Capillary evaporator

Details Capillary evaporator Capillary evaporator

1997-04-16

2001-06-05

Atkinson, Christopher (Department: 3743)

Heat exchange

Intermediate fluent heat exchange material receiving and...

Liquid fluent heat exchange material

C165S907000, C165S905000

Reexamination Certificate

active

06241008

ABSTRACT:

This invention relates to a capillary evaporator for a loop containing a working fluid in liquid form and in vapour form.
In such loops, the capillary action acts as a pump to draw condensed liquid towards a heat input structure to generate the vapour phase. Such loops are known as capillary pumped loops and are particularly valuable in satellites in which there may be a need to transport heat from equipment such as vacuum tubes, transistors or antennas to remote radiators, or to connect two radiating surfaces. Referring to
FIG. 1
, working fluid is vaporized in the evaporator
1
and the part
2
of the loop connecting the evaporator
1
and the condenser
3
contains vapour in the section adjacent to the evaporator. Vapour is condensed in the condenser
3
, as heat is rejected from it. (In the vacuum of space, heat can only be lost from a satellite by radiation). Liquid is returned at a lower temperature, than upstream of the condenser, to the evaporator
1
via a pipe
4
. A reservoir
5
is optionally provided to accommodate volume variation or to provide control. The evaporator is positioned in thermal contact with the heat generating equipment.
Referring to
FIGS. 2 and 3
, the evaporator comprises an impermeable casing
6
having a liquid input
4
and a separate vapour outlet
2
. The liquid is fed to the interior of a porous hollow body
7
(closed at one end) forming a wick, which is held by internal fins
8
. Vapour is produced at the outer periphery of the wick
7
and flows along the grooves
9
between the fins
8
to a manifold
10
communicating with the vapour outlet
2
.
The casing
6
including fins
8
form a heat input structure to the wick, in that the equipment to be cooled is put in thermal contact with the outer periphery of the casing, or a surface connected to it. Referring to
FIG. 4
, which shows a fragmentary region B of the plan view of
FIG. 3
on a larger scale, cooled liquid enters the interior of the hollow wick
7
, and vapour is formed at a liquid/vapour front or meniscus
11
, in the vicinity of the foot of each fin
8
.
Liquid is drawn across the wick from the inner to the outer diameter by means of capillary action due to the porous nature of the wick, which is typically approximately 50% porous i.e. the cavities in the wick make up around 50% of its total volume.
Conventional designs use either a high conductivity metal wick or a low conductivity plastics wick. The meniscus recedes some distance into the wick as shown, and there is a significant pressure drop for the vapour flowing in the wick, the vapour escaping in the direction of the arrows
12
. The vapour then passes along grooves
9
to the manifold region
10
to conduct the vapour out of the evaporator with minimum pressure drop.
Each type of wick has a fundamental drawback. Thus, metal wicks being conductive require a larger cooling of the incoming liquid to the interior of the hollow wick than a lower conductivity plastics wick in order to ensure the temperature at the meniscus is below the saturation temperature of the working fluid, and this in turn calls for a larger surface area of radiator (condenser) than a lower conductivity plastics wick would require. Further, such wicks only work above a certain minimum heat load in order for vapour to be produced at all and in order for the loop to transport heat at all. (The minimum heat load is strongly dependent on temperative and adverse gravitational head.)
An advantage of metal wicks is that their high conductivity means that the meniscus
11
can recede far enough for the amount by which it overlaps the fins
8
(see arrows
12
) to be large enough for the pressure drop of the vapour leaving the wick to be acceptably low.
The drawback of plastics wicks is their low conductivity, which has the result that the heat supplied to the wick from the fins
8
is localized in the region of the fins. The amount by which the meniscus retreats depends on the pressure balance in the loop. Thus, the meniscus may not recede far enough from the fins to provide an adequate overlap of meniscus relative to fins
8
, resulting in a restricted channel for the vapour to escape (arrows
12
), thereby resulting in a larger pressure drop of the vapour leaving the wick then for the metal wick. If the meniscus is able to retreat sufficiently far from the fins
8
to provide a reasonably small pressure drop of the vapour leaving the wick, because of the low conductivity of the wick, there will now be a larger temperature difference between the foot of the fin
8
and the meniscus. This means that the vapour produced from the meniscus is at a lower temperature level than if the meniscus did not recede, and the radiator will not now work so efficiently because it will contain lower temperature vapour, and again a larger radiator surface area is required. The plastics wick does not however require the sub cooling of the incoming liquid which the metal wick requires. Ceramic wicks of low or high conductivity are available, with the attendant disadvantages noted, respectively, for plastics or metal wicks.
The invention provides a capillary evaporator comprising an inlet and an outlet for communication with a loop containing a working fluid, a wick for drawing in by capillary action working fluid in liquid form received from the inlet, and a heat input structure for vaporizing working fluid in the wick for passage through the outlet, wherein the heat input structure is spaced from the wick.
The spacing avoids the need for the meniscus to recede in order to reduce the pressure drop of the vapour leaving the wick, thereby reducing the temperature drop between the heat input structure and the meniscus so that the vapour is produced at a higher temperature and needs a smaller surface area of radiating surface in the loop.
Advantageously, there is provided a conductive spacer for spacing the heat input structure from the wick, the spacer having a greater thermal conductivity than the wick and producing a lower pressure drop per unit length for a given cross-sectional area, for a given vapour, than the wick (preferably less than a tenth of that for the wick and advantageously less than one hundredth of that for the wick.) This is even better than simply having a gap between the wick and the heat input structure, since the spacer still permits a low vapour pressure drop but the meniscus temperature is higher because of the superior conducting properties of the spacer as compared with the conduction provided by the vapour itself in the case where there is simply a gap.
The invention is particularly applicable to wicks of low conductivity, such as plastics material, for example, Teflon, or ceramic material. The spacer is advantageously of metallic material, such as nickel or aluminium, and the average permeability may be at least 10 times the permeability of the wick, preferably at least 100 times the permeability of the wick.


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