EP4325156A1 - Heat pipe of the capillary pumping type, with re-entrant grooves including at least one substrate porous at the evaporator - Google Patents

Abstract

The invention relates to a heat pipe (1) of which at least the evaporator integrates within it a porous insert which extends in a connecting channel between the vapor channel and the liquid channel(s).

Description

Technical area

The present invention relates to a capillary pumping type heat pipe with reentrant grooves.

The present invention aims to improve capillary pumping, while ensuring a high surface heat flux of such a heat pipe.

Prior art

A heat pipe is a thermal device allowing a quantity of energy to be transferred from a hot source to a cold source, a certain length apart.

A heat pipe comprises a hermetically sealed enclosure, a working fluid and a capillary network. During manufacturing, all the air present in the heat pipe tube is evacuated and a quantity of pure liquid is introduced to saturate the capillary network. Once the enclosure is closed and subjected to a hot source, an equilibrium is established between the liquid phase and the vapor phase.

Under the effect of a hot source applied in a zone at one of the longitudinal ends, designated the evaporator, part of the liquid phase vaporizes and absorbs the heat flow in latent form by inducing a slight overpressure which causes the movement of the steam towards an area at the other longitudinal end, designated the condenser where the cold source is applied. At the condenser, the vapor condenses and returns to the liquid phase. The condensed fluid (condensate) circulates in the capillary network and returns to the evaporator to repeat a cycle, under the effect of capillary forces, when the heat pipe is not subject to gravity. The return of the liquid fluid from the condenser to the evaporator is obtained by capillary pumping.

Indeed, in the absence of gravity, the engine of the heat pipe is the capillary pumping of the fluid which takes place at the level of the liquid-vapor interface zones specifically configured so that a meniscus forms, resulting from the angle of contact between the fluid and the metal constituting the heat pipe (itself depending on the surface tension of the fluid, and the tensions solid/liquid and solid/vapor interfacials). The greater the capillary pumping forces in relation to the various friction forces of the fluid phases with the walls and between them, the more efficient the heat pipe is and can transport a large quantity of energy.

The heat pipes affected by this absence of gravity are for example those used in space operating conditions, such as for the thermalization of satellite telecommunications systems.

In the terrestrial regime, with gravity, the problem of liquid phase return is completely different, and other liquid-vapor/wall interface configurations are implemented.

Grooved heat pipes work on the principle of capillary pumping. They feature a tube, in which the inner surface has axial/longitudinal [1] or slightly spiral-shaped grooves. Grooved heat pipes comprise a vapor core and a peripheral capillary network in which the liquid phase circulates. Due to a variation in curvature of the liquid-vapor interface between the condenser zone and the evaporator zone, a pressure gradient appears in the liquid, which leads to a variation in capillary pressure. The smaller the width of the grooves, the greater the capillary pumping effect.

Furthermore, deep grooves make it possible to obtain a large passage section for the liquid return, and therefore to minimize pressure loss.

The maximum power that grooved heat pipes can transport is generally fixed by the capillary limit, the driving term of which is the capillary pressure, and the term essentially limiting the loss of liquid pressure in the grooves and, to a lesser extent, the pressure losses of steam flow.

Reentrant groove heat pipes are particular examples of groove heat pipes, in which the grooves have a narrow connection channel relative to the rest of the groove, which allows the capillary pumping effect to be increased while limiting heat losses. charge in the liquid phase. These heat pipes are used mainly in the space sector, for example for thermal regulation in satellites and/or spacecraft.

The known production techniques for heat pipes with grooves, and in particular heat pipes with reentrant grooves, do not make it possible to obtain grooves having a depth significantly greater than their width.

These heat pipes are made essentially by extrusion with or without a floating mandrel including the groove pattern. This process makes it possible to carry out, generally in relatively soft metals (aluminum or copper), generally cylindrical tubes with internal longitudinal or slightly spiral grooves inside the tube.

These tubes are also used in evaporators and condensers, but can also be used to make heat pipes.

The grooves are generally rectangular or trapezoidal and of an aspect ratio limited by the extrusion process. The depth of the grooves can generally go up to 0.2mm with a depth/width aspect ratio of around 1 maximum.

In the case of reentrant grooves, the manufacturing constraints are even more drastic, limiting the width, the length of the constriction and the section of the reentrant part.

Another technique uses mechanical machining, with this technique also the depth to width ratio is not significantly greater than 1. Furthermore, this technique has a relatively high cost price and is not suitable for manufacturing on average and large series.

Another technique uses chemical etching. But it also does not allow for a significant depth to width ratio.

To overcome these drawbacks, the applicant proposed in the patent application EP3553445A1 a heat pipe made by stacking plates joined together with sealing, the end plates of which forming closing plates and the intermediate plates are structured, so that their stacking delimits reentrant grooves extending over the entire length of the heat pipe. The plates can be assembled using different welding, brazing or bonding techniques.

Over the operating temperature range of a heat pipe, different physical limits can limit its performance.

The operating curve of the heat pipe, which allows us to know its maximum transport capacity, is obtained by combining the curves corresponding to the different physical operating limits which ultimately define the operating range of the heat pipe in terms of maximum power that it can to transfer.

There figure 1 illustrates the curve delimiting the operating range for an example of a capillary pumped heat pipe. On this figure 1 , we recall that the portions of curve Qviscous, Qsonic, Qentrainment, Qcapillary, Qboiling, define the viscous, sonic, entrainment, capillary and boiling limits respectively.

Regarding the boiling limit, in a capillary pumped heat pipe, the capillary driving pressure must compensate for the static pressure losses linked to the volume and dynamic forces generated by the flow of the working fluid from the heat pipe (friction between the flows and walls). This capillary driving pressure, which corresponds to the difference in capillary pressure between the evaporator and the condenser, is a function of the structure of the heat pipe, and the dynamic pressure losses which are increasing with the mass flow of the working fluid (and with the length of the heat pipe), itself a function of the thermal power imposed on the evaporator: the capillary limit is reached when the capillary driving pressure is equal to the sum of the pressure losses (liquid phase and vapor phase). In other words, a heat pipe only works if the capillary driving pressure is greater, in microgravity conditions, than the pressure losses, and therefore below the capillary limit.

This limit is generally expressed in W.m: it is inversely proportional to the effective length of the heat pipe, that is to say that for the same design (same cross section over the entire length), the capillary limit, expressed in W, of a 2 meter heat pipe is half that of a 1 meter heat pipe.

dans laquelle :

• f est le coefficient de perte de charge (dépendant du régime d'écoulement du fluide, et que l'on peut déterminer via le nombre de Reynolds à partir de corrélations),
• v la vitesse du fluide de travail (proportionnelle à la puissance thermique à transporter),
• ρ la masse volumique du fluide de travail et
• Dh le diamètre hydraulique.

Frictional pressure losses in a flow are expressed by the Darcy-Weisbach equation as follows: $$\frac{\mathit{dP}}{\mathit{dz}}=f\frac{1}{D_{\mathrm{<figure-callout\; id="2"\; label="h\; \rho \; v"\; filenames="imgf0004.png"\; state="\{\{state\}\}">h</figure-callout>}}}\rho \frac{v^{}}{}\frac{{}^2}{2}$$

in which :

• f is the pressure loss coefficient (dependent on the fluid flow regime, and which can be determined via the Reynolds number from correlations),
• v the speed of the working fluid (proportional to the thermal power to be transported),
• ρ the density of the working fluid and
• Dh the hydraulic diameter.

avec S la section du fluide et Pm le périmètre mouillé.The hydraulic diameter is itself defined by the following equation: $$D_h=\frac{4S}{P_m}$$

with S the section of the fluid and Pm the wetted perimeter.

We therefore understand that to maximize the capillary limit, it is necessary to favor liquid and vapor channels of high section, in order to minimize the fluid speeds in these channels without reducing the flow rate, which is proportional to the thermal power transferred in the heat pipe .

It is also necessary to have a low groove width at the interface, in order to have a high capillary pressure. But this groove width cannot be too small to be technologically achievable by a brazing operation according to the process of the request. EP3553445A1 . Indeed, too small a width would create a risk of blockage by the solder.

Furthermore, the ratio of sections between the channel and the groove must not be too high, in order to facilitate the evacuation of steam bubbles which may possibly appear in the channels.

Furthermore also, during the evaporation taking place at the level of the meniscus of the liquid/vapor interface in the groove which delimits a connecting channel, the meniscus moves back in the latter, all the more so as the heat flow is high. If the meniscus moves back too much, it can reach the reentrant canal. In this case, the capillary radius at the interface increases and therefore the capillary pressure, driving the heat pipe, decreases; there is then a risk of defusing the heat pipe.

