Effect of a Premixed Pilot Flame on the Velocity-Forced Flame Response in a Lean-Premixed Swirl-Stabilized Combustor

2019 ◽  
Author(s):  
Jihang Li ◽  
Stephen Peluso ◽  
Domenic Santavicca ◽  
James Blust
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Shear Layer ◽  
Heat Release Rate ◽  
Release Rate ◽  
Wall Region ◽  
Rate Fluctuation ◽  
Phase Images ◽  
Flame Response ◽  
Near Wall

Abstract The effect of a fully-premixed pilot flame on the velocity-forced flame response of a fully premixed flame in a single-nozzle lean-premixed swirl combustor operating on natural gas fuel is investigated. Measurements of the flame transfer function show that as the percent pilot is increased there is a decrease in the flame transfer function gain at all frequencies, a decrease in the frequencies at which the gain minima and maxima occurred, and a decrease in the flame transfer function phase at high frequencies. High-speed CH* chemiluminescence flame imaging is used to gain a better understanding of the mechanism(s) whereby the pilot flame affects flame dynamics and thereby the flame transfer function. Time-averaged flame images show that the location of the maximum heat release rate does not change with forcing frequency or percent pilot, although the flame extends further upstream into the inner shear layer with increasing percent pilot. Heat release rate fluctuation images show that significant heat release rate fluctuations occur in the inner shear layer, the outer recirculation zone, and the near wall region and that the primary effect of increasing the forcing frequency or the percent pilot is a shift of the heat release rate fluctuation from the near wall region to the inner shear layer. In addition, an increase in the percent pilot results in lengthening and narrowing of the inner shear layer and the near wall regions. The phase images show that the phase is less uniform as the frequency or percent pilot increase, resulting in greater interference between in phase and out of phase fluctuations which reduces the FTF gain. The phase images also show that the wavelength of the heat release rate perturbation travelling through the inner shear layer decreases with increasing frequency and percent pilot which suggests that the pilot flame alters the recirculation flow field. Flame transfer functions calculated for the heat release rate fluctuations in the inner shear layer, the near wall region and the outer recirculation zone show that the inner shear layer is the largest contributor to the global heat release rate fluctuation in the unpiloted flame and that the primary effect of the pilot flame on the reduction of the global FTF gain is a result of the pilot flame’s effect on the inner shear layer.

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

2013 ◽  
Author(s):  
Bernhard C. Bobusch ◽  
Bernhard Ćosić ◽  
Jonas P. Moeck ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Optical Measurement ◽  
Low Frequencies ◽  
Fuel Flow ◽  
Calibration Measurement

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatio-temporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The spatially and temporally resolved equivalence ratio fluctuations in the reaction zone are measured using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

scholarly journals Combustion Instability of Swirl Premixed Flame with Dielectric Barrier Discharge Plasma

Processes ◽  
2021 ◽  
Vol 9 (8) ◽  
pp. 1405
Author(s):  
Kai Deng ◽  
Shenglang Zhao ◽  
Chenyang Xue ◽  
Jinlin Hu ◽  
Yi Zhong ◽  
...  
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Combustion Instability ◽  
Premixed Flame ◽  
Oh Radicals ◽  
Acoustic Frequency ◽  
Planar Laser ◽  
Barrier Discharge Plasma

The effects of plasma on the combustion instability of a methane swirling premixed flame under acoustic excitation were investigated. The flame image of OH planar laser-induced fluorescence and the fluctuation of flame transfer function showed the mechanism of plasma in combustion instability. The results show that when the acoustic frequency is less than 100 Hz, the gain in flame transfer function gradually increases with the frequency; when the acoustic frequency is 100~220 Hz, the flame transfer function shows a trend of first decreasing and then increasing with acoustic frequency. When the acoustic frequency is greater than 220 Hz, the flame transfer function gradually decreases with acoustic frequency. When the voltage exceeds the critical discharge value of 5.3 kV, the premixed gas is ionized and the heat release rate increases significantly, thereby reducing the gain in flame transfer function and enhancing flame stability. Plasma causes changes in the internal recirculation zone, compression, and curling degree of the flame, and thereby accelerates the rate of chemical reaction and leads to an increase in flame heat release rate. Eventually, the concentration of OH radicals changes, and the heat release rate changes accordingly, which ultimately changes the combustion instability of the swirling flame.

