Publications
165 results found
Xia Y, Morgans AS, Jones WP, et al., 2017, Predicting thermoacoustic instability in an industrial gas turbine combustor: Combining a low order network model with flame LES, ASME Turbo Expo 2017
Copyright © 2017 ASME and Siemens AG. The thermoacoustic modes of a full scale industrial gas turbine combustor have been predicted numerically. The predictive approach combines low order network modelling of the acoustic waves in a simplified geometry, with a weakly nonlinear flame describing function, obtained from incompressible large eddy simulations of the flame region under upstream forced velocity perturbations, incorporating reduced chemistry mechanisms. Two incompressible solvers, each employing different numbers of reduced chemistry mechanism steps, are used to simulate the turbulent reacting flowfield to predict the flame describing functions. The predictions differ slightly between reduced chemistry approximations, indicating the need for more involved chemistry. These are then incorporated into a low order thermoacoustic solver to predict thermoacoustic modes. For the combustor operating at two different pressures, most thermoacoustic modes are predicted to be stable, in agreement with the experiments. The predicted modal frequencies are in good agreement with the measurements, although some mismatches in the predicted modal growth rates and hence modal stabilities are observed. Overall, these findings lend confidence in this coupled approach for real industrial gas turbine combustors.
Yang D, Morgans A, 2016, A semi-analytical model for the acoustic impedance of finite length circular holes with mean flow, Journal of Sound and Vibration, Vol: 384, Pages: 294-311, ISSN: 1095-8568
The acoustic response of a circular hole with mean flow passing through it is highly relevant to Helmholtz resonators,fuel injectors, perforated plates, screens, liners and many other engineering applications. A widely used analyticalmodel [M. S. Howe. On the theory of unsteady high Reynolds number flow through a circular aperture, Proc. ofthe Royal Soc. A. 366, 1725 (1979), 205223] which assumes an infinitesimally short hole was recently shown tobe insufficient for predicting the impedance of holes with a finite length. In the present work, an analytical modelbased on the Green’s function method is developed to take the hole length into consideration for “short” holes. Theimportance of capturing the modified vortex noise accurately is shown. The vortices shed at the hole inlet edge areconvected to the hole outlet and further downstream to form a vortex sheet. This couples with the acoustic wavesand this coupling has the potential to generate as well as absorb acoustic energy in the low frequency region. Theimpedance predicted by this model shows the importance of capturing the path of the shed vortex. When the vortexpath is captured accurately, the impedance predictions agree well with previous experimental and CFD results, forexample predicting the potential for generation of acoustic energy at higher frequencies. For “long” holes, a simplifiedmodel which combines Howe’s model with plane acoustic waves within the hole is developed. It is shown that themost important effect in this case is the acoustic non-compactness of the hole.
Li J, Morgans AS, 2016, Simplified models for the thermodynamic properties along a combustor and their effect on thermoacoustic instability prediction, Fuel, Vol: 184, Pages: 735-748, ISSN: 0016-2361
Accurately predicting the thermoacoustic modes of a combustor depends upon knowledge of the thermodynamic properties within the combustor; flame temperature, heat release rate, speed of sound and ratio of specific heats all have a strong effect. Calculating the global equilibrium properties resulting from fuel combustion is not straightforward due to the presence of complex multi-species and multi-step reaction mechanisms. A method which decouples the calculations of species dissociations is proposed in this work: this improves the precision of calculation when using few species and reduces the computational cost and complexity to a degree that embedding within low order thermoacoustic network codes is feasible. When used to calculate the combustion product mole fractions, temperature, heat release rate, speed of sound and ratio of specific heats for hydrocarbon-air flames, the method is found to be accurate and highly efficient across different operating conditions and fuel types. The method is then combined with improved low-order wave-based network modelling, the latter employing wave-based acoustic models which account for the variation of thermodynamic properties along the combustion chamber. For a laboratory-scale combustor with a large downstream temperature variation, it is shown that accurate prediction of thermoacoustic modal frequencies and growth rates does depend on accounting for the variation in thermodynamic properties.