It is therefore advantageous to have a large volume of liquid in a connecting channel, in order to minimize this phenomenon.

The overall thermal resistance of a heat pipe can be evaluated by making a network analogy of independent thermal resistances.

Such a network is schematized in figure 2 in which a heat flow Q emitted by a hot source SC must be evacuated by a heat pipe to a cold source SF.

• la résistance entre la source extérieure et la paroi R1, R9 respectivement à l'évaporateur et au condenseur;
• la résistance de la paroi extérieure R2, R8 respectivement à l'évaporateur et au condenseur;
• la résistance des canaux liquides (réseau capillaire) R3, R7 respectivement à l'évaporateur et au condenseur ;
• la résistance de l'interface entre liquide et vapeur R4, R6 respectivement à l'évaporateur et au condenseur et
• la résistance de l'écoulement de vapeur R5.

From a thermal point of view, the heat pipe can be considered as a set of a number of eleven thermal resistors R1 to R11 in series and/or in parallel as shown in this figure 2 . The axial resistances of the outer wall R10 and the capillary network R11 along the length of the heat pipe are immense. Consequently, the preferred heat flow path is that passing through the steam circulation section. This path is made up of five different resistances, as follows:

• the resistance between the external source and the wall R1, R9 respectively to the evaporator and the condenser;
• the resistance of the outer wall R2, R8 to the evaporator and the condenser respectively;
• the resistance of the liquid channels (capillary network) R3, R7 to the evaporator and the condenser respectively;
• the resistance of the interface between liquid and vapor R4, R6 respectively at the evaporator and the condenser and
• the resistance of the steam flow R5.

On this path, the limiting thermal resistance is that of the liquid channels (capillary network), respectively at the evaporator and the condenser (R3, R7).

For the evaporator, the author of the publication [2] proposed a thermal resistance model with a path through the liquid in the groove parallel to a conductive path in the tooth and then in the evaporation film.

In this very conservative model, the equivalent conductivity of the film is given by an empirical formula according to equation 3: $${\lambda }_{\mathit{movie}}=\frac{{\lambda }_L}{0.185\ast \mathrm{vs}}$$

In an aluminum-walled heat pipe filled with ammonia as the working fluid, given the difference in thermal conductivity between liquid ammonia (0.4 W/m/K) and aluminum (150 W/ m/K), the preferred path of the heat flow between the hot source and the steam will pass through the metal walls between liquid channels.

The same pattern occurs at the condenser, where the preferred path of the heat flow between the steam and the cold source will pass through the walls between channels.

It therefore appears that, to increase the thermal conductance at the evaporator and the condenser, it is advantageous for the walls between liquid channels to have as high a section as possible.

Finally, the film resistance depends in particular on the length of the triple contact lines between liquid, vapor and solid.

The mechanical resistance of a heat pipe with reentrant grooves can also impact its performance. In particular, in the patent application EP3553445A1 , a connecting channel between a liquid channel and a vapor channel is completely open over its entire length along the longitudinal axis of the heat pipe, the brazed, glued or welded assembly of the stacked plates can therefore only be carried out on the peripheral walls of these, which results in significant wall thicknesses to resist the internal pressure of the heat pipe.

We also know another type of heat pipe, called an artery, as disclosed in the patent US4422501A , which is a heat pipe whose liquid condensate return is physically separated from the channel where the steam circulates after evaporation.

The main advantage of an artery heat pipe is to annihilate the maximum flow of liquid particles entrained by the vapor by eliminating the liquid/vapor interfaces in the adiabatic zone of the heat pipe.

In addition, producing an artery heat pipe is much less expensive and simple to install than a heat pipe with a sintered peripheral capillary network. With a small diameter artery, depending on the fluid and materials selected, if the capillary forces are large enough, operation against gravity can be expected with good performance.

Such an artery heat pipe 1 is shown in figures 3, 3A And 3B : just like other rectilinear heat pipes, it comprises a sealed enclosure delimited by a wall 10 and extends between a first longitudinal end, intended to be heated by a thermal flow Φ _E emitted by a hot source, to form, within the enclosure, an evaporator Z _E and a second longitudinal end intended to output a thermal flow Φ _S towards a source cold to form, within the enclosure, a condenser Zc where the vapor V condenses, the sealed enclosure delimiting an adiabatic zone Z _A between the evaporator and the condenser.

Such an artery heat pipe 1 integrates a liquid channel 11 forming an artery which brings the liquid from the condenser Zc to the evaporator Z _E , which is dissociated from the vapor channel 13. In such a heat pipe, there is no liquid-vapor interface except for the evaporator Z _E and the condenser Zc. This also makes it possible to reduce the friction forces of the liquid phase. In addition, the liquid phase at the evaporator Z _E is not directly subjected to the heat flow Φ _E from the hot source, and this plays a role in moving away from the boiling limit.

A very interesting configuration for a terrestrial application, that is to say where gravity applies, consists of integrating one or more of the arteries 11 in a porous medium 14, peripheral to the steam channel 13, as illustrated in the figure 4 . This porous medium 14, also called wick, can have pores with an average size of a few hundred microns per millimeter. In addition to the loss of reduced liquid pressure that it induces, the porous medium 14 ensures a very good boiling limit at the level of the evaporator by a multiplication of lines and triple points P, that is to say solid-liquid-vapor contact points, which are very favorable to evaporation.

Heat pipes with grooves, in particular with reentrant grooves, have the major disadvantage of having a high thermal resistance to the evaporator, and a radial heat flow which it is possible to transmit in the evaporator also limited by the boiling phenomena in the grooves. Typically, this flow is limited to around 15 W/cm ² for an ammonia heat pipe.

• pour une structure réalisée par poudre frittée, il est nécessaire de fritter celle-ci à l'intérieur du caloduc, ce qui est difficilement réalisable sur des longueurs de plusieurs mètres, typiquement rencontrées dans des applications spatiales (satellites) ;
• la taille des pores de la structure capillaire poreuse étant de quelques microns, les pertes de pression du retour de liquide dans ladite structure deviennent vite rédhibitoires sur des longueurs de plusieurs mètres, ce qui limite la puissance thermique transportable.

Heat pipes with a porous capillary structure, such as sintered powder, achieve higher performance than grooved heat pipes. In particular, these heat pipes with porous capillary structure make it possible to accommodate much higher surface heat fluxes at the evaporator, and to have lower thermal resistance to the evaporator. However, they have two major disadvantages:

• for a structure made by sintered powder, it is necessary to sinter it inside the heat pipe, which is difficult to achieve over lengths of several meters, typically encountered in space applications (satellites);
• the size of the pores of the porous capillary structure being a few microns, the pressure losses of the return of liquid in said structure quickly become prohibitive over lengths of several meters, which limits the transportable thermal power.

The patent US9618275B1 describes a heat pipe with a hybrid structure, ie with grooves which extend longitudinally in the adiabatic part and the condenser and with a porous insert of cylindrical shape in the evaporator. This heat pipe with a hybrid structure partially addresses the problem mentioned above with grooved heat pipes. However, it has several disadvantages. First of all, the return of the liquid (in the evaporator zone) occurs longitudinally in the thickness of the cylindrical porous insert, which induces a high liquid pressure loss. Indeed, the size of the pores of the porous insert is intrinsically small so as to obtain a high capillary pressure, but with the counterpart of necessarily inducing a low permeability. The method of carrying out the Figure 6 of this patent provides a solution to this problem of low permeability: it provides for an enlargement of the diameter of the porous insert in the evaporator. However, such widening necessarily induces an increase in thermal resistance in this area of the heat pipe (evaporator). Furthermore, the cylindrical porous insert disclosed must ensure fluid continuity with the grooves of the adiabatic zone, otherwise the capillary pumping risks defusing. However, in concrete terms, this fluid continuity seems difficult to guarantee, and requires expensive and complex solutions (machining of fingers in the porous insert penetrating the grooves), and random.

Consequently, there is a need to further improve heat pipes with grooves, in particular reentrant grooves, in order to overcome the aforementioned drawbacks, in particular the high thermal resistance to the evaporator and the limitation of the radial heat flow to the evaporator, typically of the order of 15 W/cm ² for a heat pipe filled with ammonia.

The general aim of the invention is then to respond at least in part to this need.

