Transient and limit cycle combustion dynamics analysis of turbulent premixed swirling flames

2017 ◽  
Vol 830 ◽  
pp. 681-707 ◽  
Author(s):  
Paul Palies ◽  
Milos Ilak ◽  
Robert Cheng
Keyword(s):  
Heat Release ◽  
Flame Front ◽  
Limit Cycle ◽  
Burning Velocity ◽  
Propagation Mode ◽  
Dynamic Mode ◽  
Flame Surface ◽  
Swirl Number ◽  
Combustion Dynamics ◽  
Swirling Flames

Premixed low swirling flames (methane–air and hydrogen–methane–air) are experimentally investigated for three different regimes. Stable, local transient to instability and limit cycle regimes corresponding to three distinct equivalence ratios are considered. Dynamic mode decomposition is applied to the hydrogen–air–methane flame to retrieve the modes frequencies, growth rates and spatial distributions for each regime. The results indicate that a vortical wave propagating along the flame front is associated with the transition from stability to instability. In addition, it is shown that a key effect on stability is the location of the non-oscillating (0 Hz) flame component. The phase-averaged unsteady motion of the flames over one cycle of oscillation shows the vortical wave rolling up the flame front. The Rayleigh index maps are formed to identify the region of driving and damping of the self-sustained oscillation, while the flame transfer function phase leads to the propagation mode of the perturbations along the flame front. The second mechanism identified concerns the swirl number fluctuation induced by the mode conversion. By utilizing hypotheses for the flow field and the flame structure, it is pointed out that those mechanisms are at work for both flames (methane–air and hydrogen–methane–air) and their effects on the unsteady heat release are determined. Both unsteady heat release contributions, the vortical wave induces flame surface fluctuations and swirl number oscillation induces unsteady turbulent burning velocity, are in phase opposition and of similar amplitudes.

Structure of flame quenched by cold surfaces

1967 ◽  
Vol 296 (1447) ◽  
pp. 546-565 ◽  
Keyword(s):  
Heat Release ◽  
Heat Sink ◽  
Burning Velocity ◽  
Flame Structure ◽  
Flame Surface ◽  
Hydrogen Atoms ◽  
The Status ◽  
Rate Of Heat Release ◽  
Composition Pattern

The region of interaction of a cooled metallic heat sink with a premixed lean flat ethylene/air flame is investigated. The detailed distribution of burning velocity is measured, by means of a refinement of the particle track method. Two dimensional distributions of temperature and composition in this region are obtained from thermocouples and microprobe sampling, followed by gas-chromatographic analysis. The burning velocity-heat loss relation is found to depend on the orientation of the flame front to the heat sink. Heat release rates deduced by substituting temperature and flow data into conservation equations reveal some unexpec­ted features close to the heat sink; in particular the zone of heat release becomes narrower and the maximum rate of heat release does not decrease appreciably towards the heat sink, in spite of a considerable reduction in temperature. These features are interpreted in terms of the composition pattern, which suggests that the narrowing is due to quenching of the second stage of reaction, while the reaction rate effect is caused by diffusion of hydrogen atoms to the surface, which acts as a sink for them. It is shown that a reaction such as H + O 2 → Surface HO 2 would account for all the features observed; conclusions are drawn regarding the status of quenching theory and the role of flame-surface interactions in flame structure studies.

Effects of Hydrogen Addition on Swirl-Stabilized Flame Properties

2010 ◽  
Author(s):  
Shengrong Zhu ◽  
Sumanta Acharya
Keyword(s):  
Flame Front ◽  
Surface Density ◽  
Recirculation Zone ◽  
Burning Velocity ◽  
Mie Scattering ◽  
Reacting Flow ◽  
Flame Surface ◽  
Flame Surface Density ◽  
Hydrogen Addition ◽  
Methane Flame

The role of hydrogen addition to swirl-stabilized methane flames is studied experimentally. Of specific interest are flame properties including flame surface density and curvature. The measurements are based on Particle Image Velocimetry (PIV), Mie-scattering and CH-chemiluminescence imaging. Identification of the flame front and its geometric characterization provides an understanding of the flame properties. Compared to the non-reacting flow, the methane flame broadens the central recirculation zone. Hydrogen enriched flames reduce the central recirculation zone and scales down the characteristic length of the flow. With hydrogen addition, the distribution of the flame front curvature is broadened and flame surface density is increased. This indicates that hydrogen addition increases the reaction front thermo-diffusive instability, causing the flame front to be more wrinkled, and increasing the flame surface area leading to an increase in the burning velocity.

