les investigation and sensitivity analysis of the flow ... · pdf fileles investigation and...
TRANSCRIPT
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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)
09 February 2011 Piero Iudiciani FM Seminar Series
⎟⎟⎠
⎞⎜⎜⎝
⎛
∂
∂+
∂∂
=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
09 February 2011 Piero Iudiciani FM Seminar Series
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^
09 February 2011 Piero Iudiciani FM Seminar Series
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)
09 February 2011 Piero Iudiciani FM Seminar Series
Inner axial swirler
TARS TARS -- Geometrical detailsGeometrical details
09 February 2011 Piero Iudiciani FM Seminar Series
Intermediate axial swirler
TARS TARS -- Geometrical detailsGeometrical details
09 February 2011 Piero Iudiciani FM Seminar Series
Outer radial swirler
TARS TARS -- Geometrical detailsGeometrical details
09 February 2011 Piero Iudiciani FM Seminar Series
TOP BOTTOM
TARS TARS -- Geometrical detailsGeometrical details
Streamlines
• Complex geometry• Computations include the swirlers• To ensure the quality of the grid: mostly cuboids cells
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
- 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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
SGS sensitivity: SGS sensitivity: Turbulence SpectraTurbulence Spectra
- All the models capture a decade of the TKE spectra- ILES least dissipative
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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!
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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.
09 February 2011 Piero Iudiciani FM Seminar Series
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
Axial velocity. Arbitrary scale
)()()(),( )()0(0 xtaxatxq j
N
jkjk Ψ+Ψ= ∑
Mode 0: Average field.
09 February 2011 Piero Iudiciani FM Seminar Series
POD results:POD results: Longitudinal cross section Longitudinal cross section MODE 2 MODE 3
Axial velocity. Arbitrary scale
Modes 2-3: Pair of modes, Helical structures
)()()(),( )()0(0 xtaxatxq j
N
jkjk Ψ+Ψ= ∑
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
POD resultsPOD results
Axial velocity. Arbitrary scale
• Mode 0: Averaged field. • Staggered pattern at 45 influence of the 8 vanes. • CRZ centered in average
Inner
Intermediate
Outer
09 February 2011 Piero Iudiciani FM Seminar Series
POD resultsPOD results
Axial velocity. Arbitrary scale
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
09 February 2011 Piero Iudiciani FM Seminar Series
POD resultsPOD results
• Fourier Analysis of the time coefficients confirms the peaks at about 160 Hz
)()()(),( )()0(0 xtaxatxq j
N
jkjk Ψ+Ψ= ∑
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
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
09 February 2011 Piero Iudiciani FM Seminar Series
THANK YOU!
Questions?