The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Matthew P. Juniper - One of the best experts on this subject based on the ideXlab platform.
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Weakly nonlinear analysis of thermoacoustic instabilities in Annular Combustors
Journal of Fluid Mechanics, 2016Co-Authors: Giulio Ghirardo, Matthew P. Juniper, Jonas P. MoeckAbstract:© 2016 Cambridge University Press. Rotationally symmetric Annular Combustors are of practical importance because they generically resemble combustion chambers in gas turbines, in which thermoacoustically driven oscillations are a major concern. We focus on azimuthal thermoacoustic oscillations and model the fluctuating heat release rate as being dependent only on the local pressure in the combustion chamber. We study the dynamics of the Annular Combustor with a finite number of compact flames equispaced around the annulus, and characterize the flames' response with a describing function. We discuss the existence, amplitude and the stability of standing and spinning waves, as a function of: (i) the number of the burners; (ii) the acoustic damping in the chamber; (iii) the flame response. We present the implications for industrial applications and the future direction of investigations. We then present as an example the first theoretical study of thermoacoustic triggering in Annular Combustors, which shows that rotationally symmetric Annular chambers that are thermoacoustically unstable do not experience only stable spinning solutions, but can also experience stable standing solutions. We finally test the theory on one experiment with good agreement.
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Stability analysis of thermo-acoustic nonlinear eigenproblems in Annular Combustors. Part I. Sensitivity
Journal of Computational Physics, 2016Co-Authors: Luca Magri, Michaël Bauerheim, Matthew P. JuniperAbstract:We present an adjoint-based method for the calculation of eigenvalue perturbations in nonlinear, degenerate and non-self-adjoint eigenproblems. This method is applied to a thermo-acoustic Annular Combustor network, the stability of which is governed by a nonlinear eigenproblem. We calculate the first- and second-order sensitivities of the growth rate and frequency to geometric, flow and flame parameters. Three different configurations are analysed. The benchmark sensitivities are obtained by finite difference, which involves solving the nonlinear eigenproblem at least as many times as the number of parameters. By solving only one adjoint eigenproblem, we obtain the sensitivities to any thermo-acoustic parameter, which match the finite-difference solutions at much lower computational cost.
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Stability analysis of thermo-acoustic nonlinear eigenproblems in Annular Combustors. Part II. Uncertainty quantification
Journal of Computational Physics, 2016Co-Authors: Luca Magri, Franck Nicoud, Michaël Bauerheim, Matthew P. JuniperAbstract:Monte Carlo and Active Subspace Identification methods are combined with first- and second-order adjoint sensitivities to perform (forward) uncertainty quantification analysis of the thermo-acoustic stability of two Annular Combustor configurations. This method is applied to evaluate the risk factor, i.e., the probability for the system to be unstable. It is shown that the adjoint approach reduces the number of nonlinear-eigenproblem calculations by as much as the Monte Carlo samples.
S M Frolov - One of the best experts on this subject based on the ideXlab platform.
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Hydrogen-fueled detonation ramjet model: Wind tunnel tests at approach air stream Mach number 5.7 and stagnation temperature 1500 K
International Journal of Hydrogen Energy, 2018Co-Authors: S M Frolov, V S Ivanov, V. I. Zvegintsev, V. S. Aksenov, I. O. Shamshin, D. A. Vnuchkov, D. G. Nalivaichenko, A. A. Berlin, V. M. Fomin, A. N. ShiplyukAbstract:Abstract The mode of continuous spinning detonation (CSD) combustion of hydrogen in the Annular Combustor of a model of a hydrogen-fueled detonation ramjet under conditions of approach air stream Mach number 5.7 and stagnation temperature 1500 K is registered experimentally in a short-duration (pulsed) wind tunnel at the overall air-to-hydrogen equivalence ratio (ER) ranging from 0.7 to 1.4. The maximum values of thrust and specific impulse of the ramjet model are attained at ER = 1.25 and are estimated as 1550 N and 3300 s, respectively. At 1.4
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wind tunnel tests of a hydrogen fueled detonation ramjet model at approach air stream mach numbers from 4 to 8
International Journal of Hydrogen Energy, 2017Co-Authors: V S Ivanov, S M Frolov, V. I. Zvegintsev, V. S. Aksenov, I. O. Shamshin, D. A. Vnuchkov, D. G. NalivaichenkoAbstract:Abstract Experimental studies of an axisymmetric hydrogen-fueled detonation ramjet model 1.05-m long and 0.31 m in diameter with an expanding Annular Combustor were performed in a pulse wind tunnel under conditions of approaching air stream Mach number ranging from 4 to 8 with the total temperature of 290 K. In a supersonic air flow entering the Combustor, continuous and longitudinally pulsating modes of hydrogen detonation with the corresponding characteristic frequencies of 1250 and 900 Hz were obtained. The maximum measured values of fuel-based specific impulse and total thrust were 3600 s and 2200 N.
