Impact of Shear Flow Instabilities on the Magnitude and Saturation of the Flame Response

2013 ◽  
Author(s):  
Steffen Terhaar ◽  
Bernhard Ćosić ◽  
Christian Oliver Paschereit ◽  
Kilian Oberleithner
Keyword(s):  
Transfer Function ◽  
Shear Flow ◽  
High Speed ◽  
Transfer Functions ◽  
Flow Fields ◽  
Shear Layers ◽  
Flow Instabilities ◽  
Mean Flow ◽  
Swirl Number ◽  
Flame Response

Amplitude-dependent flame transfer functions, also denoted as flame describing functions, are valuable tools for the prediction of limit-cycle amplitudes of thermoacoustic instabilities. However, the effects that govern the transfer function magnitude at low and high amplitudes are not yet fully understood. It is shown in the present work that the flame response at perfectly premixed conditions is dominated by the growth rate of vortical structures in the shear layers. An experimental study in a generic swirl-stabilized combustor was conducted in order to measure the amplitude-dependent flame transfer function and the corresponding flow fields subjected to acoustic forcing. The applied measurement techniques included the Multi-Microphone-Method, high-speed OH*-chemiluminescence measurements, and high-speed Particle Image Velocimetry. The flame response and the corresponding flow fields are assessed for three different swirl numbers at 196 Hz forcing frequency. The results show that forcing leads to significant changes in the time-averaged reacting flow fields and flame shapes. A triple decomposition is applied to the time-resolved data, which reveals that coherent velocity fluctuations at the forcing frequency are amplified considerably stronger in the shear layers at low forcing amplitudes than at high amplitudes, an indicator for a nonlinear saturation process. The strongest saturation is found for the lowest swirl number, where the forcing additionally detached the flame. For the highest swirl number, the saturation of the vortex amplitude is weaker. Overall, the amplitude-dependent vortex amplification resembles the characteristics of the flame response very well. An application of linear stability analysis to the time-averaged flow fields at increasing forcing amplitudes yields decreasing growth rates of shear flow instabilities at the forcing frequency. It therefore successfully predicts a saturation at high forcing amplitudes and demonstrates that the mean flow field and its modifications are of utmost importance for the growth of vortices in the shear layers. Moreover, the results clearly show that the amplification of vortices in the shear layers is a dominant driver for heat release fluctuations and their saturation.

Impact of Shear Flow Instabilities on the Magnitude and Saturation of the Flame Response

2014 ◽  
Vol 136 (7) ◽  
Author(s):  
Steffen Terhaar ◽  
Bernhard Ćosić ◽  
Christian Oliver Paschereit ◽  
Kilian Oberleithner
Keyword(s):  
Transfer Function ◽  
Shear Flow ◽  
High Speed ◽  
Transfer Functions ◽  
Flow Fields ◽  
Shear Layers ◽  
Flow Instabilities ◽  
Mean Flow ◽  
Swirl Number ◽  
Flame Response

Amplitude-dependent flame transfer functions, also denoted as flame describing functions, are valuable tools for the prediction of limit-cycle amplitudes of thermoacoustic instabilities. However, the effects that govern the transfer function magnitude at low and high amplitudes are not yet fully understood. It is shown in the present work that the flame response at perfectly premixed conditions is strongly influenced by the growth rate of vortical structures in the shear layers. An experimental study in a generic swirl-stabilized combustor was conducted in order to measure the amplitude-dependent flame transfer function and the corresponding flow fields subjected to acoustic forcing. The applied measurement techniques included the multi-microphone-method, high-speed OH*-chemiluminescence measurements, and high-speed particle image velocimetry. The flame response and the corresponding flow fields are assessed for three different swirl numbers at 196 Hz forcing frequency. The results show that forcing leads to significant changes in the time-averaged reacting flow fields and flame shapes. A triple decomposition is applied to the time-resolved data, which reveals that coherent velocity fluctuations at the forcing frequency are amplified considerably stronger in the shear layers at low forcing amplitudes than at high amplitudes, which is an indicator for a nonlinear saturation process. The strongest saturation is found for the lowest swirl number, where the forcing additionally detached the flame. For the highest swirl number, the saturation of the vortex amplitude is weaker. Overall, the amplitude-dependent vortex amplification resembles the characteristics of the flame response very well. An application of a linear stability analysis to the time-averaged flow fields at increasing forcing amplitudes yields the decreasing growth rates of shear flow instabilities at the forcing frequency. It therefore successfully predicts a saturation at high forcing amplitudes and demonstrates that the mean flow field and its modifications are of utmost importance for the growth of vortices in the shear layers. Moreover, the results clearly show that the amplification of vortices in the shear layers is an important driver for heat release fluctuations and their saturation.

