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Stave Core (“Plank”) Thermal QA - UK

Stave Core (“Plank”) Thermal QA - UK. Motivation for Thermal QA: Building 24 modules to populate a plank costs a lot of effort and money (1 stave ≡ 40 transatlantic return flights*) => We should test the plank thoroughly (within reason).

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Stave Core (“Plank”) Thermal QA - UK

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  1. Stave Core (“Plank”) Thermal QA - UK • Motivation for Thermal QA: • Building 24 modules to populate a plank costs a lot of effort and money (1 stave ≡ 40 transatlantic return flights*) => We should test the plank thoroughly (within reason). • If it looks good and behaves well mechanically, it will probably be ok thermally. (?) • But of course you can’t see inside it. • Maybe you want to do some of: • Thermal QA of every plank (had better be fast: ~ 2h per plank?)….. the first few …. a selected sample • Check out thermally: irradiated / thermally cycled / otherwise stressed …planks • Infrared Thermography (of the surface) is worth ~ 105 individual contact measurements. • Is it feasible? (have had useful discussion with Dave L). * “only 3 if you travel First Class” (Adrian Bevan) Graham Beck, LBL September 2012

  2. IR Thermography At low T: -30C black body radiation intensity is still appreciable: ~ 40% room temperature level. The peak is well within the LWIR window, commonly used by the cheaper IR cameras* (e.g. QMUL, Liverpool)! Energy Balance + Kirchoff says that: emissivity + reflectivity + transmissivity = 1. =>Thermography works best on high emissivity surfaces: • Bigger signal • Lower reflected ambient radiation (inc. camera!) • The (default) plank surface is 25mm Kapton over (refective) Aluminium shield (if present). Kapton is somewhat transmissive. • For bus tape e I measure ~ 90% (above room temperature); DaveL measures ~83% at low T (-25C?). • Expect plank thermography can work. • Sensitivity to faults will depend on the “noise” due to surface quality – uniformity, flatness etc. • Even if the bus gets buried (?) there should be an electrically insulating (so high e) surface. * Nowadays you can buy a radiometric IR camera for ~ 4 transatlantic flights (10% of a stave). Graham Beck, LBL September 2012

  3. How would you make a measurement? • Flow coolant through the pipe, at ~ -30C. • Actively heat the plank? It turns out that natural convection and radiation from ambient supplies about the right thermal load. Rough estimates: Convection: Assume htc = 5W/m2K (consistent w. R.Bennett calculations and Geneva experiments). (ignore dependence on plank orientation here). Radiation: W/A = e. s * (T4 – Ta4) A=Area, Stefan’s constant s = 5.67E-8, e =emissivity: assume = 1 ≈ 4s .Tmean3 * DT ; (DT << T) i.e. (linear approx). Radiative htc ≈ 4s.Tmean3 e.g. +20C (ambient) => -25C (Tmean ≈ 270K) => 4.5 W/m2K - very comparable to convection! Ambient load per module length of plank: ≈ 125 mm × 98mm × ~ 10 W/m2K × 45C => 5.5W ≈ Hybrid Power. Graham Beck, LBL September 2012

  4. FEA Thermal Model of a section of plank +20C, 10 W/m2K Surface Film Load • Quick & Dirty: Stave130 Module FEA with sensors+hybrids stripped away. Fast and informative … not particularly accurate ! • Same htc top & bottom (clearly inaccurate) • Cable Bus top and bottom • 2mm id Ti pipe. • Material Specific Heats assigned (used in transient case) - but Facing Glue Cv omitted. - Corresponds rather roughly to Tim’s planklet measurement : Graham Beck, LBL September 2012

  5. - 25 ° C ° - 32 C - 25 ° C PERFECT PLANK in STEADY STATE: comparison with Tim’s IR plot. After a couple of iterations to tie down CO2 temperature (approx. -38.5C) (FEA Ambient Thermal Load now ~ 6.5W). ABAQUS FEA Tim’s IR (Stavelet3+masking tape) & Fit Scale: Abaqus DT looks ~ 14% low. => Some confidence in FEA feasibility studies … Graham Beck, LBL September 2012

  6. Now introduce a FAULT into the FEA model: GLUE missing between the foam and pipe Effect on STAVE (module) performance: Mean TC Headroom Sensor T (degrees) Baseline, No faults -25.23 C 21.6 Missing from one pipe, top half, DZ 10mm -25.21 21.5 " " " DZ 30mm -25.14 21.3 " FULL circumference, 30mm -24.6 19.7 • Choose as a benchmark test: glue missing over 30mm length, half the pipe circumference. • Has only a small effect on STAVE thermal performance (heat spreads around the fault) • Maybe you would want to know about it anyway. It might deteriorate with thermal cycling, etc. Graham Beck, LBL September 2012