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continuous detonation combustion of ternary hydrogen liquid propane air mixture in Annular Combustor
International Journal of Hydrogen Energy, 2017Co-Authors: V S Ivanov, S M Frolov, V. S. Aksenov, I. O. ShamshinAbstract:Abstract Experiments are performed on continuous detonation combustion of ternary hydrogen–liquid propane–air mixture in a large-scale Annular Combustor 406 mm in outer diameter with an Annular gap of 25 mm. Liquid propane is fed into the Combustor at the time when sustained continuous-detonation combustion of hydrogen–air mixture is attained therein. Mass flow rates of hydrogen, propane and air in the experiments ranged from 0.1 to 0.5 kg/s (hydrogen), 0.1 to 0.5 kg/s (propane), and 5 to 12 kg/s (air). Continuous-detonation combustion of liquid propane in air is obtained for the first time due to addition of hydrogen rather than due to enrichment of air with oxygen. Combustor operation with a single continuously rotating detonation wave (DW) for about 0.1 s has been obtained when the flow rates of propane and air remained constant while the flow rate of hydrogen was rapidly decreasing.
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three dimensional numerical simulation of the characteristics of a ramjet power plant with a continuous detonation Combustor in supersonic flight
Russian Journal of Physical Chemistry B, 2016Co-Authors: A V Dubrovskii, V S Ivanov, A E Zangiev, S M FrolovAbstract:Multi-variant three-dimensional numerical simulations demonstrate the feasibility of the continuous- detonation process in an Annular Combustor of a ramjet power plant operating on hydrogen as fuel and air as oxidant in conditions of flight at a Mach number of M 0 = 5.0 and an altitude of 20 km. Conceptual schemes of an axisymmetric power plant, 400 mm in external diameter and 1.3 to 1.5 m in length, with a supersonic intake, divergent Annular Combustor, and outlet nozzle with a frusto-conical central body are proposed. Calculations of the characteristics of the internal and external flows, with consideration given to the finite rate of turbulent-molecular mixing of the fuel mixture components with each other and with the combustion products, as well as the finite rate of chemical reactions and the viscous interaction of the flow with the bounding surfaces, have shown that, in these flight conditions, the engine of such a power plant has the following performance characteristics: the thrust, 10.7 kN; specific thrust, 0.89 (kN s)/kg; specific impulse, 1210 s; and specific fuel⋅consumption 0.303 kg/(N h). In this case, the Combustor can operate with one detonation wave traveling in the Annular channel at an average velocity of 1695 m/s, which corresponds to a detonation wave rotation frequency of 1350 Hz. It is shown that, an operating Combustor has regions with subsonic flow of detonation products, but the flow is supersonic throughout its outlet section.
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Energy efficiency of a continuous-detonation combustion chamber
Combustion Explosion and Shock Waves, 2015Co-Authors: S M Frolov, A V Dubrovskii, V S Ivanov, V. S. Aksenov, I. O. ShamshinAbstract:Systematic experimental and computational studies of the energy efficiency of continuous-detonation Combustors (CDCs) have been performed. A small-size and a large-size CDCs using hydrogen as fuel and oxygen or air as oxidizer have been developed and tested. It was first experimentally proved that the Zel’dovich thermodynamic cycle with continuous-detonation combustion of a hydrogen-oxygen mixture in an Annular Combustor is more efficient than the Brayton thermodynamic cycle with continuous combustion of the mixture, other things being equal. The specific impulse of a small-size bench-scale rocket engine with a 50 mm diameter CDC operating in the continuous-detonation mode was 6–7% higher than that in the continuous combustion mode of operation. The measured fuel-based specific impulse for the large-size CDC of 406 mm diameter running on a hydrogen-air mixture was at a level of 3000 s. Three-dimensional calculations to optimize the structure and operation mode of the large-size CDC have shown that when running on a combustible mixture with a nearly stoichiometric overall composition, the specific impulse can be increased to ≈4200 s.
