GB1602093A

GB1602093A - Two-phase thermosiphons

Info

Publication number: GB1602093A
Application number: GB2469377A
Authority: GB
Original assignee: UK Secretary of State for Industry
Current assignee: UK Secretary of State for Industry
Priority date: 1977-06-14
Filing date: 1977-06-14
Publication date: 1981-11-04
Also published as: FR2394776A1; DE2825515A1; JPS546113A

Description

(54) IMPROVEMENTS IN OR RELATING TO TWO PHASE THERMOSIPHONS (71) I, THE SECRETARY OF STATE FOR INDUSTRY, London do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to two-phase thermosiphons.

A two-phase thermosiphon is a sealed system in which a closed evaporation/condensation cycle takes place.

It is a device for transferring heat from one location adjacent an evaporator section of the sealed system to another location adjacent a condenser section of the system.

A heat pipe, as referred to in the specification, is a particular form of twophase thermosiphon in which liquid formed at the condenser section is returned to the evaporator section, at least in part, by means of a wick structure which functions by capillary action.

Heat pipes are particularly useful where it is desired to operate a thermosiphon in the absence of a gravitational field. The wick will act to return liquid to the evaporator section regardless of the orientation of the system. In a gravitational field, the effectiveness of heat pipes is limited by their capillary height i.e. the height of the column of liquid the wick can transport by capillary forces. Thermosiphons which are not heat pipes can operate satisfactorily under gravity with liquid from the condenser section draining back to an evaporator section located below it.

It is known to provide heat pipes having wick structures in which are located one or more arteries for the purpose of increasing the flow rate of liquid from the condenser section to the evaporator section. See, for example. P. D. Dunn and D. A. Reay "Heat Pipes" Pergamon 1976 pp 98-99. These known arterial wicks in heat pipes have certain disadvantages, one of which is that their effectiveness is reduced when bubbles of non-condensing gas are present within an artery. This is because the artery must be completely closed off from the vapour space by the wick structure in the known arrangement, hence leading to the possibility that a bubble of non-condensing gas will block the artery and hence render it ineffective.This invention seeks to provide a two-phase thermosiphon which is not limited to operation with its condenser section above its evaporator section, and having a passage for providing an increased flow of liquid from the condenser section back to the evaporator section whose effectiveness is less severely restricted than these known arteries by the presence of non-condensing gas within the thermosiphon.

According to the present invention there is provided a two-phase thermosiphon comprising a sealed vessel containing a liquid and its vapour, said vessel having a wall; an evaporator section comprising a first area of the wall, and a condenser section comprising a second area of the wall, said evaporator and condenser sections being connected by a vapour space within the vessel; a header space within the vessel; a wick separating the header space from the vapour space, said wick extending into and covering at least a part of the first area of the wall within the evaporator section, said wick being capable of conveying liquid from the header space to the evaporator section; and a liquid flow path from the condenser section to the header space.

By "vapour space" is meant that volume within a thermosiphon vessel in which in use there is a flux of vapour between the evaporator section of the vessel and the condenser section of the vessel. By "header space" is meant a volume which is not part of the vapour space and through which the main vapour flux does not pass during operation of the device. In some embodiments of thermosiphon according to the invention there will be a significant flux of vapour from the vapour space into the header space, but this flux will not be responsible for the bulk of the flow of heat from the evaporator section to the condenser section. In any case, the pressure of vapour within the header space must in use be significantly below that in the vapour space. In use of the thermosiphon, there will be a flow of condensate from the condenser section through the header space to the evaporator section.The header space must be such as to release from the flow and/or to condense any bubbles of gas or vapour which enter the header space.

A saturated wick can support across it a pressure difference which can be as much as its characteristic capillary height APc. In thermosiphons according to the present invention this characteristic is used -to establish in use a pressure difference between the vapour space and the header space, and this pressure difference can be as high as APc. If the pressure difference is higher or if the wick is not saturated over all its area separating the spaces, then there can be a flow of vapour through the wick out of the vapour space into the header space. On the other hand, liquid can freely pass through a saturated wick from the vapour space to the header space and is urged to do so by the pressure difference across the wick created by evaporation of liquid from the evaporator section into the vapour space.This pressure difference is available to drive the liquid out of the vapour space and into the header space in other ways, for example, upwardly along a tube connecting the vapour space with the header space. Once liquid has reached the header space, it is returned to the evaporator section of the thermosiphon to feed the evaporator by the same wick that separates the header space and the vapour space. This results from capillary wicking action by which liquid scales along the wick to replace liquid lost therefrom by evaporation in the evaporator section.

