Effect of Soret diffusion on thermodiffusively stable flames

Hydrogen is taking a pivotal role in our near-future energy economy. Because of this, many studies are investigating combustion properties of hydrogen, specifically with respect to its diffusive properties and the resulting thermodiffusive instabilities. In addition to preferential Fickian diffusion, the Soret or thermodiffusion plays an important role in hydrogen flames as well. While most studies are focussed on thermodiffusively unstable flames, either with or without Soret diffusion, this work conducts a systematic study of the impact of Soret diffusion on thermodiffusively stable flames. For rich hydrogen-air flames at atmospheric conditions, which exhibit thermodiffusively stable behaviour, including Soret diffusion generally decreases laminar unstrained flame speeds. This effect is strongest near stoichiomeric conditions and becomes weaker as the flames become richer. The opposite trend is found for strained flame speeds, where Markstein numbers generally increase at richer conditions, but the effect of Soret diffusion is weakest near stoichiometric conditions and largest for very rich flames. For a 2D perturbed rich hydrogen flame, the return to a planar shape at ϕ=6 is up to twice as fast when Soret diffusion is included, which also agrees approximately with results from linear stability analysis. The Soret diffusion flux counteracts the Fickian diffusion of the hydrogen species and creates locally leaner conditions in negatively curved parts of the flame, leading to higher heat release rates, and an enrichment of hydrogen towards the burnt gases in regions with positive curvature.