Smart Passive Control of Thermoacoustic Instability in a Bluff-Body Stabilized Combustor: A Lagrangian Analysis of Critical Structures

2020 ◽  
Author(s):  
C. P. Premchand ◽  
Manikandan Raghunathan ◽  
Midhun Raghunath ◽  
K. V. Reeja ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Wavelet Transform ◽  
Flow Field ◽  
Heat Release ◽  
Shear Layer ◽  
Heat Release Rate ◽  
Release Rate ◽  
Sound Production ◽  
Bluff Body ◽  
Passive Control ◽  
Thermoacoustic Instability

Abstract The tonal sound production during thermoacoustic instability is detrimental to the components of gas turbine and rocket engines. Identifying the root cause and controlling this oscillatory instability would enable manufacturers to save in costs of power outages and maintenance. An optimal method is to identify the structures in the flow-field that are critical to tonal sound production and perform control measures to disrupt those “critical structures”. Passive control experiments were performed by injecting a secondary micro-jet of air onto the identified regions with critical structures in the flow-field of a bluff-body stabilized, dump, turbulent combustor. Simultaneous measurements such as unsteady pressure, velocity, local and global heat release rate fluctuations are acquired in the regime of thermoacoustic instability before and after control action. The tonal sound production in this combustor is accompanied by a periodic flapping of the shear layer present in the region between the dump plane (backward-facing step) and the leading edge of the bluff-body. We obtain the trajectory of Lagrangian saddle points that dictate the flow and flame dynamics in the shear layer during thermoacoustic instability accurately by computing Lagrangian Coherent Structures. Upon injecting a secondary micro-jet with a mass flow rate of only 4% of the primary flow, nearly 90% suppression in the amplitude of pressure fluctuations are observed. The suppression thus results in sound pressure levels comparable to those obtained during stable operation of the combustor. Using Morlet wavelet transform, we see that the coherence in the dominant frequency of pressure and heat release rate oscillations during thermoacoustic instability is affected by secondary injection. The disruption of saddle point trajectories breaks the positive feedback loop between pressure and heat release rate fluctuations resulting in the observed break of coherence. Wavelet transform of global heat release rate shows a redistribution of energy content from the dominant instability frequency (acoustic time scale) to other time scales.

The Effects of Forcing Direction on the Flame Transfer Function of a Lean-Burn Spray Flame

2021 ◽  
Author(s):  
Nicholas C. W. Treleaven ◽  
André Fischer ◽  
Claus Lahiri ◽  
Max Staufer ◽  
Andrew Garmory ◽  
...  
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Fuel Injector ◽  
Lean Burn ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Good Reproduction ◽  
Small Change

Abstract The flame transfer function (FTF) of an industrial lean-burn fuel injector has been computed using large eddy simulation (LES) and compared to experimental measurements using the multi-microphone technique and OH* measurements. The flame transfer function relates the fluctuations of heat release in the combustion chamber to fluctuations of airflow through the fuel injector and is a critical part of thermoacoustic analysis of combustion systems. The multi-microphone method derives the FTF by forcing the flame acoustically, alternating from the upstream and downstream side. Simulations emulating this methodology have been completed using compressible large eddy simulations (LES). These simulations are also used to derive an FTF by measuring the fluctuations of mass flow rate and heat release rate directly which reduces the number of simulations per frequency to one, significantly reducing the simulation cost. Simulations acoustically forced from downstream are shown to result in a lower value of the FTF gain than simulations forced from upstream with a small change in phase, this is shown to be consistent with theory. Through using a slightly different definition of the FTF, this is also shown to be consistent with measurements of the heat release rate using OH* chemiluminescence however these results are inconsistent with the multi-microphone method result. The discrepancy comes from not having an accurate measurement of the acoustic impedance at the exit plane of the injector and from certain convective phenomena that alter the downstream velocity and pressure field with respect to the purely acoustic signal. All simulations show a lower gain in the FTF than the experiments but with good reproduction of phase. Previous work suggests this error is likely due to fluctuations of the fuel spray atomisation process due to the acoustic forcing which is not modelled in this study.

On the Impact of Shear Flow Instabilities on Global Heat Release Rate Fluctuations: Linear Stability Analysis of an Isothermal and a Reacting Swirling Jet

2012 ◽  
Author(s):  
Kilian Oberleithner ◽  
Sebastian Schimek ◽  
Christian Oliver Paschereit
Keyword(s):  
Stability Analysis ◽  
Heat Release ◽  
Shear Layer ◽  
Linear Stability ◽  
Heat Release Rate ◽  
Release Rate ◽  
Linear Stability Analysis ◽  
Amplitude Dependence ◽  
Swirling Jet ◽  
The Impact

The prediction of large-scale flow structures in combustor flows and their impact on the flame dynamics is of great importance to avoid thermoacoustic instabilities in modern gas turbine design. The streamwise growth of these so-called coherent structures depends on the receptivity of the shear layers, which can be predicted numerically by means of linear stability analysis. We demonstrate this approach on an isothermal swirling jet that is dominated by a self-excited helical mode that features a precessing vortex core, showing that this theoretical concept successfully predicts the frequency, the source, and the shape of this mode. The analysis is further applied to a reacting flow with a swirl-stabilized flame, pointing out important connections between the shear layer receptivity and the measured amplitude dependence of the flame transfer function. The theoretical findings suggest that the saturation of the global heat release rate fluctuations observed at moderate forcing amplitudes is caused by vanishing shear layer receptivity.

