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Tube to Foam Interface

Tube to Foam Interface. (Tim). Outline. Discuss what’s known about the tube to foam interface Describe problem Issues Theoretical Calculations Anecdotal Experience. Problem. Heat Flow

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Tube to Foam Interface

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  1. Tube to Foam Interface (Tim)

  2. Outline • Discuss what’s known about the tube to foam interface • Describe problem • Issues • Theoretical Calculations • Anecdotal Experience Tube to Foam Interface

  3. Problem • Heat Flow • The heat generated within the active components on a stave is removed through evaporating CO2 in small-bore tubes. • The heat is conducted from the facesheets to the tube via thermally-conducting carbon foam and two interfaces made using thermally-conducting adhesive. • Geometry • Tubes (S/steel so far) • 1/8” (3.175mm) OD x (0.50, 0.25, 0.22mm wall) • For the future • 2.2mm OD x 0.14mm wall (ABCN130) • Thermally-conducting Foam • Strip-staves: Two 10mm wide ‘bars’ • Pixels: Full width ‘slabs’ • Materials • Tubes • 316L / 304L stainless steel (current staves & stavelets) • CP2 titanium (only used in UK ‘nearly glue-less’ stavelet • Thermally-conducting foam • Pocofoam • Allcomp foam • Tube-to-foam adhesives • CGL (compliant) • Hysol EA9396 (30% BN by wt) (rigid) Tube to Foam Interface

  4. Issues / Concerns • Thermal Performance • Is the thermal impedance of the tube-foam interface good enough to mitigate against thermal runaway • Stave Mechanical Stability • Are the temperature-induced stave deformations (and associated stresses) small enough (stable enough) to ensure good tracking performance. • Longevity • Will the thermal impedance of the joint deteriorate over time? • Could stresses induce creep Tube to Foam Interface

  5. Comparison Compliant Adhesive Rigid Adhesive Could thermally-induced stresses lead to:- Large dimensional changes High stresses which might promote cracking & ultimate failure of thermal path Could the need to accommodate dimensional changes complicate stave mounting? Fixations in Z Mounting brackets What are the effects of long-term creep? • Could the adhesive ‘slump’? i.e. separate out under • Gravity • Capillary flow • Could the adhesive ‘migrate’ away from the interface? • Closed Cell foams • Open Cell foams • Could the adhesive become less compliant? • Irradiation induced ‘curing’ may produce a ‘rigid’ joint over time. Tube to Foam Interface

  6. Open and Closed-cell Foams • Allcomp Foam • Open structure from low density (0.05g/cc) open cell foam • 100-130ppi (0.25mm) • Pocofoam • Closed cell structure with voids typically 0.5mm diameter • Voids volume equivalent to 0.22mm thick glue layer Tube to Foam Interface

  7. Thermal Properties • Second largest impact (after fluid htc) • Doubling the thermal impedance of the foam glue reduces the coolant temperature headroom by 2⁰C (≈ 10%) Tube to Foam Interface

  8. Stave CTE Assuming Rigid Foam & Rigid Glue • Simple 1D model (Classical Laminate Theory) • 2 Face sheets (all 0/90/0) • K13D2U / RS-3 [80gsm/29%RC] • K13C2U / EX-1515 [100gsm/40%RC] • K13C2U / EX-1515 [45gsm/40%RC] • 2 Cooling tubes • S/Steel: 3.185mm OD x 0.22mm wall (x2) Effective thickness 0.037mm • Titanium: 2.2mm OD x 0.14mm wall (x2) Effective thickness 0.017mm • 2 Bus Tapes • 0.025mm Kapton cover-layer • 0.025mm / 0.05mm Aluminium screen • 0.100mm Kapton Tube to Foam Interface

  9. Stave CTE Assuming Rigid Foam & Rigid Glue Bare Facesheets Facesheets + S/S Tube Facesheets + Ti Tube Facesheets + Tape (50) Facesheets + Tape (25) Facesheets + Tape(50) + S/S Tube Facesheets + Tape(50) + Ti Tube Facesheets + Tape(25) + S/S Tube Facesheets + Tape(25) + Ti Tube Tube to Foam Interface

  10. Historical Data • Jones (2009) • Crude measurement of relative CTE of CLG & Rigid epoxy • Measurement of thermal performance vs Thermal cycling (15 cycles) • Sutcliffe (2010) • Stavelet FEA • LBNL (2010) • Thermal Cycling of 12cm rigid-glued prototypes Tube to Foam Interface

  11. Jones2009 • Clip-type extensometer • 30cm prototypes • CGL • ER2074 (rigid epoxy) • Zero thickness glue line • 0.1mm glue line • Cool down to -40C and allow to rise back to room temperature Tube to Foam Interface

