Detached Eddy Simulation of Partially Premixed Unstable Flames

Abstract

Fuel-lean combustion is an area of significant interest in the domain of numerical combustion modelling as it results in the lowering of the emissions of pollutants like CO and NOx from the combustion chamber. However, the application of fuel-lean combustion also leads to flame blowout, which is why flame stabilization mechanisms like the imparting of swirling motion to the reactants are important to bring the instabilities resulting from lean combustion under control. This work presents the numerical simulation of a benchmark experimental analysis of lean combustion in a dual swirl partially premixed combustor using the Flamelet/Progress Variable (FPV) and Eddy Dissipation Concept (EDC) combustion modelling techniques along with the Detached Eddy Simulations turbulence modelling approach. Performance of each model is analyzed based on its accuracy in capturing time-averaged parameters like axial, radial and swirl velocities, temperatures, mixture fractions and species mass fractions, along with important flow features like the development of recirculation zones. The numerical simulations were set up and executed in the ANSYS Fluent CFD solver, and the reaction kinetics for the Eddy Dissipation Concept were modelled using the GRI Mech 2.11 chemical kinetic mechanism. The FPV approach achieved a considerably higher level of accuracy in capturing the thermochemical parameters compared to EDC, which significantly overpredicted the fuel consumption rate and time-averaged temperatures, while also predicting an unconventional inner recirculation zone profile. Following the validation analysis, a parametric analysis was performed using three combustor configurations featuring distinct inner swirler angles of 60, 62, and 64 degrees respectively. The major differences between the predicted flow features of the three geometries included a broadened inner recirculation zone for the 62 degrees configuration and a considerably narrow inner recirculation zone for the 64 degrees configuration, which also predicted a highly pronounced outer recirculation zone where mixing and reactions dominated as opposed to the inner recirculation zone.