JPH0539991A - Heat exchanger for superfluid helium generation - Google Patents

Abstract

PURPOSE:To keep a sufficient performance of heat-exchanging with helium to certainly generate superfluid helium even under cryogenic temperatures at which Kapitza resistance increases remarkably. CONSTITUTION:Fins 22 and 24 are attached on the surfaces of main bodies 14, 16 and 18 made of copper. Or, particles composed of metals such as copper, etc., are sintered on the surface and a lead-tin alloy thin layer or metallic oxide layer is formed on the surface of the particles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat exchanger used in a superfluid helium generator.

[0002]

2. Description of the Related Art In recent years, along with the active use of superconductors represented by superconducting magnets, development of a pressurized superfluid helium generator, which is a refrigerant for the superconductors, has been advanced.
In this pressurized superfluid helium, as shown in FIG. 6, the helium in the normal fluid state is cooled to a point λ (point of temperature 2.17K on the saturated vapor curve L) or less, and undergoes phase transition to the superfluid state, Also, it is pressurized to a pressure higher than the saturated vapor pressure (point P _{2 in the} figure), and its thermal conductivity is extremely high, so that heat is rapidly taken away from the object to be cooled, and the temperature uniformity of the entire liquid is improved. Since it is particularly excellent and has an advantage of maintaining a liquid phase state more stably than saturated superfluid helium (point P _{1 in the} figure) at the same temperature, it is very important in ultra-low temperature technology.

Conventionally, as a method for generating such pressurized superfluid helium, the saturated superfluid helium (point P _{1 in} FIG. 6) is once cooled by cooling the helium in the normal fluid state along the saturated vapor curve L. There are known a method of generating and pressurizing this, and a method of directly generating pressurized superfluid helium (P _{2 in} FIG. 6) by cooling the normal fluid helium at a constant pressure.

Among these, an example of an apparatus for generating pressurized superfluid helium by the latter method is shown in FIG.
In the figure, a normal-flow liquid helium tank 2 contains about 4.2K of normal-flow helium 1. This ordinary fluid liquid helium tank 2 is connected to a superfluid liquid helium tank 3 via a communication path 4.
However, the communication passage 4 is normally closed by a safety valve 9 so that both tanks 2 and 3 are thermally insulated from each other and also from the outside.

A heat exchanger 5 for generating superfluid helium is provided in the superfluid liquid helium tank 3, and an inlet end portion of the superfluid liquid helium tank 3 is provided with a helium introducing pipe 11 and a JT valve 6 through which the normal fluid liquid helium tank is provided. 2 and the outlet end is connected to the top of the normal flow liquid helium tank 2 via the helium outlet pipe 12, the vacuum pump 8 and the liquefier 13. In the figure, 7 is a heat exchange section for exchanging heat between the helium introducing pipe 11 and the helium introducing pipe 12.

In such an apparatus, the superfluid liquid helium tank 3 contains the ordinary fluid liquid helium (the liquid helium to be cooled), and the vacuum pump 8 is operated to reduce the pressure in the heat exchanger 5, Liquid helium in the liquid helium tank 2 is expanded through the helium introducing pipe 11 and the JT valve 6 to generate low-temperature saturated superfluid helium in the heat exchanger 5, and superfluid by heat exchange with the saturated superfluid helium. By cooling the liquid helium in the liquid helium tank 3 while maintaining it at atmospheric pressure, pressurized superfluid helium can be generated in the superfluid liquid helium tank 3.

As described above, since superfluid helium is produced in the superfluid liquid helium tank 3 through the heat exchanger 5, the heat exchanger 5 is required to have an extremely high heat transfer performance. Therefore, conventionally, fins are formed on the heat transfer wall of the heat exchanger 5, directly on the surface of the heat transfer wall of the heat exchanger 5, or on the surface of the fin formed on the heat transfer wall, the heat conductivity is high. It has been devised to increase the surface of the heat exchanger by sintering fine metal powder.

[0008]

The Kapitza resistance (interfacial heat resistance) becomes remarkable in the extremely low temperature range, particularly in the temperature range where superfluid helium is produced as described above. The Kapitza resistance is generally a thermal resistance generated at a boundary surface in heat transfer between substances having different densities, and is considered to be caused by reflection of phonons at the boundary surface.
This Kapitza resistance is more remarkable as the acoustic impedances of the two substances are less matched, and therefore a substance such as liquid helium that has an extremely low density and a low sound velocity can be used at a low temperature when compared with a solid such as metallic copper. Will have a large resistance to Kapitza. The thermal phenomenon due to this Kapitza resistance is expressed by the following equation.

Q = h _K · ΔT where q is the heat flow from the first substance to the second substance, h _K is the Kapitza thermal conductivity (that is, the reciprocal of the Kapitza resistance), and ΔT is the first The surface temperature difference between the substance and the second substance, that is, the temperature jump at the interface is shown.
As is clear from this equation, when the Kapitza thermal conductivity h _K is small, that is, when the Kapitza resistance is large, the surface temperature difference also becomes large and the heat transfer is hindered accordingly. For example, at a refrigerant temperature of 1.8K often used in the above superfluid helium cooling device, q = 1W /
At cm ² , ΔT is 1-2K depending on the surface condition of the material
Also reaches.