Li J, Morgans AS, 2016, Feedback control of combustion instabilities from within limit cycle oscillations using H-infinity loop-shaping and the nu-gap metric, Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol: 472, ISSN: 0080-4630
Combustion instabilities arise due to a two-waycoupling between acoustic waves and unsteadyheat release. Oscillation amplitudes successivelygrow, until nonlinear effects cause saturation intolimit cycle oscillations. Feedback control, in whichan actuator modifies some combustor input inresponse to a sensor measurement, can suppresscombustion instabilities. Linear feedback controllersare typically designed using linear combustor models.However, when activated from within limit cycle,the linear model is invalid and such controllers arenot guaranteed to stabilise. This work develops afeedback control strategy guaranteed to stabilise fromwithin limit cycle oscillations. A low order model ofa simple combustor, exhibiting the essential featuresof more complex systems, is presented. Linear planeacoustic wave modelling is combined with a weaklynonlinear describing function for the flame. The latteris determined numerically using a level set approach.Its implication is that the open loop transfer function(OLTF) needed for controller design varies withoscillation level. The difference between the mean andthe rest of OLTFs is characterised using the ν-gapmetric, providing the minimum required “robustnessmargin” for an H∞ loop-shaping controller. Suchcontrollers are designed and achieve stability bothfor linear fluctuations and from within limit cycleoscillations.
Xia Y, Duran I, Morgans AS, et al., 2016, Dispersion of entropy waves advecting through combustion chambers, The 23rd International Congress on Sound & Vibration
Yang D, Morgans AS, 2016, An analytical model for the acoustic impedance of circular holes of finite length, Int. Congress on Sound and Vibration
Morgans AS, Duran I, 2016, Entropy noise: a review of theory, progress and challenges, International Journal of Spray and Combustion Dynamics, Vol: 8, Pages: 285-298, ISSN: 1756-8277
Combustion noise comprises two components: direct combustion noise and indirect combustion noise. The latter is thelesser studied, with entropy noise believed to be its main component in combusting flows. Entropy noise is generated viaa sequence involving diverse flow physics. It has enjoyed a resurgence of interest over recent years, due to its increasingimportance to aero-engine exhaust noise and a recognition that it can affect gas turbine combustion instabilities.Entropy noise occurs when unsteady heat release rate generates temperature fluctuations (entropy waves), and thesesubsequently undergo acceleration. Five stages of flow physics have been identified as being important, these being (i)generation of entropy waves by unsteady heat release rate; (ii) advection of entropy waves through the combustor; (iii)acceleration of entropy waves through either a nozzle or blade row, to generate entropy noise; (iv) passage of entropynoise through a succession of turbine blade rows to appear atthe turbine exit and (v) reflection of entropy noise backinto the combustor, where it may further perturb the flame, influencing the combustor thermoacoustics. This paperreviews the underlying theory, recent progress and outstanding challenges pertaining to each of these stages
Li J, Morgans AS, 2016, Thermoacoustic analysis of combustors consisting of long flames using a low order network model approach, Int. Symp. on Thermoacoustic Instabilities in Gas Turbines and Rocket Engines: Industry Meets Academia
Xia Y, Morgans A, Jones WP, et al., 2016, Combining low order network modelling with incompressible flame LES for thermoacoustic instability in an industrial gas turbine combustor, Joint meeting of the British, Portuguese and Spanish Sections of the Combustion Institute
Duran I, Xia Y, Morgans AS, et al., 2016, Dispersion of entropy waves advecting through combustion chambers, The 2nd Joint meeting of the British, Portuguese and Spanish Sections of the Combustion Institute
Flinois T, Morgans AS, 2016, Feedback control of unstable flows: a direct modelling approach using the Eigensystem Realisation Algorithm, Journal of Fluid Mechanics, Vol: 793, Pages: 41-78, ISSN: 1469-7645
Obtaining low order models for unstable flows in a systematic and computationally tractable manner has been a long-standing challenge. In this study, we show that the Eigensystem Realisation Algorithm (ERA) can be applied directly to unstable flows, and that the resulting models can be used to design robust stabilising feedback controllers. We consider the unstable flow around a D-shaped body, equipped with body-mounted actuators, and sensors located either in the wake or on the base of the body. A linear model is first obtained using approximate balanced truncation. It is then shown that it is straightforward and justified to obtain models for unstable flows by directly applying the ERA to the open-loop impulse response. We show that such models can also be obtained from the response of the nonlinear flow to a small impulse. Using robust control tools, the models are used to design and implement both proportional and H-infinity loop-shaping controllers. The designed controllers were found to be robust enough to stabilise the wake, even from the nonlinear vortex shedding state and in some cases at off-design Reynolds numbers.
