Recent enhancement to SI-ICE combustion models: Application to stratified combustion under large EGR rate and lean burn

Transcription

1 Recent enhancement to SI-ICE combustion models: Application to stratified combustion under large EGR rate and lean burn G. Desoutter, A. Desportes, J. Hira, D. Abouri, K.Oberhumer, M. Zellat*

2 TOPICS Introduction Challenging for stratified combustion under large EGR rate Lean burn Point out some weaknesses Present state of STAR-CD models predictions and measures to improve them. Application and validation to SI-GDI Conclusion and perspectives

3 Introduction : Schematic of Premixed Regimes Flame brush is thickened Flame structure changes High EGR Lean Mixture

4 To what extent does turbulent flame retain laminar flame structure: The Turbulent Energy Spectrum as a function of Wavelength Corrugated & Wrinkled Flamelets - Flame still retains laminar flame structure - Wrinkled or corrugated by turbulent eddies Broken Reaction Zones - Kolmogorov is now smaller than reaction sheet thickness - Flame now lacks local structure (extinguishes) Kolmogorov scale would require very fine mesh to resolve So sub-grid modelling needed

5 ECFM-CLEH MODEL : FLAME SURFACE DENSITY (FSD)* Ensemble averaged flamlet Ability of small turbulent eddies to interact with the flame front ITNFS function (Intermittent Turbulent Net Flame Stretch) Intermittent stretch function due to strain and curvature Flame wall interaction Consumption to flame propagation Change due to gas compression/expansion Change due to flame expansion Initiation due to ignition (spark or knock) * Though an exact transport equation for the FSD can be derived from the Navier-Stokes equations, the above terms are modelled.

6 Laminar flame speed Temperature = 800 K ; Pressure = 10 bar : Metghalchi without extension Metghalchi with extension Table from detailed chemistry Laminar flame speed decreases around stoichiometry

7 The laminar flame speed : EGR effect Experimental data : linear effect of EGR Level on Laminar Flame speed Laminar Flame Speed EGR Level The data above is are stoichiometric mixture : Is this linearity conserved for lean or rich mixture??

8 Laminar flame speed (LFS) : EGR Effect Comparison between Linear correction and detailed chemistry LFS (EGR) versus LFS(EGR=0) Ratio factor using linear correction Results from detailed chemistry ( freely propagating flame using detailed mechanism ) Mixture Equivalence Ratio Around Stoichiometry : Linear correlation fit with detailed chemistry BUT NOT below and above

9 Libraries Generation using DARS

10 Minimum energy for Ignition at SPARK Spark energy for ignition mj Laminar flame speed decrease Laminar flame thickness increase Require more energy for Ignition Stoichiometry Equivalence ratio

11 Breakdown Voltage Breakdown voltage for different mixture o Taken into account o Equivalence ratio mixture o Residual gases composition and concentration

12 Spark formation and burnt gas deposit Ignition v ie (t Initialization ) (Inter-electrode voltage) Breakdown! Ignition Breakdown phase Spark phase Glow phase Compute the current in the secondary circuit Compute the inter-electrode voltage Electrical Circuit V Breakdown spk V bd a spark is formed (l spk ) The Energy of the spark is transferred to the gas (E ign ) Initialize progress variable profile c ign, ign Impose burnt gas mass at the spark plug m in bg ( l ) spk If E ign Ignition is successful E crit Growth of flame kernel in the modified -equation

13 Application I : High EGR rate Spark Ignited Direct Injection EGR variation by Exhaust Valve phasing change 40% 20% 10% Side injector Air and Wall guided Increasing EGR (trapped Residual Burn Gases) Operating condition : 1300 rpm Low Load

14 Residual Burn Gases and Equivalence Ratio contour around Spark timing Residual Burnt Gas (%) Low EGR Residual Burnt Gas (%) Medium EGR Residual Burnt Gas (%) High EGR Equivalence Ratio Low EGR Equivalence Ratio Medium EGR Equivalence Ratio High EGR

15 Voltage at spark: Energy transferred to the gas. Voltage at spark Energy transferred to the gas ~0.070 Joules available in the secondary circuit Spark timing Ignition succeed

16 In-cylinder pressure history and apparent rate of heat release Dashed lines :measurement Continuous lines : prediction

17 BURN MASS FRACTION HISTORY Early stage of combustion: 0-10% Duration

18 SUMMARY Stratified combustion under high EGR level have been investigated Homogeneous LEAN BURN combustion have been investigated Laminar Flame speed is a key factor for these extreme conditions Advanced spark model : Breakdown voltage reviewed and enhanced Application and validation to SI-GDI (homogeneous ) and PFI engines Good match for global thermodynamic quantities Good match for combustion history Good match for combustion history (0-5% and 10% burn) NOx prediction match well (not presented here) Work in progress for emissions (CO and UHC)