Raman imaging of counterflow diffusion flames
Hydrogen versus air counterflow diffusion flames are studied with linewise Raman imaging system. Raman measurements are gathered over 20 second duration with high precision (2%) and high spatial resolution (160 $\mu$m). Axial strain rate values are measured by laser Doppler velocimetry. Three different fuel jet concentrations are studied: mole fractions of 21% hydrogen diluted with 79% nitrogen, 50% hydrogen diluted with 50% nitrogen and undiluted hydrogen. Axial strain rate is varied for each of the fuel dilution cases. Raman images of these flames are converted into temperature and major species mole fraction profiles. Mixture fraction and scalar dissipation rate profiles are generated from the mole fraction information. Strain rate and differential diffusion effects are clearly illustrated by the results. Numerically predicted strain rate and differential diffusion effects are experimentally matched. Peak temperatures values agree well with numerical estimates for all of the fuel dilutions except for the undiluted fuel jet cases. At least 50 K higher temperatures are observed for undiluted fuel jet flames when compared to numerically predicted temperatures of these flames. A library of mixture fraction and scalar dissipation rate is generated for hydrogen-air combustion.
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