EP3612782B1 - Evaporator having an optimized vaporization interface - Google Patents

Description

The present invention relates to evaporators usually used in two-phase working fluid heat transfer systems. US 6,227,287 B1 discloses an evaporator with the features of the preamble of claim 1.

More specifically, we are interested in the vaporization interface at the point where liquid is transformed into vapor by absorbing a large quantity of calorific energy.

This type of evaporator is usually used to cool electronic equipment, such as a processor (CPU, GPU), a power module (IGBT, SiC, Gan etc.), or any other electronic component releasing calories, or any other source heat.

This type of evaporator is used in a system that includes a condenser and return lines to circulate the fluid between the evaporator and the condenser.

The current trend in electronics leads to having to evacuate significant thermal powers on small surfaces.

In the evaporator, provision is made at the level of the interface between the capillary wick (which brings the liquid) and the organ or the calorie reception/transfer plate (in contact with the primary hot source which brings the calories) empty spaces that form channels for the escape of steam. These steam channels are arranged either in the capillary wick or in the heat-receiving member. The most common is to provide grooves of rectangular section to form such steam channels, as for example taught by the document US5725049 [NASA ].

Some have tried to increase the heat capacity by drawing steam channels of different shapes to increase the capacity in terms of heat flow. In fact, the presence of vapor channels leads to a concentration of the density of heat flow in contact with the wick, which has led the developers to favor so-called “re-entrant” grooves, as for example in the document EP0987509 [Matra Marconi Space ].

Others have tried to minimize parasitic thermal leakage, as for example in the document US6330907 [Mitsubishi ], but the formation of vapor bubbles in the contact zone with the wick is not avoided, which threatens a good supply of liquid to the vaporization zone.

But it is noted that the known vaporization interfaces do not make it possible to treat a surface heat flux beyond 20 Watts/cm ² because the heat exchange coefficients deteriorate very strongly with the increase in the heat flux density of the made of a depression of the vaporization front inside the primary wick. The increase in the number of vapor bubbles inside the wick increases the risk of drying out, that is to say the risk of an interruption of the supply of liquid to this place, a phenomenon which it is should be avoided.

However, it turns out that the needs are now greater, which is why the inventors sought to optimize the vaporization interface of the evaporators in the two-phase working fluid heat transfer loops.

• un organe de réception (1) des calories comprenant une base (10) et une pluralité de projections (11), chaque projection s'étendant à partir de la base jusqu'à un sommet (12), et dont la taille décroît avec l'éloignement par rapport à la base, chaque projection ayant des flancs latéraux (13),
• une mèche primaire (2) en premier matériau poreux et présentant une face frontale (20) adjacente au sommet des projections, les flancs latéraux des projections délimitant avec la mèche primaire des espaces vides formant des canaux de vapeur (4), caractérisé en ce que les flancs latéraux des projections sont revêtus d'une couche fine (3) de matériau poreux, de préférence d'un second matériau différent du premier matériau.

For this purpose, the subject of the invention is a capillary evaporator for a heat transfer system, the evaporator comprising:

• a calorie receiving member (1) comprising a base (10) and a plurality of projections (11), each projection extending from the base to an apex (12), and whose size decreases with distance from the base, each projection having side flanks (13),
• a primary wick (2) made of a first porous material and having a front face (20) adjacent to the top of the projections, the lateral flanks of the projections delimiting with the primary wick of the empty spaces forming vapor channels (4), characterized in that the side flanks of the projections are coated with a thin layer (3) of porous material, preferably of a second material different from the first material.

The term " thin layer" means a layer with a thickness of less than 1 mm. The inventors have found that advantageously a low thickness associated with the projections contributes to obtaining good performance.

It should be noted that the thin layer of porous material is in contact with the primary wick at the level of a junction zone, at the location of which at the location of which liquid passes from the primary wick to through the thin layer of porous material forming a so-called secondary wick.

By the term " whose size decreases with distance from the base", it should be understood that at least one dimension of the projection (11) decreases as one moves away from the base. (10).