Han X, Li J, Morgans A, 2015, Prediction of combustion instability limit cycle oscillations by combining flame describing function simulations with a thermoacoustic network model, Combustion and Flame, Vol: 162, Pages: 3632-3647, ISSN: 0010-2180
Accurate prediction of limit cycle oscillations resulting from combustion instability has been a long-standing challenge. The present work uses a coupled approach to predict the limit cycle characteristics of a combustor, developed at Cambridge University, for which experimental data are available (Balachandran, Ph.D. thesis, 2005). The combustor flame is bluff-body stabilised, turbulent and partially-premixed. The coupled approach combines Large Eddy Simulation (LES) in order to characterise the weakly non-linear response of the flame to acoustic perturbations (the Flame Describing Function (FDF)), with a low order thermoacoustic network model for capturing the acoustic wave behaviour. The LES utilises the open source Computational Fluid Dynamics (CFD) toolbox, OpenFOAM, with a low Mach number approximation for the flow-field and combustion modelled using the PaSR (Partially Stirred Reactor) model with a global one-step chemical reaction mechanism for ethylene/air. LES has not previously been applied to this partially-premixed flame, to our knowledge. Code validation against experimental data for unreacting and partially-premixed reacting flows without and with inlet velocity perturbations confirmed that both the qualitative flame dynamics and the quantitative response of the heat release rate were captured with very reasonable accuracy. The LES was then used to obtain the full FDF at conditions corresponding to combustion instability, using harmonic velocity forcing across six frequencies and four forcing amplitudes. The low order thermoacoustic network modelling tool used was the open source OSCILOS (http://www.oscilos.com). Validation of its use for limit cycle prediction was performed for a well-documented experimental configuration, for which both experimental FDF data and limit cycle data were available. The FDF data from the LES for the present test case was then imported into the OSCILOS geometry network and limit cycle oscillations of frequency 342 Hz and
Rigas G, Morgans AS, Brackston RD, et al., 2015, Diffusive dynamics and stochastic models of turbulent axisymmetric wakes, Journal of Fluid Mechanics, Vol: 778, Pages: R2-1-R2-10, ISSN: 0022-1120
A modelling methodology to reproduce the experimental measurements of a turbulent flow in the presence of symmetry is presented. The flow is a three-dimensional wake generated by an axisymmetric body. We show that the dynamics of the turbulent wake flow can be assimilated by a nonlinear two-dimensional Langevin equation, the deterministic part of which accounts for the broken symmetries that occur in the laminar and transitional regimes at low Reynolds numbers and the stochastic part of which accounts for the turbulent fluctuations. Comparison between theoretical and experimental results allows the extraction of the model parameters.
Flinois TLB, Morgans AS, Schmid PJ, 2015, Projection-free approximate balanced truncation of large unstable systems, Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Vol: 92, ISSN: 1539-3755
In this article, we show that the projection-free, snapshot-based, balanced truncation method can be applied directly to unstable systems. We prove that even for unstable systems, the unmodified balanced proper orthogonal decomposition algorithm theoretically yields a converged transformation that balances the Gramians (including the unstable subspace). We then apply the method to a spatially developing unstable system and show that it results in reduced-order models of similar quality to the ones obtained with existing methods. Due to the unbounded growth of unstable modes, a practical restriction on the final impulse response simulation time appears, which can be adjusted depending on the desired order of the reduced-order model. Recommendations are given to further reduce the cost of the method if the system is large and to improve the performance of the method if it does not yield acceptable results in its unmodified form. Finally, the method is applied to the linearized flow around a cylinder at Re = 100 to show that it actually is able to accurately reproduce impulse responses for more realistic unstable large-scale systems in practice. The well-established approximate balanced truncation numerical framework can therefore be safely applied to unstable systems without any modifications. Additionally, balanced reduced-order models can readily be obtained even for large systems, where the computational cost of existing methods is prohibitive.
