EP0832411B1 - Capillary pumped heat transfer loop - Google Patents

Description

The present invention relates to a loop with capillary heat transport pumping comprising at at least one evaporator, at least one condenser and one tank arranged to store a heat transfer fluid, said evaporator comprising an outlet connected by a line of steam at a condenser inlet, a condenser outlet being connected to the reservoir, said evaporator comprising an evaporator body and being provided with a material porous arranged to produce a capillary pressure of pumping inside the loop and exerting it on said heat transfer fluid from the surface of the material in contact with the evaporator body, said evaporator being also arranged to evaporate the heat transfer fluid by heat absorption.

Such a capillary pumping loop is known from the publication "Computer Model of satellite Thermal Control System Using a controlled capillary pumped loop "by K.A. Goncharov, E. Yu Kotlyarov and G.P. Serov published in SAE Technical Paper Series No. 932306. Such loops are for example used in satellites and allow heat transfer from a heat source, for example electronic equipment, to the condenser where the removed heat is dissipated. The loop is not good heard not limited to weightless applications because it also works in the presence of gravity. The porous material present in the evaporator has a axial channel which supplies heat transfer liquid porous material. The liquid saturation of the material porous allows the creation of capillary pressure. It is this capillary pressure which will allow circulation vapor from the evaporator to the condenser and the return of the condensed fluid to the evaporator without it use mechanical pumping means. The loop configuration allows circulation of the evaporator to the condenser and then to the tank, which in turn supplies the evaporator with heat transfer liquid. The capillary material of the evaporator is thus supplied with heat transfer liquid and is therefore constantly saturated with liquid. In this way the capillary material allows to develop capillary pumping pressures able to compensate for pressure drops in the loop. The capillary pressure obtained with capillary materials currently known allows the heat transfer fluid to be pumped from the condenser to the evaporator even over a height of several meters under a gravity field.

If, before steam circulation, the loop is at rest with the evaporator above the condenser, the heat transfer fluid completely fills the liquid line, vapor line and condenser, and partially the evaporator assembly. The liquid of the steam line and condenser will be pushed by steam generated by the evaporator to the tank. This pushed comes from a pressure difference between the evaporator and the tank caused by the external heat flow applied to the evaporator, which flux increases in one first the temperature of the evaporator. Volume of liquid vis-à-vis the volume of vapor contained by the tank therefore depends on the volume of steam vis-à-vis the volume of liquid in the vapor line and condenser. This phase change loop and capillary pumping is called "auto-start" because it does not require any related device or special procedure starting. It is indeed the heat flux applied to the evaporator which causes the loop to start.

One drawback of the known loop is that the evaporator and the tank are connected to form a indivisible together. The tank temperature is mainly dictated by parasitic heat flow flowing from the evaporator to the tank. Pressure which prevails within the reservoir depends on the temperature and thus the vaporization pressure and temperature and condensation at which heat transport occurs in the loop is equal to the temperature of the tank. The heat source temperature is thus not sufficiently regulated, because it depends on the heat balance said parasitic flux and heat losses from the tank towards the atmosphere. The solution applied by the state of the technique lies in active thermal control of the tank via a Peltier cell which links the reservoir to the evaporator or other related devices that allow to regulate the tank temperature and so the temperature of the entire transport loop heat. This solution, however, makes the loop more complex. In addition, if the heat flux supplied by the heat source is too low, the temperature of the tank equals that of the evaporator surface and it there is no steam circulation.

The object of the invention is to remedy these disadvantages.

To this end a capillary pumping loop heat transport according to the invention is characterized in that the tank and the evaporator are isolated thermally from each other and interconnected by a pipe comprising a first part formed by a capillary connection arranged to pump the heat transfer fluid from the reservoir to the porous material and a second part arranged to evacuate gas bubbles and / or vapor formed in the evaporator to the tank, which tank being arranged to be maintained at a temperature lower than that of the evaporator. Insulation the thermal value of the tank and the evaporator consequence of thermally decoupling them and allowing thus conditioning the tank to a temperature independent of that of the evaporator. The heat flow direct interference from the evaporator to the tank is thus checked. The temperature of the tank is thus mainly given by the temperature of the liquid coming from the condenser and by the temperature of the environment. These two temperatures are also stable and low, the tank and therefore the evaporator (s) are kept at a minimum temperature. This result is very widely desired because it allows heat exchange with a minimum temperature difference between the source of heat and condenser. The capillary bond that brings the heat transfer liquid from the tank to the evaporator ensures that the porous material of the evaporator is always sufficiently supplied with heat transfer liquid and so that the capillary pumping pressure can be developed to maintain circulation in the loop. The second part allows for evacuation to the tank the vapor and the non-condensable gas formed by the stray heat flow through the capillary material of the evaporator. Since the tank is one lower temperature than the evaporator, this is the temperature difference between tank and evaporator which will ensure the circulation of gas and steam in said second part towards the reservoir.

