Experimental Analysis of Transition to Higher Acoustic Mode in Syngas Combustion Dynamics

A turbulent syngas combustor is experimentally characterized for its transitionary stability characteristics by continuously varying Reynolds number (Re). The combustor transits from low-to high-frequency instability at lower and higher Re, respectively, through the stable regime. Time-series analysis based on short-time Fourier transform (STFT) and a novel wavelet-based measure are performed to analyze unsteady pressure data. This reveals the presence of higher harmonics or over-riding waves at low-frequency instability, followed by the presence of subdued higher frequency content in the stable zone. In the particular study, the stable zone is thus fundamentally different from the definition of stable combustion as noise. The OH* chemiluminescence analysis revealed the absence of large-scale structures in sustaining both types of combustion instability, with the shear layer being the flame stabilization and modulating region. The low-and high-frequency instability difference is analyzed based on the flame images across a pressure cycle and reveals distinct patterns across different Re. The nature of frequency shifts in the present work is explored in light of a reduced-order model, whereby it is seen that the frequency shifts result from the stability of the accompanying modes at the various operating conditions.