EP2956729B1 - Heat transport device with diphasic fluid - Google Patents

Description

The present invention relates to two-phase fluid heat transport devices, in particular passive devices with a two-phase fluid loop with capillary pumping or using gravity.

It is known from the document FR-A-2949642 an example of such a device used as a cooling means for electrotechnical power converter. The document US 4,576,009 discloses a heat transfer device, adapted to extract heat from a hot source and to return this heat to a cold source by means of a two-phase working fluid contained in a closed general circuit.

In established operating conditions, these devices give full satisfaction. However, it appeared that the starting phases from a 'cold' state (minimum ambient temperature and zero heat flux) could be particularly delicate for large thermal powers, and could require a preliminary conditioning step, for example by preheating the tank. Without this conditioning, the pressure in the circuit may be insufficient to ensure sufficient heat transfer.

It has therefore appeared a need to improve the availability of start-up for such two-phase loops.

• un premier circuit de communication, pour du fluide de travail essentiellement en phase vapeur, reliant la sortie de l'évaporateur à une entrée du condenseur,
• un deuxième circuit de communication, pour du fluide de travail essentiellement en phase liquide, reliant une sortie du condenseur au réservoir et à l'entrée de l'évaporateur,

caractérisé en ce que la portion gaz du réservoir comprend de la phase vapeur du fluide de travail avec une première pression partielle P1 (pression déterminée par la température du réservoir) et un gaz auxiliaire non condensable avec une deuxième pression partielle P2, cette dernière étant prédéterminée pour permettre d'obtenir une pression totale supérieure ou égale à une pression minimale de fonctionnement prédéterminée requise lorsque la portion liquide dans l'entièreté du circuit général clos est à un volume total minimum.For this purpose, the subject of the invention is a thermal transfer device, without active regulation, without an active control system, adapted to extract heat from a hot source and to restore this heat to a cold source by means of a two-phase working fluid contained in a closed general circuit, comprising at least one evaporator, having an inlet and an outlet, at least one condenser, separate and at a distance from the evaporator, a reservoir having an internal volume, with a liquid portion and a gas portion separated by a liquid-vapor interface and at least one inlet / outlet orifice arranged at the level of the liquid portion, the volume of the liquid portion being variable between a minimum volume Vmin and a maximum volume Vmax,

• a first communication circuit, for working fluid essentially in the vapor phase, connecting the outlet of the evaporator to an inlet of the condenser,
• a second communication circuit, for working fluid essentially in the liquid phase, connecting an outlet of the condenser to the tank and to the inlet of the evaporator,

characterized in that the gas portion of the reservoir comprises the vapor phase of the working fluid with a first partial pressure P1 (pressure determined by the temperature of the reservoir) and a non-condensable auxiliary gas with a second partial pressure P2, the latter being predetermined to allow to obtain a total pressure greater than or equal to a predetermined minimum operating pressure required when the liquid portion in the entirety of the closed general circuit is at a minimum total volume.

Thanks to these arrangements, in particular thanks to the second partial pressure P2, a minimum pressure is ensured in the tank due to the presence of the non-condensable auxiliary gas in the gas portion of the tank, even when the liquid portion is at its minimum. , or that the device is completely cold, without heat input on the evaporator for quite a long time. The minimum pressure linked to the presence of the non-condensable auxiliary gas in the tank makes it possible to obtain a high saturation temperature in the second communication circuit (the gas pipe), which makes it possible to obtain a minimum density of the vapor phase of the Working fluid, and since the heat transfer capacity of the loop is proportional to the density of the vapor phase, an improved heat transport capability can be obtained instantaneously upon cold start of the loop.

Moreover, thanks to these provisions, we obtain a passive regulation without the need for an active control system, which increases the reliability of this kind of device. Such a system, without active pumping and without active control system, requires no maintenance and has a very high reliability; and its energy consumption is very low or even zero.

As a non-condensable auxiliary gas, a gas which remains in the gaseous state over the entire range of temperature / pressure to which the device is subjected is preferably chosen; in addition, a gas with a low diffusion coefficient in the liquids is chosen as the auxiliary gas.

