Development and Validation of a 1-D Tool for Thermoacoustic Instabilities Analysis in Gas Turbine Combustors

2008 ◽  
Author(s):  
A. Andreini ◽  
B. Facchini ◽  
L. Mangani ◽  
F. Simonetti
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Nodal Point ◽  
Mean Flow ◽  
Combustion Heat ◽  
Thermoacoustic Instabilities ◽  
Lumped Parameter ◽  
Resonance Frequencies ◽  
Lean Premixed ◽  
Energy Equations

In the last years, the more restrictive environmental legislations have constrained gas turbine manufacturers to the development of new low-emission combustors. Lean Premixed technology has become a necessary standard to meet emissions requirements and allowing an heavy reduction of nitrogen oxides emission. This kind of technology, due to the use of lean premixed mixtures, is severely affected by thermoacoustic phenomena which cause damages to combustor components and consequently reduce the overall gas turbine life of a factor of two or more. Specifically, premixed flames pose the threat of pressure oscillations. This phenomenon is the effect of the strong interaction between combustion heat-release and fluid dynamics aspects. In order to investigate thermoacoustic instabilities, a mono-dimensional code was developed and validated. It takes into account only longitudinal frequencies and it is thought to be highly modular to modify or add blocks, corresponding to different thermoacoustic models. The tool is based on a lumped-parameter approach, which consists in considering constant mean flow quantities over each fundamental straight duct element and a nodal point at each duct interface. For each interface, where an acoustic impedance could be present, the linearized fluctuating mass, momentum and energy equations are solved including entropic waves. To validate such tool, several tests, referring to actual test rigs and experimental gas turbine combustor geometries, were performed. The results show a general agreement with empirical data and other numerical results reported in literature in terms of resonance frequencies, stability and modal shapes, both for no flame and fluctuating heat release cases.

Low-Order Modelling of the Response of Ducted Flames in Annular Geometries

2012 ◽  
Author(s):  
Owen S. Graham ◽  
Ann P. Dowling
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Network Model ◽  
Time Domain ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Fuel Injectors ◽  
Thermoacoustic Instabilities ◽  
Lean Premixed

The adoption of lean premixed prevaporised combustion systems can reduce NOx emissions from gas turbines, but unfortunately also increases their susceptibility to thermoacoustic instabilities. Initially, acoustic waves can produce heat release fluctuations by a variety of mechanisms, often by perturbing the equivalence ratio. If correctly phased, heat release fluctuations can subsequently generate more acoustic waves, which at high amplitude can result in significant structural damage to the combustor. The prediction of this phenomenon is of great industrial interest. In previous work, we have coupled a physics based, kinematic model of the flame with a network model to provide the planar acoustic response necessary to close the feedback loop and predict the onset and amplitude of thermoacoustic instabilities in a lab-scale, axisymmetric single burner combustor. The advantage of a time domain approach is that the modal interaction, the influence of harmonics, and flame saturation can be investigated. This paper extends this approach to more realistic, annular geometries, where both planar and circumferential modes must be considered. In lean premixed prevaporised combustors, fluctuations in equivalence ratio have been shown to be a dominant cause of unsteady combustion. These can occur, for example, due to velocity perturbations in the premix ducts, which can lead to equivalence ratio fluctuations at the fuel injectors, which are subsequently convected downstream to the flame surfaces. Here, they can perturb the heat release by locally altering the flame speed, enthalpy of combustion, and, indirectly, the flame surface area. In many gas turbine designs, particularly aeroengines, the geometries are composed of a ring of premix ducts linking a plenum and an annular combustor. The most unstable modes are often circumferential modes. The network model is used to characterise the flow response of the geometry to heat fluctuations at an appropriate location, such as the fuel injectors. The heat release at each flame holder is determined in the time domain using the kinematic flame model derived, as a function of the flow perturbations in the premix duct. This approach is demonstrated for an annular ring of burners on a in a simple geometry. The approach is then extended to an industrial type gas turbine combustor, and used to predict the limit cycle amplitudes.

