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.

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.

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.

scholarly journals Role of induced axial acoustics in transverse acoustic flame response

2018 ◽  
Vol 195 ◽  
pp. 140-150 ◽  
Author(s):  
Travis Smith ◽  
Benjamin Emerson ◽  
William Proscia ◽  
Tim Lieuwen
Keyword(s):  
Transverse Acoustic ◽  
Flame Response

Acoustic-Convective Interference in Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

2021 ◽  
Author(s):  
Eirik Æs⊘y ◽  
José G. Aguilar ◽  
Mirko R. Bothien ◽  
Nicholas Worth ◽  
James Dawson
Keyword(s):  
Bluff Body ◽  
Transfer Functions ◽  
Pure Hydrogen ◽  
Mean Velocity ◽  
Flow Disturbances ◽  
Flame Response ◽  
Vortex Shedding Frequency ◽  
Flame Imaging ◽  
Velocity Spectra

Abstract We investigate the occurrence of modulations in the gain and phase of flame transfer functions (FTF) measured in CH4/H2 and pure H2 flames. These are shown to be caused by flow disturbances originating from the screws used to centre the bluff body indicative of a more generalised phenomenon of convective wave propagation. Velocity measurements are performed around the injector dump plane, inside the injector pipe, and in the wake of the bluff body to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the screws appear in the unforced velocity spectra and the magnitude of these convective modes depends on the screws’ location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear axisymmetric. However, the rms fields capture a strong asymmetry due to convective disturbances. To quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. For acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cut-off frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cut-off frequency.

Large Eddy Simulation of Flame Response to Transverse Acoustic Excitation in a Model Reheat Combustor

2012 ◽  
Author(s):  
M. Zellhuber ◽  
C. Meraner ◽  
R. Kulkarni ◽  
W. Polifke ◽  
B. Schuermans
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Shear Layers ◽  
Harmonic Excitation ◽  
Single Frequency ◽  
Jet Flame ◽  
Eddy Simulation ◽  
Auto Ignition ◽  
Transverse Acoustic ◽  
Flame Response

The response of a perfectly premixed, turbulent jet flame at elevated inflow temperature to high frequency flow perturbations is investigated. A generic reheat burner geometry is considered, where the spatial distribution of heat release is controlled by auto-ignition in the jet core on the one hand, and kinematic balance between flow and flame propagation in the shear layers between the jet and the external recirculation zones on the other. To model auto-ignition and heat release in compressible turbulent flow, a progress variable / stochastic fields formulation adapted for the LES context is used. Flow field perturbations corresponding to transverse acoustic modes are imposed by harmonic excitation of velocity at the combustor boundaries. Simulations with single-frequency excitation are carried out in order to study the flame response to transverse fluctuations of velocity. Heat release fluctuations are observed predominantly in the shear layers, where flame propagation is important. The flow-flame coupling in these regions is analysed in detail with a filter-based post-processing approach, invoking a local Rayleigh index and providing insight into the interactions of flame wrinkling by vorticity and convection due to mean and fluctuating velocity.

Nonlinear Heat-Release/Acoustic Interactions in a Gas Turbine Combustor

2004 ◽  
Author(s):  
Ben Bellows ◽  
Tim Lieuwen
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbines ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Amplitude Dependence ◽  
Acoustic Oscillations ◽  
Bunsen Flame ◽  
Flame Response ◽  
Acoustic Transfer Function

This paper describes an experimental investigation of the response of the flame in a lean, premixed combustor to imposed acoustic oscillations. The ultimate objective of this work is to develop capabilities for predicting the amplitude of combustion instabilities in gas turbines. Simultaneous measurements of CH* and OH* chemiluminescence, pressure, and velocity were obtained over a range of forcing amplitudes and frequencies. These data show that nonlinearity in the heat release/acoustic transfer function is manifested in two ways. First, the flame chemiluminescence response to imposed oscillations saturates at pressure and velocity amplitudes on the order of p’/po ∼0.02 and u’/uo∼0.3. In addition, the phase between the CH* or OH* oscillations and acoustic oscillations exhibits some amplitude dependence, even at disturbance amplitudes where the amplitude transfer function is linear. We also find that the response of this swirling, highly turbulent flame exhibits similarities to those of simple, laminar flame configurations. First, the “linear”, low amplitude flame response is similar to the laminar, V-flame measurements and predictions of Schuller et al. [1]. Also, at large disturbance amplitudes, the subharmonic characteristics of the oscillations exhibit analogous characteristics to those observed by Bourehla & Baillot [2] in a conical Bunsen flame, and Searby & Rochwerger [3] in a flat flame.

