摘要:

We report high-resolution measurements of the singular contributionRbto the thermal boundary resistance between a solid surface and superfluid helium near the superfluid-transition temperatureT&lgr;.The results confirm the observation by Murphy and Meyer that a gap between the cell end and the sidewall leads to an apparent finite-current contribution toRb.In the absence of such a gap, overall agreement ofRbwith theoretical predictions is very good. Remaining small differences require further investigations. Without a sidewall gap and within our resolution we found no finite-current effects over the range3.9 &mgr;W/cm2<Q<221 &mgr;W/cm2.©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593539

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present some theoretical considerations that are relevant to the forthcoming measurements in microgravity of the specific heat of confined liquid helium (the flight experiment “CHEX”). Primary attention is devoted to the suppression near the boundaries, the “negative surface specific heat.” Above the lambda point, we compute this to lowest order in the &Vegr;-expansion: below the lambda point our discussion is more restricted. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593540

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We study a steady state, bearing a heat current,Q0.In the normal state of liquid4Hewith constraining geometry, near the lambda point, at temperaturesTgreater than the lambda temperature,T&lgr;.Critical order parameter fluctuations near the boundary are incorporated in the expression for the non-local thermal resistivity. The Kapitza resistance is manifested by the additional temperature rise at the boundary, as compared with the bulk-extrapolated value. Sensitivity of the calculations to the application of the Dirichlet boundary condition is discussed. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593541

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The appearance and dynamical behavior of a He I–He II phase interface is investigated experimentally. The experimental mode in which nearly saturated He I initially at a little bit higher temperature than the lambda temperature is cooled by sudden evaporative cooling is primarily employed in the present experiment among several possibilities. In this mode where an interface appears and propagates downward, some dynamical aspects of an interface layer can be preferably investigated. The phenomenon is investigated by the application of the schlieren visualization method, and by measuring the temperature variation by superconductive temperature sensors and the pressure variation as well as the evaporating vapor flow rate which can be converted into the cooling rate. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593542

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The results of an experimental study of the evolution of the shape of nonlinear second-sound pulses in superfluid He-II are reported. The pulses propagate in the bulk (3Dgeometry) and along a cryoacoustic waveguide filled with liquid helium (quasi-1Dgeometry) at temperatures corresponding to the negative, positive, or zero nonlinearity coefficient. A strong dependence of the shape of the propagating pulse on the dimensionality of the wave was observed. The finite size of the heater (generator of a sound) affects the profile of a short3Dpulse even at distances many times greater than the heater size, which restricts the minimal width of the excited pulse. The experimental data are compared with the results of numerical simulations. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593543

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We are currently building a prototype for a new test of critical-point universality at the lambda transition in4He,which is to be performed in microgravity conditions. The flight experiment will measure the second-sound velocity as a function of temperature at pressures from 1 to 30 bars in the region close to the lambda line. The critical exponents and other parameters characterizing the behavior of the superfluid density will be determined from the measurements. The microgravity measurements will be quite extensive, probably taking 30 days to complete. In addition to the superfluid density, some measurements of the specific heat will be made using the low-g simulator at the Jet Propulsion Laboratory. The results of the superfluid density and specific heat measurements will be used to compare the asymptotic exponents and other universal aspects of the superfluid density with the theoretical predictions currently established by renormalization group techniques. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593544

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The average hydrodynamic impulse (or Lamb impulse)JVin the counterflowing, superfluid, turbulent He-II and the kinetic energyE,which is associated with the chaotic vortex filaments, are calculated. These quantities are defined as averages of the classical quantities taken over the stochastic vortex loop distribution. The averaging procedure was performed by using the Gaussian model of the chaotic vortex tangle elaborated earlier. Calculations show that the quantityJVhas only a component in the direction of external counterflow velocity (created, e.g., by a heater). As for the energy, it is shown that besides the usual local contribution, the quantityEincludes additional terms due to a long-range interaction. Some dynamic properties of the turbulent He-II due to the impulse and energy of the vortex configuration, such as suppression of the superfluid density and decrease in the velocity of the second sound that propagates in the superfluid turbulent He-II, are discussed. ©1998 American Institute of Physics. 

ISSN:1063-777X    

DOI:10.1063/1.593545

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Helium flow in an enclosure caused by sudden heating of the enclosure walls is considered. Boltzmann’s equation and complete time-dependent Navier–Stokes equations with appropriate boundary conditions are integrated numerically. It is shown that the results of calculations obtained by using these methods for a steady-state flow are close. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593546

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Cryosorption of helium isotopes(3Heand4He)on thin layers of solid argon at low gas pressures in the bulk is studied experimentally upon a change in the layer temperature from 4.2 to2 K. It is shown that the sorption isostere for4Hedisplays anomalous behavior near the temperatureT&lgr;of transition of liquid4Heinto superfluid state. An abrupt change in pressure is observed for a4Hefilm thickness corresponding approximately to two monolayers. In experiments on cryosorption of gaseous3Heon an argon layer for a3Hefilm thickness corresponding approximately to one monolayer, the pressure in the bulk varies monotonically over the entire temperature interval. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593547

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The results on boiling of liquids and gas bubbling under simulated long-term reduced gravity are reported. The possibility of extrapolating these results to the real spacecraft conditions is considered and the program of on-board space experiments is proposed to test the current theory at the relative accelerations10−4–10−6which are unattainable in a laboratory. Tests in free-fall towers, in aircraft, or in ballistic rockets are inadequate since only prolonged (over one hour) experiments in the Earth’s orbit can provide reliable data. The advantage of cryogenic liquids, helium, in particular, in space experiments is discussed. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593548

出版商:AIP    

年代:1998

数据来源: AIP