• le gaz auxiliaire non condensable peut être de l'hélium ; moyennant quoi les propriétés physico-chimiques de l'hélium conviennent parfaitement et ce gaz présente une bonne disponibilité industrielle ;
• le fluide de travail peut être du méthanol ; ce fluide permettant de travailler dans une gamme de température satisfaisante et présente une performance capillaire satisfaisante.
• la deuxième pression partielle P2 peut être au moins plusieurs fois supérieure à la première pression partielle P1 lorsque la portion liquide est à son volume minimum ; de sorte que la pression minimale est assez élevée pour autoriser un démarrage instantané sans préparation sous charge thermique importante ;
• le volume du réservoir peut être compris entre 1,3 et 2,5 fois le volume maximum de la portion liquide ; de sorte que lorsque le volume de la portion liquide est maximal, la pression et la température dans le réservoir et dans la boucle restent limitées et restent compatible avec un prélèvement efficace des calories au niveau de l'évaporateur ;
• le dispositif peut être principalement soumis à la gravité terrestre, l'orifice d'entrée/sortie étant agencé au niveau d'au moins un point bas du réservoir ; moyennant quoi on évite que le gaz auxiliaire ne soit aspiré en direction de l'évaporateur ;
• le dispositif peut être principalement soumis à une microgravité, le réservoir comprenant une masse poreuse agencée au moins au voisinage de l'orifice d'entrée ; moyennant quoi une barrière de liquide est formée dans la masse poreuse et on évite que le gaz auxiliaire ne soit aspiré en direction de l'évaporateur ;
• l'évaporateur peut comprendre une masse microporeuse adaptée pour assurer un pompage capillaire de fluide en phase liquide ; on obtient ainsi un système passif sans entretien ;
• dans le cas où le dispositif est principalement soumis à la gravité, l'évaporateur sans structure capillaire peut être placé en dessous du condenseur et du réservoir, de sorte que la gravité est utilisée pour déplacer le liquide vers l'évaporateur ; ce qui représente une solution très simple et particulièrement robuste et fiable ;
• un clapet anti-retour peut être agencé à l'entrée de l'évaporateur ; on peut ainsi empêcher un retour de liquide en direction inverse au sens de circulation normal, et empêcher ainsi un assèchement de l'évaporateur au démarrage sous forte charge ;

In various embodiments of the invention, one or more of the following provisions may also be used:

• the non-condensable auxiliary gas may be helium; whereby the physicochemical properties of helium are perfectly suitable and this gas has a good industrial availability;
• the working fluid may be methanol; this fluid to work in a satisfactory temperature range and has a satisfactory capillary performance.
• the second partial pressure P2 may be at least several times greater than the first partial pressure P1 when the liquid portion is at its minimum volume; so that the minimum pressure is high enough to allow instant start without preparation under high thermal load;
• the volume of the reservoir can be between 1.3 and 2.5 times the maximum volume of the liquid portion; so that when the volume of the liquid portion is maximum, the pressure and temperature in the reservoir and in the loop are limited and remain compatible with efficient removal of calories at the evaporator;
• the device can be mainly subjected to Earth's gravity, the inlet / outlet orifice being arranged at at least one low point of the tank; whereby it is avoided that the auxiliary gas is sucked in the direction of the evaporator;
• the device can be mainly subjected to microgravity, the reservoir comprising a porous mass arranged at least in the vicinity of the inlet orifice; whereby a liquid barrier is formed in the porous mass and the auxiliary gas is prevented from being sucked towards the evaporator;
• the evaporator may comprise a microporous mass adapted to provide capillary pumping of fluid in the liquid phase; a maintenance-free passive system is thus obtained;
• in the case where the device is mainly subjected to gravity, the evaporator without capillary structure can be placed below the condenser and the reservoir, so that the gravity is used to move the liquid to the evaporator; which represents a very simple and particularly robust and reliable solution;
• a non-return valve can be arranged at the inlet of the evaporator; it is thus possible to prevent a return of liquid in the opposite direction to the direction of normal circulation, and thus prevent drying of the evaporator at startup under heavy load;

• la figure 1 montre une vue générale d'un dispositif selon un mode de réalisation de l'invention,
• la figure 2 illustre les fluides dans un diagramme pression-température général,
• les figures 3A et 3B montrent le réservoir avec une portion liquide respectivement minimale et maximale,
• la figure 4 montre un second mode de réalisation du dispositif,
• les figures 5A et 5B illustrent des diagrammes de pression et température de saturation en fonction de la température ambiante.

