development of advanced combustion model
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In-cylinder flow and combustion modeling using the OpenFOAM® technology
Department of Energy, Politecnico di Milano
http://www.engines.polimi.it
https://twitter.com/ICEPoliMi/
T. Lucchini, G. Montenegro, G. D’Errico, A. Onorati, L. Cornolti
Workshop "HPC enabling of OpenFOAM for CFD applications"
http://www.engines.polimi.it
Background
• Target for next IC engines generation: contemporary reduction of fuel consumption and pollutant emissions.
• Development and design focused on fuel-air mixing and combustion:
GDI engines (premixed or stratified)
Diesel engines (complex injection strategies)
New combustion modes (HCCI, PCCI)
• CFD widely used for IC engines design:
Development of a platform that can be used both for model implementation and engine design.
http://www.engines.polimi.it
Topics
• Development of new combustion models for IC engines using the OpenFOAM® technology.
• Lib-ICE, model development for in-cylinder flow simulations:
Mesh management
Fuel-air mixing
Combustion
• Examples of application on real engine cases
http://www.engines.polimi.it
Model implementation: Lib-ICE
• Mesh motion for engine geometries
• Combustion (spark-ignition, diesel)
• Lagrangian sprays + liquid film
• Unsteady flows in intake and exhaust systems:
Plenums, silencers, 1D-3D coupling
• Reacting flows in after-treatment devices
DPF, SCR, catalyst
Libraries and solvers for IC engine simulations based on OpenFOAM® technology
http://www.engines.polimi.it
OpenFOAM-2.0.x
Lib-ICE
src
• Spray • Thermo-physical models • Combustion models • Liquid film • Engine mesh management • Boundary conditions • 1D-3D coupling
solvers
• Cold-flow • Diesel • Spark-ignition • Combustion • Spray
utilities
• Pre-processing • Parallel-processing • Post-processing • Mesh-management
Lib-ICE – code structure
http://www.engines.polimi.it
MESH 1
XXX CA YYY CA
MESH 2
YYY CA ZZZ CA
1) Multiple meshes cover the entire cycle simulation.
2) Each mesh is valid in a user-specified interval.
3) During each time-step: Grid points are moved using
automatic mesh motion and/or pre-defined points motion.
Mesh topology can be eventually changed
4) Mesh-to-mesh interpolation.
Full-cycle simulation
http://www.engines.polimi.it
Automatic mesh motion
• Grid points motion computed by an automatic mesh motion solver, based on the Laplace equation.
02 u
toldnew uxx
Layout for deforming mesh simulations
Full-cycle simulation
http://www.engines.polimi.it
Topological changes
Full-cycle simulation
• Reduction of the overall number of meshes.
• Mesh is modified according to user-defined parameters, no need to specify exactly when a specific event should take place.
Dynamic mesh layering Sliding interface
http://www.engines.polimi.it
Topological changes: Adaptive Local Mesh Refinement
Full-cycle simulation
• Grid resolution increased to properly describe the most important physical and chemical processes and reduce grid dependency:
Fuel-air mixing
Combustion
http://www.engines.polimi.it
Mesh management
engine library in Lib-ICE
engine
engineTopoChangerMesh
fvMotionEngineMesh
pistonLayerAREngineMesh
twoStrokeEngineMesh
pistonRefineEngineMesh
Deforming grid for full-cycle simulations in four-stroke engines
Compression-expansion with dynamic mesh layering
Full-cycle simulation in four-stroke engines
Compression-expansion with adaptive local mesh refinement
multiCycleEngineTime
http://www.engines.polimi.it
Full-cycle simulation
1) Generation of multiple meshes 2) Definition of a template case
3) engineCaseSetUp utility case
constant
init
system
case0300
case0320
……
case0720
case1440
……
dataOutput
4) runMultiCycleCase solverName simpleColdSpeciesEngineDyMFoam;
startTimes
(/* list of startTime for each mesh */);
endTimes
(/* list of endTimes for each mesh */);
writeInterval
(/* list of writeInterval for each mesh */);
parallelRun on;
5) Post-processing
Specific utilities for case set-up
http://www.engines.polimi.it
Generation of engine meshes
Grid imported from different tools
(Gambit®, ICEM-CFD®, Pointwise®,
STAR-CD®)
Fully automatic hexahedral mesh
generation from stl file
Full-cycle simulation
http://www.engines.polimi.it
Mesh generation: new approach based on Blender®
Blender + snappyHexMesh: engine mesh generation exclusively based on open-source tools
Full-cycle simulation
http://www.engines.polimi.it
Full-cycle simulation
Hydrogeninjector
Cylinderhead
Exhaust
Intake
Tumbleplate
Laser532/266 nm
Piston
Engine block
Opticalliner
Displaced volume 560 cm3
Compression ratio 11
Intake pressure/temperature 1 bar / 36 °C
Engine speed 1500 rpm
SANDIA Hydrogen Engine (data from ECN)
Fixed zones
Deforming zones
Motored conditions. Experimental pressure and temperature profiles imposed at intake and exhaust ports.
