Visualisation of blow-off events of two interacting turbulent premixed flames

The effect of flame spacing on the blow-off behaviour of two adjacent confined lean methane-air bluff body stabilized premixed flames with and without swirl has been examined. The measurements include blow-off limits, visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, and the estimation of the average duration of the blow-off event. It was found that correlations based on a Damk̈ohler number collapse the blow-off velocity data for confined premixed flames in both single and two injector configurations with different enclosure geometry, and conditions with and without swirl. The blow-off transient was found to be dependent on the injector spacing, the presence of swirl, and the swirl direction of each injector. For closely spaced injectors with swirl, blow-off begins with disintegration of the flame at the attachment point, and a lifted flame remains located near the downstream end of the enclosure. The reaction zone for the lifted flame disintegrates gradually, eventually leading to complete extinction of the flame at the downstream locations. For non-swirling flames and swirling flames with larger injector spacing, reaction is occurring in the wake region above the bluff body. Flame disintegration begins at the downstream locations, and the final flame fragments to survive are located close to the bluff body. The average duration of the blow-off event, Τext, was found to be strongly dependent on the blow-off behaviour. When normalised with the characteristic flow time at blow-off d/UBO (where d is the bluff body diameter and UBO is the bulk flow velocity past the bluff body at blow-off), values for the two adjacent flames without swirl are in the range 60-120. When swirl is introduced, values of Τext/(d/UBO) and are within the range 90 - 200. For both cases with and without swirl, Τext/(d/UBO) was generally found to be larger for blow-off of two adjacent flames than for single flames. The results can help further understanding of blow-off in practical applications and the development of advanced turbulent combustion models. © 2013 by James Kariuki