This document presents a large eddy simulation (LES) analysis of swirling flows in gas turbine swirl burners, highlighting their advantages in flame stability and fuel-air mixing. It explores the physics of swirl jet dynamics, the numerical methods used for turbulence modeling, and the geometrical configurations of the burner setup. The findings indicate effective capturing of vortex structures and flow characteristics, validating the LES approach against experimental data while demonstrating low emissions during combustion.

1. Large Eddy Simulation Analysis on
Confined Swirling Flows in a Gas Turbine
Swirl Burner
Turbulence Assignment
ROSHAN SAH
USN :- 17AE60R01
M.Tech (1st Year)
Dept.of Aerospace Engg.
Indian Instituteof Technology
Kharagpur (IIT KGP)

2. Introduction:
• Swirl burners have been widely used in gas turbine combustors because of their flame stability and rapid fuel/air
mixing characteristics.
• Large Eddy Simulation (LES) investigation into flow fields in a model gas turbine combustor equipped with a swirl
burner has been carried out
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3. The Physics involved :
• The intensity of swirl is decided by the swirler structure and affected by the combustion
chamber shape.
• A high velocity annular jet is issued into the chamber along the expanded wall and a
central recirculation zone is formed.
• Then a shear layer is created between the high velocity jet and the Center Toroidal
Recirculation Zone (CTRZ).
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4. • The CTRZ promotes themixing of fuel/air and serves as a flame stabilizationmechanism.
• The swirl can trigger large scale unsteady motion known as Processing Vortex Structure . Strong PVC is developed
under isothermal conditions contributed to turbulence instability by triggering the formation of radial axial
eddies.
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5. Numerical Model and Method
• Large Eddy Simulation (LES) modelling was conducted by using the commercial software FLUENT (Version 12, ANSYS
Inc.,Canonsburg, PA, USA) under reactingconditions.
• Large and small scale vortexes are calculatedseparatelyusing a filter function.
• Then large scale turbulence is simulated directly and small scale turbulence is processedby isotropic assumptions.
• The sub-grid scale (SGS)turbulence models in FLUENTemploy the Boussinesq hypothesisas in the RANS models.
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6. Large Eddy Simulation (LES) Model:
Three types of ComputationalFluid Dynamics (CFD) numerical simulation are used to study the turbulentflows.
1. Direct Numerical Simulation (DNS)
2. LES
3. Reynolds AveragedNavier-Stokes (RANS).
Why use LES?
• DNS models turbulence by solving the Navier-Stokesequations directly without any turbulence model . Extensive
computingresources are needed for DNS.
• RANS can only consider time averageflow features, it cannotcapture turbulentcharacteristicsof flows.
• LES is a turbulence resolving method that,in the one hand, can provide more accuratepredictionsthan RANS
models for accuratelycapturingflow field informationand main vortex structureand in the other hand, it requires
less computationalresourcethan DNS.
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7. • LES separatelycalculateslarge and small scale vortex using a filter function.
• Then large scale turbulence is simulated directly and small scale turbulence is processedby isotropic
assumptions
• For effects of small scales turbulence on large scales, a sub-grid stress model was established for the processing.
• LES uses the following filtering function 𝑓(𝑥, 𝑡) and large scale quantities are obtained by weighted integralin
physical space :
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8. Governing Equations :
ρ, ui, p and ν represent density,velocity, pressure andkinematic viscosity. Superscript ‘-’in
the formula represents filtered physicalquantity; represents sub-grid
stress which is modeled by Smagorinskymodel :-
-----------1
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9. In equation 1
• 𝑆𝑖𝑗is the deformationrate tensor of solvable scale
• νt is the eddy viscosity
• Cs is the Smagorinskycoefficient which represent dynamic characteristics ofturbulence.
• Cs value around 0.1 hasbeen found to yield the best results for a wide range of flows, and is the
default value in this research;
• The filter size Δ is related to the cell size. The majorityof cells are smaller than 2 mm, so the
vorticityof this scale can be captured.
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10. Computational Model and Parameters
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11. • The modelled burner geometryis mainly composed ofa swirler and a combustion chamber.
• There are 12 curve vanes on the swirler and the curve angle of each vane is 45°.
• The inner and outer diameters of swirler are d0 = 50 mm and d1 = 80 mm and the length is 40 mm.
• The combustion chamberconsistsofinlet expansion section, main chamber.
• The angle of inlet expansion is 34° and the length is 60 mm.