A first preferred embodiment of a capillary pumping loop for transporting heat according to the invention is characterized in that in said pipe which connects the evaporator to the tank the first part comprises at least a first channel and the second part at least one second channel, the diameter of the first channel being less than that of the second channel. Thanks to this configuration, any gas or vapor in the second part does not hinder the circulation of heat transfer fluid from the reservoir to the capillary material of the evaporator, because the smaller diameter of the first channel allows greater pumping pressure.

A second preferred embodiment of a capillary pumping loop for transporting heat according to the invention is characterized in that the pipe that connects the evaporator to the tank extends in the central axis of the evaporator, said porous material of the evaporator being coaxially arranged with respect to the driving. This ensures an adequate supply of capillary material in heat transfer liquid and allows operation of the evaporator on all of its outer casing.

A third preferred embodiment of a loop according to the invention is characterized in that the tank is thermally connected to the minus one of the evaporators by a thermoelectric cell Peltier effect arranged to regulate the temperature of the tank. This configuration allows you to vary the temperature difference between the tank and the evaporator, while keeping the tank temperature lower to that of the loop, and thereby influence the circulation in the loop. This configuration allows also active tank temperature control and as a consequence of the vaporization temperature and loop condensation. This embodiment has the advantage of using an evaporator as a cold source of the tank rather than an auxiliary transport device heat.

Preferably it includes an evaporator auxiliary connected to a line of fluid leaving the condenser. This configuration has the advantage of avoiding a link capillary between the auxiliary evaporator and the tank. The performance of the hair bond no longer limits that of the auxiliary evaporators. Therefore the distances between the evaporator and the tank are no longer limited. The return line of the condensed fluid from of the condenser thus ensures the circulation of the non-condensable vapor and gas. These will be transported to the tank by circulation existing in the loop.

According to another preferred form of realization of the loop according to the invention said evaporator auxiliary is connected to the fluid line by a capillary bond. The auxiliary evaporator is working so in the same way with respect to the fluid line than the one that the evaporator works in relation to tank.

Preferably the end of the link capillary in contact with the fluid line is thermally connected to the auxiliary evaporator by a cell thermoelectric Peltier effect arranged to cool the line in relation to the auxiliary evaporator. Regulation fluid line temperature becomes possible.

La figure 1 illustre schématiquement un premier exemple de réalisation d'une boucle suivant l'invention;

La figure 2 illustre une coupe longitudinale de la surface du matériau capillaire;

La figure 3 a respectivement b et c montre une vue en coupe longitudinale respectivement transversale de la liaison capillaire qui relie l'évaporateur au réservoir;

La figure 4 illustre schématiquement le fonctionnement de l'évaporateur;

Les figures 5 et 6 représentent un diagramme de pression respectivement de température;

La figure 7 illustre schématiquement un deuxième exemple de réalisation d'une boucle suivant l'invention, et

La figure 8 illustre schématiquement une boucle suivant l'invention pourvue d'une cellule Peltier.

The invention will now be described in more detail with the aid of exemplary embodiments of a capillary pumped heat transport loop shown in the figures where:

Figure 1 schematically illustrates a first embodiment of a loop according to the invention;

FIG. 2 illustrates a longitudinal section of the surface of the capillary material;

FIG. 3 a respectively b and c shows a view in longitudinal cross section respectively of the capillary connection which connects the evaporator to the tank;

Figure 4 schematically illustrates the operation of the evaporator;

Figures 5 and 6 show a pressure diagram respectively temperature;

FIG. 7 schematically illustrates a second embodiment of a loop according to the invention, and

FIG. 8 schematically illustrates a loop according to the invention provided with a Peltier cell.