• Mesh validity: 15 deg each. Average cell size ranging from 300 to 600 thousand.
• Grid size: Cylinder: 2.5 mm Valve region: 0.75 mm Ducts: 2.5 – 5 mm
• Turbulence model: standard k-e
http://www.engines.polimi.it
Exp. Calc.
The main flow features at BDC are almost preserved at this time, when the intake valve is closing.
-140 CA (IVC: -140)
SANDIA Engine: velocity field comparison
Full-cycle simulation
http://www.engines.polimi.it
Full-cycle simulation
Gas pressure now higher than ambient pressure.
Still rather good agreement between computed and experimental data.
Exp. Calc.
-70 CA (IVC: -140)
SANDIA Engine: velocity field comparison
http://www.engines.polimi.it
Full-cycle simulation
Post-processing
0
1
2
3
4
5
6
7
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
435 465 495 525 555 585 615 645 675 705 735
Crank Angle [deg]
Tumble ratio
Turb. Kin. energy
SOI EOI
IVC
Cylinder pressure In-cylinder charge motions
Specific utilities to estimate relevant quantities: charge motions (tumble, swirl ratio, turbulence intensity); air-fuel mixing: homogeneity index.
http://www.engines.polimi.it
Comprehensive spray model for Diesel and GDI sprays
Turbulence induced breakup
Wave + Catastrophic
breakup
• Breakup transition: Weber number.
Spray model
http://www.engines.polimi.it
• Extending the capabilities of the standard OpenFOAM spray library, by introducing new models for:
High pressure injection
Liquid fuel atomization
Droplet-wall interaction
Liquid evaporation
• Reducing the grid-dependency and CPU time of spray simulations:
Easy development of new models.
Fast tuning of existing models.
Spray model
Comprehensive spray model for Diesel and GDI sprays
http://www.engines.polimi.it
Mesh management
dieselSpray library in Lib-ICE
dieselSpray
spray
parcel
spraySubModels
injectorModel
atomizationModel
wallModel
breakupModel
Fully re-implemented
Partially re-implemented
Huh model with and without cavitation
Huh-Gosman model
Bai-Gosman model
Bug fixes
http://www.engines.polimi.it
Liquid film model
Liquid film
• Approach proposed by Bai and Gosman (SAE-960626).
• Mass, momentum and energy equations solved for a thin film using the Finite Area Method accounting for both interaction with liquid (impact) and gas phase (evaporation).
http://www.engines.polimi.it
Spray model
Non-evaporating diesel spray
Experimental data courtesy of Dr. Montanaro and Dr. Allocca (CNR-Istituto Motori)
http://www.engines.polimi.it
Spray model
Evaporating diesel spray (ECN data)
• Fuel: n-dodecane (“Spray-A”)
• Effects of ambient conditions (temperature, density) on liquid and vapor penetration.
