1 DLES, Ercoftac Workshop, Trieste, 8-10 September 2008 LES of turbulent flow through a heated rod bundle arranged in a triangular array. S. Rolfo, J C Uribe and D. Laurence The University of Manchester, M60 1QD, UK School of Mechanical, Aerospace & Civil Engineering. CFD group The University of Manchester

2 Summary Introduction: Code_Saturne RANS/LES coupling Rod Bundle test case Results: Flow pulsations Average field Conclusions

3 Code_Saturne Code developed by EdF and now open source downloadable from:  Unstructured  Co-located, Finite Volume Formulation  Different turbulent models (both RANS and LES)  Incompressible or expandable flows with or without heat transfer  Different modules: radiative heat transfer, combustion, magneto- hydrodynamics, compressible flows, two-phase flows  Possible coupling with Code_Aster for structural analysis and with Syrthes for thermal analysis.  Parallel code coupling capabilities (awarded the Gold medal on HPCx) ‏

4 RANS-LES coupling (1)‏ The Hybrid RANS-LES method is following a usual LES decomposition in large scale and sub-grid part: The anisotropic part of the residual stress tensor and residual heat flux can be decomposed following a Schumann decomposition: Sub- grid viscosity RANS viscosity computed from the mean velocity field For the eddy conductivity a simply turbulent Prandtl number analogy is used

5 RANS-LES coupling (2)‏ The merging between the two velocity fields is done through a blending function to obtain a smooth transition Turbulent RANS length scale computed with a relaxation model based on Filter width Empirical constants computed in order to match the stress profile for channel Re = 395 Under-resolved standard LES Hybrid results with mesh Size ~ 0.1 mil cell

6 Rod Bundle arranged in a triangular array Experimental work: I.Secondary flow : Vonka “Measurement of secondary vortices in Rod Bundle” Nuclear Engineering and Design, Volume 106, Issue 2, 2 February 1988, Pages II.Flow pulsations: Krauss, Meyer “ Experimental investigation of turbulent transport of momentum and energy in a heated rod bundle”, Nuclear Engineering and Design, Volume 180, Issue 3, April 1998, Pages CFD work (URANS): 1.E. Merzari, H. Ninokata, E. Baglietto “Numerical simulation of flows in tight-lattice fuel bundles Nuclear Engineering and Design”, Volume 238, Issue 7, July 2008, Pages D. Chang, S. Tavoularis “Simulations of turbulence, heat transfer and mixing across narrow gaps between rod-bundle sub-channels” Nuclear Engineering and Design, Volume 238, Issue 1, January 2008, Pages From II

7 Rod Bundle: Cases definition Two different geometrical configuration 1. P/D = 1.06 Re = 6000 Mesh size ~ 1.6 Million cells with Heat transfer. – 0.75<Δr+<1.06 – 7.5<rΔ θ +<11 – 16<Δx+<22.5 Re = 6000 Mesh size ~ 0.35 Million cells with Heat transfer – 0.8<Δr+<1.3 – 15<rΔθ+<20 – 40<Δx+<60 Re = Mesh size ~ 0.9 Million cells no Heat transfer – 0.8<Δr+<1.2 – 20<rΔθ+<25 – 50<Δx+<70 2. P/D = 1.15 Re = 6000 Mesh size ~ 1.4 Million cells no Heat transfer – 0.8<Δr+<1.1 – 6.5<rΔθ+<10 – 16<Δx+<22.5 All the domains have a stream-wise length equal to 12 times the hydraulic diameter

8 Results: Flow Pulsation in the mid-plane Re = 6000 LES Re = 6000 Re = 6000

9 Flow Pulsation in the mid-plane LES P/D =1.06 LES P/D =1.06 LES P/D =1.15 Hybrid P/D =1.06

10 Results: Power spectra angular velocity P/D=1.06 Point A

11 Results: Power spectra angular velocity P/D=1.15 Point A

12 Results: Power spectra angular velocity P/D=1.06 (Hybrid)‏ Point A

13 Results: Power spectra angular velocity P/D=1.06 Re =39000)‏ Point A

14 Results: Average Field (LES P/D = 1.06 Re = 6000) /Ub <uθ><uθ>

15 Results: Reynolds Stress LES P/D=1.06

16 Results: Comparison average values LES/Hybrid

17 Results: Comparison Re stress Re=6000

18 Conclusions and future work  LES  Flow Fluctuations detected,  Presence of a second dominant frequency has to be verified with bigger domain.  Hybrid  Flow pulsations detected and dominant frequency in according with LES  Problem in the near wall region. Possible reason blending function  Same problem for the heat transfer  Future work  Calibration of the wall function for the Rod Bundle (in progress)  Improvement heat transfer with a dynamic turbulent Prandtl number  LES computations with bigger cross section to verify the dominant frequencies  Improved geometry with addition of wire wrapped around the pin (SFR reactor). Acknowledgements This work was carried out as part of the TSEC programme KNOO and as such we are grateful to the EPSRC for funding under grant EP/C549465/1.

19 Power spectra angular velocity P/D=1.06 Point B

20 Reynolds Stress Hybrid P/D=1.06 Re=5800

21 Average value Hybrid LES viscosity Blending function RANS viscosity

22 Average value Hybrid (Heat transfer Average temperature

23 Reynolds Stress Hybrid P/D=1.06 Re=39000