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

Flame Structure and Stabilization Mechanisms in a Stagnation-Point Reverse-Flow Combustor

[+] Author and Article Information
Mohan K. Bobba, Priya Gopalakrishnan, Karthik Periagaram, Jerry M. Seitzman

School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0150

J. Eng. Gas Turbines Power 130(3), 031505 (Apr 02, 2008) (8 pages) doi:10.1115/1.2836614 History: Received June 06, 2007; Revised July 03, 2007; Published April 02, 2008

A novel combustor design, referred to as a stagnation-point reverse-flow (SPRF) combustor, was recently developed to overcome the stability issues encountered with most lean premixed combustion systems. The SPRF combustor is able to operate stably at very lean fuel-air mixtures with low NOx emissions. The reverse flow configuration causes the flow to stagnate and hot products to reverse and leave the combustor. The highly turbulent stagnation zone and internal recirculation of hot product gases facilitates robust flame stabilization in the SPRF combustor at very lean conditions over a range of loadings. Various optical diagnostic techniques are employed to investigate the flame characteristics of a SPRF combustor operating with premixed natural gas and air at atmospheric pressure. These include simultaneous planar laser-induced fluorescence imaging of OH radicals and chemiluminescence imaging, and spontaneous Raman scattering. The results indicate that the combustor has two stabilization regions, with the primary region downstream of the injector where there are low average velocities and high turbulence levels where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product recirculation levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on the chemical time scales and the flame structure is quantified using simple reactor models. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zone regime throughout the combustor. The flame tends to become more flameletlike, however, for increasing distance from the injector.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of (a) SPRF combustor and (b) layout of the OH PLIF and PIV setups

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Figure 2

Optical layout of SRS setup

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Figure 3

Typical single shot Raman spectra recorded close to injector in the SPRF combustor at ϕ=0.58

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Figure 4

Variation of NOx and CO emissions with adiabatic flame temperature for a total loading of 8.1g∕s in comparison to a laminar flame model for NOx

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Figure 5

Velocity fields for a flow rate of 8.1g∕s: (a) mean axial velocity contours and (b) mean centerline velocity and turbulence intensity profiles

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Figure 6

Instantaneous OH images of the near injector region of the noninsulated SPRF combustor for ϕ=1 with increasing mass flow rates; (a) 0.14, (b) 0.2, (c) 0.43, (d) 1.8, and (e) 5.7g∕s

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Figure 7

(a) Instantaneous OH and (b) simultaneous chemiluminescence images at ϕ=0.58 and a loading of 8.1g∕s in the premixed SPRF combustor acquired in different widows

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Figure 8

Axial variation of mean species mole fractions: (a) CO2 and CH4, (b) O2 and H2O, and (c) temperature along the combustor centerline. Conditionally averaged values of these quantities over data points only in reactants are also plotted.

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Figure 9

Histogram of product fraction (fP) over 500 data points measured at various axial locations along the combustor centerline

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Figure 10

Variation of ignition delays in milliseconds with increasing product entrainment levels (fP) over a range of equivalence ratios. Flow times to reach various axial locations estimated from the mean velocity field are also shown for reference.

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Figure 11

Estimate of turbulent combustion regimes for the SPRF combustor: points shown for same axial locations, product fractions, and flow conditions in Tables  12

Tables

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