FR2783313A1 - HEAT TRANSFER DEVICE - Google Patents

Abstract

A device capable of performing the function of evaporator or of condensing in a fluid loop with capillary pumping and consisting of a tubular body (4) containing a capillary wick (19) at one end and a reservoir (18) at the other end. A tubular capillary link (20) drains the liquid from the reservoir (18) to the porous wick (19) so as to keep it permanently saturated with liquid. The capillary link (20) is preferably installed on the internal walls of the reservoir and extends to the wick. It can be made of plastic to reduce heat exchange between the tank and the body of the heat exchanger. When the device is used as an evaporator, the liquid is introduced through the tube (27) which passes through the tank (18) and the capillary link (20). The heat flux is transmitted, by conduction in the evaporator body (4), to the liquid contained in the porous wick (19) which evaporates and escapes through the grooves (10) which line the internal wall of the body d When the device operates in condensing mode, the heat flux released by the condensation of steam in the grooves is transmitted by conduction in the body of the condenser to the radiator from which it is extracted.

Description

This invention relates to a heat transfer device. It emanates

  the need for satellites in orbit around the earth to transfer thermal energy from a hot side, radiator facing the sun, to a cold side, radiator in the shade. This can be obtained by using a capillary pumped fluid loop consisting of one or more evaporators, hot side, and one or more condensers cold side connected together by conduits as described in FIG. 1 of French patent FR2723187. It should be noted that such a device must be designed

  to operate in a harmless environment.

  The evolution of the satellite in its orbit leads to a solar exposure of each of its faces variable as a function of time. (relative position in relation to the sun depends on the season). It is therefore necessary to duplicate the heat transfer systems so that they can operate in opposite directions. However, such a duplication has the consequence of duplicating the mass and consequently reducing the maximum mass of the payload by the same amount as the satellite can carry. Another problem linked to the use of two separate fluid loops relates to the duplication of the thermal interfaces necessary for the collection and dissipation of heat which, given the small space available on board a satellite, makes this concept little attractive The use of two fluid loops also requires the use of a quantity of heat transfer fluid twice as large, which taking into account the corrosive and toxic character of the liquid generally used (ammonia), represents an additional risk in the event of leakage on the satellite as well as during

  manufacturing, handling and testing operations.

  The French patent Ref. 2723187 describes a means of avoiding the need for duplication of such a device. Figure 2-6 describes a reversible capillary fluid loop. Using a simple loop, the heat is automatically transported, whatever the direction, from the hot side to the cold side. However, despite the fact that the desired operation is obtained with a single loop, the number and therefore the mass of the components of such a loop are not much less than those of a system consisting of two simple inverted loops. This is because such a system must include an evaporator and a condenser on the hot side and also on the cold side. In this case, the evaporator located on the temporarily cold side is useless and the condenser located on the temporarily side

hot is too.

  French patent Ref. 2723187 envisages the possibility that an evaporator can also function as a condenser, but such

  To the knowledge of the inventors, the system has never been tried in practice.

  Patent WO97 / 00416 describes a capillary pumped fluid loop having a ramp of evaporators connected in parallel on the hot side from which the heat is extracted. A tank, thermally isolated from the evaporators, is connected to each evaporator via a capillary link. This ensures a continuous supply of liquid to the capillary wick of the evaporator for

evaporation.

  The inventors have recognized that an unexpected advantage can be obtained with the use, in a reversible loop, of a capillary link whose function

  is similar to that of WO97 / 00416.

  The advantage is that when one of the exchangers is used (or will be used) in condensing mode, the capillary link guarantees that the wick is saturated with liquid and therefore cannot generate a parasitic capillary pumping effect opposing the nominal capillary pumping generated by the exchanger located on the hot side. To obtain this effect it is obviously necessary to have a

  tank on each of the two exchangers.