Characterization of Combustion Dynamics in Swirl Stabilized Flames

2009 ◽  
Author(s):  
Uyi Idahosa ◽  
Abhishek Saha ◽  
Chengying Xu ◽  
Saptarshi Basu
Keyword(s):  
Heat Release ◽  
Frequency Response ◽  
Equivalence Ratio ◽  
Swirl Number ◽  
Combustion Dynamics ◽  
Flow Rates ◽  
Fuel Flow ◽  
Air Supply ◽  
Swirl Intensity ◽  
Air Speed

This paper investigates flame frequency response relative to changes in swirl intensity and equivalence ratio in a non-premixed swirl stabilized burner. The degree of swirl in the burner is characterized by the swirl number (S) provided by circumferentially distributed air supply ports directed tangentially to the main axial air flow. Equivalence ratio variations are induced using varying constant, linear ramp and exponentially decaying fuel (propane) flow rates towards blowoff. The variations in the air speed at the exit of the burner (U) are measured with an anemometer located at the base of the flame. The emission of CH* radicals (I) is used as a marker of flame heat release and is measured using a photomultiplier (PMT). The frequency response of the PMT heat release and burner velocity signals are analyzed in the frequency domain using the Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) methods. Amplification in the power of heat release fluctuation is observed in low swirl flames close to blowoff. This effect is found to be reversed in higher swirl number flames even close to blowoff. In dynamic approaches to blowoff (using ramp and decaying fuel flow rates), the dominant heat release fluctuation frequencies are observed to be similar to perturbation frequencies in lean flames hovering at constant fuel flow rates close to blowoff.

Impact of Swirl Fluctuations on the Flame Response of a Perfectly Premixed Swirl Burner

2009 ◽  
Author(s):  
Thomas Komarek ◽  
Wolfgang Polifke
Keyword(s):  
Heat Release ◽  
Axial Position ◽  
Strong Impact ◽  
Time Lags ◽  
Swirl Number ◽  
Swirl Generator ◽  
Flame Response ◽  
Combustion Technology ◽  
Swirling Flames ◽  
The Stability

Combustion instabilities represent a long known problem in combustion technology. The complex interactions between acoustics and turbulent swirling flames are not fully understood yet, making it very difficult to reliably predict the stability of new combustion systems. For example, the effects of fluctuations of swirl number on the heat release of the flame have to be investigated in more detail. In this paper a perfectly premixed, swirl stabilized burner with variable axial position of the swirl generator is investigated. In experiments, the position of the swirl generator has a strong impact on the dynamic flame response, although it does not influence the time-averaged distribution of the heat release significantly. This phenomenon is further investigated, using computational fluid dynamics combined with system identification. The generation of fluctuations of swirl number, their propagation to the flame, and their effect on the dynamic flame response are examined. A simple model based on convective time lags is developed, showing good agreement with experiments.

Impact of Swirl Fluctuations on the Flame Response of a Perfectly Premixed Swirl Burner

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Thomas Komarek ◽  
Wolfgang Polifke
Keyword(s):  
Heat Release ◽  
Axial Position ◽  
Strong Impact ◽  
Time Lags ◽  
Swirl Number ◽  
Swirl Generator ◽  
Flame Response ◽  
Combustion Technology ◽  
Swirling Flames ◽  
The Stability

Combustion instabilities represent a long known problem in combustion technology. The complex interactions between acoustics and turbulent swirling flames are not fully understood yet, making it very difficult to reliably predict the stability of new combustion systems. For example, the effects of fluctuations of swirl number on the heat release of the flame have to be investigated in more detail. In this paper a perfectly premixed swirl stabilized burner with variable axial position of the swirl generator is investigated. In experiments, the position of the swirl generator has a strong impact on the dynamic flame response, although it does not influence the time-averaged distribution of the heat release significantly. This phenomenon is further investigated using computational fluid dynamics combined with system identification. The generation of fluctuations of swirl number, their propagation to the flame, and their effect on the dynamic flame response are examined. A simple model based on convective time lags is developed, showing good agreement with experiments.

