Thermo-Diffusive Flame Speed Adjustment and its Application to Hydrogen Engines

Practical direct injection hydrogen combustion applications typically require operating the engine in the lean regime. Lean hydrogen flames feature strong thermo-diffusive instability effects making 3D CFD simulations challenging. In particular where the calibrated model is required to operate across a range of equivalence ratios without adjustment and provide accurate results on coarse grids necessitated by the run-times of 3D CFD. In this paper we present a 3D CFD study of a Euro VI HD diesel engine converted to operate on hydrogen gas using direct injection. A scaling methodology recently proposed for conversion from constrained to freely propagating flame based on DNS data is implemented. A laminar flame speed tabulation is developed based on the conversion of 1D results obtained from direct kinetics simulations to freely propagating flame expression considering the behaviour of the thermo-diffusive instability for a wide range of pressures, temperatures and equivalence ratios. The resulting approach is applied to model engine operation under a set of fuelling conditions ranging from λ = 2.5 to λ = 3.5 within the framework of a G-equation/RANS combustion model with tabulated kinetics. Discussion of the meshing requirements is also presented. The resulting model is demonstrated to accurately predict the trends in engine performance and correctly capture the flame acceleration driven by thermo-diffusive effects.