Dynamic flame structure and Thermoacoustic instability in a hydrogen-kerosene dual-fuel afterburner under upstream flame excitation

This study investigates forced combustion oscillations in a hydrogen-kerosene dual-fuel afterburner designed for pre-cooled/intercooled aeroengines. A methane combustor serves simultaneously as a preheater and an upstream excitation source to generate low-frequency thermoacoustic oscillations. Experiments are conducted on a rectangular afterburner model featuring a pilot-stage kerosene V-gutter flame holder and main-stage hydrogen transverse jets from radial/circumferential V-gutters. Three operating modes including pilot-stage kerosene only, hydrogen only, and full dual-fuel are analyzed under various equivalence ratios. High-speed OH chemiluminescence, dynamic pressure measurements, proper orthogonal decomposition (POD), and spectral proper orthogonal decomposition (SPOD) are employed to characterize flame dynamics and thermoacoustic instability. A low-order thermoacoustic model is developed and validated using the open-source OSCILOS toolbox. Results show that the upstream methane flame dominates combustion oscillations, with net effective contribution exceeding that of the afterburner flame by a factor higher than 5. Variations in flame structures, and hydrogen flame thickness and length are identified for different operating modes. The pilot-stage kerosene flame exhibits better spatial coherence and suppresses second-harmonic oscillations, while hydrogen flames display more dispersed energy. Limit-cycle oscillations occur when sufficient heat release is present. These findings provide insights into dual-fuel afterburner operation under forced low-frequency oscillations.