Normalization reveals the role of thermodiffusive effects in turbulent premixed hydrogen–methane flames

Abstract

This paper investigates the interaction between turbulence and thermodiffusive instabilities experimentally using a large Mie-scatter imaging data set of hydrogen-enriched methane–air flames (46 different conditions) at constant flow field conditions and variable blending ratios and equivalence ratios. In methane–air lean premixed flames with unity Lewis numbers, turbulence has a strong effect on the overall flame morphology and global consumption speed. Conversely, turbulence is found to have less of an effect on the overall morphology and consumption speed of lean hydrogen–methane flames at sufficiently low (sub-unity) Lewis numbers for the conditions examined herein. The results of the present work provide experimental support to the suggestion that one-dimensional laminar flame characteristics obtained from freely-propagating flame simulations are unsuitable normalization factors when comparing thermodiffusively unstable flames of variable blending and equivalence ratios. It is demonstrated how characteristic scaling laws based on the DNS-based consumption speed parameterized by an instability parameter ω T D paint a more consistent picture of the turbulence-instability interactions in the present regimes. Novelty and Significance Statement The work herein constitutes the first application of Mie scattering for the measurement of turbulent flame characteristics (consumption speed, area, and curvature) in premixed hydrogen-enriched methane–air flames across the full range of lean conditions, providing one of the largest experimental data sets for the purpose of validation. The design of experiments involved a Karlovitz number Ka L sweep at constant flow conditions, by varying the equivalence ratio of the flame. This approach is original and differs from traditional designs (i.e., Troiani et al. (2024)) which have so far relied on modifying flow turbulence characteristics (e.g., bulk flow or turbulent rms velocity) at fixed equivalence ratio. Moreover, this work provides the first experimental evidence in favor of using characteristic normalizations which account for thermodiffusive instabilities in hydrogen-containing flames, and its importance. This has not been evidenced in past studies investigating turbulence and thermodiffusive instability interplay given turbulence was varied at constant equivalence ratio, and thus constant laminar flame characteristics (i.e., laminar flame speed and thickness).