Liquid can be transferred from the vapour space into the header space over an upward distance which is larger than the capillary height of the wick by transferring a mixture of liquid and vapour in a suitable tube or other transfer means. Such a mixture can be lifted through a greater vertical distance by a given pressure difference because it is less dense than the full liquid. One way of providing a vapourliquid mixture in the transfer means is to provide a tube communicating between a condenser section reservoir (into which liquid can flow from the condenser section) and the header space with the end of the tube in the reservoir occluded by liquid. A hole in the wall of the tube outside the reservoir allows vapour to enter the tube from the vapour space and mix with the liquid there to provide a vapour-liquid mixture in the tube.Besides allowing transfer over greater heights such an arrangement has the advantage that contaminating gases should be carried out of the vapour space into the header space where they are less detrimental to the performance of the thermosiphon.

Where the thermosiphon is to operate in any orientation it will be necessary that the transfer means should operate in any orientation. One way of ensuring that the end of the tube in the condenser reservoir is occluded at any orientation of the sealed vessel is to use a wick to separate the vapour space and the reservoir. The pressure of vapour in the vapour space will drive the liquid which condenses in the condenser section through the wick and into the reservoir to maintain the reservoir substantially full of liquid and occlude the end of the tube.

Any bubbles of contaminating gas which form in the liquid flowpath during operation of the device will be swept into the header space by the flow of liquid so that they do not permanently obstruct the operation of the liquid flowpath.

For a better understanding of the invention, reference will now be made, by way of example, to the drawings filed with the Provisional Specification, of which: Fig. 1 is an axial section of a first embodiment of two-phase thermosiphon according to the invention, Fig. 2 is an axial section of a second embodiment of two-phase thermosiphon according to the invention, and Fig. 3 is an axial section of a third embodiment of two-phase thermosiphon according to the invention.

In Figure 1 there is shown a two-phase thermosiphon constituted by a sealed vessel 2 containing a quantity of a volatile liquid 12 in contact with its vapour. The vessel contains a vapour space 4 and a header space 6 separated by a wick 8 which extends down the wall of the vessel 2. The thermosiphon operates in the orientation as shown in the drawing i.e. with the header spaced at the top. The volatile liquid thus tends to drain under gravity to a reservoir in the lower end of the vessel 2 away from the wick 8. The liquid volume is so chosen that in use the liquid occludes the lower end of a liquid flow path constituted by a tube 10 which communicates with the header space 6. The wick 8 is sealed to the tube 10 and to the interior walls of the vessel 2. A priming wick 14 connects the wick 8 to the liquid 12 and provides a means to saturate the wick 8.

The evaporator section of the thermosiphon is constituted by a section of the vessel wall covered by the wick 8 and the condenser section is constituted by a section of the vessel wall near the lower end of the vessel 2. The header space 6 is adjacent the evaporator section but not part of it.

In use, heat Q is applied to the evaporator section causing liquid to evaporate (but not in the header space 6) and increasing the pressure of vapour in the vapour space.

Heat Q is removed from the walls of the vessel 2 in the condenser section, and a flux of vapour flows through the vapour space 4 from the evaporator to the condenser.

Provided the wick 8 is saturated a pressure difference will tend to become established across the wick 8, with Ph the pressure in the header space being less than Pv the pressure in the vapour space. This pressure difference Pd (Pd=Pv-Ph) causes liquid 12 to tend to rise in tube 10 and, when the pressure difference is sufficient, liquid will flow into the header space 6 from the tube 10. However, the pressure difference Pd cannot exceed APc the capillary height of the wick 8 because this is the maximum pressure difference sustainable across the saturated wick. Thus the height of the liquid column in the tube 10 through which liquid 12 is raised from the reservoir to the header space 6 is limited to the capillary height of the wick 8.The tube 10 is less likely to become blocked by bubbles of contaminating gas than are conventional arteries because the bubbles can escape from the tube into the header space 6 where they need not significantly affect performance if the volume of such gas is small compared with that of the header space.