Effects of Intrinsic Flame Instabilities On Thermoacoustic Oscillations in Lean Premixed Gas Turbines

2022 ◽  
Author(s):  
Fangyan Li ◽  
Xiaotao Tian ◽  
Ming-long Du ◽  
Lei Shi ◽  
Jiashan Cui
Keyword(s):  
Heat Release ◽  
Gas Turbines ◽  
Lewis Number ◽  
Acoustic Mode ◽  
Longitudinal Distribution ◽  
Unstable State ◽  
Rocket Motors ◽  
Thermoacoustic Instability ◽  
Thermoacoustic Instabilities ◽  
Lean Premixed

Abstract Thermoacoustic instabilities are commonly encountered in the development of aeroengines and rocket motors. Research on the fundamental mechanism of thermoacoustic instabilities is beneficial for the optimal design of these engine systems. In the present study, a thermoacoustic instability model based on the lean premixed gas turbines (LPGT) combustion system was established. The longitudinal distribution of heat release caused by the intrinsic instability of flame front is considered in this model. Effects of different heat release distributions and characteristics parameters of the premixed gas (Lewis number Le, Zeldovich Number and Prandtl number Pr) on thermoacoustic instability behaviors of the LPGT system are investigated based on this model. Results show that the LPGT system features with two kinds of unstable thermoacoustic modes. The first one corresponds to the natural acoustic mode of the plenum and the second one corresponds to that of the combustion chamber. The characteristic parameters of premixed gases have a large impact on the stability of the system and even can change the system from stable to unstable state.

Flame Response Mechanisms Due to Velocity Perturbations in a Lean Premixed Gas Turbine Combustor

2010 ◽  
Vol 133 (2) ◽  
Author(s):  
Brian Jones ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Velocity Fluctuation ◽  
Gas Turbine Combustor ◽  
Velocity Fluctuations ◽  
Inlet Velocity ◽  
Lean Premixed ◽  
Flame Response ◽  
Second Peak

The response of turbulent premixed flames to inlet velocity fluctuations is studied experimentally in a lean premixed, swirl-stabilized, gas turbine combustor. Overall chemiluminescence intensity is used as a measure of the fluctuations in the flame’s global heat release rate, and hot wire anemometry is used to measure the inlet velocity fluctuations. Tests are conducted over a range of mean inlet velocities, equivalence ratios, and velocity fluctuation frequencies, while the normalized inlet velocity fluctuation (V′/Vmean) is fixed at 5% to ensure linear flame response over the employed modulation frequency range. The measurements are used to calculate a flame transfer function relating the velocity fluctuation to the heat release fluctuation as a function of the velocity fluctuation frequency. At low frequency, the gain of the flame transfer function increases with increasing frequency to a peak value greater than 1. As the frequency is further increased, the gain decreases to a minimum value, followed by a second smaller peak. The frequencies at which the gain is minimum and achieves its second peak are found to depend on the convection time scale and the flame’s characteristic length scale. Phase-synchronized CH∗ chemiluminescence imaging is used to characterize the flame’s response to inlet velocity fluctuations. The observed flame response can be explained in terms of the interaction of two flame perturbation mechanisms, one originating at flame-anchoring point and propagating along the flame front and the other from vorticity field generated in the outer shear layer in the annular mixing section. An analysis of the phase-synchronized flame images show that when both perturbations arrive at the flame at the same time (or phase), they constructively interfere, producing the second peak observed in the gain curves. When the perturbations arrive at the flame 180 degrees out-of-phase, they destructively interfere, producing the observed minimum in the gain curve.

Comparison Between Self-Excited and Forced Flame Response of an Industrial Lean Premixed Gas Turbine Injector

2011 ◽  
Author(s):  
Stephen Peluso ◽  
Bryan D. Quay ◽  
Jong Guen Lee ◽  
Domenic A. Santavicca
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Dynamic Pressure ◽  
Variable Length ◽  
Finite Limit ◽  
Pressure Measurements ◽  
Operating Condition ◽  
Lean Premixed ◽  
Flame Response ◽  
Dynamic Pressure Measurements