Flinois TL, Morgans AS, Schmid PJ, 2015, Projection-free approximate balanced truncation of large unstable systems, Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Vol: 92, Pages: 023012-023012, ISSN: 1063-651X
In this article, we show that the projection-free, snapshot-based, balanced truncation method can be applied directly to unstable systems. We prove that even for unstable systems, the unmodified balanced proper orthogonal decomposition algorithm theoretically yields a converged transformation that balances the Gramians (including the unstable subspace). We then apply the method to a spatially developing unstable system and show that it results in reduced-order models of similar quality to the ones obtained with existing methods. Due to the unbounded growth of unstable modes, a practical restriction on the final impulse response simulation time appears, which can be adjusted depending on the desired order of the reduced-order model. Recommendations are given to further reduce the cost of the method if the system is large and to improve the performance of the method if it does not yield acceptable results in its unmodified form. Finally, the method is applied to the linearized flow around a cylinder at Re = 100 to show that it actually is able to accurately reproduce impulse responses for more realistic unstable large-scale systems in practice. The well-established approximate balanced truncation numerical framework therefore can be safely applied to unstable systems without any modifications. Additionally, balanced reduced-order models can readily be obtained even for large systems, where the computational cost of existing methods is prohibitive.
Li J, Morgans AS, 2015, Control of combustion instabilities by a second heat source, The 22nd International Congress on Sound and Vibration (ICSV) 2015
Yang D, Morgans AS, 2015, Helmholtz resonators for damping combustor thermoacoustics, The 22nd International Congress on Sound and Vibration (ICSV) 2015
Han X, Morgans AS, 2015, Flame describing function calculations of a turbulent partially-premixed flame from LES, The 22nd International Congress on Sound and Vibration (ICSV) 2015
Duran I, Morgans AS, 2015, The effect of hydrodynamic instabilities on thepropagation of entropy waves through nozzlesand on the generation of indirect combustion noise, The 22nd International Congress on Sound and Vibration
Morgans AS, Li J, 2015, The effect of entropy noise on combustion instability in the presence of advective shear dispersion, the 22nd International Congress on Sound and Vibration (ICSV) 2015
Li J, Morgans AS, 2015, Time domain simulations of nonlinear thermoacoustic behaviour in a simple combustor using a wave-based approach, Journal of Sound and Vibration, Vol: 346, Pages: 345-360, ISSN: 0022-460X
Lean premixed combustion chambers are susceptible to combustion instabilities arising from the coupling between the heat release rate perturbations and the acoustic disturbances. These instabilities are not desirable and knowledge of this complex mechanism is necessary in order to prevent or at least suppress them. A low order model is developed comprising a linear acoustic network and an improved analytical form for a flame describing function (FDF). The latter includes both the saturation of the amplitude of heat release rate perturbations and the change of phase lag relative to oncoming acoustic velocity fluctuations when the instability grows into a limit cycle. A stability map is constructed by moving the flame along the Rijke tube based on the eigenvalues resolved from the network. The acoustic model is then converted into the time domain and combined with the flame describing function to determine the evolutions of the heat release rate disturbances and velocity perturbations within the tube. It is shown that this method can be used to capture some quite intricate nonlinear behaviour of combustion instabilities and the results in the time domain are consistent with those predicted in the frequency domain.
