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

An Acoustic Time-of-Flight Approach for Unsteady Temperature Measurements: Characterization of Entropy Waves in a Model Gas Turbine Combustor

[+] Author and Article Information
Dominik Wassmer

Chair of Fluid Dynamics,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: dominik.wassmer@tu-berlin.de

Bruno Schuermans

GE Power,
Brown-Boveri-Str. 7,
Baden 5401, Switzerland
e-mail: bruno.schuermans@ge.com

Christian Oliver Paschereit

Chair of Fluid Dynamics,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: oliver.paschereit@tu-berlin.de

Jonas P. Moeck

Combustion Dynamics,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: jonas.moeck@tu-berlin.de

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 29, 2016; final manuscript received July 28, 2016; published online October 18, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(4), 041501 (Oct 18, 2016) (8 pages) Paper No: GTP-16-1289; doi: 10.1115/1.4034542 History: Received June 29, 2016; Revised July 28, 2016

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame–acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so-called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the measurement of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are measured for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adhere well to a Strouhal number scaling for different mean flow velocities.

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Fig. 1

Principle setup of the TOF method (here: combustion rig setup)

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Fig. 2

Ceramic adapter and tips of the electrodes as seen from inside the exhaust tube

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Fig. 3

High-pass filtered microphone signals showing the arrival time of the acoustic signal of microphone 3 and microphone 4

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Fig. 4

Schematic of the experimental setup for validation

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Fig. 5

Schematic of the combustion chamber setup

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Fig. 6

Comparison of the steady temperature measurements with thermocouples and the TOF method (top); corresponding relative deviations (bottom)

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Fig. 7

Hot-wire temperature measurements of one valve injection cycle at 20 Hz at different radial locations; corresponding mean value of thermocouple measurements at different radial locations (horizontal lines)

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Fig. 8

Temperature fluctuation over a 20 Hz period of the injection valve; measured via hot-wire (solid line) and TOF method (dashed line)

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Fig. 9

Equivalence ratio measured in the mixing tube for different valve frequencies and main air mass flows

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Fig. 10

Absolute value (top) and phase (bottom) of the transfer function between the valve signal and the equivalence ratio in the mixing tube

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Fig. 11

Phase-averaged TOF measurements over one period of fuel modulation for three different valve frequencies

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Fig. 12

Absolute value (top) and phase (bottom) of the transfer function between the equivalence ratio fluctuation in the mixing tube and the temperature fluctuations at the measurement plane

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Fig. 13

Absolute value (top) and phase (bottom) of the transfer function between the equivalence ratio fluctuation in the mixing tube and the temperature fluctuation at the measurement plane plotted against the Strouhal number




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