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

On the Adequacy of Chemiluminescence as a Measure for Heat Release in Turbulent Flames With Mixture Gradients

[+] Author and Article Information
Martin Lauer

Lehrstuhl für Thermodynamik, Technische Universität München, Boltzmannstraße 15, 85748 Garching, Germanylauer@td.mw.tum.de

Thomas Sattelmayer

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

J. Eng. Gas Turbines Power 132(6), 061502 (Mar 18, 2010) (8 pages) doi:10.1115/1.4000126 History: Received April 09, 2009; Revised April 14, 2009; Published March 18, 2010; Online March 18, 2010

The determination of the heat release in technical flames is commonly done via bandpass filtered chemiluminescence measurements in the wavelength range of OH or CH radicals, which are supposed to be a measure for the heat release rate. However, these indirect heat release measurements are problematic because the measured intensities are the superposition of the desired radical emissions and contributions from the broadband emissions of CO2. Furthermore, the chemiluminescence intensities are strongly affected by the local air excess ratio of the flame and the turbulence intensity in the reaction zone. To investigate the influence of these effects on the applicability of chemiluminescence as a measure for the heat release rate in turbulent flames with mixture gradients, a reference method is used, which is based on the first law of thermodynamics. It is shown that although the integral heat release can be correlated with the integral chemiluminescence intensities, the heat release distribution is not properly represented by any signal from OH or CH. No reliable information about the spatially resolved heat release can be obtained from chemiluminescence measurements in flames with mixture gradients.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Chemiluminescence spectrum of a stoichiometric premixed methane-air flame. The narrowband radical emissions from OH∗, CH∗, and C2∗ are superimposed by the broadband emission from CO2∗.

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Figure 5

Integral heat release of the flame determined with the reference technique

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Figure 6

Time averaged reaction progress of combustion for burner air excess ratios of 0.9, 1.2, and 1.6. With increasing burner air excess ratio an increasing portion of the mixture in the shear layer between the swirled flow and the ambient air is not burnt.

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Figure 7

Axial heat release distribution determined with the reference technique

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Figure 8

Chemiluminescence spectrum with the fifth-degree polynomial fit for the CO2∗ emission

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Figure 9

Comparison of the integral chemiluminescence intensities and the heat release of the flame. All variables are normalized to their maximum value.

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Figure 3

Sketch of the test rig and exemplary axial air excess ratio distribution due to ambient air entrainment

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Figure 4

Heat release rate in the flame midplane of the λ=1.2 operation point

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Figure 10

Comparison of the axial chemiluminescence distribution of bandpass filtered and CO2∗ contributions corrected OH∗ and CH∗ with the axial heat release distribution

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Figure 11

Comparison of reaction progress of combustion (left hand side) and turbulence intensity (right hand side). The contours indicate the time averaged reaction zone with a reaction progress of 0.1, 0.5, and 0.9.

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Figure 12

Comparison of the location of maximum emission

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Figure 2

The simplified methane oxidation mechanism (16). The main portion of the carbon follows the marked reaction path. Only a small portion follows the side paths, which yield OH∗, CH∗, and C2∗. From this, Najm (16) concluded that OH∗, CH∗, and C2∗ are not reliable markers for reaction rate or heat release rate in the flame.

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