Computational Investigation of Vertically Heated Pipes using the Large-Eddy Simulations Approach Yacine Addad (PDRA) (Supervisors: Dominique Laurence & Mark Cotton) School of Mechanical, Aerospace & Civil Engineering (MACE) The University of Manchester 7th Int. ERCOFTAC Symposium-ETMM June 2008,Limassol, Cyprus

- EPS Research Council project, 6M £, 50 PhDs and postdocs, - to maintain and develop skills relevant to nuclear power generation. - largest commitment to fission reactor research in UK for over 30 years, - led by Imperial College, collaborations with industrial and governmental stakeholders and international partners.

AGR working scheme Relevance to AGR and VHTR

V gradient away from wall => Turbulence increase V gradient nearer wall => Turbulence decrease buoyancy aiding buoyancy opposing Buoyancy aiding or opposing vertical pipe flow

Nu/Nu0 against ‘buoyancy parameter’, [Hall and Jackson ]

“standard” STAR k-epsilon model (Lien Chen Leschziner)

Test Case Description Re=180 based on R and u . Boussinesq Approximation. Periodic Flow. Lz=30R Grid1: 1.61 million Grid2: 4.83 million Grid1 Resolution:  r + min =1.0,  + =6.28,  z + =18. Grid2 Resolution:  r + min =1.0,  +=6.28,  z+=7.03. Buoyancy effects on turbulence in VHTR channels: WR-LES

Simulation  r + min r  + z+z+ NcellsLz DNS Ref. [1] ,145,7005D DNS Ref. [2] ,456,44815D LES 1.61 mil ,612,80015D LES 4.83 mil ,838,40015D [1] Isothermal pipe flow DNS: F. Unger and R. Friedrich [2] Buoyant DNS: J. You, Jung Y. Yoo, H. Choi, Int. J. Heat Mass Transfer (no electronic data available other than scanned publication data) Buoyancy effects on turbulence in VHTR channels: WR-LES Grid resolution

Nusselt number as a function of the buoyancy parameter from You et al. For RANS models predictions, see the paper by Amir Keshmiri et al. Ratio of SGS to molecular viscosity

Results for Fully-Developed Forced Convection. (no buoyancy) Expt. of Polyakov & Shindin

Case1: Forced convection Gr/Re 2 =0.0 Buoyancy effects on turbulence in VHTR channels: WR-LES Data normalized by the bulk velocity

Cases from Gr/Re 2 =0.063 to Gr/Re 2 =0.241 Buoyancy effects on turbulence in VHTR channels: WR-LES Data normalized by the bulk velocity

Buoyancy effects on turbulence in VHTR channels: WR-LES Variation of the normal stress Cases from Gr/Re 2 =0.0 to Gr/Re 2 =0.241

Refined Large Eddy Simulation for Reactor Thermal-Hydraulics Studies The present results serve as confirmation of the DNS data of You et al. and as a reference data for RANS models predictions and validation. They also show that the mixed convection ‘drop’ in Nusselt number is even more sudden than the DNS points would suggest. Confirm that using fine grids allows for ‘Quasi-DNS’ calculations with an unstructured finite volume code, even under severe conditions of laminarized flows. Use the LES approach for the design of new generation reactors: Higher temperatures, passive safety (buoyancy), fluctuations … Conclusions Acknowledgements: TSEC programme 'KNOO' EPSRC funding under grant EP/C549465/1 for PDRA and Computer resources