Visualization of blow-off events in bluff-body stabilized turbulent premixed flames

Abstract

Confined and unconfined turbulent methane-air lean premixed flames stabilized on an axisymmetric bluff-body of diameter d=25 mm have been examined close to the blow-off limit and during the extinction transient, with OH∗ chemiluminescence, flame tomography and OH-PLIF operated at 5kHz, allowing a quantification of the duration of the blow-off event. Blow-off was approached by increasing the bulk velocity Ub or decreasing the equivalence ratio and the flame shape changed from conical to cylindrical with decreasing length. Close to blow-off, the flame closed on the axis and was about 2d long, and just before the blow-off condition it took an “M” shape with reaction fronts inside the recirculation zone (RZ). During the blow-off event, fresh reactants entered the RZ from the forward stagnation region and significant fragmentation of the flame occurred, with branches of the flame remaining anchored on the bluff-body edge and separate flame pockets moving randomly inside the RZ. Overall blow-off occurred with the gradual elimination of these flame fragments. The integrated OH∗ emission decreased slowly to zero as the flame surface decreased over a period of about 15d/Ub. The results suggest that the blow-off event in recirculating flames lasts long compared to the residence time in the RZ and the structure of the flame close to extinction supports the underlying assumptions behind well-stirred reactor concepts of blow-off.