Experimental and Analytical Study of the Acoustic Properties of a Gas Turbine Model Combustor With a Choked Nozzle

Combustion is now considered as a non-negligible contributor to gas turbine noise. Combustion noise can be divided into two types: direct combustion noise directly caused by flame surface fluctuations and indirect combustion noise caused by non-homogeneities in the burnt gases, which radiate sound when interacting with the first turbine stages. The aim of the present project is to obtain an extensive experimental database as well as a better understanding of the physical phenomena inside a pressurized combustion chamber with a choked exhaust nozzle. To do so, a pressurized model scale combustor has been developed, containing a tangential admission injector creating a swirling premixed flow. Satisfactory premixing is obtained in the injection device by a porous media. The combustion chamber shows large optical accesses and various ports for pressure and temperature sensors. On the upstream side, an impedance control device is installed while, downstream, the exhaust nozzle can be easily varied to study its influence on noise generation. A mean chamber pressure higher than 2 bar can be reached for the targeted operating points. The present analysis of the flame behaviour is a first step towards the study of combustion noise. The flame dynamics are characterized by spectral analysis of the dynamic pressure in the combustion chamber. The aim of this work is to determine the dominating acoustic modes during combustion operation. With the help of analytical calculations, the test bench is first modelled as a two cavity system, and later as a five cavity system, taking into account the feeding lines. The nozzle can be assumed as choked due to the pressurization of the chamber. With this method, the majority of the acoustic modes can be identified and explained. The study shows that these modes are linked to the geometry of the whole combustor including the injection tube, the combustion chamber and the feeding lines.

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Experimental Analysis of Confinement and Swirl Effects on Premixed CH4-H2 Flame Behavior in a Pressurized Generic Swirl Burner

Volume 4B: Combustion, Fuels and Emissions ◽

10.1115/gt2017-64794 ◽

2017 ◽

Author(s):

Jon Runyon ◽

Richard Marsh ◽

Daniel Pugh ◽

Philip Bowen ◽

Anthony Giles ◽

...

Keyword(s):

Combustion Chamber ◽

Gas Turbine ◽

Kinetic Modelling ◽

Dynamic Pressure ◽

Flame Stabilization ◽

Combustion Noise ◽

Combustion Stability ◽

Acoustic Response ◽

Swirl Burner ◽

Swirl Number

The introduction of hydrogen into natural gas systems for environmental benefit presents potential operational issues for gas turbine combustion and power generation applications; in particular acceptable blending concentrations are still widely debated. The use of a generic swirl burner under conditions pertinent to a gas turbine combustor is therefore advantageous to (i) provide evidence of potential design modifications to inform future gas turbine operation on hydrogen blends and (ii) validate numerical model predictions. Building on a previous experimental combustion database consisting of methane-hydrogen fuel blends under atmospheric and elevated ambient conditions, a new scaled generic swirl burner has been designed for experimental investigation of flame stability and exhaust gas emissions at combustor inlet temperatures to 573 K, pressures to 0.33 MPa, and thermal powers to 126 kW. The geometry downstream of the modular burner is developed further to enable separate investigation under isothermal and combustion conditions of the influence of combustor outlet geometry and the effect of changing geometric swirl number. The burner confinement is modified to include both a cylindrical exit quartz combustion chamber and a conical convergent exit quartz combustion chamber, designed to provide a more representative geometric and acoustic boundary at the combustor outlet. Two inlet geometric swirl numbers of industrial relevance are chosen; namely 0.5 and 0.8. Combustion stability and heat release locations of lean premixed CH4-air and CH4-H2-air combustion are evaluated by a combination of OH planar laser induced fluorescence, OH* chemiluminescence, and dynamic pressure measurements. Changes in flame stabilization location are characterized by the use of an OH* chemiluminescence intensity centroid. Notable upstream flame movement coupled with changes in acoustic response are evident, particularly near the lean operating limit as hydrogen blending shifts lean blowoff of methane flames to lower equivalence ratios with corresponding reduction in NOx emissions. The influence of increased pressure on the lean operating point stability and emissions appear to be small over the range considered, however a power law correlation has been developed for scaling combustion noise amplitudes with inlet pressure and swirl number. Indicators of flame flashback as well as combustor acoustic response are affected considerably when the convergent combustor outlet geometry is deployed. This has been shown to alter the influence of the central recirculation zone as a flame stabilizing coherent flow structure. Chemical kinetic modelling supports the experimental observations that stable burner operation can be achieved with blended methane-hydrogen up to 15% by volume.

