Effect of syngas composition on high frequency combustion instability in a non-premixed turbulent combustor

An experimental investigation on the effect of syngas composition (H 2 /CO) on thermodynamic instability of a bluff-body combustor is studied. Three syngas compositions, namely, 25%H 2 –75%CO, 50%H 2 –50%CO and 75%H 2 –25%CO, and pure hydrogen and 75%H 2 –25%CH 4 are acoustically characterized by varying the inlet Reynolds number, Re, in the range of 2200–8100. The combustor is observed to undergo two modes of thermo-acoustic instability across the above Re range for all syngas compositions and pure hydrogen, whereas for the H 2 –CH 4 composition, the instability manifests at a single frequency ∼130 Hz. For all the syngas compositions, the pressure oscillations exhibit low frequency (130 Hz) at low Re, and bifurcate to higher frequency (∼500 Hz) at mid-range Re values, while pure hydrogen exhibits bifurcation from the low frequency to the first harmonic (∼260 Hz). The high frequency observed with the syngas compositions is at the third harmonic natural acoustic mode of the combustor and matches thrice the Strouhal frequency of the hydrodynamic oscillations of the shear layer separating from the bluff body. Further investigation into the nature of the dynamic behaviour across the listed fuels is performed using simultaneous time resolved imaging of OH* and CO 2 * chemiluminescence and two-component particle image velocimetry. The peak in the CO 2 * chemiluminescent intensity is found to be axially staggered downstream of that of OH*. This is due to sequential oxidation of H 2 and CO, the former producing OH as an intermediate consumed by the latter. Spatio-temporal analysis of the time-resolved data reveals that the presence of the flame in the shear layer of the bluff-body plays a significant role in modulating the high frequency acoustic excitation. Further, a spatial Rayleigh map is derived based on both the OH* and CO 2 * chemiluminescence, which reveals the bluff-body shear layer dynamics to yield the key driving and damping regions in the present study.