EP2520889B1 - Device and system for transferring heat - Google Patents

Description

The invention relates to a device and a system, particularly cryogenic, heat transfer, and a method for cooling or pre-cooling an object by means of such a device or such a system. The device and the system of the invention make it possible in particular to achieve a thermal bond of high thermal conductivity between a cold source and an object to be cooled (hot source).

• permettre le refroidissement de l'objet, initialement à température ambiante, jusqu'à sa température de fonctionnement, c'est-à-dire la température requise pour le bon fonctionnement de l'objet ; c'est la fonction « pré refroidissement » ;
• Maintenir sa température par un transfert de chaleur vers la source froide à la température de fonctionnement ; c'est la fonction « stabilisation thermique ».

Controlling the temperature of an object to cool and / or releasing heat requires the use of a thermal link connecting the object to a "cold source" which, depending on different technological constraints, may be remote. At low temperature, that is to say at a temperature below room temperature, controlling the temperature of the object requires two essential thermal functions, namely:

• allow the object to cool down, initially at ambient temperature, to its operating temperature, ie the temperature required for the proper functioning of the object; it is the function "pre-cooling";
• Maintain its temperature by transferring heat to the cold source at the operating temperature; it is the function "thermal stabilization".

In some applications, particularly in the space field, it is interesting to have also an additional function of "thermal switch", which is to interrupt the heat transfer by an external action.

A transfer of heat over a distance of up to several meters can be achieved through different devices.

The simplest of them and the most used, the metallic braid, works by simple conduction of heat in a solid with high thermal conductivity. The major disadvantage of such a device lies in the fact that it has a large mass, and all the more so that it is desired to obtain a low thermal resistance. In addition, a gradient heat between the cold source and the object to be cooled can not be avoided. In addition, the function "thermal switch" can not be ensured.

The convective devices, exploiting the circulation of a fluid moved by a pump, therefore appear much more attractive in many applications, including cryogenic. In most cases, for reasons of reliability, mass and size, it is desired to avoid the use of mechanical pumps in which mechanical elements are moving relative to each other. Pumping without moving parts is therefore preferred. In addition, two-phase systems are generally used in which heat transfer is achieved by evaporation of an object-side liquid and condensation of the cold source side vapor. This type of system works in principle, without any difference in temperature and allows a significant heat transfer by liquid / vapor phase change due to the importance of the latent heat of phase change, which is of interest. Four main types of thermal link work on this principle: the thermosiphon, the "simple" heat pipe, the fluid loop and the oscillating heat pipe. These are passive devices, that is to say that the fluid circulates without external action.

In a thermosyphon, the two-phase fluid circulates under the effect of gravitational forces induced by the density difference between the liquid and the vapor. The system may be a single channel where the descending liquid and the rising vapor circulate against the current, or a loop with a liquid channel and a steam channel. In all cases, the object must be located lower than the cold source, which is a sometimes unacceptable constraint. In addition, by its principle, the thermosiphon does not work in microgravity and is therefore not suitable for space applications.

The "simple" heat pipe and the fluid loop make it possible to overcome these constraints by exploiting the "capillary pumping" generated by the curvature of a liquid meniscus evaporating within a capillary structure.

The "simple" heat pipe is a tube containing a capillary structure, commonly called "wick", in thermal contact with the internal walls of the tube. The liquid is located only in the wick and flows from the condenser to the evaporator. The steam produced flows back to the condenser in a channel arranged in the center of the tube.

As in the case of the thermosiphon, the function "thermal switch" can not be carried out simply; however, reliable operation in micro gravity is possible. Rather used in rectilinear geometry to homogenize the temperature of an object, its integration in a complex system of thermal control can be problematic, because requiring non-rectilinear geometries. In addition, the heat pipe is not suitable for transporting heat over long distances (several meters) because of the significant head losses caused by the flow of liquid in the porous wick and the viscous interactions between the liquid and the vapor (training losses).

The fluid loop, commonly known as the Loop Heat Pipe (LHP) or CPL (Capillary Pumped Loop), overcomes the disadvantages mentioned above of the "simple" heat pipe. It operates on the basis of the same principle as the latter, but the capillary structure is located only at the evaporator to generate the capillary pumping necessary for the circulation of the fluid; in addition, the evaporator and the condenser are connected by two separate pipes for the liquid and the steam. In this way, the pressure losses are lower, the heat transfer can take place over long distances and the use of non-rectilinear geometries is less problematic. The function "thermal switch" can be achieved by simple heating localized on the liquid line that boils the liquid and causes the defusing of the loop.

A more detailed description of the operation of a fluid loop can be found in the article of Jentung Ku "Operating Characteristics of Loop Heat Pipes", 29th International Conference on Environmental Systems, 12-15 July 1999, Denver, USA est.

The use of fluid loops at cryogenic temperatures poses the problem of pre-cooling the object ("hot source") in thermal contact with the evaporator. Indeed, for the capillary pumping to begin, it is necessary that the wick is drenched with the cryogenic fluid in the liquid state. But, in a fluid loop, the wick is only in the evaporator, that is to say in the "hot" part of the device. Pre-cooling is necessary to allow wetting of the wick. Several solutions have been proposed to produce cryogenic LHP or CPL.

A first cryogenic LC concept, called the Cryogenic Capillary Pumped Loop (CCPL), is described in the article D. Bugby and B. Marland "Flight Results from the Cryogenic Capillary Pumped Loop (CCPL) Flight Experiment on STS-95" SAE Paper No. 981814, 28th International Conference on Environmental Systems, July 13-16, 1998, Danvers, USA. A est. In this device, the pre-cooling is carried out by the expulsion to the evaporator of a cryogenic liquid contained in a cold reservoir via a pre-cooled tank line in a heat exchanger ("condenser spool"). This expulsion is achieved by electric heating on the tank. The arrival of liquid towards the evaporator causes its cooling and finishes by filling it with liquid. The system is ready to boot.

