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

Pulsation-Amplitude-Dependent Flame Dynamics of High-Frequency Thermoacoustic Oscillations in Lean-Premixed Gas Turbine Combustors

[+] Author and Article Information
Frederik M. Berger

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: berger@td.mw.tum.de

Tobias Hummel

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany;
Institute for Advanced Study,
Technische Universität München,
Garching 85748, Germany
e-mail: hummel@td.mw.tum.de

Bruno Schuermans

Institute for Advanced Study,
Technische Universität München,
Garching 85748, Germany;
GE Power, Baden 5401, Switzerland
e-mail: bruno.schuermans@ge.com

Thomas Sattelmayer

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: sattelmayer@td.mw.tum.de

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 3, 2017; final manuscript received August 1, 2017; published online November 7, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(4), 041507 (Nov 07, 2017) (10 pages) Paper No: GTP-17-1257; doi: 10.1115/1.4038036 History: Received July 03, 2017; Revised August 01, 2017

This paper presents the experimental investigation of pulsation-amplitude-dependent flame dynamics associated with transverse thermoacoustic oscillations at screech level frequencies in a generic gas turbine combustor. Specifically, the flame behavior at different levels of pulsation amplitudes is assessed and interpreted. Spatial dynamics of the flame are measured by imaging the OH chemiluminescence (CL) signal synchronously to the dynamic pressure at the combustor's face plate. First, linear thermoacoustic stability states, modal dynamics, and flame-acoustic phase relations are evaluated. It is found that the unstable acoustic modes converge into a predominantly rotating character in the direction of the mean flow swirl. Furthermore, the flame modulation is observed to be in phase with the acoustic pressure at all levels of the oscillation amplitude. Second, distributed flame dynamics are investigated by means of visualizing the mean and oscillating heat release distribution at different pulsation amplitudes. The observed flame dynamics are then compared against numerical evaluations of the respective amplitude-dependent thermoacoustic growth rates, which are computed using analytical models in the fashion of a noncompact flame-describing function. While results show a nonlinear contribution for the individual growth rates, the superposition of flame deformation and displacement balances out to a constant flame driving. This latter observation contradicts the state-of-the-art perception of root-causes for limit-cycle oscillations in thermoacoustic gas turbine systems, for which the heat release saturates with increasing amplitudes. Consequently, the systematic observations and analysis of amplitude-dependent flame modulation shows alternative paths to the explanation of mechanisms that might cause thermoacoustic saturation in high frequency systems.

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Fig. 1

Schematic of longitudinal (a) and transversal (b) acoustic mode with respective flame heat release region

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Fig. 2

Left: cut-plane reconstruction of the measured heat release oscillation from external excitation; middle and right: numerically simulated acoustic pressure and velocity distribution

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Fig. 3

Schematic of the gas turbine burner experiment with camera setup and detail of the instrumentation ports P1-6 at the face plate; contrast level in face plate detail (color map available in online version) indicates instantaneous acoustic T1 mode distribution

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Fig. 4

PDF's of the Fourier coefficients Re(f(t)) and Re(g(t)) for (a) stable and (b) unstable combustor operation, and (c) PDF of the spinratio S(t) for the former operation points, with PDF mean S¯ (solid line) and deviation (dashed line)

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Fig. 5

(a) Samples for the polar decomposed pressure signals for LS and HS operation, (b) PDF of the filtered acoustic pressures, and (c) sample amplitudes bins (color order available in online version) over the pressure envelope amplitude for OH–CL post-processing (LS configuration)

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Fig. 6

Upper diagrams: Air excess ratio over the pulsation amplitude in the fashion of a bifurcation diagram for the LS configuration (a) and HS configuration (b). Lower diagrams: Spinning ratio of the reconstructed first transverse mode, where ccw denotes in swirl direction, and cw against swirl direction.

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Fig. 7

Phase lag Δϕpq of measured acoustic pressure pulsations p′ toward heat release oscillations q′ (obtained from integral OH–CL) over the pulsation amplitude for unstable operation points HS3 and HS4

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Fig. 8

Top: Mean OH–CL distribution of the flame related to the amplitude of the dynamic pressure pulsation A¯p′ given from the dynamic pressure envelope. Bottom: Respective oscillating heat release fields obtained from experimental data (E) and numerical simulations (N).

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Fig. 9

Left: Integral heat release of the amplitude bin averaged flame Q¯A and integral heat release of the flame oscillations Q′A (absolute value of the complex reconstruction). Right: Frequency response for different pressure amplitudes.

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Fig. 10

Mean OH–CL distributions for min 〈I¯〉A¯min and max 〈I¯〉A¯max pressure amplitude level for LS1-4 and difference image of the former 〈I¯〉A¯max−〈I¯〉A¯min

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Fig. 11

Amplitude-dependent driving rates from analytic source term expressions for (left) deformation/density coupling βρ(A), (middle) displacement coupling βΔ(A) and (right) their superposition βρ(A); upper diagrams for LS configuration, lower diagrams for HS configuration

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Fig. 12

Schematic of acoustic pressure p′ and velocity u′ over the combustor radius and indicative flame positions 1–3 that expose different thermoacoustic driving contributions



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