Impact of thermal and differential-preferential diffusion on the dynamics and acoustics of hydrogen–air slit flames

The influence of thermal and differential-preferential diffusion on the flame dynamics and acoustic emission of laminar hydrogen–air slit flames is investigated using two-dimensional direct numerical simulations (DNS) and modal decomposition techniques. Simulations span a range of equivalence ratios ( ϕ = 0 . 4 –0.7) and diffusion models, including mixture-averaged diffusion with and without the Soret effect and a simplified Unity Lewis number approximation. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) reveal that dominant hydrodynamic instabilities persist across models, particularly at richer conditions. However, the inclusion of Soret and differential-preferential diffusion modifies the spectral structure of the dominant modes, such that energy is redistributed across higher-order components and a shift in the acoustic peak frequency is induced. These effects occur across all equivalence ratios, but are most evident at intermediate values where competing instabilities increase sensitivity to diffusion-driven modal interactions. At lean conditions, diffusion drives the dominant instability, while at richer conditions it modulates the spectral features of hydrodynamic modes. Neglecting thermal and differential-preferential diffusion fails to capture this behavior, potentially leading to underestimated sound levels at key hydrodynamic frequencies. These findings highlight the importance of detailed diffusion modeling to accurately predict combustion generated noise in hydrogen systems. Novelty and significance statement The present study is the first to provide a detailed numerical analysis of the effects of differential-preferential and thermal diffusion on the dynamics and acoustic emissions of hydrogen–air slit flames. The novelty of this work lies in two main contributions. First, it demonstrates that diffusion model assumptions can substantially alter predicted instability growth rates and spatial organization in slit flames. Second, it establishes a clear link between these modeling-induced changes in instability behavior and measurable differences in the resulting acoustic field, essential for accurate prediction of flame dynamics and acoustic response.