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Extrapolating Experimental Results for Model Divertor Studies to Prototypical Conditions

Extrapolating Experimental Results for Model Divertor Studies to Prototypical Conditions. M. Yoda, S. I. Abdel-Khalik, D. L. Sadowski and M. D. Hageman Woodruff School of Mechanical Engineering. Objective / Motivation. Objective

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Extrapolating Experimental Results for Model Divertor Studies to Prototypical Conditions

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  1. Extrapolating Experimental Results for Model Divertor Studies to Prototypical Conditions M. Yoda, S. I. Abdel-Khalik, D. L. Sadowski and M. D. Hageman Woodruff School of Mechanical Engineering

  2. Objective / Motivation Objective • Experimentally evaluate thermal performance of gas-cooled divertor designs in support of the ARIES team • Evaluate variants of current designs to enhance their thermal performance Motivation • Experimental validation of numerical studies • Divertors may have to accommodate both steady-state and transient heat flux loads exceeding 10 MW/m2 • Performance needs to be “robust” with respect to manufacturing tolerances and variations in flow distribution ARIES Meeting (5/10)

  3. Approach • Design and instrument test modules that closely match divertor geometries • Conduct experiments at conditions matching and spanning expected non-dimensional parameters for prototypical operating conditions • Reynolds number Re • Use air instead of He • Measure cooled surface temperatures and pressure drop • Effective and actual heat transfer coefficients • Normalized pressure drops • Compare experimental data with predictions from CFD software for test geometry and conditions ARIES Meeting (5/10)

  4. Plate-Type Divertor • Covers large area (2000 cm2 = 0.2 m2): divertor area O(100 m2) • HEMJ, T-tube cool 2.5, 13 cm2 • Accommodates up to 10 MW/m2 without exceeding Tmax 1300 °C, max 400 MPa 20 100 cm Castellated W armor 0.5 cm thick • 9 individual manifold units with ~3 mm thick W-alloy side walls brazed together ARIES Meeting (5/10)

  5. Pin-Fin Array • Can thermal performance of leading divertor designs be further improved? • Mo foam in 2 mm gap increased HTC by up to 50%, but also increased pressure drop by up to 100%[Gayton et al. 2009] • HEMP: coolant flows through pin-fin array [Diegele et al. 2003] • Combine jet impingement cooling of plate-type divertor with pin-fin array • Pin-fin array increases cooled surface area like foam, but should have less pressure drop • Pins span entire 2 mm gap, with 2 mm clear strip in center to allow jet to impinge ARIES Meeting (5/10)

  6. GT Test Module q q Test module • Jet from H = 0.5 or 2 mm  L = 7.62 cm slot • Coolant: Air • Bare and pin-covered cooled surfaces • 2 mm gap • Brass, W have similar k 6 6 6 Plate design • Jet from H = 0.5 mm  L = 20 cm slot • Coolant: He • Bare cooled surface • 2 mm gap W Armor Brass shell In In Out Out Al cartridge W-alloy ARIES Meeting (5/10)

  7. Components 7 7 7 2 mm slot (L) and 0.5 mm slot (R) Al inner cartridges “Pins” surface brass shell “Bare” surface brass shell A = 1.59103 m2 ARIES Meeting (5/10)

  8. GT Air Flow Loop Cu heater block • Three heaters • q = VI/A Measure • Coolant P, T at inlet, exit • P across module ARIES Meeting (5/10)

  9. Experimental Conditions 9 9 9 ARIES Meeting (5/10)

  10. Cooled Surface Temps. Five thermocouples embedded 1 mm inside brass shell near center of slot to avoid edge effects Temperatures extrapolated to surface, then used to determine local heat transfer coefficients Spatially averaged HTC average of five local HTC results y x 3 5 2 4 1 10 10 10 1 mm In Out ARIES Meeting (5/10)