Other aspects, objects and advantages of the invention will appear on reading the following description of two embodiments of the invention, given by way of non-limiting examples, with reference to the accompanying drawings, in which:

• the figure 1 shows a general view of a device according to one embodiment of the invention,
• the figure 2 illustrates the fluids in a general pressure-temperature diagram,
• the Figures 3A and 3B show the reservoir with a respective minimum and maximum liquid portion,
• the figure 4 shows a second embodiment of the device,
• the Figures 5A and 5B illustrate diagrams of pressure and saturation temperature as a function of the ambient temperature.

In the different figures, the same references designate identical or similar elements.

The figure 1 shows a two-phase fluid loop heat transport device. In the case of the first mode, the pumping is ensured by taking advantage of the capillarity phenomenon. The device comprises an evaporator 1, having an inlet 1a and an outlet 1b, and a microporous mass 10 adapted to provide capillary pumping. For this purpose, the microporous mass 10 surrounds a blind central longitudinal recess 15 in communication with the inlet 1a for receiving working fluid in the liquid state from a fluid line in the liquid phase.

The evaporator 1 is thermally coupled to a hot source 11, for example an assembly comprising electronic power components or any other element generating heat, for example by joule effect, or by any other process.

Under the effect of the supply of calories to the contact 16 of the microporous mass filled with liquid, fluid passes from the liquid state to the vapor state and is evacuated by the transfer chamber 17 and by a first circuit. communication 4 which conveys said vapor to a condenser 2 having an inlet 2a and an outlet 2b ; the condenser 2 being distinct and not adjacent to the evaporator 1.

In the evaporator 1, the cavities released by the evacuated vapor are filled with liquid sucked by the microporous mass 10 from the aforementioned central recess 15; it is the phenomenon of capillary pumping well known in itself. The heat flux Q taken from the hot source corresponds to the flow rate multiplied by the latent heat L of vaporization of the working fluid ( Q = L.dM / dt ) .

Inside the condenser 2, heat is transferred by the fluid in the vapor phase to a cold source 12, which causes cooling of the fluid in the vapor phase and its phase change to the liquid phase, ie its condensation.

At the condenser 2, the temperature of the working fluid is lowered below its liquid-vapor equilibrium temperature, which is also called sub-cooling ('sub cooling' in English) so that the fluid can not return to the steam state without any heat input.

The vapor pressure pushes the liquid towards the outlet 2b of the condenser 2 which opens onto a second communication circuit 5 connected to the inlet 1a of the evaporator 1. This gives a circulation loop of the two-phase fluid capable of extracting heat from the hot source 11 to return this heat to a cold source 12.

The heat transported by the vapor phase in the first communication circuit can be written as Q = pVS, where p is the density of the vapor phase, V is the displacement velocity of the vapor phase and S is the section of the communication circuit.

The second communication circuit 5 is also connected to a reservoir 3. This reservoir serves as an expansion vessel for the working fluid, and contains working fluid both in the liquid phase and in the gas phase. Said tank forms, with the first and second communication circuits 4,5, together with the evaporator 1 and the condenser 2, a closed general circuit in other words hermetic.

The reservoir 3 has at least one inlet / outlet port 31, and a certain interior volume 30 generally attached to the design for a given application. This volume may possibly be adjustable by a mechanical device operated manually or automatically. The reservoir also comprises a filling orifice 36 which allows an initial filling of the circuit, this filling orifice being closed the rest of the time. It should be noted that the reservoir 3 may have any shape, and in particular parallelepipedal, cylindrical, or other.

The heat transfer device is designed to operate within a certain range of ambient temperature; in the illustrated example, this temperature range can be: [-50 ° C, + 50 ° C]. Furthermore, it is desirable that the hot source 11 does not exceed a certain predetermined maximum temperature, regardless of the heat flow to be evacuated. This predetermined maximum temperature may be for example 100 ° C. Of course, these temperatures may depend on the intended application type, space applications in microgravity, terrestrial applications on board a vehicle or in a fixed location.

