GB2127143A

GB2127143A - Heat pipe

Info

Publication number: GB2127143A
Application number: GB08305139A
Authority: GB
Inventors: Graham Rattcliff; Peter Ian Patrickson
Original assignee: G B P HOLDINGS Ltd
Current assignee: G B P HOLDINGS Ltd
Priority date: 1982-09-07
Filing date: 1983-02-24
Publication date: 1984-04-04
Also published as: GB8305139D0

Abstract

In order to prevent the vapour flow within a heat pipe from resisting the return of condensed, heat transfer liquid towards the evaporator section 23, the heat pipe is arranged so that it has separate paths for the vapour flow and liquid return, this being achieved by means of an open-ended tube 24 located coaxially within the heat pipe. The tube 24 is supported by means of a semi-porous plug 25 which causes the liquid to collect as at 26 and slowly drain into the evaporator section. Numerous modifications of the heat pipe, including one where the condenser section is at a greater angle to the horizontal than the evaporator section, are also disclosed. Where a wick for condensate return is provided, this may be sandwiched between the wall of the heat pipe and a coaxial inner tube to increase the area of plain wettable surface contact with the wick to improve the wicking performance. <IMAGE>

Description

SPECIFICATION Heat pipe The present invention relates to heat pipes.

Heat pipes are normally limited in their power handling by the rate of liquid return (condensate return) to the evaporator section. One of the factors which limits this rate of return is the fact that the vapour flow from the evaporator to the condenser is in contraflow to, and hence tends to oppose, the return of the liquid.

Broadly, the present invention relates to a heat pipe which is so arranged that the condensate return is isolated from the vapour flow which is in the opposite direction, and the rate of return is thus enhanced. This may be achieved by providing a return path for the liquid, which is separate from the vapour path from the evaporator to the condenser section, and arranging the heat pipe so that upon the application of heat to the evaporator section, the vapour and liquid flows will be set up in the correct directions. Thus in one construction, the heat pipe may comprise an outer tube sealed at each end and containing a heat transfer medium and means defining separate vapour and liquid flow paths between the evaporator and condenser sections.

This means defining the flow paths may be in the form of an inner tube, which may be coaxial with the outer wall of the heat pipe, and be open at each end and extend almost from one end to the other of the heat pipe. The inner tube may thus define an annular gap which constitutes the return path for the condensed liquid. Towards the evaporator end of the pipe, the gap may be closed by a plug so that condensed liquid will collect behind the plug and slowly drain into the evaporator section assisted by the static liquid head of the collected liquid. The plug may additionally serve as the means to support the inner tube within the outer body of the pipe.

The inner surface of the outer wall of the heat pipe may be wicked or unwicked.

Preferably the arrangement is such that the vapour flow in the evaporator section assists in distributing the returned liquid and that the vapourflow in the condenser section is not obstructed by condensed liquid. These effects may be achieved by ensuring that the vapour flow in each section is in the same circulation direction as the liquid; this may be done by having the ends of the inner tube extending through the two sections and closely facing the ends of the outer pipe.According to a second aspect of the present invention there is provided a heat pipe having an evaporator section and a condenser section and containing a heat transfer medium and a wick for returning the condensed heat transfer medium to the condenser section, the whole, or substantially the whole, of the lateral surfaces of the wick being in contact with surfaces wettable by the condensed heat transfer medium. This aspect of the invention may be used in combination with, or independently of, the first aspect of the invention.

The surface in contact with the radially outer lateral surface of the wick may be the inner surface of the wall of the heat pipe and the surface in contact with the radially inner lateral surface of the wick may be provided by an annular member fitted coaxially within the wall of the heat pipe or by a plurality of such members arranged end to end. Where there is a plurality of such members, the members may be longitudinally spaced to permit access by the condensed heat transfer medium from the wick. The member or members may be provided with through-holes to permit access by the condensed heat transfer medium from the wick.

When used in the same heat pipe as the first aspect of the invention, the member, or members, contacting the radially inner surface of the wick may also serve as a support for the means defining the return path for the condensed heat transfer medium, particularly where this means is an inner tube coaxial with the outer wall of the heat pipe.

The basis of this aspect of the invention is that the start up characteristic of a heat pipe, particularly when subjected to very low rates of rise of temperature can be enhanced by improving the return flow of condensed heat transfer medium to the evaporator via the wick (if the condensed heat transfer medium is vapour-locked) by increasing the wicking capability. It has been demonstrated that the wicking performance of a wick is greatly enchanced when in contact with a plain wettable surface, and the second aspect of the present invention ensures that whereas previously the wick was in contact with only one wettable surface, the inner member or members ensure that both inner and outer wicking surfaces are in contact with a wettable surface.As well as increasing the area of contact between the wick and a wettable surface, the inner member or members also ensure the uniformity and integrity of the wick structure and, as mentioned above, may also be utilised to support the inner tube of the heat pipe, where provided. The second aspect of the invention will improve wicking performance throughout the total operational range of the heat pipe.

The invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a typical conventional heat pipe; Figure 2 is a corresponding view of a first embodi mentofthe present invention; Figure 3 is a cross-sectional view of a second embodiment of the present invention; Figures 4 to 7 show modifications of the embodiment of Figure 3; and Figures 8 and 9 are respectively a longitudinal section and a cross-section of part of an embodiment of the first and second aspects of the invention.

Figure 1 shows a conventional heat pipe which comprises pipe body 1 which is inclined to the horizontal so that its condenser section 2 is slightly higher than its evaporator section 3. This form of heat pipe, as mentioned earlier, suffers from the disadvantage that the vapour flow 4 between the evaporator section 3 and the condenser section 2 tends to resist the return flow of the condensed liquid 5.

Figure 2 shows in cross-section a first embodi ment of the present invention. In this, the heat pipe comprises an outer, heat conductive cylindrical tube 21 which is sealed at each end to provide a condenser end 22 and an evaporator end 23. In normal use, the pipe is upwardly inclined from the evaporator end 23 to the condenser end 22.

Extending almost from end to end within the outer tube 21 is an inner tube 24 which is held in spaced concentric relation with the outer tube 21 by a plug 25 which is porous to the liquid phase of the heat transfer medium within the heat pipe. The annular gap 27 between the outer tube 21 and the inner tube 24 provides the return path for the heat transfer liquid which has condensed in the condenser section and has run down hill towards the evaporator section. The inside of the inner tube 24 provides the forward, vapour flow path from the evaporator section 23 to the condenser section 22.

The plug 25 may be located at the junction between the evaporator section and the adiabatic longitudinal central section of the pipe and may be adapted in any one of a number of ways so that a desired rate of return of the liquid to the evaporator section, under the driving force of the head of liquid above the plug is achieved; for example, the plug may be made of a semi-porous material such as phosphor bronze and/or be provided with a clearance between it and one of the two tube walls, and/or have a milled surface and/or slots, boreholes or small tubes through it.

In operation, the condensed liquid 26 flowing back from the condenser section collects behind the plug 25 and so provides a static liquid pressure head to enable the liquid to flow at a controlled rate through the plug 25 and into the evaporator section 23.

The inner surface of the tube 21 may be wicked using conventional techniques or may be devoid of any capillary lining.

When used in the near horizontal position (i.e. with the evaporator 23 slightly lower than the condenser 22), the liquid fill of the heat pipe 20 is calculated to completely fill the annular gap between the evaporator 23 and condenser 22 when the heat pipe is working. Sufficient excess liquid to allow 'wetting' of the evaporator and condenser regions should be provided.

With the application of heat to the evaporator section 23, boiling occurs and the vapour generated takes the route of the lowest resistance, up the inner tube 24, driving out and ejecting the fluid within, into the condenser section 22. Vapour now freely flows up the inner tube 24 and condenses in the condenser section 22, the condensate returning down the annular gap 27 so forming the column 26 of liquid in the annular gap which now presents a positive driving head of liquid flow into the evaporator section 23. The intrusion of the inner tube 24 into the evaporator section 23 ensures that the vapour flow when generated, is in the same direction of circulation as the liquid flow. This means that the vapour flow assists the distribution of liquid throughout the evaporator section, whereas in conventional heat pipe techniques the flows are opposing.The same effect occurs in the condenser section 22.

As shown in Figure 3, to improve or increase the effective head of liquid driving the return liquid an upward bend in the pipe 20 towards the condensor section 22 may be made. This ensures a positive head even when the horizontal adiabatic and evaporator sections are inclined upwardly from the horizontal. The embodiment of Figure 3 is otherwise similar to that of Figure 2.

Under start-up conditions a column of liquid must be raised up the inner tube 24 and ejected into the condenser section 22. The vapour pressure required in the evaporator section 23 to achieve this may be great enough to allow vapour to pass the semiporous plug 25, and a 'start-up' failure mode will then take place. To ensure that the 'start-up' liquid heads in both the annular gap 27 and inner tube 24 are equal, one or more small 'bleed holes' 29 as shown in Figure 4 may be provided in the inner tube 24 at the level of the start-up head. This allows liquid from the inner tube 24to bleed into the annular gap 27 and maintain equal levels.