  11. Ap Af Adiabatic fin tip Cooled Surface q Brass shell Thermocouples 1 mm Abare Al cartridge ARIES Meeting (5/10)

  12. Effective vs. Actual HTC hact = spatially averaged heat transfer coefficient (HTC) associated with the geometry at the given operating conditions heff = HTC necessary for a bare surface to have the same surface temperature as a pin-covered surface subject to the same incident heat flux For pin-covered surface: Fin efficiency f depends on hact(f as hact) Ap = 9.54104 m2; Af = 5.08103 m2 A = 1.59103 m2 12 ARIES Meeting (5/10)

  13. Effective HTC: Air • Effective HTC of pin-covered surfaces 90-180% greater than actual HTC of bare surfaces • Increase is less than increase in area (f < 1; hact may be less) 2 mm Bare  2 mm Pins 0.5 mm Bare 0.5 mm Pins heff [kW/(m2K)] Re (/104) ARIES Meeting (5/10)

  14. Pressure Drops • Pressure drops rescaled to Po = 414 kPa: • Pins increase P by 40% at most • P greater for H = 0.5 mm slot 2 mm Bare  2 mm Pins 0.5 mm Bare 0.5 mm Pins P[kPa] Re (/104) ARIES Meeting (5/10)

  15. Calculating Actual HTC 15 15 15 For pin-covered surfaces, iterate since f = f(hact) Initial “guess” for hact that for corresponding bare surface Assuming an adiabatic fin tip, fin efficiency Use f to determine new value of hact Repeat Steps 2 and 3 until (hact, f) converge • Pin perimeter Per = 3.14103 m; length L = 2 103 m; tip area Ac = 7.85102 m2 • f decreases as HTC increases ARIES Meeting (5/10)

  16. Actual HTC • Actual HTC for pin-covered surfaces lower than those for bare surfaces • But pins increase cooled surface area by 276% hact [kW/(m2K)]  Bare Pins Re (/104) ARIES Meeting (5/10)

  17. HTC for Helium 17 17 17 • To predict performance of plate-type divertor at prototypical operating conditions, convert hact measured for air to hact for He • Ts = 1300 °C; Tin = 600 °C; kHe = 323×103 W/(mK); W fins • For bare surface correct for changes in thermal conductivity • For pin-covered surface, correct for changes in fand thermal conductivity ARIES Meeting (5/10)

  18. Fin Efficiency: He vs. Air flower for He because HTCs higher f  as Re Tungsten (k = 101 W/(mK)) fins for He, vs. brass (k = 115 W/(mK)) fins for air fdecreases if k decreases 18 f [%]  Air  He Re (/104) ARIES Meeting (5/10)

  19. Calculating Max. Heat Flux • Maximum heat flux • Total thermal resistance RT due to conduction through PFC, convection by coolant • Conductivity of PFC taken to be that of pure tungsten • Thickness of PFC LPFC = 2 mm ARIES Meeting (5/10)

  20. Max. Heat Flux 20 20 Fins • Increase qmax to 18 MW/m2 at expected Re, and to 19 MW/m2 at higher Re • Allow operation at lower Re for a given qmax  lower pressure drop 2 mm Bare  2 mm Pins 0.5 mm Bare 0.5 mm Pins qmax[MW/m2] Re (/104) ARIES Meeting (5/10)

  21. Conclusions H = 2 mm rectangular jet of He impinging on pin-covered surface under prototypical conditions (Re = 3.3104) can accommodate heat fluxes up to 18 MW/m2 Based only on heat transfer (vs. thermal stress) considerations Pin fins can reduce operating Re, and hence coolant pumping requirements, for a given maximum heat flux Benefits of pin fins decrease as Re increases and/or k decreases Pin-fin array Increases effective HTC by 90-180%, but decreases actual HTC Increases P by at most 40% H = 0.5 mm slot consistently gives lower heff and higher P than H = 2 mm slot 21 21 21 ARIES Meeting (5/10)

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