Effects of Soret diffusion on the intrinsic instability of premixed hydrogen/air flames

TL;DR

This study quantifies how Soret diffusion influences the stability and structure of hydrogen/air flames, revealing its effects on growth rates, wrinkle formation, and flame surface area through simulations.

Contribution

It provides the first detailed analysis of Soret diffusion's impact on hydrogen flame instabilities across linear and nonlinear regimes.

Findings

Soret diffusion increases perturbation growth rate at phi < 1.7 and decreases it at phi > 1.7.

Soret diffusion accelerates small-scale wrinkle formation in lean flames.

Global fuel consumption rate decreases despite local speed increases due to reduced flame surface area.

Abstract

Hydrogen flames exhibit multiple intrinsic instabilities. The low molar masses of H and H2 lead to significant Soret diffusion near the flame front; however, its influence on hydrogen flame instabilities remains to be quantified. This study investigates the effect of Soret diffusion on instability evolution dynamics via one-dimensional counterflow analysis and two-dimensional, high-fidelity direct numerical simulations covering both the linear growth regime and the fully developed nonlinear regime over a wide range of equivalence ratios (phi). In the linear regime, Soret diffusion increases the perturbation growth rate at phi < 1.7, especially under lean conditions, but reduces the growth rate at phi > 1.7. A similar sensitivity reversal is observed in the Markstein length near the peak equivalence ratio of unstretched laminar flame speed. In the nonlinear regime, Soret diffusion accelerates the formation of small-scale wrinkles in lean hydrogen flames and reduces the characteristic size of large-scale finger structure by one-third. An interesting observation is that, although Soret diffusion promotes preferential diffusion and increases the local flame displacement speed, the global fuel consumption rate decreases due to a reduction in the overall flame surface area. In addition, curvature-based flame segment analysis reveals a synergistic effect between Soret diffusion and Fickian diffusion that enhances/reduces the local equivalence ratio in positively/negatively curved regions of the flame front. The probability distributions of the Karlovitz number and the density-weighted displacement speed are also analyzed; results suggest that, for lean hydrogen flames, Soret diffusion broadens the distributions for both parameters, particularly on the positive side. These findings promise to advance the fundamental understanding of hydrogen flame dynamics under complex differential transport.