S. Candel

A review of active control of combustion instabilities

Abstract Combustion instabilities in modern high-performance propulsion systems are often manifested as large amplitude pressure oscillations and can result in serious performance degradation. These pressure oscillations are often coupled with oscillations in heat release as well as oscillations in the combustor through-flows. The coupling between various oscillatory phenomena has often been traced to an acoustic resonance of a system component which acts as a host for the coupled oscillations. Other phenomena which can lead to unstable oscillations have been identified as well, these include naturally occurring hydrodynamic instabilities and convectively coupled oscillations. Due to their potential harm to system performance, it is often necessary to find ways to reduce the magnitude of these oscillations in the course of developing a new combustion system. Historically, the control and suppression of combustion instabilities has been achieved through hardware design changes. These modifications have included changes in the fuel delivery system, including feedlines and pumps, changes in the fuel injection distribution pattern, and modifications to the combustor or the combustor liner geometry. In general, these design modifications have been made in an attempt to change the resonant behavior of the combustion system so as to avoid the resonantly coupled oscillations which lead to combustion instability. In recent years, much attention has been focused on the control and suppression of combustion instabilities by actively and continuously perturbing certain combustion parameters in order to interrupt the growth and persistence of resonant oscillations. The strategies used in this field of active control vary greatly in nature, both with respect to the theoretical basis for the control system and the system hardware which is employed. The purpose of this paper is to present a discussion of different methods which can be used to suppress combustion instabilities using active control, as well as to give a review of the work which has recently been performed in this area of combustion research.

Combustion control and sensors: a review

There is an increased interest in the application of control to combustion. The objective is to optimize combustor operation, monitor the process and alleviate instabilities and their severe consequences. One wishes to improve the system performance, for example by reducing the levels of pollutant emissions or by smoothing the pattern factor at the combustor exhaust. In other cases, the aim is to extend the stability domain by reducing the level of oscillation induced by coupling between resonance modes and combustion. As combustion systems have to meet increasingly more demanding air pollution standards, their design and operation becomes more complex. The trend towards reduced NOx levels has led to new developments in different fields. Automotive engines and gas turbine combustors are considered in this article. In the first case, complex exhaust aftertreatment is being applied and dedicated engine control schemes are required to ensure and maintain high pollutant conversion efficiency. For gas turbines, premixed combustors, which operate at lower local temperatures than conventional systems have been designed. In both cases, monitoring and control of the operating point of the process have to be achieved with great precision to obtain the full benefits of the NOx reduction scheme. For premixed combustors operating near the lean stability limit, the flame is more susceptible to blowout, oscillation or flashback. Research is now carried out to reduce these dynamical problems with passive and active control methods. In addition to a broad range of fundamental problems raised by Active Combustion Control (ACC) and Operating Point Control (OPC), there are important technological issues. This paper contains a review of some facets of combustion control and focuses on the sensors that take or could take part to combustion control solutions. The current status of ACC and OPC is presented together with the associated control concepts. The state of the art in sensors is reviewed and their applicability is evaluated. Research efforts in combustion diagnostics are to a certain extent devoted to the development of sensors for control applications. The objective of such developments differs from that which is pursued when one wishes to perform detailed measurements on a laboratory scale experiment. The sensor system should not necessarily provide quantitative measurements because relative data are already useful for control purposes. This change of orientation will be discussed and illustrated by examples of current interest. It is concluded that development in control will depend critically on the availability of sensors and on their reliability, robustness, immunity to noise and capacity to operate in a harsh environment. Research is needed on the fundamentals of ACC and OPC but it should also address the more technical aspects of the problem.

Vortex-driven acoustically coupled combustion instabilities

Combustion instability is investigated in the case of a multiple inlet combustor with dump. It is shown that low-frequency instabilities are acoustically coupled and occur at the eigenfrequencies of the system. Using spark-schlieren and a special phase-average imaging of the C 2 -radical emission, the fluid-mechanical processes involved in a vortex-driven mode of instability are investigated. The phase-average images provide maps of the local non-steady heat release. From the data collected on the combustor the processes of vortex shedding, growth, interactions and burning are described. The phases between the pressure, velocity and heat-release fluctuations are determined. The implications of the global Rayleigh criterion are verified and a mechanism for low-frequency vortex-driven instabilities is proposed.

