Effect of strain on thermoacoustic characteristics of lean premixed hydrogen flames

Recent research [Porcarelli et al., Int. J Hydrogen Energy 49, 2024] shows NOx can be suppressed in lean premixed hydrogen flames at high levels of strain without loss of efficiency. However, the thermoacoustic response of such flames in high strain regimes remains unexplored. The present work addresses this by investigating the thermoacoustic characteristics of very lean premixed hydrogen flames under increasing levels of applied strain rates. High-fidelity simulations are conducted in a counterflow, reactants-to-products configuration, where strain rate is controlled by inlet velocities. Several simulations are conducted under a range of different strain rates and forcing frequencies. The simulations are run using a modified version of the reactingFOAM solver within OpenFOAM-9 that accounts for the different diffusivities in hydrogen flames and uses the low-Mach formulation of the Navier-Stokes equations. The forcing was imposed by applying sinusoidal oscillations on top of the uniform velocity of the reactants at the inlet boundary with a specific frequency and an amplitude of 10% of the mean value. Results indicate that above a threshold, increasing strain raises the gain and shifts its peak to higher frequencies. This behaviour stems from the flame speed response to strain at negative Markstein lengths and can be leveraged for the development of ultralow-emission combustion devices where applied strain can be used as an additional mechanism to shift an unstable flame response to different frequencies. Moreover, the highest applied strain rate investigated triggers an inversion of the Markstein length, leading to a 180 $\circ$ phase shift in the flame dynamic response. This aspect and further implications are discussed in the paper.