Small heat exchanger using silver powder

A simple heat exchanger using silver powder for cooling pure liquid ³He is described. The temperature difference across the heat exchanger was less than 5μK at the superfluid transition of ³He with a thermal flux of 0.06 nW.     

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm²(10 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm² (5 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm² (5 MW/m²). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.     

Visualization of 3He Nucleate Boiling Below 1 K

Boiling behavior of liquid ³ He below 1 K was visualized as shadowgraph image. A light source and a video camera (or still camera) were arranged at room temperature. The light was guided to the ³ He cell by an optical fiber, and the shadowgraph image was transferred to the camera by an image fiber. The ³ He bubble formed on the heated copper surface was spheroid-like due to the low surface tension. The size at departure from the surface and the relative bubble formation rate were measured as a function of heat flux. The previously obtained heat transfer curve could be explained qualitatively by this boiling behavior.     

Phonon reflection at a silicon-3He interface

The reflection of heat pulses at an interface between the (111) surface of silicon and liquid ³He has been measured for heat pulse temperatures from 1.4 to 9K. The measurements were made with the silicon-helium temperature equal to 0.4 or 1.8K. The detectors were cadmium or aluminum superconducting bolometers. Longitudinal and transverse phonons incident nearly perpendicularly on the surface had reflection coefficients of 1 and 0.8, respectively, independent of pulse temperature. Transverse phonons incident at a large angle and diffusely reflected had reflection coefficients dropping from 0.7 at 1.5K to 0.4 at 9K. The results are in broad agreement with previous measurements on solid-⁴He interfaces obtained from heat pulse and monochromatic phonon experiments.Equipment grants and financial support provided by the National Research Council of Canada and the School of Graduate Studies and Research.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.     

Mobility of 2D Electrons on Pure 4He and on 3He–4He Dilute Solution

The mobility of 2D electrons on pure ⁴He and on 0.5 % solution of ³He in ⁴He was investigated for different electron densities in the temperature range 0.12 to 1.3 K. The electrons in the same electron density show the same transition temperature from liquid state to Wigner crystal state in both pure ⁴He and in the solution. In the high temperature range where the gas-scattering is dominant, the electrons show a smaller mobility in the solution than in the pure ⁴He due to the electron collision with ³He gas atoms which have a higher vapor pressure. In the middle temperature range where the ripplon-scattering is dominant, the mobility in the solution is smaller than in ⁴He. This is explained by a smaller surface tension caused by ³He atoms collected at the surface. In the low temperature range where electrons are in the Wigner crystal state, the mobility gradually increases with decreasing temperature in the solution, while it stays almost constant in the pure ⁴He. The mobility increase is more pronounced in the low electron density. The results are qualitatively in agreement with the existing theory which includes the bulk ³He quasiparticle reflection from surface dimples and the effect of the surface layer of ³He atoms.     

Observation and analysis of the detachment frequency of coalesced bubbles in pool boiling liquid nitrogen

The detachment frequency of coalesced bubbles is one of the important parameters in the boiling heat transfer model at high heat flux. Although some researches on detachment frequency of coalesced bubbles have been done for common liquids, the work on cryogenic liquids is relatively insufficient. A visual experimental apparatus was built for observing the boiling in liquid nitrogen. The growth process of coalesced bubbles was recorded by a high-speed camera, emphasizing on the detachment frequency of coalesced bubbles from the circular, horizontal heating surfaces. The effects of the heating surface material, the surface diameter and the heat flux on the detachment frequency were analyzed. The available empirical correlations from common liquids are compared with our experimental data to check their applicability in liquid nitrogen, where the detachment frequency is 13.47 s⁻¹.     

Instabilities of the liquid solid interface

Bodensohn et al.¹ observed that a sudden temperature change of the liquid above 1.1 K caused the interface between solid and liquid⁴He to become corrugated; they proposed that the instability of the interface was caused by the introduction of a heat current induced by the temperature gradient. Subsequently Grinfeld² showed that a non hydrostatic stress of the crystal could cause an instability; this idea was put forward by Balibar, Edwards and Saam³ to explain the observation of Bodensohn et al. A more detailed analysis has been given recently by Bowley and Nozières⁴. The two causes of instability are analyzed in the present paper for both normal³He and superfluid⁴He. The phase diagram and critical heat current for normal³He are presented. The corrugations will appear most rapidly near the minimum of the melting curve at 0.32 K and need a temperature gradient of order a few µK/cm. For superfluid⁴He the inability of the liquid to support thermal conduction (heat is transported by second sound) changes the nature of the instability, with the result that dissipation at the interface becomes important. As a consequence only the Grinfeld instability is observable in practice so that corrugations appear with wavelength between 6 and 8 mm.     

