Fusion Engineering and Design 58–59 (2001) 211–215

Pressure drop and helium inlet in ITER CS1 conductor

Pierluigi Bruzzone *CRPP-Technologie de la Fusion, 5232 Villigen-PSI, Switzerland

Abstract

A number of pressure drop measurements have been carried out at CRPP on a section of ITER cable-in-conduitconductor, using pressurized water at room temperature. The longitudinal friction factor is deduced for the strandbundle area and the central hole. Two layouts of radial helium inlet have been investigated and the measured radialpressure drop is assessed in terms of the equivalent conductor length producing the same pressure drop in axialdirection. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Cable-in-conduit conductor; Helium inlet; Pressure drop measurements

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1. Introduction

For magnets wound by forced flow conductors,it is convenient to make the distance between thecoolant inlet and outlet (‘hydraulic length’) asshort as possible to reduce the pressure drop andincrease the coolant speed. In coils wound bydouble pancakes, the electrical connections be-tween conductor sections may be convenientlyplaced at the outer radius, where the magneticfield is weaker. The hydraulic inlet is at the pan-cake transition at the inner radius and the electri-cal connections are used as hydraulic outlets,obtaining a short hydraulic length (half of theconductor sections) and providing the lowest op-erating temperature at the highest field. The fu-sion magnets planned for ITER are built fromdouble pancakes, with the coolant inlet at thepancake transition. The following investigations

are devoted to the hydraulic behavior of the ra-dial coolant inlet for the ITER conductor, withthe aim to assess the pressure drop at the inletversus the axial pressure drop per unit meter ofconductor.

2. Sample layout and test set-up

A section of the ITER CS1 conductor [1], leftfrom the winding of the second layer of the innermodule of the CS Model Coil (CS1.2 B), is usedfor the hydraulic test. A proper cut at the ends ofthe conductor section, without strand smearingand local variation of the helium cross-section, isobtained by electronic erosion. The geometry dataof the conductor are summarized in Table 1 [2,3].For the wet perimeter in the strand bundle, thefull strand perimeter is accounted.

The axial pressure drop measurements havebeen carried out on a straight, 6 m long conduc-tor section, later cut to 1 m length to extend the

* Tel.: +41-56-310-4363; fax: +41-56-310-3729.E-mail address: bruzzone@psi.ch (P. Bruzzone).

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P. Bruzzone / Fusion Engineering and Design 58–59 (2001) 211–215212

range of the operating mass flow rate. The axialtest has also been repeated after blocking thecentral hole by a rubber pipe, restricting theflow to the bundle area. For the radial pressuredrop, two layouts of the helium inlet have beencompared. In one case, a plain hole has beendrilled on one side of the jacket and finished bymanual tools. The outer cable wrap is removedby a grinding tool and a pipe, �in=16 mm, iswelded to the jacket, see Fig. 1 left. In a sec-ond, more sophisticated design, a 200 mm longsection of jacket is fully removed by a millingmachine and replaced by two half shells withmachined longitudinal and radial channels toimprove the fluid penetration through the strandbundle and avoid stagnant helium regions. Theeight longitudinal and five radial channels, seeFig. 1 right, have a cross-section of 3×1.8 mm.The outer cable wrap is removed over the 200mm, but the subcable wraps are maintained. Af-ter welding the shells together and to the jacket,the cable inside the shells is fully supported (nogap).

Pressure drop measurements on forced flowconductors must be done in the relevant rangeof Reynolds numbers. The option to use super-critical helium at the nominal operating condi-tions (mass flow rate, pressure and temperature)must be discarded for short length measure-ments as the pressure drop is in the range of 1mbar and severe accuracy problems would oc-cur. To obtain relevant Reynolds numbers withhelium at room temperature, a very large massflow rate is necessary, due to the very smalldensity [4]. Using nitrogen at room temperature,with seven-times higher density than helium, itis easier to achieve high Reynolds numbers [5].However, large pressure and temperature varia-tions occur from inlet to outlet, and the assess-ment of the results must be done using densityand viscosity values averaged over a broadrange of pressure and temperature. A convenientway to achieve high Reynolds numbers and sig-nificant pressure drop in short sections of forcedflow conductors is to use pressurized water atroom temperature [6]. The temperature is con-

Table 1Geometric data of conductor CS1.2 B

Symbol Source/methodDescription ValueFormula

38.99 mm–dcsCable space average diameter Measured on test sampleAcs �(dcs/2)2Cable space area 1194 mm2 From the above data

305 mm–PswSubcable wrap perimeter From Nicollet [2]Subcable wrap cross-section Psw0.1Asw 30.5 mm2 From the above data

Pow – 220 mm Measured on test sampleOuter wrap perimeterOuter wrap cross-section Pow0.1Aow 22 mm2 From the above dataSpiral cross-section –Asp 24 mm2 From Nicollet [2]