The volume of liquid ejected from the inner tube 24 either during start-up or when the heat pipe is running may be sufficient to raise the head in the annular gap 27 to an unacceptable level i.e. enough to blank off the entire condenser section 22. To prevent this and to maintain a predetermined level in the annular gap, both at start up and running conditions a liquid reservoir 30 (Figure 5) of sufficient volume may be provided to accept the volume of fluid ejected from the inner tube. This reservoir 30 is open to the annular gap at an appropriate predetermined level and to the inner tube at an appropriate predetermined lower level. The reservoir 30 may be an external attachment as shown, or for example a cylindrical or other suitably shaped attachment coaxial with the heat pipe.A further possibility is for it to be provided by a change in the cross-sectional area of the annular gap 27. The cross-section of the reservoir 30 is preferably sufficiently large in relation to the horizontal crosssection of the gap 27 that the liquid head remains approximately constant as liquid is ejected into the reservoir 30.

A reservoir such as 30 may be desirable because without it during "start up" under very slow rates of temperature rise of the evaporator, a mode of failure may be encountered which causes the evaporator 23 to "dry out" before actual "start up" occurs.

More specifically this failure mode is caused by the vapour generated at the evaporator 23 slowly pushing the liquid in the inner tube 24 back until the vapour/liquid interface passes beyond the evaporator region 23 and into the liquid condensate return region 26. At this time vapour will condense down on the wall of the inner tube 24 raising the temperature and hence the temperature of the working fluid in the annular gap.

Heat transfer is thus effected from the evaporator 23 to the working fluid in the annular condensate return gap 26. If this heat transfer should occur for an over-long period, vapour may eventually be generated within the annular gap 26 at the entrance to the evaporator 23. This vapour may finally block the entrance to the evaporator 23 and hence preclude any possibility of the working fluid entering the evaporator 23 thereby causing "dry out".

A similar effect may be observed if the plug 25 has too great a porosity and hence allows vapour produced in the evaporator to pass too freely through it.

This effect may be obviated either by joining the lower end of the reservoir 30 directly through to the inner tube 24 whilst keeping the upper end of the reservoir 30 joined to the annular condensate return gap 27 or by joining the lower end of the reservoir 30 directly through to the inner tube 24 alone and having no further connections between the reservoir and the heat pipe.

The failure mode described above is then preempted by the following mechanism: As the working fluid is ejected from the inner tube 24 it will collect in the reservoir 30 which being of large cross-sectional area will permit only a small rise in fluid head in the inner tube 24.

When the vapouriliquid interface in the inner tube passes beyond the porous plug 25 point in the annular gap 27 and vapour generation occurs within the annular gap 27 as previously described, the vapour generated therein will force some of the liquid in the annular gap 27 back along the gap 27 causing a rapid raise in working fluid head in the annular gap 27.

This will finally lead to an excess working fluid head in the annular gap 27 compared to the inner tube 24. Thus the excess head pressure in the annular gap 27 will force the collapse of the vapour bubble in the annular gap 27 and allow the fluid to return to the porous plug 25 and hence to supply the evaporator 23.

Under very slow "start up" conditions, the above mechanism will be cyclical thus ensuring that periodically the evaporator is always wetted.

If start-up conditions are particularly slow i.e. low evaporator heating rate, it is possible that the adiabatic section adjacent to the semi-porous plug 25 will be heated by conduction, vapour or convection at nearly the same rate as the evaporator.

Vapour generation in this region will prevent liquid reaching the semi-porous plug 25 and hence the liquid feed to the evaporator section 23. To prevent this situation arising during start-up, a one-way valve 31 as shown in Figure 6 may be sited between the semi-porous plug 25 and the adiabatic section.

This valve 31 will permit the flow of liquid under normal running conditions but prevents any reverse flow in the annular gap during start-up.

With the heat pipe in the operational mode, whether or not fitted with any of the modifications shown in Figures 3 to 6, provision may be made to improve the distribution of liquid within the evaporator section 23. This can be achieved by providing an auxiliary fluid supply throughout the evaporator section, other than via the semi-porous plug 25 or wick, if fitted. For example, a small bore pipe 32 running through the semi-porous plug and extending well into the evaporator section as shown in Figure 7 will allow liquid to be fed from the annular gap 22 direct to any selected area within the evaporator.

One version of a heat pipe of the type described, when having a diameter of 15mm and a length of 4000 mm and a liquid head of 100 mm, was capable of transporting 1.25 kW with a total temperature gradient of 5"C, with the evaporator raised 20 mm above the horizontal. Heat pipes using conventional techniques and of the same dimensions rarely exceed a power transport capability of 0.2 kW when operated in the same temperature range (20'C 150"C).

