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

Abstract

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 across the flame surface as a result of preferential diffusion. These fluctuations are found to be well-captured by the gradient of OH-PLIF intensity across 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 of lean hydrogen-air flames. This highlights the potential of OH intensity gradient magnitudes as a reliable marker of thermo-diffusive instability, and a potential surrogate to local fuel consumption speed which is inaccessible experimentally.