• D. Khruslatev, « Cryogenic loop heat pipes as flexible thermal links for cryocoolers », Proc. 12th Cyocoolers Conference, pp. 709-716 (2003 ) ;
• Q. Mo et J.T. Liang, "A novel design and experimental study of a cryogenic loop heat pipe with high heat transfer capability" IJHMT 49, pp 770-776(2006 ) ; et
• Q. Mo, J.T. Liang et C. Jinghui, «Investigation of the effects of three key parameters on the heat transfer capability of a CLHP », Cryogenics 47 pp. 262-266 (2007 ).

Another concept is to insert in the main loop a secondary capillary pump in hydraulic connection with a secondary condenser thermally bonded to the cold source. The application of an electric heater on the secondary pump generates the circulation of the fluid and therefore the liquid supply of the main evaporator, which leads to its pre-cooling. This concept is described by the following publications:

• D. Khruslatev, "Cryogenic loop heat pipes and flexible thermal links for cryocoolers", Proc. 12th Cyocoolers Conference, pp. 709-716 (2003 );
• Q. Mo and JT Liang, "A novel design and experimental study of a cryogenic loop heat pipe with high heat transfer capability" IJHMT 49, pp 770-776 (2006) ); and
• Mo, JT Liang and C. Jinghui, "Investigation of the effects of three key parameters on the heat transfer capability of HPLC", Cryogenics 47 pp. 262-266 (2007) ).

Yet another solution is to use a secondary circuit comprising a secondary fluid line, a condenser, a diphasic reservoir and a secondary capillary pump. By simple application of an electric power on the secondary pump, this circuit allows the pre-cooling of the loop, in particular the liquid filling of the main evaporator. This solution is described in the article of J. Yun, E. Kroliczek and L. Crawford "Development of a Cryogenic Loop Heat Pipe (HPLC) for Passive Optical Bench Cooling Applications", 32nd ICES 2002, SAE Paper No. 2002-01-2507, San Antonio, Texas, 2002 , as well as in the patent US 7,004,240 .

A similar concept is that of the "advanced cryogenic fluid loop"("cryogenic advanced LHP"): TTHoang, D. Khruslatev and J. Ku, "Cryogenic advanced loop heat pipe in temperature range of 20-30K" Proc. 12th International heat pipe conference, (2002), pp. 201-205 ; US 2003/0159808 ; WO 03/054469, July 3, 2003 .

Another possibility is to use gravity to start the loop. The article of H. Pereira, F. Haug, P. Silva, J. Wu, and T. Koettig, "Cryogenic loop heat pipe for the cooling of small particle detectors at CERN", Cryogenic Engineering Conf., June 28 - July 2, 2009, Tucson , United States is, describes an LHP where the liquid line is placed above the evaporator; in this way, a "gravity pumping" allows the priming and assists the capillary pumping in normal operation. Of course, this principle is not suitable for space applications.

Another passive diphasic heat transfer device is the oscillating heat pipe, also known as the "Pulsating Heat Pipe" PHP. This device consists of a single tube, of diameter less than the length capillary, forming several loops or undulations and filled with a two-phase fluid consisting of liquid, forming "liquid plugs", and steam, forming bubbles. One end of each loop or ripple is in thermal contact with a hot source and the opposite end with a cold source. Under these conditions instability is created, causing an oscillatory movement of the liquid plugs and bubbles. This results in extremely efficient heat transfer. The capillary can be closed at both ends (PHP "open"), or else loop back on itself (PHP "closed", more efficient).

The PHP principle is described by the article of MB Shafi, A. Faghri and Y. Zhang "Analysis of heat transfer in unlooped and looped pulsating heat pipes", Int. Journ. of Numerical Methods for Heat and Fluid Flow, Vol. 12, No. 5, (2002), pp. 585 - 609

A PHP adapted to cryogenic applications is described in the article of R. Chandratilleke et al., "Development of Cryogenic Loop Heat Pipe", Cryogenics 38 (1998) pp. 263-269 . Although pre-cooling is necessary, it is not mentioned in this publication.

In general, the two-phase heat transfer devices such as those described above can be described as "passive" when they perform their thermal stabilization function; however, when they are used at cryogenic temperatures, they require means - often active - pre-cooling which considerably complicate the structure and operation and / or impose constraints sometimes unacceptable (presence of a gravitational field, relative arrangement of some components).

The document US 2003/159808 A1 , considered to represent the state of the art closest to the subject of claim 1, discloses a heat transfer device comprising a reservoir for storing a two-phase fluid, an evaporator that can be thermally connected to a heat source having a higher temperature than a cold source, a first and second condenser can be thermally connected to the cold source.

The invention aims to overcome the aforementioned disadvantages of the prior art by proposing a cryogenic heat transfer device of particularly simple structure, allowing both to pre-cool an object, independently and even against gravity, over long distances, but also to evacuate the heat released by the object once pre-cooled. This device can be described as "active" because it exploits actuators (heat sources) to ensure the circulation of a two-phase fluid; however, no moving mechanical parts are necessary (except, in some embodiments, valves). It can perform alone the functions of pre-cooling and thermal stabilization, or be used only for the pre-cooling step of an object, the thermal stabilization can then be provided by a conventional passive device.

• un premier réservoir pour stocker un fluide diphasique, équipé d'un premier moyen de chauffage et connecté à une source froide par une première résistance thermique ;
• un deuxième réservoir pour stocker ledit fluide diphasique, équipé d'un deuxième moyen de chauffage et connecté à ladite ou une autre source froide par une deuxième résistance thermique ; et
• un conduit fluidique pouvant être traversé par ledit fluide diphasique, reliant lesdits premier et deuxième réservoirs, ledit conduit comprenant au moins :
• un évaporateur pouvant être connecté thermiquement à une source chaude ayant une température supérieure à celle de ladite source froide ;
• un premier et un deuxième condenseur, situés de part et d'autre dudit évaporateur et pouvant être connectés thermiquement à ladite source froide ;
  lesdits premier et deuxième moyens de chauffage et ledit conduit fluidique étant agencés de telle manière que l'activation du premier moyen de chauffage provoque l'expulsion dudit fluide diphasique dudit premier réservoir vers ledit deuxième réservoir à travers ledit conduit fluidique, et l'activation du deuxième moyen de chauffage provoque l'expulsion dudit fluide diphasique dudit deuxième réservoir vers ledit premier réservoir à travers ledit conduit fluidique.