Measurements of the Superfluid Joule–Thomson Refrigerator Using High Concentration 3He–4He Mixtures

The superfluid Joule–Thomson refrigerator (SJTR) uses a liquid superfluid ³He–⁴He mixture to provide cooling below 1 K. Performance measurements of the SJTR using 5% and 11% ³He concentration mixtures are reported. High concentration operation shows higher cooling powers at high temperature. Ultimate temperatures are seen to increase with increasing concentration due to a pinching of the temperature defect in the recuperative heat exchanger. This pinching effect is due to the variation of the heat capacity of the ³He–⁴He mixture with temperature and concentration and is discussed in detail and design changes are suggested to mitigate it.     

NMR,optical, and plastic flow experiments in bcc3He —4 He mixture crystals — in pursuit of a vacancy fluid

We describe experiments on the properties of bcc³He —⁴ He solid mixtures on the melting curve between 0.5K and 1.9K. In this paper we focus on effects related to the presence of thermal vacancies. First, we used NMR to image the³He distribution within the solid in equilibrium with the superfluid, as well as its T₁ and t₂. The most surprising result was that above about 1K, vacancy related motion of³He atoms in the solid becomes faster than in the liquid. To check the macroscopic aspects of this motion, we used the vibrating wire technique to look at plastic flow of the bcc solid phase, by moving the wire through the crystal. The temperature dependence of the plastic flow velocity indicates that the vacancy population in the bcc solid behaves like a viscous fluid. The extent to which the vacancy population causes the solid to have liquid like properties is best demonstrated through optical observations of the distillation of³He atoms out of the crystal, which takes place via formation of fluid bubbles within the solid, which then percolate into the liquid, creating a vivid impression of boiling.     

Heat capacity of helium submonolayers on a nonuniform surface

Data of McCormick, Goodstein, and Dash, and Stewart and Dash, for heat capacities of³He and⁴He films on Ar-plated copper in the submonolayer region have been analyzed on the basis of a model of a long-range variation of adsorption potential. This variation allows the adatoms to form small islands on the surface. The total heat capacity of the film is the sum of two contributions, one a Fermi-like contribution due to surface heterogeneity, and the other a two-dimensional Debye-like contribution due to molecular vibrations. The data are roughly consistent with a Debye temperature of 32K for all the⁴He data and 38 K for the³He data. The role of surface heterogeneity in the heat capacity of films is briefly discussed.     

Possible Phase Transition at Low mK Temperatures in Liquid Helium Mixture Films Adsorbed on Graphite

We report heat capacity and magnetisation measurements of ³He adsorbed on the surface of graphite plated with three atomic layers of ⁴He. For ³He coverages n ₃>4 nm^?2 the heat capacity corresponds to a 2D Fermi fluid. The inferred hydrodynamic mass of the ³He quasiparticles is 1.38±0.05 m₃. The ³He effective mass ratio increases with coverage to 2.4 at n ₃=4 nm^?2, due to Fermi liquid interactions. The heat capacity isotherm exhibits a steplike increase centred on n ₃=4.5 nm^?2, similar to that previously observed on four layers of ⁴He. This is associated with the population of the first excited Andreev surface bound state. However, in the present case, as n ₃ is increased through the step a pronounced anomalous feature develops in the temperature dependence of the heat capacity, around 10 mK. Below 5 mK the heat capacity is approximately linear in temperature. With n ₃=7 nm^?2, we find that this behaviour is very sensitive to small changes in the ⁴He third layer coverage, around the completed third layer. Measurements of the ³He magnetisation,, by continuous wave NMR methods, find a significant increase with decreasing temperature below around 20 mK. Together the data suggest that a phase transition takes place in the film at low mK temperatures.     

Film condensation heat transfer of helium in a vertical tube

Helium was condensed on the wall of a vertical copper tube (14 mm in diameter, 32 mm long) in the experimental range: vapour temperature in the tube ? 4.1–5.0 K; heat flux ? 0.5–86 mW cm^?2; temperature differences across a liquid film ? 0.01–1.3 K ;film Reynolds numbers ? 3–380.It was found that the heat flux increased linearly with the film temperature difference, yielding a constant heat transfer coefficient of about 0.065 W cm^?2 K^?1. The discrepancy between the experimental results and the laminar condensation theory is also considered.     

