Dynamics and stability of lean-premixed swirl-stabilized combustion

A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems

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

Turbulent premixed combustion: Flamelet structure and its effect on turbulent burning velocities

Combustion control and sensors: a review

A comprehensive experimental study has been conducted on combustion instabilities in a premixed swirler injector system at various equivalence ratios, chamber pressures, and inlet air temperatures. A stability map has been determined to indicate the range of operating conditions conductive to the occurrence of instabilities. The amplitude of instabilities was found to be a strong function of the equivalence ratio, with pressure oscillations as high as 20% of the mean chamber pressure and unsteady velocities comparable to the mean flow values observed for equivalence ratios around 0.6. On the other hand, beyond an initial threshold value of inlet air temperature at which instability suddenly initiated, variations in inlet air temperature had minimal effect on the strength of instabilities. Measurements of steady and unsteady flame structures carried out using CH chemiluminescence and photographic imaging techniques indicate that the onset of instabilities can potentially cause significant alterations in flame structure, sometimes even causing near extinction of the flame during certain periods of the oscillation cycle. Coupled longitudinal oscillations were observed in the combustion chamber and upstream duct. Instability characteristics such as frequency, mode shape, and phase obtained through pressure measurements were found to be in excellent agreement with predictions from a linear acoustic analysis. A strong correlation was found between the heat release and pressure fluctuations near the dump plane, indicating a possible mechanism for creating and sustaining instabilities.

An experimental study of combustion dynamics of a premixed swirl injector

The interactions between the local flame structure produced by a premixed swirl-stabilized injector with combustion instabilities were experimentally studied for a model gas turbine combustor operating at high pressure and temperature. The model gas turbine combustor studied utilizes a sudden-expansion dump combustor with a single swirler and bluff body for enhancing mixing rate and flame stabilization, respectively. Laser-based measurements were made for both stable and unstable operating conditions. The local flame front structure was visualized using planar laser-induced fluorescence (PLIF) from the OH⊙, and the global heat release zone was interpreted from flame emission measurements. For stable combustion conditions, the mean reaction rate estimated independently from both OH-PLIF and OH * chemiluminescence measurements showed good agreement, thereby indicating confidence in the use of OH-PLIF measurements for extracting the local mean reaction rate. For unstable combustion conditions, the flamefront characteristics, including flame surface density and mean reaction rate, were evaluated together with the information from the OH * chemiluminescence measurements to identify the boundary of the heat release region at discrete phases of the unstable flame. Analysis of the flame structures during combustion instability indicated significant variations during different phases of the instability. The heat release flow field, particularly in the recirculation regions appearing at the corner and inner face of the dump plane, varied substantially. Rayleigh index information indicated that the recirculation zones play an important part in driving the instability. In contrast, the high shear layer formed along the interface between reactants and hot products produced a region where the instability was damped due to a lowering of the heat release.

J.C. Broda

Papers

An experimental study of combustion dynamics of a premixed swirl injector

An experimental estimation of mean reaction rate and flame structure during combustion instability in a lean premixed gas turbine combustor