An object of the invention is therefore a device, especially a cryogenic device, for transferring heat, comprising:

• a first reservoir for storing a two-phase fluid, equipped with a first heating means and connected to a cold source by a first thermal resistance;
• a second reservoir for storing said two-phase fluid, equipped with a second heating means and connected to said or another cold source by a second thermal resistance; and
• a fluid duct through which said two-phase fluid flows, connecting said first and second reservoirs, said duct comprising at least:
• an evaporator that can be thermally connected to a hot source having a temperature greater than that of said cold source;
• a first and a second condenser, located on either side of said evaporator and being thermally connectable to said cold source;
  said first and second heating means and said fluid conduit being arranged such that activation of the first heating means causes expulsion of said two-phase fluid from said first reservoir to said second reservoir through said fluid conduit, and activation of the second heating means causes expulsion of said two-phase fluid from said second reservoir to said first reservoir through said fluid conduit.

• Le dispositif peut contenir un dit fluide diphasique en quantité au moins suffisante pour remplir, à l'état liquide, ledit conduit fluidique et une partie du volume de l'un desdits premier et deuxième réservoir, mais insuffisante pour remplir à l'état liquide les deux réservoirs et ledit conduit fluidique.
• Lesdits premier et deuxième réservoir peuvent présenter une contenance supérieure à celle du conduit fluidique. De préférence, ces réservoirs peuvent présenter une même contenance.
• Ledit fluide diphasique peut être un fluide cryogénique, présentant une température critique inférieure ou égale à 200K, voire à 120K. Il peut s'agir, par exemple, de l'hélium, de l'hydrogène, du néon, de l'azote ou de l'oxygène respectivement aux températures de 4,2K, 20K, 27K, 77K et 90K et dont les températures critiques sont respectivement de 5,2K, 33K, 44K, 126K et 154K.
• Le dispositif peut comprendre également au moins une dite source froide, comprenant des moyens de refroidissement adaptés pour l'amener à une température permettant l'existence d'une phase liquide dudit fluide à l'intérieur desdits réservoirs.
• Ledit conduit fluidique peut être connecté auxdits premier et deuxième réservoirs par des piquages respectifs réalisés aux extrémités inférieures de ces derniers. Ce mode de réalisation convient aux applications terrestres, en présence d'un champ gravitationnel.
• En variante, chacun dudit premier et deuxième réservoirs peut contenir un matériau poreux conducteur de la chaleur, mouillable par la phase liquide dudit fluide ; on entend par « mouillable » un matériau avec lequel ladite phase liquide forme un angle de contact inférieur à 90°. Ce mode de réalisation convient aux applications spatiales dans un environnement de microgravité.
• Ledit conduit fluidique peut être relié à un réservoir de réduction de la pression. Il s'agit là d'une caractéristique avantageusement présente dans la plupart des dispositifs fluidiques de transfert de la chaleur opérant à des températures cryogéniques.
• Le dispositif de l'invention peut comprendre également un dispositif de commande adapté pour activer alternativement le premier et le deuxième moyen de chauffage, de manière à provoquer un transfert dudit fluide diphasique dudit premier réservoir audit deuxième réservoir et vice-versa.

According to various embodiments of the invention:

• The device may contain a said two-phase fluid in an amount at least sufficient to fill, in the liquid state, said fluidic conduit and a portion of the volume of one of said first and second reservoir, but insufficient to fill in the liquid state the two reservoirs and said fluid conduit.
• Said first and second reservoir may have a capacity greater than that of the fluidic conduit. Preferably, these tanks may have the same capacity.
• Said two-phase fluid may be a cryogenic fluid, having a critical temperature less than or equal to 200K, or even 120K. It can be, for example, helium, hydrogen, neon, nitrogen or oxygen respectively at the temperatures of 4.2K, 20K, 27K, 77K and 90K and whose temperatures critics are respectively 5.2K, 33K, 44K, 126K and 154K.
• The device may also comprise at least one said cold source, comprising cooling means adapted to bring it to a temperature allowing the existence of a liquid phase of said fluid inside said tanks.
• Said fluid duct can be connected to said first and second tanks by respective taps made at the lower ends thereof. This embodiment is suitable for terrestrial applications, in the presence of a gravitational field.
• Alternatively, each of said first and second reservoirs may contain a porous heat-conducting material wettable by the liquid phase of said fluid; "Wettable" means a material with which said liquid phase forms a contact angle of less than 90 °. This embodiment is suitable for space applications in a microgravity environment.
• Said fluid duct may be connected to a pressure reduction tank. This is a characteristic advantageously present in most fluidic heat transfer devices operating at cryogenic temperatures.
• The device of the invention may also comprise a control device adapted to alternately activate the first and second heating means, so as to cause a transfer of said two-phase fluid from said first tank to said second tank and vice versa.

Another object of the invention is a heat transfer system comprising two devices as described above, thermally connected between said hot source and said cold source, or respective cold sources, wherein said control means are configured to activate the respective heating means periodically and in quadrature phase.

Another object of the invention is a heat transfer system, a device as described above and a passive two-phase heat transfer device, such as a fluidic loop heat pipe or an oscillating heat pipe, thermally connected between said hot source and said cold source, or respective cold sources. In a particular embodiment, said passive two-phase heat transfer device can be connected to said first and second tanks by a valve system enabling it to be filled with two-phase fluid.

Another object of the invention is a heat transfer system comprising a device as described above, in which an oscillatory heat pipe, thermally connected between said hot source and said cold source, is integrated in said fluid duct.

1. a. Relier thermiquement ledit objet à l'évaporateur dudit dispositif, de telle sorte qu'il fonctionne comme source chaude ;
2. b. Relier thermiquement lesdits premier et deuxième réservoirs, ainsi que lesdits premier et deuxième condenseurs, à ladite source froide, de manière à provoquer un remplissage au moins partiel d'au moins ledit premier réservoir par une phase liquide dudit fluide diphasique ;
3. c. Activer ledit premier moyen de chauffage, de telle sorte que ladite phase liquide dudit fluide diphasique s'écoule vers ledit deuxième réservoir à travers ledit évaporateur, où elle s'évapore au moins partiellement en refroidissant ledit objet, et ledit deuxième condenseur, où la vapeur ainsi formée retourne à l'état diphasique ;
4. d. Lorsque le premier réservoir est sensiblement vide de ladite phase liquide, éteindre ledit premier moyen de chauffage ;
5. e. Activer ledit deuxième moyen de chauffage, de telle sorte que ladite phase liquide dudit fluide diphasique s'écoule vers ledit premier réservoir à travers ledit évaporateur, où elle s'évapore au moins partiellement en refroidissant ledit objet, et ledit premier condenseur, où la vapeur ainsi formée retourne à l'état diphasique ; et
6. f. Lorsque le deuxième réservoir est sensiblement vide de ladite phase liquide, éteindre ledit deuxième moyen de chauffage ;
   les étapes c. à f. étant répétées de manière cyclique.

