The Aerodynamic Response of Fuel Injector Passages to Incident Acoustic Waves

2015 ◽  
Author(s):  
Jialin Su ◽  
Andrew Garmory ◽  
Jon Carrotte
Keyword(s):  
Numerical Simulations ◽  
Heat Release ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Experimental Measurements ◽  
Similar Data ◽  
Fuel Injector ◽  
Acoustic Characteristics ◽  
Fuel Injectors ◽  
Low Emission

Modern low emission combustion systems are more prone to combustion instabilities due to operation at lean conditions. The response of the airflow passing through the injector to incident acoustic waves is therefore of interest. Airflow fluctuations can initiate, for example, perturbations in stoichiometry and velocity that are subsequently delivered into the heat release region. In the case of liquid fuelled gas turbines the atomisation process will also be affected. Such effects can lead to further unsteady heat release and the generation of acoustic waves, thereby leading to combustion instability. This paper describes experimental measurements and the development of a numerical methodology by which the unsteady airflow response of complex, modern, low emission fuel injectors can be characterised. Single and two passage injector configurations have been investigated which broadly capture many of the features associated with modern fuel injectors. Although targeted at low emission (lean burn) liquid fuelled injector geometries, the methodology developed is thought applicable to a wide range of injector configurations. Initially experimental measurements were used to characterise the overall acoustic impedance of each injector design over a range of frequencies. Such information is also required for the low order thermo-acoustic network models, as typically used in the design process, to predict the stability of the combustion system. In addition to the experimental measurements a methodology was developed using unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations in which acoustic boundary conditions were implemented to reproduce the experimental scenarios. Interrogation of the pressure field enabled similar data analysis techniques to be applied to the numerical data for determining the injector acoustic characteristics. Fidelity of the numerical simulations is confirmed by the excellent agreement between the experimental data and numerical simulations. Furthermore, the unsteady flow field within the passages is difficult to access experimentally, but can be examined in more detail from the simulation results. In this way an improved understanding of the passage flows and their individual responses to the incident acoustic pressure waves can be obtained. The numerical approach is aimed at providing a computationally efficient and economic tool for predicting the acoustic characteristics of the complex geometries typical of modern fuel injector designs. Using this tool injector designs with different acoustic response characteristics can be developed relatively quickly.

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.

Measurement and Analysis of Flame Transfer Function in a Sector Combustor Under High Pressure Conditions

2003 ◽  
Author(s):  
W. S. Cheung ◽  
G. J. M. Sims ◽  
R. W. Copplestone ◽  
J. R. Tilston ◽  
C. W. Wilson ◽  
...  
Keyword(s):  
High Pressure ◽  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Mass Flow ◽  
Atmospheric Pressure ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Transfer Functions ◽  
Unsteady Pressure

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. A flame transfer function describes the change in the rate of heat release in response to perturbations in the inlet flow as a function of frequency. It is a quantitative assessment of the susceptibility of combustion to disturbances. The resulting fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. Flame transfer functions for LPP combustion are poorly understood at present but are crucial for predicting combustion oscillations. This paper describes an experiment designed to measure the flame transfer function of a simple combustor incorporating realistic components. Tests were conducted initially on this combustor at atmospheric pressure (1.2 bar and 550 K) to make an early demonstration of the combustion system. The test rig consisted of a plenum chamber with an inline siren, followed by a single LPP premixer/duct and a combustion chamber with a silencer to prevent natural instabilities. The siren was used to induce variable frequency pressure/acoustic signals into the air approaching the combustor. Both unsteady pressure and heat release measurements were undertaken. There was good coherence between the pressure and heat release signals. At each test frequency, two unsteady pressure measurements in the plenum were used to calculate the acoustic waves in this chamber and hence estimate the mass-flow perturbation at the fuel injection point inside the LPP duct. The flame transfer function relating the heat release perturbation to this mass flow was found as a function of frequency. The same combustor hardware and associated instrumentation were then used for the high pressure (15 bar and 800 K) tests. Flame transfer function measurements were taken at three combustion conditions that simulated the staging point conditions (Idle, Approach and Take-off) of a large turbofan gas turbine. There was good coherence between pressure and heat release signals at Idle, indicating a close relationship between acoustic and heat release processes. Problems were encountered at high frequencies for the Approach and Take-off conditions, but the flame transfer function for the Idle case had very good qualitative agreement with the atmospheric-pressure tests. The flame transfer functions calculated here could be used directly for predicting combustion oscillations in gas turbine using the same LPP duct at the same operating conditions. More importantly they can guide work to produce a general analytical model.

