Turbulent nonpremixed flames of methane near extinction: Mean structure from Raman measurements

Spontaneous Raman-Rayleigh measurements of species concentration-temperature have been made in turbulent nonpremixed flames of methane near extinction. The means and rms of fluctuations of temperature, density, mixture fraction, and the mass fractions of CH₄, O₂, N₂, H₂O, H₂, CO, and CO₂ are presented in this paper. The mean Raman-Rayleigh results compare favorably with probe measurements of species concentrations and temperature made in similar flames. At a given axial location, unreactedness (finite rate kinetics) and not unmixedness [high (ξ″²) ^{1 2}] explains almost entirely the change in the flame structure. As the flame approaches extinction, the Favre averaged mass fractions of CH₄ and O₂ increase while those of the products H₂O, H₂, CO, and CO₂ decrease. The rms of fluctuations in the temperature and mass fractions O₂, H₂O, H₂, CO, and CO₂ decrease as well. Although these results favor the concept of a diffuse flame structure, the existence of partially premixed laminar diffusion flamelets cannot be excluded.