Presentation of the invention

To do this, the invention firstly relates to a capillary pumped heat pipe with reentrant grooves, extending along a first longitudinal direction (X), comprising a sealed enclosure extending between a first longitudinal end , intended to be heated by a hot source SC to form, within the enclosure, an evaporator and a second longitudinal end intended to be cooled by a cold source SF to form, within the enclosure, a condenser, the sealed enclosure delimiting an adiabatic zone between the evaporator and the condenser, the sealed enclosure comprising a stack of plates in a second direction, orthogonal to the first direction, two of which closing plates and at least a number of n modules on top of each other with n being an integer greater than or equal to 1, each module comprising at least one intermediate plate between the closing plates, the intermediate plate(s) comprising at least one first intermediate plate comprising at least one window whose edges partially delimit at least one vapor channel extending along the first direction (X) between the evaporator and the condenser, in which the vapor is intended to circulate, and on at least one lateral side of the window in a third direction (Y) orthogonal to the first (X) and second direction (Z), at least one structure whose edges partly delimit at least one liquid channel in the evaporator and the condenser , the heat pipe comprising, at least in the evaporator, at least one exchange zone defining a liquid-vapor interface and delimited between the at least one intermediate plate and at least one other intermediate plate or a closing plate, connecting the ( s) vapor channel(s) and the liquid channel(s), each exchange zone comprising at least one connecting channel opening out.

According to the invention, at least part of the connecting channels opening out from the evaporator each housing an insert made of porous material which extends at least partly into the liquid channel(s) and into the ( s) vapor channel(s), the porous insert being adapted to bring the liquid by capillary action from the liquid channel(s) to the vapor channel(s) in which it evaporates.

According to an advantageous configuration, the condenser of the heat pipe also comprises at least one exchange zone defining a liquid-vapor interface and delimited between the at least one intermediate plate and at least one other intermediate plate or a closing plate, connecting the(s) ) vapor channel(s) and the liquid channel(s), each exchange zone comprising at least one connecting channel emerging, at least part of the connecting channels emerging from the condenser housing each an insert made of porous material which extends at least partly into the liquid channel(s) and into the vapor channel(s), the porous insert being adapted to condense the vapor in the (s) steam channel(s) and bring the condensed liquid by capillary action to the liquid channel.

The porous inserts in the evaporator can be made of the same constituent material, have the same shape and/or the same dimensions as the porous inserts in the condenser.

According to another advantageous configuration, the adiabatic zone of the heat pipe comprises at least one exchange zone between the vapor channel(s) and the liquid channel(s), defining a liquid interface. -steam and delimited between the at least one intermediate plate and at least one other intermediate plate or a closing plate, each exchange zone comprising at least one connecting channel opening out free of insert.

We can combine these two configurations and therefore have porous inserts in the evaporator and in the condenser of the heat pipe while having an adiabatic zone free of porous inserts. Preferably, the liquid channels are present in all zones of the heat pipe (evaporator, adiabatic zone, condenser), in order to minimize the pressure loss of the liquid return.

Advantageously, the porous insert of the evaporator and, where applicable, the condenser extends over the entire width of the vapor channel in the third direction (Y).

Advantageously, the porous insert of the evaporator and, where appropriate, the condenser extends over the entire width of the liquid channel in the third direction (Y).

According to an advantageous alternative embodiment, the porous insert has the shape of a flat sheet. According to an advantageous embodiment, at least part of the intermediate plates comprises, in the evaporator and where appropriate in the condenser, at least one longitudinal notch extending in the first direction (X) forming a support surface and maintaining the porous flat sheet.

Preferably, the material constituting a porous insert is based on graphite, preferably not wettable by a brazing filler metal. Such a material has the advantage of having high porosity and thermal conductivity, low density and is slightly compressible. In addition, it is not wettable by the eutectic aluminum alloy during a process of assembling the plates of the stack by eutectic brazing. Conversely, it is perfectly wettable with ammonia, which is a reference fluid for filling the heat pipe enclosure. We can cite, for example, graphite sheets marketed under the name SIGRACET ^® or even that under the name AvCarb ^® .

Preferably, the width of a porous insert is between 0.1 and 1mm, preferably of the order of 0.6mm. This width determines its thickness and is advantageously slightly greater than or equal to the width of the connecting channels, so as to ensure continuity of the flow of the liquid, and simplify the production of the intermediate plates.

Generally speaking, the porous insert is compressible and slightly thicker than the width of the connecting channels in the YZ plane so as to ensure good wedging in the spacer plates. Good wedging allows good thermal contact with the plates and thereby efficient conduction of heat between the entry of the heat flow on the plates and the evaporation zone on the faces of the porous insert in the steam channel.

In the configuration of the case where the condenser also houses one or more porous inserts, the heat flow is applied between the faces of each porous insert in the steam channel, and the extraction of the heat flow is done through the exterior faces of the heat pipe.

More preferably, the average size of the pores of a porous insert is between 1 and 100 µm, preferably of the order of 50 µm.

The material constituting the plates forming the sealed enclosure is preferably chosen from aluminum, copper, nickel, or an alloy based on at least two of these. The material(s) used for the manufacture of the heat pipe are chosen according to the constraints of mass, assembly, required robustness, compatibility with the fluid, etc.

The sheet metal assembly technique depends on the material.

For example, in the case of aluminum alloy plates, vacuum brazing with clad plates, salt bath brazing, inert gas brazing, ultrasonic welding, laser welding, gas welding can be used. friction-kneading (“Friction Stir Welding” in Anglo-Saxon language), bonding...

In the case of copper, stainless steel or superalloy plates, diffusion welding, laser welding, diffusion soldering, bonding, etc. can be used.

In the case of stainless steel plates, super alloys, diffusion welding, laser welding, diffusion brazing, bonding, etc. can be used.

For example, the assembly of aluminum alloy plates is obtained by eutectic brazing. In a known manner, aluminum alloy plates are used, one or both faces of which is or are coated with an aluminum alloy with a lower melting point.

For example, a sheet of alloy from the AA3xxxx series is used at the core, with a coating with a eutectic alloy from the AA4xxxx series comprising silicon with a lower melting point. merger. The coating is typically done using a roll-bond technique. The total thickness of the plates is typically 0.05 mm to 5 mm, with a coating typically of 5% to 10% of the total thickness on one or both sides. By hot pressing two aluminum plates coated in this way at a temperature higher than the melting temperature of the eutectic, but lower than the temperature of the core alloy, the surface eutectic alloy melts and forms a solder alloy. waterproof assembly between the two plates. Brazing is preferably carried out under pressure using a mechanical holding system, which maintains the stack under pressure during brazing in a vacuum furnace. Cutting and/or folding is required in the manufacturing process to lighten and/or shape the structure. They are preferably made after assembly. It should be noted that the cutting of the windows in the plates, for example the central windows, is carried out before assembly.

• Etape a/ : Des plaques en un matériau donné sont découpées suivant la forme extérieure souhaitée pour le caloduc. Les inserts poreux sont également découpés, avantageusement sous forme de feuilles planes.
• Etape b/ : Les plaques intercalaires sont structurées, par exemple par poinçonnage, usinage, découpe laser, par découpe au jet d'eau ou par gravure chimique traversante...afin de réaliser les fenêtres spécifiques dans les différentes plaques, de sorte qu'une fois assemblées un caloduc à rainures réentrantes soit formé.
• Etape c/ : Les plaques sont ensuite empilées dans un ordre donné, en alternant entre deux plaques intercalaires un insert poreux à l'évaporateur et le cas échéant au condenseur. Puis deux plaques de fermeture sont disposées aux extrémités de l'empilement pour fermer le ou les canaux. L'empilement est réalisé de sorte à ce qu'un insert poreux soit pris en sandwich entre deux plaques intercalaires ou entre une plaque intercalaire et une plaque de fermeture, de préférence en étant comprimé. Eventuellement des canaux de refroidissement sont prévus sur une ou les deux faces de l'empilement.
• Etape d/ : Les plaques sont alors assemblées entre elles selon une technique d'assemblage choisie en fonction du ou des matériaux des plaques, par exemple soudage, brasage, collage...L'assemblage final des plaques est étanche. Le ou les matériaux des plaques est ou sont choisis en fonction du fluide de travail, qui est lui-même choisi en fonction des spécifications de thermalisation du système à réaliser.
• Etape e/ : Le caloduc est ensuite rempli. Afin de remplir le caloduc du fluide de travail, on peut utiliser un queusot de remplissage inséré sur la tranche de l'enceinte. On peut également utiliser un queusot fixé sur un orifice ménagé à travers l'une ou l'autre des plaques de fermeture, et perpendiculairement à celle-ci. Le fluide est choisi en fonction des conditions de fonctionnement du caloduc (température de fonctionnement...) et de la compatibilité avec le ou les matériaux du caloduc.

