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TECHNICAL PAPERS: Gas Turbines: Combustion and Fuels

Combustion Oscillation Monitoring Using Flame Ionization in a Turbulent Premixed Combustor

[+] Author and Article Information
B. T. Chorpening

National Energy Technology Laboratory,  US Department of Energy, Morgantown, WV 26507-0880benjamin.chorpening@netl.doe.gov

J. D. Thornton, E. D. Huckaby

National Energy Technology Laboratory,  US Department of Energy, Morgantown, WV 26507-0880

K. J. Benson

 Woodward, Loveland, CO 80538

J. Eng. Gas Turbines Power 129(2), 352-357 (Aug 30, 2006) (6 pages) doi:10.1115/1.2431390 History: Received September 30, 2004; Revised August 30, 2006

To achieve very low NOx emission levels, lean-premixed gas turbine combustors have been commercially implemented that operate near the fuel-lean flame extinction limit. Near the lean limit, however, flashback, lean blow off, and combustion dynamics have appeared as problems during operation. To help address these operational problems, a combustion control and diagnostics sensor (CCADS) for gas turbine combustors is being developed. CCADS uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. Previous development efforts have shown the capability of CCADS to monitor flashback and equivalence ratio. Recent work has focused on detecting and measuring combustion instabilities. A highly instrumented atmospheric combustor has been used to measure the pressure oscillations in the combustor, the OH emission, and the flame ion field at the premix injector outlet and along the walls of the combustor. This instrumentation allows examination of the downstream extent of the combustion field using both the OH emission and the corresponding electron and ion distribution near the walls of the combustor. In most cases, the strongest pressure oscillation dominates the frequency behavior of the OH emission and the flame ion signals. Using this highly instrumented combustor, tests were run over a matrix of equivalence ratios from 0.6 to 0.8, with an inlet reference velocity of 25ms(82fts). The acoustics of the fuel system for the combustor were tuned using an active-passive technique with an adjustable quarter-wave resonator. Although several statistics were investigated for correlation with the dynamic pressure in the combustor, the best correlation was found with the standard deviation of the guard current. The data show a monotonic relationship between the standard deviation of the guard current (the current through the flame at the premix injector outlet) and the standard deviation of the chamber pressure. Therefore, the relationship between the standard deviation of the guard current and the standard deviation of the pressure is the most promising for monitoring the dynamic pressure of the combustor using the flame ionization signal. This addition to the capabilities of CCADS would allow for dynamic pressure monitoring on commercial gas turbines without a pressure transducer.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Functional diagram of the experimental combustor. The flame provides a slightly conductive path between the guard electrode (G), the spark plugs, and walls of the combustor. A voltage is applied to the guard electrode and the spark plugs; the corresponding current is measured.

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Figure 2

Diagram of inlet end of the Sparky combustor (dimensions in inches). The spark plug electrodes are labeled A1–A10 and B1–B10, and have a 25.4mm(1in.) spacing. The OH* detectors are labeled OH1–OH4, and are on a 50.8mm(2in.) spacing. The pressure transducers are labeled Pc (combustor), Pn1 (nozzle 1 pressure), Pn2 (nozzle 2 pressure), and Pf (fuel inlet pressure). The guard electrode is on the end of the premixer center body.

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Figure 3

Sample data from operation at a reference velocity =25m∕s with Φ=0.71. The fuel system has been tuned for smaller pressure oscillations. The electrode signals in the left column are in microamps; the OH emission signals are in arbitrary units (193Hz case).

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Figure 4

Sample data from combustor with reference velocity =25m∕s, and Φ=0.69. The fuel system has been tuned to produce large pressure oscillations. The electrode signals in the left column are in microamps; the OH emission signals are in arbitrary units (164Hz case).

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Figure 5

Potential field at an electrode pair in the combustor without flow. The electrodes have an applied potential of 5V. The figure shows three-dimensional potential surfaces.

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Figure 6

Frequency spectra from real time data in the 193Hz case. The peak magnitudes vary considerably with electrode location, but the dominant frequency is constant.

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Figure 7

Sensor statistics at Φ=0.6, Vref=25m∕s

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Figure 8

Sensor statistics from Φ=0.7, Vref=25m∕s

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Figure 9

Sensor statistics for Φ=0.8, Vref=25m∕s. Primary oscillation frequencies of the points are indicated.

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Figure 10

Correspondence between primary pressure frequency and primary frequency in guard current oscillations

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