Superconducting generator cooling system simulation

Superconducting generator cooling system simulation P.W. Eckels, J. Buttyan, J.H. Parker Jr., and AL Patterson Cryogenic Technology and Electronics, Westinghouse Research Laboratories, 1 3 1 0 Beulah Road, Pittsburgh, PA 1 5 2 3 5 , USA

Received 15 February 1985

Rotating tests have been performed to evaluate continuous liquid helium transfer and the cryogenic environment established in a test rotor. The testing was related to high speed alternator development programmes and the rotating Dewar was designed to simulate a self-pumping liquid helium cooling system for superconducting rotors. Stable pumping to < 0.6 atm* and stable helium level regulation were achieved at several different levels. The established pool was, within the limitations of steady state heat transfer, isothermal and there was no evidence of a warm bore condition. Within the rotating Dewar, heat transfer devices, level gauges and other instrumentation have been tested using onboard microprocessors to log, digitize and then transmit the data from the rotor. All of the cooling system functions required by large synchronous alternators were observed to be present and stable.

Keywords: testing

cryogenics;

helium;

cooling

Reference 1 reported the testing of a 0.9 m diameter 4000 r.p.m, rotating liquid helium Dewar. Initially the apparatus operated as a batch device being refilled with liquid He for each test. A single charge o f ~ 7 ~ of liquid helium allowed ~-,40 rain. of testing. For this test series, a continuous transfer rotating seal was adapted to the rotor with transfer line throttle valves and a T-tube type level regulator as alternative helium flow control methods. The transfer system shown in Figure / was a simple device consisting of cold end Teflon sleeve bearing and seal 4 which prevented thermoacoustic oscillations and a warm end face seal 5 which prevented helium leakage. The liquid helium depth within the rotor was indicated by an

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Sealand Bearing(flfl0n)

C01dSealSupport

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Rotating Shaft ium Inlet Tube L-Vacuum Space F0rshedaFaceSeal(XFLUOR) [

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Stationary Bayonet Figure 1

Schematic of the helium transfer system

Figure 2

The rotating heat transfer test rig

systems;

The rotating heat transfer facility reported previouslyI has been modified to provide a stable cryogenic environment to perform heat transfer tests relevant to high speed alternator cooling system development programmes. In its modified configuration the rotating Dewar simulates a self-pumping liquid helium cooling system for superconducting generator rotors. In the test series reported here we carry the superconducting level gauge work of Gamble 2 to larger diameters using different techniques and complement the earlier cooling system simulation work of lntichar and Schnapped. We have observed stable pumping to < 0.6 atm and stable helium level regulation, Measurements indicate that the pool was isothermal within the limitation of steady heat transfer and that the warm bore observed previously 1 has been eliminated. Within this environment heat transfer devices, level gauges, strain gauges and thermocouples have been tested. The apparatus, level gauges and other instrumentation performed exceptionally well.

* 1 atm = 101 kPa 0011-2275/85/080471-04 $03.00 © 1985 Butterworth 5t Co (Publishers) Ltd

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Research and technical notes

Figure 3

Convection trap on the torque tube pump outlet

American Magnetics superconducting level gauge (serial no. B030831BB) with a 100 mm active length. The data acquisition system consisted of a regulated current supply and voltage telemetry system as previously reported 1,6. Figure 2 shows the 0.26 m diameter rotor bore as well as the T-tube and helium level gauge. Previous testing I showed that the helium gas near the bore centreline was at a temperature of 13 K and that the rotor pumped to only 3.9 K at 3600 r.p.m. The observed pressure in the rotor was consistent with an isentropic radial pump having 13 K helium gas flowing into it. To try and eliminate the warm bore, three actions were taken. Additional insulation was added to the central lead duct. Copper fins were added to conduct heat from the bore to the helium pool as shown in Figure 2. Convection traps were added to the torque tube pumps as shown inFigure3. The traps were designed as Scurlock has described 7 to prevent warm gas recirculation from the torque tube to the bore. The modifications were successful in eliminating the warm bore condition but the individual contribution of each could not be established in this test series.

smallest radius regulated the pool to a 47 mm radius at 3200 r.p.m. All of the regulated levels agreed with pressure balance computations based on isentropic processes for the torque tube pump pressure rise and isothermal, hydrostatic liquid pressure rise in the pool. The pool was isothermal within 0.15 K, a difference which was attributed to steady heat transfer from and to the isothermalizing copper disc. Pool temperature was indicated by two carbon glass thermometers located at the bore and outboard radius which had their major axes oriented perpendicular to the radius. They were potted and were specially made for high g field operation by Lake Shore Cryotronics, Inc. They served to calibrate on-board silicon diode temperature sensors. After> 10 thermal and speed cycles to 4000g, no drift in their original calibration could be detected. The level gauge was tested in several modes at varying speeds of rotation with its outermost active length at 133 mm radius. Figure4 shows a voltage-current graph for the gauge as gauge current was increased from 0 to 140 mA and decreased to zero again. The ramping cycle was completed in ~ 3 s. At low current the single trace slope is indicative of the gauge heater resistance. With I increasing current, hysteresis regions are encounteredl that are associated with resistive zone propagation ~ through first gas and then liquid. These regions have been discussed by Efferson8. The middle single trace region is of interest as it is the region where the normal zone is anchored by the liquid level and is the gauge operating range. In the stationary frame the gauge operated with 65 mA but in the speed range 780 - 3600 r.p.m, the gauge could be operated at92 mA. The slope of the single trace at high current corresponds to the resistance of the fully normalized gauge.

