Experimental study of flame-hole reignition mechanisms in a turbulent non-premixed jet flame using sustained multi-kHz PIV and crossed-plane OH PLIF

The dynamics of flame-hole reignition were studied experimentally in a turbulent non-premixed CH4/H2/N2 jet flame at Red=22,800 (flame ‘DLR-B’ from the TNF workshop). Simultaneous measurements of the OH combustion radical and velocity field were performed using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) at a sustained rate of 10kHz. The dynamics of the reignition process were tracked through time and two reignition mechanisms were identified. Particular care was taken to reduce the influence of out-of-plane motion on the analyzed events by simultaneously measuring the OH distribution in crossed planes. Flame-holes reignited due to both edge-flame propagation and turbulent transport of burning flame segments. However, the edge-flame propagation mechanism was dominant and accounted for over 90% of the flame-hole reignition rate on average. Furthermore, the presence of large scale turbulent structures adjacent to a flame-hole did not necessarily result in reignition due to turbulent transport. Instead, the edge-flames propagated around the perimeter of such structures, indicating intervening regions of well mixed gas. The range of measured edge-flame propagation speeds agreed well that of highly-preheated premixed flames, with a mode of approximately 4m/s and a mean of approximately 7m/s.