Combustion dynamics and control: Progress and challenges

Combustion dynamics constitutes one of the most challenging areas in combustion research. Many facets of this subject have been investigated over the past few decades for their fundamental and practical implications. Substantial progress has been accomplished in understanding analysis, modeling, and simulation. Detailed laboratory experiments and numerical computations have provided a wealth of information on elementary dynamical processes such as the response of flames to variable strain, vortex rollup, coupling between flames and acoustic modulations, and perturbed flame collisions with boundaries. Much recent work has concerned the mechanisms driving instabilities in premixed combustion and the coupling between pressure waves and combustion with application to the problem of instability in modern low NO x heavyduty gas turbine combustors. Progress in numerical modeling has allowed simulations of dynamical flames interacting with pressure waves. On this basis, it has been possible to devise predictive methods for instabilities. Important efforts have also been directed at the development of the related subject of combustion control. Research has focused on methods, sensors, actuators, control algorithms, and systems integration. In recent years, scaling from laboratory experiments to practical devices has been achieved with some successebut limitations have also been revealed. Active control of combustion has also evolved in various directions. A number of experiments on laboratory-scale combustors have shown that the amplitude of combustion instabilities could be reduced by applying control principles. Full-scale terrestrial application to gas turbine systems have allowed an increase of the stability margin of these machines. Feedback principles are also being explored to control the point of operation of combustors and engines. Operating point control has special importance in the gas turbine field since it can be used to avoid operation in unstable regions near the lean blowoff limits. More generally, closed loop feedback concepts are useful if one wishes to improve the combustion process as demonstrated by applications to automotive engines. Many future developments of combustion will use such concepts for tuning, optimization, and emissions reduction. This article proposes a broad survey of these fast-moving areas of research.

Flame Stretch and the Balance Equation for the Flame Area

Abstract When a flame propagates in a nonuniform flow it experiences strain and curvature effects. The fractional rate of change of the flame area constitutes the flame stretch. This quantity is often used to describe the structure and extinction mechanisms of turbulent flames. It also occurs in many recent studies of premixed laminar flames. This article provides a unified view of this concept on the basis of a novel derivation of stretch in terms of strain rate and curvature. The flame stretch, the rate of change of the normal to the flame front and the rate of change of the curvature are deduced from a general transport theorem. As an illustration, the components of flame stretch are evaluated in the case of a direct numerical simulation of the interaction between a pair of vortices and a laminar flame. Another application of flame stretch concerns the determination of the available flame surface density. A balance equation is derived for this quantity and cast in various useful forms thus providing a ...

Quenching processes and premixed turbulent combustion diagrams

The structure of premixed turbulent flames is a problem of fundamental interest in combustion theory. Possible flame geometries have been imagined and diagrams indicating the corresponding regimes of combustion have been constructed on the basis of essentially intuitive and dimensional considerations. A new approach to this problem is described in the present paper. An extended definition of flamelet regimes based on the existence of a continuous active (not quenched) flame front separating fresh gases and burnt products is first introduced. Direct numerical simulations of flame/vortex interactions using the full Navier–Stokes equations and a simplified chemistry model are then performed to predict flame quenching by isolated vortices. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effect of stretch, curvature, transient dynamics and viscous dissipation can be accounted for. As a result, flame quenching by vortices (which is one of the key processes in premixed turbulent combustion) may be computed accurately. The effects of curvature and viscous dissipation on flame/vortex interactions may also be characterized by the same simulations. The influence of non-unity Lewis number and of thermo-diffusive processes in turbulent premixed combustion is discussed by comparing flame responses for two values of the Lewis number ( Le = 0.8 and 1.2). An elementary (‘spectral’) diagram giving the response of one flame to a vortex pair is constructed. This spectral diagram is then used, along with certain assumptions, to establish a turbulent combustion diagram similar to those proposed by Borghi (1985) or Williams (1985). Results show that flame fronts are much more resistant to quenching by vortices than expected from the classical theories. A cut-off scale and a quenching scale are also obtained and compared with the characteristic scales proposed by Peters (1986). Results show that strain is not the only important parameters determining flame/vortex interaction. Heat losses, curvature, viscous dissipation and transient dynamics have significant effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. It is found, for example, that the Klimov–Williams criterion which is generally advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.

A unified model for the prediction of laminar flame transfer functions : comparisons between conical and V-flame dynamics

Transfer functions of premixed laminar flames submitted to incident flow perturbations are envisaged and a unified model is derived analytically. This model, based on a linearization of the G-equation for an inclined flame, includes convective effects of the flow modulations propagating upstream of the flame. It is shown that the flame dynamics is governed by two relevant parameters, a reduced frequency, ω∗, and the ratio of the flame burning velocity to the mean flow velocity, SL/ῡ, or equivalently the flame angle α with respect to the flow direction. In the limit of low driving frequencies, the flame motion is only controlled by ω∗ and the unified model reduces to previous kinematic formulations derived for rim stabilized conical flames and V-flames anchored on a central rod. Flame transfer functions for these flame geometries with the different velocity models proposed are derived and limiting cases are examined. In the conical flame case, the low-frequency model gives a good approximation of the gain, but only a fair approximation of the phase. Convective effects are shown to induce an increasing phase lag, while low-frequency models predict a saturation phenomenon. The convective model derived in this article improves results for the gain and the phase which agree with numerical simulations and experiments. It is shown in particular that 1) the correct transfer function phase trend is retrieved and depends on the flame angle α; 2) the reduced cut-off frequency corresponds to a situation where the convective wavelength along the flame front λ = (ῡ cos α)/f equals the flame length L; and 3) the flame response is weakly affected by the amplitude of these perturbations. In the V-flame case, the low-frequency model yields a good approximation of the phase but does not feature gain values in excess of one found in the simulations. This behavior is correctly predicted by the convective model and is shown to depend on the flame angle α. A V-flame behaves as an amplifier in a certain range of frequencies. It is shown that these types of flames are more susceptible to combustion instabilities than conical flames. The V-flame response is also shown to strongly depend on the amplitude of the fluctuations even for moderate perturbation levels.