It will be obvious to those skilled in the art that many modifications to the embodiment shown in Figure 1 within the scope of the invention (as claimed in the appendant claims) are possible. Thus, the priming wick can be omitted if it is proposed to prime the wick 8 by other means such as cooling the header space to a temperature lower than that of the liquid 12 in the reservoir, or by applying external forces to the sealed vessel 2 so as to bring some of the liquid in the reservoir into contact with the wick 8. Furthermore, the height of the tube 10 can be increased if it is arranged that it transports a mixture of vapour and liquid rather than fully dense liquid. One way of providing a vapour/liquid mixture in tube 10 is to provide a hole in the cylindrical wall of the tube 10 in the vapour space so that in use vapour flows into the tube 10 and mixes with liquid in the tube.Another way is to locate the tube so that it receives some of the heat input Q whereby some of the liquid in the tube 10 can be vaporised. A further means of providing vapour in the tube 10 is to fit to the tube an electrical heater. With a liquid/vapour flow in the tube 10, heat must be removed from the header space 6 to condense the vapour issuing from the tube 10. The tube 10 need not be of cylindrical cross-section. A tendency for liquid to rise in the tube 10 by capillary action between the liquid and the walls of the tube is not required for operation of the device.

Figure 2 shows an embodiment ot twophase thermosiphon which can operate in any orientation. Many of its components are common with the embodiment of Figure 1 and like reference numerals are employed to identify those components. Thus, a sealed vessel 2 has a vapour space 4 and a header space 6 separated by a substantially cylindrical wick 8. This wick covers the vessel wall between, and is bonded to header plates 16 and 18 which are themselves bonded in gas-tight manner to the tube 10. The tube 10 provides a longitudinal liquid flow path between a condenser reservoir (below the plate 18) and the header space 6.The wick is spaced slightly from the vessel wall, at least around part of its circumference, so as to create one or more further longitudinal flow paths for liquid between the wick and the wall, although to avoid penetration through the wick by vapour it must be arranged that evaporation and condensation take place on the inner surface layer of the wick portions in the evaporator and condenser sections, i.e. on the layer thereof adjacent the vapour space. Sufficient liquid 12 is provided so that the wick 8 will always be in contact with the liquid. The length of the wick in the direction parallel to the cylindrical axis of the sealed vessel 2 i.e. to its longitudinal axis is less than the capillary height of the wick because the sealed vessel 2 is to be capable of being used inter alia in an upright position and no other means of saturating the wick on starting up are provided.

In use, the input of heat Q generates vapour in the evaporator section at the inner surface layer of the wick 8 which raises the pressure in the vapour space 4 above that in the header space 6. Liquid in the vapour space 4 is thus urged outwardly through the wick 8. As liquid in the wick evaporates from its inner surface layer in the evaporator section there will be a flow of liquid within the wick into the evaporator section. Such flow will be from the condenser section along the wick in the manner of a conventional heat pipe (i.e. by capillary action within the wick).However, as liquid is urged outwardly of the vapour space by the pressure of vapour therein, liquid is also urged to flow from the reservoir adjacent header plate 18 along the tube 10 into the header 6 and hence to the wick 8 adjacent the evaporator section or up the one or more flow paths between the wick 8 and the wall of the sealed vessel 2, which also open into the header space 6.

This is because the header space is separated by the saturated wick from the vapour space, and is hence at a lower pressure than the vapour space. Any contaminating gas within the sealed vessel 2 will tend not to reside in tube 10 nor in the aforesaid flow paths between wick and wall but will instead tend to accumulate in the header space 6 where in small quantities it need not seriously affect the performance of the device.

As with the embodiment of Figure 1, it may be appropriate to modify the tube 10 so that it carries a mixture of liquid and vapour. Contaminating gases are more likely to be carried from the vapour space into the header space with this arrangement.

