Comprehensive linear stability analysis for intrinsic instabilities in premixed ammonia/hydrogen/air flames
Terence Lehmann, Lukas Berger, T.L. Howarth, Michael Gauding, Sanket Girhe, Bassam B. Dally, Heinz Pitsch
Abstract
Two-dimensional direct numerical simulations of planar laminar premixed ammonia/hydrogen/air flames are conducted for a wide range of equivalence ratios, hydrogen (H 2 ) fractions in the fuel blend, pressures, and unburned temperatures to study intrinsic flame instabilities (IFIs) in the linear regime . For stoichiometric and lean mixtures at ambient conditions, a non-monotonic behavior of thermo-diffusive instabilities with increasing H 2 fraction is observed. Strongest instabilities occur for molar H 2 fractions of 40%. The analysis shows that this behavior is linked to the joint effect of variations of the effective Lewis number and Zeldovich number. IFIs in ammonia/hydrogen blends further show a non-monotonic trend with respect to pressure, which is found to be linked to the chemistry of the hydroperoxyl radical HO 2 . The addition of NH 3 opens new reaction pathways for the consumption of HO 2 resulting in a chain carrying behavior in contrast to its chain terminating nature in pure H 2 /air flames. Theoretically derived dispersion relations can predict the non-monotonic behavior for lean conditions. However, these are found to be sensitive to the different methods for evaluating the Zeldovich number available in the literature. The novelty of this research is the systematic identification and explanation of a non-monotonic behavior of intrinsic flame instabilities IFIs in ammonia/hydrogen/air (NH 3 /H 2 /air) flames concerning the hydrogen content in the fuel and the pressure. To the author’s knowledge, this study presents the largest parametric study for linear stability analyses of NH 3 /H 2 /air flames. Furthermore, a sensitivity analysis to the Zeldovich number is proposed to link the macroscopic effect of IFIs to the microscopic effects of chemistry. In light of possible applications of NH 3 as zero-carbon fuel, this study is significant because the fundamental understanding of IFIs in NH 3 /H 2 /air flames is key for the analysis and modeling of such flames.