LES investigation and sensitivity analysis of the flow ... · PDF fileLES investigation and sensitivity analysis of the flow dynamics in a gas ... Siemens SGT800 Background Gas Turbines

LES investigation and sensitivity LES investigation and sensitivity analysis of the flow dynamics in a gas analysis of the flow dynamics in a gas

turbine swirl combustorturbine swirl combustor

P. Iudiciani

Division of Fluid Dynamics, Lund Institute of technology, LTH, Lund, Sweden

09 February 2011

09 February 2011 Piero Iudiciani FM Seminar Series

• Background and goals• Presentation of the TARS burner• Methods: LES, PIV• Results

– Grid and SGS model sensitivity– Instantaneous LES field– Average LES fields– Comparison with experiments– POD results

• Conclusions• Ongoing/future work: reacting cases

OutlineOutline


Regulations on emissions

– Sustain flame stabilization– Achieve good mixing– Avoid flashback, blow-off

Siemens SGT800

BackgroundBackgroundGas Turbines and aero-engines will still be a vital

technology in the following years

Issues

www.airbus.com

Operation at leanconditions


Background and GoalsBackground and GoalsCommon way to hold flames is by swirl stabilization:Recirculation by aerodynamical means:

- Burnt gases towards fresh mixture- Compact flame, typical tulip shape- No hot parts (no bluff bodies)- Favors good mixing

Main goals of this work:

• Contribute to the understanding of dynamics of swirling flow

• Determine some effects of model/geometrical uncertainties


D=2 inches

Inner Intermediate

Outer

- Close to real aero-engine design- 3 concentric interchangeable swirlers- Vane angles modifiable- Flexible (gaseous/liquid fuels)

Triple Annular Research Swirler (TARS)Triple Annular Research Swirler (TARS)The Triple Annular Research Swirler (TARS)

Sketch of the TARS rig

TARS longitundinal cross section

Flow


Methods: LES and PIVMethods: LES and PIV

Large Eddy Simulations (LES):• Computational domain-includes TARS internal parts• High spatial resolution (grid sensitivity)• Second order accurate in time & space• Different Sub-Grid-Scale (SGS) models (sensitivity)

Particle Image Velocimetry (PIV):• Standard PIV system (data rate ca 7 Hz)• TiO2 seeding• 500 samples for mean and rms• multiple burner orientation (flow asymmetry)


⎟⎟⎠

⎞⎜⎜⎝

⎛

∂

∂+

∂∂

=i

j

j

iij x

uxuS

21

( ) ( )i

ijijj

jij

i

xpS

xuu

xtu

∂∂

−−∂∂

=∂∂

+∂∂

ρντ 12

LES LES -- Governing equationsGoverning equations

Unknown, to be modeled:

Eddy viscosity ( ) type model

Spaced averaged continuity and momentum equations:

Momentum

Continuity

Subgrid scale stresses (SGS)

Strain ratetensor

0=∂∂

i

i

xu

jijiij uuuu −=τ

ijijij Sk Δ−= νδτ 232

How to model ?Δν

Δν

Δν


LES LES -- Governing equationsGoverning equationsFour SGS models evaluated:

- Smagorinsky- proportional to the Strain rate- Constant to be assigned

- Filtered Smagorinsky- High pass filter to the velocity

- Large scales do not contribute to the turbulent viscosity- Constant to be assigned

- Dynamic Smagorinsky- Subtest filter ^ and subtest stresses

- Constant in (1) dynamically computed through Germano’s identity:

- Implicit LES (ILES)- No explicit SGS model is used

||2ijSC Δ= ΔΔν

|~|23 ijSC Δ=Δν

)(~i

ni uHPu =

ijijjijiij TuuuuL τ̂ˆˆ −=−=∧

∧

−= jijiij uuuuT^


PIV measurementsNumber of samples = 500Acquisition Rate ~7Hz

Operating conditionsAir flow= 7.0 g/sAngles: 50°clockwise

PIV measurementsPIV measurements

PIV camera

Conditioning pipe(5 fine mesh screens)

