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Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

Application of Fiber-Optical Microphone for Thermo-Acoustic Measurements

[+] Author and Article Information
Holger J. Konle1

 Technische Universität Berlin, Müller-Breslau-Strasse 8, 10623 Berlin, Germanyholger.konle@pi.tu-berlin.de

Christian O. Paschereit

 Technische Universität Berlin, Müller-Breslau-Strasse 8, 10623 Berlin, Germanyoliver.paschereit@tu-berlin.de

Ingo Röhle

 German Aerospace Center, Bunsenstrasse 10, 37073 Göttingen, Germanyingo.roehle@dlr.de

1

Corresponding author.

J. Eng. Gas Turbines Power 133(1), 011602 (Sep 14, 2010) (8 pages) doi:10.1115/1.4001983 History: Received April 08, 2010; Revised April 08, 2010; Published September 14, 2010; Online September 14, 2010

A high temperature resistant fiber-optical microphone (FOM) was developed and successfully applied in a combustion chamber at a thermal power of 8.4 kW to measure thermo-acoustic oscillations at a frequency of 85 Hz and a sound pressure level of 154 dB. The sensor head temperature was estimated to 1000K. The core of the optical setup used for the FOM is a Fabry–Perot interferometer. To create an acoustical sensor based on this type of interferometer, a new method of generation and postprocessing of the interference signal was developed. The simple replaceability of the sensor membrane reduces the requirements concerning the sensor handling compared with conventional condenser microphones and allows the adaptation of the sensor sensitivity to its application case changing the membrane stiffness.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Principle setup of an extrinsic, fiber based FOM

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Figure 2

Interfering reflections of a FP interferometer created between fiber end and reflector

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Figure 3

A sinusoidal reflector movement measured with the FP (black solid line) and a conventional Mach–Zehnder (red dashed line) interferometer

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Figure 4

Optical setup of the FOM based on a FP interferometer

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Figure 5

Single-frequency-excitation: acquired signals of the advanced FP setup

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Figure 6

Signal processing for the advanced FP setup

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Figure 7

FOM sensor head—photograph

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Figure 8

FOM sensor head—technical drawing

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Figure 9

Spectra of FOM and condenser microphone for sweep excitation between 200 Hz and 2000 Hz

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Figure 10

Coherence between the sensors for sweep excitation between 200 Hz and 2000 Hz

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Figure 11

Transfer function for sweep excitation: amplitude

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Figure 12

Transfer function for sweep excitation: phase

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Figure 13

Spectra of FOM and condenser microphone for low-frequency multitone excitation

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Figure 14

DLR in-house design probe microphone

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Figure 15

Combustion chamber: design drawing of flame tube (L) and photograph (R)

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Figure 16

Schematic top view of the combustion chamber

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Figure 17

Spectra FOM and probe microphone applied in a combustion chamber

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Figure 18

Pockels-cell switching frequency high (10 kHz)

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Figure 19

Pockels-cell switching frequency low (3 kHz)

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Figure 20

Polar diagram: variation of the Pockels-cell switching frequency from 3 Hz to 10 kHz

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Figure 21

Required Pockels-cell switching frequency—comparison experiment and theory

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