Significance of the Direct Excitation Mechanism for High-Frequency Response of Premixed Flames to Flow Oscillations

2020 ◽  
Author(s):  
Vishal Acharya ◽  
Tim Lieuwen
Keyword(s):  
Heat Release ◽  
Acoustic Waves ◽  
Premixed Flames ◽  
Axial Flow ◽  
Flame Length ◽  
Excitation Mechanism ◽  
Direct Mechanism ◽  
Direct Excitation ◽  
Flow Disturbances ◽  
Flame Response

Abstract Premixed flames are sensitive to flow disturbances, which can arise from acoustic or vortical fluctuations. For transverse instabilities, it is known that a dominant mechanism for flame response is “injector coupling”, whereby pressure oscillations associated with transverse waves excite axial flow disturbances. These axial flow disturbances then excite heat release oscillations. The objective of this paper is to consider another mechanism — the direct sensitivity of the unsteady heat release to transverse acoustic waves, and to compare its significance relative to the induced axial disturbances, in a linear framework. The rate at which the flame adds energy to the disturbance field is quantified using the Rayleigh criterion and evaluated over a range of control parameters, such as flame length and swirl number. The results show that radial modes induce heat release fluctuations that always add energy to the acoustic field, whereas heat release fluctuations induced by mixed radial-azimuthal modes can add or remove energy. These amplification rates are then compared to the flame response from induced axial fluctuations. For combustor centered flames, these results show that the direct excitation mechanism has negligible amplification rates relative to the induced axial mechanism for radial modes. For transverse modes, the fact that the nozzle is located at a pressure node indicates that negligible induced axial velocity disturbances are excited; as such, the direct mechanism dominates. For flames that are not centered on pressure nodes, the direct mechanism for mixed-modes, dominates for certain nozzle locations and flame angles.

Significance of the Direct Excitation Mechanism for High-Frequency Response of Premixed Flames to Flow Oscillations

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Vishal S. Acharya ◽  
Timothy C. Lieuwen
Keyword(s):  
Heat Release ◽  
Acoustic Waves ◽  
Premixed Flames ◽  
Axial Flow ◽  
Flame Length ◽  
Excitation Mechanism ◽  
Direct Mechanism ◽  
Direct Excitation ◽  
Flow Disturbances ◽  
Flame Response

Abstract Premixed flames are sensitive to flow disturbances, which can arise from acoustic or vortical fluctuations. For transverse instabilities, it is known that a dominant mechanism for flame response is “injector coupling,” whereby pressure oscillations associated with transverse waves excite axial flow disturbances. These axial flow disturbances then excite heat release oscillations. The objective of this paper is to consider another mechanism—the direct sensitivity of the unsteady heat release to transverse acoustic waves—and to compare its significance relative to the induced axial disturbances, in a linear framework. The rate at which the flame adds energy to the disturbance field is quantified using the Rayleigh criterion and evaluated over a range of control parameters, such as flame length and swirl number. The results show that radial modes induce heat release fluctuations that always add energy to the acoustic field, whereas heat release fluctuations induced by mixed radial-azimuthal modes can add or remove energy. These amplification rates are then compared to the flame response from induced axial fluctuations. For combustor-centered flames, these results show that the direct excitation mechanism has negligible amplification rates relative to the induced axial mechanism for radial modes. For transverse modes, the fact that the nozzle is located at a pressure node indicates that negligible induced axial velocity disturbances are excited; as such, the direct mechanism dominates. For flames that are not centered on pressure nodes, the direct mechanism for mixed modes dominates for certain nozzle locations and flame angles.

Flame Response to Transverse Acoustic Forcing With Minimal Axial Coupling

2017 ◽  
Author(s):  
Travis Smith ◽  
Benjamin Emerson ◽  
William Proscia ◽  
Tim Lieuwen
Keyword(s):  
Heat Release ◽  
Gas Turbines ◽  
Axial Flow ◽  
Release Mechanism ◽  
Direct Role ◽  
Flow Disturbances ◽  
Transverse Excitation ◽  
Transverse Acoustic ◽  
Flame Response

Instabilities associated with transverse acoustic modes are an important problem in gas turbines. A number of studies have reported results on the response of flames to transverse excitation, in order to understand the acoustic-velocity-heat release mechanism associated with combustion instabilities. However, all forced and self-excited transverse studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that the actual transverse disturbances play a negligible direct role in generating spatially integrated oscillatory heat release. Rather, they suggest that it is the induced axial disturbances that control the bulk of the heat release response. As such, there is a need to control the relative amplitudes of the axial and transverse disturbances exciting the flame, and determine their relative roles in the overall heat release response. This paper presents experimental results to address this issue. The flow field and flame edge were measured using 5kHz simultaneous sPIV and OH-PLIF, and the relative heat release fluctuations were measured through OH* chemiluminescence. The flame was forced with both strong transverse and axial oscillations, with various degrees of coupling between them, showing quite consistently that it is the axial flow disturbances that excite heat release oscillations. These observations demonstrate that the key role of the transverse motions is to set the “clock” for the frequency of the oscillations, but have negligible effect on the actual heat release disturbances exciting the instability. Rather, it is the axial disturbances, induced by inherent multi-dimensional effects that lead to the actual heat release oscillations.