Another subject of the invention is a method of cooling or pre-cooling an object, in particular at a cryogenic temperature, by means of a device as described above, comprising the following steps:

1. at. Thermally connecting said object to the evaporator of said device, so that it functions as a hot source;
2. b. Thermally connecting said first and second reservoirs, as well as said first and second condensers, to said source cold, so as to cause at least a partial filling of at least said first reservoir by a liquid phase of said two-phase fluid;
3. vs. Activating said first heating means, such that said liquid phase of said two-phase fluid flows to said second tank through said evaporator, where it evaporates at least partially by cooling said object, and said second condenser, where the steam thus formed returns to the two-phase state;
4. d. When the first reservoir is substantially empty of said liquid phase, turn off said first heating means;
5. e. Activating said second heating means, such that said liquid phase of said two-phase fluid flows to said first tank through said evaporator, where it evaporates at least partially by cooling said object, and said first condenser, where the steam thus formed returns to the two-phase state; and
6. f. When the second tank is substantially empty of said liquid phase, turn off said second heating means;
   steps c. to f. being repeated cyclically.

Yet another object of the invention is a method of cooling an object comprising a pre-cooling step as described above, followed by a thermal stabilization step by means of a two-phase passive transfer device. the heat.

• Les figures 1A, 1B et 1C, des schémas fonctionnels de trois dispositifs de transfert de la chaleur selon trois variantes d'un mode de réalisation de l'invention ;
• Les figures 2A et 2B, deux vues en coupe d'un réservoir pour fluide cryogénique adapté pour être utilisé en conditions de microgravité ;
• La figure 3, le schéma fonctionnel d'un premier mode de réalisation d'un système cryogénique de transfert de la chaleur selon l'invention, mettant en oeuvre deux dispositifs du type représenté sur la figure 1A, connectés en parallèle entre une source chaude et une source froide ;
• La figure 4, le schéma fonctionnel d'un deuxième mode de réalisation d'un système cryogénique de transfert de la chaleur selon l'invention, mettant en oeuvre un dispositif du type représenté sur la figure 1A et une boucle fluide, connectés en parallèle entre une source chaude et une source froide ;
• La figure 5, le schéma fonctionnel d'un troisième mode de réalisation d'un système cryogénique de transfert de la chaleur selon l'invention, mettant en oeuvre un dispositif du type représenté sur la figure 1A et un caloduc oscillant fermé, connectés en parallèle entre une source chaude et une source froide ;
• La figure 6, le schéma fonctionnel d'un quatrième mode de réalisation d'un système cryogénique de transfert de la chaleur selon l'invention, mettant en oeuvre un dispositif du type représenté sur la figure 1A auquel est intégré un caloduc oscillant ouvert ;
• Les figures 7A - 7C, des schémas illustrant la structure et le fonctionnement d'un cinquième mode de réalisation d'un système cryogénique de transfert de la chaleur selon l'invention, mettant en oeuvre un dispositif du type représenté sur la figure 1A et un caloduc oscillant ouvert, connectés en parallèle entre une source chaude et une source froide ; et
• Les figures 8A et 8B, des résultats expérimentaux illustrant le fonctionnement d'un dispositif du type illustré sur la figure 1A.

Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the accompanying drawings given by way of example and which represent, respectively:

• The Figures 1A, 1B and 1 C functional diagrams of three heat transfer devices according to three variants of an embodiment of the invention;
• The Figures 2A and 2B two sectional views of a cryogenic fluid reservoir adapted for use in microgravity conditions;
• The figure 3 , the block diagram of a first embodiment of a cryogenic heat transfer system according to the invention, implementing two devices of the type shown in the Figure 1A connected in parallel between a hot source and a cold source;
• The figure 4 , the block diagram of a second embodiment of a cryogenic heat transfer system according to the invention, implementing a device of the type shown in FIG. Figure 1A and a fluid loop, connected in parallel between a hot source and a cold source;
• The figure 5 , the block diagram of a third embodiment of a cryogenic heat transfer system according to the invention, implementing a device of the type shown in FIG. Figure 1A and a closed oscillating heat pipe, connected in parallel between a hot source and a cold source;
• The figure 6 , the block diagram of a fourth embodiment of a cryogenic heat transfer system according to the invention, implementing a device of the type shown in FIG. Figure 1A which incorporates an open oscillating heat pipe;
• The Figures 7A - 7C , diagrams illustrating the structure and operation of a fifth embodiment of a cryogenic heat transfer system according to the invention, implementing a device of the type shown in FIG. Figure 1A and an open oscillating heat pipe, connected in parallel between a hot source and a cold source; and
• The Figures 8A and 8B , experimental results illustrating the operation of a device of the type illustrated on the Figure 1A .

As shown in Figure 1A , a heat transfer device according to the invention essentially comprises two tanks R1 and R2, preferably having the same capacity, connected to each other via a fluid duct CF. This set contains a fluid in the two-phase state, liquid and vapor. It consists, in the direction R1 to R2, of a first condenser C1 (dark gray), a first section of the fluidic duct, an evaporator EV, a second section of the fluid duct (light gray), and a condenser C2.

