Characterization of differential diffusion effects during the constant volume ignition of a temperature stratified lean premixed hydrogen/air mixture subject to decaying turbulence F. Bisetti 1 , J.-Y. Chen 1 , J. H. Chen 2 and E. R. Hawkes 3 1 Dept. Mechanical Engineering, University of California at Berkeley 2 Combustion Research Facility, Sandia National Laboratory 3 School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Australia fbisetti@me.berkeley.edu http://firebrand.me.berkeley.edu October 16, 2007 October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 1 / 27
Outline Hydrogen and HCCI ignition 1 Basic definitions of differential diffusion 2 Transport equation Differential diffusion characteristics 3 Effect of differential diffusion on macro combustion characteristics The mechanisms of differential diffusion Statistics October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 2 / 27
Outline Hydrogen and HCCI ignition 1 Basic definitions of differential diffusion 2 Transport equation Differential diffusion characteristics 3 Effect of differential diffusion on macro combustion characteristics The mechanisms of differential diffusion Statistics October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 3 / 27
Hydrogen combustion Environmentally friendly . CO 2 free nature and excellent combustion characteristics Combustion . Low minimum ignition energy, high laminar flame speed and wide flammability limits HCCI . Used as primary fuel (0 . 1 < Φ < 0 . 3) or as stabilizing additive (to natural gas, alcohols, etc.) from onboard reforming. but... ✞ ☎ H 2 displays high diffusivity ⇒ differential diffusion ✝ ✆ Effect on macro characteristics? E.g. heat release rate, turbulent flame speed and burning rate October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 4 / 27
Temperature stratification and differential diffusion Effects of temperature stratification on HCCI ignition - Chen et al., 2006 Stratification promotes a smooth pressure rise High stratification (T RMS = 30 K) promotes deflagrations alongside with spontaneous ignition Differential diffusion effects in HCCI ignition Effects on burn . How does differential diffusion influence heat release rate? Controlling parameters . How is differential diffusion affected by (temperature) inhomogenities? Mechanism . How do the flow conditions promote differential diffusion? October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 5 / 27
Outline Hydrogen and HCCI ignition 1 Basic definitions of differential diffusion 2 Transport equation Differential diffusion characteristics 3 Effect of differential diffusion on macro combustion characteristics The mechanisms of differential diffusion Statistics October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 6 / 27
Transport equation for ξ H H-atom mixture fraction ξ H = Y H 2 + Y H + β OH Y OH + β H 2 O Y H 2 O + β HO 2 Y HO 2 + β H 2 O 2 Y H 2 O 2 Transport equation D ξ H = D th ∇ 2 ξ H + D th δ H 2 O ∇ 2 Y H 2 O Dt Le H 2 � � δ H ∇ 2 Y H + δ OH ∇ 2 Y OH + D th + D th δ H 2 O 2 ∇ 2 Y H 2 O 2 + D th δ HO 2 ∇ 2 Y HO 2 = ( D th / Le H 2 ) ∇ 2 ξ H + Ω p + Ω r � �� � � �� � Term I Term II October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 7 / 27
Outline Hydrogen and HCCI ignition 1 Basic definitions of differential diffusion 2 Transport equation Differential diffusion characteristics 3 Effect of differential diffusion on macro combustion characteristics The mechanisms of differential diffusion Statistics October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 8 / 27
Effect on heat release rate 0.4 1.4 15 K 3.75 K 15 K, Le = 1 15 K 30 K 0.3 30 K 1.3 30 K, Le = 1 " H,max / " 0 Q/Q max 0.2 1.2 0.1 1.1 0 1 0 0.2 0.4 0.6 0.8 1 0 0.5 1 1.5 t/ τ 0 t/ ! 0 ✞ ☎ Differential diffusion increases with temperature stratification ✝ ✆ ✞ ☎ Heat release rate is mildly affected by differential diffusion ✝ ✆ October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 9 / 27
T r . m . s . = 30 K temperature stratification October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 10 / 27