  Therefore, with this invention, a reversible capillary pumping loop is obtained used for heat transfer between two zones, comprising on each side an exchanger intended to vaporize the working fluid when its role is to extract heat and to condense this same working fluid when its role is to restore heat, each of these exchangers comprises a capillary material and one or more channels associated with the capillary material intended for collecting the vapor which escapes therefrom during vaporization when the the exchanger operates in evaporator mode and for the extraction of the liquid during condensation when the exchanger operates in condensing mode; characterized in that each exchanger comprises a reservoir in which the capillary material extends, the volume of the reservoirs and the quantity of working fluid introduced into the loop are determined so as to be sure that there is always a sufficient amount of liquid in the reservoir to maintain the

porous wick in saturation.

  By ensuring that the loop contains the right amount of working fluid, it is possible to arrange to have always enough liquid in

  the reservoir to keep the capillary wick saturated and therefore the pump primed.

  On the warm side, the steam bubbles are evacuated to the tank where they condense. There is thus little risk that the bubbles penetrate into the capillary wick. The capillary action effectively has the effect of preventing the entry of gas bubbles and, on the contrary, of promoting the absorption of the liquid. On the cold side, the saturation of the porous wick with liquid coming from the tank will ensure its

  hydrostatic passivity, ie no capillary pumping occurs.

  Each exchanger comprises a tubular envelope made of a material with high thermal conduction carrying the channels described above open on the internal wall of the envelope. This allows heat to be extracted from the steam contained in the channels when the exchanger is operating in condensing mode. The channels are preferably located in the first part of the envelope while the other part constitutes the reservoir. The heat conducting material used to make the envelope is preferably an alloy which makes it possible to obtain the grooves by extrusion and thus makes it possible to reduce the mass. The capillary wick should preferably also be cylindrical so that it can be introduced into the envelope with intimate contact at least in the

  first part as defined above.

  The capillary wick can be produced in a single part but is preferably produced in the form of a capillary wick with a fine structure (very small pore diameter) to obtain a high capillary pumping; and a separate capillary link, of coarser structure whose role is only to supply the capillary wick with the fluid coming from the reservoir. In this configuration, the link

  capillary is preferably made of plastic such as for example polyethylene.

  An advantage linked to the use of plastic is its low thermal conductivity which

  isolates the tank from the rest of the exchanger and therefore from the heat source.

  Another advantage is that plastic is lighter than other materials like stainless steel. Preferably, this hair material also covers the

  inner wall of the tank and thus isolates its contents.

  The idea of using, for this application, a porous synthetic material such as plastic, which is a thermal insulator, has an advantage in evaporator mode but also in condenser mode and therefore constitutes a second

  advantage of this invention as explained in the following paragraphs.

  In accordance with the second aspect of this invention, it is an evaporator used in a thermal transfer system of the “capillary pumped fluid loop” type consisting of a reservoir, a capillary wick intended to provide the effect of pumping, designed to transmit heat to the wick in order to evaporate the liquid it contains, and a capillary link whose role is to supply the wick with liquid coming from the reservoir whose particularity is

to be made of plastic.

  The capillary link can be machined from a rod of sintered powder, e.g. polyethylene powder, the shapes of its external surfaces are shaped so as to obtain a tight contact with the internal surfaces of the tank and the

capillary wick.

  The capillary wick is preferably bored along its length so as to provide a passage between the reservoir and the part of the envelope which

contains.

  In this configuration, an internal tube, used for the circulation of the liquid coming from the condenser, connects one end of the envelope to the other end by crossing the reservoir and the capillary link in such a way that the liquid injected by the supply line flows in the opposite direction into the above-mentioned bore before being absorbed by the capillary material. Reverse circulation pushes any bubbles back to the tank. In addition, the fact that the diameter of this tube is very small increases the speed of the fluid,

  which prevents it from being reheated and bubbles forming.

  The advantage of the internal tube mentioned above is independent of the specificities 1 and 2 of the invention described above, there is therefore a third

  specificity of this invention which is described in the following paragraphs.