Preferably the sealed vessel contains enough liquid to saturate the wick and fill the header space 6 around all sides of the vapour space 4.

Where the tube 10 carries only liquid, contaminating gases can be carried from the vapour space to the header space by a small bore tube connecting the two spaces. Such a tube must have a bore sufficiently small as not to destroy the pressure difference between the vapour space and the header space.

The embodiment of Fig. 3 has a sealed vessel 2 with a vapour space 4 and a header space 6 separated by a header plate 16 and a wick 8. The wick 8 is bonded in gas-tight manner to the plate 16 and to the wall of the vessel 2. The plate is bonded to a tube 10.

The wick 8 has a priming portion 14 which extends to the bottom of the vessel so as to contact the liquid 12 in a reservoir there.

The device is for use in an upright position, as shown in the drawing. The tube 10 has an orifice 20 in its cylindrical wall above the level of liquid therein in normal use. The upper end of the tube 10 extends into a subsidiary condenser section 22 of the vessel 12 where, in use, a relatively small quantity of heat q is removed from the vapour issuing from the tube 10 to condense the vapour.

In use, the embodiment functions in similar fashion to those already described.

The wick 8 is primed by wick portion 14.

However, in embodiments where the top of the wick 8 is higher above the level of liquid 12 than the capillary height of the wick 8 then priming can in any case be effected by cooling section 22. The narrow neck portion separating the evaporator portion from the subsidiary condenser section 22 is useful in that it provides a head of liquid above the head 16 which acts to assist flow of liquid up the tube 10 in the manner described in for example the complete specification of British Patent No. 1496327.

It will be apparent to those skilled in the art that the operation of the illustrated embodiments does not depend on capillary rise in the tube 10. Accordingly, the diameter of the tube 10 is not limited to capillary diameters. Indeed, it may be advantageous for the diameter of tube 10 to be relatively large in order to reduce the resistance to flow of fluid through the tube.

WHAT I CLAIM IS: 1. A two-phase thermosiphon comprising a sealed vessel containing a liquid and its vapour, said vessel having a wall: an evaporator section comprising a first area of the wall and a condenser section comprising a second area of the wall, said evaporator and condenser sections being connected bv a vapour space within the vessel: a header space within the vessel: a wick separating the header space from the vapour space, said wick extending into and covering at least a part of the first area of the wall within the evaporator section, said wick being capable of conveying liquid from the header space to the evaporator section; and a liquid flow path from the condenser section to the header space.

2. Thermosiphon according to claim I having a reservoir within the vessel where liquid condensate can collect.

3. A thermosiphon according to claim 2 wherein the wick separating the header space from the vapour space has a portion which extends into the reservoir.

4. A thermosiphon according to claim 2 or claim 3 including means for introducing vapour into the said liquid flow path to provide a vapour/liquid mixture therein said liquid flow path opening into said reservoir at a position below the minimum normal level of liquid therein.

5. A thermosiphon according to claim 4 wherein the means for introducing vapour into the said liquid flow path is constituted by an opening above the maximum normal level of liquid in the reservoir, said opening connecting the liquid flow path with the vapour space.

6. A thermosiphon according to claim 4 or claim 5 wherein a third area of the vessel wall adjacent the header space constitutes a subsidiary condenser section.

7. A thermosiphon according to claim 2 wherein a wick separates the reservoir from the vapour space.

8. A thermosiphon according to claim 7 wherein the vapour space is enclosed by the wick which covers the walls of the vessel in a central section thereof, the header space being located in the volume remaining within the vessel at one end thereof, and the reservoir being located in the volume remaining within the vessel at the other end thereof.

9. A thermosiphon according to claim 8 wherein the wick is spaced from the walls of

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Claims

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separated by the saturated wick from the vapour space, and is hence at a lower pressure than the vapour space. Any contaminating gas within the sealed vessel 2 will tend not to reside in tube 10 nor in the aforesaid flow paths between wick and wall but will instead tend to accumulate in the header space 6 where in small quantities it need not seriously affect the performance of the device.

As with the embodiment of Figure 1, it may be appropriate to modify the tube 10 so that it carries a mixture of liquid and vapour. Contaminating gases are more likely to be carried from the vapour space into the header space with this arrangement.

