EP1068481A1

EP1068481A1 - Heat exchanging device with active two-phase fluid and method for making same

Info

Publication number: EP1068481A1
Application number: EP99910447A
Authority: EP
Inventors: Jean Duval
Original assignee: Atmostat Etudes et Recherches
Current assignee: Atmostat Etudes et Recherches
Priority date: 1998-03-30
Filing date: 1999-03-29
Publication date: 2001-01-17
Anticipated expiration: 2019-03-29
Also published as: WO1999050607A1; FR2776764B1; DE69910996T2; DE69910996D1; EP1068481B1; FR2776764A1

Abstract

The invention concerns a heat exchange device (50) with active two-phase fluid, comprising at least a capillary pumping channel (9) and at least a gas transfer channel (6), said device enabling reversible fluid passage, between at least a capillary channel (9) and at least a gas transfer channel (6), during the liquid/gas or gas/liquid transition, resulting from temperature variations sustained by at least one zone of the device (50). It comprises at least one foil (2, 3, 4) comprising on one of its two main surfaces, at least two parallel slots, communicating longitudinally with each other and at least one foil (3, 4) capable of covering the grooves to form at least a capillary channel (9) and at least one gas transfer channel (6).

Description

"Device for heat exchange with active biphasic fluid and method for manufacturing such a device"

The present invention relates to the field of heat exchange devices with active fluid, and more specifically, those which contain a two-phase fluid and which comprise capillary channels.

By "biphasic" is meant the fact that the fluid contained in such devices is present, so that they are operational, in the form of the two liquid and gas phases. Also qualified below by “capillaries”, channels which have a very small section compared to their length, and especially which are capable of producing phenomena of pumping by capillarity on liquids.

Bi-phase fluid thermal devices are already known, comprising capillary channels capable of producing capillarity phenomena on the liquid phase, and gas transport channels in which the gas phase of the fluid is confined, the capillary channels communicating with the channels. gas transport.

These devices are used either in closed loop or in open loop. In a closed loop, the device is used as a heat pipe and operates autonomously. In this application, the device is exposed to a cold condensation zone, also called a cold source, and a hot vaporization zone, also called a hot source. The fluid condenses in its liquid phase, in the cold zone and is vaporized in its gas phase in the hot zone. The capillary forces then act on the liquid phase of the fluid to move it from the condensation zone to the vaporization zone. The gas pressure being greater in the vaporization zone than in the. condensation, a gas flow is obtained in the opposite direction to the displacement of the liquid phase. The capillary and pressure forces act alone as a motor for the circulation of the fluid

In open loop, the device is used as an evaporator and a 99/50607

pump as well as a condenser are integrated in the circuit For this device to be functional, the fluid must arrive in its liquid form in the device and leave it in its gaseous form, to be condensed in a different element of the circuit In the presence of forces gravity, it suffices to orient the device properly to conserve the liquid, which is denser than the gas, in the zone of arrival of the liquid in the device, without the latter being able to re-enter the gas circuit. downstream of the device But in the absence of gravity forces, the liquid can be in the form of droplets dispersed in the gas phase The capillary channels then make it possible to fix these droplets and to prevent them from returning to the downstream gas circuit of the device

For these applications, devices of a first type are already used, consisting of cylindrical rods with circular section stacked perpendicular to their longitudinal direction in a hexagonal network Stacked in this way, these rods define cavities between them These cavities extend longitudinally parallel to the rods and have a roughly triangular cross section These cavities contain the biphasic fluid The parts of the external surfaces of the rods, located in the vicinity of the vertices of the triangles, that is to say near the contact zones between two rods, constitute channels capable of exerting capillary forces on the liquid phase of the fluid The central zone of the cavities forms a gas transport channel For this type of device to function properly, it is essential that there is no interruption of the capillary channels along their length This requires a pre stacking cis and rigid cylindrical rods These rods are therefore housed and wedged in grooves made in a rectilinear and rigid bar A device of this type is relatively expensive to produce and has drawbacks for certain applications One of these drawbacks is for example its rigidity which is hardly compatible with the work of the parts on which it is fixed, when these are subjected to constraints On the other hand, the performance of such a device used as a heat pipe depends on its capacity to 99/50607

transport calories through the fluid. However, the movement of the fluid in a heat pipe is ensured by the capillary forces exerted on the liquid phase of the fluid, contained in the capillary channels. But in devices of this type, a large volume is occupied by the cylindrical rods themselves. Consequently, the number of capillary channels for a given volume is relatively small, which limits the performance of such devices. This low compactness also does not allow optimal integration with the electronic circuits which are equipped with it. The document FR 2 735 565, for example, presents another type of device. A device of this other type consists of aluminum tubes, internally striated to form capillary channels open on a hollow central core, serving as a gas transport channel. Again, the cylindrical geometry of these tubes does not promote compactness and optimal performance.

