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

Sensitivity of the Numerical Prediction of Turbulent Combustion Dynamics in the LIMOUSINE Combustor

[+] Author and Article Information
Mina Shahi

e-mail: m.shahi@utwente.nl

J. C. Roman Casado

Laboratory of Thermal Engineering Enschede,
Faculty of Engineering Technology,
University of Twente,
Enschede 7500 AE, Netherlands

Thomas Sponfeldner

Department of Mechanical Engineering,
Imperial College London,
London SW7 2AZ, UK

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 15, 2013; final manuscript received August 12, 2013; published online October 28, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(2), 021504 (Oct 28, 2013) (12 pages) Paper No: GTP-13-1261; doi: 10.1115/1.4025373 History: Received July 15, 2013; Revised August 12, 2013

The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. For that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ansys-cfx combustion models. To validate the predicted variable fields of the combustor's transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewed.

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References

Figures

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Fig. 2

A schematic representation of the model combustor: (a) computational domain in CFD calculation; (b) an enlarged view around the wedge; (c) cross-sectional area of the combustor (view from the top)

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Fig. 1

(a) Experimental setup; (b) LIMOUSINE burner

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Fig. 3

Mesh-dependency studies of structured grid (st) at different locations: (a) y = 10 cm, (b) y = 20 cm, (c) y = 30 cm and unstructured mesh (unst): (d) y = 10 cm, (e) y = 20 cm, (f) y = 30 cm based on the streamwise velocity

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Fig. 4

Comparison of streamwise velocity for the chosen grids taken from cross section A-A at: (a) y = 10 cm, (b) y = 20 cm, (c) y = 30 cm

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Fig. 14

Pressure and temperature monitoring points in the CFD domain: upstream and downstream of the wedge

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Fig. 11

Details of mesh around the bluff body for structured mesh

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Fig. 12

Details of mesh around the bluff body for unstructured mesh

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Fig. 5

Comparison of streamwise velocity for the chosen grids taken from cross section B-B at: (a) y = 0.5 cm, (b) y = 10 cm, (c) y = 20 cm

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Fig. 6

Comparison of structured (st) (on top) and unstructured mesh (unst) (on bottom) on the mixing behavior of CH4 concentration at: (a) y = 10 cm, (b) y = 20 cm, (c) y = 30 cm

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Fig. 7

Comparison of turbulent eddy viscosity calculated by structured (st) (on top) and unstructured mesh (unst) (on bottom) at (a) y = 10 cm, (b) y = 20 cm, (c) y = 30 cm

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Fig. 8

Streamwise velocity component for 40 kW thermal power and air factor 1.4: experiment (left), structured mesh (middle), unstructured mesh (right)

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Fig. 9

Streamwise velocity component on the cross-sectional area B-B: (a) structured mesh; (b) unstructured mesh

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Fig. 10.

Streamwise velocity component on the cross-sectional area B-B using BVM model power = 60 kW and λ = 1.2

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Fig. 13

Numerical residuals using structured (top) and unstructured mesh (bottom)

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Fig. 15

FFT for 40 kW thermal power and air factor 1.4: experiment, structured mesh, unstructured mesh for different locations: (a) p4, (b) p5, (c) p6

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Fig. 16

Pressure (black line) and velocity (gray line) mode shape at the first fundamental frequency (top) and at the third quarter wave mode (bottom) for the structured grid calculations

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Fig. 17

Pressure fluctuations time history (left column) and FFT (right column) at power = 40 kW and λ = 1.4 measured at a location 200 mm downstream of the wedge from experiment ((a) and (b)), BVM ((c) and (d)), Pdf ((e) and (f)), EDM ((g) and (h)), and EDM-FRC ((i) and (j))

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Fig. 18

Stability map of the LIMOUSINE combustor

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