Excitation of high frequency thermoacoustic oscillations by syngas in a non-premixed bluff body combustor

A laboratory-scale bluff body combustor is mapped for its stability and flame dynamics of non-premixed flames with three fuels, namely, pure H-2, H-2-CH4 mixture, and H-2-CO mixture, the last one representing syngas. Unsteady pressure measurements and high-speed OH* and CH*/CO2* chemiluminescence imaging are simultaneously performed. The combustor behaviour with syngas is markedly different than the other two in exciting high frequency oscillations, typically at the third harmonic longitudinal acoustic mode of the duct at high air flow Reynolds numbers (Re). In contrast, the H-2-CH4 excites only the fundamental longitudinal mode, and pure H-2 excites up to the first harmonic. The latter two are observed to lock on to the shear layer mode of the bluff body wake, whereas the H-2-CO case locks on to thrice the Strouhal number associated with the shear layer mode, commensurate with the excitation of the third harmonic natural acoustic mode. Time-averaged OH* and CO2* chemiluminescence images show large-scale structure for the H-2-CH4 case compared to heat release rate zones aligned with the shear layer in the pure H-2 and H-2-CO cases. Cross-sectionally averaged chemiluminescence profiles exhibit a streamwise stagger in the peaks of OH* and CO2*, suggesting two heat release rate zones, that could excite the acoustic oscillations. The instantaneous profiles indicate a convective delay between the two heat release rate zones that is close to the third harmonic acoustic time scale. The sequential of H-2 oxidation to OH followed by CO oxidation by OH to form CO2 are considered to be responsible for the high frequency excitation in the case with the H-2-CO mixture when compared to the other two cases. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.