Duran I, Morgans AS, 2015, On the reflection and transmission of circumferential waves through nozzles, Journal of Fluid Mechanics, Vol: 773, Pages: 137-153, ISSN: 1469-7645
Han X, Morgans AS, 2015, Simulation of the flame describing function of a turbulent premixed flame using an open-source LES solver, Combustion and Flame, Vol: 162, Pages: 1778-1792, ISSN: 0010-2180
Numerical simulations were used to characterise the non-linear response of a turbulent premixed flame to acoustic velocity fluctuations. The test flame simulated was the bluff body stabilised flame which has been the subject of a detailed experimental study (Balachandran et al., 2005). Simulations were performed using Large Eddy Simulation (LES) via the open source Computational Fluid Dynamics (CFD) software, , with combustion modelled by combining a Flame Surface Density (FSD) method with a fractal approach for the wrinkling factor. The cold flow field and the unforced reacting flow were used for preliminary code validation. In order to characterise the non-linear response of the unsteady heat release rate to acoustic forcing, a harmonically varying velocity fluctuation, for which both the forcing frequency and normalised forcing amplitude were varied, was imposed. The flame response was characterised via a Flame Describing Function (FDF), also known as a non-linear flame transfer function, for which the gain and phase shift depend on forcing amplitude as well as forcing frequency. The response at four frequencies was compared to experimental data in detail, confirming that the LES results captured both the qualitative flame dynamics and the quantitative response of the heat release rate with very reasonable accuracy. The full FDF was then obtained across more frequencies, again showing a good fit with the experimental data, other than for a slight under-prediction in gain, most probably due to neglecting the effect of wall heat loss and the effect of combustion modelling. The agreement was significantly better than has been obtained previously for this test case using numerical simulations. Finally, it was found that increasing combustor length had little affect on the flame response, which may prove useful for future long combustor stability and limit cycle analysis. This work thus confirms that LES, in this case via the open source Code_Saturne, provides a usefu
Luzzato CM, Morgans AS, 2015, The effect of a laminar moving flame front on thermoacoustic oscillations ofan anchored ducted V-flame, Combustion Science and Technology, Vol: 187, Pages: 410-427, ISSN: 1563-521X
Rigas G, Oxlade AR, Morgans AS, et al., 2014, Low-dimensional dynamics of a turbulent axisymmetric wake, Journal of Fluid Mechanics, Vol: 755, Pages: 1-11, ISSN: 0022-1120
The coherent structures of a turbulent wake generated behind a bluff three-dimensional axisymmetric body are investigated experimentally at a diameter-based Reynolds number of ∼2×105 . Proper orthogonal decomposition of base pressure measurements indicates that the most energetic coherent structures retain the structure of the symmetry-breaking laminar instabilities and are manifested as unsteady vortex shedding with azimuthal wavenumber 𝑚=±1 . In a rotating reference frame, the shedding preserves the reflectional symmetry and is linked with a reflectionally symmetric mean pressure distribution on the base. Due to a slow rotation of the symmetry plane of the turbulent wake around the axis of the body, statistical axisymmetry is recovered in the time average. The ratio of the time scales associated with the slow rotation of the symmetry plane and the vortex shedding is of order 100.
Morgans AS, Dahan JA, Flinois T, 2014, Feedback control for reducing the pressure drag of bluff bodies terminated by a backward-facing step, UKACC Control2014 Conf.
Li J, Morgans AS, 2014, Model-based control of nonlinear combustion instabilities, The 21st International Congress on Sound and Vibration (ICSV) 2014
Morgans AS, Goh CS, Dahan JA, 2013, The dissipation and shear dispersion of entropy waves in combustor thermoacoustics, Journal of Fluid Mechanics, Vol: 733, ISSN: 0022-1120
Luzzato CM, Assier RC, Morgans AS, et al., 2013, Modelling thermo-acoustic instabilities of an anchored laminar flame in a simple lean premixed combustor: including hydrodynamic effects, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-15
Luzzato CM, Assier RC, Morgans AS, et al., 2013, Modelling thermo-acoustic instabilities of an anchored laminar flame in a simple lean premixed combustor: Including hydrodynamic effects
Lean premixed combustors reduce oxide of nitrogen (NOx) emissions but are also prone to self sustained thermo-acoustic instabilities. Attempting to model these instabilities has become a popular research topic, and asymptotic based flame modelling allows us to capture all of the length scales involved in the instability. This paper presents a method for solving the full acoustic, hydrodynamic and flame coupling of an anchored laminar V-flame within a simple combustor. This allows us to model the Darrieus-Landau instability effects, and show how they influence the general behaviour and stability of the combustor when compared to G-Equation numerical results.
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