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Effects of Nozzle Helmholtz Number on Indirect Combustion Noise by Compositional Perturbations

Volume 2C: Turbomachinery ◽

10.1115/gt2017-63382 ◽

2017 ◽

Cited By ~ 1

Author(s):

Luca Magri ◽

Jeffrey O’Brien ◽

Matthias Ihme

Keyword(s):

Velocity Profile ◽

Combustion Chamber ◽

Gas Turbine ◽

Transfer Functions ◽

Mixture Fraction ◽

Supersonic Nozzle ◽

Noise Pollution ◽

Combustion Noise ◽

Dipole Source ◽

Thermoacoustic Oscillations

By modeling a multi-component gas, a new source of indirect combustion noise is identified, which is named compositional indirect noise. The advection of mixture inhomogeneities exiting the gas-turbine combustion chamber through subsonic and supersonic nozzles is shown to be an acoustic dipole source of sound. The level of mixture inhomogeneity is described by a difference in composition with the mixture fraction. An n-dodecane mixture, which is a kerosene fuel relevant to aeronautics, is used to evaluate the level of compositional noise. By relaxing the compact-nozzle assumption, the indirect noise is numerically calculated for Helmholtz numbers up to 2 in nozzles with linear velocity profile. The compact-nozzle limit is discussed. Only in this limit, it is possible to derive analytical transfer functions for (i) the noise emitted by the nozzle and (ii) the acoustics travelling back to the combustion chamber generated by accelerated compositional inhomogeneities. The former contributes to noise pollution, whereas the latter has the potential to induce thermoacoustic oscillations. It is shown that the compositional indirect noise can be at least as large as the direct noise and entropy noise in chocked nozzles and lean mixtures. As the frequency with which the compositional inhomogeneities enter the nozzle increases, or as the nozzle spatial length increases, the level of compositional noise decreases, with a similar, but not equal, trend to the entropy noise. The noisiest configuration is found to be a compact supersonic nozzle.

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Combustion Oscillation Monitoring Using Flame Ionization in a Turbulent Premixed Combustor

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.2431390 ◽

2006 ◽

Vol 129 (2) ◽

pp. 352-357 ◽

Cited By ~ 4

Author(s):

B. T. Chorpening ◽

J. D. Thornton ◽

E. D. Huckaby ◽

K. J. Benson

Keyword(s):

Standard Deviation ◽

Gas Turbine ◽

Gas Turbines ◽

Dynamic Pressure ◽

Pressure Oscillation ◽

Chamber Pressure ◽

Ion Distribution ◽

Combustion Dynamics ◽

Gas Turbine Combustors ◽

Flame Ionization

To achieve very low NOx emission levels, lean-premixed gas turbine combustors have been commercially implemented that operate near the fuel-lean flame extinction limit. Near the lean limit, however, flashback, lean blow off, and combustion dynamics have appeared as problems during operation. To help address these operational problems, a combustion control and diagnostics sensor (CCADS) for gas turbine combustors is being developed. CCADS uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. Previous development efforts have shown the capability of CCADS to monitor flashback and equivalence ratio. Recent work has focused on detecting and measuring combustion instabilities. A highly instrumented atmospheric combustor has been used to measure the pressure oscillations in the combustor, the OH emission, and the flame ion field at the premix injector outlet and along the walls of the combustor. This instrumentation allows examination of the downstream extent of the combustion field using both the OH emission and the corresponding electron and ion distribution near the walls of the combustor. In most cases, the strongest pressure oscillation dominates the frequency behavior of the OH emission and the flame ion signals. Using this highly instrumented combustor, tests were run over a matrix of equivalence ratios from 0.6 to 0.8, with an inlet reference velocity of 25m∕s(82ft∕s). The acoustics of the fuel system for the combustor were tuned using an active-passive technique with an adjustable quarter-wave resonator. Although several statistics were investigated for correlation with the dynamic pressure in the combustor, the best correlation was found with the standard deviation of the guard current. The data show a monotonic relationship between the standard deviation of the guard current (the current through the flame at the premix injector outlet) and the standard deviation of the chamber pressure. Therefore, the relationship between the standard deviation of the guard current and the standard deviation of the pressure is the most promising for monitoring the dynamic pressure of the combustor using the flame ionization signal. This addition to the capabilities of CCADS would allow for dynamic pressure monitoring on commercial gas turbines without a pressure transducer.