The process for producing a heat pipe according to the invention can advantageously consist of the following steps:

• Step a/ : Plates of a given material are cut according to the desired external shape for the heat pipe. The porous inserts are also cut, advantageously in the form of flat sheets.
• Step b/ : The intermediate plates are structured, for example by punching, machining, laser cutting, by water jet cutting or by through chemical etching...in order to produce specific windows in the different plates, so that once assembled, a heat pipe with reentrant grooves is formed.
• Step c/ : The plates are then stacked in a given order, alternating between two interposed plates a porous insert at the evaporator and if necessary at the condenser. Then two closing plates are placed at the ends of the stack to close the channel(s). The stack is made so that a porous insert is sandwiched between two intermediate plates or between an intermediate plate and a closing plate, preferably while being compressed. Cooling channels may be provided on one or both sides of the stack.
• Step d/: The plates are then assembled together using an assembly technique chosen according to the material(s) of the plates, for example welding, brazing, gluing, etc. The final assembly of the plates is watertight. The material(s) of the plates is or are chosen according to the working fluid, which is itself chosen according to the thermalization specifications of the system to be produced.
• Step e/ : The heat pipe is then filled. In order to fill the heat pipe with the working fluid, you can use a filling plug inserted on the edge of the enclosure. It is also possible to use a tail fixed to an orifice made through one or the other of the closing plates, and perpendicular to it. The fluid is chosen based on the operating conditions of the heat pipe (operating temperature, etc.) and compatibility with the heat pipe material(s).

• les plaques intercalaires présentent une épaisseur comprise entre 0,5 à 6mm, de préférence de l'ordre de 2mm,
• les plaques de fermeture présentent une épaisseur comprise entre 1 et 3mm, de préférence de l'ordre de 2mm,
• la largeur dans le plan YZ d'un canal de liaison est comprise entre 0,1 et 1mm, de préférence de l'ordre de 0,2mm,
• l'épaisseur de structuration dans le plan YZ délimitant l'espace entre deux canaux de liaison adjacents est comprise entre 0,2 et 2mm, de préférence de l'ordre de 0,4mm,
• la largeur dans le plan YZ d'un canal liquide est comprise entre 0,5 et 4mm, de préférence de l'ordre de 3mm.

According to one or more advantageous characteristics which can be combined with each other:

• the intermediate plates have a thickness of between 0.5 to 6mm, preferably of the order of 2mm,
• the closing plates have a thickness of between 1 and 3mm, preferably of the order of 2mm,
• the width in the YZ plane of a connecting channel is between 0.1 and 1mm, preferably of the order of 0.2mm,
• the structuring thickness in the YZ plane delimiting the space between two adjacent connecting channels is between 0.2 and 2mm, preferably of the order of 0.4mm,
• the width in the YZ plane of a liquid channel is between 0.5 and 4mm, preferably of the order of 3mm.

According to an advantageous configuration, the heat pipe comprises n modules one on top of the other, n being an integer greater than or equal to 1, defining a single vapor channel and n liquid channels on at least one lateral side, in particular on each lateral side of the vapor channel . Preferably, the heat pipe comprises a single central vapor channel arranged between n liquid channels distributed laterally on either side of the vapor channel.

According to an advantageous alternative embodiment, at least one of the end plates has a surface greater than that of the intermediate plates in a direction transverse to that (Z) of the stack so as to form thermal diffusers. With such thermal diffusers, the heat flow into and out of the heat pipe is spread over a larger area and locally reduces the maximum flux density that would be achieved without a diffuser.

According to another advantageous embodiment, the cross section in the XY plane of the evaporator integrating the porous insert and/or that of the condenser, preferably rectangular, is greater than that of the adiabatic zone of the heat pipe. In the case of a heat flow perpendicular to the main plane of the plates, this variant makes it possible to increase the exchange surface at the evaporator and/or condenser and therefore minimize external thermal resistance, while limiting the increase mass of the heat pipe.

In the case of a heat flow parallel to the plane of the plates, the size of the liquid and/or vapor channels in the YZ plane can be increased, making it possible to increase the capillary limit of the heat pipe.

If there are no liquid channels under a porous insert, but only a capillary return of the liquid in the porous insert, it is preferable to increase the width of a porous insert in the liquid zone, in order to limit losses of pressure in it.

According to yet another advantageous embodiment, the structures of the evaporator and where appropriate the condenser comprise a plurality of pads distributed, preferably uniformly, along the first direction (X). These pads make it possible to minimize the pressure loss in a porous insert, while ensuring mechanical support of the latter. This variant with pads can be made for configurations with liquid channels on the sides of the heat pipe enclosure or in the central part thereof.

According to an advantageous embodiment, the heat pipe comprises heat exchange means at the first end and/or second end, the heat exchange means at the second end advantageously comprising one or more fins in thermal contact with at least one of the closing plates.

According to this mode, the heat exchange means may comprise a fluidic circuit in thermal contact with at least one of the closing plates, said circuit being formed by a plate structured so as to delimit channels, said channels being closed by said closing plate. closure and an additional closure plate, the heat exchange means also comprising means for supplying heat transfer fluid to said fluid circuit.

In general, it is possible to integrate within the heat pipe itself at the level of the evaporator and/or the condenser a fluidic circuit forming the hot source and/or the cold source. This circuit will take the form of a pleated or perforated closing sheet to form a radiator full air. Alternatively, it can take the form of a liquid or two-phase circuit forming the hot and/or cold source.

Advantageously, the cross section in the YZ plane of the vapor channel and the liquid channel(s), preferably rectangular, is constant over the entire length of the heat pipe.

According to an alternative embodiment, cutouts of the narrow connecting channels can be made straight, or with a wavy or sawtooth shape over all or part of the length of the channels. Such a shape makes it possible to obtain walls not wetted by the condensation film, in the condenser of the heat pipe, at the tops of the undulations or saw teeth. This also makes it possible to increase the exchange coefficient in condensation by minimizing the interface resistance of the condensation film.

• une source froide (SF) ;
• une source chaude (SC) et
• au moins un caloduc à rainures réentrantes tel que décrit précédemment, le caloduc étant agencé de sorte que le flux de chaleur (φ_E ) de la source chaude (SC) sur l'évaporateur soit sur au moins une face latérale de l'enceinte en regard du canal vapeur ou sur au moins une face latérale en regard du(des) canal(ux) liquide(s), tandis que l'extraction de chaleur au condenseur vers la source froide (SF) est sur au moins une face latérale de l'enceinte en regard du(des) canal(ux) liquide(s), ou sur une face latérale perpendiculaire à celui-ci (ceux-ci).

The invention also relates to a system comprising:

• a cold source (SF);
• a hot spring (SC) and
• at least one heat pipe with reentrant grooves as described above, the heat pipe being arranged so that the heat flow (φ _E ) from the hot source (SC) on the evaporator is on at least one side face of the enclosure in facing the vapor channel or on at least one side face facing the liquid channel(s), while the heat extraction from the condenser towards the cold source (SF) is on at least one side face of the enclosure facing the liquid channel(s), or on a side face perpendicular to it (these).

The same heat pipe according to the invention can be used to carry out thermal control, heat transfer, component cooling, etc. The heat source can therefore be for example an electrical or electronic component, heat storage, an exothermic chemical reactor, fin heat sinks in forced convection, cold plates in single or two-phase flow, cold storage, an endothermic chemical reaction, etc.

The cold source can consist of a radiator with natural or forced convection.

We can consider different radiator technologies: pleated fins, extruded fins, skived fins, pinned fins, molded fins, fins fixed by knurling, fins made by 3D printing, or any other fin obtained by a production technique of surface extension known to those skilled in the art.

One or more finned radiators as described above can be used in a heat pipe according to the invention, straight single-channel or multi-channel or having any other shape.

Thus, the invention essentially consists of proposing a heat pipe of which at least the evaporator integrates within it a porous insert which extends in a connecting channel between the vapor channel and the liquid channel(s).

The production of such a heat pipe is implemented with a process as according to the patent application EP3553445 , which consists of stacking and then assembling together by gluing, welding, preferably by vacuum brazing, punched or machined metal plates to define the different heat pipe channels. The porous inserts according to the invention do not modify the assembly process, since they are inserted directly in alternation with the plates in the stack which is assembled.