Therefore, no matter how much the surface area of the heat exchanger 5 is increased as in the prior art, a large surface temperature difference occurs between the heat transfer wall of the heat exchanger and the helium inside and outside thereof due to the Kapitza resistance. As a result, sufficient heat exchange is not performed between the heat exchanger 5 and the liquid helium, and the liquid helium in the superfluid liquid helium tank 3 may undergo a phase transition to a normal flow state.

In view of the above circumstances, the present invention ensures sufficient heat exchange with liquid helium even at extremely low temperatures where the Kapitza resistance is remarkable, and can reliably generate superfluid liquid helium. The purpose is to provide a container.

[0012]

In recent years, by forming a thin film of a lead-tin alloy (so-called solder) or a metal oxide film on the surface of copper, compared with the case without such a thin film, the resistance to liquid helium and the Kapitza resistance are increased. Has been confirmed and published by research.

The present invention intends to contribute to the technology for generating superfluid helium by utilizing such a physical phenomenon. It is immersed in liquid helium to be cooled and saturated superfluid helium inside and outside In a superfluid helium generation heat exchanger that cools the liquid helium to be cooled to superfluid helium by performing heat exchange with the liquid helium to be cooled, the main body is made of copper, and its inner and outer surfaces Fins on at least one of the surfaces, or particles of copper or precious metal are sintered and lead
A thin film of tin alloy or an oxide film of copper or the like is formed.

[0014]

According to the above construction, the surface area of the heat exchanger is increased by the formation of the fins or the sintering of the metal particles, and the thin film or the metal oxide film made of the lead-tin alloy is formed on the surface thereof. Therefore, these thin films enhance the matching of the acoustic impedance with liquid helium and reduce the Kapitza resistance at the interface. Therefore, heat transfer between the heat exchanger heat transfer wall and the external liquid helium is promoted, and the liquid helium is effectively cooled by the heat exchanger.

[0015]

FIG. 1 shows a heat exchanger 5 for generating superfluid helium in one embodiment of the present invention. The overall structure of the superfluid helium generator equipped with the heat exchanger 5 is the same as that shown in FIG. 5, and the description thereof is omitted here.

The heat exchanger 5 has a cylindrical outer peripheral wall 14, and an upper portion and a lower portion of the outer peripheral wall 14 are closed by a top wall 16 and a bottom wall 18, respectively. A helium outlet pipe 20 is fixed to the center of the ceiling wall 16 so as to penetrate therethrough, and a helium introducing pipe 11 is introduced into the center of the helium outlet pipe 20. The helium outlet pipe 20 is shown in FIG. It is connected to the helium lead-out pipe 12 and the vacuum pump 5 shown. Further, a large number of vertically extending pipe-shaped fins 22 are vertically provided on both upper and lower surfaces of the top wall 16 and the bottom wall 18 (that is, both front and back surfaces), and a large number of thin plates extending vertically on the outer surface of the outer peripheral wall 14. -Shaped fins 24 are provided over the entire circumference, thereby increasing the surface area. Further, each member in the heat exchanger 5 is made of copper, preferably oxygen-free copper having particularly high thermal conductivity.

Although only the specific row of fins 22 are shown in FIG. 1, the fins 22 are actually provided over the entire surfaces of the top wall 16 and the bottom wall 18.

Then, the entire shape is 10 to ⁴ with such a shape.
Heat treatment is performed at a temperature of 900 ° C for 12 hours in a vacuum below Torr, and then the whole is immersed in a lead-tin alloy (ie, solder) solution to form a lead-tin alloy thin film on the entire surface. ..

In such a structure, the helium introduced through the helium introducing pipe 11 and the JT valve 6 expands and cools while the pressure inside the heat exchanger 5 is reduced by the operation of the vacuum pump 8 shown in FIG. It is converted into saturated superfluid helium, and the external liquid helium is cooled by heat exchange between this low-temperature saturated superfluid helium and the liquid helium to be cooled outside the heat exchanger 5, and pressurized superfluid helium is generated.
Since the fins 22 and 24 are provided on the surfaces of the heat transfer walls 14, 16 and 18 of the heat exchanger 5 and the thin film of lead-tin alloy is formed on the surfaces thereof, the thin films are Compared to the case without it, the Kapitza resistance between the heat exchanger 5 and the helium inside and outside thereof is reduced, the heat transfer property is improved accordingly, and the external liquid helium is effectively cooled, stabilizing the superfluid state. Will be kept. Moreover, in this embodiment, since the entire heat exchanger 5 is heat-treated in vacuum before the formation of the lead-tin alloy film, dislocations in the heat transfer wall of the heat exchanger 5 are reduced, and the heat conductivity is further improved. improves.

