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

Combustion control and sensors: a review

Progress in probability density function methods for turbulent reacting flows

Results from an experimental study of the mechanism of unstable combustion in a coaxial, optically accessible, bluff-body-stabilizeddumpcombustorwithnaturalgasasthefuelarereported.Aparametricstudywasperformed to investigate the effects of equivalence ratio, inletvelocity, inlet fuel distribution, inlet swirl, and centerbody recess oncombustionstability. It wasfoundthatall of theseparametershadan effectonthestability characteristicsofthis combustor.Atselectedunstableoperatingconditions,phase-resolvedCHchemiluminescenceimageswerecaptured to study the heat-release structure during one period of pressure oscillation. The e ame‐ e owe eld interaction that is depicted in these images indicates that e ame‐ vortex interactions, and the resultant e ame area changes, play a signie cant role in the instabilities that occur when there is no swirl. A simple analysis of these images, however, showedthate uctuatinge ameareaandequivalenceratioe uctuationsbothcontributetotheheatreleasee uctuations that drive the instability. Unstable combustion with swirl appears to be fundamentally different from unstable combustion without swirl in that instabilities with swirl occur near lean blowout and appear to be associated with repeated detaching and reattaching of the e ame from the centerbody.

Mechanism of Combustion Instability in a Lean Premixed Dump Combustor

Two-dimensional flame structure measurements of a freely propagating premixed turbulent flame were examined for fractal character over a range of turbulent Reynolds numbers from 52 to 1431 and Damkohler numbers from 10 to 889. The fractal dimension was found to increase with u'/SL, from 2.13 at u'/SL of 0.25 to 2.32 at u'/SL of 11.9. An inner cutoff was not identified due to limited spatial resolution, however, it was shown that the inner cutoff was smaller than the Gibson scale for the conditions studied. The outer cutoff was found to occur at scales comparable to the integral scale, however, the cutoff was a gradual, rather than sharp, transition from fractal behavior. A heuristic relationship between the turbulent flame surface fractal dimension and u'/SL was developed which agrees well with the fractal dimension measurements made in this study, as well as those made in a number of different flame configurations.

The Fractal Nature of Premixed Turbulent Flames

Surface properties of turbulent premixed propane/air flames at various Lewis numbers☆

Interference mechanisms of acoustic/convective disturbances in a swirl-stabilized lean-premixed combustor

The curvature of turbulent premixed flame fronts is an important spatial property that needs to be quantified over a range of turbulence conditions. In this study, measurements of flamelet curvature along with orientation statistics for u'/ SL = 1.42-5.71 are obtained from OH planar laser-induced fluorescence images of the flame boundary by applying a curve-tracing difference formula with interval length of the order of the inner cutoff scale. Use of this particular interval length is essential for accurate tracing of the flame boundary with implicit filtering of extraneous noise that can introduce significant errors in the curvature measurements. The distributions of flamelet curvature are found to be symmetric with respect to the zero mean, while the variance increases with increasing u'/SL. These distributions can be approximated by Gaussian distribution functions. The positive and negative mean curvatures show a nearly square-root dependence on u'/SL whereas the mean flamelet radius of curvat...

D. A. Santavicca

Papers

Mechanism of Combustion Instability in a Lean Premixed Dump Combustor

The Fractal Nature of Premixed Turbulent Flames

Surface properties of turbulent premixed propane/air flames at various Lewis numbers☆

Interference mechanisms of acoustic/convective disturbances in a swirl-stabilized lean-premixed combustor

Curvature and Orientation Statistics of Turbulent Premixed Flame Fronts