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 CH/sub 4/, O/sub 2/, N/sub 2/, H/sub 2/O, H/sub 2/, CO, and CO/sub 2/ 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 (xi-tilde''/sup 2/)/sup 1/2/) explains almost entirely the change in the flame structure. As the flame approaches extinction, the Favre averaged mass fractions of CH/sub 4/ and O/sub 2/ increase while those of the products H/sub 2/O, H/sub 2/, CO and CO/sub 2/ decrease. The rms of fluctuations in the temperature and mass fractions O/sub 2/, H/sub 2/O, H/sub 2/, CO, and CO/sub 2/ 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.