  7. PLANK FEA: GLUE FAULT (STEADY STATE) Plotted bands: -32C=> -36C 0.2C intervals (~ camera resolution) Height of bump is 0.7C (top surface - also 0.4C bump on bottom surface) Would this be visible in practice? - depends on “noise”: bus surface flatness, uniformity, convective fluctuations etc. Graham Beck, LBL September 2012

  8. Now look at Transient behaviour … Tim, re Thermal Shock: “What is the typical dT/dt seen during Stavelet Testing with CO2?” Stavelet 3: IR temperature history of two points on the surface, above the pipes, after turning on CO2 (later turning off / back on again) • At SP02 (above inlet pipe) T falls ~ 7C/s after turning on CO2… • CO2 flows around U-bend, and reaches SP01 after ~43s => speed ~ 12mm/s. • meanwhile, outlet (SP01) is cooled, slowly, by conduction across the plank. • (Interesting? ~simultaneous warming when flow stopped). Try to reproduce this in FEA - then assess potential for fault finding . . . Graham Beck, LBL September 2012

  9. Propagate change from Ambient to CO2 conditions in 1s steps Transient FEA… Sledgehammer approach: • Divide pipe surface (crudely!) into 8 x 12.25mm lengths along Z • Transient FEA 1s increments: • Change film conditions for successive sections (≡ 12.25mm/s) from +20C, 10W/m2K => -38.5C, 8000W/m2K. • Apply this to inlet pipe. • After 43s, apply (in reverse sense) to outlet pipe • Allow to settle (a further 43s). (See Perfect Plank Transient.wmv: !Range is +20C to -40C) => Plot history of surface T above centre of each pipe (as Tim) Graham Beck, LBL September 2012

  10. PERFECT PLANK: Surface Temperature transient. Rate [C/s] vs time T vs time FEA: FEA doesn’t reproduce all features of IR, and peak dT/dt is 70% higher But worth pursuing … IR: Graham Beck, LBL September 2012

  11. FEA TRANSIENT – with and without GLUE FAULT T vs time Rate [C/s] vs time PERFECT FAULTY (see Faulty Plank Transient.wmv Range: 0C to -40C) Peak Rate is unchanged on inlet side (~11C/s), but ~25% lower over centre of fault. Graham Beck, LBL September 2012

  12. A Trivial QA Algorithm:• Define lines L1,L2 on the surface above the pipes. • Record T on L1, L2 for duration of test (this would be ~ 5 mins for a full stave) • For each Z, evaluate dT/dt(t). • Plot Peak dT/dt vs Z. • Look for anomalies (bumps)The plots are derived in this way from the FEA Temperatures at 2x20 nodes along the bus tape.[Fluctuations due to jerky FEA! Ignore Z extremes – wrong BCs!]=> Inlet behaviour is fairly level. . .=> Outlet shows expected bump due to fault. So there is an appreciable signal !! . . .BUT . . . . . . there will be background conduction from ± Z . . .and what is the noise like in practice???Many Questions: Best answered by experiment! Graham Beck, LBL September 2012

  13. Alternative (avoiding thermal feedback) - FLOW routed through independent chiller bath - RETURN from stave routed directly to Vaporiser (Poss.re-locate second valve). T1 T2 STAVE CYLINDER T3 T4 Tn located as per Nikhef Manual. T4 redundant if HEX omitted. T5 could be un-monitored/low precision. T1,T2,T3: 4-wire PT100? (expensive but accurate). OR Pico 8-chan. Thermocouples (incs. Labview) Vaporiser (Heater) Dial + DAQ. (Re-locate upstream of vaporiser) T5 QMUL will build a CO2 blow-off cooling system to evaluate these QA ideas (so far have a list of parts to purchase – but need a decision on temperature monitoring)...and I believe Oxford are building an intentionally “faulty” stave. Graham Beck, LBL September 2012

  14. coverlayer bus tape BACKUP: Measurement of IR Emissivity of Bus Tape Bus sample + thermal grease on (warmed) block of copper. Black tape added (v. small correction to T, estimated by adding more layers). Spot radiometer measures T of tape and bus: Bus e estimated assuming 94% for black tape. - e seems high – as hoped. Note: Kapton thickness is 25mm, cf radiometer sensitive in 8-13mm window. At low temperature (-25C?) , Dave Lynn finds ~ 83%. Graham Beck, LBL September 2012

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