Sebastien Candel - One of the best experts on this subject based on the ideXlab platform.
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leading point behavior during the ignition of an Annular Combustor with liquid n heptane injectors
Proceedings of the Combustion Institute, 2019Co-Authors: Théa Lancien, Sebastien Candel, Daniel Durox, Kevin Prieur, Ronan VicquelinAbstract:Abstract Experimental and numerical investigations of ignition in Combustors with multiple burners have recently emerged and have provided new insights on the last phase of ignition in gas turbine-like Annular geometries where the flame propagates from burner to burner. Previous comparisons between calculations and experiments of light-round in a laboratory scale Annular combustion chamber have demonstrated the ability of large-eddy simulation to predict such processes for perfectly premixed conditions and, more recently, for n-heptane spray injection. The present analysis focuses on two additional operating points with liquid n-heptane sprays and the turbulent flame propagation in the two-phase mixture is examined through the behavior of its leading points. The validation of the light-round process is characterized in terms of ignition delays. The detailed analysis of the propagation through the definition of a leading point enables to highlight some key phenomena responsible for the flame behavior, such as the influence of the liquid droplet spray and its vaporization in the chamber. Calculations indicate that the volumetric expansion due to the chemical reaction at the flame induces a strong azimuthal flow in the fresh stream at a distance of several sectors ahead of the flame, which modifies conditions in this region. This creates heterogeneities in the gas composition and wakes on the downstream side of the swirling jets formed by the injectors, with notable effects on the motion of the leading point and on the absolute flame velocity.
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Large Eddy Simulation of Light-Round in an Annular Combustor With Liquid Spray Injection and Comparison With Experiments
Journal of Engineering for Gas Turbines and Power, 2018Co-Authors: Théa Lancien, Sebastien Candel, Daniel Durox, Kevin Prieur, Ronan VicquelinAbstract:The light-round is defined as the process by which the flame initiated by an ignition spark propagates from burner to burner in an Annular Combustor, eventually leading to a stable combustion. Combining experiments and numerical simulation, it was recently demonstrated that under perfectly premixed conditions this process could be suitably described by large eddy simulation (LES) using massively parallel computations. The present investigation aims at developing light-round simulations in a configuration that is closer to that found in aero-engines by considering liquid n-heptane injection. The large-eddy simulation of the ignition sequence of a laboratory scale Annular combustion chamber comprising sixteen swirled spray injectors is carried out with a mono-disperse Eulerian approach for the description of the liquid phase. The objective is to assess this modeling approach of the two-phase reactive flow during the ignition process. The simulation results are compared in terms of flame structure and light-round duration to the corresponding experimental images of the flame front recorded by a high-speed intensified CCD camera and to the corresponding experimental delays. The dynamics of the flow is also analyzed to identify and characterize mechanisms controlling flame propagation during the light-round process.
