The role of thermodiffusion and dimensionality in the formation of cellular instabilities in hydrogen flames

Hydrogen is quickly becoming one of the most important fuels for combustion applications. However, compared to conventional hydro-carbon flames, the high diffusivity of hydrogen makes lean hydrogen flames prone to form cellular instabilities. In this work, the formation of cellular structures on a lean hydrogen–air flame is studied numerically in a laminar flow with prescribed initial perturbation. The flame is fully resolved and a detailed reaction mechanism as well as detailed diffusion models are utilized. In the literature, most numerical works directly studying cell formation are limited to two-dimensional setups. However, the additional principal curvature direction in three dimensions can have a strong impact on the cell formation and flame propagation. Because of this, simulations are performed both in 2D and 3D to directly quantify the effect of dimensionality on flame propagation. In the 3D simulations, higher local curvatures yield local heat release rates that exceed the ones from 2D simulations by 80%. In addition, simulations with and without thermo or Soret diffusion are carried out. While Soret diffusion leads to a decrease in flame speed for freely propagating flames, it accelerates the formation of thermodiffusively unstable cells as well as increases local heat release rates. This can be explained by an increase of local equivalence ratios in the reaction and post-oxidation zone due to the altered focusing of diffusive fluxes, leading to locally increased heat release rates for positively curved flame segments. The efficiency factor is evaluated to model the effect of the cellular structures on the local burning rate. increases during the formation of primary cells and reaches a quasi-steady value once the secondary structures are formed, which can present an approach for modeling the effect of cellular structures on hydrogen flame dynamics.