Measurement of Flame Frequency Response Functions Under Exhaust Gas Recirculation Conditions

2012 ◽  
Vol 134 (9) ◽  
Author(s):  
Joseph Ranalli ◽  
Don Ferguson
Keyword(s):  
Transfer Function ◽  
Carbon Capture ◽  
Transfer Functions ◽  
Combustion Process ◽  
Exhaust Gas Recirculation ◽  
Exhaust Gas ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Flame Response ◽  
Gas Recirculation

Exhaust gas recirculation has been proposed as a potential strategy for reducing the cost and efficiency penalty associated with postcombustion carbon capture. However, this approach may cause as-yet unresolved effects on the combustion process, including additional potential for the occurrence of thermoacoustic instabilities. Flame dynamics, characterized by the flame transfer function, were measured in traditional swirl stabilized and low-swirl injector combustor configurations, subject to exhaust gas circulation simulated by N2 and CO2 dilution. The flame transfer functions exhibited behavior consistent with a low-pass filter and showed phase dominated by delay. Flame transfer function frequencies were nondimensionalized using Strouhal number to highlight the convective nature of this delay. Dilution was observed to influence the dynamics primarily through its role in changing the size of the flame, indicating that it plays a similar role in determining the dynamics as changes in the equivalence ratio. Notchlike features in the flame transfer function were shown to be related to interference behaviors associated with the convective nature of the flame response. Some similarities between the two stabilization configurations proved limiting and generalization of the physical behaviors will require additional investigation.

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.

On the Propagation of Sound in a High-Speed Non-Isothermal Shear Flow

2009 ◽  
Vol 8 (3) ◽  
pp. 199-230 ◽  
Author(s):  
L.M.B.C. Campos ◽  
M.H. Kobayashi
Keyword(s):  
Wave Equation ◽  
Shear Flow ◽  
Acoustic Wave ◽  
Sound Speed ◽  
High Speed ◽  
Shear Flows ◽  
Critical Layer ◽  
Acoustic Wave Equation ◽  
Mean Flow ◽  
Flow Vorticity

The propagation of sound in shear flows is relevant to the acoustics of wall and duct boundary layers, and to jet shear layers. The acoustic wave equation in a shear flow has been solved exactly only for a plane unidirectional homentropic mean shear flow, in the case of three velocity profiles: linear, exponential and hyperbolic tangent. The assumption of homentropic mean flow restricts application to isothermal shear flows. In the present paper the wave equation in an plane unidirectional shear flow with a linear velocity profile is solved in an isentropic non-homentropic case, which allows for the presence of transverse temperature gradients associated with the ***non-uniform sound speed. The sound speed profile is specified by the condition of constant enthalpy, i.e. homenergetic shear flow. In this case the acoustic wave equation has three singularities at finite distance (besides the point at infinity), viz. the critical layer where the Doppler shifted frequency vanishes, and the critical flow points where the sound speed vanishes. By matching pairs of solutions around the singular and regular points, the amplitude and phase of the acoustic pressure in calculated and plotted for several combinations of wavelength and wave frequency, mean flow vorticity and sound speed, demonstrating, among others, some cases of sound suppression at the critical layer.

Flame and Flow Topologies in an Annular Swirling Flow

2013 ◽  
Author(s):  
I. Chterev ◽  
C. W. Foley ◽  
S. Kostka ◽  
A. W. Caswell ◽  
N. Jiang ◽  
...  
Keyword(s):  
High Speed ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Flow Fields ◽  
Shear Layers ◽  
Flame Stabilization ◽  
Preheat Temperature ◽  
Breakdown Region ◽  
Flame Shape ◽  
Annular Swirling Flow

A variety of different flame configurations and heat release distributions, with their associated flow fields, can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle life, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper discusses the flame and flow topologies that exist in these flows. These results illustrate the importance of the sensitivity of flame configurations to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel air ratio) parameters. We particularly emphasize the centerbody shape as differentiating between two different families of flame shapes. Results are shown illustrating the time averaged and instantaneous flame shape and flow fields, using high speed PIV, OH-PLIF, and luminosity imaging.

Impact of Boundary Conditions on the Reconstructed Flame Transfer Function for Gas Turbine Combustion Systems

2008 ◽  
Author(s):  
Krzysztof Kostrzewa ◽  
Axel Widenhorn ◽  
Berthold Noll ◽  
Manfred Aigner ◽  
Werner Krebs ◽  
...  
Keyword(s):  
Transfer Function ◽  
Boundary Conditions ◽  
Gas Turbine ◽  
Transfer Functions ◽  
Feedback Mechanism ◽  
Pressure Amplitude ◽  
Flame Response ◽  
Gas Turbine Combustion ◽  
Combustion Systems ◽  
The Impact

