The effect of dynamic operating conditions on the thermoacoustic response of hydrogen rich flames in an annular combustor

Abstract

Self-excited thermoacoustic instabilities in symmetric annular combustion chambers typically give rise to spinning, standing and mixed azimuthal modes which are time-varying in nature and occur in a highly noisy environment due to turbulent combustion. We investigate the effect of linear ramps of increasing and decreasing equivalence ratio on the operating limits and thermoacoustic dynamics, from near lean blow-off to near flashback, in a laboratory scale annular combustor. The combustor features 12 lean-premixed hydrogen–methane flames at a fuel composition of 70% hydrogen and 30% methane by power. Equivalence ratio ramps were conducted for different thermal powers P=4, 6 and 8 kW per burner and three different ramp times tramp=5, 20, 60 s to simulate dynamic operation. It was found that ramping leads to self-excited instabilities that exhibit repeatable modal dynamics which depend on thermal power, ramp direction and duration. Different types of hysteresis were observed between the upward and downward ramps which affected the amplitudes and the stable operating range. The hysteresis phenomena also showed repeatable behaviour in terms of the nature and orientation angle. In one specific case, the simultaneous existence of two spinning modes was observed before the appearance of mode hopping leading to both an increase in frequency and doubling of the amplitude. High-speed OH* chemiluminescence of the flames showed that the mode hopping was accompanied by a change in the flame shape which becomes more compact and distributed.