Preferably the sealed vessel contains enough liquid to saturate the wick and fill the header space 6 around all sides of the vapour space 4.

Where the tube 10 carries only liquid, contaminating gases can be carried from the vapour space to the header space by a small bore tube connecting the two spaces. Such a tube must have a bore sufficiently small as not to destroy the pressure difference between the vapour space and the header space.

The embodiment of Fig. 3 has a sealed vessel 2 with a vapour space 4 and a header space 6 separated by a header plate 16 and a wick 8. The wick 8 is bonded in gas-tight manner to the plate 16 and to the wall of the vessel 2. The plate is bonded to a tube 10.

The wick 8 has a priming portion 14 which extends to the bottom of the vessel so as to contact the liquid 12 in a reservoir there.

The device is for use in an upright position, as shown in the drawing. The tube 10 has an orifice 20 in its cylindrical wall above the level of liquid therein in normal use. The upper end of the tube 10 extends into a subsidiary condenser section 22 of the vessel

12 where, in use, a relatively small quantity of heat q is removed from the vapour issuing from the tube 10 to condense the vapour.

In use, the embodiment functions in similar fashion to those already described.

The wick 8 is primed by wick portion 14.

However, in embodiments where the top of the wick 8 is higher above the level of liquid 12 than the capillary height of the wick 8 then priming can in any case be effected by cooling section 22. The narrow neck portion separating the evaporator portion from the subsidiary condenser section 22 is useful in that it provides a head of liquid above the head 16 which acts to assist flow of liquid up the tube 10 in the manner described in for example the complete specification of British Patent No. 1496327.

It will be apparent to those skilled in the art that the operation of the illustrated embodiments does not depend on capillary rise in the tube 10. Accordingly, the diameter of the tube 10 is not limited to capillary diameters. Indeed, it may be advantageous for the diameter of tube 10 to be relatively large in order to reduce the resistance to flow of fluid through the tube.

WHAT I CLAIM IS: 1. A two-phase thermosiphon comprising a sealed vessel containing a liquid and its vapour, said vessel having a wall: an evaporator section comprising a first area of the wall and a condenser section comprising a second area of the wall, said evaporator and condenser sections being connected bv a vapour space within the vessel: a header space within the vessel: a wick separating the header space from the vapour space, said wick extending into and covering at least a part of the first area of the wall within the evaporator section, said wick being capable of conveying liquid from the header space to the evaporator section; and a liquid flow path from the condenser section to the header space.
2. Thermosiphon according to claim I having a reservoir within the vessel where liquid condensate can collect.
3. A thermosiphon according to claim 2 wherein the wick separating the header space from the vapour space has a portion which extends into the reservoir.
4. A thermosiphon according to claim 2 or claim 3 including means for introducing vapour into the said liquid flow path to provide a vapour/liquid mixture therein said liquid flow path opening into said reservoir at a position below the minimum normal level of liquid therein.
5. A thermosiphon according to claim 4 wherein the means for introducing vapour into the said liquid flow path is constituted by an opening above the maximum normal level of liquid in the reservoir, said opening connecting the liquid flow path with the vapour space.
6. A thermosiphon according to claim 4 or claim 5 wherein a third area of the vessel wall adjacent the header space constitutes a subsidiary condenser section.
7. A thermosiphon according to claim 2 wherein a wick separates the reservoir from the vapour space.
8. A thermosiphon according to claim 7 wherein the vapour space is enclosed by the wick which covers the walls of the vessel in a central section thereof, the header space being located in the volume remaining within the vessel at one end thereof, and the reservoir being located in the volume remaining within the vessel at the other end thereof.
9. A thermosiphon according to claim 8 wherein the wick is spaced from the walls of

the vessel sufficiently to permit a flow of liquid between the wick and the walls from the reservoir to the header space.
10. A thermosiphon according to any one preceding claim wherein the header space is located in use above the vapour space.
11. A thermosiphon substantially as hereinbefore described with reference to Figure 1, Figure 2 or Figure 3 of the drawings filed with the Provisional Specification.