The two-phase fluid LC has a critical temperature lower than the operating temperature of the object O to be cooled, and is partly in the liquid state at the temperature of the cold source. Depending on the application in question it may be for example water, ammonia, or a cryogenic fluid such as nitrogen, oxygen, hydrogen, neon or liquid helium. The device of the invention is particularly suitable for cryogenic applications (object temperatures of less than or equal to 200K or even 120K), or more generally to applications in which the object to be cooled must be brought to an operating temperature of less than the ambient temperature (conventionally, 20 ° C). The quantity of two-phase fluid contained in the device must be sufficient to fill, in the liquid state, the fluidic conduit and at least a portion (typically 50% or 75%) of the internal volume of one of the reservoirs. At the same time, the device must not be completely filled with liquid, because in this case no circulation of the two-phase fluid could occur.

The first condenser C1, which is in the immediate vicinity of the first tank R1, is in thermal contact with a cold source SF, having means (for example a bath of cryogenic fluid or a cryo-refrigerator) capable of bringing its temperature T _F at a value less than or equal to the saturation temperature of the two-phase fluid. Thus, this condenser C1 is filled with two-phase fluid in the liquid state.

The evaporator EV, located in the central portion of the fluidic conduit CF, is in thermal contact with a "hot plane" PC, which is a good thermal conductor element thermally connected to an object O to be cooled. The hot PC plan serves as a "hot spring"; its temperature T _C is greater than or equal to the saturation temperature of the two-phase fluid. Thus, the evaporator EV contains fluid in the liquid state, then diphasic, then - possibly - entirely in the vapor state.

The first tank R1 is connected to the cold source via a first thermal resistor RTH1; similarly, the second tank R2 is connected to the cold source via a second thermal resistance RTH2. The values of these resistors constitute important parameters for the dimensioning of the device of the invention, as will be discussed later. In addition, the two tanks are equipped with respective heating means, MC1, MC2, for example electrical resistors. A DC control device (computer, microprocessor card, etc.) transmits signals SMC1, SMC2 for controlling the two heating means MC1 and MC2.

A "hot" pressure reduction tank, RRP, is connected to the fluidic conduit CF. This is a conventional feature of cryogenic systems, designed to prevent excessive pressure build-up when the system is at ambient temperature. In other embodiments, the RRP reservoir may be absent: in this case, the device is under pressure at room temperature, in the supercritical domain; its cooling is then long, because the fluid must cool first by conduction in the gas, before condensing in the cold parts of the system.

To describe the operation of the device of the Figure 1A the initial situation is considered in which the device, excluding the evaporator EV, is "cold" at the temperature T _F. The two tanks R1 and R2 are partially filled with liquid with a sky of vapor above. Both condensers C1 and C2 are completely full of liquid, while the rest of the duct, including the evaporator EV, is filled with steam. At the beginning, the two heating means MC1, MC2 are extinguished and the system is thermalized: the tanks R1, R2 and the condensers C1, C2 are at the temperature of the cold source, T _F , while the evaporator EV is located at the temperature of the hot plane, T _C.

At time t = t ₀ the first heating means MC1 is activated to inject heat into the first tank R1. This causes the evaporation of a small portion of the liquid contained therein, and therefore an increase in pressure which causes the expulsion of another large portion of liquid in the conduit CF to the second reservoir. The liquid cools in the condenser C1. Under the effect of the overpressure induced by heating, the liquid then goes to the evaporator EV where it heats up, then evaporates partially or totally. In the latter case, the steam is superheated, that is to say that its temperature is higher than the saturation vapor temperature T _SAT at the pressure prevailing in the fluid duct, at the outlet of EV. In doing so, the fluid extracts heat from the hot plane PC and object O. The vapor (or the two-phase liquid / vapor) leaving the evaporator continues to flow to the second tank R2. Before doing so, however, it passes through the second condenser C2, where it gives heat to the cold source by condensing. At the output of C2, the fluid is two-phase. The vapor phase component of this fluid, entering the tank R2, condenses thanks to the cold power passing through the thermal resistance RTH2. The incoming liquid thus fills the tank R2.

Over time, the first reservoir R1 is empty of the liquid component of the two-phase fluid. Since the outlet tapping of the tank R1 is located in the lower part, when the liquid level passes below this tapping, the tank is almost empty. Pure steam comes out of R1, which is depressurized; therefore, its temperature drops. This drop in temperature is detected and is the signal that triggers the extinction of MC1 and the ignition of MC2. From this moment, the reservoir R2, which has partially filled with liquid, becomes the reservoir "source", while R1 becomes the reservoir "recuperator". The flow rate of fluid in CF reverses. R2 empties under the effect of MC2. The cycle ends when R2 is almost empty. Then a new cycle can begin again to be repeated as much as necessary.

In a variant, the signal triggering the extinction of MC1 and the ignition of MC2 could be the increase of the temperature of the tank R1 which occurs after the latter has emptied completely of liquid.

• Une partie Q_{RS_F} de la puissance chauffante Q_RS injectée dans le réservoir source (R1 ou R2, en fonction de la phase de fonctionnement) est perdue à travers la résistance thermique RTH1, RTH2. La différence Q_RS - Q_{RS_F} sert à générer le débit de fluide ṁ dans le conduit fluidique, et à générer par évaporation la vapeur remplaçant le liquide qui sort du réservoir source. Maximiser les résistances thermiques RTH1 et RTH2 permet de limiter la puissance perdue, et donc d'améliorer l'efficacité énergétique du dispositif, mais augmente le temps de mise en froid, c'est-à-dire le temps nécessaire pour atteindre les conditions initiales décrites plus haut. La puissance perdue ${\dot{Q}}_{R1\_F},{\dot{Q}}_{R2\_F}$
  
  à travers les résistances thermiques RTH1 et RTH2 vaut ${\dot{Q}}_{R1/2\_F}=\frac{T_{\mathit{SAT}}-T_F}{R_{\mathit{TH}1/2}}\mathrm{.}$
  
