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TECHNICAL PAPERS: Gas Turbines: Combustion and Fuels

# Experimental Study on the Role of Entropy Waves in Low-Frequency Oscillations in a RQL Combustor

[+] Author and Article Information
J. Eckstein, E. Freitag, C. Hirsch, T. Sattelmayer

Lehrstuhl für Thermodynamik,  Technische Universität München, 85747 Garching, Germany

J. Eng. Gas Turbines Power 128(2), 264-270 (Mar 01, 2004) (7 pages) doi:10.1115/1.2132379 History: Received October 01, 2003; Revised March 01, 2004

## Abstract

“Rumble” is a self-excited combustion instability, usually occurring at the start-up of aero-engines with fuel-spray atomizers at sub-idle and idle conditions, and exhibiting low limit frequencies in the range of $50Hzto150Hz$. Entropy waves at the (nearly) choked combustor outlet are supposed to be the key feedback mechanism for the observed self-excited pressure oscillations. The experimental study presented here aims at clarifying the role of entropy waves in the occurrence of rumble. A generic air-blast atomizer with a design prone to self-excitation has been incorporated into a thermoacoustic combustor test rig with variable outlet conditions. The thermoacoustic response of the flame was characterized by recording the $OH*$ chemiluminescence, the dynamic pressures, the dynamic temperatures, and by applying PIV. The measurements have shown the occurrence of periodic hot spots traveling with the mean flow with considerable dispersion. Measurements have been conducted with an open-ended resonance tube in order to eliminate the impact of entropy waves on the mechanism of self-excitation. The oscillation obtained, comparable in amplitude and frequency, proved that self-excitation primarily depends on convective time delays of the droplets in the primary zone and thus on the atomization characteristics of the nozzle.

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## Figures

Figure 1

Schematic representation of the generic combustor with air distribution and the measurement positions

Figure 2

High speed camera recording of the flame OH* chemiluminescence during one combustion oscillation cycle at 126Hz

Figure 3

Time traces of the relative combustor pressure p′∕p (grey) and the normalized OH* chemiluminescence (black) obtained with the Venturi nozzle

Figure 4

Phase-resolved, averaged droplet velocities at the injector outlet measured by PIV for one oscillation cycle at 126Hz. The velocity magnitude refers to the absolute value of the two component velocity vector u2+v2 in the center plane of the combustor.

Figure 5

Time traces of the relative combustor pressure p′∕p (grey) and the gas temperature near the nozzle inlet (Position T4 in Fig. 1, black)

Figure 6

Fourier spectra of the relative pressure amplitudes obtained for the operation with Venturi nozzle (black) and resonance tube (grey)

Figure 7

Time traces of the relative combustor pressure p′∕p (grey) and the normalized OH* chemiluminescence (black) obtained with the resonance tube

Figure 8

Schematic representation of the feedback cycle for entropy waves with the phase relations of the quantities involved

Figure 9

Side bands of the relative pressure amplitude for the limit cycle of the self-excited oscillation with Venturi. The graph is an enlargement of Fig. 6.

Figure 10

Limit cycle properties of the combustion oscillations with Venturi. Limit cycle frequencies over the injector pressure loss for variable equivalence ratios.

Figure 11

Normalized pressure amplitude Δp′∕p of the self-excited oscillation over the relative static pressure drop Δp∕p across the injector

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