LAUSR

Abstract The heat release distribution, combustion instability characteristics and flow dynamics of lean swirl-stabilized flames of hydrogen enriched natural gas were studied using time-resolved (10 kHz) stereo PIV, OH* chemiluminescence imaging and acoustic pressure measurements. The technically premixed gas turbine model combustor was operated at elevated pressure up to 5 bar with preheated air. The H2 vol. fraction in the fuel was varied up to 50%. An M-shaped aerodynamically stabilized flame persisted at 1 bar up to low H2 enrichment ratios. Increasing H2 enrichment caused the M-flame to first transition to a bistable flame then to a shear layer stabilized V-flame. Increasing pressure and H2 enrichment both decreased the flame length. The combustion instability frequency and thermoacoustic amplitude varied and were mapped across operating conditions. The effect of H2 enrichment on the thermoacoustic amplitude was explained by examining the convective coupling between the Helmholtz acoustics and heat release. H2 enrichment was shown to increase the phase delay between the pressure and heat release by decreasing the pressure wave travel times in the plenum and swirler sections of the burner and by decreasing the flame length while simultaneously increasing the pressure wave speed within the burner. Thus H2 enrichment increased the thermoacoustic amplitude when the phase delay at the initial condition was negative, and decreased it when the initial phase delay was positive. Flow dynamics were studied by characterizing the precessing vortex core frequency and energy. The PVC structure existed across all conditions, and its frequency reduced to a different Strouhal number for the M-flame and V-flame. H2 enrichment consistently slightly decreased the PVC energy.