Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Why Nonuniform Density Suppresses the Precessing Vortex Core

[+] Author and Article Information
Kilian Oberleithner

Laboratory for Turbulence
Research in Aerospace & Combustion,
Monash University,
Clayton, VIC 3800, Australia
e-mail: kilian.oberleithner@pi.tu-berlin.de

Christian Oliver Paschereit

Institut für Strömungsmechanik und Technische
Akustik, Chair of Fluid Dynamics,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 9, 2013; final manuscript received July 21, 2013; published online September 23, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(12), 121506 (Sep 23, 2013) (9 pages) Paper No: GTP-13-1251; doi: 10.1115/1.4025130 History: Received July 09, 2013; Revised July 21, 2013

Linear stability analysis is applied to a swirl-stabilized combustor flow with the aim to understand how the flame shape and associated density field affects the manifestation of self-excited flow instabilities. In isothermal swirling jets, self-excited flow oscillations typically manifest in a precessing vortex core and synchronized growth of large-scale spiral-shaped vortical structures. Recent theoretical studies relate these dynamics to a hydrodynamic global instability. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density inhomogeneities created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a perfectly premixed steam-diluted detached flame featuring a strong precessing vortex core. The second represents a perfectly premixed dry flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change in instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows for relating the flame position and the resulting density field to the emergence of a precessing vortex core.

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Grahic Jump Location
Fig. 1

Sketch of the experimental setup

Grahic Jump Location
Fig. 2

Mean properties of the dry V-flame configuration (left) and the steam-diluted annular flame configuration (right): (a) and (b) mean normalized axial velocity component; (c) and (d) mean OH* chemiluminescence indicating the mean flame location; (e) and (f) mean normalized density distribution; (a)–(f ) streamlines computed from the axial and radial mean velocity component

Grahic Jump Location
Fig. 3

POD analysis of the annular flame configuration: (a) POD spectrum showing energy distribution among the POD modes; (b) power spectrum of the first three POD coefficients; (c) time plot of the first two POD coefficients

Grahic Jump Location
Fig. 4

Dominant coherent flow structures of the annular flame configuration: (a) and (b) first two spatial POD modes; (c) phase difference between the first two modes (weighted by the oscillation amplitude) revealing how the waves emanate from the jet center at x/Dh≈1.2 and r = 0, as indicated by the black cross

Grahic Jump Location
Fig. 5

Spatiotemporal stability analysis of the annular flame configuration. (a) and (b) Contours and profiles of the mean axial velocity component and density; (c) and (d) absolute growth rate ω0,i and absolute frequency ω0,r of the isothermal (chainlines) and stratified (solid lines) analysis; the dashed horizontal line refers to the measured global oscillation frequency; the dashed vertical lines refer to the wavemaker location xs of the stratified analysis

Grahic Jump Location
Fig. 6

Connection of the global mode wavemaker location and streamwise wave propagation derived from PIV measurements: (a) measured phase distribution of the coherent vorticity; (b) normalized wavenumber and phase velocity of the measured coherent vorticity along the jet centerline; (c) and (d) absolute growth rate ω0,i and absolute frequency ω0,r of the isothermal (chainlines) and stratified (solid lines) analysis; the dashed vertical lines refer to the wavemaker location xs of the stratified analysis

Grahic Jump Location
Fig. 7

Spatiotemporal stability analysis of the V-flame configuration. (a) and (b) Contours and profiles of the mean axial velocity component and density; (c) and (d) absolute growth rate ω0,i and absolute frequency ω0,r of the isothermal (chainlines) and stratified (solid lines) analysis; the dashed horizontal line refers to the measured global oscillation frequency; the dashed vertical lines refer to the wavemaker location xs of the stratified analysis




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