An experimental study was conducted to compare the relationship between self-excited and forced flame response in a variable-length lean premixed gas turbine (LPGT) research combustor with a single industrial injector. The variable-length combustor was used to determine the range of preferred instability frequencies for a given operating condition. Flame stability was classified based on combustor dynamic pressure measurements. Particle velocity perturbations in the injector barrel were calculated from additional dynamic pressure measurements using the two-microphone technique. Global CH* chemiluminescence emission was used as a marker for heat release. The flame’s response (i.e. normalized heat release fluctuation divided by normalized velocity fluctuation) was characterized during self-excited instabilities. The variable-length combustor was then used to tune the system to produce a stable flame at the same operating condition and velocity perturbations of varying magnitudes were generated using an upstream air-fuel mixture siren. Heat release perturbations were measured and the flame transfer function was calculated as a function of inlet velocity perturbation magnitude. For cases in this study, the gain and phase between velocity and heat release perturbations agreed for both self-excited and forced measurements in the linear and nonlinear flame response regimes, validating the use of forcing measurements to measure flame response to velocity perturbations. Analysis of the self-excited flame response indicates the saturation mechanism responsible for finite limit amplitude perturbations may result from nonlinear driving or damping processes in the combustor.

Model Based Active Combustion Control Concept for Reduction of Pulsations and NOx Emissions in Lean Premixed Gas Turbine Combustors

2008 ◽  
Author(s):  
Ernst Schneider ◽  
Stephan Staudacher ◽  
Bruno Schuermans ◽  
Haiwen Ye
Keyword(s):  
Gas Turbine ◽  
Gaussian Processes ◽  
Nox Emissions ◽  
Combustion Control ◽  
Model Based Control ◽  
Thermoacoustic Instabilities ◽  
Gas Turbine Combustors ◽  
Model Based ◽  
Lean Premixed ◽  
Control Concept

Strict environmental regulations demand gas turbine operation at very low equivalence ratios. Premixed gas turbine combustors, operated at very lean conditions, are prone to thermoacoustic instabilities. Thermoacoustic instabilities cause significant performance and reliability problems in gas turbine combustors, so their prevention is a general task. Splitting the fuel mass flow between different burner groups, i.e. using a burner group fuel staging technique, is a possibility to control the thermoacoustic instabilities. The resulting combustion perturbations have also effects on the gas turbine NOx emissions making it necessary to find an optimum balance between pulsations and emissions. This paper presents a model based active combustion control concept for the reduction of pulsations and emissions in lean premixed gas turbine combustors. The model is integrated in an observer structure derived from a Luenberger observer. The control logic is based on a PID algorithm allowing either the direct command of the pulsation level with a continuous monitoring and a potential limit setting of the NOx emission level or vice versa. The gas turbine pulsations and emissions are modelled using Gaussian Processes. - Gaussian Processes are stochastic processes related to Neural Networks that can approximate arbitrary functions. Based on measured gas turbine data they can be implemented in an easy and straightforward manner. The model provides the control system with real time data of the outputs resulting from settings of the staging ratio that is the actuating variable of the system. A model based control concept can significantly alleviate the effects of time delays in the system. The model based control concept allows for fast adaptation of the burner group staging ratio during static and transient operations to achieve an optimum of the pulsation and emission levels. During tests the model based control concept gave good results and proved to be robust even at high disturbance levels.

Thermo-Acoustic Interactions in a Premixed Dump Combuster

2002 ◽  
Author(s):  
S. Galvin ◽  
J. A. Fitzpatrick
Keyword(s):  
Heat Release ◽  
Research Effort ◽  
Lean Premixed Combustion ◽  
Thermoacoustic Instabilities ◽  
The Past ◽  
Lean Premixed ◽  
Prediction And Control ◽  
Modelling Techniques ◽  
And Control ◽  
Phase Relationships

There is significant interest in the interaction of parameters associated with lean premixed combustion because the demand for reduced emissions has led to an increased usage of this type of system. Thermoacoustic instabilities, which arise as a consequence of unsteady pressure and heat release interactions, are known to occur frequently for these lean premixed configurations. There has been a substantial research effort in the past decade directed at the development of modelling techniques for the prediction and control of these instabilities. Tests have been performed in an optically accessible dump combustor for a range of equivalence ratio with two different dump or expansion ratios and a number of flow rates. Both thermoacoustic and flow/acoustic interactions are observed over a large operating range. The pressure and heat release autospectra and cross-spectra were calculated and their coherence and phase relationships are examined to determine their interaction including the applicability of the Rayleigh criterion.

