For velocity sensitive premixed flames, intrinsic thermoacoustic (ITA) feedback results from flow-flame-acoustic interactions as follows: perturbations of velocity upstream of the flame result in modulations of the heat release rate, which in turn generate acoustic waves that travel in the downstream as well as the upstream direction. The latter perturb again the upstream velocity, and thus close the ITA feedback loop. This feedback mechanism exhibits resonance frequencies that are not related to acoustic eigenfrequencies of a combustor and generates — in additional to acoustic modes — so-called ITA modes. In this work spectral distributions of the sound pressure level (SPL) observed in a perfectly premixed, swirl stabilized combustion test rig are analyzed. Various burner configurations and operating points are investigated. Spectral peaks in the SPL data for stable as well as for unstable cases are interpreted with the help of a newly developed simple criterion for the prediction of burner intrinsic ITA modes. This criterion extends the known −π measure for the flame transfer function (FTF) by including the burner acoustic. This way, the peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. It is found that ITA modes are prevalent in this particular combustor. Their frequencies change significantly with the power rating (bulk flow velocity) and the axial position of the swirler, but are insensitive to changes in the length of the combustion chamber. It is argued that the resonance frequencies of the ITA feedback loop are governed by convective time scales. For that reason, they arise at rather low frequencies, which scale with the bulk flow velocity.

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Spectral distributions of the sound pressure level (SPL) observed in a premixed, swirl stabilized combustion test rig are scrutinized. Spectral peaks in the SPL for stable as well as unstable cases are interpreted with the help of a novel criterion for the resonance frequencies of the intrinsic thermo-acoustic (ITA) feedback loop. This criterion takes into the account the flow inertia of the burner and indicates that in the limit of very large flow inertia, ITA resonance should appear at frequencies where the phase of the flame transfer function (FTF) approaches −π/2. Conversely, in the limiting case of vanishing flow inertia, the new criterion agrees with previous results, which state that ITA modes may arise when the phase of the FTF is close to −π. Relying on the novel criterion, peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. Various combustor configurations are investigated over a range of operating conditions. It is found that in this particular combustor, ITA modes are prevalent and dominate the unstable cases. Remarkably, the ITA frequencies change significantly with the bulk flow velocity and the position of the swirler but are almost insensitive to changes in the length of the combustion chamber (CC). These observations imply that the resonance frequencies of the ITA feedback loop are governed by convective time scales. A scaling rule for ITA frequencies that relies on a model for the overall convective flame time lag shows good consistency for all operating conditions considered in this study.

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It is understood that so-called “entropy waves” can contribute to combustion noise and play a role in thermoacoustic instabilities in combustion chambers. The prevalent description of entropy waves generation regards the flame front as a source of heat at rest. Such a model leads — in its simplest form — to an entropy source term that depends exclusively on the unsteady response of the heat release rate and upstream velocity perturbations. However, in the case of a perfectly premixed flame, which has a constant and homogeneous fuel / air ratio and thus constant temperature of combustion products, generation of entropy waves (i.e. temperature inhomogeneities) across the flame is not expected. The present study analyzes and resolves this inconsistency, and proposes a modified version of the quasi 1-D jump relations, which regards the flame as a moving discontinuity, instead of a source at rest. It is shown that by giving up the hypothesis of a flame at rest, the entropy source term is related upto leading order in Mach number to changes in equivalence ratio only. To supplement the analytical results, numerical simulations of a Bunsen-type 2D premixed flame are analysed, with a focus on the correlations between surface area, heat release and position of the flame on the one hand, and entropy fluctuations downstream of the flame on the other. Both perfectly premixed as well as flames with fluctuating equivalence ratio are considered.