Modeling Unsteady Flow of a Lean Partially Premixed Flame on a Dry Low NOx Combustion System

2013 ◽  
Author(s):  
Salvatore Matarazzo ◽  
Hannes Laget ◽  
Evert Vanderhaegen ◽  
Jim B. W. Kok
Keyword(s):  
Gas Turbine ◽  
Limit Cycle ◽  
Gas Turbines ◽  
Premixed Flame ◽  
Computational Domain ◽  
Pressure Oscillations ◽  
Combustion Dynamics ◽  
Partially Premixed ◽  
Partially Premixed Flame ◽  
Limit Cycle Behavior

The phenomenon of combustion dynamics (CD) is one of the most important operational challenges facing the gas turbine (GT) industry today. The Limousine project, a Marie Curie Initial Training network funded by the European Commission, focuses on the understanding of the limit cycle behavior of unstable pressure oscillations in gas turbines, and on the resulting mechanical vibrations and materials fatigue. In the framework of this project, a full transient CFD analysis for a Dry Low NOx combustor in a heavy duty gas turbine has been performed. The goal is to gain insight on the thermo-acoustic instability development mechanisms and limit cycle oscillations. The possibility to use numerical codes for complex industrial cases involving fuel staging, fluid-structure interaction, fuel quality variation and flexible operations has been also addressed. The unsteady U-RANS approach used to describe the high-swirled lean partially premixed flame is presented and the results on the flow characteristics as vortex core generation, vortex shedding, flame pulsation are commented on with respect to monitored parameters during operations of the GT units at Electrabel/GDF-SUEZ sites. The time domain pressure oscillations show limit cycle behavior. By means of Fourier analysis, the coupling frequencies caused by the thermo-acoustic feedback between the acoustic resonances of the chamber and the flame heat release has been detected. The possibility to reduce the computational domain to speed up computations, as done in other works in literature, has been investigated.

System Level Analysis of Acoustically Forced Nonpremixed Swirling Flames

2014 ◽  
Vol 6 (3) ◽  
Author(s):  
Uyi Idahosa ◽  
Saptarshi Basu ◽  
Ankur Miglani
Keyword(s):  
Heat Release ◽  
Flow Rate ◽  
Ccd Camera ◽  
Time Dependent ◽  
Fuel Flow Rate ◽  
Fuel Flow ◽  
Flame Response ◽  
Rate Oscillations ◽  
Swirling Flames ◽  
Flame Sheet

This paper reports an experimental investigation of dynamic response of nonpremixed atmospheric swirling flames subjected to external, longitudinal acoustic excitation. Acoustic perturbations of varying frequencies (fp = 0–315 Hz) and velocity amplitudes (0.03 ≤ u′/Uavg ≤ 0.30) are imposed on the flames with various swirl intensities (S = 0.09 and 0.34). Flame dynamics at these swirl levels are studied for both constant and time-dependent fuel flow rate configurations. Heat release rates are quantified using a photomultiplier (PMT) and simultaneously imaged with a phase-locked CCD camera. The PMT and CCD camera are fitted with 430 nm ±10 nm band pass filters for CH* chemiluminescence intensity measurements. Flame transfer functions and continuous wavelet transforms (CWT) of heat release rate oscillations are used in order to understand the flame response at various burner swirl intensity and fuel flow rate settings. In addition, the natural modes of mixing and reaction processes are examined using the magnitude squared coherence analysis between major flame dynamics parameters. A low-pass filter characteristic is obtained with highly responsive flames below forcing frequencies of 200 Hz while the most significant flame response is observed at 105 Hz forcing mode. High strain rates induced in the flame sheet are observed to cause periodic extinction at localized regions of the flame sheet. Low swirl flames at lean fuel flow rates exhibit significant localized extinction and re-ignition of the flame sheet in the absence of acoustic forcing. However, pulsed flames exhibit increased resistance to straining due to the constrained inner recirculation zones (IRZ) resulting from acoustic perturbations that are transmitted by the co-flowing air. Wavelet spectra also show prominence of low frequency heat release rate oscillations for leaner (C2) flame configurations. For the time-dependent fuel flow rate flames, higher un-mixedness levels at lower swirl intensity is observed to induce periodic re-ignition as the flame approaches extinction. Increased swirl is observed to extend the time-to-extinction for both pulsed and unpulsed flame configurations under time-dependent fuel flow rate conditions.

