large eddy simulation as a design toolfor aero …...ce document et les informations qu’il...
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Large Eddy Simulation as a design tool for
aero-engine combustors
3rd MUSAF Conference, 28th October 2016
S. Richard J. Lamouroux G. Taliercio T. Lederlin
Safran Turbomeca became Safran Helicopter Engines in may 2016
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Not a scientific - but maybe useful - presentation (!), more a high fidelity
CFD end-user vision
• Scope of CFD for combustor design
• Relevancy and feasibility of LES
• State of the art of LES (for combustion) at Safran HE
• Challenges for LES of real combustors
• Outlooks for Hifi CFD methods in the aero-propulsion industry
Objectives of this talk
3rd MUSAF Conference 20162
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Combustor design, a complex trade-off
• Thermodynamic efficiency (pressure drop, combustion
efficiency)
• HP core lifetime (temperature profiles@combustor exit,
combustor liner temperature/gradient maps)
• Pollutant emissions (focus on NOx and soot)
• Operability (Lean-Blow Off, Ignition/Relight, combustion
stability)
• Non-CFD related constraints : engine integration,
manufacturing, costs, maintenance
CFD in the combustor design process
3rd MUSAF Conference 20163
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3 main types of CFD exploitation
• Physics analysis and understanding (after or in parallel to
experimental campaigns / engine failure or malfunction) -
Nsimulation~O(1-10)
• Iterative design – selective design (prior to experimental campaigns
– reduction of the design space) - Nsimulation~O(10-100)
• Optimization – robust design (in replacement /drastic reduction of
experiments – best configuration for given targets) -
Nsimulation~O(100-1000)
CFD in the combustor design process
3rd MUSAF Conference 20164
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Matrix representation of the possible CFD scope
3rd MUSAF Conference 20165
CFD type of use Physics analysis Iterative /
selective designOptimization/RD
Pressure drop
Combustion
efficiency
OTDF/RTDF
profiles
Liner
temperature
NOx
Soot
Lean-blow-off
Combustion
stability
Ignition
Thermodynamic
efficiency
HP core lifetime
Pollutant
emissions
Operability
Design targe vs CFD use
Design targets
Some questions to be addressed for each cell
of this matrix
- Relevancy of HiFi CFD (precision vs needs) ?
- Feasibility of HiFi CFD in an industrial
context (cost & return time) ?
- Availability of Hifi CFD tools?
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Relevancy of LES for combustor design
3rd MUSAF Conference 20166
Combustion in an aero-engine
• Is first a complex (and anisotropic) problem of mixing!
Progressive introduction of air in the burner (~60-70% of
air do not directly participate to combustion)
• Is a “moderate” turbulent Reynolds number problem
(Ret~5000-10 000) : ratio of the largest to the smallest
scales of the flow ~ 500 – 1000 in the vein � narrow
turbulence spectrum
• Has highly unsteady and intermittent characters
(turbulence intensity~10-20%) � non-linear coupling
between turbulence and chemistry difficult to model
Combustor physics features all the ingredientsto efficiently apply Large Eddy Simulation
Primary zone view (2000 frame/s)
liner cooling
primary holesinjectordilution hole
80%
12%
8%
20%
20%40%
1/r
Turb
ule
nce
Sp
ect
rum
lt ηk
resolved
modelled
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Industrial feasibility of LES
3rd MUSAF Conference 20167
Equivalent CPU hours on Intel
Ivy Bridge architecture
• LES can be predictive (depends
on the numerics, models, user)
but is ALWAYS expensive
• Optimization/Robust Design for
operability through the use of
“brute force” LES is today not a
reachable target
Pushing LES for design is a strategic choice implying de dicated means. There’s still an important need for lower-order modelli ng (or multifidelity) in which LES can play a role
Physics analysis Iterative /
selective designOptimization/RD
Pressure drop
Combustion
efficiency
OTDF/RTDF
profiles
Liner
temperature
NOx
Soot
Lean-blow-off
Combustion
stability
Ignition
Thermodynamic
efficiency
HP core lifetime
Pollutant
emissions
Operability
Design targe vs CFD use
CPUhReturn time (base 2000 cores)
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Fidelity vs Quantity
• Natural trend to refine the grid for reducing CAD
simplifications and sensitivity to modelling
• Maturity of LES yields a rapid growth of the
number of configurations to compute (incremental
design improvement, multiple design points
simulation…)
• CPU needs for productive LES has already started to
exceed supercomputer power evolution ���� CPU
consumption for combustion has been multiplied by
100 in the last five years at SAFRAN HE
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Link with meshing capabilities
0,01
0,1
1
10
100
1000
10000
100000
1995 2000 2005 2010 2015 2020 2025
Standard
Challenge
Number of elements per burner sector
(10-20 sectors in the full burner)
X1 sim.
X5 sim.
X50 sim.
