Aimee S. Morgans

The Influence of Entropy Waves on the Thermoacoustic Stability of a Model Combustor

Thermoacoustic instability can be a major problem for aero-engine combustors, particularly lean-premixed burners designed for low NOx emissions. The instability is caused by the interaction between unsteady heat release and acoustic waves within the combustion chamber. Unsteady combustion generates acoustic waves directly, as well as entropy fluctuations that are quiescent. The subsequent acceleration of these entropy waves at the combustor exit creates further acoustic waves known as indirect combustion noise. In this article, a thermoacoustic model is extended to study the effects of dissipation and dispersion of the entropy waves on the stability of the combustor. Four combustor configurations are discussed: a stable combustor that may be destabilized due to the presence of entropy noise, an unstable combustor that may be stabilized by indirect combustion acoustics, an unstable combustor that experiences a “mode switch” to oscillations at a different frequency, and a combustor that is driven to instability due to entropy waves but is not necessarily accompanied by an instability in the heat release. The different scenarios that may arise from the inclusion of entropy noise provide motivation for further work on the influence of entropy waves in combustors.

Acoustic Damping of a Helmholtz Resonator with an Oscillating Volume

Combustion instabilities are caused by a coupling between acoustic waves and unsteady heat release. Helmholtz resonators are widely used as acoustic dampers to stabilize unstable combustion systems. Such dampers are typically subjected to a low Mach number grazing flow and are normally only effective over a narrow frequency range, close to the resonant frequency. To increase the effective frequency range, a Helmholtz resonator with an oscillating volume, implemented via an electromagnetic shaker and vibrating backplate, was designed and experimentally tested at the University of Loughborough. It was found that volume oscillation can either increase or decrease the acoustic power being absorbed by the resonator, depending on the phase with which it is driven. A nonlinear numerical model of a Helmholtz resonator with an oscillating volume was then developed to simulate the experiments. Excellent agreement between the numerical and experimental results is found. Furthermore, insight into how to obtain maximum power absorption was provided by the numerical model and validated by the experiments. Finally, to optimize the phase in real time (by minimizing the amplitude of the pressure oscillations), active control of the backplate vibration was experimentally investigated. For the low Mach number grazing-flow regime investigated, this was found to give increased damping and to increase the effective frequency range of the resonator.

Transonic Helicopter Noise

Helicopter noise is an increasingly important issue, and at large forward-flight speeds transonic rotor noise is a major contributor. A method for predicting transonic rotor noise, which is more computationally efficient than previous methods and which furthermore offers physical insight into the noise generation, is developed. These benefits combine to make it of potential use to helicopter rotor designers. The permeable surface form of the Ffowcs Williams-Hawkings (FW-H) equation is used to express the sound field in terms of a distribution of monopole and dipole sources over a permeable control surface and a distribution of quadrupole sources over the volume outside of this surface. By choosing the control surface to enclose the transonic flow regions, the noise from the quadrupole distribution becomes negligible. Only the more straightforward surface sources then need be considered, making the acoustic approach computationally efficient. By locating the control surface close to the blade subject to enclosing the transonic flow regions, efficiency in the computational-fluid-dynamics (CFD) approach is also attained. To perform noise predictions, an Euler CFD method to calculate the flowfield was combined with an acoustic method incorporating the retarded time formulation of the FW-H equation. Several rotor blades in hover and steady forward flight were considered, all of which involved transonic flows but for which shock delocalization did not occur. The predictions showed very good agreement with experimental data and with predictions obtained using more computationally intensive methods.

Entropy noise: A review of theory, progress and challenges

Combustion noise comprises two components: direct combustion noise and indirect combustion noise. The latter is the lesser studied, with entropy noise believed to be its main component. Entropy noise is generated via a sequence involving diverse flow physics. It has enjoyed a resurgence of interest over recent years, because of its increasing importance 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 these subsequently undergo acceleration. Five stages of flow physics have been identified as being important, these being (a) generation of entropy waves by unsteady heat release rate; (b) advection of entropy waves through the combustor; (c) acceleration of entropy waves through either a nozzle or blade row, to generate entropy noise; (d) passage of entropy noise through a succession of turbine blade rows to appear at the turbine exit; and (e) reflection of entropy noise back into the combustor, where it may further perturb the flame, influencing the combustor thermoacoustics. This article reviews the underlying theory, recent progress and outstanding challenges pertaining to each of these stages.

