A combined oscillation cycle involving self-excited thermo-acoustic and hydrodynamic instability mechanisms

Abstract

The paper examines the combined effects of several interacting thermo-acoustic and hydrodynamic instability mechanisms that are known to influence self-excited combustion instabilities often encountered in the late design stages of modern lowemission gas turbine combustors. A compressible large eddy simulation approach is presented, comprising the flame burning regime independent, modelled probability density function evolution equation/stochastic fields solution method. The approach is subsequently applied to the PRECCINSTA (PREdiction and Control of Combustion INSTAbilities) model combustor and successfully captures a fully self-excited limit-cycle oscillation without external forcing. The predicted frequency and amplitude of the dominant thermo-acoustic mode and its first harmonic are found to be in excellent agreement with available experimental data. Analysis of the phaseresolved and phase-averaged fields leads to a detailed description of the superimposed mass flow rate and equivalence ratio fluctuations underlying the governing feedback loop. The prevailing thermo-acoustic cycle features regular flame lift-off and flashback events in combination with a flame angle oscillation, as well as multiple hydrodynamic phenomena, i.e. toroidal vortex shedding and a precessing vortex core. The periodic excitation and suppression of these hydrodynamic phenomena is confirmed via spectral proper orthogonal decomposition and shown to be controlled by an oscillation of the instantaneous swirl number. Their local impact on the heat release rate, which is predominantly modulated by flame-vortex roll-up and enhanced mixing of fuel and oxidiser, is further described and investigated. Finally, the temporal relationship between the flame 'surface area', flame-averaged mixture fraction and global heat release rate are found to be directly correlated.

Longitudinal and azimuthal thermo-acoustic instabilities in an industrial gas turbine combustor operating at elevated pressure

“…It may therefore be concluded that weak thermo-acoustic effects, in combination with heat losses to the base plate wall, likely contribute to the partial lift-off of the outer flame branch. Similar (thermo-acoustically induced) flame lift-off events in a gas turbine model combustor were previously studied by Fredrich et al 88 . Their work demonstrated how periodic vortex shedding, which was also observed experimentally in the present test case, 37,39 can enhance flame lift-off from the burner rim.…”

“…39 The exact role of mass flow rate and equivalence ratio oscillations in a gas turbine model combustor with similar design to the SGT-100 test case was recently studied and quantified numerically in literary works. 88,90,91…”

“…39 The exact role of mass flow rate and equivalence ratio oscillations in a gas turbine model combustor with similar design to the SGT-100 test case was recently studied and quantified numerically in literary works. 88,90,91 While the frequency of the predicted limit-cycle oscillation is almost identical for Cases A and B, the oscillation amplitude is clearly affected by the combustor's operating pressure. Case A exhibits pressure fluctuations with a peak amplitude of about 3 kPa.…”

Large Eddy Simulation of a Reacting Kerosene Spray in Hot Vitiated Cross-Flow

“…The ensemble average over all stochastic fields finally yields the filtered mean value of each scalar quantity ̃ . Note that this modelling approach has previously been applied to test cases involving premixed (Jones et al 2012a), partially premixed (Fredrich et al 2019(Fredrich et al , 2021b, non-premixed (Jones and Prasad 2010) and spray (Gallot-Lavallée et al 2017 flames and does not require any tuning of the default model parameters.…”

Combustion dynamics analysis of a pressurized airblast swirl burner using proper orthogonal decomposition

“…Preliminary cold flow experimental analysis of the burner conducted in our laboratory at the University of Twente hinted at the presence of a precessing vortex core (PVC) which is a coherent vortical structure that rotates around the burner axis and has a single or double helical structure. The PVC has been observed in both reacting and non-reacting or cold flows in swirl-stabilized combustors [16][17][18] and it has great implications on their performance due to having an effect on turbulent mixing and thermoacoustic characteristics. The precession frequency of the PVC in similar swirl-stabilized combustors has been shown to be highly sensitive to inlet air velocity and the gradient of rotational velocity and located in the inner shear layer of the vortex breakdown region.…”