Thermoacoustic feedback mechanisms for premixed burners with a reacting hydrogen-methane jet in cross-flow

Understanding the interaction mechanisms between acoustic pressure pulsations and heat release of the flames is essential for preventing thermoacoustic instabilities in gas turbines. This study focuses on an industrial burner design under high-pressure operating conditions with a first and a second stage, the latter realized as a reacting jet in cross-flow. The premixed hydrogen-methane-air flames cause high-frequency instabilities of several thousand Hertz. The hydrodynamic structure of the reacting jet opens up new pathways for establishing feedback loops for instabilities. Large eddy simulations are performed for a technically premixed and a perfectly premixed second stage to evaluate the role of equivalence ratio fluctuations. The technically premixed simulation demonstrates that the displacement of the flame front orthogonal to the jet axis, caused by the acoustic velocity of a longitudinal mode, can significantly affect the flame surface, producing delayed fluctuations in heat release that can amplify pressure oscillations in accordance with the Rayleigh criterion. In the case of a perfectly premixed axial stage, the system exhibits a rotating transverse-longitudinal mixed mode. This mode modulates at much higher frequency and is accompanied by strong shear layer activity. Phasor diagrams demonstrate that the instability is driven by the compression of the cold gases. Surprisingly, flame surface modulations are out of phase with heat release fluctuations. Those results demonstrate two distinct feedback mechanisms for high-frequency instabilities in staged combustion. Novelty and significance statement For the first time, this study investigates a technically and perfectly premixed reacting jet in cross-flow with hydrogen addition under self-excited thermoacoustic instabilities. A technically relevant configuration at elevated pressure (7 bar) is considered in numerical simulations as well as experiments. Due to the asymmetry of the jet trajectory, the jet can respond to orthogonal velocity perturbations relative to the jet axis at the same frequency. The mechanism triggered by these orthogonal velocity fluctuations amplifies in regions where the acoustic velocity is ahead of the pressure and thus depends on the chamber impedance. This criterion must be considered when placing the second stage. Furthermore, we show that in the absence of equivalence perturbations, the reacting transverse jet strongly responds to the density fluctuation of the transverse mode. Thus, the heat release perturbations are inevitably in phase with the pressure fluctuations.