Dual-modification transported probability density function framework for hydrogen combustion simulation: Resolving differential diffusion effects

Accurate modeling of differential diffusion effects is critical for advancing hydrogen combustion simulations in next-generation propulsion systems. This study develops an enhanced Eulerian stochastic fields (ESF) method within a transported probability density function (TPDF) framework, incorporating two key innovations: species-specific diffusion coefficients calculated via the Chapman–Enskog theory and modified stochastic transport terms accounting for differential Wiener processes. The model resolves the inherent limitations of unity Lewis number assumptions, particularly crucial for hydrogen's high diffusivity. Validation in a vitiated coflow burner demonstrates a 15% improvement in flame lift-off height predictions, with a 93% error reduction in OH radical concentrations, resolving long-standing overmixing issues. Application to a high-pressure rocket combustor reveals 18% shorter low-OH zones, indicating enhanced H2/O2 mixing through modified stochastic transport. The dual modification of both deterministic diffusion and stochastic terms proves essential for capturing preferential radical transport in near-field regions, turbulence–chemistry interactions at subgrid scales, and mixing-limited combustion dynamics. These findings establish that differential diffusion significantly impacts flame stabilization through competing mechanisms—enhancing fuel-oxidizer mixing while promoting radical dispersion into inert flows. The demonstrated framework provides critical capabilities for optimizing hydrogen combustor designs across atmospheric and supercritical pressure regimes.