The invention particularly finds application in the space domain.

• à l'évaporateur, généralement de faible longueur, il est requis des flux radiaux élevés à cause des composants électroniques que l'on cherche à refroidir, dont les densités de puissance ne cessent d'augmenter. De plus, il faut une résistance thermique faible et, afin d'assurer une pression motrice au caloduc, il faut une pression capillaire élevée assurée par un faible rayon capillaire à l'interface liquide/vapeur ;
• dans la zone adiabatique, généralement de grande longueur, il faut limiter les pertes de pression dans le canal vapeur et dans les canaux liquides, afin d'avoir une limite capillaire de transport de chaleur axiale élevée.
• au condenseur, qui peut être également de grande longueur, il faut également limiter les pertes de pression, mais aussi assurer une condensation efficace pour minimiser la résistance thermique dans cette zone.

For a heat pipe configured for an application in the space domain, the constraints encountered are of several types which can be listed as follows:

• at the evaporator, generally of short length, high radial flows are required because of the electronic components that we seek to cool, whose power densities continue to increase. In addition, low thermal resistance is required and, in order to ensure a driving pressure to the heat pipe, a high capillary pressure is required ensured by a small capillary radius at the liquid/vapor interface;
• in the adiabatic zone, generally of great length, it is necessary to limit the pressure losses in the vapor channel and in the liquid channels, in order to have a high capillary limit for axial heat transport.
• at the condenser, which can also be of great length, it is also necessary to limit pressure losses, but also to ensure effective condensation to minimize thermal resistance in this area.

The invention described here makes it possible to respond to these constraints, by allowing different configurations in each heat pipe zone in order to optimize it as best as possible according to the desired performance criteria.

More particularly, the porous inserts inserted longitudinally in the evaporator of the heat pipe will allow the liquid to be transported by capillary action between the liquid channel(s) and the vapor channel. The evaporation of the liquid on the surface of a porous insert allows obtaining a high capillary pressure, while ensuring a higher surface heat flow than a heat pipe with reentrant grooves or a sintered capillary, according to the state of the art.

For example, the practical limit of surface heat flux with a heat pipe filled with ammonia, with reentrant grooves according to the state of the art is of the order of 15 W/cm ² . With a heat pipe according to the invention, this practical limit can rise to more than 50 W/cm ² .

Thermal resistance is also improved in the evaporator thanks to a larger liquid evaporation surface.

In the evaporator and possibly condenser zones, there are liquid channels, the circulation of liquid in the porous taking place only over the small distances between liquid channel and vapor channel, making it possible to limit pressure losses in the porous.

In the adiabatic zone, there are no porous parts; longitudinal grooves defining the connecting channels make it possible to minimize the loss of liquid pressure, even over large distances.

Also arranging porous inserts longitudinally in the condenser makes it possible to reduce the thermal resistance in this zone, even if this can be done at the expense of an increased liquid pressure loss in this zone.

Furthermore, a heat pipe according to the invention has a high capillary pressure thanks to the small pore size of a porous insert. And so, it is likely to work without gravity, or even against gravity. This makes the heat pipe according to the invention particularly suitable for space missions near celestial objects where there may be non-zero gravity (moon, mars, asteroids, etc.).

Other advantages and characteristics will become clearer on reading the detailed description, given for illustrative and non-limiting purposes, with reference to the following figures.

Brief description of the drawings

• [ Figure 1 ] there figure 1 illustrates the curve delimiting the operating range for an example of a capillary pumped heat pipe.
• [ Figure 2 ] there figure 2 is a symbolic representation of a network of thermal resistors that are established for a heat pipe.
• [ Figure 3 ] there Figure 3 is a schematic view of an artery heat pipe according to the state of the art (Example 1).
• [ Fig 3A ], [ Fig 3B ] THE figures 3A And 3B are cross-sectional views of the Figure 3 , respectively at the level of the evaporator and the condenser of the artery heat pipe according to the state of the art (Example 1).
• [ Figure 4 ] there Figure 4 is a cross-sectional view of a variant of the arterial heat pipe according to the state of the art.
• [ Figure 5 ] there Figure 5 is a photographic reproduction of an example of a heat pipe with reentrant grooves with an Omega cross section according to the state of the art (Example 1).
• [ Figure 6 ] there Figure 6 is a schematic side view of a heat pipe with reentrant grooves, according to the state of the art (Example 2).
• [ Figure 7 ] there Figure 7 is a perspective and cross-sectional view of the Figure 6 , at the level of the evaporator of the heat pipe with reentrant grooves according to the patent application EP3553445 (Example 2).
• [ Figure 8 ] there figure 8 is a detailed and cross-sectional view of the Figure 7 .
• [ Figure 9 ] there Figure 9 is an exploded view of a heat pipe with reentrant grooves with transverse connecting channels produced by stacking assembled metal plates, with porous inserts at the evaporator, according to a third example of the invention with a heat pipe aspect ratio square.
• [ Figure 10 ] there Figure 10 is a detailed and cross-sectional view of the Figure 9 , at the level of the evaporator, according to the third example of the invention.
• [ Figure 11 ] there Figure 11 is a detailed and cross-sectional view according to a third example of the invention, at the level of the condenser and the adiabatic zone of the heat pipe.
• [ Figure 12 ] there Figure 12 is a detailed view in cross section, at the level of the evaporator, of a heat pipe with reentrant grooves with transverse connecting channels produced by stacking metal plates assembled according to a fourth example of the invention, with a ratio of appearance of flat rectangular heat pipe.
• [ Figure 13 ] there figure 13 is an exploded view of a heat pipe with reentrant grooves with transverse connecting channels produced by stacking metal plates assembled according to a fifth example of the invention.
• [ Figure 14 ] there Figure 14 is a detailed and cross-sectional view of the Figure 13 , at the level of the evaporator according to the fifth example of the invention.
• [ Figure 15 ] there Figure 15 is a detailed view in cross section, at the level of the condenser and the adiabatic zone of the heat pipe according to the fifth example of the invention.
• [ Figure 16 ] there Figure 16 is a detailed view in cross section, at the level of the evaporator, of a heat pipe with reentrant grooves with transverse connecting channels produced by stacking metal plates assembled according to a sixth example of the invention.
• [ Figure 17 ] there Figure 17 illustrates in the form of curves the capillary limits of the different examples of heat pipes according to the state of the art and according to the invention.
• [ Figure 18 ] there Figure 18 is an exploded view of a heat pipe with reentrant grooves with transverse connecting channels produced by stacking metal plates assembled according to another embodiment of the invention.
• [ Figure 19 ] there Figure 19 is a detailed and cross-sectional view of the Figure 18 , at the evaporator level.
• [ Figure 20 ] there Figure 20 is a detailed and cross-sectional view of the Figure 18 , at the level of the adiabatic zone of the heat pipe.
• [ Figure 21 ] there Figure 21 is a perspective and detail view of an alternative embodiment of the evaporator of a heat pipe according to the invention, comprising heat diffusers integrated at the level of the evaporator.
• [ Figure 22 ] there Figure 22 is a detailed perspective and exploded view of another alternative embodiment of the evaporator of a heat pipe according to the invention.
• [ Figure 23 ] there Figure 23 is a detailed perspective and exploded view of another alternative embodiment of the evaporator of a heat pipe according to the invention.
• [ Fig 24], [Fig 25 ] THE figures 24 and 25 are perspective views of other examples of embodiment of the connection channels at the level of the central window with intermediate plates of a heat pipe according to the invention.
• [ Figure 26 ] there Figure 26 is a perspective view of another embodiment of a heat pipe according to the invention arranged in several planes.
• [ Fig 27A ], [ Fig 27B], [Fig 27C], [Fig 27D ], [ Fig 27E], [Fig 27F], [Fig 27G ], [ Fig 27H ] THE figures 27A to 27H are perspective views of examples of fins applicable to a heat pipe according to the invention.
• [ Figure 28 ] there Figure 28 is an exploded view of a heat pipe according to an exemplary embodiment comprising a heat exchanger at the level of the condenser.

detailed description

THE figures 1 to 4 have already been commented on in the preamble. They are not detailed below.

For the sake of clarity, the same element according to the prior art and the invention is designated by the same numerical reference.

It is specified here that the reference SC used in the figures designates the hot source or, by extension, the zone of application of the heat flux emitted by the hot source directly on the side face of a heat pipe enclosure. Φ _E designates the heat flow emitted by the hot source towards a heat pipe, and Φ _S designates the heat flow emitted by a heat pipe towards a cold source.