TARS

Dump plate

x/D

y/D

laser sheetfor PIV (532 nm)


Inner axial swirler

TARS TARS -- Geometrical detailsGeometrical details


Intermediate axial swirler



Outer radial swirler



TOP BOTTOM


Streamlines

• Complex geometry• Computations include the swirlers• To ensure the quality of the grid: mostly cuboids cells


Computational Domain Computational Domain

• Includes TARS and pipe upstream (avoid BC issues).• Low conical co-flow allowing entrainment (unconfined).• Refined grid inside and downstream the TARS

- 5.6 millions of cells(5.1 millions cuboids)

- Finest resolution: D/162- y+~1-2


Boundary ConditionsBoundary Conditions

InletU: top-hat axial profile, 0-crossp: zero-gradient

Pipe, TARS walls, plateU: no-slipp: zero-gradient

Entrainment coneU: uniform, low co-flowp: zero-gradient

OutletU: zero-gradientp: uniform = ref pressure

Given mass flow distribution among the three swirlers


Grid SensitivityGrid Sensitivity

• Small differences in the mean values• Larger variations in the RMS, mostly downstream

Coarser grid: good compromise between resolution and comp. time

• Two grids are considered • SGS: ILES (more sensitive to resolution)

Coarse FineNo. CellsResolution

5.6 millions 8.1 millionsD/162 D/188

Mean RMSAxial Velocity Radial Velocity Axial Velocity Radial Velocity


- Less dissipative, finer structures- Structures in the intermediate and the outer passages

SGS model Sensitivity: SGS model Sensitivity: Fluctuating VelocityFluctuating Velocity

Magnitude of the instantaneous fluctuating velocity . Longitudinal cross section><− ii uu

- Larger structures- Turbulence develops only inside the inner swirler


SGS model Sensitivity: SGS model Sensitivity: Mean velocityMean velocity

- Deviations over azimuthal angle

- Less azimuthalvariations- More symmetric- Look similar to each other

• Filtered and Dynamic Smagorinsky less dissipative

Smagorinsky ILES

Filtered Smagorinsky Dynamic Smagorinsky

Φ: Azimuthal angle


SGS sensitivity: SGS sensitivity: Turbulence SpectraTurbulence Spectra

- All the models capture a decade of the TKE spectra- ILES least dissipative


Instantaneous LES flow field Instantaneous LES flow field

Axial velocity. Longitudinal and perpendicular cross section

Tangential velocity. Longitudinal cross section

• Stretched, separated pockets of high tangential velocity

• Recirculation zone is captured well

• Three swirling jets and relative shear layers


AnimationsAnimations

Axial velocity. Longitudinal cross section Axial velocity. Perpendicular cross section

• The flow inaccessible by experiments is characterized• Reverse flow also inside the TARS • High turbulence, wide range of scales • Dynamics of the three swirling jets and relative shear layers:

Jets merging at about the outlet of TARS


Instantaneous PIV dataInstantaneous PIV dataAxial velocity contour Out of plane vorticity

Largely in agreement with the PIV data (downstream the TARS):

• CRZ surrounded by a swirling jet

• Shear layers with the CRZ and air at rest

• Flow asymmetry in the experiments!


Averaged LES flow fieldAveraged LES flow field

• Reverse flow at the tips of the air passages

Average axial velocity. Longitudinal and perpendicular cross sections

• Flow non uniform in azimuthal direct. downstream the TARS

Helps the merging of the three swirling jets

• Recirculation extends in the inner swirler


Averaged LESAveraged LES flow fieldflow field

Iso-surface of mean axial velocity.U=-1m/s (blue); U=4.5 m/s (red)

Central recirculation zone

Traces of the passage in the (8) vanes of the swirler:Staggered pattern every 45°

Reverse flow at the entrance of the outer radial swirler

Toroidal recirculation zone at the tip of the inner swirler


Comparison with experimentsComparison with experiments

• Qualitative agreement: showing the vortex breakdown bubble surrounded by a swirling jet

