Effects of turbulence on species mass fractions in methane/air jet flames

TLDR: In this article, the authors used the combination of Raman scattering and laser-induced fluorescence to obtain simultaneous measurements of CO, OH, H2, and NO along with the major species, temperature, and mixture fraction in a series of six piloted methane/air jet flames.

Abstract: It is important that combustion models capture the effects of turbulent mixing on reaction zone structure in non-premixed and partially premixed flames. A more complete understanding of the response of species mass fractions to turbulent mixing is needed to improve predictive capabilities, particularly with regard to combustion intermediates and minor species. Using the combination of Raman scattering. Rayleigh scattering, and laser-induced fluorescence, simultaneous measurements of CO, OH, H2, and NO are obtained along with the major species, temperature, and mixture fraction in a series of six piloted methane/air jet flames. Flame conditions vary from laminar to turbulent with significant localized extinction. Two-photon laser-induced fluorescence (TPLIF) is used to determine instantaneous CO concentrations, providing an improvement over Raman scattering measurements of CO in methane flames. Conditional probability density functions (cpdf's) of species mass fractions in the six flames are compared. Significant changes are observed in the mass fraction cpdf's of several species. Results for H2O, CO2, H2, and OH are consistent with the concept that turbulent transport becomes dominant over molecular diffusion within the range of Reynolds numbers and axial locations considered in these experiments. The cpdf's of CO mass fraction are broadened in the turbulent flames relative to the laminar flame. However, there is not an increase in the maximum conditional mean value of the CO mass fraction as suggested by some previously reported measurements in methane flames. The cpdf's of NO mass fraction at a given streamwise location in the turbulent flames show NO levels decreasing significantly as jet velocity increases.