We describe below different examples of heat pipes with reentrant grooves according to the state of the art and according to the invention in order to be able to compare their performances, as detailed below.

We specify that for all these examples, the liquid channels are present both in the evaporator and in the condenser.

All the heat pipes according to these examples have the same total length, the same evaporator length, the same length of the adiabatic zone, and the same length of the condenser.

The external cross section defining the lateral dimensions of each heat pipe according to these examples is equal to 13.2mm x 13.2mm.

Example 1 (according to the state of the art) : the heat pipe with reentrant grooves 1 is obtained by extrusion according to the state of the art with reentrant grooves, as shown in the Figure 5 .

The evaporator of such a heat pipe 1, axisymmetric of generally cylindrical shape, comprises a peripheral wall 10 in which are made a plurality of channels 11 with a cross section in the general shape of Omega, regularly distributed angularly and opening via a connecting channel 12 onto a central cylindrical hollow 13 in which the vapor phase circulates. The peripheral wall 10 is in contact with a hot source (SC).

Example 2 (according to the state of the art) : On the figures 6 to 8 , we can see an example of a heat pipe 1 with capillary pumping with reentrant grooves produced according to the patent application EP3553445 .

This example of heat pipe 1 with capillary pumping extending along a longitudinal axis Figure 6 .

The heat pipe 1 comprises a sealed enclosure 2 extending along the longitudinal axis within the enclosure an evaporator Z _E. The second longitudinal end 4 is intended to be cooled by a cold source SF to form a condenser Zc within the enclosure.

The sealed enclosure 2 internally delimits an adiabatic zone Z _A between the evaporator and the condenser.

The hot source is for example an electrical or electronic component, a heat storage unit, an exothermic chemical reactor. The cold source is for example a radiative surface, fins in forced convection, cold plates in single or two-phase flow, cold storage, an endothermic chemical reaction, etc.

The sealed enclosure 2 is produced by stacking and assembling end plates 22 also called closing plates and intermediate plate modules 20 arranged between the end plates 22, according to a process described in the patent application EP3553445 .

A module comprises at least two intermediate plates, the plates of the different intermediate plate modules 20 comprising windows or other structures, being stacked so as to delimit channels 11, 12, 13 as detailed below. A module can also include a single plate machined on its two main faces.

The production, stacking and assembly of the plates is not detailed here, we can refer to the aforementioned request EP3553445 . However, the plates 20, 22 are preferably made of aluminum alloy and assembled by vacuum brazing. Aluminum alloy plates can preferably be coated with a eutectic clading.

A preferred embodiment consists of machining clad plates 20 on their two main faces, then assembling these sheets by vacuum eutectic brazing. As a variant, machining can be carried out on a single main face of the clad plates.

For assembly, different processes are possible: salt bath brazing, inert gas brazing, ultrasonic welding, friction stir welding, bonding, etc.

The external dimensions of heat pipes range from a few centimeters to a few meters. The maximum size of heat pipes is generally limited by the tooling available. Indeed, the assembly of sheets by vacuum brazing requires large vacuum furnaces, several meters long.

For sheet metal cutting and machining, large machines are also required. In addition, the mechanical strength of sheets with small width and long length cuts must be taken into account.

For example, windows are made by punching, cutting, for example by laser or water jet.

In the example illustrated, all the plates 20 have the same external dimensions, the stack defining the sealed enclosure 2 is then of rectangular parallelepiped shape with four longitudinal faces parallel to the XY plane or the XZ plane, each having a large surface area favoring heat exchanges with the hot source SC and the cold source SF. In the example illustrated, the longitudinal face 21 parallel to the plane XY is that which receives the heat flow (φ _E ) from the hot source (SC).

The stack of plates 20 with their windows or structures 14, internally delimits, in the evaporator Z _E , liquid channels 11, a vapor channel 13 connected to the liquid channels 11 by connecting channels 12.

More _precisely , the vapor channel 13 of constant rectangular cross section extends along the longitudinal axis adiabatic Z _A .

A liquid channel 11 may or may not be connected to the vapor channel 13 depending on the area of the heat pipe. When connected to the vapor channel 13, as in the evaporator Z _E , a liquid channel 11 is connected by a connecting channel 12 of section in the plane XZ smaller than that of the liquid channel. Each liquid channel 11 is intended for the circulation of liquid from the condenser Zc to the evaporator Z _E.

A connecting channel 12 forms a reentrant groove by defining an exchange zone between the vapor and the liquid. In other words, a connection channel 12 defining at least one liquid-vapor interface.

Thus, in the state of the art, as illustrated in figures 7 and 8 , the connecting channels 12 extend in the longitudinal direction X of the heat pipe. As symbolized on the figure 8 , this production of the channels 12 has the particular disadvantage that the brazing zones B between intermediate plates 20 within the stack and with the end plates 22 can only be physically present on one longitudinal side of the liquid channels 11.

By way of example, in particular for an aluminum alloy heat pipe using ammonia as working fluid, the intermediate plates 20 have a thickness of between 0.5 mm and 6 mm, preferably equal to 2 mm.

The end plates 22 have a thickness of between 1 mm and 3 mm, preferably equal to 2 mm.

The width in the YZ plane of a connecting channel 12 is between 0.1 and 1mm, preferably of the order of 0.2mm.

The width in the YZ plane of a liquid channel 11 is between 0.5 and 4mm, preferably of the order of 3mm.

Example 3 (according to the invention) : In this example illustrated in figures 9 to 11 , the evaporator comprises a plurality of porous graphite inserts 15, each preferably in the form of a flat sheet.

A porous insert 15 rests inside a longitudinal notch 16 provided for this purpose in an intermediate plate 20.

The stacking of the intermediate plates 20 is carried out by alternating between two intermediate plates a porous insert 15 at the evaporator Z _E. Then, the two closing plates 22 are arranged at the ends of the stack to close the channel(s) 11, 12, 13. The stack is made so that a porous insert 15 is sandwiched between two plates. spacers 20 or between an intermediate plate 20 and a closing plate 22 while being compressed.

In this example 3, the porous insert 15 extends across the entire width in the Y direction of both the liquid 11, connection 12 and vapor 13 channels. The porous insert 15 makes it possible, by capillarity, to bring the liquid from each liquid channel 11 to the vapor channel 13 where it evaporates on the surface.

The adiabatic zone Z _A and the condenser Zc are free of inserts.

The heat flow Φ _E coming from the hot source reaches the side face 23 of the enclosure 2 of the heat pipe, that is to say being parallel to the plane of the plates 20, 22 and the liquid channels 11 are therefore at opposite face 23.

In this example 3, the fact of having several channels 11 in parallel makes it possible to increase the transmitted heat flow and/or the exchange surface. In this example 3, the return of the liquid therefore takes place on the side opposite to that 23 through which the heat flow Φ _E is supplied, which repels the appearance of bubbles in the liquid channels 11 (boiling limit).

At the condenser Zc, the heat flow Φ _S is extracted by the lateral face 24, that is to say that on the side of the liquid channels 11. We can also envisage heat extraction on the face 23 on the vapor channel side, or even on the other sides of the heat pipe.

Example 4 (according to the invention) : This example illustrated in Figure 12 is identical to Example 3 except that the aspect ratio between the width and height of the heat pipe is different so as to have a flatter heat pipe.

Example 5 (according to the invention) : In this example illustrated in figures 13 to 15 , the evaporator also comprises a plurality of porous graphite inserts 15, each preferably in the form of a flat sheet.

A porous insert 15 rests inside a longitudinal notch 16 provided for this purpose in an intermediate plate 20.

The stacking of the intermediate plates 20 is carried out by alternating between two intermediate plates a porous insert 15 at the evaporator Z _E. Then, the two closing plates 22 are arranged at the ends of the stack to close the channel(s) 11, 12, 13.

The stack is made so that a porous insert 15 is sandwiched between two intermediate plates 20 or between an intermediate plate 20 and a closing plate 22 while being compressed.

In this example 5, the porous insert 15 extends across the entire width in the Y direction of both the liquid 11, connection 12 and vapor 13 channels. To maintain the porous insert 15 even better, we can consider a longitudinal notch provided for this purpose on the outer edge of the liquid channels 11.

Here, each lateral end of a porous insert 15 in the Y direction is above a liquid channel 11. In other words, two liquid channels 11 are arranged symmetrically on either side of the vapor channel 13 inside from which the porous inserts 15 extend parallel to each other.

The adiabatic zone Z _A and the condenser Zc are free of inserts and are therefore identical to those of example 2 according to the state of the art.

