Analysis of acoustic, entropy and vorticity waves in a non-uniform annular combustor

Abstract

Annular ducts with axial cross-sectional surface area gradient are typical elements in aero-engines and land-use gas turbines. Acoustic, entropy and vorticity waves propagating in these regions are generally coupled and may affect the thermoacoustic instabilities. However, due to the highly coupled mechanism, there has been no analytical solutions; the numerical solutions of typical linearized Euler equations (LEEs) cannot clearly identify the contributions from the coupled three waves. This work presents two asymptotic methods to isolate the acoustic, entropy and vorticity waves coupled in these kinds of annular ducts. These methods are validated by comparing their predicted results to those numerically resolving the LEEs. Further analyses are conducted in a model annular combustor. Results show that the interaction among the three waves cannot be neglected in the combustion with a large inlet Mach number flow. The interaction between acoustic and vorticity waves is stronger than that between acoustic and entropy waves. Also, this work shows that the entropy noise and the vortex noise have similar mechanisms.