超流氦传热中的几个特殊物理问题相界面问题以及压力效应

根据传热实验及理论研究,提出了超流氦传热中的几个特殊物理问题的研究课题,本文深入研究了超流氦传热中的相界面问题以及压力效应。     

Preliminary Results from a Shuttle Mission to Measure the Heat Capacity of Confined Helium near the Lambda Point

We describe the preliminary results from a Shuttle mission to measure the heat capacity of helium confined within a stack of evenly spaced silicon plates at temperatures very close to the superfluid transition. Recently developed high resolution thermometry has substantially improved our ability to study dimensional cross-over effects in well-defined geometries. These effects have been of interest to theorists and esperimentalists for decades. The main part of the apparatus consists of a high purity copper calorimeter containing a stack of 408 silicon plates spaced 57 microns apart, and a pair of high resolution, fast response, paramagnetic salt thermometers with a noise level of < 10^–10 K in a 1 Hz bandwidth. The resolution of the heat capacity measurements was about 5 × 10^–9 K, allowing the finite size peak to be mapped in detail. In addition, wide range data containing information on the behavior of the surface specific heat was collected. The preliminary analysis shows fair agreement with theory. The results can also be combined with supplementary ground measurements on smaller length scales to perform additional tests of scaling predictions for cross-over to lower dimensional behavior.     

Study of a Helium Film in 10 μm Porous Glass by Torsional Oscillator in Free Decay Mode

Dissipation and superfluid density of a thin helium film (superfluid transition at temperature T_c=0.88 K), placed in 10 m porous glass are studied as a function of oscillation amplitude or AC amplitude at fixed temperatures near superfluid transition. The measurements are performed using a high-Q torsional oscillator in the free decay mode as well as constant drive mode. Results show decreasing superfluid density with increasing velocity and non-monotonic behavior of the dissipation in the film.     

Gravitational Effects on Nonlinear Heat Transport Near the Superfluid Transition in 4He

We have recently observed nonlinear heat transport within 30 nK of the superfluid transition temperature using heat flux, Q, in the range 0.1 < Q < 2 erg/(s cm ² ). While Haussmann and Dohm (HD) accurately predict the initial departure of the thermal conductivity, , from the linear response region, is greater than expected very close to T . We anticipate that the nature of the thermal conductivity's nonlinearity may depend upon Earth's gravity in the low heat flux limit (Q < 0.5 erg/(s cm ² )). Comparison of our data to similar data to be taken in a microgravity laboratory will provide an experimental determination of the effect of gravity on nonlinear heat transport near the superfluid transition. The microgravity measurements will also permit the first experimental test of theories that do not consider gravitational effects, such as those by HD.     

The Vapor Film Evolution at Superfluid Helium Boiling in Conditions of Microgravity

The results of numerical modeling of the vapor film temporal evolution on the cylindrical heater at film boiling of the superfluid helium (HeII) in conditions of microgravity are presented. The methods of molecular-kinetic theory have been applied to describe the vapor near the interface: vapor-HeII. For this purpose the Boltzmann equation have been solved. The system of equations of conservation for the liquid has been used. Different models of heat transfer in superfluid helium have been discussed and studied.     

Superfluid Transition of 4He for Two-Dimensional Crossover, Heat Capacity, and Finite-Size Scaling

We report heat capacity measurements of confined films of ⁴ He. These studies were undertaken to test predictions of correlation-length scaling. They are the first measurements for completely confined films over a range of confinements, and represent a geometry where criticality changes from 3-dimensions (3D) to 2D. The finite system is realized with a ⁴ He film confined between two, 2 diameter, silicon wafers, which are separated by a small gap. A new technique was developed to bond these wafers at a uniform separation. The gap size, which determines the film thickness, ranges from 0.05 to 0.7 m in the present work, and has better than 1% uniformity. The bonded cells are used to conduct high precision heat capacity measurements using a modified ac technique. This involves oscillating the sample temperature, as in conventional ac calorimetry, but with simultaneous dc regulation of the average temperature. The data are analyzed using a modified Sullivan–Seidel equation, which takes into account in an empirical way the finite conductivity of the cell. This procedure yields heat capacity data with good absolute accuracy and high resolution. Scaling analysis of the data both above and below the bulk transition temperature shows collapse onto universal curves determined only by the ratio of the correlation length to the confinement size. This is true everywhere except near the heat capacity maximum. Here, and into the superfluid side there is lack of scaling which might be associated with 2D crossover. We compare this result with calculations of scaling functions and find that these tend to underestimate the effect of confinement. Comparison with earlier results for cylindrical confinement shows differences which are most striking in the region of the specific heat maximum. The cylindrical and planar confinement data follow similar trends above the superfluid transition of bulk helium. Below the transition, however, the present data show much more structure. Fits of the scaled planar data above the transition to an empirical scaling function yield a correlation length exponent of _eff=0.674±0.001.     

