Heat Transfer Device

Abstract

A heat transfer device (heat pipe) in which the layer of material having a capillary structure for the transport of condensate from the condensor to the evaporator is formed by helically wound wire which has grooves transversely to the axis of the wire.

Description

BACKGROUND OF THE INVENTION

The invention relates to a heat transfer device comprising a closed tubular container having at one end a first tube wall section, at the other end a second tube wall section, and an intermediate third tube wall section. The container comprises a heat transfer medium which absorbs thermal energy from the first tube wall section while changing from the liquid phase into the vapour phase and delivers thermal energy to the second tube wall section while changing from the vapour phase into the liquid phase. The container furthermore comprises a layer of material having a capillary structure and covering the tube wall for transporting condensed medium from the second to the first tube wall section.

Devices of this type are known from the U.S. Pats. Specifications Nos. 3,229,759 and 3,402,767. With such devices, often termed heat pipes, large quantitites of thermal energy can be transmitted substantially without temperature drop, without using a pumping device and without further moving parts. Liquid heat transfer medium which evaporates near the first tube wall section (evaporation zone) moves in the vapour phase to the second tube wall section (condensation zone) as a result of the lower vapour pressure there due to the slightly lower temperature at that area. The vapour condenses on the second tube wall section while delivering the heat of evaporation, after which the condensate is returned through the layer of material having a capillary structure on the basis of capillary action to the first tube wall section to be evaporated there again. On its way, the condensate passes the third tube wall section (transport zone).

The layer of material having a capillary structure ensures that condensate can flow back in all circumstances from the second to the first tube wall section, even against gravity or without gravity field. Gauzes of wire or band-shaped material often serve as a layer of material having a capillary structure in heat pipes.

A drawback of the use of gauzes is that the condensate transport capacity and hence the heat transfer capacity of the device is restricted by it. This is caused by the fact that the large number of wires of the gauze structure which extend transversely to the direction of condensate transport inhibit flow of condensate.

For the transport of condensate, use is sometimes made of grooves which are especially provided for that purpose in the tube wall. The drawback of this construction is that it is difficult, time-consuming and expensive to provide the grooves in the wall with the required precision.

It is the object of the present invention to provide a heat transfer device of the type described in which a large condensate transport capacity of the material layer having a capillary structure is associated with a simple and cheap manufacture and construction of the layer.

SUMMARY OF THE INVENTION

In order to realize this objective the heat transfer device according to the invention is characterized in that the layer of material is formed by at least one wire which is wound helically against the tube wall in the direction of the tube axis and which is provided with several capillary grooves which are distributed over the length of the wire, and extend transversely to the axis of the wire over at least that part of the circumference of the wire which faces the adjacent tube wall surface.

The grooves can be provided in the wire in an easy and admissible manner. The wire which is provided with grooves may then be wound helically and be inserted into the tubular container in the wound condition so as to fit accurately. It is alternatively possible to arrange the wire against the tube wall in the container while winding.

In this manner a layer of material having a capillary structure is obtained in a simple manner in which the capillary grooves in the wire also bounded by the tube wall extend mainly in the axial direction, namely the condensate transport direction, of the device. The present system of grooves may have the same large condensate transport capacity as heat transfer devices having axial grooves in the wall of the tubular container.

Of course, the wire should be wound with such a pitch that gaps between the turns do not become too large. When the gap width is too large, heat transfer medium condensate remains no longer caught in the capillary structure and the operation of the device is disturbed.

In a favorable embodiment of the device according to the invention the turns of the wire section which covers the third tube wall section mutually engage each other. This presents a few advantages. First of all, the winding of said wire section is much simpler but the engaging turns also constitute a closed surface which separates medium condensate in the transport zone from medium vapour present in said zone. Due to the absence of a free phase surface area of heat transfer medium in vapour and in liquid form in the transport zone, no liquid particles from the layer of material having capillary structure can be dragged along by medium vapour. As a result of this a large heat transfer capacity is maintained.

A further favorable embodiment of the heat transfer device according to the invention is characterized in that the capillary grooves in the wire section which covers the first part of the tube wall extend throughout the circumference of the wire.

In the first section of the tube wall forming the evaporation zone, it must be possible for heat transfer medium to evaporate freely from the capillary structure. By providing the wire section which covers the first section of the tube wall with circumferential grooves, the advantage is obtained from a point of view of winding technology that the turns can engage each other while medium evaporating on the first section of the tube wall can nevertheless freely reach the vapour space since the circumferential grooves ensure apertures in the continuous row of turns of wire.

According to the invention, the capillary grooves in the wire section which covers the second section of the tube wall may also extend throughout the circumference of the wire. It must be be possible for medium vapour to condense freely on the second tube wall section forming the condensation zone. This remains the case when the turns of the wire section which covers the second tube wall section engage each other, since the circumferential grooves again constitute apertures in the closed row of turns.