It has also been proposed, for example in document US Pat. No. 5,697,428, a device in which a continuous groove is engraved in a first metal plate. This groove has rectilinear portions parallel to each other, connected by curved portions, the whole having a serpentine shape. A second metal plate is placed on the first plate so as to close the groove and form a tube. In such a structure, the zones where the fluid is in its liquid phase and those where the fluid is in its gas phase, follow one another along the path of the fluid in the tube. The internal dimension of the tube is the same in all the zones where the fluid moves. This device therefore does not allow the circulation of each phase of the fluid to be optimized independently. Pumping by capillarity, in particular, is not carried out and the qualifier "capillary" here essentially relates to the geometry of the tube which has a very small cross section compared to its length, which allows the gas to remain in bubbles in the liquid and push it.

An object of the invention is to provide a thermal device with active biphasic fluid, flat, flexible, having a compactness and high performance, which includes in its thickness at least one channel of sufficiently large section so that a gas passes easily without a liquid being able to obstruct it, and at least one channel sufficiently small so that a liquid can be there propagate by capillarity. Another object of the invention is also to provide a device having low risks of stopping operation by local drying of the capillary channels.

These aims are achieved by means of a thermal device with active biphasic fluid, comprising at least one capillary channel and at least one gas transport channel, each capillary channel having a section adapted so that the liquid phase of the fluid can be pumped there by forces. capillaries, each gas transport channel having a cross section greater than that of a capillary channel, this device allowing a reversible passage of fluid, between at least one capillary channel and at least one gas transport channel, during the liquid transition / gas or gas / liquid, consecutive temperature variations undergone by at least one zone of the device, characterized in that it comprises at least one sheet comprising on one of its two main faces, at least two parallel grooves, communicating longitudinally with each other and at least one sheet capable of covering the grooves to form at least one capillary channel and at least one transport channel t gaseous.

Thus, a device according to the invention has a sheet structure which allows it to be flat. On the one hand, this shape makes it possible to have large contact surfaces between the device and the structures which are equipped with the device. Thus the heat exchanges between the device and these structures are facilitated.

On the other hand, this structure allows rational optimization of capillary pumping and gas flow.

In fact, in this device, the capillary channel is made "flat", by forming a groove in a sheet, before being integrated into the mass of the device. As a result, the dimension perpendicular to the main surface of the sheet, on which the groove which constitutes it is flush, can be optimized. This dimension, which will be called "thickness of the channel "may be as weak as necessary. However, the capillary pressure tends towards a maximum when the thickness of the capillary channel tends towards zero and only the “flat” embodiment makes it possible to obtain the few microns or tens of microns necessary for a great height of lifting of the liquid.

Advantageously, the thickness of the capillary channel is less than 100 μm for a high capillary pressure. But more preferably, this thickness of the capillary channel is approximately between 30 and 70 μm. Furthermore, the dimension parallel to the main surface of the sheet determines what will be called the "capillary channel width". However, the width of the capillary channel induces the pressure drop and therefore makes it possible to obtain the necessary flow of liquid. The “flat” arrangement makes it possible to increase this width as much as necessary and therefore allows a large flow and a high thermal power.

Preferably, a capillary channel has a width of the order of 0.3 to 1 mm for a sufficient flow rate and a limited pressure drop. Among the heat pipes of the prior art, the wetting heat pipes have a large section but the capillary pressure is very low, which does not allow inclined use of the heat pipe. For wicking heat pipes or old micro-pipes, the section in which the thickness of the capillary is optimal has a very small width.

In addition, the section of each gas transport channel is determined in thickness, by the number of stacked sheets and the thickness of each sheet, and in width by the width of the corresponding groove, engraved on the entire thickness of each sheet. . This section is large enough to reduce the gas velocity and allow a flow with low pressure drop. This thus makes it possible to avoid limiting the performance of the micro-heat pipe by reaching the speed of sound by the gases in the gas transport channels.

In addition, in the device according to the invention, the grooves are made directly on the sheets. A rigid structure is therefore not necessary, unlike devices with stacking cylindrical rods of the prior art. The thickness and the nature of the material of the sheets can therefore be chosen to give flexibility to the device. The fact of being able to choose thin sheets also makes it possible to gain in compactness and to optimize the ratio capacity of transport of heat on size of the device, to obtain high performances. At a given power, the device according to the invention has a much smaller thickness than traditional heat pipes, typically 3 to 5 times thinner, which induces a significant lightening potential. Because of its very small thickness, the device according to the invention can also be easily deformed, which allows bending with a very small radius of curvature, close to folding. This possibility makes it possible to generate non-planar contact surfaces, in particular cylindrical, to generate changes of planes by change of altitude or angular direction, or even to generate geometries in "bellows" making it possible to make flexible connections with wells thermal.

On the other hand, the transfer of gas between the capillary channels and the gas transport channels must be optimized, inter alia, to avoid drying. In the heat pipes of the prior art, with surface wetting or wicks, this exchange is permanent insofar as the two types of channels are integrated one into the other. However, in the prior art heat pipe improvement tests, the search for capillary pumping performance has resulted in isolation of the capillary channel, which means that the gas circulates in the capillary, promoting drying. The device according to the invention makes it possible to optimize both the transfer between capillary channels and gas transport channels and the performance in capillary pumping.

