Intrinsic instabilities in premixed hydrogen flames: parametric variation of pressure, equivalence ratio, and temperature. Part 2 – Non‐linear regime and flame speed enhancement

Abstract

The propensity of lean premixed hydrogen flames to develop intrinsic instabilities is studied by means of a series of simulations at different equivalence ratios [0.4–1.0], unburned temperatures [298 K–700 K], and pressures [1 bar–20 bar]. In addition to the Darrieus-Landau or hydrodynamic instability, lean premixed hydrogen flames are prone to thermodiffusive instabilities, which lead to significant flame front wrinkling and a chaotic process of formation and destruction of cellular structures along the flame front. In part 1 of this work (L.Berger et al., Combust. Flame, 2022), the stability of planar flames that are exposed to weak harmonic perturbations has been studied. In this part, the long-term dynamics of these flames, which become strongly corrugated and show strong flame speed enhancements and variations of the local reaction rates, are studied. In particular, local extinction events and strong peaks of the reaction rates, sub- and super-adiabatic temperatures in the burned gas, and variations of the local flame thickness are observed such that the flame reactivity significantly deviates from an unstretched laminar flamelet. Consistent with the stability analysis in part 1, the impact of instabilities increases if the equivalence ratio or unburned temperature are decreased or the pressure is increased. In particular, the propensity of intrinsic instabilities to increase at high pressure is relevant to several combustion applications operating at elevated pressures. Further, the effect of the global flame parameters such as the expansion ratio, Zeldovich number, and Lewis number on the flame speed enhancement is quantitatively assessed. Finally, the flame speed enhancement due to instabilities is found to correlate well with the maximum growth rates of the perturbed flames that have been measured in part 1 indicating a strong link between the two different flame regimes.