Shreenivasan O., Chakravarthy S., Dreizler A., Janicka J.

An experimental set-up of a half-dump combustor with optical access from all sides, has been developed in the present work, to investigate the transition from low-amplitude broadband noise generation to large-amplitude discrete tones due to combustion instability, as the flow conditions are gradually varied. The length of the combustor duct and the axial location of the step relative the acoustic mode excited in the duct are independently varied. Methane is used as fuel, and it is injected at the corner of the step tangential to the inlet air flow in the present work. The inlet Reynolds number is varied in the range of 6000-60000. The overall equivalence ratio is varied in the range of 0.03-0.13. High-amplitude discrete tones are excited under certain conditions when the observed dominant frequency jumps from that of a natural acoustic mode to a vortex shedding mode. This condition is denoted as the onset of combustion instability, and an experimentally determined stability map is obtained of the conditions of the onset of instability. High-speed imaging of the chemiluminescence fluctuations in the flame stabilization zone is performed at different conditions such as far away from onset of instability and after at the onset. Fourier transform of the time variation of the fluctuation in the total chemiluminescent intensity in the combustion zone is performed, and the resulting spectra is correlated with the acoustic spectra observed at different locations in the duct. It is observed that away from the onset conditions, the total chemiluminescence spectra primarily indicate the vortex shedding frequency as dominant, but exhibit dominant frequencies coinciding with those of the acoustic spectra under onset and maximum amplitude excitation conditions. The full paper will quantify this correlation as the above conditions are closely varied across the onset of instability in the stability map.