1. 1 KNOO Annual Meeting 2009 CFD analysis of Fuel Rod Bundles S. Rolfo D. Laurence School of Mechanical, Aerospace & Civil Engineering (MACE) The University of Manchester Manchester, M60 1QD cfd.mace.manchester.ac.uk/Main/StefanoRolfo The University of Manchester

2. 2 Summary  Introduction  Scale separation and turbulence modelling  Code_Saturne  Flow in rod bundles arranged into triangular array: refined LES  A proposal for next generation IV reactors: Sodium Fast Reactor (SFR) fuel assembly  Conclusions

3. 3 DNS LES RANS Scales separation and levels of approximation. (low Reynolds Number) (low Re if wall resolved) (usual Eng. solution)

4. 4 Code_Saturne CFD Code developed by EdF, open source & downloadable from: www.code-saturne.org  Unstructured, Co-located, Finite Volume Formulation  Range of turbulence models (RANS and LES), in collab. with U o Man  Incompressible or expandable flows with or without heat transfer  Different modules: radiative heat transfer, combustion, magneto- hydrodynamics, compressible flows, two-phase flows, ALE methods.  Possible coupling with Code_Aster for FE structural analysis and with Syrthes for thermal analysis.  Parallel code coupling capabilities (awarded the Gold medal on HPCx) ‏  TWiki collaborative website with forum and developments: http://cfd.mace.manchester.ac.uk/twiki/bin/view/Saturne/WebHome

5. 5 Rod Bundle arranged in a triangular array Large computational Domain  Geometrical configuration:  P/D = 1.06  Mesh size ~ 7 mil cells  Thermal Hydraulic regime  Re = 6 000  Heat transfer (q w =60 W/m2)  Cases on-going  Re = 6 000 with 7 different scalars (varying Prandtl number and temperature BC )  Re=12 000 (~14 mil cells) Small computational Domain (prev runs)  P/D = 1.06 & 1.15 @ Re = 6 000  Mesh size ~ 1.8 mil.

6. 6 Power spectra in the gap region

7. 7 Statistical Results (time & streamwise averaging)

8. 8 8 SFR fuel assembly: Case presentation Flow parameters: P/D = 1.1 Re = 11000 (Bulk vel = 1 m/s) Working fluid liquid sodium  = 847 kg/m3 µ = 2.55 10 -4 Kg/m/s Pr = 5 10 -3

9. 9 SFR test case

10. 10 SFR test case: Eddy visc. vs. Re stress tensor transport Axial Vel k-  Axial Vel Rij k-  Rij Cross Flow Cross Flow T/Tb kk

11. 11 Toward high Reynolds number: Hybrid RANS/LES A solution to reduce the mesh constrain for LES is to hybridize with RANS. The model consists in performing RANS in the near wall region and LES far from it The model and the way in which the two velocity fields are coupled is presented in: S. Rolfo, J. C. Uribe, D. Laurence, “LES and Hybrid RANS/LES of turbulent flow in fuel rod bundle in a triangular array”, to appears in Quality and Reliability of LES - Trieste Sept 2008., (2008)

12. 12 Conclusions  SFR fuel assembly  Preliminary RANS calculations shows a large effect of the spiral wire on velocity & temperature field inhomogeneties in cross section.  Re stress model predicts larger differences between sub-channels whereas k-  model gives an underestimation of the maximum axial velocity and secondary motion of about 20%.  Need to run LES or Hybrid RANS/LES calculations to arbitrate.  Rod bundle (smooth configuration)  Flow Fluctuations in the mid plane of the geometry were successfully detected for the configuration characterised by P/D = 1.06. However P/D = 1.15 did not show any significant behaviour from channel or pipe flows. Dominant frequency is in according with the experiments. However there are extra higher frequencies that might be due to the low Re tested. Higher Re calculations are on going. Acknowledgements : This work was carried out as part of the ‘Towards a Sustainable Energy Economy’ (TSEC) programme ‘Keeping the Nuclear Option Open’ (KNOO) and as such we are grateful to the UK Engineering and Physical Sciences Research Council for funding under grant EP/C549465/1. Many thanks to C. Peniguel for help on the SFR test case and EDF R&D for use of their HPC facilities (BlueGene)