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Combustion Oscillation Monitoring Using Flame Ionization in a Turbulent Premixed Combustor

Volume 2: Turbo Expo 2004 ◽

10.1115/gt2004-53881 ◽

2004 ◽

Cited By ~ 1

Author(s):

B. T. Chorpening ◽

J. D. Thornton ◽

E. D. Huckaby ◽

K. J. Benson

Keyword(s):

Standard Deviation ◽

Gas Turbine ◽

Gas Turbines ◽

Dynamic Pressure ◽

Pressure Oscillation ◽

Chamber Pressure ◽

Ion Distribution ◽

Combustion Dynamics ◽

Gas Turbine Combustors ◽

Flame Ionization

To achieve very low NOx emission levels, lean-premixed gas turbine combustors have been commercially implemented which operate near the fuel-lean flame extinction limit. Near the lean limit, however, flashback, lean blowoff, and combustion dynamics have appeared as problems during operation. To help address these operational problems, a combustion control and diagnostics sensor (CCADS) for gas turbine combustors is being developed. CCADS uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. Previous development efforts have shown the capability of CCADS to monitor flashback and equivalence ratio. Recent work has focused on detecting and measuring combustion instabilities. A highly instrumented atmospheric combustor has been used to measure the pressure oscillations in the combustor, the OH emission, and the flame ion field at the premix injector outlet and along the walls of the combustor. This instrumentation allows examination of the downstream extent of the combustion field using both the OH emission and the corresponding electron and ion distribution near the walls of the combustor. In most cases, the strongest pressure oscillation dominates the frequency behavior of the OH emission and the flame ion signals. Using this highly instrumented combustor, tests were run over a matrix of equivalence ratios from 0.6 to 0.8, with an inlet reference velocity of 25 m/s. The acoustics of the fuel system for the combustor were tuned using an active-passive technique with an adjustable quarter-wave resonator. Although several statistics were investigated for correlation with the dynamic pressure in the combustor, the best correlation was found with the standard deviation of the guard current. The data show a monotonic relationship between the standard deviation of the guard current (the current through the flame at the premix injector outlet) and the standard deviation of the chamber pressure. Therefore, the relationship between the standard deviation of the guard current and the standard deviation of the pressure is the most promising for monitoring the dynamic pressure of the combustor using the flame ionization signal. This addition to the capabilities of CCADS would allow for dynamic pressure monitoring on commercial gas turbines without a pressure transducer.

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Numerical Modeling of Thermoacoustic Oscillations in a Gas Turbine Combustion Chamber

Volume 1: Combustion and Fuels, Education ◽

10.1115/gt2006-90945 ◽

2006 ◽

Cited By ~ 1

Author(s):

Dariusz Nowak ◽

Valter Bellucci ◽

Jan Cerny ◽

Geoffrey Engelbrecht

Keyword(s):

Heat Release ◽

Combustion Chamber ◽

Gas Turbine ◽

Expert Knowledge ◽

Combustion Process ◽

Acoustic Mode ◽

Test Facility ◽

Mode Shapes ◽

Flame Surface ◽

Gas Turbine Combustion

The prediction of high-frequency acoustic oscillations in gas turbine combustors is an important issue, related to engine performance, NOx emissions, component lifetime and engine operational flexibility. Different methods with increasing complexity and predictive ability have been discussed in a number of papers. Application of these methods requires large computational capacity and long computational times. Therefore, a limited number of variants of small combustor models or small sectors can be analyzed in a reasonable time. This paper presents an approximate approach, applicable under certain specific conditions. It is based on an understanding that the acoustic pressure oscillations are tied to the oscillation in heat release rate. The interaction is taking place in the heat release zone, independent of the type of the feedback mechanism. For a typical gas turbine combustion chamber, many acoustic modes exist in the frequency range of interest. However, only a few of these modes are excited by the combustion process and thus are relevant. The mode excitation depends both on combustion noise (due to flame excitation contribution independent of the acoustic field) and combustion instability (acoustic mode made unstable by the flame transfer function). With a flame surface obtained from steady state CFD simulation, and with acoustic mode shapes obtained from a Finite Element package, the forced acoustic response of the combustion system to the flame excitation was calculated. In a first validation step, this method has been tested on a single burner atmospheric test facility. In a second step, the method will be applied to an annular SEV combustion chamber of a GT26 ALSTOM gas turbine. The strength of this approach is that large models can be analyzed quickly to show the influence of changes in a flame position and effect of the combustor geometry. The weakness is that combustion instabilities can not be addressed by such a method. Furthermore, the phase relation of the excitation between different parts of the flame is frequency dependant and needs to be given as an input, which requires an experience and expert knowledge.

