Comprehensive linear stability analysis for intrinsic instabilities in premixed ammonia/hydrogen/air flames

TL;DR

This study conducts a detailed linear stability analysis of premixed ammonia/hydrogen/air flames, revealing non-monotonic behaviors of intrinsic flame instabilities influenced by fuel composition, pressure, and chemical pathways.

Contribution

It provides a comprehensive linear stability framework for ammonia/hydrogen/air flames, highlighting the effects of fuel mixture ratios, pressure, and chemical reactions on flame instabilities, with theoretical predictions validated against simulations.

Findings

Instabilities peak at 40% hydrogen molar fraction.

Non-monotonic instability trends with pressure linked to HO2 chemistry.

Dispersion relations predict behavior but are sensitive to Zeldovich number evaluation methods.

Abstract

Two-dimensional direct numerical simulations of planar laminar premixed ammonia/hydrogen/air flames are conducted for a wide range of equivalence ratios, hydrogen ($\rm H_2$) 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 ($\rm H_2$) fraction is observed. Strongest instabilities occur for molar ($\rm H_2$) 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 $\rm HO_2$. The addition of $\rm NH_3$ opens new reaction pathways for the consumption of $\rm HO_2$ resulting in a chain carrying behavior in contrast to its chain terminating nature in pure $\rm H_2$/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.