Failure of thermoacoustic instability control due to periodic hot gas ingestion in Helmholtz dampers

Passive damping devices such as Helmholtz dampers are often employed in gas turbines to mitigate thermoacoustic instabilities. The periodic ingestion of the combustion chamber hot gas in the resonant cavity of Helmholtz dampers can be a serious issue, if not considered in the design phase of these devices. The periodic hot gas ingestion modulates the density in the damper neck and thereby the damper's resonance frequency. This effect, combined with the nonlinearities in the acoustic damping caused by the periodical reversal of the flow direction in the neck, can prevent the intended control of the thermoacoustic feedback. Despite its relevance for gas turbine design, this topic has not received significant attention in literature. This study presents experimental and theoretical investigations of that problem. A physics-based model is derived with a few empirical parameters, calibrated with data which were collected from an experimental setup comprising a tunable Helmholtz damper connected to a combustor operated at atmospheric pressure. This model of noise-driven coupled oscillators with nonlinear damping and stiffness is capable of reproducing the bistable dynamics observed in the experiments for specific ranges of equivalence ratios. The model is then used to draw general conclusions about the robustness of dampers with respect to this unwanted phenomenon. In particular, the influence of the geometry and flow conditions on the damping and stiffness nonlinearities, which both participate to possible failure of the passive control, is described. We show that in the case of broadband dampers, the effect of the nonlinear stiffness is weaker than the one of damping nonlinearity, and that it can become significant for more narrow-band dampers. The results can be used for designing acoustic dampers that are robust with regard to the risk of spontaneous loss of thermoacoustic stability due to periodic hot gas ingestion.

Automated summary

The ingestion of hot gas in the resonant cavity of Helmholtz dampers can affect the damper's resonance frequency and prevent the control of thermoacoustic feedback in gas turbines. This study presents experimental and theoretical investigations on this issue and develops a physics-based model for dampers. The results can be used to design acoustic dampers that are robust against the risk of hot gas ingestion.