Thermal-Hydraulics Studies of Helium-Cooled Divertors

1. Thermal-Hydraulics Studies of Helium-Cooled Divertors M. Yoda, S. I. Abdel-Khalik, B. Zhao and S. A. Musa July 28, 2016

2. He-Cooled Divertors • “Divert” ~20% of energy from burning plasma away from first wall onto divertor target surfaces • Target surfaces subject to very high heat fluxes (>10 MW/m2 steady state; much higher transient values) • Helium-cooled (solid) tungsten-alloy divertors most developed divertor concept • Concepts studied by KIT (FZK), ARIES: HEMJ, HEMP, T-tube, HCFP, integrated plate-finger design • Issues with W surfaces exposed to plasma containing He: erosion/redeposition, nanobubbles, fuzz, D retention, … • Only divertor concept experimentally shown to remove >10 MW/m2 steady-state Fusion Matls Workshop (7/16)

3. Introduction • Aim • Evaluate thermal-hydraulic performance of leading He-cooled divertor designs at prototypical conditions using dynamically similar experiments at nearly prototypical conditions and numerical simulations • Research Objectives • Experimentally test modules that closely mimic current divertor designs at nearly prototypical conditions • Perform numerical simulations validated by experimental data • Estimate maximum heat flux and He pumping power requirements • Determine numerically whether divertor designs can be simplified and/or improved Fusion Matls Workshop (7/16)

4. He-Cooled Multi-Jet Divertor • HEMJ: design proposed for DEMO • Jet impingement cooling: 6.8 g/s He at (600 °C,10 MPa) exits from 25 (24 0.6 mm dia. + one 1.04 mm dia.) holes into H = 0.9 mm gap • ~106 modules to cool O(100m2) divertor • KIT/Efremov experiments Norajitraet al. 15 • Single module at prototypical conditions withstood >103 cycles at 914 MW/m2 • Nine-finger module at (500 °C, 10 MPa) tested to 6 MW/m2 • Complex geometry  need to develop new fabrication methods: deep drawing thimbles, powder injection molding (PIM) tiles W Tile 18 mm W-alloy shell 15 mm 18 mm Steel cartridge Fusion Matls Workshop (7/16)

5. Experimental Approach • Test at near-prototypical conditions in GT helium loop • He mass flow rate ṁ 10 g/s, inlet temperature Ti< 420 °C (vs. 600 °C), inlet pressure pi  10 MPa • RF induction heater [INL STAR facility]: heat flux q from He energy balance (less than actual incident flux due to losses) • q  6.6 MW/m2: decreases as Tidue to thermal losses • Match dimensionless ṁ (Reynolds number Re) and fraction of heat removed by convection, vs. conduction (Biot number Bi thermal conductivity ratio   ks/kfor geometrically similar studies) • Measure cooled surface and coolant temperatures, pressure drop • Determine correlations for dimensionless HTC (Nusselt number) Nu = f(Re, ) [neglecting Prandtl number effects] and loss coefficient KL = g(Re) Fusion Matls Workshop (7/16)

6. Current GT Helium Loop Induction heater Testsection • Evacuate loop, then charge to 10 MPa from 41.3 MPa source tanks • Control mass flow rate ṁ with bypass • Filters remove >7 μm particulates • Heat He with recuperator + electric heater P P T T Cooler T He source tank Heater Buffer tank Buffer tank Compressor Venturimeter P Recuperator Vacuum pump Fusion Matls Workshop (7/16)

7. HEMJ (J1-c) Test Section q • WL10 (99% W, 1% La2O3) outer shell + stainless jets inner cartridge with adjustable gap width H • Thermocouples (TCs) embedded 0.5 mm from cooled surface of outer shell  average cooled surface temperature Tc • Experiments at H = 0.470.03 mm, 0.860.02 mm, 1.280.04mm • Focus on higher inlet temperatures Ti = 300400 C • Determine average and standard deviation from 10 measurements taken with air-dry clay at a given setting • Reynolds number Re= 1.31045.4104 (ṁ  38 g/s) • Prototypical ṁ = 6.8 g/s Rep = 2.14104at Ti = 634 °C Dimensions in mm Fusion Matls Workshop (7/16)

8. Experimental Results •  Ti = 300C • 400 C • q 4.5 MW/m2 • Nu less than previous results at lower Tiand correlation • Nu independent of H • Re-examine correlation at Ti 300 C Mills et al. 15 • KL 1.8 for H = 0.47, 0.86 mm • KL 1.86for 1.28 mm • Pressure transducer recalibrated • Simulations predict KL 1.77 Nu [-] H = 0.47,0.86, 1.28 mm Re [/104] Fusion Matls Workshop (7/16)