Experimental Development of On-Line Flame Transfer Function Measurements for Fielded Gas Turbines

2021 ◽  
Author(s):  
Austin Matthews ◽  
Anna Cobb ◽  
Subodh Adhikari ◽  
David Wu ◽  
Tim Lieuwen ◽  
...  
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Gas Turbines ◽  
High Speed ◽  
Transfer Functions ◽  
Full Scale ◽  
Fuel Injector ◽  
Experimental Development ◽  
On Line

Abstract Understanding thermoacoustic instabilities is essential for the reliable operation of gas turbine engines. To complicate this understanding, the extreme sensitivity of gas turbine combustors can lead to instability characteristics that differ across a fleet. The capability to monitor flame transfer functions in fielded engines would provide valuable data to improve this understanding and aid in gas turbine operability from R&D to field tuning. This paper presents a new experimental facility used to analyze performance of full-scale gas turbine fuel injector hardware at elevated pressure and temperature. It features a liquid cooled, fiber-coupled probe that provides direct optical access to the heat release zone for high-speed chemiluminescence measurements. The probe was designed with fielded applications in mind. In addition, the combustion chamber includes an acoustic sensor array and a large objective window for verification of the probe using high-speed chemiluminescence imaging. This work experimentally demonstrates the new setup under scaled engine conditions, with a focus on operational zones that yield interesting acoustic tones. Results include a demonstration of the probe, preliminary analysis of acoustic and high speed chemiluminescence data, and high speed chemiluminescence imaging. The novelty of this paper is the deployment of a new test platform that incorporates full-scale engine hardware and provides the ability to directly compare acoustic and heat release response in a high-temperature, high-pressure environment to determine the flame transfer functions. This work is a stepping-stone towards the development of an on-line flame transfer function measurement technique for production engines in the field.

Experimental Investigation of Spray and Combustion Performances of a Fuel-Staged Low Emission Combustor: Part II — Effects of Venturi Angle

2018 ◽  
Author(s):  
Cunxi Liu ◽  
Fuqiang Liu ◽  
Jinhu Yang ◽  
Yong Mu ◽  
Gang Xu
Keyword(s):  
Flow Field ◽  
High Altitude ◽  
Gas Turbines ◽  
Fuel Injection ◽  
Operating Conditions ◽  
Combustion Stability ◽  
Main Stage ◽  
Fuel Injector ◽  
Lean Burn ◽  
Low Emission

In order to reduce NOx emissions, modern gas turbines are often equipped with lean burn combustion systems, where the high-velocity fuel-lean conditions that limit NOx formation in combustors also inhibit the ability of ignition, high altitude relight, and lean combustion stability. To face these issues, an internally staged scheme of fuel injection is proposed. The pilot and main fuel staging enable fuel distribution control and high turn-down ratio, multi-injections of main fuel leads to a fast and efficient fuel/air mixing. A fuel-staged low emission combustor in the framework of lean burn combustion is developed in the present study, the central pilot stage of fuel injector working singly at low power operating conditions is swirl-cup prefilming atomization and main stage is jet-in-crossflow multi-injection atomization, a combination of pilot and main stage injection is provided for higher power operating conditions. A significant amount of the air mass flow utilised for fuel preparation and initiation is adverse to the operability specifications, such as ignition, lean blow-out, and high-altitude relight etc. The spray characteristics of pilot spray and flow field are one of the key factors affecting combustion operability. This work investigates the effects of the venturi angle on combustion operability, the ignition and lean blow-out performances were evaluated in a single dome rectangular combustor. Furthermore, the spray patterns and flow field are characterized by kerosene-planar laser induced fluorescence and particle image velocimetry to provide insight into the correlation between spray, flow field and combustion operability performances.

Using CFD for Time-Delay Modeling of Premix Flames

2001 ◽  
Author(s):  
Peter Flohr ◽  
Christian Oliver Paschereit ◽  
Bart van Roon ◽  
Bruno Schuermans
Keyword(s):  
Time Delay ◽  
Heat Release ◽  
Time Delays ◽  
Flame Temperature ◽  
Model Fitting ◽  
Operating Conditions ◽  
Fuel Injector ◽  
Fuel Injectors ◽  
Flame Shape ◽  
Good Agreement

This paper presents a refined model of the transfer function of a premix burner, compares the model with experiments, and discusses how the model can be used to map stability characteristics of a combustion system. The model is based on the assumption that acoustic velocity fluctuations cause modulations of fuel concentration at the fuel injector which, after a time delay, result in fluctuating heat release rates at the flame. Here, the time delay is modeled as a multitude of single time delays. The distribution of these time delays can be found either from model fitting to experimental data, or can be obtained directly from numerical simulations of the burner. The effect of distributed time delays is caused by axially distributed fuel injectors, turbulent diffusion, and a non-planar flame shape. As a consequence, heat release fluctuations at higher frequencies cancel, an effect which is also observed experimentally. It is found that the model is generally in good agreement with experiments. It is also demonstrated that the model can be used to map the burner stability charactistics for various operating conditions, e.g. for variations in power and flame temperature. A stability analysis is performed by incorporating the flame model into a combustor network model.