Advantageously, the fluid in the liquid phase is pumped by capillarity from the primary wick into the thin layer which coats the projections at the point where the vaporization takes place; the exchange surface is increased. Thanks to these provisions, an evaporation interface is obtained capable of processing a heat flux greater than 50 Watts/cm ² , with heat exchange coefficients W/(m ² K) much higher than those of the known art and , depending on the various possible configurations, the evaporation interface will even be capable of processing several tens, or even hundreds, of Watts/cm ² .

Also, we note that in the area of the top of the projections, the heat flux transferred directly to the primary wick is greatly reduced compared to the total heat flux (we mainly vaporize on the sides) and therefore we avoid creating a boiling phenomenon in the zone of contact with the primary wick, in other words, overheating of the the primary wick. Thus, the transfer of parasitic flow is limited both by very strongly limiting the penetration of the vaporization front into the primary wick and also by limiting the overheating of the receiving member while promoting the extraction of the vapor created in the channels. dedicated.

In various embodiments of the invention, one and/or the other of the following provisions may also be used:
According to one option, the thin layer can have a substantially uniform thickness. According to this configuration, a relatively simple method of manufacture and assembly can be provided by using a metallic woven weft which is intimately bonded to the surface of the receiving member.

According to one option, the thin layer may have a non-uniform thickness, the thickest part (31) of the thin layer being placed in contact with the primary wick in the vicinity of the top of each projection, and the thickness (EC) of said thin layer decreasing away from the primary wick. This configuration makes it possible to obtain better overall performance in terms of power dissipated per unit area.

According to one option, the heat-receiving member may comprise a plate, which corresponds to a planar configuration for the heat source to be cooled.

According to another option, the heat-receiving member can be generally shaped like a cylinder, which can correspond to a cylindrical configuration for the heat source to be cooled, which happens to be as common as the flat configuration. This cylindrical configuration is common when using a high pressure fluid, such as ammonia for space applications; in this case it is possible to have a flat plate, generally made of aluminium, assembled on the external surface of the cylindrical evaporator.

According to one option, the projections can advantageously be formed in the form of rectilinear ribs of section trapezoidal (even triangular); the calorie receiving member is thus easy to manufacture by extrusion or simple machining (milling). Furthermore, such a trapezoidal section allows robust transmission of the mechanical forces, in particular induced by the assembly in compression of the power modules on the evaporator by screwing (which the thin conventional fins which have a substantially constant thickness do not allow. on their height, especially with copper).

According to one option, the projections are adjacent to each other and each vapor channel (4) has a generally triangular section with one of its points directed towards the base of the receiving member. The density of the zones covered by the thin layer is thus maximized and consequently the heat exchanges too, for a given overall available surface.

According to one option, the section of the projections forms a symmetrical isosceles trapezium (in other words a "tooth"), with the short side having a length at most 20% compared to the length of the long side. In other words D3 < 0.2 W ; thus are formed steam channels of sufficient size, in particular their width between the peaks of the projections allows rapid flow of steam without excessive pressure drops.

According to one option, the short side D3 (that is to say the width of the top) has a dimension <0.3 mm. The inventors have noticed that, contrary to the prejudices of those skilled in the art, the thinness of the peaks is not problematic and is even an advantage if it is combined with the presence of the thin layer because it avoids the appearance of the vapor phase in the liquid supply zone and limits the transfer of parasitic flux through the primary wick.

According to one option, with regard to the geometry of the section of the projection, the half-angle of opening at the top α is less than 45°, and preferably comprised between 5° and 30°.

This corresponds to the fact that the height of the projections H2 is greater than 1/2 of their influence W on the base, which partly explains the increase in efficiency of the exchanges by an increase in the effective surface.

According to one option, the primary wick is preferably obtained from a material that is a poor thermal conductor, such as nickel, stainless steel, ceramic or Teflon; typically with a thermal conductivity of less than 100 W/mK. This avoids reheating the liquid located on the other side of the primary wick and greatly limits parasitic thermal leaks.