An Experimental Validation of Heat Release Rate Fluctuation Measurements in Technically Premixed Flames

2013 ◽  
Author(s):  
Poravee Orawannukul ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Premixed Flames ◽  
Reconstruction Technique ◽  
Chemiluminescence Intensity ◽  
Lean Premixed ◽  
Rate Of Heat Release

Knowledge of the effects of inlet velocity and inlet equivalence ratio fluctuations on the rate of heat release in lean premixed gas turbine combustors is essential for predicting combustor instability characteristics. This information is typically obtained from independent velocity-forced and fuel-forced flame transfer function measurements, where the global chemiluminescence intensity is used as a measure of the flame’s overall rate of heat release. The flame in an actual lean premixed combustor is referred to as a technically premixed flame and is exposed to both velocity and equivalence ratio fluctuations. Under these conditions the chemiluminescence intensity does not provide a reliable measure of the flame’s rate of heat release. The objective of this work is to experimentally assess the validity of a technique for making heat release rate measurements in technically premixed flames based on the linear superposition of fuel-forced and velocity-forced flame transfer function measurements. In the absence of a technique for directly measuring the heat release rate fluctuations in an air-forced technically premixed, the heat release reconstruction is validated indirectly by comparing measured to reconstructed chemiluminescence intensity fluctuations. Results are reported for a range of operating conditions and forcing frequencies which demonstrate the capabilities and limitations of this technique. A variation of this technique, referred to as a reverse reconstruction, is proposed which does not require a measurement of the fuel-forced flame transfer function. The air-forced flame transfer function gain and phase obtained using the reverse reconstruction technique are presented and compared to the results from the direct reconstruction technique.

scholarly journals Effects of the Injector Design on the Transfer Function of Premixed Swirling Flames

2017 ◽  
Author(s):  
M. Gatti ◽  
R. Gaudron ◽  
C. Mirat ◽  
T. Schuller
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Transfer Functions ◽  
Thermal Power ◽  
Phase Lag ◽  
Swirl Number ◽  
Injector Design

This article reports a series of experiments on the dynamics of lean-premixed swirl-stabilized flames submitted to harmonic flowrate modulations. The flame transfer function is analyzed for different injector designs with a specific focus on conditions leading to the lowest heat release rate response for a given flowrate perturbation. Experiments are carried out at a fixed equivalence ratio and fixed thermal power. Transfer functions are measured for radial swirling vanes by modifying the diameter of the swirler injection holes, the diameter of the injection tube at the top of the swirler and the end piece diameter of a central insert serving as a bluff body. It is found that the lowest response depends on the forcing frequency and is obtained when the injector design features the largest swirl number. The transfer function of the studied flames features a minimum gain value which decreases for increasing swirl levels. This minimum value is found to be independent of the velocity forcing level and is only controlled by the level of swirl. An excessive swirl level however leads to flash-back of the perturbed flames inside the injector. The way the flame behaves at this forcing frequency is analyzed for a set of injectors featuring the same radial swirling vane design and different injection tube diameters or conical end pieces. It is found that at the condition corresponding to the lowest FTF gain, i.e. the injector with the largest swirl number, the upper and lower parts of the flame contribute to out of phase heat release oscillations, but they also both feature a reduced level of fluctuations. When the swirl number decreases, the FTF gain increases due to a reduction of the phase lag between heat release rate oscillations in the lower and the upper parts of the flame and more importantly due to a general increase of the level of heat release oscillations in both parts of the flame.

scholarly journals The Effect of Fuel Staging on the Structure and Instability Characteristics of Swirl-Stabilized Flames in a Lean Premixed Multi-Nozzle Can Combustor

2017 ◽  
Author(s):  
Janith Samarasinghe ◽  
Wyatt Culler ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca ◽  
Jacqueline O’Connor
Keyword(s):  
Heat Release ◽  
Flow Rate ◽  
Heat Release Rate ◽  
Release Rate ◽  
Pressure Fluctuation ◽  
Fuel Flow Rate ◽  
Fuel Flow ◽  
Rate Fluctuation ◽  
Fuel Staging ◽  
Fluctuation Amplitude

