Mixed acoustic–entropy combustion instabilities in gas turbines

Abstract

A combustion instability in a combustor terminated by a nozzle is analysed and modelled based on a low order Helmholtz solver. A Large Eddy Simulation (LES) of the corresponding turbulent, compressible and reacting flow is first performed and analysed based on Dynamic Mode Decomposition (DMD). The mode with the highest amplitude shares the same frequency of oscillation as the experiment (approx. 320 Hz) and shows the presence of large entropy spots generated within the combustion chamber and convected down to the exit nozzle. The lowest purely acoustic mode being in the range 700 − 750 Hz, it is postulated that the instability observed around 320 Hz stems from a mixed entropy/acoustic mode where the acoustic generation associated with entropy spots being convected throughout the choked nozzle plays a key role. The DMD analysis allows to extract from the LES results a low-order model that confirms that the mechanism of the low-frequency combustion instability indeed involves both acoustic and convected entropy waves. The Delayed Entropy Coupled Boundary Condition (Motheau et al. 2014) is implemented into a numerical Helmholtz solver where the baseline flow is assumed at rest. When fed with appropriate transfer functions to model the entropy generation and convection from the flame to the exit, the Helmholtz/DECBC solver predicts the presence of an unstable mode around 320 Hz, in agreement with both LES and experiments.

Solution of the quasi-one-dimensional linearized Euler equations using flow invariants and the Magnus expansion

“…Figure 8 shows the upstream-propagating acoustic wave at the inlet generated by an entropy wave. This wave is responsible for low-frequency instabilities in combustion chambers, as mentioned by Goh & Morgans (2011b) and already observed in some industrial cases (Motheau et al 2012).…”

Navier-Stokes Characteristic Boundary Conditions Using Ghost Cells

“…For example, the conversion of entropy waves to acoustic waves at the outlet of a combustion chamber may trigger a combustion instability in an aeronautical gas turbine. 1 This physical phenomenon is very sensitive to the geometry and small fluctuations of the flow, thus it is essential in numerical studies of this type of flow to accurately represent the outflow boundary conditions. Several approaches have been proposed to derive consistent boundary conditions that would tackle the problems cited above.…”

Investigation of the transmitted noise of a combustor exit nozzle caused by burned hydrogen-hydrocarbon gases

“…Hydrodynamic mechanisms, such as vorticity generation and breakdown as well as interaction between flow and boundaries, can decay the entropy wave strength and thus, they can reduce the noise generation [11,12]. Transfer of heat can also modify acoustic and entropic waves.…”