Performance Assessment of Modeling Approaches for Moderate or Intense Low-Oxygen Dilution Combustion in a Scale-Bridging Burner

The present work provides deep insights into methane oxidation in a cyclonic flow field chamber whose strong internal recirculation of burnt products enables the attainment of a moderate or intense low-oxygen dilution (MILD) combustion regime over a wide range of operating conditions. Steady-state Favre-averaged Navier-Stokes (FANS) simulations are performed to evaluate the thermochemical processes taking place within the reactor and to assess the suitability of existing computational fluid dynamics (CFD) models to describe the turbulence-chemistry interactions in such a scale-bridging configuration. A sensitivity analysis is carried out with respect to different turbulence closure models [i.e., renormalization group (RNG) k-ϵ, realizable k-ϵ, and Reynolds stress model], kinetic mechanisms (i.e., KEE-58 and GRI-Mech 2.11), as well as different turbulent combustion approaches, i.e., the flamelet generated manifold (FGM), the eddy dissipation concept (EDC), and the partially stirred reactor (PaSR) models. The accuracy of numerical predictions is assessed by a direct comparison to obtained experimental results in terms of temperature profiles locally collected within the reactor and flue gas compositions monitored at the exit of the burner. Results highlight that the EDC and PaSR methods are the most suitable modeling paradigms for the investigated MILD combustion conditions, although the former may likely predict an extinction of the combustion process; conversely, the FGM model shows the largest discrepancies with respect to experimental data, highlighting the need for improvements.