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

Flow Inhomogeneities in a Realistic Aeronautical Gas-Turbine Combustor: Formation, Evolution, and Indirect Noise

[+] Author and Article Information
Andrea Giusti

Department of Engineering,
University of Cambridge,
Trumpington Street,
Cambridge CB2 1PZ, UK

Luca Magri

Department of Engineering,
University of Cambridge,
Trumpington Street,
Cambridge CB2 1PZ, UK
e-mail: lm547@cam.ac.uk

Marco Zedda

Combustion Aerothermal
Methods Rolls-Royce plc,
P.O. Box 31,
Derby DE24 8BJ, UK

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 March 23, 2018; final manuscript received June 15, 2018; published online October 26, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(1), 011502 (Oct 26, 2018) (11 pages) Paper No: GTP-18-1140; doi: 10.1115/1.4040810 History: Received March 23, 2018; Revised June 15, 2018

Indirect noise generated by the acceleration of combustion inhomogeneities is an important aspect in the design of aero-engines because of its impact on the overall noise emitted by an aircraft and the possible contribution to combustion instabilities. In this study, a realistic rich-quench-lean (RQL) combustor is numerically investigated, with the objective of quantitatively analyzing the formation and evolution of flow inhomogeneities and determining the level of indirect combustion noise in the nozzle guide vane (NGV). Both entropy and compositional noise are calculated in this work. A high-fidelity numerical simulation of the combustion chamber, based on the large-eddy simulation (LES) approach with the conditional moment closure (CMC) combustion model, is performed. The contributions of the different air streams to the formation of flow inhomogeneities are pinned down and separated with seven dedicated passive scalars. LES-CMC results are then used to determine the acoustic sources to feed an NGV aeroacoustic model, which outputs the noise generated by entropy and compositional inhomogeneities. Results show that non-negligible fluctuations of temperature and composition reach the combustor's exit. Combustion inhomogeneities originate both from finite-rate chemistry effects and incomplete mixing. In particular, the role of mixing with dilution and liner air flows on the level of combustion inhomogeneities at the combustor's exit is highlighted. The species that most contribute to indirect noise are identified and the transfer functions of a realistic NGV are computed. The noise level indicates that indirect noise generated by temperature fluctuations is larger than the indirect noise generated by compositional inhomogeneities, although the latter is not negligible and is expected to become louder in supersonic nozzles. It is also shown that relatively small fluctuations of the local flame structure can lead to significant variations of the nozzle transfer function, whose gain increases with the Mach number. This highlights the necessity of an on-line solution of the local flame structure, which is performed in this paper by CMC, for an accurate prediction of the level of compositional noise. This study opens new possibilities for the identification, separation, and calculation of the sources of indirect combustion noise in realistic aeronautical gas turbines.

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Figures

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Fig. 1

Schematic of the combustor. The arrows represent the different streams entering the combustion chamber.

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Fig. 2

Schematic of the computational approach used in this work

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Fig. 3

Nondimensional radial temperature profile at the combustor's exit from experiments (EXP) and LES-CMC computation. The experimental data is provided by Rolls-Royce plc.

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Fig. 4

LES-CMC solution in a stream-wise cross section: (a) mean mixture fraction, (b) rms of the mixture fraction fluctuations, and (c) rms of the temperature fluctuations. The white iso-line indicates the stoichiometric mixture fraction. The locations of the probes used for the analysis of local quantities is shown in(a).

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Fig. 5

Root mean square fields of the fluctuations of the passive scalars Zi(a) associated with the air flows (see Fig. 1)

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Fig. 6

Conditionally filtered temperature (black line, left axis) and conditionally filtered CO mass fraction (blue line, right axis) in the mixture fraction space computed by LES-CMC at (a) probe 2 (primary region) and (b) probe 21 (combustor's exit). Each line corresponds to a different time, which is uniformly sampled over a period of 4 ms.

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Fig. 7

Mean and rms of the fluctuations of temperature and mixture fraction at the combustor's exit (TM is the mean temperature averaged over the cross section and T30 is the temperature at the inlet of the combustor)

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Fig. 8

Instantaneous LES-CMC solution in a stream-wise cross section: (a) mixture fraction, (b) heat release rate in MW/m3, (c) time history of the heat release rate integrated over the combustor's exit plane. In (a), the white line is the stoichiometric mixture fraction. In (b), the red line is the iso-line of HRR = 100 MW/m3.

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Fig. 9

Correlation between the mixture fraction, Z, of probe 18 at the combustor's exit and the mixture fraction of the other probes (see Fig. 3(a) for the numbering). In red, the same quantity is shown for the temperature.

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Fig. 10

Correlation between the mixture fraction, Z (denoted by 0 in the axes), and the passive scalars, Zi(a), of the dilution air streams (see Fig. 3(a) for the numbering) for selected locations at the combustor's exit: (a) probe 19, (b) probe 17, (c) probe 21, and (d) probe 22

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Fig. 11

Correlation between the mixture fraction, Z (denoted by 0 in the axes), and the passive scalars, Zi(a), of the dilution air streams (see Fig. 3(a) for the numbering) at selected locations in the primary combustion region and downstream of the dilution ports: (a) probe 2 and (b) probe 13

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Fig. 12

Strengths of the acoustic sources due to a compositional inhomogeneity at Z¯=0.02 at the combustor's exit, which corresponds to the nozzle inlet. Quantities are cross-sectional averaged.

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Fig. 13

Mixture fraction (a) and temperature (b) fluctuations at the combustor's exit. The temperature fluctuation is normalized by the mean temperature, T¯. (Fig. 3(a) shows where the probes are placed in the schematic.)

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Fig. 14

Species that most contribute to the strengths of the acoustic sources due to a compositional inhomogeneity

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Fig. 15

Nozzle transfer functions to ((a),(b)) compositional inhomogeneity and ((c),(d)) entropy inhomogeneity coming from the combustor and entering the nozzle. πb+ is the forward-propagating pressure wave at the nozzle's outlet. In (a), the lines correspond to Ψ¯+ℵ¯=−4.24, where the min/max values of the bars correspond to Ψ¯+ℵ¯=−3.92/Ψ¯+ℵ¯=−4.56.

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