According to one option, the thin layer is obtained from a good thermal conductor, such as copper or aluminum; typically with a coefficient greater than 100 W/mK and preferably greater than 380 W/mK.

This promotes good diffusion of the heat in the thin layer and good distribution of the places of vaporization.

According to one option, the diameter of the pores of the fine layer is smaller than the diameter of the pores of the primary wick. This promotes the supply of liquid to the thin layer from the primary wick and inside the thin layer from the thickest part of said thin layer.

According to one option, the thickness EC of the thin layer is less than 0.5 mm, preferably wherever the thin layer is in contact with the calorie receiving plate 1. The inventors have found that advantageously such a low thickness was enough to get a good performance. Furthermore, it is noted that the calorie receiving plate is not flat (presence of projections 11) unlike certain embodiments of the prior art.

According to one option, the thickness H1 of the base is between 0.5 and 5 mm. This thickness is adjusted to obtain sufficient rigidity and solidity for assembly, for example by screwing, of the component to be cooled.

According to one option, the height H2 of the projections is between 0.5 and 3 mm. This height is adjusted to obtain a sufficient passage section for the steam channels to avoid potential pressure drop problems.

According to one option, the projections are formed in the form of circular ribs. This can be used in case the evaporator is in a disc shape.

According to one option, the projections are formed in the form of a conical stud or a pyramidal stud. The surface efficiency can be further improved and, depending on the manufacturing methods adopted, the cost price of the coated heat-receiving plate can remain reasonable.

According to one option, the thickness E2 of the primary wick is constant and preferably between 1 and 8 mm. Such a simple primary wick is available and inexpensive material. According to one option, the top of the projections is in contact with the primary wick over a surface less than 20% of the useful surface of the primary wick.

Furthermore, the invention also relates to a heat transfer system comprising an evaporator as described above, a condenser, fluid pipes with either gravity pumping, namely a thermosiphon configuration (including the so-called "pool boiling" configurations) either capillary pumping only or in combination with a jet, or even an evaporator powered by a mechanical pump.

• la figure 1 est une vue générale schématique d'un système de transfert thermique incluant un évaporateur selon l'invention,
• la figure 2 est une vue en coupe transversale partielle d'un évaporateur selon un premier mode de réalisation, selon un plan de coupe II-II visible à la figure 1,
• la figure 3 représente une vue schématique partielle en perspective de l'évaporateur,
• la figure 4 représente plus en détail une portion de la coupe transversale illustrant une projection et son revêtement poreux,
• la figure 5 représente un second mode de réalisation de type évaporateur cylindrique (au lieu de plan),
• la figure 6 représente la répartition du flux de vaporisation le long du flanc des projections revêtues de la couche fine de matériau poreux,
• la figure 7 représente le flux de chaleur à l'intérieur de la projection ainsi que le flux d'approvisionnement de liquide le long de la couche fine,
• la figure 8 illustre l'agencement des canaux vapeur dans une coupe horizontale selon un plan de coupe VIII-VIII visible à la figure 2,
• la figure 9 est une vue en coupe horizontale schématique d'un évaporateur à plots qui représente une autre variante de réalisation.
• la figure 10 illustre deux variantes de réalisation concernant la configuration de la couche fine de matériau poreux.

Other aspects, objects and advantages of the invention will appear on reading the following description of an embodiment of the invention, given by way of non-limiting example. The invention will also be better understood with regard to the attached drawings in which:

• the figure 1 is a schematic general view of a heat transfer system including an evaporator according to the invention,
• the picture 2 is a partial cross-sectional view of an evaporator according to a first embodiment, along a section plane II-II visible at figure 1 ,
• the picture 3 represents a partial schematic perspective view of the evaporator,
• the figure 4 shows in more detail a portion of the cross-section illustrating a projection and its porous coating,
• the figure 5 represents a second embodiment of the cylindrical evaporator type (instead of plan),
• the figure 6 represents the distribution of the vaporization flux along the side of the projections coated with the thin layer of porous material,
• the figure 7 represents the heat flux inside the projection as well as the liquid supply flux along the thin layer,
• the figure 8 illustrates the arrangement of the steam channels in a horizontal section according to a cutting plane VIII-VIII visible at figure 2 ,
• the figure 9 is a schematic horizontal sectional view of a stud evaporator which represents another alternative embodiment.
• the figure 10 illustrates two variant embodiments concerning the configuration of the thin layer of porous material.