• La puissance échangée dans l'évaporateur EV vaut : $$Q_{\mathit{EV}}=\dot{m}\left[c_{\mathit{PL}}\left(T_{\mathit{SAT}}-T_F\right)+h_{\mathit{LV}}+c_{\mathit{PV}}\left(T_{\mathit{VC}}-T_{\mathit{SAT}}\right)\right]$$
  
  
  où ṁ est le débit dans le circuit fluidique, c_PL et c_PV, respectivement, la chaleur spécifique de la phase liquide et de la phase vapeur à pression constante, h_Lv la chaleur latente d'évaporation, T_VC la température du fluide dans le conduit du coté du réservoir récupérateur.
• Le débit ṁ dans le circuit fluidique vaut : $\dot{m}=\frac{{\dot{Q}}_{\mathit{RS}}-{\dot{Q}}_{\mathit{R}1/2\_F}}{h_{\mathit{LV}}}\left(\frac{{\rho }_l}{{\rho }_v}-1\right)$
  
  où ρ _l et ρ _v sont, respectivement, les densités
  de la phase liquide et de la phase vapeur à saturation à la température des réservoirs, qui sont supposées égales. Ce débit est essentiellement imposé par la valeur de puissance échangée Q_EV requise par l'application considérée.
• Le débit ṁ dans CF étant donné, le fluide sortant de C_1/2 permet le refroidissement de l'objet. Le fluide sort de l'évaporateur EV à température T_VC inférieure, mais proche de T_O. Le fluide, s'il est à l'état vapeur, se refroidit sur une très faible surface et se condense sur la quasi-totalité de la surface froide qui est à T_F, La quasi-totalité de la puissance est échangée à travers cette surface. En première approximation Q_EV ≈ H_CONDS _C2/1(T_SAT―T_F ) où H_COND et S _C2/1 représentent respectivement le coefficient de transfert de chaleur en condensation et la surface d'échange S _C2/1 du condenseur récupérateur C_2/1. Cette surface est donc fondamentale dans le dimensionnement. Elle fixe la température de saturation, c'est-à-dire la température des réservoirs donc leur pression.

The main criteria for sizing a device of the type shown on the Figure 1A are the following :

• Q _{RS_F} part of the heating power Q _RS injected into the source tank (R1 or R2, depending on the phase of operation) is lost through the thermal resistance RTH1, RTH2. The difference Q _RS - Q _{RS_F is} used to generate the flow of fluid ṁ in the fluid duct, and to generate by evaporation the vapor replacing the liquid coming out of the source reservoir. Maximizing the thermal resistances RTH1 and RTH2 makes it possible to limit the power lost, and thus to improve the energy efficiency of the device, but increases the cooling time, ie the time necessary to reach the initial conditions. described above. The lost power ${\dot{Q}}_{R1\_F},{\dot{Q}}_{R2\_F}$
  
  through the thermal resistors RTH1 and RTH2 is worth ${\dot{Q}}_{R1/2\_F}=\frac{T_{\mathit{SAT}}-T_F}{R_{\mathit{TH}1/2}}\mathrm{.}$
  
• The power exchanged in the evaporator EV is: $$Q_{\mathit{EV}}=\dot{m}\left[{\mathrm{vs}}_{\mathit{PL}}\left(T_{\mathit{SAT}}-T_F\right)+h_{\mathit{LV}}+{\mathrm{vs}}_{\mathit{PV}}\left(T_{\mathit{VC}}-T_{\mathit{SAT}}\right)\right]$$
  
  
  where ṁ is the flow rate in the fluidic circuit, c _PL and c _PV , respectively, the specific heat of the liquid phase and the vapor phase at constant pressure, h _Lv the latent heat of evaporation, T _VC the temperature of the fluid in the duct on the side of the recovery tank.
• The flow ṁ in the fluidic circuit is: $\dot{m}=\frac{{\dot{Q}}_{\mathit{RS}}-{\dot{Q}}_{\mathit{R}1/2\_F}}{h_{\mathit{LV}}}\left(\frac{{\rho }_l}{{\rho }_v}-1\right)$
  
  where ρ _l and ρ _v are, respectively, the densities
  the liquid phase and the saturation vapor phase at the reservoir temperature, which are assumed to be equal. This flow rate is essentially imposed by the exchanged power value Q _EV required by the application in question.
• The flow ṁ in CF being given, the fluid leaving C _1/2 allows the cooling of the object. The fluid leaves the evaporator EV at lower temperature T _VC , but close to T _O. The fluid, if it is in the vapor state, cools on a very small surface and condenses on almost the entire cold surface which is at T _F. Almost all the power is exchanged through this area. As a first approximation Q _EV ≈ H _COND S _{C 2/1} ( T _SAT -T _F ) where H _COND and S _{C 2/1} represent respectively the heat transfer coefficient in condensation and the exchange surface S _{C 2/1} of the recovery condenser C _2/1 . This surface is therefore fundamental in sizing. It sets the saturation temperature, that is to say the temperature of the tanks and their pressure.

The Figures 1B and 1 C relate to variants of the device of the Figure 1A . In the case of Figure 1A , the two condensers are integrated in the same room, interposed between the tanks and the cold source. In the case of Figure 1B , the condensers are independent of each other, but always interposed between the tanks and the cold source. In the case of figure 1C , the two condensers are independent of each other and tanks.

The Figures 8A and 8B show experimental results illustrating the operation of a device of the type of the figure 1C , using helium as a two-phase fluid and a cold source consisting of a cryogenic bath of helium (T _F = 4.3K approximately). The figure 8A shows the evolution of the temperature T _C of the full hot, which goes from 70K to 4.3K in less than one hour, for a thermal mass of 400J. The Figure 8B shows the fluctuations of the temperatures T _R1 , T _R2 of the reservoirs and those, much less important, of the temperature T _F of the cold source.

The example of Figure 1A refers to the case of a device operating in a gravitational field. Under these conditions, the heating means MC1 and MC2 are preferably located in the upper part of each tank, while the connections connecting the pipe CF to the tanks are in the lower part thereof. This arrangement makes it possible to ensure that the increase in pressure in the reservoir causes an injection of liquid, and no steam, into the conduit CF.

In the absence of gravity (space applications) there is the problem of locating the liquid / vapor interface, which is necessary to ensure that only liquid is injected into the fluidic conduit CF. The illustrated solution on Figures 2A and 2B is to use a porous material MP, wettable by the cryogenic liquid so as to be gorged by it, completely (almost) filling each R reservoir. In the example of Figures 2A / 2B the heating means MC is located in the center of the tank, in contact with the porous material. When this heating means is activated, a temperature gradient is created in the porous material, with temperatures higher than the saturated vapor temperature (ie at the boiling or liquefying temperature of the fluid). ) in the center and lower on the periphery. This requires that the steam is in the center, close to the heating, and the liquid in the peripheral part of the tank. The increase of the pressure in the vapor, due to evaporation in a closed volume, forces the liquid to escape via RP peripheral grooves provided for this purpose. The use of a conductive porous material (metal, for example) is preferable for the heat flow to go directly to the liquid / vapor interface, instead of generating a large temperature gradient that would be useless.

