Application of LES to CFD simulation of Diesel combustion

1. Application of LES to CFD simulation of Diesel combustion 3604A058-2 Fumio KUWABARA

2. Background Diesel Combustion Internal conditions Turbulent flow etc. CFD code Prediction Method RANS Now LES Future ? Calculation Results Process Ignition, Combustion, products

3. Key aspects of turbulence • Unsteady, aperiodic motion • Turbulence is characterizedby eddies or instabilities • Largest eddies are the same scale as the flow and are often anisotropic • Smaller eddies form off the larger eddies and become more isotropic at smaller scales

4. What is Eddy? Small Eddies • Large eddies: anisotropic • Large eddies extract energy from the flow • Large eddies are and carry most of the turbulent energy • Directly affecting the mean fields • Small eddies: isotropic • Smaller eddies extract energy from larger eddies • The smaller scales act mainly as a sink for the turbulent energy Large Eddies

5. What is Turbulence Model? Turbulence Simulation resolved flow turbulent flow Turbulence Model not resolved flow Operation: Separate the flow field

6. Turbulence Simulation • Direct Numerical Simulation (DNS) • Resolves the whole spectrum of scales • No modeling is required • Large Eddy Simulation (LES) • Large eddies are directly resolved • Smaller eddies are modeled • Reynolds -Averaged Numerical Simulation （RANS） • Solves “averaged” Navier-Stokes equations • The most widely used approach for industrial flows

7. Turbulence Simulation （comparison） Reynolds -Averaged Numerical Simulation More Computational Effort & Precision Large Eddy Simulation More useful Direct Numerical Simulation

8. Navier - Stokes Equations Navier - Stokes Equations for an incompressible fluid: Unsteady Advection Pressure Viscosity

9. RANS : What is RANS? fluctuating parts mean Time Decompose velocity into mean and fluctuating parts: Reynolds -Average RANS doesn’t resolve any scales of turbulence at all !

10. RANS : RANS equation Reynolds -Averaged Navier -Stokes Equations Additional term Reynolds stresses Closure Problem Turbulence Model

11. RANS : Eddy viscosity model RANS equations require closure for Reynolds stresses: Mean velocity Turbulent Viscosity: Dissipation Rate of Turbulent Kinetic Energy: Turbulent Kinetic Energy:

12. RANS : k-εmodel Turbulent viscosity is determined from Transport equations for turbulent kinetic energy and dissipation rate are solved so that turbulent viscosity can be computed for RANS equations. k equation  equation empirical constants

13. RANS : Result Before After

14. LES : What is LES? This technique resolves the largest scales of turbulence and models the smaller scales. important Large eddies directly resolved turbulent flow not so important Small eddies modeled Spatial filter

15. LES : Spatial filter • Select a spatial filter function G • Define the resolved-scale (large-eddy): • Find the unresolved-scale (small-eddy ): GridScale SubGridScale All Scale

16. LES : LES equation The Filtered Equations Additional term Subgrid Scale （SGS）Stress SGS Closure Problem Smagorinsky model

17. LES : Smagorinsky model LES equations require closure for SGS stresses. SGS eddy Viscosity empirical constants （theory value） need for adjustment to turbulent flow !

18. LES : Result Before After

19. A Study of application of LES About Nishiwaki’s Study Table 1 Calculation conditions SGS model Cs =0.2 Fuel : isooctane Fig. 1 Computational grid system Reactions:29, Chemical species20

20. Results Temp. RANS LES RHR Fig. 2 Fields of Temp and RHR at TDC calculated by RANS（Left） ,LES（Right）

21. Criticism • RANSモデルでは捕らることができない自着火空間分布を予測できる可能性がある． • モデル定数の補正が必要となるスマゴリンスキーモデルを導入しているため，モデルの変更を考える必要がある． • LESでは，噴流の濃度・空間的変化について把握することが重要．

22. Future prospect on LES • エンジン内流れのサイクル平均ではない非定常流れとして直接解析できる．そのため，ノッキングなどのサイクル変動に起因する現象メカニズムの解明につながる． • 乱流中の噴霧，燃焼過程を普遍性のある物理モデルで表すことができる．流れパターンなどに一貫したモデルを使用することで，新しい機構･代替燃料の導入に際しても適用可能． • NOX ，すすなどの微量有害物質の生成予測に対しては，瞬時・局所の温度（濃度）分布の予測が可能．

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