Heat management issues A perspective view centered on the design of superconducting devices Opinions...
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Transcript of Heat management issues A perspective view centered on the design of superconducting devices Opinions...
Heat management issuesA perspective view centered on the
design of superconducting devices
Opinions of L. BotturaAt the mWorkshop on Thermal Modeling and Thermal
Experiments for Accelerator Magnets
Sept. 30th, Oct. 1st, 2009
Outline Matters of steady state heat removal
from coils, magnets and other superconducting systems
Stability of superconductors Magnet quenches, and not only
Outline Matters of steady state heat removal
from coils, magnets and other superconducting systems
Stability of superconductors Magnet quenches, and not only
Cooling in steady operation - 1 Coil heat transfer
What is the maximum power that can be removed from a coil ?
Focus on strand-to-helium heat transfer, through the insulation
Mainly motivated by LHC operation, NIT, HFM on the longer term
Magnet heat transfer Same motivation as above, extend the analysis to
the proximity cryogenics
We will hear extensively on both
Cooling in steady operation - 2 Force-flow cooling
Pressure drop, flow and heat transfer in supercritical helium flow
Relevant for the SC link foreseen for the Phase I upgrade (and FCM ?)
For SC link, two options being designed and prototyped
NbTi (Tmax < 5.5 K !) through APUL at FNAL MgB2 (Tmax < 10…15 K) at CERN
A long SC link, possibly vertical geometry, is being designed within the scope of WP 7.5 of EuCARD program
NIT - SC link8 x 0.6 kA
3 kA
14 kA
7 x 14 kA + 7 x 3 kA + 8 x 0.6 kA
Cable R&D by courtesy of A. Ballarino, CERN-TE-MSC
Aha ! FCMInternally cooled cable prototype
CERN/BNG Dstrand: 0.6 mm Jc(4.2 K, 5 T) > 2500 A/mm2
Cu:CuMn:NbTi = 2.4 Deff < 3 m < 1 ms Number of strands = 32 Cable twist pitch < 80 mm Bnom = 2 T Tnom = 4.5 K Inom = 5800 A Ic = 11500 A Tcs > 6.5 K Iop/Ic = 50 % Tmargin > 2.0 K IDpipe = 4.5 mm ODconductor = 7.6 mm Ra > 100 Massflow = 5 g/s Pressure drop (60 m) < 0.1 bar
Present state-of-the-art in pressure drop
The customary approach to friction data modeling is to plot in dimensionless
form f(Re), fit a model, and compare results
Data for small-size CICC’sDcable = 12 mmDstrand = 0.81 mmCable: 3 x 3 x 4 x 4
Kat
hede
r
Alternative approach based on porous media
Straight correlation plot to check accuracy:
Average relative error f ≈ 20 %
A tortuous matter - 1 Tortuosity is the ratio of
the length l of a flow streamline between two points x1 and x2, and the distance of the two points d = | x2- x1 |
Larger tortuosity implies larger pressure drop
Length effect Flow effect
d
l
A tortuous matter - 2 Tortuosity in porous media depends on void fraction
(porosity) - we already take into account this effect In addition tortuosity in CICC’s depends on the
cabling pattern - we do not take into account this effect !
Cable tomography by courtesy of ENEA and PSI
Scales and issues Small temperature increase, up to the temperature margin (< 1
K) The heat transfer process affects coil, magnet and proximity
cryogenics (1 mm …100 m) Slow time scales, comparable to operation (1 s … 1 h) He-I and He-II are both of relevance Today, much of the experimental focus is on cable/coil in
He II, but very little data in all other areas (magnet, powering cables)
Modeling work is lagging behind experimental results, which is normal, but there may be a lack of basic understanding of the dominating physical mechanisms (micro/macro porosity in cables and insulations)
Outline Matters of steady state heat removal
from coils, magnets and other superconducting systems
Stability of superconductors Magnet quenches, and not only
Superconductors stability
Measurements by M. de Rapper, CERN-TE-MSC
Figure 3
Figure 2
Scales and issues Small temperature increase up to the decision
recovery/quench (≈ 1 K) The heat transfer process is local to the strand in the
cable (1 mm …1 cm) Fast time scales for the decision between
recovery/quench (10 s … 10 ms) He-I and He-II are both of relevance We do not have a complete, consistent and
validated model of heat transfer for these processes
Outline Matters of steady state heat removal
from coils, magnets and other superconducting systems
Stability of superconductors Magnet quenches, and not only
Quench propagation Single components (e.g. magnet, bus-
bars (!?!)) require knowledge of heat transfer at the level of the cable
Systems (e.g. strings of magnets) require knowledge of heat and mass transfer at the level of the proximity cryogenics
Clean gap
≈ 45 mm
Quench in an LHC bus-bar Mock-up of defects in the LHC interconnects
are tested to find the boundary between stable quenches (can be protected by a current dump) and thermal runaways (can lead to over-temperatures)
Sample manufactured by C. Urpin, H. Prin, CERN-TE-MSC
Run 090813.15Stable quench: a normal zone is established and reaches approximate steady-state conditions (T ≈ 30…40 K)
stable
Measurements by G. Willering, G. Peiro, A. Verweij, CERN-TE
Run 090813.20Runaway quench: the temperature in the normal zone increases over a time scale of the order of few s to R.T.
runaway
trunaway
Measurements by G. Willering, G. Peiro, A. Verweij, CERN-TE
Effect of heat transfer @ 1.8 K
Adiabatic interconnect
Wet interconnect
Measurements by G. Willering, G. Peiro, A. Verweij, CERN-TE
Effect of heat transfer @ 4.3 K
Wet interconnect
Adiabatic interconnect
Measurements by G. Willering, G. Peiro, A. Verweij, CERN-TE
Heat transfer coefficient
Measurements by D. Richter, CERN-TE-MSCAnalysis by P. Granieri and M. Casali, CERN-TE-MSC
?
A string of LHC magnets model of the regular
LHC cell: D quadrupole and
lattice correctors 3 dipoles F quadrupole and
lattice correctors 3 dipoles
QV9202SIQV920
Pressure evolution computed pressure
Quench propagation reasonable
match of quench
propagation MB3-MB2-MB1
quench propagation MB3-
MB4 too fast
Scales and issues Large temperature increase, potentially up to room-
temperature (≈ 100 K) The heat transfer process is local to the cable in the
magnet (1 cm …1 m) Moderate time scales for the evolution of the
temperature profile during quench (0.1 s … 100 s) He-I (including pressure waves and mass transport)
is of most relevance The process spans several orders of magnitude,
and involves transport phenomena, with uncertainties at each level of the multiple scales
Summary
space
time
tem
pera
ture
10-3
10-2
10-1
110
1001000
10-3
10-2
10-1
110
100
0.1
1
10
100
Quench of magnet and busses
Magnet cooling,Flow issues
Stability
Cable/Coil cooling