In the various figures, the same references designate identical or similar elements. For reasons of clarity of the description, certain dimensions are not represented to scale.

The figure 1 shows a heat transfer system comprising an evaporator 7 comprising receiving member 1 which makes it possible to evacuate a flow of calories Qin received by the evaporator 7 from a dissipative component ('hot source') in the direction of a condenser COND able to receive those calories and evacuate them Qout to a 'cold source' (ambient air, lukewarm or cold water, radiant panel, etc....).

A steam pipe 8 makes it possible to convey the steam produced in the evaporator to the condenser. A liquid pipe 9 allows the liquid condensed in the condenser to be brought back to the evaporator 7. The condenser and the pipes are assumed to be known per se and will not be described here in more detail. The evaporator, the condenser and the pipes form a heat transfer loop, which works either by using gravity (thermosyphon) or by using capillary pumping, a solution which works both on land and in a zero-gravity configuration or against an acceleration field (gravity, movement of a vehicle) or pumping assisted by a mechanical pump.

In the example shown in figure 1 , there is shown a tank RES which serves as an expansion vessel for the liquid (thermal expansion of the liquid and variation in the volume of vapor outside the tank); if this tank is present as a separate element, it is called a 'CPL' loop (Capillary Pumped Loop). According to another configuration, the reservoir function is provided inside the evaporator and in this case we speak of a so-called 'LHP' loop (Loop Heat Pipe). In the case of a “thermosyphon” configuration, the presence of the tank is not necessary.

The operation of the loop in general, with in particular the vapor line, the liquid line and the condenser is known per se, it will not be detailed below. In the following, the description will focus on the evaporator and its internal structure.

The evaporator 7 comprises a calorie receiving member marked 1; in the first example illustrated, it is a plate 1 against which is leaned an element to be cooled (not shown) which provides a flow of calories labeled Qin. This plate has a special structure on the inside to the evaporator, this will be detailed later.

The evaporator 7 in question is a capillary type evaporator, that is to say it contains a wick, in other words a porous mass, which sucks in by capillarity liquid which is in a liquid compartment 5 in communication with the liquid line 9 and the expansion tank RES.

It should be noted that, if we take a broader view than that of the evaporator, the term “transfer member ” 1 could be used instead of the term “receiving member”. In the following, the term "receiving member" may also be replaced in certain cases by the term "hot plate" or "receiving plate".

Structurally, the evaporator 7 comprises the aforementioned hot plate 1 , a capillary structure which will be detailed later, the aforementioned liquid compartment 5 and a casing-cover which makes it possible to assemble the whole and to delimit a sealed interior space of the evaporator. which hermetically contains the working fluid.

More precisely, the capillary structure comprises a primary wick marked 2 completed by a capillary coating structure which forms a thin layer of porous material (reference 3) which will be discussed in more detail below.

According to a first embodiment illustrated in particular in figures 2 to 4 , the hot plate, in other words the calorie receiving member 1 , comprises a base 10 which extends along a plane YZ along 2 directions Y, Z perpendicular to the depth axis denoted X , and a plurality of projections 11 , each extending from the base 10 to a top 12 , with side flanks marked 13 .

Advantageously, the size (dimension) of each of said projections 11 decreases with distance from the base. In other words, at least one dimension of the projection 11 decreases as one moves away from the base 10. In other words, in practice, the lateral sides 13 are not not parallel to each other.

