Spark ignition of single bluff-body premixed flames and annular combustors

The ignition behaviour of single and multiple premixed burners was investigated exper- imentally. For the single-burner experiments, the burner used a conical bluff body in a larger pipe open to the atmosphere. Visualization of the flame with a 5kHz OH* chemi- luminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work in non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability, Pign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high Pign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. The annular combustion chamber consisted of 18 bluff-body premixed burners each fitted with a swirler. The chemi- luminescence films showed that propagation of the flame from burner to burner usually did not follow a purely azimuthal direction, but rather a "sawtooth" pattern with a down- stream and sideways motion from one burner, bringing the flame to the downstream part of the recirculation zone of the adjacent burner that then convected it upstream to fully ignite this adjacent burner. This pattern was more pronounced at high velocities. With an increase in velocity, the possibility of failed ignitions increased and the time taken for the overall combustor to ignite also increased. The results can be used for validation of numerical models of transient combustion processes and offer insights into the ignition of realistic devices. © 2013 by E. Mastorakos