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Self-Excited High-Frequency Transverse Limit-Cycle Oscillations and Associated Flame Dynamics in a Gas Turbine Reheat Combustor Experiment

10.1115/gt2021-59540 ◽

2021 ◽

Author(s):

Jonathan McClure ◽

Frederik M. Berger ◽

Michael Bertsch ◽

Bruno Schuermans ◽

Thomas Sattelmayer

Keyword(s):

Heat Release ◽

Combustion Chamber ◽

Gas Turbine ◽

Limit Cycle ◽

High Frequency ◽

Dynamic Pressure ◽

High Amplitude ◽

Limit Cycle Oscillations ◽

Flame Dynamics ◽

Auto Ignition

Abstract This paper presents the investigation of high-frequency thermoacoustic limit-cycle oscillations in a novel experimental gas turbine reheat combustor featuring both auto-ignition and propagation stabilised flame zones at atmospheric pressure. Dynamic pressure measurements at the faceplate of the reheat combustion chamber reveal high-amplitude periodic pressure pulsations at 3 kHz in the transverse direction of the rectangular cross-section combustion chamber. Further analysis of the acoustic signal shows that this is a thermoacoustically unstable condition undergoing limit-cycle oscillations. A sensitivity study is presented which indicates that these high-amplitude limit-cycle oscillations only occur under certain conditions: namely high power settings with propane addition to increase auto-ignition propensity. The spatially-resolved flame dynamics are then investigated using CH* chemiluminescence, phase-locked to the dynamic pressure, captured from all lateral sides of the reheat combustion chamber. This reveals strong heat release oscillations close to the chamber walls at the instability frequency, as well as axial movement of the flame tips in these regions and an overall transverse displacement of the flame. Both the heat release oscillations and the flame motion occur in phase with the acoustic mode. From these observations, likely thermoacoustic driving mechanisms which lead to the limit-cycle oscillations are inferred. In this case, the overall flame-acoustics interaction is assumed to be a superposition of several effects, with the observations suggesting strong influences from autoignition-pressure coupling as well as flame displacement and deformation due to the acoustic velocity field. These findings provide a foundation for the overall objective of developing predictive approaches to mitigate the impact of high-frequency thermoacoustic instabilities in future generations of gas turbines with sequential combustion systems.

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Absolute and Convective Instability in Gas Turbine Fuel Injectors

Volume 2: Combustion, Fuels and Emissions, Parts A and B ◽

10.1115/gt2012-68253 ◽

2012 ◽

Cited By ~ 10

Author(s):

Matthew P. Juniper

Keyword(s):

Combustion Chamber ◽

Gas Turbine ◽

Hydrodynamic Instabilities ◽

Fuel Injector ◽

Predictive Tool ◽

Fuel Injectors ◽

Acoustic Modes ◽

Single Stream ◽

Lean Premixed ◽

Global Modes

Hydrodynamic instabilities in gas turbine fuel injectors help to mix the fuel and air but can sometimes lock into acoustic oscillations and contribute to thermoacoustic instability. This paper describes a linear stability analysis that predicts the frequencies and strengths of hydrodynamic instabilities and identifies the regions of the flow that cause them. It distinguishes between convective instabilities, which grow in time but are convected away by the flow, and absolute instabilities, which grow in time without being convected away. Convectively unstable flows amplify external perturbations, while absolutely unstable flows also oscillate at intrinsic frequencies. As an input, this analysis requires velocity and density fields, either from a steady but unstable solution to the Navier–Stokes equations, or from time-averaged numerical simulations. In the former case, the analysis is a predictive tool. In the latter case, it is a diagnostic tool. This technique is applied to three flows: a swirling wake at Re = 400, a single stream swirling fuel injector at Re ∼ 106, and a lean premixed gas turbine injector with five swirling streams at Re ∼ 106. Its application to the swirling wake demonstrates that this technique can correctly predict the frequency, growth rate and dominant wavemaker region of the flow. It also shows that the zone of absolute instability found from the spatio-temporal analysis is a good approximation to the wavemaker region, which is found by overlapping the direct and adjoint global modes. This approximation is used in the other two flows because it is difficult to calculate their adjoint global modes. Its application to the single stream fuel injector demonstrates that it can identify the regions of the flow that are responsible for generating the hydrodynamic oscillations seen in LES and experimental data. The frequencies predicted by this technique are within a few percent of the measured frequencies. The technique also explains why these oscillations become weaker when a central jet is injected along the centreline. This is because the absolutely unstable region that causes the oscillations becomes convectively unstable. Its application to the lean premixed gas turbine injector reveals that several regions of the flow are hydrodynamically unstable, each with a different frequency and a different strength. For example, it reveals that the central region of confined swirling flow is strongly absolutely unstable and sets up a precessing vortex core, which is likely to aid mixing throughout the injector. It also reveals that the region between the second and third streams is slightly absolutely unstable at a frequency that is likely to coincide with acoustic modes within the combustion chamber. This technique, coupled with knowledge of the acoustic modes in a combustion chamber, is likely to be a useful design tool for the passive control of mixing and combustion instability.

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