The impact of a surrounding co-flow on the thermoacoustic stability of a premixed CH4/H2 flame

Abstract The present study focuses on a radially staged injector, with a bluff-body stabilized inner pilot stage, and a trapped-vortex stabilized outer main stage. The influence of a non-reacting main stage flow (pure air) on the thermoacoustic stability of the premixed methane/hydrogen pilot flame was investigated for a fixed pilot flame equivalence ratio and bulk inlet velocity. High speed pressure, PMT, and high-speed imaging measurements were performed to characterize the pilot flame response. Thermoacoustic instabilities were characterized over a wide range of main stage flow velocities and pilot flame hydrogen power fractions. During stable operation, the flame structure and length changed with the addition of hydrogen and main stage air flow rate. At hydrogen power fractions beyond 0.4, large amplitude thermoacoustic instabilities were observed. Increasing the main stage air flow rate was observed to first increase and then decrease the amplitude of these instabilities, due to changes in bulk convective time delays, the phase of the heat release rate oscillations, their frequency, and the mode shape within the combustor. The additional main flow air surrounding the pilot flame was observed to change the unsteady flame structure, which also affects the global response. These observations illustrate how hydrogen addition and the main stage flow alter the pilot flame response, which may be useful for understanding stability in radially staged configurations.