Materials Performance Centre Seminars, 12/09/2006

1. Materials Performance Centre Seminars,12/09/2006 • Application of Large Eddy Simulation • to thermal-hydraulics • in the Nuclear Power Generation Industry. • Dominique.Laurence@manchester.ac.uk • School of Mech, Aero & Civil Eng. • Fluids AIG / CFD group • Dominique.Laurence@edf.fr • EDF R&D Chatou The University of Manchester Contributions: Y. Addad, I. Afgan, S. Benhamadouche, S. Berrouk, N Jarrin C. Moulinec, T. Pasutto, 1

2. Turbulence, Reynold Number Re = UD/visc. Low speed jet Osborne Reynolds 1868, became a professor of engineering at Owens College (now University of Manchester) Higher speed jet Reynolds tank, G. Begg building 2

3. Kolmogorov Energy Cascade Андре́й Никола́евич Колмого́ров Moscow State Uni. 1939 3

4. Direct Numerical Simulation (DNS) • Solve the standard (Navier Stokes) flow equation on a very fine mesh • All scales are resolved on that mesh (down to Kolmogorov scale) • No model needed • Results considered as valid as experimental data • Enormous computer resources, for even modest speed-domain size (Reynolds number) Nx=6144, Ny=633, Nz=4608 points = 17,921,212,416 cells 6 Million processor hours on 2048 processors, Barcelona Supercomputing Center Sergio Hoyas and Javier Jimenez, (2006) "Scaling of velocity fluctuations in turbulent channels up to Re_tau = 2000", Phys. of Fluids, vol 18, 4

5. Luckily, only Large Scales matter(most of the time) • Large Scales & Human activity • Drag, mixing, heat transfer, • Large Scales dictate flow physics • Generated by/scale with obstacle • Impose dissipation rate • Exceptions: noise, combustion, Weather forecast (we are the small scales !) 5

6. Large Eddy Simulation = Filtering Time Filter Space Filter CFD codes naturally induce filter D = 2 dx 6

7. DHIT: Decay of Hom. Iso. Turb. • MANDATORY test case for first time LES ! • Reveals numerical dissipation, stability, 7

8. 3 Levels of CFD approaches to turbulent flow • Direct Numerical Simulation (DNS) databases <=> Experiments • = “Costly”Fluid Dyn., exceptional, limited to zoom effect, • - 100% accurate, back to 1st principles, NO modelling hypothesis • Large Eddy Simulation (LES) • = “Colourful”Fluid Dyn., much detail, fluctuations, spectra, • - Applicable Eng. problems, at some cost • - Almost as reliable as DNS, but know-how required, not well established • Reynolds Averaged (RANS) • = “Conventional”Fluid Dyn., used daily in Eng., only mean values (B&W) • - Economical, full reactor or sub-component design (parametric) possible • - Problem: wide range of models to choose from, • - needs improvement & validation for new range of applications • (high temperature, buoyancy, conjugate heat transfer, ) • Future : Coupling of RANS and LES, using DNS for insight & validation 8

9. Industrial LES applications to reactor thermal hydraulics • LES is mostly about numerical methods • Grid able to capture most turbulent scales • Easier in Power Industry (confined, non-streamlined geometry) • Local/embedded grid refinement, polyhedral scales • Boundary conditions • Walls => quasi DNS (wall functions not ideal) • Some real periodic geometry. pb. in Power Gen (tube bundles) • Synthetic inlet turbulence • Target values • Order of Mag. (within 10%), not 0.01 on Cd • Thermal mixing & loading, spectra, vibrations…. • Need Fast Unstructured FV Solver • EDF code Saturne & Star-CD very similar • High Accuracy Numerical Scheme • No numerical dissipation ( Central differencing, Second order in time) • Avoid any mesh distortion 9

10. LES at EDF: Thermal stresses in T junction - Experimental mock-up (both thermalhydraulics and thermal fatigue mechanical aspects) - models and numerical tools to gain a better understanding. Qhot Qcold Configuration studied : Thot = 168°C , Tcold = 41°C Flow rate = 1000 m3/h, ratio 20% Length of the numerical simulation : 11 seconds 10

11. Fluid results Instantaneous fluid temperature field (Code_Saturne) (Peniguel et al. ASME-PVP Cleveland 2003) Shortcut to aaPVP_anim.mpg.lnk 11

12. Solid and fluid meshes Solid mesh : 958 975 nodeswidth of the first solid element : 100 microns Fluid mesh: 401 472 cells 12

13. Solid temperature fluctuations Instantaneous solid temperature field (Syrthes) (location C12) C12 Instantaneous fluid temperature Instantaneous solid temperature Time (s) 13

14. Temperature spectrum • - On site meas • L.E.S. • Mock-up CEX1 Frequency (Hz) Location CEX1 14

15. T Junction, stratified case • Fluid temperature and flow structure VH = 3,37 m/s TH = 204°C VC = 0,77 m/s TC = 41°C Temperature In the symmetry plane (case 1) C12 15 Temperature near the wall (case 1) Recirculation zone

17. Analysis of the results • Attenuation of the fluctuations by • the wall thermal inertia ( Ring C12 – 50 °) 17

18. Mesh too coarse for Re=1 Million • temperatures (ring C12) • fluid:probe located at 10° probe located at 50° • Solid: Exp LES 18 underestimation of the fluctations (especially at 10°)  limitation of the wall function approach ?

