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

Optimization of the Aerodynamic Flame Stabilization for Fuel Flexible Gas Turbine Premix Burners

[+] Author and Article Information
Stephan Burmberger1

Lehrstuhl für Thermodynamik, Technische Universität München, Boltzmannstraße 15, 85748 Garching, Germanystephan@burmberger.com

Thomas Sattelmayer

Lehrstuhl für Thermodynamik, Technische Universität München, Boltzmannstraße 15, 85748 Garching, Germany

That is, the rotational motion, which was created in the swirler.

According to Ref. 28, also the outward convective transport of vorticity contributes to a decreasing circumferential velocity.

The vorticity field also affects the turbulence characteristics of the flow, causing a potential increase in turbulence intensity due to the energy cascade from large scale coherent vortical structures to the smaller turbulence scales. Therefore, changes in the vorticity field can also affect flame flashback through the turbulent flame speed or through changes in the local flow velocity.

See typical velocity profiles published in Refs. 3,20.

Available for public download.

1

Corresponding author.

J. Eng. Gas Turbines Power 133(10), 101501 (Apr 25, 2011) (10 pages) doi:10.1115/1.4003164 History: Received May 10, 2010; Revised December 02, 2010; Published April 25, 2011; Online April 25, 2011

A frequently employed method for aerodynamic flame stabilization in modern premixed low emission combustors is the breakdown of swirling flows; with carefully optimized tailoring of the swirler, a sudden transition in the flow field in the combustor can be achieved. A central recirculation zone evolves at the cross-sectional area change located at the entrance of the combustion chamber and anchors the flame in a fixed position. In general, premixed combustion in swirling flows can lead to flame flashback that is caused by combustion induced vortex breakdown near the centerline of the flow. In this case, the recirculation zone suddenly moves upstream and stabilizes in the premix zone (Kröner, 2007, “Flame Propagation in Swirling Flows—Effect of Local Extinction on the Combustion Induced Vortex Breakdown,” Combust. Sci. Technol., 179, pp. 1385–1416). This type of flame flashback is caused by a strong interaction between the flame chemistry and vortex dynamics. The analysis of the vorticity transport equation shows that the axial gradient of the azimuthal vorticity is of particular importance for flame stability. A negative azimuthal vorticity gradient decelerates the core flow and finally causes vortex breakdown. Based on fundamental fluid mechanics, guidelines for a proper aerodynamic design of gas turbine combustors are given. These guidelines summarize the experience from several previous aerodynamic and combustion studies of the authors.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Cylindrical coordinate system with sign convention

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Figure 2

Decreasing azimuthal vorticity causes ur>0

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Figure 3

Increasing azimuthal vorticity causes ur<0

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Figure 4

Fuel flexible prototype burner with aerodynamic flame stabilization at atmospheric conditions. Premixed natural gas flame (left) and premixed pure hydrogen flame (right) both with a thermal power of 50 kW under stoichiometric conditions stabilized by the identical burner geometry. Left image: The entrainment of cold ambient air quenches the flame in the outer shear layer close to the burner rim. Vortex breakdown on the axis anchors the natural gas flame slightly above the burner nozzle. Right image: The high reactivity of the hydrogen flame prevents quenching of the outer shear layer. The flame is attached to the burner rim. Due to the fuel flexible aerodynamic design of the flow field, the approximate position of the vortex breakdown zone remains substantially unchanged.

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Figure 5

Configuration A with nozzle diameter D=40 mm: Time averaged profiles showing lower half of the midplane bounded by the z-axis as symmetry line at the upper end: (a) mean axial velocity u¯z; (b) mean circumferential velocity u¯φ; (c) mean azimuthal vorticity ω¯φ

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Figure 6

Configuration B with nozzle diameter D=32 mm: Time averaged profiles showing lower half of the midplane bounded by the z-axis as symmetry line at the upper end. (a) mean axial velocity u¯z; (b) mean circumferential velocity u¯φ; (c) mean azimuthal vorticity ω¯φ

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