Impact of Hydrogen Addition on the Thermoacoustic Instability and Precessing Vortex Core Dynamics in a CH₄/H₂/Air Technically Premixed Combustor

We study the impact of $H2$ enrichment on the unsteady flow dynamics and thermoacoustic instability in the prediction and control of instabilities in industrial turbines (PRECCINSTA) swirl combustor. The experiments were performed at atmospheric conditions with $H2/CH4$ fuel mixtures at a global equivalence ratio of 0.65 and a constant thermal power of 20 kW. We analyze data with three fuel compositions: $0%, 20%$⁠, and $50% H2$ in two operating modes, premixed (PM) and technically premixed (TPM). A new multiresolution modal decomposition method, using a combination of wavelet transforms and proper orthogonal decomposition (WPOD) is performed on time resolved flow velocity and OH planar laser induced fluorescence (OH planar laser induced fluorescence (OH-PLIF)) measurements. Thermoacoustic oscillations are observed in the TPM operating mode alone, indicating that the primary heat release driving mechanism is due to fuel-air ratio oscillations. WPOD results for the $0% H2$ TPM case reveal intermittent helical precessing vortex core (PVC) oscillations along with axisymmetric hydrodynamic flow oscillations due to the thermoacoustic oscillations. These oscillations cause local flame extinction near the nozzle centerbody resulting in liftoff. A PVC then develops in the flow and enables intermittent flame reattachment. In the 0% $H2$ premixed case, the flame remains lifted off the centerbody despite the presence of PVC oscillations. $H2$ enrichment results in the suppression of flame liftoff and the PVC in both operating modes. We show from flow strain rate statistics and extinction strain rate calculations that the increase of the latter with $H2$ addition, allows the flame to stabilize in the region near the centerbody where the pure CH₄ cases show lift off.