Fuel staging, or fuel splitting, is a commonly used strategy for the suppression of combustion instabilities in gas turbine engines. In multi-nozzle combustor configurations, this is achieved by varying the fuel flow rate to the different nozzles. The effect of fuel staging on flame stabilization and heat release rate distribution (referred to as flame structure), and self-excited instability characteristics is investigated in a research can combustor employing five small-scale lean-premixed industrial nozzles. The nozzles are arranged in a “four-around-one” configuration and fuel staging is achieved by injecting additional fuel to the middle nozzle. An operating condition was identified where all five nozzles were fueled equally and the combustor was subject to a self-excited instability. At the operating condition considered, the self-excited instabilities are suppressed with fuel staging: this is true for cases where overall equivalence ratio is increased by staging (by only increasing the fuel flow rate to the middle nozzle) as well as cases where overall equivalence ratio is kept constant while staging (by simultaneously decreasing the fuel flow rate of the outer nozzles while increasing the fuel flow rate to the middle nozzle). Fuel staging causes variations in the distribution of time-averaged heat release rate in the regions where adjacent flames interact. The locations of highest heat release rate fluctuation are not altered with increased fuel staging but the fluctuation amplitude is reduced. A breakup in the monotonic phase behavior that is characteristic of convective disturbances is observed with increased fuel staging, resulting in a lower pressure fluctuation amplitude. In particular, the monotonic variation in phase in the middle flame and the region where adjacent flames interact is out-of-phase with that of the outer flames, resulting in a cancellation of the global heat release rate oscillations. The distribution of local Rayleigh integral within the combustor shows that during a self-excited instability, the regions of highest heat release rate fluctuation are in phase-with the pressure fluctuation. When staging fuel is introduced, these regions fluctuate out-of-phase with the pressure fluctuation, further illustrating that fuel staging suppresses instabilities by altering the phase relationship of convective disturbances that travel along the flame front.

Amplitude-Dependent Flow Field and Flame Response to Axial and Tangential Velocity Fluctuations

2012 ◽  
Author(s):  
Sebastian Schimek ◽  
Bernhard Ćosić ◽  
Jonas P. Moeck ◽  
Steffen Terhaar ◽  
Christian Oliver Paschereit
Keyword(s):  
Flow Field ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Amplitude Dependence ◽  
Mean Flow ◽  
Modeling Tools ◽  
Velocity Fluctuations ◽  
Flame Response ◽  
The Mean

Lean premixed combustion is prone to thermoacoustic instabilities. These mostly self-excited instabilities are caused by a feedback mechanism between the acoustic field, hydrodynamic structures, and the heat release rate in the flame. While various modeling tools are available for the linear analysis of thermoacoustic systems, a detailed knowledge of the governing nonlinearities, responsible for the saturation in the flame response, is still missing. The fundamental understanding of the flow field–heat release interaction is of crucial importance for the prediction of the pressure oscillation amplitude. To improve the understanding of these interaction processes, the current paper investigates the nonlinear interaction of the flow field and the unsteady heat release rate and the role of swirl fluctuations. The test rig that is used in the present work consists of a generic swirl-stabilized combustor fed with natural gas and equipped with a high-amplitude forcing device. The influence of the phase between axial and azimuthal velocity oscillations is assessed on the basis of the amplitude and phase relations between the velocity fluctuations at the inlet and the outlet of the burner. These relations are determined in the experiment with the Multi-Microphone-Method and a two component laser-Doppler velocimeter. Particle image velocimetry and OH*-chemiluminescence measurements are conducted to study the interaction between the flow field and the flame. For several frequency regimes, characteristic properties of the forced flow field and flame are identified, and a strong amplitude dependence is observed. It is found that the convective time delay between the swirl generator and the flame has an important influence on swirl-number oscillations and the flame dynamics in the low-frequency regime. For mid and high frequencies, significant changes in the mean flow field and the mean flame position are identified for high forcing amplitudes. These affect the interaction between coherent structures and the flame and are suggested to be responsible for the saturation in the flame response at high forcing amplitudes.

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

2013 ◽  
Vol 136 (2) ◽  
Author(s):  
Bernhard C. Bobusch ◽  
Bernhard Ćosić ◽  
Jonas P. Moeck ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Optical Measurement ◽  
Low Frequencies ◽  
Fuel Flow ◽  
Calibration Measurement

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatiotemporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The equivalence ratio fluctuations in the reaction zone are measured spatially and temporally resolved using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that, despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

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