On the Use of Helmholtz Resonators for Damping Acoustic Pulsations in Industrial Gas Turbines

2001 ◽  
Author(s):  
Valter Bellucci ◽  
Christian Oliver Paschereit ◽  
Peter Flohr ◽  
Fulvio Magni
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Structural Integrity ◽  
Combustion Process ◽  
Low Frequency ◽  
Operating Regime ◽  
Pressure Pulsations ◽  
Helmholtz Resonators ◽  
Resonance Frequencies

In modern gas turbines operating with premix combustion flames, the suppression of pressure pulsations is an important task related to the quality of the combustion process and to the structural integrity of engines. High pressure pulsations may occur when the resonance frequencies of the system are excited by heat release fluctuations independent of the acoustic field (“loudspeaker” behavior of the flame). Heat release fluctuations are also generated by acoustic fluctuations in the premixed stream. The feedback mechanism inherent in such processes (“amplifier” behavior of the flame) may lead to combustion instabilities, the amplitude of pulsations being limited only by nonlinearities. In this work, the application of Helmholtz resonators for damping low-frequency pulsations in gas turbine combustion chambers is discussed. We present a nonlinear model for predicting the acoustic response of resonators including the effect of purging air. Atmospheric experiments are used to validate the model, which is employed to design a resonator arrangement for damping low-frequency pulsations in an ALSTOM GT11N2 gas turbine. The predicted damper impedances are used as the boundary condition in the three-dimensional analysis of the combustion chamber. The suggested arrangement leads to a significant extension of the low-pulsation operating regime of the engine.

Flame Response Mechanisms Due to Velocity Perturbations in a Lean Premixed Gas Turbine Combustor

2010 ◽  
Author(s):  
Brian Jones ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca ◽  
Kwanwoo Kim ◽  
...  
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Velocity Fluctuation ◽  
Modulation Frequency ◽  
Gas Turbine Combustor ◽  
Velocity Fluctuations ◽  
Inlet Velocity ◽  
Lean Premixed ◽  
Flame Response

The response of turbulent premixed flames to inlet velocity fluctuations is studied experimentally in a lean premixed, swirl-stabilized, gas turbine combustor. Overall chemiluminescence intensity is used as a measure of the fluctuations in the flame’s global heat release rate and hot wire anemometry is used to measure the inlet velocity fluctuations. Tests are conducted over a range of mean inlet velocities, equivalence ratios and velocity fluctuation frequencies, while the normalized inlet velocity fluctuation (V′/Vmean) is fixed at 5% to ensure linear flame response over the employed modulation frequency range. The measurements are used to calculate a flame transfer function relating the velocity fluctuation to the heat release fluctuation as a function of the velocity fluctuation frequency. At low frequency, the gain of the flame transfer function increases with increasing frequency to a peak value greater than one. As the frequency is further increased, the gain decreases to a minimum value, followed by a second smaller peak. The frequencies at which the gain is minimum and achieves its 2nd peak are found to depend on the convection time scale and the flame’s characteristic length scale. Phase-synchronized CH* chemiluminescence imaging is used to characterize the flame’s response to inlet velocity fluctuations. The observed flame response can be explained in terms of the interaction of two flame perturbation mechanisms, acoustic velocity fluctuations and vorticity fluctuations. Analysis of the phase-synchronized flame images show that when both perturbations arrive at the flame at the same time (or phase) they constructively interfere, producing the 2nd peak observed in the gain curves. And when the perturbations arrive at the flame 180 degrees out-of-phase, they destructively interfere, producing the observed minimum in the gain curve.