Combustion Dynamics and Instabilities: Elementary Coupling and Driving Mechanisms

Elementary processes that can be involved in the development of combustion instabilities in gas turbine combustors are described. The premixed mode of combustion is considered more specie cally because it is used in most advanced gas turbine systems. The processes envisaged portray the combustion dynamics of real systems, but they are analyzed in simple laboratory cone gurations. Among the many possible interactions, the most relevant mechanisms are those that generate e uctuations in heat release or induce pressure perturbations. Some typical paths are highlighted to help in the understanding of the multiple links that can exist between elementary processes. Processes involving acoustic/e ame coupling, unsteady strain rates, e ame response to inhomogeneities, interactions of e ames with boundaries, and e ame/vortex interactions are specie cally examined. For each process, a driving or a coupling path is proposed relating heat release e uctuations to acoustic variables in certain cases or leading from acoustic variables to heat release e uctuations in other cases. Stress is also put on characteristic time lags, which are key parameters in the triggering and development of instabilities. Well-controlled experiments illustrate the many possibilities and can serve to guide the modeling effort and to validate computational tools for combustion dynamics.

Theoretical and experimental determinations of the transfer function of a laminar premixed flame

The dynamical behavior of laminar premixed flames is investigated in this article. The flame response to incident perturbations is characterized with a transfer function relating the flow velocity modulations and the heat release fluctuations. This function is obtained using the assumptions introduced in previous studies by Fleifil et al. , but the model is extended to account for any flame angle (i.e., any operating condition). The modeling shows that phenomena can be described using a single control parameter taking the form of a reduced frequency ω* . This quantity is derived as ωR/S L cos α 0 , where ω is the angular frequency, R is the burner radius S L is the laminar burning velocity, and α 0 is the half-cone angle of the steady flame. this parameter may be used to describe the response of the burner to acoustic modulation, knowing its geometry and the flame properties. Two characteristic times have been determined. The first one defines the cut-off frequency of the low-pass filter associated with the flame response. The second one enables the prediction of the time lag between the velocity modulation at the burner exit and the flame heat release the exact transfer function and an approximation in the form of a first-order model are compared with an extensive set of experimental data corresponding to a range of equivalence ratios and two burner diameters. Good agreement is obtained for low values of the reduced frequency. In an intermediate range of frequencies, the experimental phase exceeds the theoretical values by a significant amount, the difference between theory and experiment is due to the simplifying assumptions used in the model.

Stochastic Approach to Noise Modeling for Free Turbulent Flows

A new approach to noise modeling for free turbulent flows is presented. The equations governing the sound field are obtained in two steps. The first step consists of treating the mean and turbulent components of the flow while the acoustic perturbations are neglected. In the second step, a set of equations is derived for the acoustic variables. On the left-hand side of this system, one finds the linearized Euler equations, whereas the right-hand side exhibits source terms related to the turbulent fluctuations and their interactions with the mean flow. These terms are modeled using a stochastic description of the three-dimensional turbulent motion. This is achieved by synthesizing the velocity field at each point in space and for all times with a collection of discrete Fourier modes. The synthesized field posesses the suitable one- and two-point statistical moments and a reasonable temporal power spectral density. The linearized Euler equations including a stochastic description of noise sources are solved numerically with a scheme based on a fractional step treatment. Each one-dimensional problem is solved with a weak formulation. A set of calculations are carried out for a simple freejet. Comparisons between calculations and experiments indicate that a spatial filtering of the source terms is required to obtain the expected level in the far field. Realistic pressure signals, power spectral densities, and sound field patterns are obtained. It is indicated that the stochastic noise generation and radiation (SNGR) approach may be applied to more complex flows because the numerical codes used to calculate the mean flowfield and the wave propagation are not specific of jet configurations. The limitations of the present model lie in the statistical properties of the synthetic turbulent field and in the use of an axisymmetric modeling of the acoustic propagation.