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TECHNICAL PAPERS: Gas Turbines: Combustion and Fuels

# Investigation of Entropy Noise in Aero-Engine Combustors

[+] Author and Article Information
Friedrich Bake

Department of Engine Acoustics, Institute of Propulsion Technology, German Aerospace Center (DLR), Mueller-Breslau-Str. 8, 10623 Berlin, Germanyfriedrich.bake@dlr.de

Ulf Michel, Ingo Roehle

Department of Engine Acoustics, Institute of Propulsion Technology, German Aerospace Center (DLR), Mueller-Breslau-Str. 8, 10623 Berlin, Germany

J. Eng. Gas Turbines Power 129(2), 370-376 (Feb 01, 2006) (7 pages) doi:10.1115/1.2364193 History: Received October 01, 2005; Revised February 01, 2006

## Abstract

Strong evidence is presented that entropy noise is the major source of external noise in aero-engine combustion. Entropy noise is generated in the outlet nozzles of combustors. Low-frequency entropy noise, which was predicted earlier in theory and numerical simulations, was successfully detected in a generic aero-engine combustion chamber. It is shown that entropy noise dominates even in the case of thermo-acoustic resonances. In addition to this, a different noise generating mechanism was discovered that is presumably of even higher relevance to jet engines: There is strong evidence of broad band entropy noise at higher frequencies (1 to $3kHz$ in the reported tests). This unexpected effect can be explained by the interaction of small scale entropy perturbations (hot spots) with the strong pressure gradient in the outlet nozzle. The direct combustion noise of the flame zone seems to be of minor importance for the noise emission to the ambiance. The combustion experiments were supplemented by experiments with electrical heating. Two different methods for generating entropy waves were used, a pulse excitation and a sinusoidal excitation. In addition, high-frequency entropy noise was generated by steady electrical heating.

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Copyright © 2007 by American Society of Mechanical Engineers
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## Figures

Figure 1

Sketch of the entropy wave generator (EWG)

Figure 2

Photo of the entropy wave generator (EWG)

Figure 3

Isometric view of the combustor test rig

Figure 4

Sketch of the combustion chamber setup

Figure 5

Sketch of the probe microphone. The microphone is installed in the cylindrical pressure cell.

Figure 6

Phase averaged time series of trigger (above) and thermocouple (below) signal in the pulse excitation mode

Figure 7

Phase averaged time series of combustion chamber microphone (above) and exhaust duct microphone (below) signal in the pulse excitation mode

Figure 8

Phase averaged time series of entropy-wave-generator microphone signals in the pulse excitation mode at different bulk velocities

Figure 9

Phase averaged time series of EWG microphone signals in the pulse excitation mode for different tube lengths Δx between heating module (EWG) and nozzle

Figure 10

Power spectrum of the thermocouple signal in the combustion chamber in a self-oscillating mode

Figure 11

Cross spectra between thermocouple and combustion chamber microphone (gray) and the exhaust duct microphone (black) at a self-oscillating mode

Figure 12

Phase relation of cross spectra between heating current and microphone signals downstream of nozzle for different tube length between heating module (EWG) and nozzle

Figure 13

Power spectra of combustion chamber and exhaust duct microphones up to 5kHz

Figure 14

Sketch of noise generation by small scale entropy perturbations in the entropy wave generator at constant heating

Figure 15

Power spectrum of EWG microphone downstream of the nozzle with and without constant heating at nozzle Mach number 0.35

Figure 16

Increase of frequency band power (1–4.5kHz) between nonheating and constant heating for changing nozzle Mach numbers

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