Also, a device according to the invention can comprise several capillary channels communicating longitudinally with a gas transport channel. In this way, if a local heating dries up one of the capillary channels, another of these channels can continue to ensure the circulation of the liquid phase. In addition, communication between channels 7

capillaries and gas channels, over their entire length, makes it possible not to limit the zones of exchange between these two types of channels and to have correct operation in a closed loop, whatever the respective dimensions of the vaporization and condensation zones Advantageously, the device according to the invention comprises a number of sheets stacked on each other, equal to or greater than two, each having at least one groove capable of forming a gas transport channel communicating over its entire length with a homologous groove d another sheet Advantageously also, the device according to the invention comprises at least one circuit of channels operating in closed loop and ensuring, without motor, the circulation of the fluid contained in the circuit, between an evaporation zone and a condensation zone , the capillary forces exerted on the liquid phase of the fluid contained in the capillary channels playing a pumping role on the fluid In this case, the di spositif according to the invention constitutes a heat pipe Such a heat pipe can be composed of several subsets of sheets, each subset comprising a circuit of channels, isolated from the circuit of each other subset, each circuit being charged with a fluid whose thermodynamic properties allow the fluid to work on different temperature ranges

But in another embodiment, the device according to the invention comprises at least one circuit of channels, open on a circuit comprising a pump and a condenser, the device according to the invention then playing the role of an evaporator and the forces capillaries exerted on the liquid phase of the fluid making it possible to fix it in the capillary channels, and to distribute it by capillary pumping, in these channels

Furthermore, the heat transfer must be optimized to avoid temperature gradients in hot and cold transfer zones. According to an advantageous variant of the device according to the invention, this quality of thermal transfer is optimized thanks to a particular geometry of the ends A “staircase” arrangement of the liquid-gas limits makes it possible to spread your menisci from these surfaces, and thereby to favor 8

heat exchanges

According to another aspect, the invention is a method for producing devices according to the invention

According to this method, the main operations of construction of the device according to the invention are the cutting and engraving of the sheets, the welding "flat, with the press", and the trimming This allows the simultaneous production of a large number of parts , which is favorable to mass production This is not possible for traditional heat pipes which must be machined one by one The work, by cutting and engraving flat, of the sheets allows a large freedom of drawing at reduced cost, for the device according to the invention, and facilitates the networking of the conduits constituted by a gas transport channel and the capillary channels which are associated with it This particularity is all the more remarkable as the production of the etching and cutting takes place done simultaneously everywhere on a whole sheet The corresponding cost is therefore not proportional to the length of the necessary conduits

Other aspects, aims and advantages of the invention will appear on reading the detailed description which follows The invention will also be better understood with the help of the references to the drawings in which - Figure 1 schematically represents all the steps of a nonlimiting example of a process used to produce a device according to the invention,

- Figure 2 is an elevational view from above of a basic sheet of an embodiment of a device according to the present invention, - Figure 3 is an elevational view from below of a sheet intermediate of the device according to the invention corresponding to the same embodiment as that of FIG. 2,

FIG. 4 is a top elevation view of an intermediate sheet of the device according to the invention corresponding to the same embodiment as that of FIG. 2n,

- Figure 5 is a top elevational view of an upper sheet of the device according to the invention corresponding to the same embodiment as that of Figure 2;

- Figure 6 shows schematically in section, along line A- A, a stack of sheets shown in Figures 2, 3, 4 and 5; FIG. 6a represents such a stack with a very expanded scale in the direction perpendicular to the plane of the sheets; Figure 6b shows in more detail the cross section of a gas transport channel and adjacent associated capillary channels;

- Figure 7 schematically shows a perspective view of an example of a device according to the invention, based on a mounting tool;

- Figure 8 shows schematically, top view, an application of a device according to the invention, to the cooling of components on electronic circuits;

- Figure 9 shows schematically, in longitudinal section, a device according to the invention, sandwiched between two printed circuits;

- Figure 10 schematically shows an example of use of a device according to the invention, for cooling a detector;

- Figure 1 1 is a schematic transverse section of a capillary channel in a variant of the device shown in Figure 6; - Figure 12 is a schematic cross section of a gas transport channel surrounded by its associated capillary channels, in a variant of the device shown in Figure 6;

- Figure 13 is a top view, in transparency, along a plane parallel to the main plane of a variant of the device corresponding to the embodiment shown in Figures 2 to 6, of a condenser part in this variant;

- Figure 14 is a schematic cross section along the line B- B of Figure 13, of a branch, located in the. condenser portion shown in Figure 13; and - Figure 15 is a top view, in transparency, along a plane parallel to the main plane of a variant of the device corresponding to the embodiment shown in Figures 2 to 6, of a part forming 10

tank in this variant. Preferably, but not limited to, a device according to the invention can be produced according to the method illustrated in FIG. 1.

This method comprises a step of etching a grooves in blank sheets 1, a step of localized deposition b of an assembly material, a step of stacking c sheets previously prepared according to steps a and b, and a step mounting d to weld together the sheets stacked according to step c and form for example a heat pipe 50. A blank sheet 1 consists of a plate preferably with a thickness between 0.1 and 1 mm. The material of these sheets is for example a metal. It can be copper, nickel, iron, aluminum or even one of their alloys, such as aluminum-beryllium or stainless steel. The nature of the metal of the sheets depends on the active fluid used.

