Boundary conditions for direct simulations of compressible viscous flows

Dynamics and stability of lean-premixed swirl-stabilized combustion

A review of active control of combustion instabilities

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.

Combustion dynamics and control: Progress and challenges

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 ...

Flame Stretch and the Balance Equation for the Flame Area

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.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112087000958

Vortex-driven acoustically coupled combustion instabilities

Suppression of combustion instabilities by active control

An active instability control (AIC) system is used to suppress low-frequency combustion instabilities in a non-premixed multiple-flame combustor and to study their initiation. The AIC device is based on a feedback control loop: The signal provided by a microphone is filtered, processed and sent to driver units plugged on the air duct. It allows the suppression of all unstable modes of the combustor and also provides a new and powerful method to study the instability initiation. Starting from a controlled flame with AIC, the control system is switched off and the growth of the instability is analysed through high speed Schlieren cinematography and sound pressure and reaction rate measurements. Nonlinear spectral analysis is used to extract spectral information during the oscillation growth from the initiation to the limit cycle. Results reveal that five different phases take place between stable combustion and fully established instability. The source of the first flame movements is the flow excitation by longitudinal acoustic waves. When the flow oscillation amplitude is high enough to allow interactions between flame sheets, combustion becomes periodically enhanced. This coupling leads to a fast instability growth until a limit cycle is reached.

Initiation and suppression of combustion instabilities by active control

Experimental determination of the reflection coefficient of a premixed flame in a duct

This paper describes some aspects of our effort to analyze turbulent combustion on the basis of an extension of the coherent flame model initially proposed by Marble and Broadwell.

E. Esposito

Papers

Vortex-driven acoustically coupled combustion instabilities

Suppression of combustion instabilities by active control

Initiation and suppression of combustion instabilities by active control

Experimental determination of the reflection coefficient of a premixed flame in a duct

Coherent Flame Description of Turbulent Premixed Ducted Flames