Profiles across igniting kernels (T r . m . s . = 30 K) 20 1.2 1400 1.2 ξ H / ξ 0 T Term I 15 ξ H / ξ 0 Term II 1350 1.15 1.15 Term I, Term II (1/s) 10 1300 1.1 1.1 5 1250 ξ H / ξ 0 T (K) ξ H / ξ 0 0 1200 1.05 1.05 -5 1150 1 1 -10 1100 1050 0.95 -15 0.95 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.2 0.4 0.6 0.8 1 1.2 1.4 x/ δ F x/ δ F D ξ H = ( D th / Le H 2 ) ∇ 2 ξ H + Ω p + Ω r Dt � �� � � �� � Term I Term II ✞ ☎ Igniting kernels draw excess ξ H due to reactions ✝ ✆ October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 11 / 27
Temperature and ξ H (T r . m . s . = 30 K) Snapshot at 50% H.R. Along combustion fronts 1400 S d /S L < 1.1 1500 S d /S L > 1.1 1350 Conditional mean (c) 1400 (b) T (K) T (K) 1300 1300 (a) 1200 1250 1100 1200 1000 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 0.7 0.8 0.9 1 1.1 ξ H / ξ 0 ξ H / ξ 0 ✞ ☎ Temperature correlates with mixture fraction ✝ ✆ ✞ ☎ Stronger correlation holds along fronts ✝ ✆ October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 12 / 27
Curvature effects (T r . m . s . = 30 K) H.R.R. along fronts (Le � = 1) 60 n S d /S L < 1.1 burnt 50 S d /S L > 1.1 div(n) < 0 − H.R.R. 10 9 (W/m 3 ) Conditional mean 40 unburnt 30 n 20 div(n) > 0 10 0 −10 −5 0 5 10 δ F ∇ ⋅ n κ = div ( n ) = ∇ · n H.R.R. along fronts (Le = 1) 60 ρ |∇ Y i |−∇ · ( ρ D i ∇ Y i ) ω i ˙ S d /S L < 1.1 S d = − 50 S d /S L > 1.1 ρ |∇ Y i | − H.R.R. 10 9 (W/m 3 ) Conditional mean 40 ☛ ✟ 30 Diff-diff increases H.R.R. 20 in regions of ∇ · n < 0 10 ✡ ✠ 0 −10 −5 0 5 10 δ F ∇ ⋅ n October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 13 / 27
Summary 1 Diff-diff is most prevalent for large temperature stratification 2 It develops early in the combustion process at igniting kernels 3 Diff-diff enhances H.R.R. at negatively curved deflagration fronts 4 Diff-diff doesn’t have an effect on spontaneous ignition fronts 5 Overall the heat release rate is mildly affected ( ≈ 10%) October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 14 / 27
Thanks for your attention! Any questions? October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 15 / 27
Homogeneous Charge Compression Ignition (HCCI) Homogeneous Charge Compression Ignition engine technology Thermodynamic efficiency higher then spark ignited (high compression ratios) Ultra-low NO x : 2-25 ppm and 0.04 g/kWh (compare to 8-18 g/kWh for Diesel) Virtually non-existent particulate matter October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 16 / 27
How do we conceptualize (model) HCCI ignition? Lumped model In reality. . . Charge is homogeneous Composition and temperature fields are Reaction rates control heat inhomogeneous release rate, not transport Level of turbulence mixing affects ignition T(x) high reaction rate T > Tc Tc t+ � t t T < Tc low reaction rate x, position ✞ ☎ Coupling between transport and chemistry? ✝ ✆ October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 17 / 27
Inhomogeneities and modeling Two regimes . Spontaneous ignition and deflagration fronts (dependin on level of stratification) Engine regimes . thin reaction zones and corrugated flamelets ⇒ turbulence effects on the preheat zone Hydrogen combustion . Lewis number effects and differential diffusion ⇒ area increase vs. flame speed effects ✞ ☎ Accounting for inhomogenities is challenging ✝ ✆ October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 18 / 27
DNS captured the effect of temperature stratification Major conclusions 1 Temperature inhomogeneities (stratification) create a smooth pressure rise 2 High stratification (T RMS = 30 K) promotes deflagrations alongside with spontaneous ignition 3 Stratification can locally switch the regime from spontaneous ignition to flame propagation Figures from: Chen, Hawkes, Sankaran, Mason, Im, in Combustion and Flame (145) 2006 pp. 128-144 October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 19 / 27
Direct Numerical Simulation to understand ignition Direct Numerical Simulation (2D) Fuel H 2 performed by J. Chen, E. Hawkes and Oxidizer Air Φ 0.1 others (Sandia NL). Compression Ratio 15:1 Initial pressure, atm 1:41 Initial temperature, K 400:1070 Ignition of extremely lean mixture of ∆ x DNS , mm 4.1 H 2 /Air with temperature L, mm 1.0 U, m/s 0.5 stratifications Re L O (100) T RMS , K 3.75, 15, 30 Goal. Parametric study aimed at clarifying ignition mechanism October 16, 2007 Fabrizio Bisetti (UC Berkeley) Differential diffusion 20 / 27
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