In the figures, the same reference has been attributed to the same element or to an analogous element.

Figure 1 schematically illustrates a first example of a pumping loop heat transport capillary. This loop has a tank 1 in which a heat transfer liquid is stored. Tank 1 is thermally isolated from an evaporator 2. This keeps the tank at a lower temperature than the evaporator as in will be described below. The connection between tank 1 and the evaporator 2 is provided by a line 3 which comprises a first part 18 formed by a connection capillary and a second part 4 formed by a channel axial.

The evaporator 2 includes a capillary material porous 5 arranged to produce capillary pressure within the evaporator. An evaporator outlet is connected by a steam line 6 to an inlet of a condenser 9. An output of the condenser is connected by a line 10 for the fluid which brings the fluid under form of liquid condensed in the condenser towards the tank thus closing the loop. If applicable the line fluid can also be directly connected to the evaporator. The loop can contain one or more evaporators. In the example shown in Figure 1 the loop comprises a second evaporator 8 connected by a pipe 7 at a tank outlet 1. The second evaporator 8 is also thermally dissociated from the tank.

The operation of the evaporator will be described using Figure 2. The evaporator 2 has a body 13 evaporator which forms the outer envelope of this last. The evaporator body is in contact with the capillary material 5 which is arranged coaxially by relative to the central axis of the evaporator. The material capillary 5 contains heat transfer liquid from of the tank. The capillary material 5 is provided with grooves 12 vapor collectors at the interface between this material and the evaporator body 13. The grooves 12 are in contact with steam line 6 to allow the evacuation of the vapor formed in the evaporator to the vapor line.

σl =

tension superficielle du liquide caloporteur.

R =

rayon de courbure du ménisque liquide à l'interface liquide/vapeur

When the evaporator body 13 is subjected to a flow of heat Qe coming from an external source such as for example an electronic device, the heat Qe evaporates the heat transfer liquid contained in the capillary material 5. The vapor 15 thus produced will be released towards the vapor collecting grooves 12, then entering the vapor line 6. In the evaporator there is therefore both liquid and vapor producing a liquid / vapor interface 17 on the surface of the porous capillary material in contact with the body evaporator. This liquid / vapor interface has a radius of curvature. The value of the radius of curvature of the liquid meniscus contained between the particles 16 of solid material of the porous material causes the capillary pressure P _E - P _{D to be} produced by the surface tension of the heat-transfer liquid. This pressure P _E - P _D is illustrated in FIG. 5 which represents a pressure diagram. This capillary pumping pressure is exerted on the heat transfer fluid. The liquid is under vacuum in the porous material at the interface 17, which causes suction of the liquid upstream of the porous material. The vapor is overpressure relative to the liquid and will therefore direct the latter from the interface 17 towards the vapor line. Capillary pressure meets the following equation: ΔP = 2σl R with

σl =

surface tension of the heat transfer liquid.

R =

radius of curvature of the liquid meniscus at the liquid / vapor interface

Using capillary pressure a circulation of the heat transfer fluid is produced in the capillary material and across the entire loop. This pressure is such that it can defeat all of the pressure drops in the loop as long as the capillary material remains supplied with liquid.

To maintain capillary pressure in the loop so it is necessary to power the evaporator in heat transfer liquid so that the evaporated liquid is replaced by liquid from the tank. As mentioned before the tank is connected to the evaporator via line 3, a sectional view of which is illustrated in Figure 3c. Figures 3 a + b illustrating a sectional view through the evaporator. Driving involves a first part 18 formed by a capillary connection whose structure is comparable to that of the material capillary 5 present in the evaporator but whose permeability and pore size of the capillary material is greater than that of porous material 5. The porous material 5 and the capillary material are of preferably arranged coaxially with respect to channel 4. An axial channel 4 and the capillary link 18 which extend in the central axis of the evaporator. The material capillary 18 joins the porous material 5 of the evaporator. Thus the heat transfer fluid contained in the tank 1 circulates by capillarity in the capillary connection 18 to reach the porous material 5 of the evaporator. Continuity between the capillary connection and the material porous ensures a supply of heat transfer liquid on the entire length of the link.

