DNS of Lean Premixed Combustion Assisted by Partial Fuel Stratification for Assessment of G-Equation Modeling Approach*

This study focuses on understanding the detailed flame structures associated with partial fuel stratification (PFS)-assisted lean premixed combustion using two-dimensional direct numerical simulation (DNS). The DNS is performed using the spectral element CFD solver Nek5000 with detailed chemistry, experimental pressure trace, and realistic initial conditions mapped from a separate large-eddy simulation (LES), replicating a lean spark ignition engine condition. Numerical results indicate the prevalence of conventional triple flame structures during the initial stage of flame kernel growth. Both premixed and non-premixed combustion modes are present with the premixed mode contributing dominantly to the total heat release. Detailed analysis reveals non-negligible effects of flame stretch and fuel pyrolysis on the flame displacement speed. Based on the DNS findings, the accuracy of a hybrid G-equation/well-stirred reactor combustion model is assessed for PFS-assisted operation in the LES context. Implications of the DNS analysis for other advanced engine combustion concepts with low-carbon fuels, such as H₂, are further discussed.