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Gas Turbines: Combustion, Fuels, and Emissions

Measurement of Flame Frequency Response Functions Under Exhaust Gas Recirculation Conditions

[+] Author and Article Information
Joseph Ranalli, Don Ferguson

National Energy Technology Laboratory,Morgantown, WV 26507

J. Eng. Gas Turbines Power 134(9), 091502 (Jul 18, 2012) (10 pages) doi:10.1115/1.4006877 History: Received October 31, 2011; Revised May 18, 2012; Published July 17, 2012; Online July 18, 2012

Exhaust gas recirculation has been proposed as a potential strategy for reducing the cost and efficiency penalty associated with postcombustion carbon capture. However, this approach may cause as-yet unresolved effects on the combustion process, including additional potential for the occurrence of thermoacoustic instabilities. Flame dynamics, characterized by the flame transfer function, were measured in traditional swirl stabilized and low-swirl injector combustor configurations, subject to exhaust gas circulation simulated by N2 and CO2 dilution. The flame transfer functions exhibited behavior consistent with a low-pass filter and showed phase dominated by delay. Flame transfer function frequencies were nondimensionalized using Strouhal number to highlight the convective nature of this delay. Dilution was observed to influence the dynamics primarily through its role in changing the size of the flame, indicating that it plays a similar role in determining the dynamics as changes in the equivalence ratio. Notchlike features in the flame transfer function were shown to be related to interference behaviors associated with the convective nature of the flame response. Some similarities between the two stabilization configurations proved limiting and generalization of the physical behaviors will require additional investigation.

Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Sketch of closed-loop coupling process responsible for thermoacoustic instabilities. q′ represents oscillations in the flame heat release rate and u′ represents the acoustic velocity.

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Figure 2

Sample images of flames for each injector type with the two applicable length scales notated. Note the different origin for base-to-tip flame length in the case of the LSI flame.

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Figure 3

Sketch of the rig configured for LSI flame FRF measurements. Approximate locations of diagnostics shown.

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Figure 4

(a) and (b) CAD drawing of the high and low-swirl injectors used in this study. The arrow on the LSI indicates the flow direction.

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Figure 5

(a) Time-averaged image of HSI flame at Q = 100 l/min, Φ = 0.80; (b) Abel inversion of previous; (c) time-averaged image of LSI flame at Q = 100 l/min, Φ = 0.80; and (d) Abel inversion of previous

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Figure 6

Flame lengths for HSI and LSI flames measured at varying dilution and equivalence ratio conditions. All data shown is for a total flow rate of 100 l/min.

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Figure 7

Comparison of normalized flame lengths for both injector types. Flow rate of 100 l/min. Legend is the same as in Fig. 6.

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Figure 8

Flame length and center of mass are proportional. Points show data for all test conditions for the HSI.

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Figure 9

Flame length and center of mass are proportional for short flames. Points show data for all test conditions for the LSI.

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Figure 10

Flame transfer functions for (a) HSI and (b) LSI with respect to variation in equivalence ratio. Mean flow rate for all cases shown is 100 l/min, with pure methane as fuel. All Strouhal numbers use flame length and average velocity in the nozzle for scaling.

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Figure 11

Phase as a function of Strouhal number based on flame center of mass position. Both LSI and HSI are shown. All cases are for undiluted methane at flow of 100 l/min.

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Figure 12

Flame transfer functions subject to varying flow rates for (a) HSI and (b) LSI. Fuel for all cases was pure methane with no dilution. All Strouhal numbers use flame length and average velocity in the nozzle for scaling.

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Figure 13

Flame transfer functions subject to varying levels of dilution for (a) HSI and (b) LSI. Mean flow rate for all cases shown is 100 l/min. Dilution levels are reported as a percentage of the total volumetric flow. All Strouhal numbers use flame length and average velocity in the nozzle for scaling.

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Figure 14

dc gain versus flame length computed for varying dilution and equivalence ratio conditions. All data shown is for a total flow rate of 100 l/min. Notably, gain and length can be changed independently with introduction of dilution.

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Figure 15

Steady images averaged over the entire cycle, showing the axial 50–50 split location for the intensity: (a) HSI and (b) LSI

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Figure 16

Amplitude of oscillation of the flame split axially into upstream (US) and downstream (DS) halves. Left: HSI 100 l/min, Phi = 0.80, no diluent at frequencies 110 and 160 Hz. Right: LSI 100 l/min, Phi = 0.90, no diluent at frequencies 90 and 140 Hz. Frequencies correspond to the first notch and the first rebound peak after the notch for their respective transfer functions.

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Figure 17

Line-of-sight image based phase maps for left: HSI, and right: LSI, at the same pairs of frequencies as Fig. 1. Scale ranges from π to –π in rad. Note that sharp edges may exhibit simple wrapping of the phase from –π to π, rather than actual rapid changes in the phase.

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