Effects of hydrogen on the thermo-acoustics coupling mechanisms of low-swirl injector flames in a model gas turbine combustor

Abstract The effects of hydrogen on the dynamic response of self-excited oscillations in a model low-swirl injector (LSI) gas turbine combustor have been studied by high-speed imaging of flame light emissions and OH ∗ chemiluminescence on two thermoacoustically coupled unsteady flames burning CH 4 (113 kW) and a fuel blend of 0.9H 2 /0.1CH 4 (74 kW). The two flames have different shapes. The CH 4 flame is lifted and bowl-shaped and the 0.9H 2 /0.1CH4 flame is attached and M-shaped. From phase-resolved PIV, the dominant unsteady flow structures of both flames are ring vortices shed from the LSI exit rim. The vortices of the CH 4 flame burn intermittently and those in the 0.9H 2 /0.1CH 4 flame burn consistently. Proper orthogonal decomposition analysis of OH ∗ and calculation of Rayleigh indices show different flame structures are responsible for generating flame oscillations and acoustic coupling. CH 4 flame oscillations are associated with flame folding near the standing ring vortex below the trailing edge of the lifted flame, intermittent burning of the shear layer, and irregular acoustic driving. The 0.9H 2 /0.1CH 4 flame has regular and stronger forcing. A key contributor to the 0.9H 2 /0.1CH 4 flame driving is merging of the central flat flame with the reacting rolled-up vortices, resulting in an instantaneous narrow band of localized high heat release density that is in phase with the pressure oscillation. Cyclic variations in the flow of the reactants also contribute to non-linear effects. High reactivity of H 2 generates a situation where the geometry of the attached shear layer flame and central disk-shaped flame creates a very favorable situation for exciting higher-pressure oscillations at a lower total heat release than CH 4 flames. To control combustion oscillations in a fuel-flexible LSI gas turbine combustor, the mitigation of flame attachment and/or formation of the outer shear layer would be necessary.