Understanding blowoff dynamics of bluff body stabilized turbulent flames in a prototypical combustor

Near blowoff dynamics of turbulent premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using simultaneous PIV and OH PLIF diagnostics. It has been proposed that a partial or total extinction of flame within the shear layers is the major factor that influences the final blowoff event. Observation of flame kernels within the recirculation zone, which under normal burning conditions contain only combustion products, is further evidence of the shear layer extinction. For understanding blowoff behavior comparisons were made between normal flames and flames near blowoff. 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 bluff-body. This overlap induces high local stretch rates that exceed the extinction stretch rates, resulting in local flame extinction along the shear layers. Following extinction along the shear layers, fresh reactants reach into the recirculation zone and react there, 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 reestablished for a short period. This shear layer extinction and reignition 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.