numerical simulation study of turbulent combustion phenomena · numerical simulation study of...
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Rixin YuDivision of fluid Mechanics
Department of Energy ScienceLTH
Numerical simulation study of turbulent combustion phenomena
-INTEGRATE Advanced Study Group (ASG)
9/27/20161
IntroductionSome combustion related keywords
Energy, heat and power, transportation.Emission, clean combustion, global warming.Fossil fuel, Hydrogen, biomassCombustion efficiency, combustion instabilityTurbulence combustion
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IntroductionTo understand turbulent reacting system, a challenge
Turbulence + Chemistry Wide range of scales
Da Vinci’s sketch
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Research methodologyMathematical equations describing turbulent reacting flow
Flow turbulenceNaiver-Stokes equations(Conservation of mass, momentum, energy), assuming low-Mach number.
Chemistry reactionConservation of N species mass
o Yk , k=1,..N (YH, YO2, YCH3, …)
Reaction rates ωk is given by M elementary chemical reactions
Reactions for CH4/AirH+O2↔O+OHO+H2↔OH+HH2+OH↔H2O+HOH+OH↔H2O+OH+O2+M→HO2+MH+HO2→OH+OHH+HO2→H2+O2OH+HO2→H2O+O2CO+OH↔CO2+HCH4+M↔CH3+H+MCH4+H↔CH3+H2CH4+OH↔CH3+H2OCH3+O→CH2O+HCH2O+H→HCO+H2CH2O+OH→HCO+H2OHCO+H→CO+H2HCO+M→CO+H+MCH3+O2→CH3O+OCH3O+H→CH2O+H2CH3O+M→CH2O+H+MHO2+HO2→H2O2+O2H2O2+M↔OH+OH+MH2O2+OH↔H2O+HO2OH+H+M→H2O+MH+H+M→H2+M
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Research methodologyNumerical simulations of P.D.E governing turbulent reacting flow
Partial-resolution methods (filtered P.D.E., need models)
Reynolds Averaged Navier-Stokes(RANS) Cheap; favored by industrial; Applied approach.
Large Eddy simulations(LES) more accurate, capturing dynamic features
Full-resolution method (no model)Direct(Detailed) numerical simulations(DNS)
Computational expensive, limited to very small sizelog(k)
logE
(k)
resolved LES modelled LES
modelled RANSresolved DNS
-5/3
2π/l0 2π/ηintegral Kolmogorov
2π/δ
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Data analysis : three case studies1: DNS studies of homgenouslycharged compression ignition (HCCI) engine combustion.
2: DNS of turbulent premixed flame propagation
3: Fractal flame front structures due to flame (Landau-Darrieus) instability
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*Animation edited by G. Leung, Original from Mitsubushi Corp
Case 1: Internal combustion(IC) engine
Four strokes Intake CompressionExpansionExhalation
HCCI engineLow NOx emission and high efficiencyControl problem
Types of IC engineDiesel engineSpark ignition(SI) engine
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Case 1: How does in-cylinder flow looks like?Full cycle LES of turbulent flow inside a typical diesel engine with curved-bowl shaped piston
Complicated flow, inhomogeneous mixing9/27/20168
Case 1: In-cylinder turbulent flow and T fieldPiston with a squared bowl
C
n
VR
=Vm+VRV
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Case 1: Some ways of engine-combustion
No spark
Strong turbulence Weak turbulence
Add spark(combining flame propagation + igntion)
_______________________________________
Homogenously charged compression ignition (HCCI)
Sensitive to T fluctuation
Engine combustion inside engine starts from an inhomogeneity in Temperature field and fuel/air concentration
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Case 1: What is HCCI combustion?2D-DNS of H2/Air auto-ignition in a small square (periodic) domain
9/27/2016Initial condition: Averaged T: 1070K; T rms fluctuation: 50K Pressure : 41 bar
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Case 1: What is HCCI combustion?3D-DNS of Lean H2/Air auto-ignition in a small cubic (periodic) domain
Yu. R et al, Comb. Flame, 2013
Domain size: 53 mm; Result plotting durations: 2 - 3 ms.Initial condition: Averaged T: 1070K; T r.m.s. fluctuation: 50K Pressure: 41 bar
ObservationsMultiple ignition kernels (initial hot spots)Two combustion modes
Slow deflagration waveRapid auto-ignition mode
Complicate topological surface for the reaction front
Saddle surfacesSpherical surfaces
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Case 1: Data analysis (Displacement speed: Sd)
Yu. R et al, Comb. Flame, 2013
= − Diffusion( )+ Reaction( ) ∗
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Case 1: Data analysis (Displacement speed: Sd)= − Diffusion( )+ Reaction( ) ∗
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Case 1: Data analysis Surface topology and its contribution to combustion process
Saddle surface
Spherical surface
Cylindrical surface
1: Both strong saddle and spherical fronts occupy small fraction of total flame area, however they contributes more to the flame stretch, due to the associcated larger Sd.2: In early stage small spherical fronts contribute to a large fraction of total flame stretch, saddles surfaces changes from convex to concave quite early, they are reasonable for reduction of flame front area in late stage of combustion process.
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Case 2: DNS of turbulent, statistical planar, premixed flame
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Case 2: More distributed layer of radical species with increased turbulence intensity
YH*
YHCO*
YHO2*
Ka = 65 Ka = 125 Ka =550 Ka =900 Ka = 3350 9/27/201617
Case 2: DNS of turbulent premixed combustionAt Intensive turbulence (Ka =3350) , two different fuel/air reactants
H. Carlsson, R.Yu, X. Bai, Int. J. Hydrogen ,2014H. Carlsson, R.Yu, X. Bai, Proceeding of combustion Institute, 2015.
CH4/air H2/air
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Case 2: Data analysisObservations: High heat release rate at low temperature region
Non-conventional chemical pathway!Caused by turbulent transport of the most-diffusive H radical !Elementary reaction R(9) is greatly enhanced:
H+O2+M HO2+M
Heat release contributed from R(9) conditioned at T<700K
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Case 3: Intrinsic flame instabilityDarrieus-Landau (DL) instability (caused by density difference)
Cusp shapes or cellular flames shapes,developed with no (or weak) turbulence
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Case 3: Simulations of LD instability in periodic channel
Increasing channel width
Curved flame, but stationary.
Non-stationary flamecusp formation
Fractal flame structure.
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Case 3: Data analysis-Fractal flame
Fractal cascadingstructures:
(1) 4-6 (intermediate) cells on a (largest) cell
(2) 4-7 (minor) cells on a (intermedia) cell
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Case 3: Data analysisThe Fractal flame cascading concept
Koch snowflake
1
4/3
(4/3)2
Gostintsev, et al. 1988
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Case 3: Data analysis: Fractal dimension (1+d)power-law fitting of the velocity amplification (Uw)
Power law fitting of Uw(Propagation speed after reaching statiscal staionary)~≈ 0.65 − 0.75≈ . − .
No analytical theory aviable for large channel width9/27/201624
Case 3: Data analysis: Fractal dimensions (1+d’)Box-counting of fractal images
Box counting≈ 0.07 − 0.1249/27/201625
Thanks for you attention
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