Blowoff mechanism of two dimensional bluff-body stabilized turbulent premixed flames in a prototypical combustor

Near blowoff dynamics and characteristics of turbulent premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction were observed. It has been proposed that a partial or total extinction of the flame sheet along the shear layers is the major factor that determines the final blowoff event. Observation of flame kernels within the recirculation zone, which under stable conditions contain only combustion products, is further evidence of the shear layer extinction. For a stably burning planar V flame away from blowoff, the flame front envelopes the Kelvin Helmholtz vortices. Near blowoff, the two flame fronts become more aligned with the flow direction due to reduction in flame speed and interact with the vortices emanating from the shear layer. This overlap induces high local stretch rates that exceed the extinction stretch rates, resulting in local flame extinction along the shear layers. Following extinction, fresh reactants are entrained into and react within the recirculation zone, with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for time scales of about one hundred milliseconds, potentially reigniting the shear layers such that the entire flame is re-established for a short period. This shear layer extinction and re-ignition event can happen several times before final blowoff, which occurs when the flame kernel fails to reignite the shear layers before being extinguished itself, thus leading to global flame extinction. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.