Number of elements per burner sector
(10-20 sectors in the full burner)
1
10
100
1000
10000
100000
1990 1995 2000 2005 2010 2015 2020 2025 2030
CPU need for
productive LES
CPU power
increase (moore)
No
rma
lise
dC
PU
co
nsu
mp
tio
n
Both fidelity and quantity will continue growing in the coming years but maybe not together (multifidelity LES?)
(in M elements)
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LES code availability for industrial use
3rd MUSAF Conference 20169
A recurrent debate : Engineering vs First principles LES
First Principles LES Engineering LES
Expert user Broad range of users
High quality grid Poor quality grid
Non-dissipative numericsRobust (and dissipative)
numerics
Massively parallel and
scalableMinor parallelism
Comprehensive physics Main phenomenology
Basic rules
• Robustness / user dependency : mesh generation
should obey strict rules (grid skewness, refinement)
– LES requirements drive grid generation, not the
contrary
• Numerics : A LES code should have proven good
turbulence transport and dissipation capabilities on
simple configurations (vortex convection, HIT…)
• HPC capabilities are mandatory
• Modelling : Physical models implementation should
be validated on basic experiments AND confronted
to complex applications within the same software
(strong coupling with numerics)Is this still LES?
LES is not a quick-win business but necessitates long-term investments and
partnerships with experts from academia
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3rd MUSAF Conference 2016
AVBP YALES2
Equations Compressible Low-Mach var. density
Numerics2nd/3rd/4th order in space,
2nd/3rd order in time
2nd/4th order in space,
4th order in time
Parallelism Massive Massive
Grid Hybrid unstructured grids Hybrid unstructured grids
Turbulence
modellingSmagorinsky/Wale/Sigma Smagorinsky/Wale/Sigma
Walllaw-of-the wall/specific
effusion cooling model
law-of-the wall/specific
effusion cooling model
Turbulent
combustion
TFLES/PCM-
FPI/CFM/FTACLESTFLES/PCM-FPI/FTACLES
NOx : tabulated NOMANI
model
NOx : tabulated NOMANI
model
Soot : Hybrid Tabulated /
Leung model
Two-phase flowEulerian and Lagrangian
formalism with fuel filmingLagrangian
Pollutant
State of the art of LES at Safran HE
10
Codes… … and workflow
• More than 1000 degrees of freedom to set-up
for a combustor sector calculation (initial &
boundary conditions, models parameters) � Hifi
production needs Hifi set-up
• Complete engineering work-flow from
combustor sketch to post-processing developed
with CERFACS and CORIA
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LES field of use in combustion
3rd MUSAF Conference 201611
Computational Readiness Level
• Use of both compressible (AVBP)
and low-Mach (YALES2) LES
solvers
• CPU-limited deployment of
optimization and Robust Design
• Return time challenged by in-
house experimental capabilities
for iterative design applications
Operability is still an issue from bothmodelling and CPU cost points of view
HiFi CFD maturity
Compatible
with Design
Activities
Currently
unfeasible
Physics analysis Iterative /
selective designOptimization/RD
Pressure drop
Combustion
efficiency
OTDF/RTDF
profiles
Liner
temperature
NOx
Soot
Lean-blow-off
Combustion
stability
Ignition
Design targe vs CFD use
Thermodynamic
efficiency
HP core lifetime
Pollutant
emissions
Operability
Current zone of possible
applicability
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Some achievements of LES applied to combustion
3rd MUSAF Conference 201612
Lean blow-off limit prediction
• Simulation of a fuel-air ratio transient from stable
combustion to extinction (like in experiments)
• Extinction detected by LBO precursors like
unburnt fuel at the combustor exit
• Detailed analysis of the blow-off mechanisms
Pre
curs
or
Case 1
Case 2
LBO limit
Time
�
�
� �
Normalized LBO limits for
several combustors
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Some achievements of LES applied to combustion
Low NOx / Low-Smoke combustor technology analysis
• NOMANI tabulated model for NOx (prompt, thermal)
Side viewT
NOx formation
very sensitive to :
• temperature
• FAR
• residence timeLP
injectors
combustor 1
combustor 2
combustor 3
combustor 4
combustor 5
FAR
Pilot
injector
Trends and order of magnitude can be predicted
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Some achievements of LES applied to combustion
Soot mass fraction
Soot number density
Injection wheel
cumulated mean
instantaneous
• Intermittent formation/oxydation of soots
• No negligible source term : balance makes soot level prediction very difficult
Low NOx / Low-Smoke combustor technology analysis
• Hybrid tabulated (gaseous species) / modified Leung (soot) model
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Some achievements of LES applied to combustion
Soot concentration converted into smoke number for comparison to experimental data
obtained at the engine test (ICAO correlation)
• Impact of injection technology and
operating condition are well described
• Exact smoke number values are still
difficult to predict (lack of detailed
physics)
• Fuel composition impact is correctly
accounted thanks to detailed chemistry
if
if
GTL (paraffinic
fuel)
Jet A (low
aromatic content)
Jet A (high
aromatic content)
smok
enu
mbe
r
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Some achievements of LES applied to combustion
3rd MUSAF Conference 201616
RTDF profile set-up
����(�) =��� (�)− �4
�4 −�3
combustor inlet combustor exit
RTDF[%]0Hub
Tip
Radius
(normalised temperature)
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Challenges for LES of complete combustors
3rd MUSAF Conference 201617
Many efforts in the past for turbulence / turbulent combustion modelling
• High level of fidelity reached on swirler aerodynamics
• Temperature and main species concentrations fields also predicted
• Grid convergence demonstrated
axial velocity
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Challenges for LES of complete combustors
3rd MUSAF Conference 201618
… but restricted to isolated injectors and often “low pressure/pre-heated”
conditions hiding major bottlenecks to be found as soon as LES is the subject
of an industrial use
• How to account for cooling devices (effusion cooling, rings…) or properly validate the
associated models? � represents up to 60% of the combustor air-flow rate!