From the above data24 mmAsp/1Spiral half perimeter Psp

Spiral inner/outer diameter –dspi/dspo 9.8/11.8 mm Measured on test samplecos � –Mean twist angle in strands 0.925 From ITER QA [3]

From ITER QA [3]0.81 mm–Strand diameter dst

Ast 1152�(dst/2)2/cos �Twisted strand cross-section 642 mm2 From the above dataHelium cross-section overall Acs−Ast−Asw−A0w−AspAHeov 472 mm2 From the above data

AHeH �(dspi/2)2Helium in central hole 76 mm2 From the above dataHelium in bundle AHeB AHeov−AHeH 396 mm2 From the above data

VB From the above dataVoid fraction in bundle 36.5%AHeB/(Acs−�(dspo/2)2)From the above data3.61 m1152�dst/cos �+Psw+Pow+PspWet perimeter Pw

dov 4AHeov/Pw 0.523 mm From the above dataAverage hydraulic diameter overalldB 4AHeB/Pw 0.438 mm From the above dataAverage hydraulic diameter in bundle

P. Bruzzone / Fusion Engineering and Design 58–59 (2001) 211–215 213

Fig. 1. The two helium inlet layouts: the plain hole in the jacket, �=16 mm (left) and the steel halves with machined radial andlongitudinal channels (right).

stant and hence the density and the viscosity areconstant too in a liquid, allowing good accuracyin the results.

At CRPP, the pressure drop measurementshave been carried out using a closed circuit ofde-mineralized water at 28 °C. The nominal pres-sure of the circuit is 11 bar. With the largest massflow rate, the actual pressure drop available at thesample is in the range of 5 bar. Table 2 comparesthe parameters of the test at CRPP and the oper-ating conditions of the CS Model Coil. As thegeometry is identical, the comparison can be donein terms of the mass flow rate divided by viscosity,(dm/dt)/�, instead of the Reynolds number, Re=(dm/dt)d/AHe� (in a conductor with dual channelthe assessment of the hydraulic diameter, d, isquite arbitrary). Due to the high viscosity of thewater at 28 °C and the limited water flow (pres-sure) of the circuit at CRPP, the CSMC operating(dm/dt)/� is obtained only with the central chan-nel closed: for the overall conductor (centralchannel open) an extrapolation to the actual (dm/dt)/� value is acceptable.

In the test set-up, the water flow rate is mea-sured by four, parallel operated floating ball flowmeters, ranging up to 3000 l/h, with overall accu-racy better than 5%. The pressure drop is mea-sured between the inlet and outlet by twodifferential pressure devices ranging up to 0.7 and1.5 bar. For larger pressure drops, the read-out oftwo absolute pressure meters is used. The pressuresensors overlap with good agreement over abroad range.

3. Results and discussion

The test results are discussed in terms of pres-sure drop per unit length, �p/l, and friction fac-tor, �, linked by the basic relation

�=�pl

2d�A2

(dm/dt)2 (1)

where d is the hydraulic diameter, A is the coolantcross-section, � is the coolant density and dm/dtis the mass flow rate. The read-out of the axialpressure drop versus water mass flow rate is plot-ted in Fig. 2 for both ‘open channel’, i.e. overallconductor, and ‘blocked channel’, i.e. only bundlearea. Assuming that, at any cross-section, thepressure is identical in the bundle area and thecentral channel, the fraction of fluid flowing in thebundle area, �, can be found from the polynomialfitting curves of Fig. 2 as a function of the overallflow, x, imposing

10567.19(�x)2+3818.15�x=2147.48x2+376.31x(2)

At very low mass flow rate, a power law fit isbetter suited to match the experimental data [7].The flow partition is plotted in Fig. 3 as a func-tion of the mass flow rate divided by the viscosity.Extrapolating the measured results to the CSmodel coil operating point, the fraction of coolantflowing in the strand bundle is expected in therange of 36%. From the flow partition, the ratioof the speed in the central hole, �H, to the speed inthe strand bundle, �B, is

P. Bruzzone / Fusion Engineering and Design 58–59 (2001) 211–215214

Table 2Operation parameters for CS1.B conductor

Test at CRPPCS Model Coil

Temperature �5 K 28 °CHeliumFluid Water

996.2141.1Density (kg/m3)4.45×10−6Viscosity (Pa s) 822×10−6

Inlet pressure (bar) �8�9�0.003bundle 0.85Overall0.007Overall 0.53bundleMass flow rate (kg/s)

674bundle 1034Overall(dm/dt)/� (kg/Pa s2) 644bundle1573Overall

�H

�B

=(1−�)

�

AHeB

AHeH

=9.26

For the CSMC at 7 g/s, we obtain �H=41.7 cm/sand �B=4.5 cm/s.

