Experimental study of transition in dynamical states of thermo-acoustic oscillations in a turbulent bluff body combustor

This study investigates the shift in dynamical states of the thermo-acoustic oscillations for the turbulent syngas combustor having a bluff body for the flame anchoring. In this paper, an analysis was conducted to look into the effects of three different syngas compositions on the variation of Reynolds number (Re) in the range of 2289 to 8009. The analysis involved simultaneous, unsteady pressure measurement and OH* chemiluminescence. The investigation reveals that as the controlled parameters vary, the system exhibits a sequence of dynamic states characterized by distinct nonlinear oscillations. This study aims to explore the infrequently observed transitions from low-frequency instability (LFI) to high-frequency instability (HFI) by examining various time-series data and post-processing techniques. Additionally, it aims to understand how these transitions ultimately lead to the emergence of combustion noise as a result of a change in Reynolds number. To ascertain the characteristics of thermo-acoustic oscillations under investigation, a comprehensive analysis is conducted utilizing nonlinear time-series analysis techniques like phase portrait and recurrence plots. The investigation of flame behavior in response to changes in Reynolds number has been conducted using time-resolved OH* chemiluminescence. The results obtained from this study reveal distinct flame behavior patterns. The combustion instability of syngas at HFI is driven by flame modulated by small-scale structures and its anchoring in the shear layer of the bluff-body whereas the LFI is due to larger flame modulations near the wall of the combustion chamber. In addition, the recurrence analysis method is employed to monitor the progression of the dynamical states to understand the nature of the dynamical states. Such analysis will ultimately contribute to the establishment of a stable or nearly stable combustion system.