The first part of line 3 includes at least a first channel formed between the particles of solid material of the capillary material 18. The second part 4 has at least one second channel. The diameter dl of the first channel being less than that of d2 of the second channel to allow greater capillary pressure in the first channel and therefore ensure the supply of liquid to the evaporator.

The fact that the tank 1 is thermally isolated from the evaporator does not prevent the circulation of the fluid towards the evaporator. Indeed, it is the capillary pressure produced by the porous material 5 supplied with liquid by the material 18 which ensures circulation in the loop. The insulation of the reservoir with respect to the evaporator makes it possible to maintain the reservoir at a temperature T _A lower than that of T _F of the evaporator as illustrated in FIG. 6. The reservoir being in connection with the condenser it receives the fluid condensed which is at a temperature T _I when it leaves the condenser. In this context, it should be noted that a temperature difference between the tank and the porous material of the evaporator has already been suggested in the article cited in the preamble. However, nothing in this article suggests separating the tank and the evaporator which, according to the article, must remain indivisible. The thermal insulation between tank and evaporator allowing the temperature difference between the two has a positive influence on the operation of the loop which will be described below.

The lower tank temperature by compared to the evaporator also allows to store in the tank a large amount of non-condensable gas. A large amount of non-condensable gas produced after several years of operation of the loop, generates significant partial pressure. In this case the increase partial pressure must be compensated by a decrease in partial pressure of the heat transfer fluid. The latter can be obtained by reducing the tank temperature compared to that of the evaporator.

The external heat flow Q _e will not only cause the evaporation of the heat transfer liquid at the liquid / vapor interface 17 but also a production of vapor at the level of the pipe 4 at the other interface between the first and the second part of the pipe up to its extension in the evaporator.

The heat flow Q _E also causes a parasitic heat flow Q _P which passes through the capillary material 5 of the evaporator and evaporates the heat transfer liquid present in the capillary connection 18 connecting the tank and the evaporator and more particularly in the evaporator. This is schematically illustrated in FIG. 4. The presence of a capillary material 18 in the pipe 3 within the evaporator will cause a capillary pressure P _C - P _B (FIG. 5) on the vapor produced by Q _P in l 'evaporator. The temperature T _A of the tank being lower than that T _C at the level of the second part of the pipe a heat pipe will form between the evaporator and the tank.

The capillary link 18 will operate as a heat pipe if T _C reaches a temperature equal to or greater than the saturation temperature. Otherwise the channel 4 of the evaporator is filled with liquid and there is no risk of drying of the capillary material. If non-condensable gas is dissolved in the fluid carried by the capillary link, bubbles of non-condensable gases emerge from the liquid by the contribution of parasitic heat Q _P. The saturated steam produced at the capillary link at a temperature T _C higher than that T _A of the tank. It follows that the pressure P _C is greater than P _A at the reservoir. This difference in saturation pressure will cause the transport of the vapor and the non-condensable gas from the evaporator to the reservoir via the channel 4 formed by the second part of the pipe 3. The steam condenses on contact with the cooler fluid present. in tank 1. The non-condensable gas is transported to the tank by steam. The gas bubbles then escape to the top of the tank left free by the liquid.

The drying of the capillary link is caused by both the parasitic heat flow Q _P and the flow Q _E - Q _P. This drying causes capillary pumping pressures to arise which cause a depression of the liquid in the capillary link 18 and an overpressure of the gas and the vapor in the channel 4 relative to the reservoir 1 (P _B <P _A ). This pressure difference then causes pumping by the capillary link 18 of the fluid from the reservoir to the evaporator. It is therefore thanks to the fact that the temperature of the tank is lower than that of the evaporator that the non-condensable gas and the vapor produced by Q _p is transported to the tank.

To allow the circulation of the fluid in the loop, the pressure P _B at the inlet of the evaporator must be lower than the pressure P _E at the outlet of the evaporator. It is the porous material 5 which makes it possible to support this pressure difference thanks to the capillary pressure which it can generate. As the pressure P _A at the tank is dictated by the temperature T _A and the pressure P _E at the evaporator is dictated by its temperature T _E according to the saturation curve of the heat transfer fluid, it is thanks to the fact that the temperature of the tank is lower than that of the evaporator that the circulation of the fluid in the loop can be realized.

