This study investigated the effect of tangential strain on the stability of perturbed laminar lean premixed hydrogen flames in a counterflow reactants-to-products configuration. Laminar premixed flames are highly susceptible to intrinsic instabilities, including hydrodynamic and thermodiffusive instabilities, which can cause perturbations in the flame front to grow. However, the impact of tangential strain, introduced by velocity gradients along the flame front in a counterflow setup, had not been fully explored in the context of flame stability. The present work bridged this gap by combining the counterflow setup used in previous studies of NOx reduction with the perturbation analysis commonly employed in flame stability research.

Direct Numerical Simulations were performed to study the behaviour of sinusoidal perturbations imposed on the flame front, focusing on the growth rates of these perturbations under two different strain rates (2000 1/s and 4000 1/s). The results indicated that the tangential strain improved the stability of the flame front, as the strain rate led to a reduction in both the maximum observed growth rate and the growth rate after the perturbation reached its peak. The growth rates observed were in the non-linear regime, characterised by continual variation over time. At higher strain rates, the flame front stabilised more quickly, suggesting a strong correlation between strain rate and perturbation growth dynamics. The strain-induced velocity gradients displaced the flame front, reducing its effective curvature and wavelength, further contributing to the stabilisation process.

In addition to the strain rate, the study investigated the effects of varying the amplitude and wavelength of initial perturbations. It was found that the amplitude of the initial perturbation had little impact on the maximum growth rate, but variations in the initial wavelength significantly influenced both the growth dynamics and the maximum growth rate of the perturbations.

Overall, the results provided a comprehensive understanding of how tangential strain affected flame stability and highlighted the importance of wavelength variations in determining perturbation growth rates. These findings provide insights into potential strategies to improve flame stability in practical combustion systems aimed at reducing emissions, such as in lean hydrogen combustion.