Several types of sheets are required to form a heat pipe 50 according to the invention. From a blank sheet 1, basic sheets 2, intermediate sheets 3 and upper sheets 4 can be made.

The etching step a is preferably a chemical etching with a savings mask. The mask defines the areas of the grooves to be engraved. These grooves are differently etched on the base sheets 2, the intermediate sheets 3 and the upper sheets 4. This cutting step a can be carried out in several successive operations making it possible to selectively engrave, on the one hand, the areas etched all over the thickness 5 of a sheet, and on the other hand zones engraved on a smaller thickness.

Thus, the zones engraved over the entire thickness 5 of the sheets are intended to provide gas transport channels 6. The zones engraved on a smaller thickness form a step between a first level 7, located on the upper surface of each sheet , and a second level 8. This step is intended for the formation of capillary channels 9. The chemical attack baths used for etching, adapted to the nature of the material 11

sheets, are conventional and known to those skilled in the art.

Preferably, the zones engraved between the first 7 and second 8 levels, are produced parallel to the zones engraved over the entire thickness 5 and over the entire length of the latter. These engraved zones up to the second level 5 are located on at least one edge of the engraved zones over the entire thickness 5, so as to pass, transversely with respect to the longitudinal direction of the channels 6, 9, from the first level 7, to the second level 8, then in the zones engraved over the entire thickness 5, without going back to the first level 7. Optionally, holes 10 and scaffolds 11 are also engraved in the sheets, to pass pins 12 and sockets 13 or respectively plugs 14 (these elements are not shown in FIG. 1). Holes 10 and notches 11 are shown in FIGS. 2 to 5. The step of depositing b of an assembly material is carried out according to strips suitable for obtaining a tight assembly of the sheets 2, 3, 4 therebetween and a longitudinal separation of the gas channels 6, while maintaining communication of the gas channels 6 with one another, at the ends thereof. When the sheets 2, 3, 4 are made of metal, this assembly material is also preferably a metal. Advantageously, this metal is deposited by electroplating, with a geometry determined by a savings mask. The metal thus deposited is suitable for the type of mounting envisaged. This metal deposition 15 can be different depending on whether the subsequent mounting step is carried out, for example, by thermo-compression or by brazing. This metal is also chosen according to the nature of the material of the sheets 2, 3, 4. Thus, for a mounting step d carried out by brazing, the deposition metal must have a melting temperature lower than that of the metal constituting the sheets. 2, 3, 4. With copper sheets 2, 3, 4, gold and silver can be used for diffusion brazing. With sheets 2, 3, 4 made of stainless steel, nickel and gold can be used for diffusion brazing. The nature of the metal deposited also depends on the active fluid used. For example, when 12

"Freon" is used as the active fluid, the deposited metal can be copper or silver. The thickness of the deposited metal is typically between 5 and 10 μm. The metal deposition 15 is carried out, on the upper face of the sheets, at the edge of the assembly formed by an area etched over the entire thickness 5 and at least one capillary channel 9, on either side of this assembly ( Figures 2 and 4). The metal deposit 15 is also made on the periphery of the sheets (Figures 2 and 4). The metal is deposited in small quantities so that it does not come to fill, during assembly, the zones intended to form the capillary channels 9. Typically, the thickness of the metal deposit 15 is 5 to 10 μm.

The stacking step c of the sheets, previously prepared according to steps a and b, is for example carried out by successively vertically placing three intermediate sheets 3 on a base sheet 2 and an upper sheet 4 on the intermediate sheet 3 from above. The sheets 2, 3, 4 are stacked, according to step c, presenting the engraved areas up to the second level 8, facing upwards. The zones engraved over the entire thickness 5 are placed opposite one another and define the gas transport channels 6. When the zones engraved up to the second level 8 are covered by the sheet which is immediately above it , they constitute capillary channels 9. The stack of sheets 2, 3, 4 defines a heat pipe 50. As shown in FIG. 7, to make the assembly, this heat pipe 50 can also be placed on a support 16 ( tools) and cover the whole with a sheet 17 making it possible to isolate the heat pipe 50 from the weights required for assembly. The pins 12 are optionally arranged in the holes 10, so as to keep the sheets strictly aligned during the subsequent step of mounting d

The mounting step d is preferably carried out by brazing. In this way, the brazing metal forms a liquid phase which wets the areas on which it is deposited and the areas of the adjacent sheet, located opposite them. It thus ensures the connection of the pressed sheets one on the other to ensure contact. This soldering can be 13

carried out under vacuum (10 ^"5 mbar) or under a gaseous atmosphere, but preferably under a non-oxidizing atmosphere. An under layer is possibly deposited between the sheet and the brazing metal. The sheets are thus joined together, in a sealed manner, all around of each sheet and between each set consisting of a gas transport channel and at least one capillary channel.

Caps 13 and plugs 14 are arranged in the orifices produced by superposition of the notches 11.

