RANS-Based Numerical Assessment of Mixture Stratification, Flame Topology, and Relative Emissions Trends of Turbulent Flames Stabilized in a High-Velocity-Ratio Coaxial Burner

Abstract In this study, turbulent stratified propane–air flames, stabilized in a high-velocity-ratio (HVR) coaxial burner, are investigated numerically. Two reacting cases are considered, corresponding to different upstream fuel-injection locations and different radial equivalence-ratio distributions in the downstream region. Steady two-dimensional Reynolds-averaged Navier–Stokes (RANS) simulations are performed using the SST k - ω turbulence model, while two turbulence–chemistry interaction closures, namely, a Eulerian probability density function (PDF) approach and a flamelet-generated manifold (FGM) model, are assessed against available experimental data. Both closures reproduce the main reacting-flow features of the burner, including the initial centerline deceleration, flame-induced acceleration, and postflame reverse-flow region. However, the Eulerian PDF formulation provides a more consistent representation of the measured mean velocity field and is therefore used for the comparative assessment of the two experimentally investigated, stratified, cases. The results indicate that the steeper, center-rich stratification produces a more compact and strongly curved reaction layer, whereas the smoother stratification leads to a broader and flatter reacting region. Algebraically reconstructed OH * chemiluminescence fields and progress-variable-based flame-sensor distributions provide consistent qualitative markers of these topology changes, although the computed reaction layers remain thinner than the experimentally inferred turbulent flame brush. The postprocessed thermochemical fields further suggest that the smoother equivalence-ratio gradient, higher global equivalence ratio ( Φ global ), operating condition produces a wider high-temperature and CO 2 -rich product region, together with a more spatially extended NOx-forming zone, while the steeper equivalence-ratio gradient, lower- Φ global , condition is associated with more localized CO persistence. These trends are interpreted comparatively, given the absence of corresponding pollutant measurements and the limitations of the steady RANS framework. Overall, the results demonstrate that a carefully resolved RANS/PDF approach can provide useful engineering-level insight into the mean stabilization, flame-topology response and relative pollutant-formation trends in stratified HVR flames, supporting preliminary assessment of aerodynamically stabilized combustion concepts for cleaner energy applications.