A systematic study of nonlinear coupling of thermoacoustic modes in annular combustors

Combustion-acoustic interactions through cross-talk area between adjacent model gas turbine combustors

Low-emissions can-annular gas turbines are prone to develop low-frequency self-excited thermoacoustic oscillations. Such oscillations arise from the coupling between adjacent combustors and can increase wear and thermal stresses. In this experimental study, we explore the mutual synchronization of two thermoacoustic oscillators (i.e., two model combustors) interacting via dissipative and time-delayed coupling, as introduced via a cross-talk section. Unlike most previous studies, our study makes use of a turbulent lean-premixed flame in each combustor, bringing the system configuration closer to that of practical gas turbines. Using stationary and transient measurements, we examine the effect of the cross-talk diameter and length so as to gain insight into the effect of dissipative and time-delayed coupling. We find that strengthening the dissipative coupling promotes mutual synchronization, but that weakening the dissipative coupling leads to weakly coupled or desynchronized oscillations. On operating the two combustors at different conditions, we find a significant reduction in their overall oscillation amplitude for some coupling conditions. On varying the combustor length and examining the transient response, we find elaborate changes in the pressure-heat-release-rate coupling, spontaneous mode transitions between coupled thermoacoustic modes, and the emergence of a rhomboid structure in the phase plane owing to the coexistence of in-phase and out-of-phase synchronization. In the combustion community, these two types of synchronization are known to be associated with push-push modes and push-pull modes. These findings offer new insight into the mutual synchronization of low-frequency, self-excited thermoacoustic oscillations in can-annular gas turbines, paving the way for the development of improved control strategies.

Mutual synchronization of two flame-driven thermoacoustic oscillators: Dissipative and time-delayed coupling effects.

Combustion dynamics of lean fully-premixed hydrogen-air flames in a mesoscale multinozzle array

Optimising the acoustic damping of multiple Helmholtz resonators attached to a thin annular duct

Transverse mean flows and transverse acoustic perturbations are factors that may influence the flame response, and thus change the frequency, growth rate and mode nature (standing, spinning or mixed) of the azimuthal thermoacoustic modes in annular combustors. Previous analytical and low-order network models for annular combustors usually consider only axial flame response. This work identifies the linear response of an asymmetric planar Bunsen flame under simultaneous axial and transverse perturbations. A linearized analytical method for studying the response of an inclined flame under perturbations using G-equation is applied. The relation between the flame responses under a 2-D perturbation and separate axial/transverse perturbations is studied to identify a linear 2-D flame response superposition, with the response under simultaneous perturbations being equal to the sum of two responses under each single perturbation. The effect of this linear 2-D flame response on the azimuthal thermoacoustic modes is then investigated by incorporating it into a linear low-order network methodology [19]. An annular combustor both with and without mixing burners and transverse mean flows is studied. The results are verified by comparing with FEM (Helmholtz solver) simulations. It is found that even though the transverse flame response is usually much smaller than the axial flame response, it may strongly affect the spin ratio of the azimuthal thermoacoustic modes. The asymmetries (e.g. due to burner difference) in both the transverse and axial flame response, and the effect of transverse mean flow on the acoustic propagation may all significantly change the spin ratio of the azimuthal modes, and thus need to be considered simultaneously.

An analytical study of the effect of flame response to simultaneous axial and transverse perturbations on azimuthal thermoacoustic modes in annular combustors

The effective utilization of low-grade energy sources generated from steel-making processes provides not only excellent opportunities for low cost power generation but also a significant means for the reduction of greenhouse gas emissions. In this paper, the work was carried out to study the static and dynamic combustion instabilities for gas turbine combustors burning low-calorific-value blast furnace gas (BFG). A burner was designed to stabilize the BFG flame with central pilot flames. A diagnostic system was set up to detect the characteristics of flame dynamics. In the experiments, the fuel ratio between the pilot and main burner and, the equivalence ratio of the main flame and the annular flow velocity were varied for the investigation of the combustion lean blowout (LBO) limits. The flame dynamics near LBO were investigated. The dynamic responses of the flame to flow perturbations were also measured. A network model was employed to study and validate the blowout mechanisms. The LBO limits were calculated and compared with experimental results for various equivalence ratios.Copyright © 2015 by ASME

Investigations of the Stabilities of Piloted Flames Using Blast Furnace Gas

Measurements and Modeling of the Dynamic Response of a Pilot Stabilized Premixed Flame Under Dual-Input Perturbation

Mean transverse flow can affect azimuthal modes in annular combustions due to the effect of the mean transverse flow on the flame responses. This study measured the flame responses of an asymmetric Bunsen flame subjected to transverse and axial acoustic waves. An experimental rig was set up to provide simultaneous transverse and axial perturbations. The experimental results were compared with analytical models based on the linearized G-equation. The analytical and experimental results both show that the axial FTF has a low-pass characteristic and the transverse FTF has a band-pass characteristic. The results also show the linear superposition of transverse and axial FTFs. Cases with different transverse mean flows show that the transverse flame response is caused by asymmetric flame surfaces. Flame positions and flame surface areas obtained from a camera also confirm the flame response mechanism. INTRODUCTION Lean-premixed combustion technologies are used to reduce the emissions of gas turbines. However, the techniques frequently cause combustion instabilities in the gas turbines which can severely damage the combustion chambers (Lieuwen et al., 2005). Azimuthal thermoacoustic modes have been widely observed in the combustor because the circumference is longer than the length (O'Connor et al., 2015). Numerical (Staffelbach et al., 2009; Bauerheim et al., 2015a) and experimental (Worth and Dawson, 2013; Prieur et al., 2017) results have shown that the azimuthal instabilities shift randomly among standing, spinning and mixed modes. A linear quasi-one dimensional model (Bauerheim et al., 2014; Bauerheim et al., 2015b) was developed to predict the azimuthal modes, which assumes the acoustic waves are one dimensional and uses Flame Transfer Functions (FTF) flame models. The model showed (Bauerheim et al., 2015b) that a mean azimuthal flow in the chamber can affect the azimuthal mode. The flame models used in the linear quasi-one dimensional model only show flame responses to the axial velocity perturbation. However, some experimental results (Bourgouin et al., 2015; Bourgouin, 2014) have shown that the flame is affected by both axial and azimuthal modes. The flame model should be improved to incorporate the transverse perturbation effects. Experiments for a lab-scale annular chamber are useful to study the acoustic mode characteristics. Flame dynamics and heat release rate were studied (Dawson and Worth, 2014) for standing and spinning self-excited azimuthal modes. The heat release rate of each flame varied according to the location, acoustic mode and flame separation distance. Flame dynamics for different spinning directions (Nygård et al., 2019) were studied and a novel averaging method was used to decompose the standing mode into ACW and CW spinning components. Some results (Worth and Dawson, 2017) also proved that the equivalence ratio can strongly affect the azimuthal modes.

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Chunyan Li

Papers

An analytical study of the effect of flame response to simultaneous axial and transverse perturbations on azimuthal thermoacoustic modes in annular combustors

Investigations of the Stabilities of Piloted Flames Using Blast Furnace Gas

Measurements and Modeling of the Dynamic Response of a Pilot Stabilized Premixed Flame Under Dual-Input Perturbation

Experimental Study of the Dynamic Responses of an Asymmetric Two-Dimensional Bunsen Flame