Adaptive Feedback Control of Combustion Instability in Annular Combustors

An adaptive feedback control strategy for axisymmetric annular combustors is developed. The controller uses the pressure perturbation at a fixed axial location downstream of the flame—provided by multiple pressure measurements around the combustor circumference—as the control input. Control is achieved via modulation of the fuel flow rate at multiple fuel valve locations. The controller is implemented in a computational low-order thermoacoustic network model of an annular combustor. Time domain simulations show that the controller stabilizes both longitudinal and circumferential unstable modes (including the simultaneous control of multiple unstable modes), even from within the limit cycle. By changing the flame transfer function with time, it is shown that control is retained following a large change in the combustor operating conditions.

Projection-free approximate balanced truncation of large unstable systems

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.

Feedback control for form-drag reduction on a blu body with a blunt trailing edge

The objective of the present numerical study is to increase the base pressure on a backward-facing step via a simple feedback control method; to be ultimately translated to a drag reduction on a blunt-based blu body representative of a road vehicle. Two cases are considered: a simpli ed 2D ow at low Reynolds number and a fully turbulent 3D ow at a Reynolds of Re = 1500, deemed su cient to be representative of automobile applications. Using Large Eddy Simulation (LES), system identi cation is performed to characterize the ow response to actuation. The control is e ected by a full-span slot jet, with zero-net-massux, located near separation and injecting at an angle of 45 . In the two cases, a broad range of frequencies is tested with harmonic inputs. The 2D and 3D ows are found to respond di erently to actuation, yielding open-loop responses with di erent dynamics. The open-loop characterization is used to synthesize a feedback controller. The control target is set to the instantaneous pressure uctuations on the base of the step, which in turn is expected to give a reduction in time-mean pressure. For both the 2D and 3D cases, a substantial pressure increase, and hence drag reduction, is obtained with a simple controller based on disturbance attenuation.

Acoustic Models for Cooled Helmholtz Resonators

Helmholtz resonators are commonly used to damp thermoacoustic oscillations in aeroengine and gas turbine combustors. In practice, the Helmholtz resonator is often maintained at a cooler temperature...

The Effect of a Laminar Moving Flame Front on Thermoacoustic Oscillations of an Anchored Ducted V-Flame

When investigating combustion instabilities using analytical models, it has previously been assumed that the compact flame assumption implied that the flame-front movement did not need to be taken into account to solve the acoustics. This article shows that this is not necessarily the case. This article presents a generalization of such models of anchored V-flames to allow the flame “source” of acoustic waves to vary its position in time so as to track the flame-front location. A method for solving this problem is then presented. It is found that accounting for the flame front movement can alter both the linear stability of the combustor, and (for cases that remain unstable) the limit cycle amplitude. Significant changes in limit cycle amplitude are observed across a large range of operating conditions. The flame front movement has so far only been seen to provide a stabilizing effect, reducing the Rayleigh source term. Self-tuning regulator adaptive control methods appear to be unaffected by accounting for the moving flame front.

Tuned Passive Control of Combustion Instabilities Using Multiple Helmholtz Resonators

In this work, tuned passive control is used to damp unstable combustion systems, with particular emphasis on systems which exhibit multiple unstable modes. Helmholtz resonators are used as passive dampers. The frequency at which they offer maximum damping is varied by altering their geometry; in this work, geometry changes are achieved by varying the area of the Helmholtz resonator neck. For each unstable mode exhibited by the combustion system, a separate Helmholtz resonator has its neck area tuned. Two algorithms are developed, one for identifying the characteristics of all modes present in real-time, and another for tuning the neck areas of the Helmholtz resonators. These algorithms are successfully implemented in numerical simulations of a longitudinal combustor exhibiting two unstable modes. The algorithms result in both modes being stabilised as long as two Helmholtz resonators are used. Experiments are then conducted on a Rijke tube with its upper part split into two branches of differing lengths, shaped like a ‘Y’. The differing lengths give rise to two unstable modes at different frequencies. A Helmholtz resonator is attached to each branch; the neck area of both can be varied by means of an ‘iris’ valve, which opens and closes like a camera lens. On implementing the procedure for tuning the neck areas, both unstable modes are stabilised, and stability is maintained for large changes in operating condition. This confirms that the procedure developed is sufficiently robust for use in real combustion systems exhibiting multiple unstable modes.