Exp data from ECN: www.ca.sandia.gov/ecn
http://www.engines.polimi.it
• Air/fuel mixing and velocity comparison
• Comprehensive spray model validation absolutely necessary for a correct prediction of the combustion process.
Spray model
Evaporating diesel spray (ECN data)
Exp. Calc.
20
30
40
50
Dis
tan
ce [m
m]
(a) (b) (c)
• Fuel: n-heptane (Spray-H)
Exp data from ECN: www.ca.sandia.gov/ecn
http://www.engines.polimi.it
Spray model
Impinging gasoline spray
(a) t = 400 μs (b) t = 2300 μs
Liquid film formation on the plate
Experimental data courtesy of Dr. Montanaro and Dr. Allocca (CNR-Istituto Motori)
http://www.engines.polimi.it
Background
• Combustion models for IC engines. Requirements:
Flexibility multiple-injections stratified charge combustion different combustion regimes
Detailed chemistry pollutant formation (soot) auto-ignition multi-component fuels
Interacting processes Energy transfer during spark-ignition Turbulence-chemistry interaction
http://www.engines.polimi.it
Background
• Implementation of combustion models in Lib-ICE
• Design-oriented tools:
Fast, robust and suitable for industrial simulations
Reduced chemistry
• Diagnostic-oriented tools:
Allowing a detailed analysis of the flame structure and study of new combustion modes
Detailed kinetics
http://www.engines.polimi.it
Combustion models for SI Engines
• Combustion phases: spark-ignition and turbulent combustion.
• State of the art: different combustion models available (G-Equation, Weller, ECFM) and extensively validated.
• However, initial flame development plays a big role and modeling efforts are still required to describe such phase. Influence of local flow, turbulence conditions and energy transfer from the electrical circuit to be accounted for.
• Proposed approach: Lagrangian ignition model + turbulent combustion model (Eulerian)
http://www.engines.polimi.it
Combustion models for SI Engines
• Lagrangian ignition model
1) The spark channel is represented by a set of Lagrangian particles, convected by the mean flow.
http://www.engines.polimi.it
Combustion models for SI Engines
KaKa
2) Flame kernels are launched at the particle locations satisfying the ignition criterion (Karlovitz). For each particle mass and energy equations are solved.
• Lagrangian ignition model
plasma
u
p
tup
pssr
dt
dm
4
ppp
spk
bp
p
pp
cm
QTT
m
m
dt
dT
,
http://www.engines.polimi.it
Combustion models for SI Engines
3) Flame surface density distribution reconstructed from particle distribution and their size.
cell
N
i
ii
spkV
Sf
1
• Lagrangian ignition model
http://www.engines.polimi.it
Combustion models for SI Engines
• Lagrangian ignition model
4) Initial thermal transient of the plasma channel correctly described by solving the heat conduction equation on it.
ri
Ti (>10000 K) Tu (300-600 K)
Qel
Temperature [K]
a [
m2/s
]
Temperature [K]
c p [
kJ/j
kgK
]
plp
pl
plp
elpl
pl
Vc
titVT
Vc
QT
t
T
a
a
22
http://www.engines.polimi.it
Combustion models for SI Engines
• Lagrangian ignition model
5) Simplified model for the secondary circuit to estimate the amount of energy transferred to the gas phase
titVQ
lVVtV
L
Eti
titVtiRdt
dE
spk
spkgcacs
s
ss
sssss
2
2
Rp Rs
Lp Ls Spark
gap
http://www.engines.polimi.it
Combustion models for SI Engines
• Lagrangian ignition model
6) Ignition source term included in the flame surface density transport equation:
k
it
t
ii
i PDPxScScxx
u
t
Possibility to choose different expressions for P and D depending on the selected FSD approach.
