Large Eddy Simulation of turbulent flames in a Trapped Vortex Combustor (TVC) – A flamelet presumed-pdf closure preserving laminar flame speed

Flow and flame dynamics inside a trapped vortex combustor are analyzed from Large Eddy Simulation (LES) results compared against measurements. The Navier–Stokes equations are solved in their fully compressible form over a Cartesian grid resorting to immersed boundaries to account for the complex geometry, composed of an annular flow impacting a set of axisymmetric rods (flame holders) before interacting with a cavity. Various cases are considered, varying the main flow rate, the length of the cavity, injecting secondary-air and also adding a swirling motion. From these cases, three main cavity flow regimes emerge. The modeling of molecular diffusion in LES with presumed probability density function (pdf), as filter of premixed flamelets, is also discussed. It is shown that a dynamic correction to molecular diffusion may be computed from the pdf control parameters to ensure the correct laminar flame speed, whatever the mesh used. Finally, studying the turbulent flame evolution within the cavity in the various cases, suggests that swirling motion is mandatory to favor the global burner stability.