More precisely, if we consider the section of the projection in the XY plane ( Figs 2 and 4 ), it has a trapezoidal shape with a wide base of dimension denoted W and a narrow top of dimension denoted D3. The base and the top are parallel, here parallel to the Y axis, and the lateral flanks 13 of the projection extend obliquely with an angle β relative to the base.

Looking at the section, this projection 11 can also be called a "tooth".

In the example shown, there is a symmetry of the trapezoidal shape, more precisely with a symmetrical isosceles trapezium shape, with D3 < 0.2 W.

This shape can also be described as frustoconical with a half-angle of opening at the top noted α. Preferably, α <45°, or in other words β >45°, is chosen.

Preferably, it will be possible to choose for half-angle of opening at the top α comprised between 5° and 30°.

According to a particular embodiment, the small side D3 will have a size <0.3 mm.

As seen at picture 3 , the projections extend at a constant section along the direction Z. Thus, between said projections, are formed empty spaces, shaped like grooves 4 and also called in the present context "vaporization channels" 4 or "steam channels". ".

Advantageously, it is provided that the projections 11 are adjacent to each other, each neighboring projection being separated by a steam channel 4 ; we therefore note a repeating pattern along the Y axis with a pitch corresponding to the dimension W which is none other than the width of the projection 11 at its base.

The height of the vaporization channels is marked H2. In this example, the projections are formed as straight ribs with a trapezoidal section and W represents the pitch pattern taken along the Y axis.

The primary wick, marked 2, is formed as a thick layer of porous material; in the example illustrated, the thickness E2 of this layer is constant over the entire surface of the evaporator, which makes it possible to use an inexpensive standard product. One can typically choose for the thickness E2 of this primary wick a value comprised between 1 and 8 mm, preferably comprised between 2 mm and 5 mm.

The primary wick 2 has a front face 20 facing the receiving plate 1, and a rear face 25 in contact with the liquid 5. Optionally, the flat primary wick can be completed by internal walls 28 which form a rigid structure allowing to reinforce the mechanical strength of the evaporator. These internal walls can be porous or non-porous according to the possible needs of liquid distribution functions by capillarity.

It is not excluded to have a primary wick of non-constant thickness, as will be seen later.

For this primary wick 2, a rather poor thermal conductor material such as nickel, stainless steel or Teflon is preferably chosen. In general, a material having a thermal conductivity of less than 70 W/mK, preferably less than 20 W/mK, will preferably be chosen.

Advantageously according to the invention, the sides 13 of the projections are coated with a thin layer 3 of porous material.

By thin layer, it is generally necessary to understand a layer with a thickness of less than 1 mm.

By interface plane P is meant a plane parallel to YZ adjacent to the top 12 of the projections, and which in the assembled state of the evaporator, also coincides with the front face 20 of the primary wick.

It is noted that the sides 13 of the projections provided with their coating delimit with the front face 20 of the primary wick the passage section of the steam channels 4.

Going back to the thin layer 3 of porous material, according to the first embodiment, particularly illustrated in figure 4 , its thickness is not constant on the flanks 13 of the projections and preferably varies along the flanks away from the primary wick; the thickest part 31 is placed in contact with the primary wick, at the level of an interface 23 placed in the plane P in the vicinity of the top of each projection 12, and the thickness EC of said thin layer decreases as it moves away of the primary wick, to the vicinity of the bottom 41 of the groove where the end portion of the thin layer marked 32 has an almost zero thickness.

Advantageously, the thickness EC of the thin layer is everywhere less than 0.5 mm.

According to another possibility, one can choose for upper limit of thickness EC a value lower than 0,2 x W.

In a preferred theoretical configuration, starting from the interface 23 in contact with the primary wick 2, an axis L is defined along the flank 13 of the projection, the thickness EC equals EC1 at the abscissa L1 and decreases when moves along L towards the bottom 41 of the groove, where the thickness EC3 is almost zero or at the very least notably thinner than the part EC1, passing through intermediate thicknesses EC2.