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A hysteresis phenomenon leading to spinning or standing azimuthal instabilities in an Annular Combustor
Combustion and Flame, 2017Co-Authors: Kevin Prieur, Thierry Schuller, Daniel Durox, Sebastien CandelAbstract:Abstract Thermo-acoustic instabilities in Annular Combustors equipped with swirling turbulent injectors are most often coupled by azimuthal modes with a spinning or a standing structure. Experiments, and recent large eddy simulations, indicate that switching takes place between these two types of modes while in other cases a single mode type prevails. Why and how one type arises is a subject of ongoing discussions in recent theoretical and numerical investigations. The present article considers this intriguing issue by making use of well controlled experiments carried out in an Annular combustion system comprising 16 identical matrix injectors operating in a laminar premixed mode and allowing full optical access to the flame region. This setup is used in a first stage to determine regions of instability as a function of equivalence ratio and injection velocity. It is shown that regions corresponding to spinning and standing azimuthal modes are well separated in this diagram, but with some overlap giving rise to a “dual mode” domain. For the same operating conditions, this Annular system thus exhibits self-sustained instabilities with stable limit cycles coupled by a spinning or a standing mode. It is next shown that the mode which appears in that region depends on the path followed to reach the operating point. Starting from a lean operating condition, the system first develops a chugging instability with a broad frequency spectrum, which then gives rise to a well-established spinning oscillation with a narrow peak frequency in the dual mode region. When operation begins under rich conditions one first observes another chugging mode that finally yields a standing mode when the equivalence ratio is diminished. Away from these regimes, well defined slanted modes and longitudinal modes can also be triggered. In the dual mode region, the type of instability is controlled by an hysteresis phenomenon and the respective chugging modes act as precursors to these established azimuthal modes. It is found that the trajectories in a state space map contain indications on the kind of azimuthal oscillation that will be observed when the target operating point is reached. Beyond the various theoretical explanations of the prevalence of one type of mode on the other, the present observations indicate that spinning or standing modes may also appear in Annular Combustors as a result of the path used to enter the region of azimuthal instability.
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Ignition dynamics in an Annular Combustor for liquid spray and premixed gaseous injection
Proceedings of the Combustion Institute, 2017Co-Authors: Kevin Prieur, Thierry Schuller, Jerome Beaunier, Daniel Durox, Sebastien CandelAbstract:Ignition is of importance in many combustion applications and raises fundamental and practical issues. The light-round process corresponding to the flame spreading phase in the ignition of Annular Combustors is examined in this article by performing experiments in a model scale configuration "MICCA-Spray". This system features 16 swirling injectors each comprising a hollow cone pressurized injector. Experiments are carried out with premixed gases as well as n-heptane and dodecane sprays. The flow, spray and flame are first characterized in a single injector configuration. Propagation from the initial kernel created by a spark plug is then observed using high speed light emission imaging. This provides flame structures at various times during the process and gives access to the time delays for flame merging. With n-heptane and dodecane fuel injection, it is found that the light-round process is similar to the one observed under fully premixed propane/air experiments but the duration of the process is augmented especially for the less volatile fuel. It is also confirmed that the delay is notably influenced by thermal conditions prevailing in the chamber at the moment of ignition, injection process and fuel composition. Making use of a flamelet like model of the combustion process, the relative changes in light-round time delay are found to be, to the first order, proportional to the relative changes in laminar burning velocity induced by the fuel spray in the air flow.
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Flame Describing Function analysis of spinning and standing modes in an Annular Combustor and comparison with experiments
Combustion and Flame, 2017Co-Authors: Davide Laera, Thierry Schuller, Daniel Durox, Kevin Prieur, Sergio M. Camporeale, Sebastien CandelAbstract:This article reports a numerical analysis of combustion instabilities coupled by a spinning mode or a standing mode in an Annular Combustor. The method combines an iterative algorithm involving a Helmholtz solver with the Flame Describing Function (FDF) framework. This is applied to azimuthal acoustic coupling with combustion dynamics and is used to perform a weakly nonlinear stability analysis yielding the system response trajectory in the frequency-growth rate plane until a limit cycle condition is reached. Two scenarios for mode type selection are tentatively proposed. The first is based on an analysis of the frequency growth rate trajectories of the system for different initial solutions. The second consid- ers the stability of the solutions at limit cycle. It is concluded that a criterion combining the stability analysis at the limit cycle with the trajectory analysis might best define the mode type at the limit cy- cle. Simulations are compared with experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global FDF and it is considered that the spacing between burners is such that coupling with the mode takes place without mutual interac- tions between adjacent burning regions. Depending on the nature of the mode being considered, two hypotheses are made for the FDFs of the burners. When instabilities are coupled by a spinning mode, each burner features the same velocity fluctuation level implying that the complex FDF values are the same for all burners. In case of a standing mode, the sixteen burners feature different velocity fluctua- tion amplitudes depending on their relative position with respect to the pressure nodal line. Simulations retrieve the spinning or standing nature of the self-sustained mode that were identified in the exper- iments both in the plenum and in the combustion chamber. The frequency and amplitude of velocity fluctuations predicted at limit cycle are used to reconstruct time resolved pressure fluctuations in the plenum and chamber and heat release rate fluctuations at two locations. For the pressure fluctuations, the analysis provides a suitable estimate of the limit cycle oscillation and suitably retrieves experimental data recorded in the MICCA setup and in particular reflects the difference in amplitude levels observed in these two cavities. Differences in measured and predicted amplitudes appear for the heat release rate fluctuations. Their amplitude is found to be directly linked to the rapid change in the FDF gain as the velocity fluctuation level reaches large amplitudes corresponding to the limit cycle, underlying the need of FDF information at high modulation amplitudes.