http://www.engines.polimi.it
Combustion models for SI Engines
sparkIgnition library in Lib-ICE
flameKernelIgnition
flameKernel
electricalCircuit
plasmaChannelModel regionModel
particle
Cloud<flameKernel>
sparkIgnitionSubModels ignition, restrike, wall, dispersion, efficiency
Solver: lagrangianPlasmaFlameKernelEngineFoam
flameSurfaceDensityModels CFM, ECFM, CFM-2, Chang-Huh, ..
http://www.engines.polimi.it
Combustion models for SI Engines
• Pre-chamber with optical access. • Fuel: propane; laminar flame speed:
Gulder
• Tested operating conditions:
1) Effect of engine speed 2) Effect of air/fuel ratio 3) Effect of spark-plug position
• Grid generation: snappyHexMesh
Engine geometry details, operating conditions and complete set of experimental data available in SAE Paper 922243.
Experimental validation: optical pre-chamber engine
http://www.engines.polimi.it
Combustion models for SI Engines
Experimental validation: optical pre-chamber engine
Non-combusting conditions
Correct prediction of turbulence intensity and velocity very important for realistic simulation of the combustion process
0
4
8
12
16
310 320 330 340 350 360
no
n-d
ime
nsi
on
al ra
tio
Crank Angle [deg]
Calc.
Exp.
u'/up Umean/up
velocity Turbulence intensity
Ignition locations
http://www.engines.polimi.it
Combustion models for SI Engines
Experimental validation: optical pre-chamber engine
Computed vs experimental burned volume
Effect of engine speed Effect of air/fuel ratio at 1000 rpm
http://www.engines.polimi.it
Diesel combustion
• Complex process involving:
Detailed fuel chemistry (auto-ignition, pollutant formation)
Spray evolution
Multi-component fuels
Turbulence-chemistry interaction
• Implemented approaches:
CTC (reduced chemistry) Well-mixed model (detailed chemistry) Flamelet-combustion models (detailed chemistry)
http://www.engines.polimi.it
Diesel combustion: CTC model
• 11 chemical species (fuel, O2, N2, CO, CO2, H2O, O, OH, NO, H, H2)
• Auto-ignition computed by the Shell auto-ignition model; turbulent combustion simulated accounting for both laminar and turbulent time scales:
• Temperature threshold to switch between auto-ignition (a=1)
and turbulent combustion (a=0).
CTCiShellii YYY ,, 1 aa
Characteristic time-scale combustion model
http://www.engines.polimi.it
Validation: passenger-car, common-rail engine
Operating conditions:
• Full load with post-injection to reduce soot
• Medium high load with:
Variable EGR rate
Three injection events (pre, main, post)
Bore 85 mm
Stroke 88 mm
Compression ratio ~15
Models:
• Modified Huh-Gosman for atomization. Diesel fuel approximated as C12H26
• CTC+Shell for combustion. C12H26 Shell-model constants proposed by Reitz.
Diesel combustion: CTC model
http://www.engines.polimi.it
Cylinder pressure validation at full load
0%
100%
EGR Qpil Qmain Qpost
Full load, single
injection, No EGR
Diesel combustion: CTC model
http://www.engines.polimi.it
Soot emissions, full load
0.5
0.6
0.7
0.8
0.9
1
0 1 3 5 7 9
No
n d
ime
nsi
on
al s
oo
t
Mass of fuel in the post-injection [mg/stroke]
Effect of post injection on soot-emissions Measured
Computed
Post-injection effect on soot emissions
Diesel combustion: CTC model
http://www.engines.polimi.it
Medium-high load, 0% EGR
3 injections
Cylinder pressure validation at mid-load
0%
100%
EGR Qpil Qmain Qpost
2300 rpm imep = 10.5 bar
Diesel combustion: CTC model
http://www.engines.polimi.it
Medium-high load, 34% EGR
3 injections
Cylinder pressure validation at mid-load
0%
100%
EGR Qpil Qmain Qpost
2300 rpm imep = 10.5 bar
Diesel combustion: CTC model
http://www.engines.polimi.it
Soot emissions, medium-high load
Injection strategy and external EGR effects on soot and NOx emissions
0
0.2
0.4
0.6
0.8
1
3.5 19.5 23.5 33.5 (std.)