The biphasic fluid is introduced into the evaporator, using the pipes 13 before these are closed.

The fluid used depends on the intended operating temperature range. It can be H ₂ 0, NH ₃ , acetone, "Freon", methane, ethane, etc.

Many variants of the process described above can be envisaged. Thus, for example, a method has been described above in which the mounting step c is carried out by brazing. It can also be carried out by thermo-compression. In this case, it is preferably carried out under vacuum to avoid passivation of the surface, by fixing non-metallic compounds (O ₂ , N ₂ , H ₂ O, volatile fats, etc.). The thermo-compression temperature is located approximately 50 ° C below the melting temperature of the metal deposited in step b. The pressure exerted on the areas to be welded is approximately 0.1 N / mm ² .

An example of a device according to the invention is described below in more detail. It is a heat pipe 50. It comprises a basic sheet 2, three intermediate sheets 3 and an upper sheet 4.

As shown in Figure 2, the base sheet 2 has an elongated shape. It has an overall size of 215 mm long, 69 mm wide and 0.25 mm thick. It includes engraved zones from the first level 7 to the second level 8. The distance between the first 1 and second 8 levels is 70 μm. The width of these areas is approximately 1 mm. A metal deposit 15 is made, on the first level 7, on the periphery of the sheet and along equidistant lines, 14

parallel and generally longitudinal. Four holes 10 are etched throughout the thickness of the base sheet 2, outside the line formed by the metal deposit 15, at the periphery.

As shown in Figures 3 and 4, the intermediate sheets 3 have the same shape as the basic sheet 2. They also have an overall size of 215 mm long, 69 mm wide, but a thickness of 200 microns.

As shown in FIG. 3, an intermediate sheet comprises zones engraved over its entire thickness 5. These zones are located at its longitudinal ends to form holes 10, at the ends of its longitudinal edges to form notches 11 and at the level of equidistant parallel and generally longitudinal lines. The latter are seven in number and are intended to form gas transport channels 6. The three most central lines are longer than the others and are extended deeper in the area between the two notches 11 arranged on the two edges opposite longitudinal of the intermediate sheet 3. All these lines lead, at each of their ends, to a zone which is transverse to them and engraved from the first level 7 to the second level 8. Thus, these zones engraved from the first level 7 to the second level 8 define capillary zones, which when bathed in the liquid phase of the fluid condensed at this level, redistribute the liquid in all the capillary channels 9.

As shown in FIG. 4, an intermediate sheet 3 also includes engraved zones from the first level 7 to the second level 8. The distance between the first 7 and second 8 levels is 70 μm. Around each zone engraved over the entire thickness 5, defining a gas transport channel 6, zones are engraved up to the second level 8, while leaving on the periphery and between each channel 6, non-engraved zones, at the first level 7. The areas engraved up to the second level 8 communicate with each other and with the notches 11.

The metal deposition 15 is carried out at the periphery of the sheet and according to 15

generally longitudinal lines, on the first level 7, according to the same geometry as the metal deposition 15 of the base sheet 2.

As shown in FIG. 5, an upper sheet 4 has an elongated shape, identical to that of the basic sheet 2 and of the intermediate sheets 3. Its overall length and width are identical to those of the basic sheets 2 and of the intermediate 3. Its thickness is 200 μm. It comprises two holes 10 at each of its longitudinal ends.

A base sheet 2, three intermediate sheets 3 and an upper sheet 4 are assembled, for example according to the method described above, to form a heat pipe 50 having a thickness of the order of a millimeter (Fig. 6a). This heat pipe 50 comprises seven gas transport channels 6. Eight capillary channels 9 open onto each gas transport channel 6 (FIG. 6b), or 56 capillary channels 9 in all. Each capillary channel 9 has a section of approximately 70 μm by 1 mm. The ribs of the stacked structure schematically shown in Figure 6, are not to scale. FIG. 6a in particular, has a very dilated scale in the direction perpendicular to the plane of the sheets, to reveal the capillary channels 9. However, if there are three intermediate sheets 3 of 0.2 mm thick, in which are made of engraved zones over the entire thickness 5 of 1 mm wide and of a base sheet 2 in which grooves are engraved of 70 μm deep and 3 mm wide and which are stacked by putting the zones engraved in coincidence, one obtains seven gas transport channels having a section of 1 mm wide by 0.6 mm thick. As shown in Figure 7, this heat pipe 50 is provided with pipes

13, as well as plugs 14 and is transferred to a support 16 and covered with a sheet 17. The support 16 consists of a plate 220 mm long, 76 mm wide and 10 mm thick. The sheet 17 has an overall length and width of 219 and 73 mm, respectively. Its thickness is 1 mm.

The heat pipe 50 is held on the support 16 with the sheet 17 by means of pins 12. It is loaded with weights isolated from the sheet 17 by shims in 16

alumina which make it possible to avoid welding the weights on the sheet 17. Other variants of the device according to the invention can be envisaged. Such a device can, for example, include more intermediate sheets 3. For example, in total the number of sheets stacked to form a heat pipe 50 can be 10 or 20. Likewise, the capillary channels 9 intended for transporting the phase liquid of the fluid by capillarity and the gas transport channels 6 can be produced in different ways. For example, a heat pipe 50 has been described above with a capillary channel 9 located on either side of each zone etched over the entire thickness 5 of the sheets. However, a capillary channel 9 may only be provided on one side of each zone etched over the entire thickness 5. It is also possible to superpose several heat pipes 50, one on top of the other.