  According to the third aspect of this invention, it is an evaporator used in a heat transfer system of the “capillary pumped fluid loop” type consisting of a capillary wick having an external surface from which the vaporized fluid is captured and then absorbed by capillary action up to the heart of the wick, and a reservoir whose particularity is to allow the introduction of the liquid at the end of the cavity, located far from the reservoir, in such a way that at least part of this liquid crosses the cavity going towards the

  tank before being absorbed by the wick.

  A concrete description of this invention will now be given to the

  means of examples which refer to the attached drawings: Figure I shows a reversible fluid capillary pumping loop consisting of 2 exchangers, each constructed according to the principles of this invention and designed to be installed in different places in a satellite so as to

  ensure heat transfer from the warmest side to the coldest side.

  Figure 2 is a section along line II-II of Figure 1, and Figure 3 is a detailed view, on a larger scale, of two of the channels marked with the reference 10 in Figure 2. To simplify the explanations, the elements located on the left side of line X in Figure 3 are considered to provide the evaporator function while the elements on the right side are considered to provide the condenser function. It should be understood that, in practice, all channels can be used at any time to

  evaporation or for condensation.

  The illustrated heat transfer equipment comprises 2 exchangers 1 located respectively in positions 2 and 3 of FIG. 3. The exchanger 1 being identical, only one of them will be described. It consists of a metallic body 4, the shade of which is chosen for its thermal conductivity and for its ability to be able to be extruded. It generally has a cylindrical shape with a first end carrying an orifice 5 and a second end carrying an orifice 6 machined in a threaded flange 7. The tubular casing 4 has a portion 8 of small diameter and carries a sole 9 serving as a support mechanical which also allows heat to be transferred to or from part or equipment of the satellite. The internal surface of the tube 8 is covered with a large number of grooves 10 as shown in FIG. 2. These grooves are parallel to the axis of the

body 4.

  The configuration of the grooves is better illustrated in Figure 3, they

  are designed to be obtainable either by extrusion or by electro-

  erosion. Each groove has an open side 1l 1 whose width is slightly

  less than half the width of the teeth 12 located between two grooves.

  Two divergent planes 13 coming from the open side 11 come to tangent at point P the circular bottom 14 of the groove. In this way the surfaces 13 and 14 meet without forming any sharp angle With reference to FIG. 1, an internal bore is machined in the body 4 in such a way as to form shoulders 15, 16 and 17. The shoulder 17 forms a acute angle with the axis of the body 4 and is aligned with the corresponding shoulder 17A of the external surface of the body 4. The shoulders 17 and 17A separate the cylindrical part 8, of small diameter, located at one end of the body 4 of part 18, of larger diameter, located on the opposite side. This part 18, which is wider, serves as a reservoir for the working fluid. Its volume is greater than that of the neighboring part 8 of the body 4 and the total volume of all the tanks of the

  loop reaches more than half the volume of the fluid contained in the loop.

  The body 4 contains a capillary device consisting of two separate tubular parts 19, 20. The part 19, referred to as the 'wick', is a relatively fine capillary structure (small pore diameter) so as to allow the development of '' a high capillary pumping effect. In the concrete illustration, it is made from sintered PTFE powder in the form of a rigid porous part whose pore diameter is of the order of 2.5 mm. Other materials which may be suitable are sintered metals such as, for example, copper, stainless steel fabrics, molybdenum, tungsten, titanium or nickel; as well as sintered ceramics and in general all structures with open porosity, foams and thermo-plastics (polymers) which

can be loaded with glass.

  The drill bit 19 is machined to the requested shape. It has a shoulder 21 on its external surface which comes into contact with the shoulder 15 of the internal surface of the body 4 so as to position it coaxially. The internal bore of the drill bit 19 is flared at its open end to receive a conical ferrule 22. A threaded plug 23 is screwed into a threaded part of the body 4 located between the shoulders 16 and 17 so as to exert an axial pressure on the ferrule 22, in order to ensure a seal by pressing the porous wick 19

against the inner wall of the body 4.

