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

摘要

In this study, we experimentally investigate large-scale vortex structures, fuel-air mixing and reactionprocesses, which occur in a partially premixed swirl-stabilized ethylene/air flame in a laboratory-scalegas turbine model combustor. Time-resolved stereo-PIV, OH-PLIF, and acetone/PAH-PLIF systems with arepetition rate of 10 kHz are used to investigate the flame behavior at 3 bar pressure and an equivalenceratio of Φ= 1.2. For isothermal conditions, the results strongly indicate that vortex-induced roll-up offuel is a main driver of the mixing process. For the reacting case, a precessing vortex core (PVC) and adouble helical vortex (DHV), both co-rotating with the swirl direction and occurring simultaneously in theinner 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 thatresembles a strong axial flapping at the frequency of the DHV at reacting conditions when considering themeasurement plane. The measurements show that the PVC and DHV cause a regular sequence of flameroll-up, mixture of burned and unburned gases, and the subsequent ignition of this mixture. The pocketsof rich burned gas formed by mixing and reaction can eventually be oxidized by OH-rich lean burnedgas regions that have the potential to prevent soot formation; however, this process strongly depends onintermittent motion within the inner recirculation zone. Based on the additional visualization of PAH-PLIFdistributions, the present study further reveals for the first time the complete sequence of formation ofrich burned gas, PAH and soot in a GT combustor. It is shown that soot forms only in certain regions ofthe high-PAH rich burned gas pockets. This indicates that other factors, such as the detailed compositionof PAHs or local temperatures, which cannot be resolved here, likely also play an important role in sootformation.