Rotational CARS thermometry in diffusion flames: On the influence of nitrogen spectral line-broadening by CH4 and H2

Rotational CARS N2 thermometry has been developed for improved accuracy in spatial flame regions with high concentrations of either H2 or CH4, which are typical conditions on the fuel side of diffusion flames. In initial validation experiments up to 700 K, rotational CARS measurements were performed in binary mixtures of H2 and N2. Subsequent spectral evaluation with commonly used N2–N2 Raman line widths clearly underestimated the real temperatures. Inclusion of recently calculated N2–H2 spectral line-broadening coefficients weighted by the concentration of hydrogen resulted in CARS temperatures in much better agreement with the true temperatures. Based on this validation work, improved temperature accuracy was achieved in flame measurements on the fuel side of a hydrogen diffusion flame by probing nitrogen simultaneously with the detection of the hydrogen S0-transition at 354 cm−1, and weighting the species specific Raman line widths with the information of relative concentration. The validation experiments were repeated for binary mixtures of nitrogen and methane up to 500 K. It was shown that N2–N2 Raman line widths gave reproducible temperatures independent of methane concentration in binary mixtures with nitrogen, thus no explicit knowledge on N2–CH4 Raman line widths is needed for this temperature region. In general, the impact of Raman line widths for accurate rotational CARS thermometry is demonstrated.