The Reynolds number in the bundle area,ReB= (dm/dt)BdB/AHeB�, can be assessed usingthe average hydraulic diameter in the bundle, dB,and the helium area, AHeB, from Table 1. Thefriction factor, �B, in the bundle area, definedaccording to Eq. (1) with index B, and derivedfrom the pressure drop results, �pB/l, is plotted inFig. 4 versus the Reynolds number. The resultsfor CS1 conductor are well matched by the gen-eral correlation proposed by Katheder [8], withonly 5% deviation at the CSMC operating point.The dotted line is a correlation proposed byNicollet [5] from pressure drop measurements car-ried out on an ITER conductor using nitrogen atroom temperature.

From the flow partition �, it is possible toderive the friction factor in the central hole, �H

�H=�pov

l

2dspi�AHeH2

((1−�)(dm/dt)ov)2

The results for �H are higher than predicted by theBlasius correlation for turbulent flow in smoothpipes, �=0.3164Re−0.25. As it was already ob-served in the conductor of the QUELL experi-ment [9], the roughness of the central spiral isresponsible for the increased turbulence comparedwith a smooth pipe.

The radial pressure drop results are gathered inFig. 5 versus the water flow rate. The solid anddotted lines correspond to the axial pressure drop,as measured in Fig. 2, multiplied by 50 and 10,

respectively. The pressure drop with the plain holein the jacket is equivalent to about 50 m axialpressure drop in the conductor. With the ma-chined steel shells, the pressure drop at the inletdecreases by a factor of five, corresponding toabout 10 m axial pressure drop.

4. Conclusions

The pressure drop measurements on the ITERconductor can be conveniently carried out usingpressurized water at room temperature, as theReynolds numbers are close enough to the actualoperating range.

The flow partition between central hole andbundle area can be deduced from the measure-ments of axial pressure drop. The friction factorin the strand bundle can be satisfactorily fitted bythe general correlation proposed by Katheder [8].

Fig. 2. Axial pressure drop per unit length vs. water mass flowrate in the CS1 conductor with open and closed centralchannel.

P. Bruzzone / Fusion Engineering and Design 58–59 (2001) 211–215 215

Fig. 3. Fraction of fluid flowing in the strand bundle vs. theoverall mass flow rate divided by the fluid viscosity.

No new correlation is required. Due to the rough-ness of the spiral, the axial friction factor in thecentral hole is higher compared with the Blasiuscorrelation for smooth pipes.

The pressure drop at the radial inlet can bereduced by a factor of 5 using machined steel shellsinstead of a plain hole in the jacket. The machinedsteel shells also provide a uniform cooling in thestrand bundle, avoiding stagnant helium regions.The radial pressure drop is equivalent to 10 mconductor in the case of machined shells andincreases, with a plain hole as helium inlet, up to50 m conductor, i.e. has a non-negligible impact onthe overall pressure drop for hydraulic length offew hundreds of meters.

Acknowledgements

The author is very indebted to Thomas Gloor forthe pressure drop measurements. The technicalsupport of the Paul Scherrer Institute is acknowl-edged.

References

[1] P. Bruzzone, N. Mitchell, M. Steeves, M. Spadoni, Y.Takahashi, V. Sytnikov, Conductor fabrication for theITER model coils, IEEE Mag. 32 (1996) 2300.

[2] S. Nicollet, J.L. Duchateau, H. Fillunger, A. Martinez, S.Parodi, Dual channel cable-in-conduit conductors thermo-hydraulics: influence of some design parameters, IEEE Appl.Supercond. 10 (2000) 1102.

[3] ITER Personnel Communication, CSMC QA documents,1997.

[4] P. Bruzzone, Fabrication of a short length of wind-and-reactconductor, Final Report to the NET Team, Final ReportContract 88-745 A, ABB HIM 2042, May 1990.

[5] S. Nicollet, H. Cloez, J.L. Duchateau, J.P. Serries, Hy-draulics of the ITER toroidal field coil cable-in-conduitconductors, Proceedings of the SOFT 98, p. 771.

[6] H. Katheder, Reynolds numbers for cable-in-conduit con-ductors, NET Internal Report, N7I70221/1/A, November1992.

[7] R. Zanino, personal communication.[8] H. Katheder, Optimum thermohydraulic operation regime

for cable-in-conduit superconductors, Cryogenics 34, ICECSupplement, 1994, p. 595.

[9] K. Hamada, et al., Thermal and hydraulic measurement inthe ITER QUELL experiments, Adv. Cryog. Eng. 43 (1998)197.

Fig. 4. Friction factor in the strand bundle vs. Reynoldsnumber. The solid line is the general correlation proposed byKatheder [7]. The dotted line is from Nicollet [5].

Fig. 5. Radial pressure drop at the helium inlet for plain hole(triangles) and machined shells (circles). The solid line is theaxial pressure drop for 10 m conductor length, the dashed linefor 50 m conductor length.