H. Kobayashi

Kobayashi,  H.

NIFS-Repository (National Institute for Fusion Science)

Fig.2 Reduced Teu vs. bath pressure PTeu is averaged for the period of fluctuations.Solid line is an eye guide.100channeleu wiretherms.5.8 cJ>(b): :3010SUStubevacuumheater cell,x-aXIS40gA 20=uv0o P.t 50P [kPa](a)Fig. 1 Schematics ofHe II channel(a) the vacuum-insulated He II-channel and positionofthermometers. The heated surface is at x = O.(b) enlargement of the heater cell, HI, H2: heatersx=L .....x=o·..·.........to be confined in 1 mm, which is larger than the thickness ofthe film-boiling state [2], TXI 3 K, Tx2 < TL Alternativedisplacement of the A-layer or the piston effect also seems beaffected on the heat transfer. The range of the displacementis not negligible compared with L at lower P. Although theslow piston motion develops the efficient heat evacuationfrom the channel, the heat transfer from the heater does notpromoted. The piston motion with considerabledisplacement does not occur above 20 kPa. More rapidexcitation rather than the slower displacement is expected tooccur around 20 kPa by the help of successive evaporation atreduced Tsvp. The work-flow may be excited by the variationin volume and pressure near the heated surface. Theoscillatory vibration with the frequency of harmonicsinherent in channel length suppresses largely the rise in Tcu'Reference1. Kobayashi, H. Proc. ofICEC-19 (2003) 7592. Fouaidy, M. Advances in Cryogenic Engineering 33(1988) 407§ 15. Heat Transfer at the Critical Heat Fluxin He IITo protect a superconducting magnet cooled in He II,the heat transfer with larger critical heat flux qA has beenstudied in concern with the phase transition. The propertiesof the heat transfer as a function of bath pressure have beenanalyzed by detecting the thermo-acoustic oscillation in thisstudy.Fig.la shows the channel made of a vacuum-insulatedstainless-steel tube with the length L = 150 mm of 5.8 mm indiameter. The channel is closed at one end with a heater cell,and the other end opens in the subcooled He II in a cryostatof two-bath system. The heater cell consists of a copperblock, heaters and thermometers. The temperature of thecopper block Tell was mede a measurement both with athermocouple and a resistance thermometer of copper wire.Temperatures Tx (T Txs) along L are measured withchip-resistors arrayed on x-axis. Power input q is suppliedthrough heater-wires HI, H2 wound on the copper block andon the stainless-steel tube. To restrict the circulation of He IIdue to the buoyancy, the channel is inverted so that theheated surface is directed downwards. In the configuration,the sound of the thermoacoustic oscillation [1] becomesaudible enough to detect with an ordinal microphoneattached to the cryostat wall. Bath pressure P was controlledwithin ± 0.1 kPa by pumping the upper bath of the cryostat.Properties of the heat transfer are outlined as follows.When qA at the atmospheric pressure reaches, Teu starts torise monotonically. During q is fixed at qA' TXI at x = 0.5 mmkeeps just below TAin spite of high Tcu' By applying theartificial disturbance immediately after the A-transition,however, Teu stays at 20 K. Both Teu and Tx are kept at theirlevels as long as the thermoacoustic oscillation lasts, Txlholds the temperature of subcooled He 1. Temperaturegradient VT=(Tx-Tb)/L maintains entirely the Gorter-Mellinkstate, the position x = 1 mm remains in He II, but x = 0.5mm is in He 1.The transition below 40 kPa accompanies with a suddendrop in Txl unlike in higher P. The drop in Tx is caused byisolating thermally the heated surface with the A-layer andthe gas film of the poor thermal conductance at loweredboiling point Tsvp' After the drop, the reduced Teu traverseson lower temperature level around 10K.Below 20 kPa, the heat transfer exhibits the morecomplicated process, the frequency of several seconds of Txcomes out near A-point. Above PA, say 6 kPa, Teu risesmonotonically without stopping at the mid-state around 10K. The pulsatory oscillation with intermittent emission ofthe sound prevents the heated surface from dry out even forq of several W/cm2• The dry out easily occurs rather withheat input through H2• Inputting q with H2 alone, no soundhas been detected above PAso far. Averaged minimum Teureduced by the thermoacoustic oscillation is obtained not atPAbut around at 20 kPa.The length of turbulence next to the heated surface seemsKobayashi, H. (rnst. of Quantum Sci., Nihon Univ.)107

Heat Transfer at the Critical Heat Flux in He II

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