In order to achieve low levels of pollutants modern gas turbine combustion systems operate in lean and premixed modes. However, under these conditions self-excited combustion oscillations due to a complex feedback mechanism between pressure and heat release fluctuations can be found. These instabilities may lead to uncontrolled high pressure amplitude oscillations which can damage the whole combustor. The flame induced acoustic source terms are still analytically not well described and are a major topic of thermo-acoustic investigations. For the analysis of thermo-acoustic phenomena in gas turbine combustion systems flame transfer functions can be utilized. The purpose of this paper is to introduce and to investigate modeling parameters, which could influence a novel computational approach to reconstruct flame transfer functions known as the CFD/SI method. The flame transfer function estimation is made by application of a system identification method based on Wiener-Hopf formulation. Varying acoustic boundary conditions, combustion models and time resolutions may strongly affect the reconstructed flame response characterizing overall system dynamics. The CFD/SI approach has been applied to a generic gas turbine burner to derive a flame response. 3D unsteady simulations excited with white noise have been performed and the reconstructed flame transfer functions have been validated with experimental data. Moreover, the impact on the reconstructed flame transfer functions because of different boundary condition configurations has been examined.

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).

scholarly journals Algebraic Growth and Streak Formation in Shear Flows

1990 ◽  
Vol 43 (5S) ◽  
pp. S218-S218
Author(s):  
Marten T. Landahl
Keyword(s):  
Shear Flow ◽  
Velocity Component ◽  
High Speed ◽  
Shear Flows ◽  
Three Dimensional ◽  
Shear Layers ◽  
Turbulent Shear ◽  
Algebraic Instability ◽  
Horizontal Pressure ◽  
The Difference

By examination of the long-term behavior of an initial three-dimensional and localized disturbance in an inflection-free shear flow a detailed study of the algebraic instability mechanism of an inviscid shear flow (Landahl, 1980) is carried out. It is shown that the vertical velocity component will tend to zero at least as fast as 1/t whereas, as a result of a nonzero liftup of the fluid elements, the streamwise disturbence velocity component will tend to a limiting finite value in a convected frame of reference. For an initial disturbence having a nonzero net vertical momentum along a streamline, the streamwise dimension of the disturbed region is found to grow indefinitely at a rate set by the difference between the maximum and minimum velocities in the parallel flow. The total kinetic energy of the disturbence therefore grows linearly in time through the formation of continuously elongating high-speed or low-speed regions. In these, internal shear layers are formed that intensify through the mechanism of spanwise stretching of the mean vorticity. The effect of a small viscosity is felt primarily in the shear layers so as to make them diffuse and eventually cause the disturbence to decay on a viscous time scale. For the streaky structures near a wall the horizontal pressure gradients are found to be small, making possible a simple approximate treatment of nonlinearty. Such an analysis suggests the possibility of the appearance of a rapid outflow event (“bursting”) from the wall that may occur at a finite time inversely proportional to the amplitude of the initial disturbance. On basis of the analysis presented it is proposed that algebraic growth is the primary mechanism for the formation of streaks in laminar and turbulent shear flows.

Validation of Flame Transfer Function Reconstruction for Perfectly Premixed Swirl Flames

2004 ◽  
Author(s):  
A. Gentemann ◽  
C. Hirsch ◽  
K. Kunze ◽  
F. Kiesewetter ◽  
T. Sattelmayer ◽  
...  
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Dynamic Behavior ◽  
Broad Band ◽  
Combustion Process ◽  
Single Frequency ◽  
Mean Flow ◽  
Swirl Number ◽  
Absolute Value ◽  
The Absolute

The introduction of lean premix combustion increases the susceptibility of the combustor to thermoacoustic instabilities. To control these instabilities, information about the dynamic behavior of the combustion process is necessary. The flame transfer function offers one possibility to describe the dynamic behavior of the combustion process. It relates velocity fluctuations through the burner to an overall heat release fluctuation caused by the flame. As the transfer function for turbulent premix swirl flames can not be derived accurately from first principles, an alternative approach is needed. This paper introduces and validates a method, based on computational fluid dynamics (CFD), to reconstruct flame transfer functions. A transient simulation of the turbulent reacting flow is performed with broad band excitation of the flow variables on the boundaries. On the basis of the resulting time series for velocity and heat release, the transfer function of the flame is reconstructed by application of a system identification procedure based on the Wiener-Hopf equation. This method is applied to a lean perfectly premixed swirl burner. The resulting transfer function is validated with experimental data up to frequencies of f = 400 Hz. Good qualitative agreement is observed between the two approaches. Remarkably, the absolute value of the flame transfer function (the ‘gain’ of the flame) is found to be larger than unity over a range of frequencies, even though fluctuations of heat release and velocity are normalized with their mean flow values. To gain insight into this phenomenon, the dynamic behavior of the flame is investigated in detail. This concerns in particular the interaction of velocity, heat release fluctuations, the swirl number, and fluctuations of flame position and shape. Instead of broad band excitation, single frequency excitation is applied on the boundary for these investigations. It is found that swirl number fluctuations are convected into the flame. At the frequency where the wavelength of those fluctuations agrees with the length scale of the flame, unburned gases accumulate in the combustor. The excess heat is released periodically, which causes the overshoot in the absolute value of the flame transfer function.

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