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 A Comparison of the Transfer Functions and Flow Fields of Flames With Increasing Swirl Number

2018 ◽  
Author(s):  
M. Gatti ◽  
R. Gaudron ◽  
C. Mirat ◽  
L. Zimmer ◽  
T. Schuller
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Flow Fields ◽  
Phase Lag ◽  
Swirl Number ◽  
Gain Curve ◽  
Cold Flow ◽  
Flow Response ◽  
Swirled Flames

The frequency response of premixed swirled flames is investigated by comparing their Transfer Function (FTF) between velocity and heat release rate fluctuations. The equivalence ratio and flow velocity are kept constant and four different swirling injectors are tested with increasing swirl numbers. The first injector features a vanishing low swirl number S = 0.20 and produces a flame anchored by the recirculating flow in the wake of a central bluff body. The three other swirling injectors produce highly swirled flows (S > 0.6) leading to a much larger internal recirculation region, which size increases with the swirl level. When operating the burner at S = 0.20, the FTF gain curve smoothly increases to reach a maximum and then smoothly decreases towards zero. For the highly swirled flames (S > 0.6), the FTF gain curve shows a succession of valleys and peaks attributed to interferences between axial and azimuthal velocity fluctuations at the injector outlet. The FTF phase-lag curves from the vanishing low and highly swirled flames are the same at low frequencies despite their large differences in flame length and flame aspect ratio. Deviations between the FTF phase lag curves of the different swirled flames start above the frequency corresponding to the first valley in the FTF gain of the highly swirled flames. Phase averaged images of the axial flow fields and of the flame chemiluminescence are used to interpret these features. At forcing frequencies corresponding to peak FTF gain values, the cold flow response of all flames investigated is dominated by large coherent vortical structures shed from the injector lip. At forcing frequencies corresponding to a valley in the FTF gain curve of the highly swirled flames, the formation of large coherent structures is strongly hindered in the cold flow response. These observations contrast with previous interpretations of the mechanisms associated to the low FTF response of swirled flames. It is finally found that for flames stabilized with a large swirl number, heat release rate fluctuations result both from large flame luminosity oscillations and large flame volume oscillations. For conditions leading to a small FTF gain value, both the flame luminosity and flame volume fluctuations are suppressed confirming the absence of strong perturbations within the flow at these frequencies. The experiments made in this work reveal a purely hydrodynamic mechanism at the origin of the low response of swirling flames at certain specific frequencies.

4D Measurements and Analysis of Heat Release Rate in a Model Gas Turbine Combustor

2020 ◽  
Author(s):  
Rongxiao Dong ◽  
Qingchun Lei ◽  
Yeqing Chi ◽  
Qun Zhang ◽  
Wei Fan
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Surface Area ◽  
Heat Release Rate ◽  
Release Rate ◽  
Flame Surface ◽  
Global Instability ◽  
Gas Turbine Combustor ◽  
Model Gas Turbine Combustor ◽  
Edge Contours

Abstract Time-resolved volumetric measurements (4D measurements) were performed to study the heat release rate characteristics in a model gas turbine combustor at 10 kHz. For this purpose, a high-speed camera combined with an image intensifier and a set of customized fiber probes were employed to continuously capture the CH* chemiluminescence signals from nine different viewing angles. Based on the measurements, the computed tomography program was performed to reconstruct the shot-to-shot 3D distributions of the CH* signals. Specific focuses have been made to demonstrate the capabilities of the current tomographic technique in applications of a realistic combustor, in which the full optical access was usually not available for every viewing angle. The results showed that the 3D reconstruction can successfully retrieval the flame edge contours rather than the signal intensity. The flame surface area was then calculated based on the reconstructed flame edge contours and used to infer the heat release rate. The fluctuation of global/local flame surface area indicated that there existed distinct difference between the global instability and local instabilities at various locations in the non-symmetric combustor. The global instability appears to be an integration of those local instabilities.

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