Numerical simulation study of turbulent combustion phenomena -INTEGRATE Advanced Study Group (ASG)

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1 Numerical simulation study of turbulent combustion phenomena -INTEGRATE Advanced Study Group (ASG) Rixin Yu Division of fluid Mechanics Department of Energy Science LTH 1

2 Introduction Some combustion related keywords Energy, heat and power, transportation. Emission, clean combustion, global warming. Fossil fuel, Hydrogen, biomass Combustion efficiency, combustion instability Turbulence combustion 2

3 Introduction To understand turbulent reacting system, a challenge Turbulence + Chemistry Wide range of scales Da Vinci s sketch 3

4 Research methodology Mathematical equations describing turbulent reacting flow Flow turbulence Naiver-Stokes equations(conservation of mass, momentum, energy), assuming low-mach number. Chemistry reaction Conservation of N species mass o Y k, k=1,..n (Y H, Y O2, Y CH3, ) Reactions for CH4/Air H+O2 O+OH O+H2 OH+H H2+OH H2O+H OH+OH H2O+O H+O2+M HO2+M H+HO2 OH+OH H+HO2 H2+O2 OH+HO2 H2O+O2 CO+OH CO2+H CH4+M CH3+H+M CH4+H CH3+H2 CH4+OH CH3+H2O CH3+O CH2O+H CH2O+H HCO+H2 CH2O+OH HCO+H2O HCO+H CO+H2 HCO+M CO+H+M CH3+O2 CH3O+O CH3O+H CH2O+H2 CH3O+M CH2O+H+M HO2+HO2 H2O2+O2 H2O2+M OH+OH+M H2O2+OH H2O+HO2 OH+H+M H2O+M H+H+M H2+M 4 Reaction rates ω k reactions is given by M elementary chemical

5 Research methodology Numerical 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 size loge(k) resolved LES integral 2π/l 0 modelled RANS resolved DNS -5/3 modelled LES Kolmogorov 2π/η 2π/δ log(k) 5

6 Data analysis : three case studies 1: DNS studies of homgenously charged compression ignition (HCCI) engine combustion. 2: DNS of turbulent premixed flame propagation 3: Fractal flame front structures due to flame (Landau-Darrieus) instability 6

7 Case 1: Internal combustion(ic) engine Four strokes Intake Compression Expansion Exhalation 7 *Animation edited by G. Leung, Original from Mitsubushi Corp HCCI engine Low NOx emission and high efficiency Control problem Types of IC engine Diesel engine Spark ignition(si) engine

8 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 8 Complicated flow, inhomogeneous mixing

9 Case 1: In-cylinder turbulent flow and T field Piston with a squared bowl n V R V =V m +V R 9 C

10 Case 1: Some ways of engine-combustion Engine combustion inside engine starts from an inhomogeneity in Temperature field and fuel/air concentration No spark Homogenously charged compression ignition (HCCI) Add spark (combining flame propagation + igntion) Sensitive to T fluctuation Strong turbulence Weak turbulence 10

11 Case 1: What is HCCI combustion? 2D-DNS of H 2 /Air auto-ignition in a small square (periodic) domain 11 Initial condition: Averaged T: 1070K; T rms fluctuation: 50K Pressure : 41 bar

12 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 Observations Multiple ignition kernels (initial hot spots) Two combustion modes Slow deflagration wave Rapid auto-ignition mode Complicate topological surface for the reaction front Saddle surfaces Spherical surfaces 12 Domain size: 5 3 mm; Result plotting durations: 2-3 ms. Initial condition: Averaged T: 1070K; T r.m.s. fluctuation: 50K Pressure: 41 bar

13 Case 1: Data analysis (Displacement speed: S d ) = Diffusion( )+ Reaction( ) 13 Yu. R et al, Comb. Flame, 2013

14 Case 1: Data analysis (Displacement speed: S d ) = Diffusion( )+ Reaction( ) 14

15 Spherical surface Case 1: Data analysis Surface topology and its contribution to combustion process Cylindrical surface Saddle surface 15 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 S d. 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.

16 Case 2: DNS of turbulent, statistical planar, premixed flame 16

17 Case 2: More distributed layer of radical species with increased turbulence intensity Y HO2 * Y HCO * Y H * 17 Ka = 65 Ka = 125 Ka =550 Ka =900 Ka = 3350

18 Case 2: DNS of turbulent premixed combustion At Intensive turbulence (Ka =3350), two different fuel/air reactants CH4/air H2/air H. Carlsson, R.Yu, X. Bai, Int. J. Hydrogen,2014 H. Carlsson, R.Yu, X. Bai, Proceeding of combustion Institute,

19 Case 2: Data analysis Observations: 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+O 2 +M HO 2 +M Heat release contributed from R(9) conditioned at T<700K 19

20 Case 3: Intrinsic flame instability Darrieus-Landau (DL) instability (caused by density difference) Cusp shapes or cellular flames shapes, developed with no (or weak) turbulence 20

21 Case 3: Simulations of LD instability in periodic channel Curved flame, but stationary. Increasing channel width Non-stationary flame cusp formation Fractal flame structure. 21

22 Case 3: Data analysis -Fractal flame Fractal cascading structures: (1) 4-6 (intermediate) cells on a (largest) cell (2) 4-7 (minor) cells on a (intermedia) cell 22

23 Case 3: Data analysis The Fractal flame cascading concept 1 4/3 (4/3) 2 Gostintsev, et al Koch snowflake 23

24 Case 3: Data analysis: Fractal dimension (1+d) power-law fitting of the velocity amplification (U w ) Power law fitting of U w (Propagation speed after reaching statiscal staionary) ~ No analytical theory aviable for large channel width

25 Case 3: Data analysis: Fractal dimensions (1+d ) Box-counting of fractal images Box counting

26 Thanks for you attention 26