The heat flow Φ _E coming from the hot source reaches the side face 21 of the enclosure 2 of the heat pipe being perpendicular to the plane of the plates 20, 22.

At the condenser Zc, the heat flow Φ _S is extracted through the side faces 23 or 24. We can also consider extraction through the side face 21 or the one opposite.

Example 6 (according to the invention) : In this example illustrated in Figure 16 , the evaporator also comprises a plurality of porous graphite inserts 15, each preferably in the form of a flat sheet.

A porous insert 15 rests inside a longitudinal notch 16 provided for this purpose in an intermediate plate 20.

The stacking of the intermediate plates 20 is carried out by alternating between two intermediate plates a porous insert 15 at the evaporator Z _E. Then, the two closing plates 22 are arranged at the ends of the stack to close the channel(s) 11, 12, 13.

The stack is made so that a porous insert 15 is sandwiched between two intermediate plates 20 or between an intermediate plate 20 and a closing plate 22 while being compressed.

In this example 6, the porous insert 15 extends across the entire width in the Y direction of both the liquid 11, connection 12 and vapor 13 channels.

Here, the central part of a porous insert 15 in the Y direction is above a single liquid channel 11 which is central and which is connected to two vapor channels 13 on either side via a connection channel 12.

The porous insert 15 allows the liquid to be brought by capillarity from the central channel 11 to the two vapor channels 13 where it evaporates on the surface.

The adiabatic zone Z _A and the condenser Zc are free of inserts.

The heat flow Φ _E coming from the hot source reaches the two lateral faces 23, 24 of the enclosure 2 of the heat pipe while being parallel to the plane of the plates 20, 22. The central liquid channels 11 are therefore distant from the faces 23, 24 where the heat flows arrive and therefore have a reduced risk of having a boil within them.

At the condenser Zc, the heat flow Φ _S can be extracted through any side face 21, 23, 24 of the enclosure 2 of the heat pipe.

The performances of Examples 1 and 2 according to the state of the art are compared with those of Examples 3 to 6 according to the invention.

The comparison criteria chosen are respectively the capillary limit, the boiling limit, the thermal resistance at the evaporator, as stated in the publication [3].

The capillary limit can be defined as the limiting power determined by the difference between the driving pressure at the liquid/vapor interface at the evaporator and the pressure losses of the liquid and vapor phases migrating inversely in the heat pipe. This limit is calculated using thermo-hydraulic models, and calculated in absolute value and relative to the heat pipe of Example 1 according to the state of the art (cylindrical heat pipe with reentrant grooves).

The boiling limit can be defined as the flow beyond which bubbles can arise in the grooves and hinder the movement of the liquid therein. This limit can be evaluated by calculating using a finite element model the maximum overheating in the grooves, and is expressed relative to the overheating in the heat pipe of example 1 according to the state of the art (cylindrical heat pipe with reentrant grooves).

Finally, the thermal resistance to the evaporator R can be evaluated by the following formula: $$\mathrm{R}=\left(\mathrm{sole}-\mathrm{steam}\right)/\mathrm{flow}$$

• tvapeur : température de la vapeur du caloduc (°C)
• flux : flux de chaleur sur la semelle (W/m²).

In which tsole: average temperature of the wall on which the heat flow is applied (°C)

• steam: heat pipe steam temperature (°C)
• flow: heat flow on the sole (W/m ² ).

We will also give this value relative to Example 1 according to the state of the art (cylindrical heat pipe with reentrant grooves).

There Figure 17 illustrates the different examples in the form of capillary limit curves as a function of the temperature of the heat pipe.

Table 1 below summarizes the gains in terms of capillary limit, boiling limit and thermal resistance at the evaporator of the different examples of heat pipes compared to the heat pipe of Example 1 according to the state of the art (heat pipe cylindrical with reentrant grooves) whose reference values are equal to 1. It should be noted that the values indicated were calculated for an adiabatic temperature of 60°C. [Table 1] Examples 1 (State of the art) 2 (State of the art) 3 (Invention) 4 (Invention) 5 (Invention) 6 (Invention) Earnings capillary limit 1 3.9 3.4 4.7 4.0 3.1 boiling limit 1 1.3 4.8 4.3 2.3 6.9 thermal resistance to the evaporator 1 1.3 2.7 2.7 2.0 3.1

From this table, it appears that whatever the example according to the invention, all the parameters measuring performance are significantly increased compared to examples 1 and 2 according to the state of the art. Example 6 according to the invention presents the best performance.

Another embodiment of the invention is illustrated in figures 18 to 20 .

In this other mode, the evaporator Z _E comprises a plurality of porous graphite inserts 15, each preferably in the form of a flat sheet.

A porous insert 15 rests inside a longitudinal notch 16 provided for this purpose in an intermediate plate 20.

The condenser Zc also comprises a plurality of porous graphite inserts 17, each preferably in the form of a flat sheet, as shown in the figure. Figure 18 . The constituent material of the porous inserts 17 may be the same as that of the porous inserts 15.

The stacking of the intermediate plates 20 is carried out by alternating between two intermediate plates a porous insert 15 at the evaporator Z _E , and a porous insert 17 at the condenser Zc. Then, the two closing plates 22 are arranged at the ends of the stack to close the channel(s) 11, 12, 13. The stack is made so that a porous insert 15 and a porous insert 17 are taken sandwiched between two intermediate plates 20 or between an intermediate plate 20 and a closing plate 22 while being compressed.

In this other mode, the porous insert 15 and the porous insert 17 extend across the entire width in the Y direction of both the liquid 11, connection 12 and vapor 13 channels.

Here, the central part of a porous insert 15 or a porous insert 17 in the Y direction is above a single liquid channel 11 which is central and which is connected to two vapor channels 13 on either side. another via a link channel 12.

The porous insert 15 allows the liquid to be brought by capillarity from the central channel 11 to the two vapor channels 13 where it evaporates on the surface.

The adiabatic zone Z _A is free of inserts.

This other mode is advantageous compared to examples 3 to 6 mentioned above because even if the capillary limit is reduced, it makes it possible to improve the thermal resistance to the condenser Zc.

There Figure 21 illustrates an advantageous variant according to which the closing plates 22 of a stack defining a heat pipe enclosure according to the invention, have a larger surface area in the XY plane, in order to use them as thermal diffusers to spread the heat flow over a wider area, at the level of the evaporator and/or condenser.

In the context of the invention, a more or less wide heat pipe can be produced depending on the evaporator, adiabatic and condenser zones, which makes it possible to optimize the mass of the heat pipe.

In the case of a flow perpendicular to the plates 20, 22, this makes it possible to increase the exchange surface at the evaporator, and/or the condenser and therefore minimize external thermal resistance.

In the case of a heat flow parallel to the plane of the plates 20, 22, the size of the liquid and/or vapor channels is increased, making it possible to increase the capillary limit of the heat pipe.

In the absence of liquid channels 11 under a porous insert 15 or 17, and therefore with a capillary return of the liquid only in said insert 15 or 17, it is preferable to increase the width of the porous insert 15 where there are the liquid channels 11, in order to limit pressure losses therein, as illustrated in Figure 22 .

There Figure 23 illustrates an advantageous variant in which the structures 14 of the evaporator and, where appropriate, the condenser comprise a plurality of pads 18 uniformly distributed in the connecting channels 12 along the direction (X). These pads 18 ensure mechanical retention of the porous insert. The presence of these pads 18 can be achieved for examples with liquid channels on the sides or central as in this Figure 23 .

The cutouts of the connecting channels 12 can be made straight, or with a wavy shape ( Figure 24 ) or sawtooth ( Figure 25 ) over all or part of the length of the channels.

This shape produced with the ZC condenser makes it possible to obtain walls not wetted by the condensation film at the tops of either the undulations or the saw teeth. This also makes it possible to increase the exchange coefficient in condensation by minimizing the interface resistance of the condensation film.

A heat pipe according to the invention can be rectilinear or arranged in different planes. It is possible to make one or more changes of direction by the shape of the plates 20, 22 and/or bends on the enclosure 2 in order to conform to the application for which the heat pipe is intended. Folds can be made in one or more planes as needed, at any folding angle.

A limitation on folding would come from a folding radius that is too small compared to the thickness of the heat pipe which could lead to the crushing of the channels during folding, and to the delamination of the plates assembled together.

It is also not possible to bend the heat pipe in the evaporator zone, due to the fragility of the porous inserts 15.