Of course, other geometries are possible; for example, the heating means may be disposed at one end of the reservoir and tapping of the CF conduit at the opposite end.

The device of the Figure 1A can be used alone as a thermal link allowing the pre-cooling of the object O, from an arbitrarily high temperature towards the temperature of the cold source, as well as its maintenance at low temperature (thermal stabilization). Thanks to the active nature of the device, the thermal switch function is performed very simply: simply do not activate the tank heating means.

The device can also be a component of a more complex cryogenic heat transfer system.

A first example of such a system is illustrated on the figure 3 . This system consists of two devices according to the Figure 1A , identified by the references Da, Db. The different components of these devices are identified by the letters "a" and "b", thus, for example, "R1a" is the first reservoir of the device "a" and so on. The control devices Dca, DCb emit signals sMC1a / sMC2a, sMC1b / sMC2b of the four heating means MC1a / MC2a, MC1b / MC2b which are in "quadrature phase", that is to say shifted temporally by a quarter (or three quarters, which is equivalent) of the duration of a complete cycle. Ideally, the two devices are identical, and have equal cycle times.

If the power Q _EV generated by the object O is constant over time, heaters management must be such that the sum of the flow rates ṁ _a + M _b may be, it also constant. The temperature of the object will then be stable. On the other hand, if the power Q _EV is not stable in time, it is necessary to vary the sum of the flow rates ṁ _a + ṁ _b by means of a suitable regulation.

The two control devices Dca, DCb can be made in the form of a single device.

In systems according to other embodiments of the invention, the device of the Figure 1A is used to perform the pre-cooling of the object O and hot plane PC, the thermal stabilization function being provided by a passive device of conventional type connected in parallel between the cold source SF and said hot plane.

The figure 4 shows such a system, in which the thermal stabilization function is provided by a fluid loop LHP comprising an evaporator EVc, in thermal contact with the hot plane PC and containing the capillary wick M, a compensation chamber CC arranged upstream of said evaporator, a fluid duct CFc connected to a hot pressure reduction chamber PRPc. In the system of figure 4 , the device of the invention is used to pre-cool the hot plane to a temperature allowing the presence of liquid in the compensation chamber and in the evaporator of the fluid loop LHP. Once initiated, it takes over.

In the embodiment of the figure 5 , the thermal stabilization function after pre-cooling is provided by a closed-type heat pump, PHPF.

In the embodiments of Figures 3 to 5 each device is equipped with its own pressure reduction tank RRP, RRPa, RRPb, RRPc, RRPd. Indeed, depending on the operating regime of the system, these tanks can be at different temperatures. The use of a common "hot" tank would require the use of a complex system of valves.

• premièrement, le conduit fluidique CF - ou au moins sa partie centrale, formant le caloduc oscillant - doit être de type capillaire (diamètre inférieur à quelques fois la longueur capillaire du liquide) et très long, ce qui augmente les pertes de charges. Il s'ensuit que la température du réservoir source T_RS doit être plus importante que dans le cas d'un dispositif « simple » tel que celui de la figure 1A, avec l'augmentation du flux thermique de fuite qui en résulte ;
• deuxièmement, le caloduc oscillant doit être de type ouvert, moins efficace que le PHP fermé de la figure 5.
• troisièmement, il n'est pas assuré que la fraction volumique de la phase liquide sera proche de la valeur optimale de 50% pour un bon fonctionnement du PHP
• quatrièmement, le grand nombre d'aller retour du conduit fluidique du PHPO entre la source froide et la source chaude impose de prévoir un grand volume de réservoir.

In the system illustrated by the figure 6 , an open oscillating heat pipe PHPO is "integrated" with the fluidic conduit CF of a device according to the invention. The evaporator EV is thus split into several hot regions of the oscillating heat pipe, alternating with cold regions of the latter. The system functions as explained above, with reference to the Figure 1A during the pre-cooling phase; then the two heating means MC1, MC2 are deactivated and the PHP, passive, takes over for the stabilization phase. This concept is interesting because it is more compact than the one of the figure 5 (In particular, only one pressure reduction tank is necessary), and because the filling of the PHP can be done directly from the tanks R1 and R2. However, it also presents some constraints:

• firstly, the fluid duct CF - or at least its central part, forming the oscillating heat pipe - must be of capillary type (diameter less than a few times the capillary length of the liquid) and very long, which increases the losses of charges. It follows that the temperature of the source tank T _RS must be greater than in the case of a "simple" device such as that of the Figure 1A , with the increase of the resulting thermal leakage flow;
• secondly, the oscillating heat pipe must be of open type, less efficient than the closed PHP of the figure 5 .
• thirdly, there is no guarantee that the volume fraction of the liquid phase will be close to the optimal value of 50% for the proper functioning of PHP
• fourthly, the large number of round-trips of the PHPO fluid duct between the cold source and the hot source makes it necessary to provide a large reservoir volume.

These disadvantages can be avoided, at least in part, by the system of Figures 7A - 7C , wherein the device of the invention is an integral part of a closed oscillating heat pipe. The counterpart is the use of two three-way valves V3V1, V3V2, and thus mechanical elements with moving parts.

The system of Figures 7A - 7C comprises a device D of the type illustrated on the Figure 1A and an oscillating heat pipe PHPF 'connected in parallel between the cold source and the hot plane. The two ends of the oscillating heat pipe are connected to the first and second condensers of the device via the three-way valves V3V1, V3V2; in this way, the heat pipe is looped on itself via the fluidic conduit CF.

Initially ( Figure 7A ) the valves are in a first position isolating the oscillating heat pipe, which is filled with steam. The device D operates in the manner described above to pre-cool the hot plane PC and the object O.