The devices described above include metal sheets, but one will not depart from the spirit of the invention if the sheets are made of plastic, composite material, etc. The assembly material is then chosen accordingly. It can be a polymer adhesive, for example. It can even be envisaged to make welds between the sheets, by fusion, without assembly material.

Devices according to the invention have been described above, the capillary channels 9 of which are formed by chemical etching of grooves in a sheet. But it can also be envisaged to produce these grooves by depositing an extra thickness material on the sheets.

Devices according to the invention can find numerous applications in space thermal, avionics, electronics, data processing, etc.

The methods used in the process described above, in particular electroplating and chemical etching, allow all kinds of geometries to be produced with complex networks of channels, without increasing the number of manufacturing steps. Whatever the number of sheets making up the device according to the invention, it may be implemented only one welding step.

In addition, a device according to the invention, both by its shape and 17

by the type of process which makes it possible, allows their easy integration into electronic circuits 20.

As illustrated in FIG. 8, heat pipes 50 arranged on electronic circuits 20 make it possible to cool hot zones 21 on which are installed components 22, heat generators, by transporting the heat to return zones 23, even s '' orifices or other components must be bypassed 22.

As illustrated in FIG. 9, a printed circuit 20 made of epoxy resin can be glued, flat on each main face of a heat pipe 50, by sandwiching the latter. Thus, the gas 6 and capillary 9 transport channels of the heat pipe 50 directly transfer the heat, from the areas of the printed circuit 20 where components 22 are to be cooled, to a heat exchanger rack 40 or a radiator. A thermal clamp 41 conducts the heat between the heat pipe 50 and the rack 40 or the radiator. The heat pipe 50 therefore plays the role here of support for the printed circuit 20 in addition to its function of thermal conductor. With a structure similar to that described in detail above, the thickness of which is less than

3 mm, it is possible to evacuate around 10 W / cm ² over at least 5 cm ² .

As indicated above, the small thickness of the devices according to the invention makes it possible to deform them for certain applications. Thus, as illustrated in FIG. 10, a heat pipe 50 can be configured as a bellows, for example to cool a mobile detector 30. It suffices to place this bellows so as to have folds of this bellows perpendicular to the plane in which they take place. both the movement generated by a vertical displacement device 31 and the movement generated by a horizontal displacement device 32, the heat pipe 50 connecting the detector 30 to a heat return element 33.

Furthermore, tests carried out with a device according to the invention, of the type shown in FIG. 1, that is to say four stages, have shown that the heat pipe effect was only obtained with good efficiency. by accepting a high temperature difference between the hot source and the cold source, in particular for high flux heat transfers. 18

The analysis of the exchange mechanisms, confirmed by numerical calculations, shows that this temperature differential essentially comprises three components:

- a temperature gradient between the surface of the device at the level of the hot source and the evaporation surface (= 46% of the total);

- a temperature differential between the evaporation surface and the condensation surface of the liquid (= 8%);

- a temperature gradient between the condensing surface and the surface of the device at the level of the cold source (= 46% of the total). The importance of the first and third components are a consequence of the low thermal conductivity of the fluid and of the concentration of the flow in the vicinity of the limit between capillary channels 9 and gas transport channels 6. The second component is the only one fundamentally linked to the process physical generator of the operation of the device according to the invention, in its heat pipe function.

To reduce the harmful effects of the first and third components, it is possible to modify the section of the capillary channels 9 as described below. In transverse section, a capillary channel 9 has an overall U shape with two parallel side walls 25 corresponding to the branches of the U and a bottom wall 26. The bottom wall 26 is perpendicular to the side walls 25 between which it s extends. Thus, each side wall 25 has a longitudinal edge linked to the bottom wall 26 and a free longitudinal edge 27 or 28 parallel to the previous edge.

As can be understood from FIG. 11, the more the free longitudinal edges 27, 28 are offset relative to each other, the more the meniscus of separation of the liquid phase and the gas phase gives by wetting a large area. This meniscus surface corresponds to a larger evaporation surface S. The heat flux F is defined by: F = ^ -

S where P is the thermal power supplied to the fluid in the channel 19

capillary 9.

This makes it possible to obtain the following relationship between the flow S, the thermal conduction λ of the fluid in the capillary channel 9 and the temperature gradient Aθ between the walls 25, 26 of the capillary channel 9 and the evaporation surface S:

S e where e is the thickness of fluid ensuring thermal conduction (the thickness e is equal to half the width of the capillary channel 9 in it and decreases as one moves away from the bottom wall 26, from one free edge to the other).

This relationship can still be written:

It is therefore noted that the reduction in thickness e and / or the increase in the evaporation surface S (which takes place squared) leads (feels) to a significant reduction in Aθ.