The working fluid of the loop is chosen to be always potentially two-phase in the range of temperatures and pressures of the fluid of the two-phase loop, as a function of the above-mentioned temperature range (see footnote 14). Fig. 2 ).

Thus, the working fluid can be chosen from a list comprising in particular ammonia, acetone, methanol, water, dielectric fluids of the type HFE7200 or any other suitable fluid. In the detailed example below, preference will be methanol.

Inside the tank 3, there is a liquid portion 6 essentially comprising working fluid (in this case methanol) in the liquid phase and a gas portion 7 comprising fluid in the vapor phase, but also, as will be seen in more detail. a non-condensable auxiliary gas 8. The non-condensable auxiliary gas 8 (denoted 'NCG' of the English Non-Condensible Gas) remains confined in the gas portion of the tank without directly participating in the heat exchanges; it has the effect of creating a minimum pressure in this gas portion. The partial pressure of this non-condensable auxiliary gas 8 is denoted P2. Over the range of temperatures and pressures of the application, this non-condensable auxiliary gas remains in the gaseous state as it appears in FIG. figure 2 , in the right part.

It should be noted here that according to the prior art known, the presence of non-condensable gas in the working circuit is undesirable because if non-condensable gas bubbles reach the area of the capillary evaporator, this reduces the thermal performance of vaporization and this can even go as far as defusing the capillary evaporator, which in some critical applications can be catastrophic.

In an environment where gravity is exerted, the gas portion 7 is located above the liquid portion 6 and a liquid-vapor interface 19 generally horizontal separates the two phases (free surface of the liquid in the tank).

In an environment where microgravity is exerted (in weightlessness), the liquid portion is contained in porous material and the gas portion occupies the rest of the volume of the reservoir; there is also in this case a liquid-vapor interface 19, but it is not flat.

The temperature of this separation surface 19 is uniquely connected to the partial pressure P1 of working fluid in the gas portion, this pressure corresponds to the saturation pressure Psat of the fluid at the temperature Tsat prevailing at the separation surface 19, as this appears at the figure 2 in left part.

In practice, the temperature of the liquid portion, the gas portion and the shell of the reservoir are relatively homogeneous; there is little temperature gradient inside the tank. The temperature of the tank is also not far from the ambient temperature in which it is located.

According to an advantageous aspect of the present invention, the inlet / outlet orifice 31 is arranged at the level of the liquid portion, so that the gas portion is never directly in communication with the liquid communication circuit 5. configuration of the capillary link between the reservoir and the porous mass maybe like that described in the patent EP0832411 .

According to a particular aspect, especially in the case of use in microgravity (a case not shown in the drawings) but not exclusively, there can be provided a porous mass 9 arranged in the vicinity of the inlet / outlet orifice 31, of which the function is to retain liquid, and therefore to form a barrier preventing components of the gas phase to be sucked towards the liquid communication circuit 5.

In terrestrial applications gravity is exerted, the inlet / outlet port 31 is arranged at a low point of the tank. It should be noted that there may be several low points in the tank.

The volume of the liquid portion 6 in the reservoir may vary between the minimum volume ( 'Vmin' ) represented in FIG. figure 3A which corresponds to a minimum total volume of liquid in the entire general circuit, and a maximum volume ( 'Vmax' ) represented at figure 3B which corresponds to a maximum total volume of liquid in the whole of the general circuit.

The difference between Vmax and Vmin is at least equal to the sum of 2 volumes, which are respectively called the expansion volume V0c and the purge volume Vpurge, which respectively represent, on the one hand, the thermal expansion of the liquid and, on the other hand, the evacuation of the liquid driven by the presence of steam in the steam pipe 4 and a portion of the condenser 2 of the loop. In other words, when the two-phase loop has been at rest for some time, there is no more steam in the loop and the liquid occupies the entire inner volume of the loop, which gives a small volume of liquid portion in the reservoir ; conversely, when the heat flow is maximum ( Q = Qmax ), the first communication circuit 4 is entirely occupied by steam and a part of the condenser circuit 2, and therefore the liquid is pushed back in the tank where it occupies a large volume. The volume of liquid portion is also influenced by the ambient temperature, which leads to the volume of expansion V0c.