Thermoacoustic Oscillations in an Annular Combustor

2001 ◽  
Author(s):  
Simon R. Stow ◽  
Ann P. Dowling
Keyword(s):  
Heat Release ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Radial Dependence ◽  
Sustained Oscillations ◽  
Resonant Modes ◽  
Annular Combustor ◽  
Propagation Matrix ◽  
Flame Shape ◽  
Thermoacoustic Oscillations

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. Acoustic waves produce fluctuations in heat release, for instance by perturbing the fuel–air ratio or flame shape. These heat fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. A linear model for thermoacoustic oscillations in LPP combustors is described. A thin annular combustor is assumed and so circumferential modes are included but radial dependence is ignored. The geometry consists of straight ducts joined by short regions of area change. Perturbations to the flow can be thought of as a combination of acoustic, entropy and vorticity waves. The development of these waves along the straight ducts is found using a propagation matrix approach. At the entrance to the combustion chamber, a flame model is used in which the unsteady heat release is related to fluctuations in fuel–air ratio. Various possible inlet and outlet conditions are described. The model is then applied to a simplified example based on a sector rig. The resonant modes are found numerically and compared with the frequencies that occurred in experiments.

The Influence of Geometric and Aerodynamic Boundary Conditions on Fuel Injector Feed and External Aerodynamics for Lean Burn Combustors

2019 ◽  
Author(s):  
Y. Li ◽  
P. A. Denman ◽  
A. D. Walker
Keyword(s):  
Flow Field ◽  
Gas Turbines ◽  
Combustion System ◽  
Fuel Injector ◽  
Flow Uniformity ◽  
Lean Burn ◽  
Fuel Injectors ◽  
Systematic Assessment ◽  
Flame Tube ◽  
External Combustion

Abstract Lean burn combustion is currently a preferred technology to meet the future low emission requirements faced by aero gas turbines. Previous work has shown that the increased air mass flow and size of lean burn fuel injector alters the necessary redistribution of the airflow leaving the high-pressure compressor. This can lead to flow field non-uniformities in the feed to combustor annuli and the fuel injectors which have the potential to impact the overall performance of the combustion system. This paper presents a systematic assessment of the effect of several aerodynamic parameters on the air flow feed to the fuel injectors and the external combustion system aerodynamics for a generic lean burn system. This includes the effect of changes to the flow splits between various combustor cooling features and annulus flows and the effect of a biased compressor exit profile. Flow field data are generated using an isothermal RANS CFD model which is validated against test rig data. The data show that changes in the flow split between the annuli modified the flow uniformity and loss to both the combustor annuli and the fuel injector feed. Changes in the compressor exit profile have a larger effect introducing more notable variations in both flow uniformity and loss. Changes to the angle of the flame tube did not greatly affect the pre-diffuser but did modify annulus loss. Further analysis showed that changes to the combustor annulus flow split, compressor exit profile and flame tube angle modified the location, at compressor exit, of the flow captured by the annuli or each fuel injector passage. The loss to each of these depends on the flow quality (total pressure and uniformity) and from the source more than the flow uniformity delivered.

Development of Flashback Resistant Low-Emission Micro-Mixing Fuel Injector for 100% Hydrogen and Syngas Fuels

2009 ◽  
Author(s):  
Howard Lee ◽  
Steve Hernandez ◽  
Vincent McDonell ◽  
Erlendur Steinthorsson ◽  
Adel Mansour ◽  
...  
Keyword(s):  
Axial Flow ◽  
Elevated Pressure ◽  
Small Scale ◽  
Fuel Injector ◽  
Fuel Injectors ◽  
Lean Blowout ◽  
Conflicting Demands ◽  
Fuel Flexibility ◽  
Low Emission ◽  
Air Mixing