Nonlinear Self-Excited Combustion Oscillations of a Premixed Laminar Flame

2011 ◽  
Vol 110-116 ◽  
pp. 1150-1154 ◽  
Author(s):  
Dan Zhao
Keyword(s):  
Heat Release ◽  
Acoustic Waves ◽  
Laminar Flame ◽  
Mode Selection ◽  
Kinematic Model ◽  
Classical Equation ◽  
Flow Disturbances ◽  
Flame Response ◽  
Premixed Laminar ◽  
Premixed Laminar Flame

Self-excited combustion oscillations are caused by a coupling between acoustic waves and unsteady heat release. A premixed laminar flame in a Rijke tube, anchored to a metal gauze, is considered in this work. The flame response to flow disturbances is investigated by developing a nonlinear kinematic model based on the classical-equation, with the assumption of a time-invariant laminar flame speed. Unsteady heat release from the flame is assumed to be caused by its surface variations, which results from the fluctuations of the oncoming flow velocity. The flame is acoustically compact, and its presence causes the mean temperature undergoing a jump, whose effect on the dynamics of the thermo-acoustic system is discussed. Coupling the flame model with a Galerkin series expansion of the acoustic waves present enables the time evolution of the flow disturbances to be calculated. It was found that the model can predict the mode shape and the frequencies of the excited combustion oscillations very well. Moreover, the fundamental mode is found to be the easiest one to be triggered among all acoustic modes. To gain insight about the mode selection and triggering, further numerical investigation is conducted by linearizing the flame model and recasting into the popular formulation.

Response of Non-Axisymmetric Premixed, Swirl Flames to Helical Disturbances

2014 ◽  
Author(s):  
Vishal Acharya ◽  
Timothy Lieuwen
Keyword(s):  
Heat Release ◽  
Surface Area ◽  
Local Area ◽  
Flame Surface ◽  
Flow Disturbances ◽  
Flame Response ◽  
Flow Oscillations ◽  
Phase Variations ◽  
The Mean ◽  
Flame Shape

Flow oscillations associated with hydrodynamic instabilities comprise a key element of the feedback loop during self-excited combustion driven oscillations. This work is motivated in particular by the question of how to scale thermoacoustic stability results from single nozzle or sector combustors to full scale systems. Specifically, this paper considers the response of non-axisymmetric flames to helical flow disturbances of the form u^i′∝expimθ where m denotes the helical mode number. This work closely follows prior studies of the response of axisymmetric flames to helical disturbances. In that case, helical modes induce strong flame wrinkling, but only the axisymmetric, m = 0 mode, leads to fluctuations in overall flame surface area and, therefore, heat release. All other helical modes induce local area/heat release fluctuations with azimuthal phase variations that cancel each other out when integrated over all azimuthal angles. However, in the case of mean flame non-axisymmetries, the azimuthal deviations on the mean flame surface inhibit such cancellations and the asymmetric helical modes (m ≠ 0) cause a finite global flame response. In this paper, a theoretical framework for non-axisymmetric flames is developed and used to illustrate how the flame shape influences which helical modes lead to net flame surface area fluctuations. Example results are presented which illustrate the contributions made by these asymmetric helical modes to the global flame response and how this varies with different control parameters such as degree of asymmetry in the mean flame shape or Strouhal number. Thus, significantly different sensitivities may be observed in single and multi-nozzle flames in otherwise identical hardware in flows with strong helical disturbances, if there are significant deviations in time averaged flame shape between the two, particularly if one of the cases is nearly axisymmetric.

Direct Numerical Simulation Study of Premixed Flame Response to Fuel-Air Ratio Oscillations

2011 ◽  
Author(s):  
Santosh Hemchandra
Keyword(s):  
Heat Release ◽  
Equivalence Ratio ◽  
Flame Length ◽  
Acoustic Oscillations ◽  
Lean Premixed Combustion ◽  
Fuel Injectors ◽  
Reduced Order ◽  
Hydrodynamic Coupling ◽  
Flame Response ◽  
Mass Flow Rates

The coupling between heat release oscillations produced by equivalence ratio fluctuations with combustor acoustic modes in lean premixed combustion systems, is a serious problem that limits the operation envelope of these devices. Such oscillations are produced by an oscillating pressure drop across air inlets and/or fuel injectors due to the presence of acoustic oscillations. This results in fluctuations in mass flow rates of air and/or fuel entering the combustor, thereby, changing the local equivalence ratio of the mixture at these injector/inlet locations. These perturbations in equivalence ratio are advected by the flow into the flame, causing its heat release to oscillate. Detailed reduced order models for the heat release response of premixed flames to equivalence ratio oscillations, based on this phenomenological picture, have been developed in the past. A key problem in validating these models is the ambiguity of interpretation of chemiluminescence signals when, the length scale of equivalence ratio fluctuations is smaller than the characteristic flame length. As such, the present work performs a DNS of a premixed methane-air flame, subject to unsteady forcing in upstream methane mass fraction. Predictions from prior reduced order modelling approaches are compared with present DNS results. The agreement between modelling and DNS predictions in the characteristics of flame response is good at low excitation frequencies and amplitudes. This agreement, however, degrades as forcing amplitude and frequency increase due to the influence of hydrodynamic coupling between the flow-fields on either side of the flame as well as damping of equivalence ratio perturbations by diffusion, on the dynamics of the flame.

