Investigation of Oscillatory States Involving Acoustic Mode Shifts in a Turbulent Syngas Combustion using Non-stationary Time-series Analysis

Abstract

The transition of thermo-acoustic oscillations in a turbulent bluff-body syngas combustor is analyzed experimentally in the present work. The analysis was carried out for three syngas compositions using simultaneous, unsteady pressure measurement and OH* chemiluminescence across the variation of Reynolds number, Re, over 2289–8009 range. It is observed that across the variation of controlled parameters, the system undergoes series of dynamical states having different nonlinear oscillations. In this paper, we investigate the seldom observed transitions from low-frequency instability to high-frequency instability through two-scale oscillations, and finally, it attains the combustion noise state in response to the variation in Re. To identify the nature of these thermo-acoustic oscillations, time-series analysis based on wavelet transformation, phase portrait, and a novel wavelet-based measure is performed. Based on the results of time-resolved OH* chemiluminescence, the distinct flame behavior is observed in response to the change in Re. It is observed that syngas combustion instability is driven by small-scale structures due to flame stabilization and modulation in the shear layer. Further, the wavelet-based analysis also tracks the evolution of the dynamical state by quantifying the stable state and other non-stable states to be composed of continuously varying phase shifts resulting in stable/quasi-stable combustion.