Statistics and Dynamics of Intermittent Boundary Layer Flashback in Swirl Flames

Abstract

Flame behavior at conditions approaching boundary layer flashback was studied in fuel-lean premixed swirl flames with a central bluff-body flame holder. High-speed chemiluminescence images were collected at over 450 different combinations of fuel composition (hydrogen/methane blends), equivalence ratio, flow rate, and reactant temperature. A selected group of conditions was further examined using simultaneous high-speed stereoscopic particle image velocimetry and OH planar laser induced fluorescence. Over a range of conditions between stable burning in the combustor and total flashback, the flame would intermittently propagate through the bluff-body boundary layer into the reactant feed tube for a period of time before retreating back to the combustion chamber. Statistical characteristics of the flame dynamics, such as the depth and duration of the flashback events, showed consistent relationships across all combinations of operating parameters. Hence, tracking these statistics provides a potential means of anticipating an upcoming flashback event. The number of transient flashback events per second showed particular promise as an early warning sign due its rather gradual change with flame propagation depth. Laser diagnostics revealed local reductions in axial velocity ahead of the tip of the flame protrusions. The strength of these reductions increased as the flame moved farther upstream; however, no total flow reversal was observed.