• Experiments with strong asymmetry:– Improper alignment– Non uniform flow upstream the TARS– Exhaust downstream

• Comparison not straightforward• New experiments with rotated burner

LESPIV


Smagorinsky ILES

Filtered Smagorinsky Dynamic Smagorinsky

Comparison with experimentsComparison with experiments

• Reasonable agreement with experiments (peak amplitude and position, gradient in the shear layer)

• SGS model (and grid) differences in the range of the experimental asymmetry


Proper Orthogonal DecompositionProper Orthogonal Decomposition (POD)(POD)

Goal: Highlight large scale coherent structures dynamics:

• Sirovich’s method of snapshots is applied and the modes are linear combinations of the instantaneous realizations:

( )

0

1( ) ( )N

n n iin

ix c u x

Nλ =

Φ = ∑

• Find a set of orthogonal functions (modes) for a modal decomposition of an ensemble of data.

)(xjΦ

Average

Fluctuations

)()()()()(),( )()()0(0 xtaxtaxatxq j

Njkj

jN

jkjk Ψ+Ψ+Ψ= ∑∑

∞

=

coherent incoherent

• Modes are ordered so to maximize TKE content


POD results:POD results: Longitudinal cross section Longitudinal cross section MODE 0 MODE 1

Axial velocity. Arbitrary scale

)()()(),( )()0(0 xtaxatxq j

N

jkjk Ψ+Ψ= ∑

Mode 0: Average field.


POD resultsPOD results: Longitudinal cross section : Longitudinal cross section

Mode 1: Although a bit noisy, seems to capture the shear layers between the CRZ and the swirling jet

MODE 0 MODE 1


)()()(),( )()0(0 xtaxatxq j

N

jkjk Ψ+Ψ= ∑

Mode 0: Average field.


POD results:POD results: Longitudinal cross section Longitudinal cross section MODE 2 MODE 3


Modes 2-3: Pair of modes, Helical structures

)()()(),( )()0(0 xtaxatxq j

N

jkjk Ψ+Ψ= ∑


POD resultsPOD results)()()(),( )()0(

0 xtaxatxq jN

jkjk Ψ+Ψ= ∑

Fourier Analysis on the sequences of the time coefficients givesinformation about the frequency of the modes:

• Peaks at about 160 Hz


POD resultsPOD results


• Mode 0: Averaged field. • Staggered pattern at 45 influence of the 8 vanes. • CRZ centered in average

Inner

Intermediate

Outer




Mode 0: Averaged field. Staggered pattern at 450 , 8 vanes

• Modes 1-2: Pair of modes, Precessing Vortex Core (PVC).• The core of the CRZ-instantaneously off-axis and precessing. • The PVC originates in the inner swirler

Inner

Intermediate

Outer



• Fourier Analysis of the time coefficients confirms the peaks at about 160 Hz

)()()(),( )()0(0 xtaxatxq j

N

jkjk Ψ+Ψ= ∑


Summary and ConclusionsSummary and ConclusionsReasonable agreement between simulations and available PIV data

Characterization of the flow not accessible by experiments:• CRZ extends inside the inner swirler• Reverse flow at the tips favoring merging of the jets• Wide range of turbulent structures up to very fine

The ILES seem to be the least dissipative

SGS model and grid induce differences in the range of the experimental uncertainties: further work to understand the origin of asymmetry

POD highlights helical structures leading to PVC at 160 Hz


Ongoing/future work: reacting casesOngoing/future work: reacting cases

• Small scale wrinkling captured• Stabilization inside the TARS• Promising comparison with PLIF

• Premixed Air/Methane flame Φ=0.8

Methane

+

air

OH PLIF

binary


THANK YOU!

Questions?

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LES investigation and sensitivity analysis of the flow ... · PDF fileLES investigation and sensitivity analysis of the flow dynamics in a gas ... Siemens SGT800 Background Gas Turbines