10 -

_GaugeFully Resistive

Normal Zone Punches Through Liquid

o

I

100.7 ma 29. 2 Q 2. 92 V

Results The continuous transfer system increased the helium consumption by the equivalent of 5 W. It performed very well and exhibited no oscillations. Transfer losses were not determined experimentally but computation indicated that they did not exceed 2 W. The transfer line had 3-4 W of radiative heat transfer losses, according to computation, indicating 1-2 W loss in the transfer system. The level regulator was tested at three different levels by drilling holes at decreasing radii in the T-tube. Level regulation was demonstrated for each set of holes from the lowest regulating speed to 3600 r.p.m. The hole set at the

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Vol 25 August

Operating Range Constant NormalZone Lockedto LiquidVapor Interface

t _Normal Zone Punches Through Vapor

0

Figure 4

0

- ' ~ C u r r e n t 100 200 300 American MagneticsGauge

Performance of the helium level gauge at 3200 r.p.m.

Research and technical notes

600 Full I

Empty I

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360 Empty Full

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60.0 r/s /

41.6 r/s

480 240

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012345 Volts

Figure 6 Level gauge voltage trace for a sudden opening of the inlet valve which admits a quantity of warm gas

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1 ~34 folts

Figure 5 Voltage trace of the level gauge at three different speeds showing depletion of the liquid pool

Level GaugeVoltage Empty I Fu" I

I !

Figure 5 shows a trace of the level gauge voltage for three different speeds with an operating current of 95 mA. During these tests the liquid helium transfer line valve was closed and the pool was being depleted. A linear decay of pool radius such as shown in Figure 5 would not be expected upon superficial examination of the helium consumption rate, however, the traces are nearly linear and exemplify the complex heat transfer and thermodynamic processes characteristic of a rotating Dewar. Figure5 also shows that the heat leak is speed dependent. The increasing slope of the voltage-time curves indicates an increasing rate of depletion of liquid with speed. Figure 6 shows the capability of the gauge to follow transients as the inlet throttle valve is opened admitting warm helium gas from the transfer line which evaporates a substantial amount of the liquid pool. The speed is 2600 r.p.m, in this test and voltage polarity on the recorder is reversed l¥om Figure 5. Figure 7 shows the level gauge voltage as the T-tube regulates the pool at 69 mm radius and 3200 r.p.m, speed. The steps in the level gauge trace corresponding to heater current pulses may be frothing of the pool due to firing the heaters in an on-board heat transfer experiment. During this test, stable bore and pool temperatures of 5.3 and 3.7 K, respectively, were observed. From the liquid surface in the bore to the outer radius, a temperature increase of 0,15 K was also observed. This is 23% of the adiabatic neutral lapse rate value and is readily attributable to the steady state heat flows within the rotor.

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Figure 7 Voltage traces of the level gauge and heater shunt at 3 6 0 0 r.p.m. The level is regulated at 69 mm radius

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Research and technical notes

Conclusions A subatmospheric pressure, two-phase flow, selfregulating rotating Dewar containing a liquid helium pool similar to that described by Bejan 9 has been shown to be an exceptionally stable, efficient cooling environment for superconducting windings under steady rotation. Major heat inputs do not disrupt its stable operation and the pool, in accordance with theory, may be rendered nearly isothermal. Level gauges have been shown to function at various speeds, temperatures and radii relevant to superconducting generator operation. Potted carbon glass thermometers have been shown to operate at 4000 g. All of the functions required by large synchronous alternators were observed to be present and stable in these experiments.

References 1 2

3

4

5 6 7

Acknowledgements We thank Mrs Marilyn B. Cross for her expert typing and preparation of this paper. The work was supported by the US Air Force Aero Propulsion Laboratory, WrightPatterson Air Force Base, OH 45433, USA under US Air Force Contract no. F33615-81-C-2014.

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Litz, D.C. et al., High tip speed test rig to study natural convection in liquid helium, Adv Cryog Eng (1981) 27 799 Gamble, B.B The development of a rotating liquid helium flow circuit, ASME Paper 77- WA/HT-37 (1977) ASME Winter An nual Meeting, Atlanta. GA. USA Intiehar, L. and Sehnapper, C. Experimental simulation of a helium cooling system for a superconducting generator, 8th Int ConfMagnet Technology MT-8 September 1983 Lagodmos, G. Rotory joint coupling for cryogenic cooling of high speed rotating telescope assembly, Cryogenic Engineering Conference, Colorado Springs, August 1983 Merkel Forsheda Seal Corp., Forsheda XFLUOR Rubber Seal, G-185, 5375 Aiman Pkwy, Cleveland, OH, USA Eekels, P.W. et aL, Heat transfer correlations lbr a cryostable alternator field winding, Adv Cryog Eng (19811 27 357 Seurioek, ILG. et al., The zero mechanical work thermal pump in the rotating frame and its associated instabilities, Proc ICEC 7 IPC Science and Technology Press, Guildford, UK (1978) 373

8

Efferson, K.R. A superconducting (Nb-Ti) liquid helium level detector, Adv Crvog Eng (1969) 15 124

9

Bejan, A. Improved thermal design of the cryogenic cooling system for a superconducting synchronous generator. PhD Thesis Massachusetts Institute of Technology, USA (1974)