Structure of High-Pressure Premixed Ammonia, Hydrogen, and Methane Flames With 100 kHz OH-PLIF Measurements

Abstract The impact of fuel composition on flame dynamics is investigated at 10 bar using 100 kHz hydroxyl planar laser induced fluorescence (OH-PLIF) measurements. The PLIF images are synchronized with high-frequency pressure measurements at distinct locations in the chamber to investigate mechanisms for thermoacoustic feedback with the flame heat release and resonant acoustic modes of the combustor. Experiments were conducted over a range of fuel compositions spanning a hydrogen fuel fraction (χ) of 30% to 90%, and an ammonia decomposition efficiency (η) from 40 to 100%. In predominantly hydrogen-rich fuel mixtures (χ, η > 0.8), flame stabilization occurs along the shear layer, and flame holding is observed closer to the burner face. As methane composition increases (χ < 0.8), significant flame corrugation and commensurate high heat release rate is observed as chemical kinetic rates and thermo-diffusive effects decrease. In these cases, OH radical production and transport between neighboring injector elements is observed. The flame heat release and transport of combustion products in this region occurs during the compression phase of the acoustic cycle and is responsible for ignition of fresh reactants. This periodic ignition, heat release and flame displacement at the resonant acoustic mode frequency of the combustor results in self-excited longitudinal combustion instabilities (p′RMS/pCC ∼ 1–7%). With significant concentrations of methane and ammonia in the fuel blend (χ < 0.9, η < 0.6), the amplitude of this instability increases (p′RMS/pCC > 7%) and periodic global extinction and reignition of flames is observed with acoustic expansion and compression.