  The capillary link 20 is, in the example illustrated, manufactured according to the same process as that used for the wick 19 but from sintered polyethylene powder and has a pore diameter of the order of 105 mm, the porous structure is therefore coarser. It has a relatively narrow part 24 in intimate contact with the internal bore of the part 19, which can be obtained thanks to the natural elasticity of the plastic, this part extends from an open end to the shoulder. 26 which is located against the shoulder 17 of the body 4. The shoulder 26 leads to a wide part 25 of the capillary link 20 which is located against the internal wall of the wide part of the body 4. The link 20 is held in the axial position through

the cap 7.

  In alternative solutions, the capillary link 20 can be manufactured at

  from other shades of plastic that can be loaded into glass.

  The plug 7 also supports the pipe 27 which extends along the axis of the body 4 by connecting the orifice 6 to a point close to the open end of the capillary link 20. Its outside diameter is significantly less than that of the inside diameter of the narrow part of the link 20 so as to provide a passage between these two parts for the circulation of the fluid. It is kept in its axial position thanks to its twisted shape which brings it into contact with the internal surface of the link 20 while maintaining a space between this tube 27 and said internal surface. The operation of the device will now be described by making

  the assumption that the starting condition is that everything is at the same temperature.

  The amount of working fluid, which is ammonia in the system presented, is sufficient to ensure that over a wide range of temperatures there is always enough liquid in the reservoir 18 to saturate the porous wick 19 and the drain

capillary 20.

  Assuming that the exchanger located in position 1 becomes hotter than that in position 2, the heat is transmitted from the hot source, to

  through the sole 9, the teeth 12 in the form of fins between the grooves 10.

  The natural elasticity of the plastic used for the manufacture of the wick 19 keeps it in contact with the fins 12 and the heat is therefore perfectly transmitted by conduction of the fins to the liquid contained in the pores of the capillary wick 19. This causes evaporation liquid under the fins in the region 28 near the surface of the wick 19, as shown by the zone above the line 28A on the left of FIG. 3. Note that in the figures, the zones occupied

  by the liquid are represented by horizontal lines.

  The vapor is collected, as indicated by the arrows in FIG. 3, through the channels 10 and then transmitted by these same longitudinal channels to the orifice 5. The capillary pumping brings the additional liquid into the capillary wick 19, this liquid arrives partly by the capillary link 20 supplied by the reservoir 18 and partly directly by the tube 27. Because the quantity of liquid in the reservoir is constantly sufficient to saturate the porous elements, any vapor bubbles being in the narrow part 24 of link 20 cannot be absorbed there. Such bubbles will be evacuated to the cooler tank, where they will condense. The link 20 thus plays the role of a simple heat pipe between the relatively hot body 4 and the colder reservoir 18. It maintains a flow of liquid in the porous wick 19 and promotes the extraction of the bubbles of vapor conveyed to the reservoir, this last effect acts concomitantly

  with the liquid flow rate from tube 27 (described later).

  The temperature of the reservoir 18, located on the hot side, determines the saturation temperature of the working fluid and must therefore be controlled. In the illustrated device, this temperature is self-controlled by the thermal equilibrium obtained between the parasitic fluxes transferred: in the body 4; by vapor bubbles pushed back to the tank; by the sub-cooled liquid coming from the condenser; and by thermal leaks resulting from exchanges with the external environment. A notable feature is that the heat flow from the sole 9 and transmitted to the liquid contained in the reservoir is limited by the use for the link 20 of an insulating material. It also constitutes an insulating coating on the internal surface of the tank. In some applications an active temperature control system can be used. In this case, these are Peltier cells, heaters, dedicated radiative surfaces, etc. The materials and the design of this device must therefore be able to limit

parasitic heat exchanges.