Implementation of the superfluid helium phase transition using finite element modeling: Simulation of transient heat transfer and He-I/He-II phase front movement in cooling channels of superconducting magnets

In the thermal design of high magnetic field superconducting accelerator magnets, the emphasis is on the use of superfluid helium as a coolant and stabilizing medium. The very high effective thermal conductivity of helium below the lambda transition temperature significantly helps to extract heat from the coil windings during steady state and transient heat deposition. The layout and size of the helium channels have a strong effect on the maximum amount of heat that can be extracted from the porously insulated superconducting cables. To better understand the behavior of superfluid helium penetrating the magnet structure and coil windings, simulation based on a three dimensional finite element model can give valuable insight. The 3D geometries of interest can be regarded as a complex network of coupled 1D geometries. The governing physics is thus similar for both geometries and therefore validation of several and different 1D models is performed. Numerically obtained results and published experimental data are compared. Once the viability of the applied methods is proven, they can be incorporated into the 3D geometries. Not only the transport properties in the bulk of the helium are of interest, but also the strong non-linear behavior at the interfaces between solids and superfluid helium (Kapitza conductance) is important from an engineering point of view, since relatively large temperature jumps may occur here.In this work it is shown how He-II behavior in magnet windings can be simulated using COMSOL Multiphysics. 1D models are validated by experimental results taken from literature in order to improve existing 2D and 3D models with more complete physics. The examples discussed include transient heat transfer in 1D channels, Kapitza conductance and sub-cooling of normal liquid helium to temperatures below the lambda transition in long channels (phase front movement).     

Specific Heat and Superfluid Density of 4He near T λ of a 33.6 nm Film Formed Between Si Wafers

We report measurements of superfluid density and specific heat of a 33.6 nm film near the superfluid transition. The film is formed between two patterned and directly bonded silicon wafers. These measurements were undertaken with the primary purpose of understanding coupling and proximity effects in a situation when the film was in contact with helium in a larger confinement (Perron et al. in Nat. Phys. 6:499, 2010; Perron and Gasparini in Phys. Rev. Lett. 109:035302, 2012). However, these data are also relevant to issues of correlation-length finite-size scaling. This is the thinnest hard-wall confined film for which such scaling has been tested for the specific heat and superfluid density. One expects that at some small thickness such scaling should fail. We compare our results with previous data of helium in a similar confinement but at larger thickness. We find good agreement with scaling in regions where previous data scaled, and confirm the lack of scaling where previously reported. In our analysis we consider a native oxide growth between the etching and bonding steps of cell fabrication and its effect on our scaling analysis.     

Superfluid helium in porous media

The superfluid properties of ⁴He absorbed in porous media are discussed. Emphasis is given to the nature of the superfluid transition for helium contained in a variety of different porous structures. Recent heat capacity and superfluid measurements for helium in Vycor glass and other porous media are presented in some detail. The onset of superfluidity as a function of adsorbed helium coverage at zero temperature, or boson localization, is touched on briefly. The problem of dissipation of superflow in porous media, especially in the vicinity of the superfluid transition, is discussed.     

Second Sound Horizon

We study an analogy between black hole physics and the phase boundary between normal and superfluid liquid helium in a gravitational field or in the presence of a constant heat current. We investigate the propagation of second sound near the boundary between superfluid and normal fluid. The speed of second sound should approach zero as it propagates toward the phase boundary where the reflection and transmission of the second sound will become ambiguous. This is analogous to the propagation of light near a singular black hole. When the phase boundary moves it is analogous to a black hole horizon: a second sound horizon. The analogy may lead us to infer a thermal radiation noise of amplitude <50 pico Kelvin from the boundary between the super and normal fluid. This thermal noise may be dramatize by applying a constant heat current from the bottom. We find a scaling behavior TQ ^1/6 for the thermal radiation of the phase boundary as a function of heat current Q. A feasible experiment to measure this thermal noise is suggested.     

Dynamic and Gravitational Effects on the Correlation Volume: Experimental Methods

We report on the design of a new prototype flight instrument that will be used to repeat previous Earth-based measurements of nonlinear heat transport near the superfluid transition in the microgravity laboratory. Since this nonlinear conductivity is associated with dynamic limitations to the divergent correlation length, and since gravitational acceleration also limits the correlation length's divergence, we anticipate that the nonlinear conductivity will depend strongly on gravitational acceleration. The apparatus, data taking procedure, systematic corrections, and error sources are discussed here.     

Second-sound velocity and superfluid density in3He-4He mixtures near superfluid transition points—Applicability of universality and scaling laws

From second-sound velocity measurements, superfluid densities near superfluid transition points were determined in ³ He- ⁴ He mixtures under saturated vapor pressure. The critical exponent of the superfluid density thus obtained increases about 15% with increasing ³ He concentration up to 40 mole percent, which contradicts the universality concept. Furthermore, the critical exponents obtained here and the exponents of the specific heat do not satisfy the scaling laws. A new concept of universality is introduced and discussed.Submitted in partial fulfillment of the requirements for the Ph.D. degree at The University of Tokyo.     