With the above-described measures a heat transfer device is thus possible in which all the wire turns engage each other which makes the device extremely suitable for simple and cheap series production.

A further favorable embodiment of the heat transfer device according to the invention is characterized in that the capillary grooves in the wire section which covers the first tube wall section have a smaller hydraulic diameter and center distance than the capillary grooves in the wire section covering the third tube wall section.

Within this scope, the hydraulic diameter is defined as 4 .times. (surface cross-section/circumference) of the groove. Since comparatively many and small grooves are present per unit of length of wire in the wire section which covers the first tube wall section (evaporation zone) a large capillary suction force is produced, the driving force for condensate transport, relative to the wire section which covers the third tube wall section (transport zone) and which comprises comparatively few and large grooves. Due to the few large grooves the flow losses in the transport zone are small. All this is of particular advantage in heat transfer devices having transport zones and/or evaporation zones of a large length.

In many small grooves in the evaporation zone which extend throughout the circumference of the wire, a large liquid vapour phase area is available in said zone. Thus, a large heat flux (heat flow per unit of tube wall surface) in the evaporation zone and hence comparatively small dimensions of the first tube wall section are possible.

According to the invention, the capillary grooves in the wire section which covers the second tube wall section may have a smaller hydraulic diameter and center distance than the capillary grooves in the wire section covering the third tube wall section.

Due to the comparatively many and small grooves in the wire section which cover the second tube wall section, the condensation zone, a large capillary force is produced in said zone which ensures the transport of condensate to the third tube wall section, the transport zone. Since comparatively few and large grooves are present in the transport zone, transport of condensate through said zone again takes place easily, substantially without flow losses. All this is of particular advantage in heat transfer devices having a condensation zone and/or a transport zone of a large length.

Furthermore, a good removal of condensate in the condensation zone is possible, in particular when a large vapour/liquid phase area is available as a result of the many and small circumferential (annular) grooves.

In a further favorable embodiment of the heat transfer device according to the invention, the helically wound wire is a helical spring which engages the tube wall by resilience. This offers the advantage that an extra connection of the wire against the tube wall, for example by sintering, is not necessary.

In certain circumstances, however, it may occur that the resilience of the helical spring diminishes under the influence of the usually high operating temperatures of the heat transfer device in such manner that the pressure force becomes too small and the helical spring works loose from the tube wall during operation. As a result of this the helical spring is no longer useful for the return of condensate. Hence the operation of the device is disturbed while danger for boiling-dry and tearing of the first tube wall section serving as evaporation occurs.

In order to avoid this drawback, in a favorable embodiment of the invention, the helical spring is internally hollow. The closed cavity contains a filling medium the pressure of which, at least at the operating temperature of the device, is higher than the pressure in the container, and the helical spring remains pressed against the tube wall under the influence of the pressure differential across it.

When the heat transfer device is out of operation and at room temperature, the pressure of the filling medium in the helical spring need not always be higher than the pressure which prevails in that case in the container but it may then be equal to or even lower than the pressure in the container. The resilience of the helical spring at room temperature usually is sufficient to keep the spring pressed against the tube wall. At room temperature the pressure in the container is otherwise usually low since the container is often evacuated in order that the evaporation-condensation process of the heat transfer medium can run off smoothly.

All kinds of material which are solid, liquid or gaseous at room temperature may be considered as a filling medium, provided the vapour pressure and gas pressure, respectively, of said materials is higher than the pressure in the container at any rate at the operating temperature of the device and possibly also at room temperature.

For example, if sodium is present as a heat transfer medium in the otherwise evacuated container, for example, potassium or calcium may be used as a filling medium.

In a favorable embodiment of the invention the filling medium is an inert gas. This offers the advantage that in the case of an unexpected leakage of the helical spring, no chemical reactions occur between the heat transfer medium and the filling medium. In that case the device is not damaged.

The invention will be described in greater detail with reference to the drawings diagrammatically and not to scale a few embodiments of the heat transfer device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows in elevation view in section the heat transfer device of this invention.

FIG. 1b shows a cross-sectional view of the device taken along lines 1b--1b in FIG. 1a.

FIG. 1c shows a longitudinal sectional view of the grooves of FIG. 1a.

FIG. 1d is a sectional view of the wire taken along line 1d--1d in FIG. 1c.

FIG. 2a, 3a, 4a and 5 show other embodiments of the heat transfer device in sectional, elevation views.

FIG. 2b is a cross-sectional view taken on the line IIb--IIb in the evaporation zone of the device shown in FIG. 2a.

