Swirl rotation direction effects on lean direct injection pilot mixer performance: Experiments and LES modeling

In order to improve the understanding of the effects of various practical design parameters on the performance of Lean Direct Injection (LDI)-based combustors, the impact of varying the relative rotation direction of the outer air swirler (OAS) and inner air swirler (IAS) on the performance of a methane-fueled LDI-type mixer/injector is investigated under non-reacting and reacting conditions by both experiments and computational fluid dynamics (CFD) simulations. Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) models are used and compared in CFD modeling of counter- and co-swirling LDI mixers, while the adoption of a nonpremixed Flamelet Generated Manifold (FGM) method to predict reacting flows is investigated for comparison with experimental measurements of the mean and fluctuating velocity fields and OH* chemiluminescence. In addition, a self-consistent and robust CFD mesh generation strategy is developed to create notionally grid-independent CFD meshes for non-reacting and reacting flow simulations. The experimental non-reacting flow fields and flame lift-off limits demonstrate that while the basic structure of the flow field does not change by altering the relative rotation direction of IAS and OAS, the details of the center recirculation zone including its width and length do, resulting in slightly reduced flame lift-off equivalence ratio for the counter-rotating configuration. Although LES-FGM is able to capture the major flow and flame patterns in both counter- and co-swirling reacting flows, some substantial disparities are apparent, particularly in the region within/near the venturi that adversely impact the predictive abilities of this LES-FGM approach.