Figures 8 and 9 illustrate an embodiment of both the first, and the second, aspects of the present invention. In the embodiment of Figures 8 and 9, the heat pipe is provided with an annular wick 41 of any suitable material for returning the condensed heat transfer medium to the evaporator section of the heat pipe. The radially outer lateral surface of the wick 41 is pushed into contact with the inner surface of the outer wall 21 of the heat pipe by means of a series of inner tubes 42a, 42b which are tightly fitted inside the wick coaxially with the outer wall 21 of the heat pipe.The inner surface of the outer wall 21 and the outer surface of the inner tubes 42 provide plain surfaces wettable by the condensed heat transfer medium contacting respectively the radially outer and radially inner lateral surfaces of the wick so as to improve its wicking performance as described above. Where, as in Figures 8 and 9, the heat pipe also embodies the first aspect of the invention, one or more of the inner tubes 42 (e.g. 42b) may be used to support the innert tube 22 which is used to define the return path for the condensed heat transfer medium. This may, for example, by achieved by providing the member 42 with a radially inturned end as at 43. In order to improve access for the condensate, the members 42 may be spaced from one another lengthwise of the heat pipe and/or a numberof apertures such as 44 may be provided.

Instead of a number of discrete inner tubes 42a, 42b, etc, a single, continuous inner tube would be provided.

Figure 9 shows at 45 a pool of the condensed heat transfer medium.

Claims

1. A heat pipe having an evaporator section, a condenser section and a vapour flow path for vapourised transfer medium passing from the evaporator section to the condenser section, in which a return path, isolated from the vapour flow path, is provided for the return of condensed heat transfer medium from the condenser section to the evaporator section.

2. A heat pipe according to claim 1 which is provided with a return path for liquid heat transfer medium which is separate from the vapour path and which is arranged so that upon application of heat to the evaporator section, the vapour and liquid flows will be set up in the correct directions.

3. A heat pipe according to claim 1 or 2 wherein the vapour and liquid paths are coaxial with one another.

4. A heat pipe according to any one of the preceding claims wherein the arrangement is such as to ensure that in the condenser section and/or the evaporator section the vapour flow is in the same direction of circulation with respect to the recirculation of heat transfer medium as the the liquid flow.

5. A heat pipe according to any one of the preceding claims and which comprises an outer tubular body sealed at each end and containing therein a coaxial, open-ended tube extending from adjacent one end of the outer tube to adjacent the other end of the outer tube.

6. A heat pipe according to claims 4 and 5 wherein the desired flow directions are assisted by the fact that the inner tube extends through the evaporator and condenser sections and has its ends closely facing the respective end walls of the outer tubular body.

7. A heat pipe according to claim 5 or 6 wherein the inner and outer tubes are bent so that the condensing section may be inclined an angle to the vertical different from that of the evaporator section.

8. A heat pipe according to claim 5, 6 or 7 wherein there is at least one aperture in the inner tube to enable the level of liquid in the inner and outer tubes to equalise.

9. A heat pipe according to any one of claims 5 to 8 wherein a plug, porous to the liquid phase of the heat transfer medium, is arranged to close the space between the inner and outer tubes adjacent the evaporator section of the heat pipe.

10. A heat pipe according to any one of the preceding claims wherein the arrangement is such that in use a body of condensed liquid collects upstream of the evaporator section and provides a static pressure head for delivery of the liquid to the evaporator section.

11. A heat pipe according to claims 9 and 10 wherein the plug causes the body of liquid to collect.

12. A heat pipe according to any one of claims 5 to 11 and including at least one further tube arranged to conduct liquid returning to the evaporator section, to a desired area thereof.

13. A heat pipe according to any one of the preceding claims and including a one-way valve in the liquid return path for permitting liquid flow only in the direction from the condenser section to the evaporator section.

14. A heat pipe having an evaporator section and a condenser section and containing a heattransfer medium and a wick for returning the liquefied heat transfer medium to the condenser section, the whole, or substantially the whole, of the lateral surfaces of the wick being in contact with surfaces wettable by the condensed heat transfer medium.

15. A heat pipe according to claim 14 wherein the surface in contact with the radially outer lateral surface of the wick is the inner surface of the wall of the heat pipe and the surface in contact with the radially inner lateral surface of the wick is provided by an annular member fitted coaxially within the wall of the heat pipe or by a plurality of such members arranged end to end.

16. A heat pipe according to claim 15 wherein, there being a plurality of such members, the members are longitudinally spaced to permit access by the condensed heat transfer medium to the wick.

17. A heat pipe according to claim 14,15 or 16 wherein the, or at least one such, member is provided with through-holes to permit access by the condensed heat transfer medium to the wick.

18. A heat pipe according to any one of claims 14 to 17 and having the features set forth in any one of claims 1 to 13.

19. A heat pipe according to claim 18 wherein the, or at least one such, member serves to support means defining the return path of the condensed heat transfer medium.

20. A heat pipe constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figure 2 of the accompanying drawings.

21. A heat pipe according to claim 10 when modified substantially as hereinbefore described with reference to and as illustrated in any one of Figures 3 to 7 of the accompanying drawings.

22. A heat pipe constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figures 8 and 9 of the accompanying drawings.