Likewise, by increasing the condensation surface, by an offset relative to each other of the free longitudinal edges 27, 28, on the side of the cold source of the device according to the invention, a reduction in the gradient of temperature between the condensation surface and the walls 25, 26 of the capillary channel 9.

The increase in the areas of evaporation S and of condensation makes it possible to reduce the first and third components mentioned of the temperature differential between the hot source and the cold source, thanks to a reduction in the concentration of the flux in the vicinity of the limit between the capillary channel 9 and the gas transport channel 6.

This makes it possible to proportionally increase the temperature differential between the evaporation surface S and that of condensation, that is to say the second component mentioned above. Consequently, the performance of the device according to the invention is improved. Likewise, the different capillary channels 9 can all have 20

identical dimensions. But, according to an advantageous variant, they can also have different dimensions, for example, with the specific aim of optimizing the conduction of heat to the vaporization zones where the evaporation surfaces are located. In practice, the offset between the free longitudinal edges 27, 28 can be variable or constant over the entire length of the capillary channel 9.

In a device according to the present invention, consisting of several intermediate sheets 3, it is advantageous to minimize the heat conduction paths in the mass of the base sheets 2, intermediate 3 and upper 4, between the face of the base sheet 2 or that of the upper sheet 4 and the side walls 25 and bottom 26 of each capillary channel 9.

As illustrated in FIG. 12, an arrangement and a stack of sheets 2, 3, 4 so as to form a gas transport channel 6 whose transverse section is generally triangular, with free longitudinal edges 27, 28 in steps, constitutes a configuration which makes it possible to minimize the heat conduction paths mentioned above.

Furthermore, on the side of the cold source, there is another possible advantageous variant for the device according to the invention. Indeed, the heat collection and transfer systems which serve as heat sinks generally only allow the heat to be removed in the form of a very low heat flux, at the level of the exchange surface between the device and these collection and heat transfer systems. So to increase the heat power exchanged, it is necessary to increase this exchange surface.

This can for example be obtained by multiplying, near the cold source, the number of conduits 51 consisting, for each of them, of a gaezux transport channel 6 and of its associated capillary channels 9. Figure 13 shows an example where two of these conduits 51 are split. In this example, each duct 51 is divided, at the level of the cold source, into two branches 52. Each of these two branches 21

52 leads to a manifold 53 connecting together, at the level of the cold source, all the branches 52 of all the conduits 51. All of the ramifications 52 opening into a conduit 51 must have a total section of capillary channels 9 sufficient to that all of the condensed fluid can return by capillarity, from the cold source to the hot source, in the various capillary channels 9 of the conduits 51 located between these two sources. Typically, the total section of these different capillary channels 9 of the different branches 52 opening into a conduit 51 is equal to that of all of the capillary channels 9 of this conduit 51.

The set of ramifications 52 constitutes a condenser. As shown in FIG. 14, the capillary channels 9 of each branch are advantageously superposed on each other so that the longitudinal free edges 26, 27 are offset from one another, as described above, in order to '' increase the condensation surface. As also shown in FIG. 14, an arrangement and a stacking of the sheets 2, 3, 4 so as to form a triangular gas transport channel 6, at the level of the branches 52, constitutes an advantageous configuration which makes it possible to minimize the conduction paths mentioned above. As indicated by the arrows, in this figure, the heat flux is very distributed.

Furthermore, for the device according to the invention to function properly, it is necessary to fill it with heat transfer fluid with precision. Indeed, - if the filling was too low, part of the capillary channels

9 would be dried and, taking into account the operating principle of the device according to the invention, this drying would occur in the part of the device according to the invention forming an evaporator, which would make the device inoperative; but - if the filling was too great, part of the gas transport channels 6 would be invaded by the excess fluid and, taking into account the same principle, the excess would be located in the part of the device according to 22

the invention forming the condenser, which also made it inoperative.

However, the volume of the capillary part is of the same order of magnitude as that corresponding to the gas transport channels 6. And the filling is done with a fluid "under vacuum", that is to say under the only saturated vapor pressure. This favors the appearance of vapor bubbles almost everywhere in the filling circuit. Handling the liquid phase of the heat transfer fluid, in small quantities, is therefore very delicate. Therefore, the filling accuracy is no better than plus or minus ten percent, relative to the target amount of liquid fluid. Which is still not enough to avoid the problems mentioned above.

To obtain correct operation of the device according to the invention despite this imprecision on the filling quantities, the Applicant proposes to arrange at least one reservoir 54 having a volume comparable to that of a gas transport channel 6 and onto which open out capillary channels 9, which put it in communication with the rest of the device according to the invention.

The volume of the whole of the reservoir (s) 54 must preferably be approximately equal to twenty percent of the quantity of liquid fluid, intended for filling the device according to the invention, that is to say also approximately twenty for cent of the capillary volume of the device according to the invention. Thus, the reservoir 54 constitutes a reserve but also makes it possible to accommodate the excess fluid.

Each reservoir 54 must be located in the cold part of the device according to the invention. But it should not be located at the coldest point, because if it did, it would help to reduce the capillary pressure bringing the liquefied fluid from the part of the device according to the invention forming the condenser to that forming the evaporator.

