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.

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.

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.

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.

Modeling the Response of Premixed Flames to Mixture Ratio Perturbations

2003 ◽  
Author(s):  
Ju Hyeong Cho ◽  
Tim C. Lieuwen
Keyword(s):  
Transfer Function ◽  
Equivalence Ratio ◽  
Flame Length ◽  
Flame Speed ◽  
Heat Of Reaction ◽  
Mean Flow ◽  
Mixture Ratio ◽  
Flame Response ◽  
The Mean ◽  
Flame Position

Combustion instabilities continue to cause significant reliability and availability problems in low emissions gas turbine combustors. It is known that these instabilities are often caused by a self-exciting feedback loop between unsteady heat release rate and reactive mixture equivalence ratio perturbations. We present an analysis of the flame’s response to equivalence ratio perturbations by considering the kinematic equations for the flame front. This response is controlled by three processes: heat of reaction, flame speed, and flame area. The first two are directly generated by equivalence ratio oscillations. The third is indirect, as it is generated by the flame speed fluctuations. The first process dominates the response of the flame at low Strouhal numbers, roughly defined as frequency times flame length divided by mean flow velocity. All three processes play equal roles at Strouhal numbers of O(1). The mean equivalence ratio exerts little effect upon this transfer function at low Strouhal numbers. At O(1) Strouhal numbers, the flame response increases with decreasing values of the mean equivalence ratio. Thus, these results are in partial agreement with heuristic arguments made in prior studies that the flame response to equivalence ratio oscillations increases as the fuel/air ratio becomes leaner. In addition, a result is derived for the sensitivity of this transfer function to uncertainties in mean flame position. For example, a sensitivity of 10 means that a 5% uncertainty in flame position translates into a 50% uncertainty in transfer function. This sensitivity is of O(1) for St<<1, but has very high values for St∼O(1).

Premixed Swirling Flame Response to Acoustic Forcing Studied With High-Speed PIV and OH* Chemiluminescence

2015 ◽  
Author(s):  
Alexey Denisov ◽  
Abhishek Ravi
Keyword(s):  
Heat Release ◽  
High Speed ◽  
Decomposition Analysis ◽  
Acoustic Oscillations ◽  
Acoustic Frequency ◽  
Precessing Vortex Core ◽  
Technological Advances ◽  
Flame Response ◽  
Swirling Flame ◽  
Acoustic Forcing

Studies of swirling flames have been boosted by technological advances in high-speed lasers and cameras. Temporal and spatial evolution of swirling flows has been revealed by high-speed particle image velocimetry (PIV). We have studied the response of a perfectly premixed swirling flame to weak acoustic perturbations induced by a pair of loudspeakers upstream of the burner. Phase-resolved response of the flame was observed with PIV and OH* chemiluminescence measurements running at 12 times the forcing frequency. The flow dynamics was not affected by the flame compared to non-reacting conditions and the flame responded to flow variations by changing its angle. Proper orthogonal decomposition analysis revealed that the strongest coherent structure in the flow was precessing vortex core that caused transversal variation of the heat release without producing acoustic oscillations. Axisymmetric vortices were not observed at this level of acoustic forcing, but precession modes were modulated at acoustic frequency as additional frequency peaks appeared at the sum and the difference of precession and forcing frequencies. Average time of vortex convection from the burner to the flame is close to the delay of the flame response to acoustic forcing, measured by microphones. This supports the importance of vortex propagation to acoustic modulation of flame heat release.

Fuel-Forced Flame Response of a Lean-Premixed Combustor

2011 ◽  
Author(s):  
Poravee Orawannukul ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Heat Release ◽  
Flow Rate ◽  
Equivalence Ratio ◽  
Operating Conditions ◽  
Chemiluminescence Intensity ◽  
Combustion Dynamics ◽  
Lean Premixed ◽  
Flame Response ◽  
Rate Of Heat Release ◽  
The Relationship

The response of a swirl-stabilized flame to equivalence ratio fluctuations is experimentally investigated in a single-nozzle lean premixed combustor. Equivalence ratio fluctuations are produced using a siren device to modulate the flow rate of fuel to the injector, while the air flow rate is kept constant. The magnitude and phase of the equivalence ratio fluctuations are measured near the exit of the nozzle using an infrared absorption technique. The flame response is characterized by the fluctuation in the flame’s overall rate of heat release, which is determined from the total CH* chemiluminescence emission from the flame. The relationship between total CH* chemiluminescence intensity and the flame’s overall rate of heat release is determined from a separate calibration experiment which accounts for the nonlinear relationship between chemiluminescence intensity and equivalence ratio. Measurements of the normalized equivalence ratio fluctuation and the normalized rate of heat release fluctuation are made over a range of modulation frequencies from 200 Hz to 440 Hz, which corresponds to Strouhal numbers from 0.4 to 2.8. These measurements are used to determine the fuel-forced flame transfer function which expresses the relationship between the equivalence ratio and rate of heat release fluctuations in terms of a gain and phase as a function of frequency. In addition, phase-synchronized CH* chemiluminescence images are captured to study the dynamics of the flame response over the modulation period. These measurements are made over a range of operating conditions and the results are analyzed to identify and better understand the mechanisms whereby equivalence ratio fluctuations result in fluctuations in the flame’s overall rate of heat release. Such information is essential to guide the formulation and validation of analytical fuel-forced flame response models and hence to predict combustion dynamics in gas turbine combustors.

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.

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