  The steam leaving the orifice 5 is transported via a tube to the homologous orifice of the exchanger located in position 3 from where it enters the grooves 10. At the start of each groove, the fluid is fully vaporized but gradually condenses along the grooves to end up completely liquid at the other end. The situation near the steam inlet is illustrated on the right of Figure 3, where we can observe the formation of a very fine condensing film denoted C over the entire surface of the groove thus freeing during the phase change a large amount of thermal energy which is transmitted by conduction to the body 8. The uniformity of this film represents a considerable advantage compared to the concepts of evaporators with rectangular grooves or in dovetails whose angles are points of accumulation of liquid. In these concepts, the accumulation of liquid in the corners reduces the size of the surface through which the thermal energy passes from the liquid to the wall and increases the thickness of the liquid film which then acts as an insulator limiting the exchanges between the vapor and the

  body 4, which interferes with heat transfer.

  In the concept presented, the only corners capable of attracting the liquid are those, shown at C, formed between the metal part 8 and the capillary wick 19, in this way, the metal surface in thermal contact with the vapor is as large as possible and the fluid after condensation is removed as quickly as possible. The liquid then enters the wick which is fully saturated given its temperature, likewise part 8 of the body is also cooler than the saturation temperature of the fluid. It is therefore not possible that a capillary pumping effect can develop in the elements 19 and 20 which remain completely passive when the exchanger operates in condenser mode. The condenser is therefore from a completely passive hydraulic point of view, the liquid circulates there only under the effect of the capillary forces which have

developed in the evaporator.

  The condensed liquid is drained towards the reservoir or passes directly into the tube 27 from where it is extracted by the orifice 6 of the exchanger located at 2. The small diameter of the tube 27 ensures that the liquid passes through the reservoir, relatively hot, with a relatively high speed which prevents it from evaporating there. The risk of bubbles entering the porous elements of the evaporator is thus considerably reduced and possible bubble formation would be transported by the reverse flow of the liquid from the open end of the tube.

27 to the tank.

  When the temperature gradient between the exchangers 1 reverses, their role is automatically reversed, the element 20 quickly ensures saturation

of the capillary microstructure 19.

Claims (27)

