The Influence of Entropy Waves on the Thermoacoustic Stability of a Model Combustor

Abstract

Thermoacoustic instability can be a major problem for aero-engine combustors, particularly lean-premixed burners designed for low NO_x emissions. The instability is caused by the interaction between unsteady heat release and acoustic waves within the combustion chamber. Unsteady combustion generates acoustic waves directly, as well as entropy fluctuations that are quiescent. The subsequent acceleration of these entropy waves at the combustor exit creates further acoustic waves known as indirect combustion noise. In this article, a thermoacoustic model is extended to study the effects of dissipation and dispersion of the entropy waves on the stability of the combustor. Four combustor configurations are discussed: a stable combustor that may be destabilized due to the presence of entropy noise, an unstable combustor that may be stabilized by indirect combustion acoustics, an unstable combustor that experiences a “mode switch” to oscillations at a different frequency, and a combustor that is driven to instability due to entropy waves but is not necessarily accompanied by an instability in the heat release. The different scenarios that may arise from the inclusion of entropy noise provide motivation for further work on the influence of entropy waves in combustors.

Keywords:

• Entropy wave dissipation and dispersion
• Indirect combustion noise
• Thermoacoustic instability

ACKNOWLEDGMENT

Chee Su Goh gratefully acknowledges the funding received from the Overseas Research Scholarship and from the Imperial College Student Opportunities Fund.

Notes

¹These flame model parameters, as well as those for Cases 2, 3 and 4, were chosen to be of the same order of magnitude as those suggested by Dowling (1997), Fleifil et al. (1996), Kaufmann et al. (2002), Hield et al. (2009). Changes in the exact values of these parameters, within reasonable limits, do not affect the qualitative results represented by the four cases.