There Figure 26 illustrates such a variant with a heat pipe comprising a rectilinear part 25 integrating the condenser ZC of the heat pipe and another rectilinear part 26, bent at 90° from the rectilinear part 25, which integrates over part of its length the evaporator Z _E. The adiabatic zone therefore extends in the two rectilinear parts 25, 26 at 90° from one another.

This Figure 26 also shows the presence of fins 27 forming a radiator to the condenser ZC.

It finally shows the presence of a filling tail 28 which allows the heat pipe to be filled with the working fluid. The filling plug can be fixed to an orifice provided on the closing plates 22, and perpendicular to it.

THE figures 27A to 27H illustrate different fin shapes that can be used as a radiator, namely extruded straight fins, skived fins, pleated fins: straight with rectangular section, with triangular section, corrugated, OSF type, straight perforated, louvered .

A liquid or two-phase circuit can be integrated directly into a heat pipe as a hot and/or cold source.

There Figure 28 illustrates such an integration: a cooling circuit 30 in which a heat transfer fluid is intended to circulate is directly in contact with the condenser. In the example shown, the cooling circuit 30 is formed by an additional plate 31, in which grooves 32 are made defining the side walls of the circuit, and the closing plate 22 and an additional closing plate 22 form the walls of the circuit. end of the cooling circuit. The closure plate 22 has two orifices 29 each opening at one end of the circuit and allowing the circulation of the heat transfer fluid.

Other advantages and improvements could be made without departing from the scope of the invention.

The invention is not limited to the examples which have just been described; In particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants.

All variants can be combined or not with each other: for example, non-opening longitudinal grooves 16 can be combined with transverse connecting channels 12 over the entire height of the intermediate plates 20, for example in a staggered manner, and where appropriate with longitudinal brazing grooves at the interface between plates 20 or 20, 22.

A heat pipe is filled with a two-phase fluid, it may be a fluid well known to those skilled in the art. This is chosen for example according to the operating and storage temperature range of the device, depending on the constraints due to pressure, flammability, toxicity of the fluid and chemical compatibility between the fluid and the material. forming the heat pipe.

In addition, certain fluids are not compatible with certain materials, oxy-reduction reactions can lead to corrosive phenomena involving reaction products, for example non-condensable gases, degrading the hydrodynamic operation of the heat pipes.

By way of example, for a heat pipe according to the invention made of nickel aluminum alloy, copper or an alloy based on a combination of them, assembled by eutectic brazing, the fluid can be used as fluid. ammonia, water, acetone, methane.... The preferred filling fluid is ammonia (NH3).

Due to the limits of use, between the working fluids and the metals mentioned, the couples envisaged can be as follows: Working fluid Heat pipe metal(s) Ammonia Aluminum, steel, stainless steel, nickel Methanol Copper, stainless steel Acetone Aluminum, stainless steel Water Copper, nickel, titanium

List of cited references

• [1]: Christine Hoa: “Thermal of heat pipes with axial grooves: studies and achievements for space applications”. University of Poitiers, 2004 .
• [2]: SW Chi, "Heat Pipe Theory and Practice," McGraw-Hill, NY, USA, 1976 .
• [3]: Jocelyn BONJOUR et al. “Two-phase thermal control systems - Thermosyphons and heat pipes” Engineering technique, BE9545, 2015 .

Claims (15)

Capillary pumped heat pipe (1) with reentrant grooves, extending along a first longitudinal direction (X), comprising a sealed enclosure (2) extending between a first longitudinal end (3), intended to be heated by a hot source SC to form, within the enclosure, an evaporator and a second longitudinal end (4) intended to be cooled by a cold source SF to form, within the enclosure, a condenser, the sealed enclosure delimiting an adiabatic zone between the evaporator and the condenser, the sealed enclosure comprising a stack of plates (20, 22) in a second direction (Z), orthogonal to the first (X) direction, including two closing plates (22 ) and at least a number of n modules on top of each other with n being an integer greater than or equal to 1, each module comprising at least one intermediate plate (20) between the closing plates, the intermediate plate(s) comprising at least a first intermediate plate comprising at least one window whose edges partially delimit at least one vapor channel (13) extending along the first direction (X) between the evaporator and the condenser, in which the vapor is intended to circulate, and on at least one lateral side of the window in a third direction (Y) orthogonal to the first (X) and second direction (Z), at least one structure (14) whose edges partly delimit at least one liquid channel (11) in the evaporator and the condenser, the heat pipe comprising, at least in the evaporator, at least one exchange zone defining a liquid-vapor interface and delimited between the at least one intermediate plate and at least one other plate interlayer or a closing plate, connecting the vapor channel(s) and the liquid channel(s), each exchange zone comprising at least one opening connecting channel (12) , at least part of the connecting channels (12) emerging from the evaporator each housing an insert made of porous material (15) which extends at least partly into the liquid channel(s) and into the vapor channel(s), the porous insert being adapted to bring the liquid by capillary action from the liquid channel(s) to the vapor channel(s) in which one(s) it evaporates. Capillary pumped heat pipe (1) with reentrant grooves according to claim 1, the condenser of the heat pipe also comprising at least one exchange zone defining a liquid-vapor interface and delimited between the at least one intermediate plate and at least one other intermediate plate or a closing plate, connecting the vapor channel(s) and the liquid channel(s), each exchange zone comprising at least one connecting channel opening (12), at least part of the opening connecting channels (12) of the condenser each housing an insert of porous material (17) which extends at least partly into the liquid channel(s) and in the steam channel(s), the porous insert being adapted to condense the steam in the steam channel and bring the condensed liquid to the liquid channel by capillary action. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the adiabatic zone of the heat pipe comprising at least one exchange zone between the steam channel(s) and the channel(s). (ux) liquid(s), defining a liquid-vapor interface and delimited between the at least one intermediate plate and at least one other intermediate plate or a closing plate, each exchange zone comprising at least one connecting channel opening out ( 12) free of insert. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the porous insert of the evaporator and, where appropriate, the condenser extending over the entire width of the vapor channel in the third direction (Y). Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the porous insert having the shape of a flat sheet preferably at least part of the intermediate plates comprising, in the evaporator and where appropriate in the condenser, at least one longitudinal notch (16) extending in the first direction (X) forming a support and holding surface for the porous flat sheet. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the material constituting a porous insert (15, 17) being based on graphite, preferably not wettable by a filler metal for brazing. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the width of a porous insert being between 0.1 and 1mm, preferably of the order of 0.6mm. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the average size of the pores of a porous insert being between 1 and 100 µm, preferably of the order of 50 µm. Heat pipe (1) with capillary pumping with reentrant grooves according to one of the preceding claims, the material constituting the plates forming the sealed enclosure, being chosen from aluminum, copper, nickel, or an alloy based on at minus two of these. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims comprising n modules one on top of the other, n being an integer greater than or equal to 1, defining a single vapor channel and n liquid channels on at least one side lateral, in particular on each lateral side of the steam channel,
preferably comprising a single central vapor channel arranged between n liquid channels distributed laterally on either side of the vapor channel. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, at least one of the end plates having a surface greater than that of the intermediate plates in a direction transverse to that (Z) of the stack so to form thermal diffusers. Heat pipe (1) with capillary pumping with reentrant grooves according to one of the preceding claims, the cross section in the XY plane of the evaporator integrating the porous insert and/or that of the condenser, preferably rectangular, being greater than that of the adiabatic zone of the heat pipe. Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, the structures of the evaporator and where appropriate the condenser comprising a plurality of pads distributed, preferably uniformly, along the first direction (X) . Capillary pumped heat pipe (1) with reentrant grooves according to one of the preceding claims, comprising heat exchange means at the first end and/or second end, the heat exchange means at the second end comprising advantageously one or more fins in thermal contact with at least one of the closing plates.
the heat exchange means preferably comprising a fluidic circuit (32) in thermal contact with at least one of the closing plates (12), said circuit being formed by a plate (36) structured so as to delimit channels (38) , said channels (38) being closed by said closing plate (12) and an additional closing plate (40), the heat exchange means also comprising means for supplying heat transfer fluid to said fluidic circuit. System comprising: - a cold source (SF); - a hot spring (SC) and - at least one heat pipe (1) according to one of the preceding claims, the heat pipe being arranged so that the heat flow (φ _E ) from the hot source (SC) on the evaporator being on at least one side face ( 21) of the enclosure facing the vapor channel or on at least one side face facing the liquid channel(s), while the heat extraction from the condenser towards the cold source (SF) being on at least one side face of the enclosure facing the liquid channel(s), or on a side face perpendicular to it (these).

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