I. O. Shamshin - One of the best experts on this subject based on the ideXlab platform.
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Hydrogen-fueled detonation ramjet model: Wind tunnel tests at approach air stream Mach number 5.7 and stagnation temperature 1500 K
International Journal of Hydrogen Energy, 2018Co-Authors: S M Frolov, V S Ivanov, V. I. Zvegintsev, V. S. Aksenov, I. O. Shamshin, D. A. Vnuchkov, D. G. Nalivaichenko, A. A. Berlin, V. M. Fomin, A. N. ShiplyukAbstract:Abstract The mode of continuous spinning detonation (CSD) combustion of hydrogen in the Annular Combustor of a model of a hydrogen-fueled detonation ramjet under conditions of approach air stream Mach number 5.7 and stagnation temperature 1500 K is registered experimentally in a short-duration (pulsed) wind tunnel at the overall air-to-hydrogen equivalence ratio (ER) ranging from 0.7 to 1.4. The maximum values of thrust and specific impulse of the ramjet model are attained at ER = 1.25 and are estimated as 1550 N and 3300 s, respectively. At 1.4
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wind tunnel tests of a hydrogen fueled detonation ramjet model at approach air stream mach numbers from 4 to 8
International Journal of Hydrogen Energy, 2017Co-Authors: V S Ivanov, S M Frolov, V. I. Zvegintsev, V. S. Aksenov, I. O. Shamshin, D. A. Vnuchkov, D. G. NalivaichenkoAbstract:Abstract Experimental studies of an axisymmetric hydrogen-fueled detonation ramjet model 1.05-m long and 0.31 m in diameter with an expanding Annular Combustor were performed in a pulse wind tunnel under conditions of approaching air stream Mach number ranging from 4 to 8 with the total temperature of 290 K. In a supersonic air flow entering the Combustor, continuous and longitudinally pulsating modes of hydrogen detonation with the corresponding characteristic frequencies of 1250 and 900 Hz were obtained. The maximum measured values of fuel-based specific impulse and total thrust were 3600 s and 2200 N.
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continuous detonation combustion of ternary hydrogen liquid propane air mixture in Annular Combustor
International Journal of Hydrogen Energy, 2017Co-Authors: V S Ivanov, S M Frolov, V. S. Aksenov, I. O. ShamshinAbstract:Abstract Experiments are performed on continuous detonation combustion of ternary hydrogen–liquid propane–air mixture in a large-scale Annular Combustor 406 mm in outer diameter with an Annular gap of 25 mm. Liquid propane is fed into the Combustor at the time when sustained continuous-detonation combustion of hydrogen–air mixture is attained therein. Mass flow rates of hydrogen, propane and air in the experiments ranged from 0.1 to 0.5 kg/s (hydrogen), 0.1 to 0.5 kg/s (propane), and 5 to 12 kg/s (air). Continuous-detonation combustion of liquid propane in air is obtained for the first time due to addition of hydrogen rather than due to enrichment of air with oxygen. Combustor operation with a single continuously rotating detonation wave (DW) for about 0.1 s has been obtained when the flow rates of propane and air remained constant while the flow rate of hydrogen was rapidly decreasing.