It should be noted that in the various figures, the bottom of the groove 41 is considered to be “punctual”. In reality, due to machining opposites and/or to facilitate the creation of the thin layer 3, there may exist a zone not covered by the thin layer 3 whose dimension is comparable to D3.

The thin layer 3 is ideally obtained from a good thermal conductor material, relative to the material constituting the primary wick 2, such as copper, aluminum or nickel, having a thermal conductivity greater than 180 W/mK and preferably greater than 380 W/mK.

According to an advantageous aspect, the diameter of the pores of the thin layer is smaller than the diameter of the pores of the wick primary ; which makes it possible to supply the liquid from the primary wick and promote the release of vapor on the surface of the thin layer.

The base 10 of the receiving member has a thickness H1 , typically between 0.5 mm and 5 mm.

Note that the top 12 of the projections is in contact with the primary wick in a plane P on a surface (D3xZ2) less than 20% of the useful surface of the primary wick.

As seen at picture 3 , the top of the projection 12 and the primary wick are continuously in contact with one another along the direction Z2 ; in other words, there is no break in contact between the top of the projections and the underside of the primary wick.

As regards the contact surface between the primary wick and the thin layer, on each side of the section, there is a width noted D1 with $$\mathbf{<figure-callout\; id="1"\; label="D"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">D</figure-callout>}1=\mathbf{<figure-callout\; id="1"\; label="CE"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">CE</figure-callout>}1/\cos \left(\mathrm{\alpha }\right).$$

The total surface contacted between the primary wick and the coated receiving plate is therefore represented D2: $$\mathbf{<figure-callout\; id="2"\; label="D"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">D</figure-callout>}2=\mathbf{<figure-callout\; id="1"\; label="D"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">D</figure-callout>}1+\mathbf{<figure-callout\; id="3"\; label="D"\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">D</figure-callout>}3+\mathbf{<figure-callout\; id="1"\; label="D"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">D</figure-callout>}1$$

Note that D2 typically extends over 10% to 50% of the base width W. It is not excluded to go up to 80% in the case where the assembly of the primary wick on all of the teeth is made with fillets ( Fig. 10 right side). This configuration is interesting in the case of a need for high mechanical strength or increased drainage of the two-phase liquid.

Furthermore, D3 < 0.3 mm.

Furthermore, it is possible to have D3=0, or even a non-contact between the tooth and the primary wick, on condition of having a thin layer thickness 3 between the top and the primary wick 2. This configuration would make it possible to increase the thermal insulation effect of the liquid transfer zone between the primary wick and the fine layer.

The Figure 6 and 7 present the operation of the vaporization surface with progressive section (that is to say the thin layer 3 of porous material). The thickness of this projection 11 being large, its effectiveness as a fin is close to 1 and its thermal resistance is at least an order of magnitude lower than that due to vaporization through the thin layer 3. This amounts to, as a first approximation, to consider that the temperature of the trapezoidal projection-fin varies little.

The thermal resistance of the thin layer, saturated or partially saturated with liquid, is inversely proportional to its thickness which varies, for example, linearly between EC1 and EC3 ( Fig.4 ). Consequently, the flow rate vaporized locally in layer 3 follows a curve 61 like that illustrated in Figure 6 .

The local flow (expressed in W/cm ² ) is extremely high at the location of the smallest thickness EC3 , that is to say at the base of the trapezoidal tooth 11. Due to the proposed geometry, the flow density heat decreases as one approaches the contact zone 23 with the primary wick. In the illustrated example which also corresponds to the Figure 4 , at the level of the projection vertex 12 , the heat flux density is divided by 20 with respect to the parietal flux, whereas on the evaporators with straight projection or else with reentrant groove of the prior art, devoid of a thin layer 3, the heat flow is multiplied by a factor greater than 1.

A phenomenon of boiling at the interface between the top 12 of the projections and the primary wick 2 is thus avoided, or very greatly limited. at 50 Watts/cm ² on average on the external surface of the evaporator.

