Learning and Characterizing Chaotic Attractors of a Lean Pre-Mixed Combustor

This paper is about the characteristics of, and a method to recognize the onset of limit cycle thermoacoustic oscillations in a gas turbine like combustor with a premixed turbu-lent methane/air flame. Information on the measured time series data of the pressure and the OH* chemiluminescence is acquired and postprocessed. This is done for a com-bustor with variation of two parameters: fuel/air equivalence ratio and combustor length. It is of prime importance to acknowledge the nonlinear dynamics nature of these instabilities. A method is studied to interpret thermoacoustic instability phenom-ena and assess quantitively the transition of the combustor, from a stable to an unstable regime. In this method, three-phase portraits are created on basis of data retrieved from the measured acoustics and flame intensity in the laboratory scale test combustor. In the path to limit cycle oscillation the random distribution in the three-phase portrait con-tracts to an attractor. The phase portraits obtained when changing operating conditions, moving from the stable to the unstable regime and back are analyzed. Subsequently the attractor dimension is determined for quantitative analysis. On basis of the trajecto-ries from the stable to unstable and back in one run, a study is done of the hysteresis dynamics in bifurcation diagrams. Finally, the onset of the instability is demonstrated to be recognized by the 0-1 criterium for chaos. The method was developed and demonstrated on a low power atmospheric methane combustor with the aim to apply it subsequently on a high-power pressurized Diesel combustor.