A satisfactory arrangement consists in placing each reservoir 54 with the ramifications 52 of the condenser. In this situation, each reservoir 54 is kept cold by the contact of the device according to the invention with the external cold source. Not being heated by the circulation of gas, it is cooler than the ramifications 52 of the condenser. However, being in 23

their neighborhood, it is warmed by them and cannot be much colder. Figure 15 illustrates such an arrangement. According to this arrangement, a set of two reservoirs 54 is located between two sets of two ramifications 52. Each reservoir 54 is surrounded by a zone of capillary channels 9, opening onto the manifold 53 communicating with the four ramifications 52.

Claims

24 CLAIMS

1. Thermal device with active biphasic fluid, comprising at least one capillary channel (9) and at least one gas transport channel (6), each capillary channel (9) having a section adapted so that the liquid phase of the fluid can be there pumped by capillary forces, each gas transport channel (6) having a cross section greater than that of a capillary channel (9), this device allowing a reversible passage of fluid, between at least one capillary channel (9) and at least a gas transport channel (6), during the liquid / gas or gas / liquid transition, consecutive to the temperature variations undergone by at least one zone of the device (50), characterized in that it comprises at least one sheet (2, 3, 4) comprising on one of its two main faces, at least two parallel grooves, communicating longitudinally with one another and at least one sheet (3, 4) capable of covering the grooves to form at least one capillary channel (9) and at least a gas transport channel (6).

2. Device according to claim 1, characterized in that each capillary channel (9) has a dimension less than approximately 100 μm, perpendicular to the main surface of the sheet (2, 3), on which the groove which constitutes it is flush.

3. Device according to one of the preceding claims, characterized in that each capillary channel (9) has a dimension of approximately between 50 and 70 μm, perpendicular to the main surface of the sheet (2, 3), on which the groove is flush which constitutes it.

4. Device according to one of the preceding claims, characterized in that it comprises a number of sheets (2, 3, 4) stacked on each other, equal to or greater than two, each having at least one ^" suitable groove forming a gas transport channel (6) communicating over its entire length with a groove homologous to another sheet (3).

5. Device according to one of the preceding claims, characterized in that the sheets (2, 3, 4) are made of copper, nickel, iron or 25

aluminum or one of their alloys, such as aluminum-beryllium or stainless steel.

6. Device according to one of the preceding claims, characterized in that it comprises at least one circuit of channels (6, 9) operating in closed loop and ensuring, without motor, the circulation of the fluid contained in the circuit, between a evaporation zone and a condensation zone, the capillary forces exerted on the liquid phase of the fluid contained in the capillary channels (9) acting as a pump on the fluid.

7. Device according to claim 6, characterized in that it is composed of several sub-assemblies of sheets (2, 3, 4), each sub-assembly comprising a circuit of channels (6, 9) isolated from the circuit of each another sub-assembly, each circuit being charged with a fluid whose thermodynamic properties allow the fluid to work on different temperature ranges.

8. Device according to one of claims 1 to 5, characterized in that it comprises at least one channel circuit (6, 9) open on a circuit comprising a pump and a condenser, the device playing the role of a evaporator and the capillary forces exerted on the liquid phase of the fluid making it possible to fix the latter in the capillary channels (9) and to distribute it by capillary pumping.

9. Device according to one of the preceding claims, characterized in that it comprises a capillary channel (9) having an overall U-shape with two side walls (25) parallel corresponding to the branches of the U and a bottom wall (26), each side wall (25) having a longitudinal edge linked to the bottom wall (26) and a free longitudinal edge (27 or 28), parallel to the preceding edge, the free longitudinal edges (27, 28) being offset from each other.

10. Device according to one of the preceding claims, characterized in that the sheets (2, 3, 4) are arranged and stacked so as to form a gas transport channel (6) whose cross section is generally triangular, with longitudinal free edges (27, 28) in steps, to minimize heat conduction paths. 26

11. Device according to one of the preceding claims, characterized in that it comprises a conduit (51), consisting of a gas transport channel (6) and at least one capillary channel (9), dividing in two ramifications (52).

12. Device according to one of the preceding claims, characterized in that it comprises a reservoir (54) located in a cold part of the device, to constitute a reserve of fluid or collect the excess fluid.

13 A method for producing devices according to one of the preceding claims, characterized in that it comprises a step of chemical etching of the grooves in a sheet (2, 3).

14. The method of claim 13, characterized in that it comprises a step of assembly by brazing under a non-oxidizing atmosphere.

15. Method according to one of claims 13 and 14, characterized in that it comprises a step of depositing an assembly material according to strips suitable for obtaining a tight assembly of the sheets (2, 3, 4) between them and a longitudinal separation of the gas channels (6), while maintaining communication of the gas channels (6) with each other, at the ends thereof.

16. The method of claim 15, characterized in that the joining material is a metal whose melting point is lower than that of the metal of the sheets (2, 3, 4).

17. Method according to one of claims 15 and 16, characterized in that the assembly material is deposited by electroplating, with a geometry determined by a savings mask.