Time-resolved study of mixing and reaction in an aero-engine model combustor at increased pressure

In this study, we experimentally investigate large-scale vortex structures, fuel-air mixing and reaction processes, which occur in a partially premixed swirl-stabilized ethylene/air flame in a laboratory-scale gas turbine model combustor. Time-resolved stereo-PIV, OH-PLIF, and acetone/PAH-PLIF systems with a repetition rate of 10 kHz are used to investigate the flame behavior at 3 bar pressure and an equivalence ratio of Phi=1.2. For isothermal conditions, the results strongly indicate that vortex-induced roll-up of fuel is a main driver of the mixing process. For the reacting case, a precessing vortex core (PVC) and a double helical vortex (DHV), both co-rotating with the swirl direction and occurring simultaneously in the inner and outer shear layer, respectively, are identified by using proper orthogonal decomposition (POD) and phase-averaging techniques. Acetone-PLIF measurements show that the fuel exhibits a motion that resembles a strong axial flapping at the frequency of the DHV at reacting conditions when considering the measurement plane. The measurements show that the PVC and DHV cause a regular sequence of flame roll-up, mixture of burned and unburned gases, and the subsequent ignition of this mixture. The pockets of rich burned gas formed by mixing and reaction can eventually be oxidized by OH-rich lean burned gas regions that have the potential to prevent soot formation; however, this process strongly depends on intermittent motion within the inner recirculation zone. Based on the additional visualization of PAH-PLIF distributions, the present study further reveals for the first time the complete sequence of formation of rich burned gas, PAH and soot in a GT combustor. It is shown that soot forms only in certain regions of the high-PAH rich burned gas pockets. This indicates that other factors, such as the detailed composition of PAHs or local temperatures, which cannot be resolved here, likely also play an important role in soot formation. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.