Acoustic-Convective Interference In Transfer Functions of Methane/Hydrogen and Pure Hydrogen Flames

2021 ◽  
Author(s):  
Eirik Æsøy ◽  
José G. Aguilar ◽  
Mirko R. Bothien ◽  
Nicholas A. Worth ◽  
James R. Dawson
Keyword(s):  
Bluff Body ◽  
Transfer Functions ◽  
Pure Hydrogen ◽  
Mean Velocity ◽  
Flow Disturbances ◽  
Flame Response ◽  
Vortex Shedding Frequency ◽  
Flame Imaging ◽  
Velocity Spectra

Abstract We investigate the occurrence and source of modulations in the gain and phase of flame transfer functions (FTF) measured in perfectly premixed, bluff body stabilised CH4/H2 and pure H2 flames. The modulations are shown to be caused by flow disturbances originating from the upstream geometry, in particular the grub screws used to centre the bluffbody, indicative of a more generalised phenomenon of convective wave propagation. Velocity measurements are performed at various locations around the injector dump plane, inside the injector pipe, and in the wake of the bluffbody to provide detailed insight into the flow. Peaks corresponding to natural shedding frequencies of the grub screws appear in the unforced velocity spectra and it is found that the magnitude of these convective modes depends on their location. Flame imaging and PIV measurements show that these disturbances do not show up in the mean velocity and flame shape which appear approximately axisymmetric. However, the urms and vrms fields capture a strong asymmetry due to convective disturbances. To further quantify the role of these convective disturbances, hydrodynamic transfer functions are constructed from the forced cold flow, and similar modulations observed in the FTFs are found. A strong correlation is obtained between the two transfer functions, subsequently, the modulations are shown to be centered on the vortex shedding frequency corresponding to the first convective mode. The reason behind the excitation of the first mode is due to a condition that states that for acoustic-convective interaction to be possible, the shedding (convective) frequency needs to be lower than the cut-off frequency of the flame response. This condition is shown to be more relevant for hydrogen flames compared to methane flames due to their shorter flame lengths and thus increased cut-off frequency.

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.

Large Eddy Simulation of Flame Response to Transverse Acoustic Excitation in a Model Reheat Combustor

2013 ◽  
Vol 135 (9) ◽  
Author(s):  
Mathieu Zellhuber ◽  
Christoph Meraner ◽  
Rohit Kulkarni ◽  
Wolfgang Polifke ◽  
Bruno Schuermans
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Shear Layers ◽  
Harmonic Excitation ◽  
Single Frequency ◽  
Jet Flame ◽  
Eddy Simulation ◽  
Transverse Acoustic ◽  
Flame Response ◽  
Compressible Turbulent Flow

The response of a perfectly premixed, turbulent jet flame at elevated inflow temperature to high frequency flow perturbations is investigated. A generic reheat burner geometry is considered, where the spatial distribution of heat release is controlled by autoignition in the jet core on the one hand, and kinematic balance between flow and flame propagation in the shear layers between the jet and the external recirculation zones on the other. To model autoignition and heat release in compressible turbulent flow, a progress variable/stochastic fields formulation adapted for the LES context is used. Flow field perturbations corresponding to transverse acoustic modes are imposed by harmonic excitation of velocity at the combustor boundaries. Simulations with single-frequency excitation are carried out in order to study the flame response to transverse fluctuations of velocity. Heat release fluctuations are observed predominantly in the shear layers, where flame propagation is important. The flow-flame coupling in these regions is analyzed in detail with a filter-based postprocessing approach, invoking a local Rayleigh index and providing insight into the interactions of flame wrinkling by vorticity and convection due to mean and fluctuating velocity.

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