  1. A reversible capillary pumped fluid loop used to transport heat between two sites (2,3) and comprising, at each site, a heat exchanger which evaporates a working fluid when the exchanger is used to extract heat and which condenses a working fluid when this exchanger is used to release heat, each of these exchangers comprises capillary elements (19,20) and one or more channels (10) associated with the capillary elements, intended for collecting the fluid (vapor) which escapes (from the capillary elements) when the exchanger is operating in evaporator mode and for the supply of fluid (liquid) to the capillary elements when the exchanger is operating in condenser mode. Device characterized in that each exchanger comprises a reservoir (18) in which the capillary material extends, the overall volume of the reservoirs and the volume of working fluid contained in the fluid loop are determined to guarantee that there is always enough liquid in   the reservoir to saturate the capillary elements.   2. Apparatus in accordance with claim I comprising a tubular envelope (4) in which the capillary element is installed, the channel or channels (10) defined between the envelope (4) and the capillary elements (19,20) ; and   the reservoir (18) formed by one of the ends of the envelope (4).   3. Apparatus according to claim 2 wherein the envelope   tubular has an enlarged portion (18) forming the reservoir.   4. Apparatus according to claim 2 or 3 wherein the capillary element comprises a tubular part of absorbent material whose axis   extends in the same direction as the longitudinal axis of the tubular casing.   5. Apparatus according to claim 4 wherein the capillary element comprises a wick (19) of absorbent material with small pore diameter to obtain a high capillary pumping pressure when the exchanger is used in evaporator mode and a capillary link (20) of absorbent material with a coarser porous structure whose role is to supply the wick   (19) with the liquid from the reservoir (18).   6. Apparatus according to claim 5 wherein the link   capillary (20) is a rigid preformed element.   7. Apparatus according to claim 5 or 6 wherein the wick (19) and the capillary link (20) are both tubular, the capillary link (20) having a relatively narrow portion (24) which is placed in the wick (19) and having   a relatively large portion (25) which is placed in the tank (18).   1] 8. Apparatus according to claim 5,6 or 7 comprising an annular fixing system (22,23) forming a seal between the   reservoir (19) and the channel (s) (10).   9. Apparatus according to claim 8 wherein the capillary link (20) passes through the annular attachment system (22,23).   10. Apparatus in accordance with one of claims 5 to 9   comprising a tube (27) which extends from a first orifice (6) located at one end of the tubular casing, passing through the reservoir (8) and the capillary link (20) to supply liquid to the capillary elements (19,20), to the end farthest from the tank.   11. Apparatus according to claim 10 comprising a second orifice (5) at the opposite end of the tubular casing (4) and   communicating with the channel (s) (10).   12. Apparatus according to one of claims 2 to 11 comprising a number of channels defined as being grooves   extended along the internal surface (12) of the tubular part.   13. Apparatus according to claim 12 wherein the   grooves are obtained either by extrusion or by EDM.   14. Apparatus in accordance with one of the claims   in which the quantity of fluid in the loop is sufficient to ensure that there is a reserve of liquid in the reservoir (18) throughout the range of   system operating temperatures.   15. An evaporator used in a fluid capillary pumping loop comprising a reservoir (18), a capillary wick (19) for supplying a pumping pressure, elements (8, 9) for bringing the heat to the wick and thus cause the evaporation of the liquid which it contains, and a capillary link (20) arranged so as to supply the wick (19) with the liquid from the reservoir (18) characterized in that this capillary link (20) is plastic. 16. An evaporator in accordance with claim 15 characterized by an envelope (4) having a first portion (8) containing the capillary wick (19) and serving as a means of bringing the heat to the wick and   a second portion (18) serving as a reservoir.   17. An evaporator according to claims 15 or 16   in which the capillary link (20) at least partially covers the internal wall of the reservoir (18).   18. An evaporator according to claims 15,16 or   17 in which the capillary link (20) extends inside the wick (19).   19. An evaporator in accordance with claim 18 when dependent on claim 17 in which the capillary link is machined so as to be mounted with contact in the internal bore of the wick (19) and in the tank (18).   20. An evaporator according to claims 18 or 19   in which the capillary link (20) is tubular and in which the evaporator comprises a liquid supply duct (27) which passes through the reservoir (18) and the capillary link (20) to its open end in such a way that the liquid which comes out returns to the tank.   21. An evaporator in accordance with any of   Claims 15 to 20 in which the capillary link (20) is produced in powder form   sintered. 22. An evaporator used in a capillary pumped fluid loop comprising a capillary wick (19) having an external surface on which the vaporized liquid is extracted by the capillary forces which develops therein, and a reservoir (18) characterized by element (27) whose role is to introduce liquid into a cavity remote from the reservoir (18) so that at least part of the liquid travels along this cavity while moving towards the   tank before being absorbed by the wick (19).   23. An evaporator according to claim 22 characterized in that the means for introducing the liquid comprises a tube (27) which extends from an inlet duct, passing through the cavity, to a exhaust duct.   24. An evaporator according to claim 23 characterized in that the tube (27) passes through the tank (18) before passing through the wick cavity (19).   25. An evaporator in accordance with any of   claims 22 to 24 comprising a capillary link (20) whose role is   the supply of liquid from the reservoir to an internal surface which constitutes the wick cavity (19).   26. An evaporator according to claim 25 characterized in that the capillary link (20) is a tubular part having a relatively narrow portion assembled in the cavity of the wick and a   relatively large portion assembled against the internal wall of the reservoir (18).   27. An evaporator according to claim 26 including   the means of spacing the tube (27) from the internal wall of the capillary link (20).