7Flashback of an open turbulent, premixed flame in a swirl burner with central bluff-body is considered. The aim 8 is to obtain further understanding of the physical mechanisms responsible for the upstream flame propagation. 9Previous studies on the same configuration hypothesised that there is an adverse pressure gradient in the 10 direction of flame propagation. In this paper this is further investigated experimentally and theoretically. Static

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In this paper it is shown that prior to flashback, small dynamical changes appear in the system. These changes appear as a drift in the recurrence plots and are found to be associated with a gradual increase in the determinism and recurrence rate. Thus, this study indicates that precursors to flame flashback exist and can be detected in the multidimensional phase space reconstructed from pressure measurements acquired during flashback. This observation could have broad academic as well as industrial implications.PACS numbers: 05.45. Tp, 47.70.Pq Flame flashback is a complex phenomenon resulting from interactions involving multiple physical processes, each governed by its own inherent temporal and spatial scale. In addition, flashback is also a transient event, with the actual event lasting only fractions of a second. Investigation of flashback requires multiple, simultaneous diagnostics without prior knowledge of the relevant time and length scales of the physical processes involved. Detection, accordingly, deals with postevent characterization. In this paper, an attempt to detect subtle dynamics prior to flashbackwhich linear, time and frequency-domain methods cannot reveal-is made through recurrence analysis of the multidimensional phase space reconstruction from pressure time traces. Recurrence analysis is superior to the common techniques (e.g. spectra) used for the analysis of short, noisy datasets for the purpose of identifying dynamical changes. Following conventional recurrence analysis methods, different sections of pressure time traces from reacting flow conditions are analyzed. Recurrence plots and recurrence quantification measures immediately before flashback are compared to those long before flashback during the normal operation of the combustor. A comparison between recurrence characteristics at isothermal flow conditions and at reacting flow conditions is included to a) highlight the dynamics of the thermoacoustically unstable condition at which flashback occurs and b) to establish the limits of fluctuations in recurrence quantification analysis measures. It is observed that small drifts exist in the dynamical behavior of the system before flashback and are made apparent by recurrence methods. It is found that the changes span 2-4 seconds prior to flashback. Within this time a) Electronic address: l.christodoulou.1@research.gla.ac.uk b) Electronic address: lipika.kabiraj@tu-berlin.de frame, determinism and recurrence rate increase gradually as flashback is approached.