Abstract
We investigate the effect of hydrogen enrichment on the non-linear response and saturation of premixed methane/hydrogen flames submitted to acoustic forcing. The thermal power is kept fixed to preserve the nozzle velocity whilst increasing the flame speed through hydrogen enrichment. The flame describing function is measured for a fixed frequency and three hydrogen-methane blends ranging from 10% to 50% by power. When the flame is forced at the same frequency at similar power and bulk velocities, hydrogen enrichment increases the saturation amplitude of the flame. To provide insight into the flame dynamics, the flames were investigated using high-speed imaging. At lower hydrogen concentrations, the flame is stabilised along the inner shear layer and saturation in the heat release rate occurs at lower forcing amplitudes due to large-scale flame-vortex interactions causing flame annihilation. At increased levels of hydrogen enrichment, distinctly different flame dynamics are observed. In these cases, the flame accelerates and propagates across to the outer shear layer which acts to suppress large-scale flame annihilation during roll-up of both the inner and outer shear layers. This results in a coherent increase in flame surface area with forcing amplitudes significantly increasing the saturation amplitude of the flame. These results show that high levels of hydrogen increase the amplitude response to acoustic forcing leading to higher saturation amplitudes. This suggests that substituting natural gas with hydrogen in Gas Turbines increases the risk of much higher limit-cycle amplitudes if self-excited instabilities occur.