More specifically, the minimum volume Vmin corresponds to a minimum ambient temperature and a zero heat flux (Q = 0) on the evaporator; this situation is represented in Figures 5A-5B by points 61. Note that the pressure in the gas portion is mainly due to the presence of the auxiliary gas 8 (pressure P2) and not the partial pressure P1 of the working fluid which is very low. The total pressure in the reservoir is Pres = P1 + P2 ; it is also substantially the pressure that prevails everywhere else in the two-phase loop.

Still without caloric ratio on the evaporator (zero heat flow, Q = 0), but with a maximum ambient temperature, there is a liquid dilation which gives a volume of liquid portion noted V0c, greater than Vmin. This situation is represented in the Figures 5A-5B by the points 62.

In circumstances where the ambient temperature is maximum and the heat flow is also maximum Q = Qmax, the volume of the liquid portion is increased by the volume corresponding to the Vpurge purge , which leads to the illustrated case. figure 3B . This situation is represented in the Figures 5A-5B by the points 64.

It can therefore be seen that when the liquid portion 6 is at its minimum volume ( Vmin ), which corresponds to a minimum total volume of liquid in the entire general circuit, the second pressure P2 is such that it makes it possible to obtain a total tank pressure greater than or equal to a predetermined minimum required operating pressure (shown at 0.7 bar in the Figure 5B in a non-limiting way, in fact this minimum value can be determined according to the application considered).

It can also be noted that, in an illustrative example, when the liquid portion 6 is at its minimum volume ( Vmin ), the second partial pressure P2 (NCG) is greater than the first partial pressure P1. This condition remains true over most of the ambient temperature range at Q = 0 and even when Q = Qmax in the cold temperature zone.

It can also be noted that when the liquid portion 6 is at its minimum volume (V min ), the second partial pressure P2 (NCG) may be several times, for example 5 times or 10 times greater than the first partial pressure P1 (cf points 61).

The minimum pressure related to the presence of gas non-condensable auxiliary in the tank (0.7 bar in example la Figure 5B ) makes it possible to obtain a high saturation temperature in the second communication circuit (50 ° C. in the example Figure 5A ), which makes it possible to obtain a minimum density p of the vapor phase of the working fluid, and since the heat transport capacity of the loop is proportional to the density of the vapor phase (Q = pVS), can obtain instantly from the cold start of the loop sufficient heat transport capacity to avoid defusing the evaporator and obtain and good loop performance.

To maintain a satisfactory thermal evacuation performance in the most constrained thermal case (maximum ambient temperature and maximum heat flow), illustrated by the points 64, it is necessary to provide a sufficient volume of the gas portion 7 over the volume of liquid portion Vmax.

Advantageously, provision may be made that the total volume 30 of the tank is between 1.3 and 2.5 times said maximum volume Vmax of the liquid portion (case of the maximum total volume of liquid phase). Thus, the saturation temperature Tsat, for an ambient temperature of 50.degree. C. and a maximum flow Qmax, remains below 90.degree. this allows to continue to take calories at the hot spring 11.

As regards the choice of non-condensable auxiliary gas 8, this gas must remain in the vapor phase throughout the operating range of the loop and in particular the pressure and temperature conditions in the tank, it must have a very low boiling point; moreover, its diffusion coefficient inside the liquids and its Oswald coefficient must also be low in order to prevent this auxiliary gas from seeping inside the liquid portion 6 of the reservoir and in the rest of the loop. .

Advantageously, helium can be chosen as an auxiliary gas. Helium is chemically neutral and its industrial availability is satisfactory. However, it is not excluded to use other gases such as nitrogen, argon or neon.

The figure 4 illustrates a second embodiment of thermosiphon type, in which the condenser 2 is placed above the evaporator 1 so that the gravity naturally leads the liquid towards the evaporator; under these conditions the role of the porous material in the evaporator is to promote heat exchange and vaporization rather than perform the actual capillary pumping function. Apart from the source of the liquid movement and the relative position of the elements that differ, everything else and in particular the operating principle is identical to the first mode described above, and will not be repeated.