Once the pre-cooling is complete, the valves move to a second position, in which they connect the oscillating heat pipe to the two reservoirs R1, R2 of the device D ( Figure 7B ). Thus, the activation of the heating means of the source reservoir (R1, in this case) causes the expulsion of liquid from the latter and the filling of the oscillating heat pipe.

Finally ( Figure 7C ) the valves pass into a third position in which the fluidic conduit CF of the device D is connected to the oscillating heat pipe to form an additional loop or wave of the latter. The heating means are inactivated and the system operates passively, as a conventional oscillatory heat pipe.

In the case of Figures 7A-7C , the fluidic conduit CF is capillary, as shown by the alternation of liquid plugs and bubbles visible on the Figure 7C ; however, its length is much less than that of the duct of the figure 6 (which alone forms an oscillating heat pipe), therefore the pressure losses are lower.

In another embodiment, not shown, the device of the invention could be used for pre-cooling and filling a CPL or LHP type fluid loop.

So far, we have always considered the case where there is only one cold source and one hot plane / object to be cooled. Of course, this is not an essential limitation: it is quite possible to use, for example, a separate heat sink for each device or tank, although this complicates the control of the heating means.

It is also possible to envisage more complex systems, comprising one or more devices according to the invention cooperating with one another (as in the case of the figure 3 ) and / or with one or more heat transfer devices of different types (as in the case of figure 4 and 5 ). We can also design more complex devices than the one of the figure 1 , comprising more than two tanks and a plurality of fluid conduits, condensers and evaporators.

Claims (16)

1. A heat transfer device including:

   - a first reservoir (R1) for storing a diphasic fluid (LC), equipped with first heating means (MC1) and connected to a cold source (SF) via a first thermal resistance (RTH1);

   - a second reservoir (R2) for storing said diphasic fluid, equipped with second heating means (MC2) and connected to said cold source or to another cold source via a second thermal resistance (RTH2); and

   - a fluidic pipe (CF) able to be traversed by said diphasic fluid, connecting said first and second reservoirs, said pipe including at least:

   - an evaporator (EV) able to be thermally connected to a hot source (PC, O) at a temperature higher than that of said cold source;

   - a first condenser (C1) and a second condenser (C2) situated on either side of said evaporator and able to be thermally connected to said cold source;

   said first and second heating means and said fluidic pipe being arranged in such a manner that activation of the first heating means causes expulsion of said diphasic fluid from said first reservoir toward said second reservoir via said fluidic pipe and activation of the second heating means causes expulsion of said diphasic fluid from said second reservoir toward said first reservoir via said fluidic pipe.
2. The heat transfer device claimed in claim 1, containing a diphasic fluid in an amount at least sufficient, in the liquid state, to fill said fluidic pipe and part of the volume of one of said first and second reservoirs, but insufficient, in the liquid state, to fill both reservoirs and said fluidic pipe.
3. The heat transfer device claimed in either of the preceding claims, wherein said and second reservoirs have a capacity greater than that of the fluidic pipe.
4. The heat transfer device according to any one of the preceding claims, wherein said first and second reservoirs have the same capacity.
5. The heat transfer device claimed in any one of the preceding claims, wherein said diphasic fluid is a cryogenic fluid having a critical temperature less than or equal to 200K.
6. The heat transfer device claimed in any one of the preceding claims further including at least one cold source (SF) including cooling means adapted to bring it to a temperature enabling the existence of a liquid phase of said fluid inside said reservoirs.
7. The heat transfer device claimed in any one the preceding claims wherein said fluid pipe is connected to said first and second reservoirs via respective bleeds produced at the lower ends thereof.
8. The heat transfer device claimed in any one of claims 1 to 6 wherein each of said first and second reservoirs contains a thermally conductive porous material (MP), wettable by the liquid phase of said diphasic fluid, in thermal contact with said heating means.
9. The heat transfer device claimed in any one of the preceding claims wherein said fluidic pipe is connected to a pressure reduction reservoir (RRP).
10. The heat transfer device claimed in any one of the preceding claims further including a control device (DC) adapted to activate alternately the first heating means and the second heating means in such a manner as to cause a transfer of said diphasic fluid from said first reservoir to said second reservoir and vice versa.
11. The heat transfer system claimed in claim 10 including two devices (Da, Db) thermally connected between said hot source and said cold source, or respective cold sources, wherein said control devices (Dca, DCb) are configured to activate the respective heating means periodically and in phase quadrature.
12. The heat transfer system claimed in any one of claims 1 to 10 including a device (D) and a heat transfer passive diphasic device (LHP, PHPF, PHPO, PHPF'), such as a fluidic loop heat pipe or a pulsating heat pipe, thermally connected between said hot source and said cold source, or respective cold sources.
13. The heat transfer system claimed in claim 12 wherein said heat transfer passive diphasic device is connected to said first and second reservoirs via a system of valves (V3V1, V3V2) enabling it to be filled with diphasic fluid.
14. The heat transfer system claimed in any one of claims 1 to 10, including a device wherein a pulsating heat pipe (PHPO) thermally connected between said hot source and cold source is integrated into said fluidic pipe.
15. A method of cooling or precooling an object by means of the device claimed in any one of claims 1 to 10, including the following steps:

    a. Thermally connecting said object to the evaporator of said device so that it functions as a hot source;

    b. Thermally connecting said first and second reservoirs and said first and second condensers to said cold source or to respective cold sources in such a manner as to cause at least partial filling of at least said first reservoir with a liquid phase of said diphasic fluid;

    c. Activating said first heating means so that said liquid phase of said diphasic fluid flows toward said second reservoir via said evaporator, in which it evaporates at least partially, cooling said object, and said second condenser, where the vapor formed in this way returns to the diphasic state;

    d. Deactivating said first heating means when the first reservoir is substantially empty of said liquid phase;

    e. Activating said second heating means so that said liquid phase of said diphasic fluid flows toward said first reservoir via said evaporator, where it is evaporated at least in part, cooling said object, and said first condenser, where the vapor formed in this way returns to the diphasic state; and

    f. Deactivating said second heating means when the second reservoir is substantially empty of said liquid phase;

    the steps c. to f. being repeated cyclically.
16. The method claimed in claim 15 of cooling an object, including a precooling step followed by a step of thermal stabilization by means of a heat transfer passive diphasic device.

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