Electron escape from the image-potential-induced surface states on liquid helium

Electron escape from the two-dimensional surface state is studied for liquid ⁴He and ³He surfaces. Experimental conditions are established under which proper signals are obtained to give the intrinsic escape rates. The escape rates in the temperature range 1.1 > T > 0.9 K for the ⁴He surface and 0.6 > T > 0.44 K for the ³He surface show the theoretically expected thermalactivation-type temperature dependence. In the present experimental configuration, the measured escape rate at higher electron densities explicitly demonstrates the electron correlation effect. A simple model is presented to account for the correlation effect on the binding energy of the surface electrons.     

Heat capacity of microscopic liquid3He droplets in silver

We report on heat-capacity measurements of liquid³He droplets in a Ag matrix at 15 mKT1 K. The samples have been prepared by implanting about 0.1 at.%³He in 42 µm thick Ag Foils. Annealing of the foils results in high-pressure³He gas bubbles of diameters between 50 Å and 150 Å; the³He in the bubbles liquefies when the metal foils are cooled to low temperatures. After each annealing step we measured the low-temperature thermal relaxation time of the foils and determined in this way the heat capacity of the³He droplets. The results differ from the specific heat of bulk liquid³He. We will discuss possible origins of this deviation.     

A-B Phase Conversion and Coexistence of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He at 2.4 MPa in cylindrical aerogel by NMR method. When the liquid is cooled down from the normal state, the A-like phase appears below superfluid transition temperature T _c ^a which is suppressed in comparison with the transition temperature of the bulk liquid. With further cooling below the certain temperature T _ab,c ^a , the A-like phase is converted into the B-like phase gradually. Both phases stably coexist within about 90 μK. When you keep the temperature constant in which both phases coexist, the A-B phase conversion stops. With furthermore cooling, the whole liquid becomes the B-like phase. The cwNMR spectra at the coexistence state suggest that the B-like phase is not uniformly distributed in the A-like phase like a large number of small bubbles in a liquid, but separated as a whole from the A-like phase. By applying a field gradient which changes as a function of square of radius, we found that the A-like phase is in the edge part with a cylindrical shape and the B-like phase is in the central part with a columnar shape.      

Experiments with 4He Heated from Above, Very Near the Lambda Point

We report on new transport phenomena observed within a column of ⁴He heated from above and close to its transition to superfluidity. In this configuration the helium sample will self-organize to a state where the temperature gradient across the column remains equal to the pressure-induced gradient of T _λ (1.273 µK/cm) even as the heat flux is varied by a factor of about a thousand. On this self-organized state we have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q. This propagating mode is due to a non-linear temperature dependence of the thermoconductance of the helium near criticality. Such a mode had been predicted to exist in this state for Q less than about 100 nW/cm². We confirm that this mode exists in this regime, however we also observe that it propagates at higher values of Q, even when the helium is pushed away from the self-organized state into the normal state.     

Heat Capacity of Adsorbed Helium-3 at Ultra-Low Temperatures

We report on direct measurements of the heat capacity of monolayers of ³He adsorbed on the surface of a copper cell filled with superfluid ³He. We found that at ultra low temperatures the surface ³He heat capacity dominates over the heat capacity of the bulk liquid ³He. The replacement of adsorbed ³He by ⁴He changes the heat capacity of the sample by an order of magnitude. These investigations were made in the framework of the “ULTIMA” project, a dark matter detector based on superfluid ³He in the limit of ultra low temperatures.     

Anomalous thermal and NMR behavior of liquid 3He in contact with an antiferromagnet

Anomalous thermal and NMR behavior has been observed in liquid ³He in contact with a single crystal of Cu(NH₃)₄ · SO₄ · H₂O, which is a magnetically one-dimensional substance. In the presence of a temperature gradient in the vertical direction and parallel to the crystal surface, the temperature of the system and the ³He NMR signal change abruptly at a temperature just above the antiferromagnetic transition temperature of the salt (0·43 K). From the experimental results it is suggested that at the onset of the anomaly the spin-lattice relaxation time of ³He is reduced and a flow is induced in liquid ³He near the surface of the crystal.This work was supported by a Research Grant of the Iwatani Naoji Foundation and a Grant-in-Aid for Scientific Research (Japan).