Effects of intrinsic instabilities in the local burning rate of lean premixed hydrogen/air laminar flames

Flame surface instability has a strong effect on the flame speed of turbulent flames in the flamelet and corrugated flamelet regimes. These effects can either increase the burning rate or cause local extinction. In this work, direct numerical simulations are used to evaluate the linear and nonlinear growth of hydrodynamic and thermo-diffusive instabilities caused by a spatial perturbation pattern along a lean hydrogen/air premixed front. The numerical simulations use detailed chemistry and transport in a two-dimensional channel geometry with periodic boundary conditions along the vertical boundaries. Both stabilized Bunsen flames and transient flames are simulated. The increase in local combustion rate due to flame stretch in both cases is compared, revealing the transient effects. Then, a diagram of the dispersion relation for the transient flame is calculated, showing the growth rate of the disturbance and the corresponding wavenumbers. The results show that in regions where the stretch is weaker, the spectrum of curvature and wavelengths tends to follow the relationships between flame speed and curvature as predicted by linear theory, despite the chaotic behavior of the fold front. On the other hand, when the curvature of the flame front tends to high negative values, significant deviations from the linear model are observed. The results demonstrate the role of hydrodynamic and thermo-diffusive instabilities in enhancing the combustion rate of turbulent lean hydrogen flames. This enhancement of combustion rate is driven by the increased flame surface area resultant from these instabilities. Hydrodynamic instability induces wrinkles on the flame surface, thus increasing its area. Similarly, thermo-diffusive instability, resulting from differential diffusion effects between heat and species, leads to variations in flame surface area, consequently affecting the combustion rate. These phenomena collectively boost the combustion process by maximizing the reactive surface area.