Pulsation-Amplitude-Dependent Flame Dynamics of High-Frequency Thermoacoustic Oscillations in Lean-Premixed Gas Turbine Combustors

2017 ◽  
Author(s):  
Frederik M. Berger ◽  
Tobias Hummel ◽  
Bruno Schuermans ◽  
Thomas Sattelmayer
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Growth Rates ◽  
High Frequency ◽  
Individual Growth ◽  
Mean Flow ◽  
Flame Dynamics ◽  
Pulsation Amplitude ◽  
The Mean ◽  
Thermoacoustic Oscillations

This paper presents the experimental investigation of pulsation-amplitude-dependent flame dynamics associated with transverse thermoacoustic oscillations at screech level frequencies in a generic gas turbine combustor. Specifically, the flame behavior at different levels of pulsation amplitudes is assessed and interpreted. Spatial dynamics of the flame are measured by imaging the OH* chemiluminescence signal synchronously to the dynamic pressure at the combustor’s face plate. First, linear thermoacoustic stability states, modal dynamics, as well as flame-acoustic phase relations are evaluated. It is found that the unstable acoustic modes converge into a predominantly rotating character in the direction of the mean flow swirl. Furthermore, the flame modulation is observed to be in phase with the acoustic pressure at all levels of the oscillation amplitude. Second, distributed flame dynamics are investigated by means of visualizing the mean and oscillating heat release distribution at different pulsation amplitudes. The observed flame dynamics are then compared against numerical evaluations of the respective amplitude-dependent thermoacoustic growth rates, which are computed using analytical models in the fashion of a non-compact flame-describing function. While results show a nonlinear contribution for the individual growth rates, the superposition of flame deformation and displacements balances out to a constant flame driving. This latter observation contradicts the state-of-the-art perception of root-causes for limit-cycle oscillations in thermoacoustic gas turbine systems, for which the heat release saturates with increasing amplitudes. Consequently, the systematic observations and analysis of amplitude-dependent flame modulation shows alternative paths to the explanation of mechanisms that might cause thermoacoustic saturation in high frequency systems.

Linearised Theory for LPP Combustion Dynamics

2003 ◽  
Author(s):  
C. A. Armitage ◽  
R. S. Cant ◽  
A. P. Dowling ◽  
T. P. Hynes
Keyword(s):  
Heat Release ◽  
Combustion Chamber ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Broad Band ◽  
Mode Shapes ◽  
Complex Frequency ◽  
Mean Flow ◽  
Combustion Dynamics ◽  
Forcing Function

Gas turbines which are operated under lean, premixed, pre–vaporised (LPP) conditions are notoriously susceptible to self–excited oscillations. In the combustion chamber the unsteady heat released by combustion processes interacts with pressure fluctuations. The challenge is to develop a tool which can determine the frequency and stability characteristics of self–excited oscillations in realistic gas–turbine geometries. To this end, the flow through the gas turbine is described as far as possible by taking advantage of linearised theory and analytical models of the behaviour in the combustion chamber. First, a steady, mean flow solution for an idealised axi–symmetric combustor geometry is calculated using the inviscid Euler equations for continuity, momentum and energy with a specified distributed mean heat release. Superimposed on this is a linearised, three–dimensional perturbed flow in which the time and circumferential variation are described by a complex frequency and mode number respectively. Within this numerical model of the combustor a ‘flame model’ is used to describe the change in the rate of combustion due to inlet flow perturbations. The flame model may be given by an analytical expression—for example using a simple time lag with an expression proportional to the mean heat release in order to describe the unsteady heat release. An alternative approach would be to use a localised and detailed unsteady CFD calculation to determine the flow downstream of a generic premix duct geometry. If the flow is perturbed at the inlet a relationship between these fluctuations and the unsteady heat release may be obtained. In order to capture the response of the system to a wide frequency range an appropriately chosen broad–band forcing function may be used to perturb the flow. System identification techniques allow the transfer function to be extracted and a suitable flame model for the linearised Euler calculations may be constructed. Sample calculations of each aspect of the research will be presented to demonstrate the capabilities of each technique and the viability of combining the approaches towards the goal of aiding the design of gas–turbine combustors. Calculations using the linearised Euler methodology with analytical expressions for the flame model will demonstrate the capability of the approach to identify the frequencies of oscillation, mode shapes and zones of stability of particular combustor geometries.

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