An experimental marker of thermo-diffusive instability in hydrogen-enriched flames

The structure of hydrogen-enriched methane-air flames in a Bunsen burner at low turbulence is investigated using OH planar laser-induced fluorescence (PLIF). Three flames are investigated at identical unstretched laminar flame speeds and turbulence conditions, while hydrogen enrichment is varied up to 70% by volume. An increase in global flame consumption speed is recorded with hydrogen addition, and is attributed to both an increase in flame surface area and fluctuations in stoichiometry along the flame surface as a result of differential diffusion. These fluctuations are found to be well-captured by the gradient of OH-PLIF intensity along the flame front and it is hence identified as a promising experimentally-accessible marker of thermo-diffusive instability. Its correlation with curvature is hereby examined for the first time experimentally. No correlations are found in absence of hydrogen, while increasingly positive correlations are recorded with hydrogen enrichment, consistent with the behavior of local fuel consumption in direct numerical simulations (DNS) of lean hydrogen-air flames. This highlights the potential of OH intensity gradient magnitudes as a marker of thermo-diffusive instability, and a potential surrogate for local fuel consumption speed which is inaccessible experimentally.