9. HEMJ Optimization • Can HEMJ design be modified to simplify manufacture while keeping similar thermal performance? • 25 jets of different diameters impinging on curved surface • Consider instead fewer and larger impinging jets, all of same diameter, impinging on and cooling flat surface • Numerical simulations • Coupled CFD / thermal stress analysis: model 60° wedge using standard k turbulence model • Validate with experimental data from He loop • Investigate effect of jet exit diameter D(all jets same dia.), spacing s on thermal performance quantified in terms of Nu, maximum h, maximum Tc, thermal stresses • Thermal stress analysis: one-way coupling, with CFD results used as loads for FEM model • Test “best” result in He loop to validate simulations Thimble Jets Cartridge Curved Flat Fluid Flat Fusion Matls Workshop (7/16)

10. Jet Array Parameterization • Vary number of rows and jets • Total jet area, V kept constant • Only designs with projected row (s/D) and jet (j/D) spacings within ~1 • Rows spaced evenly cartridge diameter for flat surfaces • Hexagonal array of jets: 30°120° wedges, depending on symmetry • Up to 7106 cells • Vary H for each geometry • H = 0.5, 0.75, 0.9, 1.25, 1.5 mm j s CR4H25 FR4H25 Fusion Matls Workshop (7/16)

11. Preliminary Results r  z • Results for HEMJ at prototypical conditions suggest: • Temperature differences over cooled surface as great as ~127 °C • Maximum von Mises stress on cooled surface ~388 MPa • Change in H due to diff. thermal expansion as great as 0.22 mm (vs. 0.9 mm) • Optimizations  can achieve similar h, lower pwith fewer jet holes and/or rows in some cases • Expansion • [mm] • 0.318 • v. Mises stress [MPa] • T [°C] • 997 • 7724 • 0 • Structural Analysis (HEMJ) • CFD Analysis (HEMJ) • 870 • 0 60° • Re = 2.2104Ti = 600 C • q = 10 MW/m2 • h = 35.3 kW/(m2·K)

12. New GT He Loop • Design and build larger He loop with ṁ  100 g/s • Experimental studies of other divertor designs: HCFP, 9-finger HEMJ module • Space for loop under renovation • Biggest challenge: creating nearly prototypical heat fluxes on test section • Current induction heater cannot provide 10 MW/m2 on large (at least 20 cm2) areas • Try reversed heat flux approach: reverse direction of heat transfer by heating plasma-facing surface with hot He, cooling with water: estimate heat transfer and hfrom energy balance of coolant, i.e., waterOvchinnikovet al. 05 • Pros: Suitable for large areas; greatly reduces maximum T loop components and test sections can be fabricated from standard materials • Cons: Only characterizes heat transfer; no information on materials Fusion Matls Workshop (7/16)

13. Design Simulations • Initial study: numerical simulations of HEMJ test section • Current configuration: heat transferred to He: Ti 600 C; To  700 C • Reversed configuration: He heats water, which cools surface by jet impingement: Ti 700 C; To  600 C • Estimate impinging jet parameters required to remove q = 510 MW/m2 without boiling T T Cooler Testsection P P Heater Fusion Matls Workshop (7/16)

14. Impinging Jet “Cooler” • Simulate impinging jet of water in pressurized tank • Jet issues from 2 cm dia. pipe into tank at 2 MPa, 27 C: fully-developed turbulent pipe flow at exit at Rew = 7.51043.7105 (ṁw= 15 kg/s) • Impinges upon brass disk modeling HEMJ plasma-facing surface (2 cm dia.  0.4 cm thickness): impose uniform heat flux BC over disk • Numerical model = radial slice of entire tank + pipe  2.3105 cells • Standard k- turbulence model • Temperature-dependent properties (NIST) Exit Tank Pipe Jet 4 cm Disk q Fusion Matls Workshop (7/16)

15. Initial Results q = 5,7.5, 10 MW/m2 • Based on h from HEMJ simulations at prototypical conditions, T  365 C  TL  335 C for Ti  700 C • Can cool heat fluxes up to 7.5 MW/m2 • Temperatures on impingement surface Tu< Tsat(2 MPa) = 212 C  no boiling TL [C] TU TL q Rew [/104] Fusion Matls Workshop (7/16)

16. Current Status • Helium loop upgraded to higher inlet temperatures: Ti 400 °C • Challenges in measurements at Ti300 C: consistency in H, seals, achieving steady-state conditions • Replacing Cu with Inconel X-750 gaskets (PHENIX) • Numerical simulations of HEMJ “variants”: thermal stress analysis and CFD • How significant is differential thermal expansion at higher Ti? • Can HEMJ design be simplified (fewer jets, flat surface)? • Experimental studies on HEMJ at Ti 300 C, H = 0.51.5 mm • Acquire more data to evaluate correlation • Designing larger He loop: ṁ  100 g/s • Initial design simulations for HEMJ suggest q = 7.5 MW/m2possible with reversed heat flux approach using jet-impingement cooling • Validate simulations with small-scale impinging jet experiment Fusion Matls Workshop (7/16)