The present work extends previous efforts using “micro-mixing” fuel injectors operating on hydrogen fuel to elevated pressure and temperature and includes initial evaluation of a second injector concept. A micro-mixing fuel injector consists of multiple, small and closely spaced mixing cups, within which fuel and air mix rapidly at a small scale. The micro-mixing injection strategy offers inherent flexibility for the accommodation of staging, dilution, and fuel flexibility, and the manufacturing technology employed for building the cups affords great flexibility to address the conflicting demands of superior fuel-air mixing and flash-back avoidance. In the present work, both radial and axial flow micromixing concepts are investigated using experiments and computational fluid dynamics. The hydrogen/air reaction structure is captured using OH* chemiluminescence at 308 nm, recorded using a 16-bit thermoelectrically cooled ICCD with a UV sensitive phosphor. Instantaneous images are used to assess flashback tendencies at pressures up to 8 atm for reaction temperatures approaching 2000 K. Emissions are measured at the exit of the combustor liner using EPA certified methodologies. The results demonstrate that both concepts can produce low NOx emissions while remaining robust relative to flashback and lean blowout. The radial concepts offer superior emissions performance, while the axial concepts offer superior flashback tendencies. Based on the results obtained to date, the micro-mixing approach appears promising relative to achieving flashback free operation with low emissions at pressures up to 8 atm while maximizing scalability and fuel flexibility.

Influence of Flame and Burner Transfer Matrix on Thermoacoustic Combustion Instability Modes and Frequencies

2010 ◽  
Author(s):  
Giovanni Campa ◽  
Sergio Mario Camporeale
Keyword(s):  
Heat Release ◽  
Combustion Chamber ◽  
Transfer Matrix ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Combustion Instability ◽  
Experimental Tests ◽  
Full Scale ◽  
Operating Conditions ◽  
Actual Geometry

The main origin of combustion instability in modern gas turbines is considered to be related to the interaction between acoustic waves and flame perturbations. An important role is played by the characteristics of combustion chamber and burners, because they influence the operating conditions at which the instability may occur. Experimental tests carried out on single burner arrangements fail to give adequate indications for the design of a full scale combustion chamber, due to the interaction of the local flame fluctuations with the propagation of the pressure waves, that have a wavelength of the same order of magnitude of the main dimensions of the chamber. Therefore there is a large interest on developing techniques able to make use of the data gathered from tests carried out on a single burner for predicting the thermoacoustic behavior of the combustion chamber at full scale with its actual geometry. A three dimensional finite element code has been developed for predicting acoustically driven combustion instabilities in combustion systems with complex geometries. The code allows one to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability, under the hypothesis of linear behaviour of the acoustic waves. The code permits to represent heat release fluctuations through an n–τ Flame Transfer Function (FTF) model and to adopt the transfer matrix method for modelling the burners. The FTF and the burner transfer matrix (BTM), as well as the temperature field and the flame location, needed for the simulation, can be obtained from experimental tests. Moreover, the code is able to make use of the local distribution of n and τ that can be evaluated from computational fluid dynamic studies on the single burner. The paper shows the importance of the flame characteristics, such as dimensions and shape of the heat release zone and its location within the combustor, underlying their influence on the instability of the modes and so the potential of the proposed method as a design tool for defining the burner characteristics and the acoustic impedance at the boundaries of the combustion chamber.

Ignition Delay Time Modulation as a Contribution to Thermo-Acoustic Instability in Sequential Combustion

2000 ◽  
Author(s):  
Alexander Ni ◽  
Wolfgang Polifke ◽  
Franz Joos
Keyword(s):  
Pressure Drop ◽  
Heat Release ◽  
Ignition Delay ◽  
Acoustic Waves ◽  
Delay Time ◽  
Ignition Delay Time ◽  
Operating Conditions ◽  
Flame Velocity ◽  
Fuel Injector ◽  
Time Modulation

Pressure pulsations due to combustion instabilities have been encountered in a premixed sequential gas turbine combustor. Measured noise spectra display one or several distinct peaks at Strouhal numbers significantly larger than unity. Height and location of the peaks depend in a sensitve manner on fuel type and/or operating conditions. The paper identifies a possible mechanism of the observed combustion instability and presents a mathematical model of acoustic self-excitation. The mechanism of self-excitation comprises interactions between the acoustic field in the fuel injector / burner with the ignition delay time of the fuel-air mixture and the heat release intensity: • pressure drop in the fuel injector nozzle changes with variations of the acoustic pressure in the burner, • variations of pressure drop and air flow velocity modulate the fuel concentration, • acoustic perturbations in the pre-flame region influence the delay time for self-ignition and consequently lead to fluctuations of flame velocity and -position. • fluctuations of flame velocity influence the refracation of acoustic waves at the flame front. • fuel inhomogeneities modulate the heat release rate and consequently the rate of volume production by the flame. Based on this structure of a self-excitation mechanism, an analytical model has been developed and used to compute eigenfrequencies and growth rates of instabilities. Some characteristics of the suggested self-excitated instabilities as they are predicted by the model match well with empirical information.

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