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Energy efficiency of a continuous-detonation combustion chamber
Combustion Explosion and Shock Waves, 2015Co-Authors: S M Frolov, A V Dubrovskii, V S Ivanov, V. S. Aksenov, I. O. ShamshinAbstract:Systematic experimental and computational studies of the energy efficiency of continuous-detonation Combustors (CDCs) have been performed. A small-size and a large-size CDCs using hydrogen as fuel and oxygen or air as oxidizer have been developed and tested. It was first experimentally proved that the Zel’dovich thermodynamic cycle with continuous-detonation combustion of a hydrogen-oxygen mixture in an Annular Combustor is more efficient than the Brayton thermodynamic cycle with continuous combustion of the mixture, other things being equal. The specific impulse of a small-size bench-scale rocket engine with a 50 mm diameter CDC operating in the continuous-detonation mode was 6–7% higher than that in the continuous combustion mode of operation. The measured fuel-based specific impulse for the large-size CDC of 406 mm diameter running on a hydrogen-air mixture was at a level of 3000 s. Three-dimensional calculations to optimize the structure and operation mode of the large-size CDC have shown that when running on a combustible mixture with a nearly stoichiometric overall composition, the specific impulse can be increased to ≈4200 s.
Jonas P. Moeck - One of the best experts on this subject based on the ideXlab platform.
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Weakly nonlinear analysis of thermoacoustic instabilities in Annular Combustors
Journal of Fluid Mechanics, 2016Co-Authors: Giulio Ghirardo, Matthew P. Juniper, Jonas P. MoeckAbstract:© 2016 Cambridge University Press. Rotationally symmetric Annular Combustors are of practical importance because they generically resemble combustion chambers in gas turbines, in which thermoacoustically driven oscillations are a major concern. We focus on azimuthal thermoacoustic oscillations and model the fluctuating heat release rate as being dependent only on the local pressure in the combustion chamber. We study the dynamics of the Annular Combustor with a finite number of compact flames equispaced around the annulus, and characterize the flames' response with a describing function. We discuss the existence, amplitude and the stability of standing and spinning waves, as a function of: (i) the number of the burners; (ii) the acoustic damping in the chamber; (iii) the flame response. We present the implications for industrial applications and the future direction of investigations. We then present as an example the first theoretical study of thermoacoustic triggering in Annular Combustors, which shows that rotationally symmetric Annular chambers that are thermoacoustically unstable do not experience only stable spinning solutions, but can also experience stable standing solutions. We finally test the theory on one experiment with good agreement.
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Weakly nonlinear analysis of thermoacoustic instabilities in Annular Combustors
'Organisation for Economic Co-Operation and Development (OECD)', 2016Co-Authors: Ghirardo Giulio, Juniper Matthew, Jonas P. MoeckAbstract:Rotationally symmetric Annular Combustors are of practical importance because they generically resemble combustion chambers in gas turbines, in which thermoacoustically driven oscillations are a major concern. We focus on azimuthal thermoacoustic oscillations and model the fluctuating heat release rate as being dependent only on the local pressure in the combustion chamber. We study the dynamics of the Annular Combustor with a finite number of compact flames equispaced around the annulus, and characterize the flames’ response with a describing function. We discuss the existence, amplitude and the stability of standing and spinning waves, as a function of: (i) the number of the burners; (ii) the acoustic damping in the chamber; (iii) the flame response. We present the implications for industrial applications and the future direction of investigations. We then present as an example the first theoretical study of thermoacoustic triggering in Annular Combustors, which shows that rotationally symmetric Annular chambers that are thermoacoustically unstable do not experience only stable spinning solutions, but can also experience stable standing solutions. We finally test the theory on one experiment with good agreement.European Research Council (Project ALORS
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Characterization and modeling of a spinning thermoacoustic instability in an Annular Combustor equipped with multiple matrix injectors