Characterization of Forced Flame Response of Swirl-Stabilized Turbulent Lean-Premixed Flames in a Gas Turbine Combustor

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Kyu Tae Kim ◽  
Jong Guen Lee ◽  
Hyung Ju Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Transfer Function ◽  
Natural Gas ◽  
Gas Turbine ◽  
Strouhal Number ◽  
Premixed Flames ◽  
Flame Length ◽  
Gas Turbine Combustor ◽  
Flame Response ◽  
Flame Shape ◽  
Flame Angle

Flame transfer function measurements of turbulent premixed flames are made in a model lean-premixed, swirl-stabilized, gas turbine combustor. OH∗, CH∗, and CO2∗ chemiluminescence emissions are measured to determine heat release oscillation from a whole flame, and the two-microphone technique is used to measure inlet velocity fluctuation. 2D CH∗ chemiluminescence imaging is used to characterize the flame shape: the flame length (LCH∗ max) and flame angle (α). Using H2-natural gas composite fuels, XH2=0.00–0.60, a very short flame is obtained and hydrogen enrichment of natural gas is found to have a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of M flames is much smaller than that of V flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St=LCH∗ max/Lconv), the flame length (LCH∗ max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the nondimensional parameters, which collapse all relevant input conditions into the flame shape and the Strouhal number.

Effect of Cooling Liner on Acoustic Energy Absorption and Flame Response

2010 ◽  
Author(s):  
Umesh Bhayaraju ◽  
Johannes Schmidt ◽  
Karthik Kashinath ◽  
Simone Hochgreb
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Acoustic Waves ◽  
Bluff Body ◽  
Acoustic Energy ◽  
Combustion Instabilities ◽  
Acoustic Damping ◽  
Lean Combustion ◽  
Bias Flow ◽  
Flame Response

Gas turbine combustors with lean combustion injectors are prone to thermo-acoustic/combustion instabilities. Several passive techniques have been developed to control combustion instabilities, such as using Helmholtz resonators or viscous dampers using perforated liners that have potential for broadband acoustic damping. In this paper the role of single-walled cooling liners is considered in the damping of acoustic waves and on the flame transfer function in a sample bluff-body burner. Three liner geometries are considered: no bias flow (solid liner), normal effusion holes, and grazing effusion holes at 25° inclination. Cold flow experiments with speaker forcing are carried out to characterise the absorption properties of the liner and compared with an acoustic network model. The results show that whereas the bulk of the acoustic losses is due to the vortex recirculation zones, the liners contribute significantly to the absorption over a wide area of the frequency range. The flame transfer function gain is measured as a function of bias flow for a given operating condition of the burner. The experiments show that for the geometry considered, the global flame transfer function is little affected by cooling except in the case of the normal flow holes. Further analysis shows that whereas the total flame transfer function is not affected, the flame heat release becomes more spatially distributed along the axial length, and a 1D flame response shows distinct modes corresponding to the modal heat release locations.

Characterization of Forced Flame Response of Swirl-Stabilized Turbulent Lean-Premixed Flames in a Gas Turbine Combustor

2009 ◽  
Author(s):  
Kyu Tae Kim ◽  
Jong Guen Lee ◽  
Hyung Ju Lee ◽  
Bryan D. Quay ◽  
Domenic Santavicca
Keyword(s):  
Transfer Function ◽  
Natural Gas ◽  
Gas Turbine ◽  
Strouhal Number ◽  
Premixed Flames ◽  
Flame Length ◽  
Gas Turbine Combustor ◽  
Flame Response ◽  
Flame Shape ◽  
Flame Angle

Flame transfer function measurements of turbulent premixed flames were made in a model lean premixed, swirl-stabilized, gas turbine combustor. OH*, CH*, and CO2* chemiluminescence emissions were measured to determine heat release oscillation from a whole flame, and the two-microphone technique was used to measure inlet velocity fluctuation. 2-D CH* chemiluminescence imaging was used to characterize the flame shape: the flame length (LCH* max) and flame angle (α). Using H2-natural gas composite fuels, XH2 = 0.00 ∼ 0.60, very short flame was obtained and hydrogen enrichment of natural gas had a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of “M” flames is much smaller than that of “V” flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St = LCH* max / Lconv), the flame length (LCH* max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the non-dimensional parameters which collapse all relevant input conditions into the flame shape and the Strouhal number.

INTERACTIONS BETWEEN ACOUSTIC WAVES AND PREMIXED FLAMES IN POROUS CHAMBERS

1994 ◽  
Author(s):  
CHO-FAN TSENG ◽  
I. TSENG ◽  
WEN-WEI CHU ◽  
VIGOR YANG
Keyword(s):  
Acoustic Waves ◽  
Premixed Flames
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