33.5 (adv.)
No
n-d
ime
nsi
on
al s
oo
t
Measured
Computed
0
100
200
300
400
500
600
700
3.5 19.5 23.5 33.5 (std.)
33.5 (adv.)
NO
x[p
pm
] Measured
Computed
External EGR [%] External EGR [%]
Diesel combustion: CTC model
http://www.engines.polimi.it
Diesel combustion: well-mixed model
• Direct integration of complex chemistry in each computational cell. Each cell is an homogeneous reactor and species reaction rates are computed by means of an ODE stiff solver:
tt
t
iiii dtW
tYttY '*
t
tYttYY ii
i
*
• Detailed approach but very time-consuming, mainly when a realistic mechanism (>50 species) is employed.
http://www.engines.polimi.it
Diesel combustion: TDAC
• TDAC: Tabulation of Dynamic Adaptive Chemistry
• Combination of mechanism reduction and tabulation techniques
• Work developed in collaboration with Dr. F. Contino (Vrije Universiteit Brussel) and Prof. H. Jeanmart (Univ. of Louvain)
CPU time speed-up compared to direct-integration is of the order of n x m (n: number of species, m: number of reactions) .
Very general approach, flexible with respect to the mechanism and fuel composition.
Up to 300 species and 1000 reactions can be used in a reasonable amount of time.
http://www.engines.polimi.it
Combustion models for SI Engines
chemistryModelPolimi library in Lib-ICE
chemistryModelPolimi
chemistryModel
chemistrySolversTDAC
mechanismReduction
tabulation
Solvers: dieselFoamAutoRefine, dieselEngineDyMFoam
ISAT
DAC, DRG, DRGEP, ..
Developed in collaboration with Dr. F. Contino and Prof. H. Jeanmart.
http://www.engines.polimi.it
Diesel combustion: well-mixed model
Auto-ignition and flame lift-off in SANDIA ECN Spray-A Diesel flame
103 species, 370 reactions mechanism for C12H26 (Sarathi et al.)
Effects of oxygen concentration and ambient temperature: IGN. DELAY
http://www.engines.polimi.it
Diesel combustion: well-mixed model
Auto-ignition and flame lift-off in SANDIA ECN Spray-A Diesel flame
103 species, 370 reactions mechanism for C12H26 (Sarathi et al.)
Effects of oxygen concentration and ambient temperature: LIFT-OFF
http://www.engines.polimi.it
Diesel combustion: well-mixed model
Flame structure in the SANDIA ECN Spray-A Diesel flame
300 species, 8900 reactions mechanism for C12H26 (Ranzi et al.)
Temperature OH Acetylene BIN1-A
http://www.engines.polimi.it
Diesel combustion: well-mixed model
Air-fuel mixing model validation
7 mm, exp 12 mm, exp 18 mm, exp
7 mm, calc 12 mm, calc 18 mm, calc
Combustion model validation
PRF30 fueled engine:
ALMR employed to better predict both the fuel-air mixing process and flame propagation.
Detailed mechanism for PRF fuels oxidation
PCCI combustion in an optical engine
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
• Multiple unsteady flamelets: a set of unsteady diffusion flames represents Diesel combustion.
• First model towards the definition of a generic model for partially-premixed combustion
xH~
pst ,~
xT~ xYhxH ii
~~
dtYtxftxY izi ,~
,;~
,~
1
0
CFD Code
xZxZ 2'',
~
FlameletCode
tZYi ,~
Detailed chemistry and turbulence-chemistry interaction
Flamelet Equations
Y
Z
YZ
t
Y ii
2
2
2
chem
ss qZ
hZ
t
h
2
2
2
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
Detailed chemistry and turbulence-chemistry interaction
• CFD domain: transport equations are solved for mixture fraction Z
and variance Z’’2 , momentum, mass, enthalpy h.