Advantageously, exchange coefficients heat of the order of 30,000 W/(m ² K) or higher (reference: contact surface of the receiving plate).

The inventors have been able to observe thermal powers transferred per unit area (of the receiving plate) in excess of 110 W/cm ² .

On the Picture 7 , it can be seen that the thin layer makes it possible to transfer a high flow rate of liquid, much greater than the quantity of liquid vaporized at the level of the top 12 of the tooth; the liquid transfer rate in the thin layer is illustrated on curve 62; this curve 62 represents the ratio QLid(h)/QLiq(L1).

The abscissa of the Picture 7 is the normalized height, i.e. the h/H2 ratio. H is a variable representing the height relative to the base. H2 is the total height of the projection.

The conductive flux QT(h) in the body of the tooth 11 , with respect to the normalized height, follows the curve marked 63 ; this curve 63 represents the ratio QT(h)/QT(0) or expressed QT(h)/QT(L2) if it is considered that the abscissa L2 corresponds to the base of the projection.

It is noted that the majority of the thermal power passes through the lower part of the tooth and through the thinnest portion 32 of the thin layer 3.

This proportion and the natural variations in the thickness of the thin layer 3 during manufacture as well as the presence of defects can cause these profiles to vary. The permeability and the distribution of the pores of the thin layer 3 are adapted accordingly to allow vaporization as close as possible to the base 10 in order to limit the vaporization in the primary wick. Similarly, it is possible to vary the thickness of the thin layer in a non-linear manner to improve the hydraulic and/or thermal properties. The linear variation is only an illustrative and simplified case of the present invention.

Note that the thin layer may present, either because manufacturing imperfections, either intentionally, double porosity, ie first areas with larger pores compared to other areas where the pores are smaller; in the same spirit, it is not excluded that there are discontinuities in the thin layer 3 that is to say grooves or isolated zones devoid of thin layer 3 on the lateral flank 13 of the projection 11 .

Furthermore, it should be noted that with regard to the assembly of the evaporator, the trapezoidal section proposed allows robust transmission of the mechanical forces, in particular in compression (assembly of the power modules by screwing).

According to another embodiment shown in figure 5 , the general arrangement of the evaporator is cylindrical. The base 10 is a cylinder receiving the flow Qin, however arrangements similar to those already described are applied, mutatis mutandis, for the projections 11, the grooves 4 and the thin layer 3. The primary wick 2 is presented as a tubular sleeve. The liquid compartment 5 is formed by the central zone of the cylindrical interior space. The operation at the level of the vaporization interface and the advantages conferred by the thin layer are not described in detail, they are completely similar to what has been described previously.

With reference to the figure 8 , each of the grooves or each vaporization channel 4 is fluidly connected (in vapor or liquid phase) to a collecting channel 40, itself connected to the outlet of the evaporator ( Vap_Out reference) which is connected to the external vapor pipe 8 .

According to another exemplary embodiment shown in figure 9 , according to a section plane similar to that of the figure 8 , the projections 11 are arranged in the form of a conical stud or a pyramidal stud. The steam channels 4 are then formed by the intervals between the pads. According to an advantageous option, the decreasing thickness from the top of the studs confers the advantages in terms of efficiency already described above.

According to another exemplary embodiment not shown in the figures, the projections can be formed in the form of circular ribs, in the case of an evaporator in the form of a pancake or disc.

On the figure 10 , are represented two variants, one on the left part of the figure (10-L) another on the right part of the figure (10-R).

On the right part, according to another exemplary embodiment, the thickness EC of the thin layer is almost constant. In general, in this configuration, a thickness EC of the thin layer of between 0.1 mm and 0.8 mm will be chosen. The operation and efficiency of such a configuration are quite satisfactory without however equaling those of the thin layer with decreasing thickness as described above. In a region close to the bottom of the groove (item 33), the thickness of the thin layer decreases rapidly to 0, in other words the bottom of the groove is not coated with material the base plate is bare .