Journal of Engineering for Gas Turbines and Power, 2015Co-Authors: Jeanfrancois Bourgouin, Jonas P. Moeck, Thierry Schuller, Daniel Durox, Sebastien CandelAbstract:Oscillations in fully Annular systems coupled by azimuthal modes are often observed in gas turbine Combustors but not well documented. One objective of the present study is to characterize this type of oscillation in a laboratory scale system, allowing detailed pres- sure measurements and high speed visualization of the flame motion. The experiment is designed to allow detailed investigations of this process at a stable limit cycle and for an extended period of time. Experiments reported in the present article are carried out in the MICCA facility which was used in our previous work to analyze instabilities arising when the chamber backplane was equipped with multiple swirling injectors (Bourgouin et al., 2013, “Self-Sustained Instabilities in an Annular Combustor Coupled by Azimuthal Acoustic Modes,” ASME Paper No. GT2013-95010). In the present study, these units are replaced by a set of matrix injectors. The Annular plenum feeds 16 such devices confined by two cylindrical quartz tubes open to the atmosphere. The multiple flames formed by the matrix injectors are laminar and have a well documented describing function. This constitutes an ideal configuration allowing systematic investigations of thermo-acoustic oscillations coupled by longitudinal or azimuthal modes while avoid- ing complexities inherent to swirling turbulent flames studied previously. Optical access to the chamber allows high speed imaging of light emission from the flames providing instantaneous flame patterns and indications on the heat release rate fluc- tuations. Eight waveguide microphones record the pressure signal at the Combustor injection plane and in the plenum. Among the unstable modes observed in this setup, this analysis focuses on situations where the system features a spinning azimuthal mode. This mode is observed at a frequency which is close to that associated with the 1A mode of the plenum. A theoretical analysis is then carried out to interpret the angular shift between the nodal lines in the plenum and chamber, and the meas- ured flame describing function (FDF) is used to quantify this shift and determine the linear growth rate.
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A new pattern of instability observed in an Annular Combustor: The slanted mode
Proceedings of the Combustion Institute, 2015Co-Authors: Jeanfrancois Bourgouin, Jonas P. Moeck, Thierry Schuller, Daniel Durox, Sebastien CandelAbstract:In Annular combustion chambers of aero-engines and gas turbines, acoustic coupling may arise from azimuthal modes which are less well damped than axial modes. Also, since the circumference is the largest length in the Combustor, the azimuthal modes have the lowest resonance frequencies and are most prone to instability. Such a coupling raises many scientific issues which are considered in a small number of funda- mental experiments. The present investigation focuses on this problem and provides experimental data on a special type of combustion instability in which the thermo-acoustic resonant coupling involves a combina- tion of modes. This produces an unusual pattern of flame responses in which the distribution of heat release rate is slanted. Data are provided in the form of free radical light intensity patterns (interpreted as heat release rate distributions) and microphone signals detected in the plenum and chamber. It is shown that the slanted pattern is the signature of a combination of two modes with coinciding frequencies, the first being a standing azimuthal mode while the second is an axial mode. Measurements of the flame describing function on a single matrix burner at the fundamental frequency are used to explain the observed phase shift and amplitude in the flame responses of the different injectors in the Annular Combustor.
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self sustained instabilities in an Annular Combustor coupled by azimuthal and longitudinal acoustic modes
ASME Turbo Expo 2013: Turbine Technical Conference and Exposition, 2013Co-Authors: Jonas P. Moeck, Jeanfrancois Bourgouin, Thierry Schuller, Sebastien CandelAbstract:Annular Combustors may give rise to various types of combustion instabilities. Some of the resulting oscillations coupled by transverse acoustic modes are commonly observed in practice and their suppression or reduction is an important issue which needs to be considered. The present study is carried out in a system comprising an Annular plenum feeding 16 swirling injectors confined by two cylindrical quartz tubes opened to the atmosphere. Calculations based on a Helmholtz solver provide a suitable estimate of frequencies observed experimentally and reveal the modal structure corresponding to the longitudinal and transverse oscillations. High speed images obtained under reactive conditions are then processed to extract the structure of heat release rate perturbations and match this structure with that of the coupling acoustic mode. It is found that the transverse instability is coupled by a first azimuthal mode which is characterized by a time varying spin ratio. This index gives the respective levels of rotating components in the azimuthal mode. Another instability arising at a lower frequency is coupled by a longitudinal acoustic mode giving rise to high-amplitude oscillations in heat release rate in which most of the flames (but not all) are synchronized and in phase with the pressure perturbation.© 2013 ASME