• Flamelet domain takes the average scalar dissipation rate conditioned on the stoichiometric mixture fraction (st) and use it in the flamelet equations for mass and energy in the mixture fraction space.
• Possibility to use multiple flamelets, with two possible criteria for division: mass or scalar dissipation rate.
• Chemical composition in CFD cells:
N
f
fifi dZZpZYIY1
1
0,xx
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
Implementation of a flamelet combustion model
• Use of any pre-implemented capability to exploit the entire potentialities that are offered by the OpenFOAM technology:
Chemistry model + in-house developments (ISAT, DAC, …)
Thermodynamics
Parallelization using existing domain decomposition techniques.
Pre-implemented functions (average, integration, matrix algebra…) to solve flamelet equations and to exchange data between the phase space and the physical domain.
http://www.engines.polimi.it
• regionModel approach for flamelet combustion models:
Flamelet mesh, representing the Z space
Flamelet equations (species and enthalpy) solved on the flamelet mesh, relying on native OpenFOAM matrix algebra and ODE solvers.
PDF averaging:
Standard OpenFOAM functions (integration, averaging) applied on the flamelet mesh, that requires accurate grading at the Z = 0 and Z = 1 boundaries.
Diesel combustion: unsteady flamelets
Implementation of a flamelet combustion model
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
flameletCombustionModels
RIF
flameletCombustionModel
scalarDissipationRate
Peters
scalarDissipationRate
pdfFunctions
Abstract class
Implementation of Representative Interactive Flamelet Model
Operations to compute scalar dissipation rate (st, (Z))
Solver: RIFdieselFoam
flameletCombustionModels library in Lib-ICE
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
Simulation: SANDIA ECN Spray H flame (n-heptane)
Flamelet mesh
0 1 z
The grid is refined at the boundaries (Z=0, Z=1), to correctly integrate the PDF function in presence of high mixture fraction variances, as suggested by Liu et al.
http://www.engines.polimi.it
Initial conditions in the flamelet domain: • Temperature in Z-domain • Species in the Z-domain
Diesel combustion: unsteady flamelets
Flamelet mesh
Temperature in Z-domain
Species
Simulation: SANDIA ECN Spray H flame (n-heptane)
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
Flamelet mesh
Temperature in Z-domain
Species
Scalar dissipation
rate
Cool flame
Simulation: SANDIA ECN Spray H flame (n-heptane)
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets Simulation: SANDIA ECN Spray H flame (n-heptane)
Flamelet mesh
Temperature in Z-domain
Species
Scalar dissipation
rate
Main ignition
http://www.engines.polimi.it
t = 0.9 ms
Diesel combustion: unsteady flamelets Simulation: SANDIA ECN Spray H flame (n-heptane)
Mixture fraction Mixture fraction variance
Stoichiometric scalar dissipation rate Temperature
Flame structure in CFD domain computed at steady state conditions
http://www.engines.polimi.it
Diesel combustion: unsteady flamelets
Simulation: SANDIA ECN Spray H flame (n-heptane)
0
0.5
1
1.5
2
8 12 16 20 24
Ign
itio
n d
ela
y [m
s]
Oxygen concentration [%]
Calc.
Exp.
amb= 14.8 kg/m3
amb= 30 kg/m3
0
0.3
0.6
0.9
1.2
800 1000 1200 1400
Ign
itio
n d
ela
y [m
s]
Ambient temperature T [K]
Calc.
Exp.
http://www.engines.polimi.it
Acknowledgments
• Ing. Rita Di Gioia, Ing. Giovanni Bonandrini, Ing. Mario Picerno, Ing. Luca Venturoli, Magneti Marelli, Bologna.
• Dr. Francesco Contino, Vrije Universiteit Brussel.
• Ing. Marco Fiocco, Politecnico di Milano.
• Paolo Colombi, MSc. Student, Politecnico di Milano
• Dr. Luigi Allocca, Dr. Alessandro Montanaro, CNR-Istituto Motori
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