FIGS. 2c and 2d are a longitudinal cross-sectional view and a cross-sectional view of wire which is provided with circumferential grooves, which wire is present on the first and the second tube wall section 2 and 3, respectively.

FIG. 3b is a sectional view taken on the line IIIb--IIIb of FIG. 3a.

FIG. 4a shows in sectional, elevation view another embodiment of this invention.

FIG. 4b is a sectional view of the device taken along line IVb--IVb in FIG. 4a.

FIG. 5 is an elevation view in section of another embodiment of the device of this invention.

Reference numeral 1 in FIG. 1a denotes a closed cylindrical container having at one end a first tube wall section 2 and at the other end a second tube wall section 3 separated from each other by an intermediate third tube wall section 4. The container 1 comprises a suitably chosen quantity of sodium as a heat transfer medium and is otherwise evacuated.

On the cylindrical inner wall of the container 1 is present a wire 5 which is wound helically in the axial direction of the cylinder. The wire comprises grooves which extend transversely to the wire axis over half the circumference of the wire and which face the adjoining tube wall surface. The grooves in the wire are shown in detail in FIG. 1c which is a longitudinal sectional view of the wire. The turns of the wire 5 in the container 1 engage each other very closely in this case so that there are very narrow gaps between the turns.

During operation, liquid sodium absorbs thermal energy through the first tube wall section 2 serving as an evaporator from a heat source not shown, as a result of which said, sodium evaporates. Via successively the narrow gaps between the turns of the wire section which covers the first tube wall section 2 and the duct inside the wire turns, sodium vapour then flows to the second tube wall section 3 (condensor) as a result of the lower vapour pressure there due to a slightly lower temperature at that area.

The sodium vapour condenses on the second tube wall section while giving off thermal energy. The condensate then flows through the grooves in the wire 5 on the basis of capillary action, while using the surface tension of the condensate, back to the first tube wall section 2 to be evaporated there again. The third tube wall section 4 constitutes a transport zone.

In the heat transfer devices shown in FIGS. 2 to 5, the same reference numerals are used for parts corresponding to the device shown in FIG. 1 except that numerals of FIG. 2 have (') added, FIG. 3 have (") added, FIG. 4 have (r) added, and FIG. 5 have (s) added. In the heat transfer device shown in FIG. 2a, the turns of wire 5' readily engage each other. The wire sections which cover the first and the second tube wall section 2' and 3' have grooves which extend throughout the circumference of the wire as is shown in detail in the longitudinal sectional view of FIG. 2c and in a cross-sectional view of the wire shown in FIG. 2d. The wire section which covers the third tube wall section 4' on the contrary has grooves which extend only over a part of the circumference of the wire. The last-mentioned wire section has fewer and larger grooves than the wire sections which cover the first and the second tube wall section 2' and 3'. Since all the wire turns engage each other mutually, winding is very simple. The continuous row of turns at the area of the third tube wall section 4' beautifully forms a partition between sodium condensate and sodium vapour. Sodium vapour on its way from the first to the second tube wall section cannot drag along condensate drops of the third tube wall section 4', which would mean a reduction of the heat transfer capacity of the device. Due to the few coarse grooves in the wire section on the third tube wall section 4' the condensate flowing through it will have small flow losses.

The many small circumferential grooves extending transversely to the wire axis over the circumference of the wire in the wire section which covers the first tube wall section provide apertures in the continuous row of turns so that sodium can freely evaporate via said apertures of the relevant wall section. They furthermore ensure a large liquid vapour phase surface area so that a large heat flow can be conveyed through a surface unit of the tube wall section. Finally they produce a large capillary suction force which ensures the transport of condensate.

Analogous construction of the wire section which covers the second tube wall section provides the advantage of an easy removal of condensate owing to on the one hand the many apertures in the closed row of turns and on the other hand the large capillary suction force.

The heat transfer device shown in FIG. 3 comprises a tubular container 1" which has parts of different diameters and which is rectangular in cross-section. Instead of one helically wound wire, two helical springs 6 and 7 are present which remain pressed against the walls of the container by resilience. The helical spring 6 covers the first tube wall section 2" while the helical spring 7 covers the second tube wall section 3" and the third tube wall section 4". Helical spring 6 only has circumferential grooves divided over the whole wire length transversely to the wire axis; helical spring 7 only over the wire section which covers the second tube wall section 3".

FIG. 4a shows a cylindrical heat transfer device in which the first tube wall section 2r bounds a cylindrical space which is present within the dimensions of the heat transfer device. The device is otherwise the same as that of FIG. 2.

