Flame and instability characteristics of high hydrogen content gas mixtures

In this study, initially, 50%H2–25%CH4–25%CO2, 60%H2–20%CH4–20%CO2 and 70%H2–15%CH4–15%CO2 mixtures were combusted in a laboratory scale-swirl stabilized-premixed combustor so as to specify combustion and emission characteristics of respective gas mixtures. Later on, such mixture flames were exposed to acoustic perturbations at different frequencies between the values of 65–205 Hz and under these conditions, flame response was evaluated. The acoustic perturbations were intentionally created by loudspeakers located in side arms of the T-shaped combustor. Two openings on the combustor walls, and photodiodes and pressure sensors assembled in the experimental apparatus enabled interpreting flame response. During experiments, inlet parameters (such as thermal power – 4 kW, equivalence ratio – 0.5, mixture temperature, etc.) and geometric swirl number (1.2) were kept constant, and tested flames were acoustically forced for 30 s at each frequency step. Results showed that thermal Zeldovich mechanism doesn’t dominate NOx formation for tested flames, and 50%H2–25%CH4–25%CO2 mixture emits nearly thirty times of magnitude of higher NO than 70%H2–15%CH4–15%CO2 mixture. Besides, the onset of flame instabilities depending on the forcing frequency isn’t controlled by gas composition. However, flame and combustion chamber acoustics weakly couple, hence pressure and heat release rate oscillations weaken as hydrogen amount in gas mixture increases.