The lead-tin alloy has a lower thermal conductivity than copper, and if the lead-tin alloy thin film is thick, the heat transfer property of the heat exchanger 5 as a whole is impaired. It is desirable to set the size to be as small as possible within the range where the matching of the acoustic impedance with liquid helium can be enhanced by the presence of the above thin film, and specifically, about 20 μm.
m is preferred.

Next, a second embodiment will be described. here,
As shown in FIG. 2, the fin 2 according to the first embodiment.
2, 24 are omitted, and instead of this, the outer peripheral wall 14 and the ceiling wall 1
6, and by sintering particles of copper or a noble metal, that is, a metal having high thermal conductivity, on the surface of the bottom wall 18,
The surface area is increasing. If the diameter of the metal particles is too small, the flow of helium on the surface of the heat exchanger 5 is hindered,
On the contrary, since the heat transfer property is reduced, the diameter of the metal particles is 1 mm.
It is desirable to set it to a degree.

Then, after heat-treating the entire heat exchanger provided with the sintered body in a vacuum as described above, a further 200
Heat treatment is carried out for 40 minutes in the atmosphere of 0 ° C or oxygen gas to form an oxide film of about 100 nm on the surface.

Even in such a structure, since the metal oxide film is formed on the surface, the Kapitza resistance with liquid helium is reduced and the thermal conductivity is improved as compared with the conventional heat exchanger without the oxide film. To do.

In this embodiment, the heat exchanger having no fins 22 and 24 is provided with the sintered body, but the surface of the fins 22 and 24 as shown in FIG. 1 is sintered. The body may be arranged. Further, the copper oxide film may be formed directly on the surfaces of the fins 22 and 24 as shown in FIG. 1, or a thin film of lead-tin alloy may be formed on the surface of the sintered body.

Next, a third embodiment is shown in FIG. Here, the heat exchanger 5 is composed of a circular tube 26 wound in a spiral shape, the helium introducing tube 11 is connected to one end of the circular tube 26, and the helium outlet tube 12 is connected to the other end. ing. A large number of fins 28 as shown in FIG. 4 are formed on the surface of the circular pipe 26 along the axial direction. After the whole is heat-treated in vacuum, the entire surface of the circular pipe 26 including the fins 28 is A lead / tin alloy film or a copper oxide film is formed in the same manner as in the embodiment.

As described above, in the present invention, the present invention can be applied to those having various shapes for performing heat exchange between liquid helium inside and outside thereof, regardless of the specific shape of the heat exchanger 5. .. For example, metal particles may be sintered on the surface of the circular pipe 26 shown in FIG. 3 instead of the fins 28.

Further, in the present invention, if a thin film of lead-tin alloy or a metal oxide film is provided on at least one of the inner and outer surfaces of the heat exchanger, higher heat transfer performance than conventional can be obtained. It goes without saying that it will be more effective if the thin films are provided on both sides.

[0028]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, the surface of the heat exchanger whose surface area has been increased by disposing the fins and the sintered body has a lead
By forming a thin film of tin alloy or a metal oxide film to reduce the Kapitza resistance with liquid helium, it is possible to secure good heat exchange between saturated superfluid helium inside and liquid helium outside. By effectively cooling the external liquid helium to be cooled, the liquid helium can be stably brought into a superfluid state.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional perspective view of a heat exchanger according to a first embodiment of the present invention.

FIG. 2 is a partial cross-sectional perspective view of a heat exchanger according to a second embodiment of the present invention.

FIG. 3 is a perspective view of a heat exchanger according to a third embodiment of the present invention.

FIG. 4 is a cross-sectional view of a circular pipe that constitutes the heat exchanger.

FIG. 5 is an overall configuration diagram showing an example of a superfluid helium generator.

FIG. 6 is a state diagram of helium.

[Explanation of symbols]

 5 heat exchangers 22, 24, 28 fins

Claims (2)

[Claims] 1. A liquid helium to be cooled is immersed in the liquid helium to be cooled, and heat exchange is performed between the saturated superfluid helium inside and the liquid helium to be cooled outside to cool the liquid helium to be cooled. In a heat exchanger for generating superfluid helium that uses helium, the body is made of copper, and fins are formed on at least one of the inner and outer surfaces of the body, and a thin film of lead / tin alloy or a copper oxide film is formed on the surface of the fin. A heat exchanger for generating superfluid helium, characterized in that 2. The liquid helium to be cooled is superfluid by being immersed in the liquid helium to be cooled and causing heat exchange between the saturated superfluid helium inside and the liquid helium to be cooled outside. In a superfluid helium heat exchanger that uses helium, the main body is made of copper, and at least one of its inner and outer surfaces is sintered with particles of copper or noble metal, and the surface of this sintered body is made of lead, A heat exchanger for generating superfluid helium, characterized in that a thin film of tin alloy or an oxide film of a sintered body is formed.