19. FAATER exp. : Meshes • Fluid meshes • 1st mesh : ~ 500 000 hexaedric cells y+~ 300 • 2nd mesh : ~ 1 000 000 hexaedric cells y+~ 170 (first node at 0,38 mm from the wall) (1 000 000 cell mesh) 19

20. Trust & Quality-CFD project: hot wall jet (Magnox case) Buoyancy: none medium high Velocity: high medium low Addad Y. , D. Laurence and S. Benhamadouche. The Negative Buoyant Wall Jet: LES Results, I.J. Heat and Fluid Flow, 25, 795-808, 2004 20

22. Grid generation (buoyant case) IntegralLength scale from k-epsilon • Pre- k-eps simulation • cell Volume near jet inlet (V)1/3=0.002 • y+=1 • In mixing region (V)1/3=0.007 • NCELLS=770 000 • StarCD code 22

23. Horizontal Velocity Comparison 23

24. LES DB => Analytic Wall Function development 24 (from A. Gerasimov)

25. Thermal hydraulics of reactors Mixed convection in co-axial pipes (Y. Addad PhD, M. Rabitt British Energy) Study the physics of the flow in the decay heat inlet pen Examine the LES solution of the code Star-CD for the natural/mixed convection cases. Validate further the analytical wall functions developed at UMIST by Gerasimov et al. 25

26. Cold Inlet Pipe in vessel => stratification trap Streamlines coloured by temperature K-eps pre-study 26

27. Coaxial heated cylinder study • LES validation and parametric test cases: • Case1-Natural convection in square cavity (Ra=1.58  109) • Case2-Natural convection in annular cavity (Ra=1.8109) • Case3- annular cavity single coaxial cylinder (Ra=2.381010) • Case4- annular cavity with 3 coaxial cylinders (Ra=2.381010) • Case5- Flow in actual penetration cavity (bulk Re=620,000). Bishop 88, McLeod 89 27

28. Natural Convection in coaxial cylinders CASE-3: Ra=2.3810E+10 Case 2: Ra=1.810E+9 SGS visc/Molecular visc.<1 CASE-4: Ra=2.3810E+10 28

30. 3 Cylinders 30

31. Flow through in-line tube bundles Objectives: Flow induced vibrations in heat exchangers (Lift & Drag coef.) Staggered: studied 10 years ago Current: in line Large heat exchanger => Homogeneous conditions => Periodic subset considered Mean pressure gradient Direction is in-line Re=45 000, P/D= 1.5 STAR CCM grid 31

32. In-line tube bundle • - Fully symmetric conditions, • but non-symmetric solutions • - Coanda effect ? • Star-CCM LES launched • to confirm EDF finding • (Benhamadouche et al. NURETH 11, Avignon 2005) Time averaged velocity field => 32 I.Afgan@postgrad.manchester.ac.uk, with STAR-CCM

33. In-line tube bundle P/D=1.5 mean pressure STAR CCM Mean velocity (Afgan) EDF Code-Saturne Mean velocity (Benhamadouche) 33

34. LES in a 180° U-Bend Pipe -Vortex Method for Inlet -Polyhedral Cells (670,000) -Re=57,400 -Y+=2 (Prisms) -L1=3D; L2=5D - Smagorinsky Model, Van Driest Damping 34

36. U-Bend Pipe Cross Sections: Mean flow • 45 degrees 135 degrees 177 degrees 90 degrees 36

37. Research: Reconstructing fluctuations Real Eddies in Channel flow Cost = 5 days computing Synthetic Eddies in Channel flow Cost = 5 seconds computing Matches all rms values and given spectrum KNOO project: Develop similar technique to reconstruct temperature fluctuations at solid wall Link with materials ageing research 37

38. Research: Mesh strategy for LES a) Possible FV near-wall refinements: a) dichotomy, b) non-conforming, c) & d) polyhedral & zoom. • Non dissipative Finite Volume Methods • Optimal meshing strategy for LES • Quality criteria for LES • General but essential issue. Collaboration with: • - CD – Adapaco (STAR-CD code) • - EDF R&D (Saturne code) • - Health & Safety Labs, CFD for Nuclear Reactor licensing ? 38

39. Research: RANS – LES coupling U Under-resolved LES RANS – LES coupling Wall distance LES RANS LES: ~ 0.1 Million cells J. Uribe, Manchester 39

40. Conclusions – Industrial LES • LES of Industrial flow • Much more information:Thermal stresses, fatigue, Acoustics, FIV (vibrations) • Cost-wise accessible when limited to subdomain • (synthetic turbulence for inlet) • Complex geometry possibly easier than smooth channel flow (academic overkill ?) • Flexible flexibility with professional/commercial software: • Opens new range of applications for LES • Medium Re number : DNS near wall resolution possible • Greater breakthrough than elaborate SGS models? • Further developments: • More meshing control (total cell size control from pre-simulation) • High Re : RANS –LES coupling, embedded LES • Cross-discipline research: Fluids / Structure-Mech/ Materials ? • Cracks, Thermal stripping, ageing, corrosion … 40