  12. Jones2009 • 30cm single-tube prototypes • Equivalent thermal performance for CGL and rigid epoxy • Thermal cycling shows no deterioration of average temperature above cooling tube at -40⁰C Tube to Foam Interface

  13. Sutcliffe (2010) • Standard UK build • 0/90/0 K13D2U/RS3 (80gsm,29%RC) • 1/8” s/steel tubes • CTEs • No Al screen • 0.02mm contraction • CTE = 1e-6 • 0.05mm Al screen • 0.042mm contraction • CTE = 2e-6 Tube to Foam Interface

  14. Critical Stresses • Foam stress (likely) to cause failure – but structures survive! • Two explanations • Glue bridging between facesheet and tube • Simple FEA assumes linear material properties but materials testing reveals otherwise • UK Stave core design employs end close-outs to protect foam Tube to Foam Interface

  15. Mechanical Materials Measurements • Pocofoam has different properties in orthogonal directions and a failure stress of typically 0.8MPa • Allcomp (K9) – 130ppi has uniform characteristics and failure stress >2.5MPa Failure 1 Tensile loading of foam 2 Unloading of foam Stress (MPa) 1 2 Strain Tube to Foam Interface

  16. LBNL (2010) • Construction • Length 12cm • Hysol 9396/BN(30% by weight) to bond tube & facings to foam • K7 foam • One SS tube(2.8mm OD) • One Ti tube(2.2 mm OD) • Thermal cycle and irradiation(time sequence) • 900 cycles (20C<->-35C) then • 1 cycle to -70C then • 1 cycle to about -175C with LN2 then • Irradiation to 50 MRad, then to 150 Mradtotal • No change in Thermal performance • No difference in SS and Ti tubes (with given ID/OD). Not a surprise (from FEA). • No significant change in thermal performance for any sample after any thermal cycle sequence, including LN2 • Effect of irradiation up to 1 GRad is <10% increase in T. • Thermal performance with K9 foam is significantly better than with K7 foam, by about 25% Tube to Foam Interface

  17. Tube to Foam Interface

  18. Tube to Foam Interface

  19. LBNL/BNL (2012) Stave 1.3m x 0.12cm 0/90/0 250 um thick K13D2U facings, 80-100 gsm pre-preg CGL around s/steel pipe, Allcomp foam Stave 1.2m x 0.12m Co-cured facings, low density 45 gsmcf pre-preg. 0/90/0 370 um thick K13C2U + bus facings CGL around steel pipe, Allcomp foam Tube to Foam Interface

  20. Comparison of Stave Stiffness, Room Temp and Chilled Stave stiffness independent of temperature (simple support 120 cm apart) Stave is about 10% stiffer when chilled with -30 deg-C coolant. Bus cable glue layers responsible??? (simple support 120 cm apart) Tube to Foam Interface

  21. Co-cure stave contraction Tube to Foam Interface

  22. 1.3 m stave and 1.2 m co-cure stave contraction • Stave Contractions • 1.2m co-cure stave contracts 0.2mm • CLT predicts 0.118mm assuming completely free tube • NB Stave is held together with 2 x 15g of Hysol – equivalent to 0.082mm thickness spread over stave area • 1.3 m stave expands ~ 20-35 um • CLT predicts 0.040mm assuming completely free tube • Similar glue mass / thickness • Tube Length Changes • Pipe moves into co-cure stave ~ 100 um, into 1.3 m stave ~ 160 um • Free stainless steel pipe should contract ~ 1mm when cooled ~ 50 deg-C • Question is: does pipe really contract 1 mm? If so, see analysis of 1.3 m stave on next slide Tube to Foam Interface

  23. 1.3 m stave contraction analysis But we expected pipe to be mostly fixed at U-bend Tube to Foam Interface

  24. Summary & Conclusions • Stresses in staves come from • Bus tapes (primarily the aluminium screen) • Core assembly adhesive • Cooling tubes (if assembled with rigid epoxy) • Evolving stave design reduces potential stresses • Smaller bore tubes (ABCN130) • Titanium (Progress in joining technology) • Bus-tape screen (0.05mm -> 0.025mm -> ‘0’ ?) • CGL • Experience since 2008 • Many staves built showing good thermo-mechanical performance • Concerns about migration into foam structure addressed by lining channel with rigid epoxy • Reliance on ‘sliding’ properties over long service life in high radiation environment • Some evidence that tube is not completely ‘free’ • Rigid Adhesive (Hysol9396/30%BN) • Experience since 2009 • Thermal cycling (tens to many hundreds) • No failures for ‘nominal’ cycling (Room temp to -40C) • FEA shows stresses in all components (in particular the foam) have large safety margins for ‘nominal’ excursions and indicate that structures will survive large (160C) excursions. Tube to Foam Interface

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