Due to the pressurization exerted by the presence of the auxiliary gas 8, it is possible to overcome the presence of a heating element to condition the two-phase loop before the actual thermal start.

It should also be noted that such a two-phase loop can be devoid of active regulation, which is a decisive advantage in terms of reliability.

Advantageously according to the invention, the device is devoid of any mechanical pump although the invention does not exclude the presence of a mechanical booster pump.

It should be noted that the proportions of the elements in the drawings are not necessarily representative of the proportions or relative dimensions of the various organs.

The first and second fluid communication circuits 4,5 are preferably tubular conduits, but could be other types of fluid communication conduits or channels (rectangular conduits, flexible, etc.). Likewise, the inlet / outlet port 31 could be a separate inlet and outlet.

The two-phase loop may advantageously be equipped with a non-return valve 18 located at the inlet of each evaporator so as to increase the maximum starting power. Indeed, the nonreturn valve 18 prevents a return of liquid in the opposite direction to the normal flow direction, and thus prevents drying of the evaporator at startup under heavy load.

In an application subjected to gravity, the non-return valve may be formed by a floating element recalled by the buoyancy thrust against a bearing to close the passage and thus prevent a return of liquid.

Note that, advantageously according to the invention, the two-phase fluid system presented here is fully self-adaptive, it requires no control law, no sensor. The result is a particularly simple design, particularly simple manufacturing, no need for maintenance, and incomparable reliability.

Claims (10)

1. Device for heat transfer, without active regulation, without active control, suited for extracting heat from a hot source (11) and for returning this heat to a cold source (12) by means of a two-phase working fluid contained in a general closed circuit, comprising:

   - at least one evaporator (1), having an inlet and an outlet,

   - at least one condenser (2), separate from and away from the evaporator,

   - a tank (3) with an internal volume (30), a liquid portion (6) and a gas portion (7) separated by a liquid-vapor interface (19), and at least one inlet/outlet orifice (31) laid out near the liquid portion, where the volume of the liquid portion can vary between a minimum volume (Vmin) and a maximum volume (Vmax),

   - a first connection circuit (4), for the working fluid essentially in vapor phase, connecting the outlet of the evaporator to the inlet of the condenser;

   - a second connection circuit (5), for the working fluid essentially in liquid phase, connecting an outlet from the condenser to the tank and to the inlet of the evaporator;

   characterized in that the gas portion (7) from the tank includes the vapor phase of the working fluid with a first partial pressure (P1) and a non-condensable auxiliary gas (8) with a second partial pressure (P2), where this partial pressure is predetermined to make it possible to obtain a total pressure (Pres) greater than or equal to a preset minimum operating pressure required when the liquid portion in the entire general closed circuit is at a minimum total volume.
2. Device according to claim 1, wherein the non-condensable auxiliary gas is helium.
3. Device according to one of claims 1 to 2, wherein the working fluid is methanol.
4. Device according to one of claims 1 to 3, wherein the total volume (30) of the tank is included between 1.3 and 2.5 times said maximum volume (Vmax) of the liquid portion.
5. Device according to one of claims 1 to 4, mainly subject to terrestrial gravity, wherein the inlet orifice is arranged in the area of a low point of the tank.
6. Device according to one of claims 1 to 4, mainly subject to microgravity, wherein the tank comprises a porous mass (9) laid out at least in the area of the inlet orifice.
7. Device according to one of claims 1 to 6, wherein the evaporator includes a micro-porous mass (10) adapted for assuring capillary pumping of the liquid phase fluid.
8. Device according to one of claims 1 to 5, subject mainly to gravity, wherein the evaporator is placed below the condenser and the tank, whereby gravity is used for moving the liquid towards the evaporator.
9. Device according to one of claims 1 to 8, wherein an anti-backflow valve (18) is laid out at the inlet of the evaporator (1).
10. Device according to one of claims 1 to 9, wherein the second partial pressure (P2) is at least several times greater than the first partial pressure (P1) when the liquid portion is at the minimum volume (Vmin).

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