methane near extinction is investigated. Simultaneous point measurements of hydroxyl radical concentration, temperature, and major species concentrations are performed using the combination of laser-induced fluorescence, Rayleigh scattering, and spontaneous Raman scattering. Three flames having increasing degrees of local extinction are considered. Average mass fractions, conditional on both mixture fraction and temperature, are similar in the three flames for all measured species. This similarity suggests that two scalars, mixture fraction and temperature, may be sufficient to describe the chemical states in the turbulent flames, independent of the complex mixing and reaction history associated with the local extinction process. Mass fractions of OH and H₂ in the turbulent flames are higher than predicted by steady strained laminar flame calculations. Consequently, the turbulent flame results cannot be represented in detail by an ensemble of steady laminar flame solutions. The relationship among the species OH, H₂, and O₂ is examined by considering the ratio R_OH=[OH]/(K_EQ[O₂][H₂])^1/2. In both the laminar flame calculations and the turbulent flame data, R_OH is near unity (indicating partial equilibrium) for temperatures above ≈1600 K and for the interval in mixture fraction were the highest OH mass fractions are observed. The agreement on this chemical kinetic aspect of the results shows that the high OH concentrations in the turbulent flames can be attributed to high H₂ concentrations and suggests that the mixing characteristics of the turbulent flames are not adequately represented by steady strained laminar flame calculations.