FIG. 5 shows a heat transfer device in which a helical spring 8 is present in a closed container is which spring comprises circumferential grooves which extend transversely to the wire axis and are distributed over the whole length of the wire. Helical spring 8 is internally hollow. The cavity 9 constitutes a closed space in which a quantity of argon is present as a filling medium. When the operating temperature of the heat transfer device is, for example, 1100.degree.K, the vapour pressure of the sodium in the otherwise evacuated container 1 is 450 Torr (1 Torr = 1 mm mercury pressure). By a suitable chosen quantity of argon in the helical spring 8 it is achieved that at the said high temperature the argon pressure in the spring is higher than 450 Torr, for example, 2 atmosphere. When the resilient of the helical spring 8 at the operating temperature of 1100.degree.K is insufficient to ensure that the helical spring remains engaging the container wall, as a result of which the operation of the device would be disturbed, the difference in pressure across the helical spring 8 ensures that the spring nevertheless remains positively pressed against the container wall. Should leakage of the helical spring 8 occur, the outflowing argon as an inert gas will not enter into chemical reactions with the sodium. The heat transfer device then remains free from further damage. Of course, all kinds of other embodiments of the heat transfer device are possible within the scope of the present invention, in addition to those shown.

When several wires are used in one heat transfer device, said wires may mutually have different diameters and/or be manufactured from different materials. Wires having a comparatively small diameter could then cover the first and the second tube wall section while a wire having a comparatively large diameter is used for the third tube wall section. Furthermore, one or more wound wires can be combined with one or more helical springs in one heat transfer device.

Claims

1. A heat transfer device comprising a closed tubular container having at one end a first tube wall section, at the other end a second tube wall section, and an intermediate third tube wall section, said container comprising a heat transfer medium which absorbs thermal energy from the first tube wall section while changing from the liquid phase into the vapour phase and delivers thermal energy to the second tube wall section while changing from the vapour phase into the liquid phase, the container furthermore comprising a layer of material having a capillary structure and covering the tube wall for the transport of condensed medium from the second in the first tube wall section, characterized in that the layer of material is formed by at least one wire which is wound helically against the tube wall in the direction of the tube axis and which comprises several capillary grooves distributed over the length of the wire and extending transversely to the wire axis over at least that part of the

2. In a heat-pipe heat transfer device formed as a closed container having walls whose inner surfaces define a tubular bore which has a longitudinal axis, said bore comprising a first section extending from one end of the bore inward, a second section extending from inward from the other end of the bore, and a third section intermediate said first and second sections, the device further including within said bore a heat transfer medium which changes from liquid to vapor phase upon absorbing thermal energy and changes from vapor to liquid phase while giving up thermal energy, the improvement in combination therewith, a helically wound wire within said bore with the outer surface of the wire adjacent said bore surface and said helical axis thereof generally aligned with said bore axis, said wire further comprising on at least that part of its outer surface adjacent said bore surface, capillary grooves which extend generally transversely of the wire axis, whereby said grooves and bore surface comprise a capillary structure and said medium when condensed to a liquid will be

3. A heat transfer device according to claim 2 wherein the capillary grooves in the wire section which covers the first tube wall section have a smaller hydraulic diameter and center distance than the capillary

4. A heat transfer device according to claim 2 wherein the capillary grooves in the wire section which covers the second tube wall section have a smaller hydraulic diameter and center distance than the capillary

5. A heat transfer device according to claim 2 wherein the helical wound wire is a helical spring whose resilience urges same outward to engage

6. A heat transfer device according to claim 5 wherein the helical spring is tubular having walls which define a closed cavity, the device further comprising in said cavity a filling medium the pressure of which, at least at the operating temperature of the device, is higher than the pressure of said medium in the container for urging the helical spring against said bore surface due to the influence of the pressure differential across the

7. Apparatus according to claim 2 wherein said wound wire comprises turns and the turns in said third section of the bore are mutually engaged which thereby form a tubular section that separates the bore through said turns

8. Apparatus according to claim 2 wherein said capillary grooves in the wire situated in said first section extend throughout the circumference of

9. A heat transfer device according to claim 8 wherein the filling medium

10. A heat-pipe heat transfer device comprising a closed container having walls whose inner surfaces define a tubular bore which has a longitudinal axis, said bore comprising a first section extending from one end of the bore inward, a second section extending inward from the other end of the bore, and a third section intermediate said first and second sections, the device further comprising within said bore a heat transfer medium which changes from liquid to vapor phase upon absorbing thermal energy and changes from vapor to liquid phase while giving up thermal energy, and a helically wound wire within said bore with the outer surface of the wire adjacent said bore surface and said helical axis thereof generally aligned with said bore axis, said wire further comprising on at least that part of its outer surface adjacent said bore surface, capillary grooves which extend generally transversely of the wire axis, whereby said grooves and bore surface comprise a capillary structure and said medium when condensed to a liquid will be transportable along said bore by capillary action therein.