The effect of hydrogen enrichment on thermoacoustic instabilities in laminar conical premixed methane/air flames

We perform a conceptual study about the linear stability of a hydrogen-enriched methane-air premixed laminar flame inside a cylindrical duct with both ends open. The flame and acoustic subsystems are coupled using the Flame Transfer Function (FTF). The power output is set to a constant value and the hydrogen molar fraction of the fuel mixture is varied. We show that adding hydrogen reduces the flame length and impacts the FTF in two ways: (i) gain drop-off shifts towards higher frequencies and (ii) phase lag reduces. Then, we compute the eigenvalues of the thermoacoustic system varying the hydrogen molar fraction. We show that adding hydrogen to the fuel mixture can stabilise linearly unstable systems and vice versa, depending on the duct natural frequencies. The results show that the hydrogen molar fraction can be a tunable parameter for controlling thermoacoustic instabilities.