• Excessive filtering/thickening of the flames at relevant pressure/temperature
conditions � validity of the sgs closure? / imply huge residence time modifications
• Fuel air ratio at the combustor exit far from flammability limits � chemistry
validation under high dilution (especially for tabulated approaches – slow
oxidation/post-oxydation chemical paths not included in look-up tables)?
• Pre/post-processing : mesh generation, local refinement / data storage and transfer
for analysis (0.5 Tb/simulation)
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Cooling devices modelling
3rd MUSAF Conference 201619
Hole resolution generally limited to primary /
dilution holes
• Correctly resolving the pressure loss (and
the air-mass flow rate) within holes
requires around 25-30 points following the
literature
~O(5mm) ~O(0,5mm)
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3rd MUSAF Conference 201620
1
10
How far can we solve cooling systems?
Big hole
Small hole
Today
… do we have to wait for 2039
and keep relying on
homogeneous “porous wall”
approaches
• Only few and recent attempts
at CERFACS and UNIFI to
propose heterogeneous
models based on local suction
and injection
Zone of interest
thickened holeapproach
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Filtering/thickening of flames at relevant pressure and temperature
3rd MUSAF Conference 201621
The flame (heat release, main species
gradients) should be resolved on a few
grid points (~5-10)
• The real flame thickness δflame is by far
lower than the grid size and needs to be
filtered or thickened at a scale ∆comb~5-
10∆x
• The higher the combustor pressure (and
therefore temperature), the greater the
flame is thickened
• Some concerns about the validity of sgs
models at high pressure levels / strong
modification of the residence time in
reaction zones (impact on polluants having
long chemical time scales)
Operating pressure (bar)
0
50
100
150
200
250
0 10 20 30 40 50
Série1
Série2Primary zone
Injector vicinity
∆co
mb
/δfl
am
e
∆x~200µm
∆x~600µm
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Conclusion
3rd MUSAF Conference 201622
Physics (unsteadiness, Turbulent Re) makes LES particularly relevant for being intensively applied to
aero-engine combustors design
A strong interaction between numerics development, model implementation and validation is
needed to assess the codes validity before applying them to design activities
The LES industrial maturity perimeter extends rapidly, more than CPU capabilities offered by the
Moore law � dedicated resources will be needed in near the future : LES is not a simple evolution
of CFD but a strategic orientation
Efficient workflows dedicated to HiFi methods are mandatory : LES has been a tool for CFD experts
in the last 20 years but bridges should now be established for end-users
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Outlooks for Hifi-methods in the aero-propulsion industry
3rd MUSAF Conference 201623
The extension of LES to the fields of Robust Design and Uncertainty Quantification will need for a
drastic reduction of grid generation efforts (CAD and meshing software, automatic/dynamic &
robust mesh refinement strategies)
• This is particularly true with the arising of 3D printing in the aeronautic market
• The time needed for setting up a simulation is still close to the one for running it
LES is today mature for simulating nominal engine operating conditions, but important modelling
improvements are still required for addressing operability issues and accounting for the fuel
composition influence : dedicated two-phase flow combustion models, multicomponent approaches for both
physical and chemical behaviors, ignition, …
Data management (including storage…) and post-processing is becoming an issue : • Methodologies have to be developed for getting much than averaged (and RMS) fields from unsteady
simulations, with objective criteria to help analyzing/comparing cases
• Multi-fidelity approaches for data analysis (?) : resolution requirements for numerical accuracy are not
necessarily the same as for physics analysis
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Acknowledgments
Many thanks to our partners CERFACS and CORIA daily supporting SAFRAN in the
development and use of LES for design activities
Acknowlegments to our colleagues from SAFRAN Aircraft Engines for providing part
of the material of this talk
Special thanks to
A. DauptainCERFACS
G. StaffelbachCERFACS
V. MoureauCORIA
J. DombardCERFACS
Y. MerySAFRAN
Aircraft Engines
A. FiguerSAFRAN
Aircraft Engines
Q. MaléSAFRAN
Helicopter Engines
R. MercierSAFRAN Tech
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