A Preliminary Heat Capacity Measurement of 3He in Aerogel near its Superfluid Transition

We report simultaneous heat capacity and torsional oscillator measurement of ³ He in aerogel near the superfluid transition. The heat capacity has a peak at the temperature T_c where the torsional oscillator shows the onset of superfluid decoupling. The coincidence of these signatures suggests that ³ He in aerogel does undergo a true thermodynamic transition.     

New Results for the Realization of the Superfluid Transition Temperature of Helium

A two-chamber sealed cell has been developed to realize the superfluid transition temperature of helium. A series of temperature plateaus are obtained while a series of small heat flows are applied to the capillary connecting the two chambers. The plateau temperatures are extrapolated to determine the transition temperature at zero heat flow. This paper reports the new results for the realization of the transition temperature of helium using seven cells. More than 30 measurements have been made in two laboratories since 2002, as five cells sealed in 2000 and two cells sealed in 2009 are used. The standard deviation of the measurements is ~0.02 mK.     

Third-sound resonance and thermal resistance in unsaturated helium films

We have obtained a thermodynamic state of thin helium films which permits precision measurements of their properties. Third-sound resonances indicate a transition in the superfluid density of these films. This transition is accompanied by an exponential rise in the thermal resistance of the helium film which indicates the formation of high-energy excitations of 0.08 eV. The dissipation at resonance indicates that thermodynamic fluctuations are the principal mode of third-sound damping.A revised approach to the data in this paper appeared inPhysical Review Letters, Vol. 25, No. 11, page 711ff, September 14, 1970.This research was supported in part by the Advanced Research Projects Agency under Contract SD-131 and in part by the Alfred P. Sloan Foundation.Alfred P. Sloan Fellow.     

Homogeneous Turbulence in Superfluid 4He in the Low-Temperature Limit: Experimental Progress

Selected advances in the research on the dynamics of tangles of quantized vortices in superfluid helium with little normal component during the last 50 years are briefly reviewed. The main emphasis is on the experimental techniques of generating and probing homogeneous one-component superfluid turbulence of various energy spectra in superfluid ⁴He in the low-temperature limit. The most recent experimental progress, modern theoretical concepts and future outlook are summarized.     

Development of a small He II cryostat with optical windows for a microgravity experiment

In order to study film-boiling phenomena in saturated superfluid helium (He IIs) under a microgravity environment, a very compact visualization setup was designed and fabricated at High Energy Accelerator Research Organization (KEK). It consists of a cryostat, a vacuum pump, a high-speed video camera and electrical circuits for measurement. The cryostat in the setup is equipped with optical windows for the visualization of film boiling in He IIs. The setup was tested to verify its thermal and safety performance under a microgravity environment using a 10 m free-drop tower at the Hokkaido Center of the National Institute of Advanced Industrial Science and Technology (AIST). Successful system operation from 1.94 to 2.05 K under microgravity conditions below 1 × 10⁻³ g was confirmed. The design and test results are described in this technical note.     

Effect of channel restrictions on the axial heat transport in subcooled superfluid helium

The present study investigates the influence of partial restrictions on the axial heat transport and critical heat flux limits in subcooled superfluid helium (helium II) channels. Different size orifices are used to simulate partial plugging of superconducting magnets cooling channels by frozen oxygen, nitrogen, hydrogen, neon or moisture during the cool down process. Thin stainless steel sharp edged orifices of sizes 0.5, 1, 2 and 5 mm id are mounted between stainless steel flanges attached to 9 mm diameter (helium II) channel. The helium II channel is heated at one end with a copper block heater while the other end heat sinks to an atmospheric superfluid helium heat exchange. Temperature drop across the restriction is measured by two calibrated carbon resistors. Measurements are carried out at both temperatures ranging from 1.8 to 2.2 K.As the orifice/channel area ratio decreases, data show a considerable decrease in the axial heat transported by internal convection process resulting in lower critical heat flux at the phase transition from helium II to helium I by the destruction of superfluidity and inititation of boiling. A linear correlation between critical channel heat flux and orificeI channel area ratio gives a good fit to the experimental data. For heat fluxes higher than the critical heat flux, transient temperature measurements for a step heat input are correlated with the time required to reach the phase transition.     

Linear and nonlinear heat transport near the superfluid transition of4He

A brief summary of the present status of the renormalization-group theory of heat transport near the superfluid transition of⁴He is presented. This includes the thermal conductivity, second-sound damping, mutual friction coefficients, boundary resistance and nonlinear effects on the temperature profile in the presence of a finite heat current.