Observations on the Role of Auto-Ignition in Flame Stabilization in Turbulent Non-Premixed Jet Flames in Vitiated Coflow

Abstract

Turbulent combustion of non-premixed jets issuing into a vitiated coflow is studied at coflow temperatures that do not significantly exceed the fuel auto-ignition temperatures, with the objective of observing the global features of lifted flames in this operating temperature regime and the role played by auto-ignition in flame stabilization. Three distinct modes of flame base motions are identified, which include a fluctuating lifted flame base (mode A), avalanche downstream motion of the flame base (mode B), and the formation and propagation of auto-ignition kernels (mode C). Reducing the confinement length of the hot coflow serves to highlight the role of auto-ignition in flame stabilization when the flame is subjected to destabilization by ambient air entrainment. The influence of auto-ignition is further assessed by computing ignition delay times for homogeneous CH4/air mixtures using chemical kinetic simulations and comparing them against the flow transit time corresponding to mean flame liftoff height of the bulk flame base. It is inferred from these studies that while auto-ignition is an active flame stabilization mechanism in this regime, the effect of turbulence may be crucial in determining the importance of auto-ignition toward stabilizing the flame at the conditions studied. An experimental investigation of auto-ignition characteristics at various jet Reynolds numbers reveals that turbulence appears to have a suppressing effect on the active role of auto-ignition in flame stabilization.