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

Prediction of Blow-Offs of Bluff Body Stabilized Flames Utilizing Close-Coupled Injection of Liquid Fuels

[+] Author and Article Information
Ben. T. Zinn

Georgia Institute of Technology,
Atlanta, GA

This assumption was proven to be valid by the fact that variation of the local equivalence ratio at any given point in the flame did not exceed the global average by more than ±5% over entire RZ on the C2 */CH* images used for calibration.

The equivalence ratio pseudocolor palette, attached to Fig. 7, is universally shared by all C2 */CH* pseudocolor images in the report. The color palette associated with heat release is also universal. Both palettes were implemented such that all images share the same minimum and maximum values to allow for qualitative visual comparison. No alteration was made to the data values used for numerical analysis.

Not all the obtained data are presented in Fig. 11; removals were made only to clarify the plot visually by making the φRZ versus φglobal map less busy, while preserving all distinct observed trends.

This fact provides the benchmark and proof of accuracy for the imaging and processing methodology applied to the data obtained with CCI.

The exponential form of the transfer function was arbitrarily chosen due to the experimentally observed general decay trend between increasing momentum flux ratio, J, and decreasing φRZ.

It is worth noting that the DZP(T) stability envelope was translated without modification from φglobal to φRZ due to their equality under the premixed conditions at which the classical data were obtained.

Contributed by International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 9, 2012; final manuscript received July 30, 2012; published online November 30, 2012. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 135(1), 011504 (Nov 30, 2012) (8 pages) Paper No: GTP-12-1267; doi: 10.1115/1.4007371 History: Received July 09, 2012; Revised July 30, 2012

This paper describes the development of an empirical approach that attempts to predict blow-out of bluff body stabilized flames using global flow parameters in systems where liquid fuel injectors are located a short distance upstream of the wake. This approach was created on the hypothesis that flame stability in such a combustion system (referred to as a close-coupled injection) is determined by the strength of the heat source developed in the bluff body recirculation zone and by the availability of sufficient contact time with fresh mixture for its ignition, similar in nature to premixed combustion systems. Based on this concept, global equivalence ratio on the classical DeZubay stability map was replaced by local equivalence ratio in the recirculation zone of the bluff body. This local equivalence ratio was determined experimentally using a chemiluminescence measurement system. Tests were conducted using a single bluff body with a close-coupled injection system in a 76 × 152 mm (3 × 6 in.) combustion tunnel. A wide range of fuel–air ratios and velocities were achieved by variation of the global equivalence ratio, incoming flow velocity, and injector size. The obtained experimental dataset was used to develop a transfer function that allowed calculation of the local equivalence ratio in the recirculation zone based on the global flow parameters. Equivalence ratio in the recirculation zone was found to be exponentially dependent upon the square root of the fuel to air momentum flux ratio such that increasing the momentum flux ratio led to a reduction in the recirculation zone equivalence ratio. Additional adjustment of this general trend by the diameter of injector and air flow velocity was necessary to improve the quality of the prediction. The developed approach demonstrated a good prediction of the globally rich blow-out of the flame. In fact, the recirculation zone lean blow-out limit (corresponding with globally rich blow-out) predicted for close coupled injection using the developed transfer function closely coincided with the lean blow-out line of the classical DeZubay envelope and with results obtained with premixed injection using the same bluff body. On the contrary, globally lean (locally rich) blow-out was predicted ∼20% below the DeZubay rich blow-out line, possibly because of the limited range of the fuel flow rates on the experimental rig used.

Copyright © 2013 by ASME
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Lovett, J., Cross, C., Lubarsky, E., and Zinn, B. T., 2011, “Mechanisms Controlling Bluff Body-Stabilized Flames With Closely Coupled Fuel Injection,” Proceedings of ASME Turbo Expo 2011, Vancouver, Canada, June 6–10, ASME Paper No. GT2011-46676, pp. 1275–1287. [CrossRef]
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Fig. 1

Diagrams of fuel injection modes for BB stabilized flames: (a) premixed injection [1], (b) CCI [2]

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Fig. 2

Stability maps for BB stabilized flames. Lines designate: (1) original DeZubay (DZP) stability envelope from Ref. [3]; (2) DZP stability envelope corrected for increased flow temperature marked from Ref. [4]; (3) close-coupled injection (CCI) experimental data from [5]; (4) CCI data with increased diameter of injectors [5].

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Fig. 3

Bluff body flame holder

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Fig. 4

Diagram of injection modes available in the rig

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Fig. 5

Schematic of the chemiluminescence imaging system

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Fig. 6

Jet-A + air flame emission spectra. Bands collected by cameras labeled.

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Fig. 7

Typical C2 */CH* image with color palette and RZ volume of integration marked with dotted line

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Fig. 8

Heat release (CH*) and equivalence ratio (C2*/CH*) distributions with varying φglobal for the most stable configuration (six injectors ×0.61 mm in diameter)

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Fig. 9

Heat release (CH*) and equivalence ratio (C2 */CH*) distributions with varying φglobal for the most unstable configuration (six injectors ×0.48 mm in diameter)

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Fig. 10

Comparison of the heat release (CH*) and equivalence ratio (C2 */CH*) distributions obtained with different injectors (i.d. = 0.48 mm, i.d. = 0.61 mm, i.d. = 0.65 mm, and i.d. = 0.74 mm) at nearly constant global φ

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Fig. 11

Data summary on the map φRZ versus φglobal. Injector diameters: (1) upstream injection 16 × 0.4572 mm, (2) CCI 6 × 0.48 mm V = 246 m/s, (3) CCI 6 × 0.61 mm V = 219 m/s, (4) CCI 6 × 0.65 mm V = 219 m/s, (5) CCI 6 × 0.74 mm V = 219 m/s.

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Fig. 12

Dependence of the local equivalence ratio in the RZ, φRZ, upon the entrainment parameter EP

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Fig. 13

DeZubay stability map with the superimposed local equivalence ratios in the recirculation zone DZP(T) calculated from the flow parameters using φRZ = f(EP) transfer function



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