In the part in contact with the primary wick, there may be provided a fillet zone 39 as illustrated by a zone shown in dotted lines, which makes it possible to increase the contact surface with the primary wick. In fact, it can be seen that the distance denoted D1' is substantially greater than the distance denoted D1.

On the left part 10L, according to another exemplary embodiment, the thickness EC of the thin layer is constant, including in the lower zone 34 and at the bottom of the groove 35. Continuing to the left, there is the portion 36 of the same thickness that covers the flank of the next tooth.

A possible solution to form such a thin layer of constant thickness ( FIG. 10 , side 'L') is to use a mesh 38 in the form of a matrix metal sheet unidirectional. The lattice is shaped on the projections including on their sides and is found in intimate contact with the receiving member 1.

For this particular assembly process, the contact with the lower zone 34 may have a cavity with a generally triangular section.

Regarding the manufacturing process, and in a non-exhaustive manner, the preparation of the primary wick 2 consists in cutting out a sheet of porous material of chosen thickness with the correct dimensions, length and width. For the receiving member 1, one starts with a copper plate (or nickel, stainless steel or aluminum) of thickness H1+H2 then proceeds to the formation of grooves and projections by removal of material , either by electro-erosion or by conventional machining or by extruding, stamping or by stamping.

Then the thin layer 3 of non-uniform thickness (first embodiment) is formed, for example, by atmospheric plasma spraying or by additive manufacturing (3D printing) or by laying a mesh as illustrated above. An assembly by diffusion makes it possible to link the two porous surfaces at the level of the contact plane P.

A contact assembly under compression is another possible option.

It should also be noted that the thin layer 3 could also cover the top 12 of the tooth before assembly of the primary bit 2.

Claims (11)

1. A capillary evaporator for a heat transfer system, the evaporator comprising:

   - a heat energy receiving member (1) comprising a base (10) and a plurality of projections (11), each projection extending from the base to a top (12), and whose size decreases with the distance from the base, each projection having lateral flanks (13),

   - a primary wick (2) made of a first porous material and having a front face (20) adjacent to the top of the projections, the lateral flanks of the projections delimiting with the primary wick empty spaces forming vapour channels (4),

   characterised in that the lateral flanks (13) of the projections are coated with a thin layer (3) of porous material, made of a second material different from the first material.
2. The evaporator according to claim 1, wherein the thin layer (3) has a substantially uniform thickness.
3. The evaporator according to claim 1, wherein the thin layer (3) has a non-uniform thickness, the thickest part (31) of the thin layer being disposed in contact with the primary wick near the top of each projection, and the thickness (EC) of said thin layer decreasing away from the primary wick.
4. The evaporator according to one of claims 1 to 3, wherein the projections are formed in the shape of rectilinear ribs of trapezoidal section.
5. The evaporator according to claim 4, wherein the projections are adjacent to each other and each vapour channel has a generally triangular section with the tip directed towards the base of the receiving member.
6. The evaporator according to claim 5, wherein the section is formed as a symmetrical isosceles trapezoid, with a base W and a short side D3 such that D3 < 0.2 W, and the short side D3 has a size < 0.3 mm.
7. The evaporator according to one of claims 4 to 6, wherein the half-angle of opening at the top α is less than 45° and is preferably comprised between 5° and 30°.
8. The evaporator according to one of claims 1 to 7, wherein the primary wick (2) is obtained from a first material that is a poor heat conductor, such as ceramic, stainless steel or Teflon.
9. The evaporator according to one of claims 1 to 8, wherein the thin layer (3) is obtained from a second material that is a good heat conductor, such as copper, aluminium or nickel.
10. The evaporator according to one of claims 1 to 9, wherein the diameter of the pores of the thin layer (3) is smaller than the diameter of the pores of the primary wick (2).
11. A heat transfer system comprising an evaporator according to one of the preceding claims, a condenser, fluid pipes with either gravity pumping, namely a thermosiphon configuration, or capillary pumping only or in combination with a jet or mechanical pumping.

Images