• The main chamber are cylindricalwith the length of L = 360 mm and the diameter of main chamber is D =
160 mm.
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12. Grid Generation and Grid Independence :
(a) The grid of whole burner; (b) O-style grid of cross section planes; (c) grid
refinement near to the wall.
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13. • LES computationaldomain is discretized using a hexahedral grid and consistsof a totalof four
million cells and 63 blocks.
• The cell number was increased from 2.8 million to four million to investigatethe influence of grid
density on the calculationreliability.
• Axial velocity variationof both grids was everywhere less than 5% and the simulation results were
closer to the experimental datawith the fine grid.
• The majority of cells are smaller than 2 mm and the largestcells are locatedin the middle section of
chamber.
• The grid is refined, especially along the wall, to ensure the suitable of Y+ value(less than 4) and
accuracyof results.
• As , the RANS grid requirements are relatively low and the change of axial velocity profile less than
5% when cell number is more than 2.1 million ,the RANS computationaldomain is finally discretized
using 5 mm cells.
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14. Investigation specifications:
• The gas flow was simulated in the Eulerian framework while the liquid kerosene fuel is
injected in the form of droplets using a discrete phase model.
• The interaction between gas and liquid phases are representedby the coupled Eulerian
and Lagrangian (liquid phase flow) formulation.
• Kerosene is representedby a C12H23 simplified chemicalmechanism and the equilibrium
assumption ofprobabilitydensityfunction (PDF) methodologyis used to reduce the
calculation time greatly.
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15. The minimum diameter of droplets is set to 10 μm, maximum diameter is 100 μm and the mean
diameter is 40 μm which refer to the experiment data of Hadef .The fuel is injected at (0,0,5) position
with a cone angle of 60 degree and the injection velocityis 60 m/s
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16. Model Validation :
Comparison of axial velocity and temperature between experimentaland simulatingresults. (a)Axial
velocity at Z/L = 0.3; (b)Axial velocity at Z/L = 0.5; (c) Temperature as function ofZ/L.
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17. Results and Discussion :
• Flow Characteristics:-
Contourplots ofthe axial velocityon the cutting
plane—Instantaneous flowbyLES (top) and time
averaged flowby RANS
(a) Radial distribution ofthe mean axial velocity(U); (b) Mean radial
velocity(V); (c) Mean tangential velocity(W)at different positionunder
reactivecondition.
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18. (a) LES results for radial distribution of RMS of the axial velocity
fluctuations (U’); (b) RMS of the radial velocity fluctuations (V’); (c) RMS
of the tangential velocity fluctuations (W’) under isothermal and
reactive condition.
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19. Combustion Processes and Temperature Distributions :-
Instantaneous iso-surface of combustion flame of different temperature.
(a) T = 1000 K; (b)T = 1500 K; (c)T = 1800 K.
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20. (a) Radial distributions ofmean temperature and (b)RMS temperature.
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21. Conclusions :
• In this study, the flow and vortex characteristics in a swirl combustor were examined by using
the LES method for which the model was validated with experimental results.
• Simulation results showed that the heat release during combustion accelerated the axial
velocity motion and made the recirculation zone more compact. As the combustion was
taking place under lean burn conditions,NO emissions were less than 10 ppm.
• The detailed vortex structure in isothermal flow fields were well captured by the filter
function of the LES method. Mean and RMS of the velocity fluctuations profiles were
presented for describing the turbulence and the typical flow field characteristics.
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22. References:
1.Lefebvre, A.H. Gas TurbineCombustion,2nd ed.; Taylor and Francis:London, UK, 1999.
2. Valeramedina, A.; Syred,N.; Bowen, P. Central recirculationzone visualization in confined swirl
combustorsforterrestrialenergy. J. Propuls.Power2013,29,195–204.
3. Coghe, A.; Solero,G.; Scribano,G. Recirculationphenomena in a natural gas swirl combustor.Exp.
Therm. FluidSci. 2004,28,709–714.
4. Syred, N. A review of oscillationmechanisms and the role of the processing vortexcore (PVC) in swirl
combustionsystems.Prog.EnergyCombust.Sci. 2006,32,93–161.
5. Valera-Medina,A.; Syred,N.; Griffiths,A. Visualizationof isothermal large coherent structuresin a swirl
burner. Combust.Flame2009,156,1723–1734.
6. https://pdfs.semanticscholar.org/23e0/cca5280a6fe31f1ee707343d33a434b7c256.pdf
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23. Thank you!!
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