The gas and vapor flow in channel 4 against the current does not prevent the circulation of the fluid to the evaporator due to the presence of the capillary link 18.

The configuration of the capillary link 18 is preferably that described in Belgian patent n ° 903187. This configuration has the advantage of releasing bubbles from gas towards the center of the canal.

F: sortie de l'évaporateur

P_E - P_F: perte de charge au niveau de l'évaporateur

G: entrée du condenseur

P_F - P_G: perte de charge dans la ligne de vapeur

H: limite de condensation de la vapeur dans le condenseur

I: sortie du condenseur

T_H - T_I: baisse de température due à un sous-refroidissement

K: entrée du réservoir

T_K - T_I: augmentation de température dans la ligne du fluide vers le réservoir

P_I - P_A: chute de pression dans la ligne du fluide

T_J - T_I: diminution de température dans la ligne du fluide vers le réservoir

In FIGS. 5 and 6, the other temperature and pressure values will not be described in more detail since they represent known values of a capillary pumped heat transport loop. However for the sake of clarity the different points in the loop will be named:

F: evaporator outlet

P _E - P _F : pressure drop at the evaporator

G: condenser inlet

P _F - P _G : pressure drop in the steam line

H: steam condensation limit in the condenser

I: condenser output

T _H - T _I : temperature drop due to sub-cooling

K: tank inlet

T _K - T _I : temperature increase in the fluid line to the reservoir

P _I - P _A : pressure drop in the fluid line

T _J - T _I : decrease in temperature in the fluid line to the reservoir

Point J in Figure 6 represents a situation where the fluid has been further cooled before entering the tank. As illustrated in Figure 7 an auxiliary evaporator is connected to the line of fluid which connects the condenser 9 to the tank 1. All like evaporator 2, auxiliary evaporator 21 can be connected to the fluid line by a capillary link. he it is also possible to mount the auxiliary evaporator 21 on line 10 of fluid so that the fluid passes through the auxiliary evaporator.

The heat transfer fluid leaving the condenser and flows through the fluid line 10 is colder than the one at points 22 and 23 in the evaporator auxiliary 21. Thus the capillary link of the evaporator auxiliary works in a heat pipe in a similar way evaporator 2. The vapor bubbles are condensed in the laundry 10 and those of non-condensable gases are entrained by the circulation of the liquid towards the tank. This configuration has the advantage of avoiding a link capillary between the auxiliary evaporator and the tank without limiting the performance of the evaporator auxiliary. Therefore the distance between tank and evaporator is not limited.

Figure 8 shows a preferred example a capillary pumped heat transport loop according to the invention. The configuration of the evaporator assembly and tank compared to Figure 1 is more particularly dedicated to transport applications weightless heat for spacecraft.

The evaporator assembly comprises, according to the example, three evaporators 2, 31 and 32 connected in parallel. The capillary links 3 guarantee following the invention the supply of coolant tank 1 to the evaporators. During the ground tests, the coolant supply to evaporator B located slightly above the tank is carried out thanks to the capillary pumping pressure developed by the capillary link 3.

The heat flow q _e produces a vapor flow which is conveyed by the steam line 6 to the condensers 9 and 30. The heat flow q _e absorbed at the evaporators by vaporization of the heat transfer liquid is transferred to the condensers by condensation of the flow of steam.

The condensation formed on the walls of condensers is conveyed along capillary grooves 36 to the ends of the condensers. A structure capillary only allows the passage of condensed liquid to the liquid line 33.

Preferably, according to the invention, the tank 1 is thermally controlled by a cell thermoelectric (Peltier effect) 33. A sole 34 linking the Peltier cell to the evaporator 2 allows the supply or the extraction of thermal energy 35 from the reservoir to the evaporator. It is the Peltier 33 cell which performs the temperature difference between tank 1 and the sole 34 to direct the heat energy in the direction wish. The tank temperature control is thus realized. The pressure in the tank is dependent of the tank temperature following the curve of saturation of the heat transfer fluid and therefore the vaporization and condensation pressure and temperature in the loop is identical to that of the tank.

Tank 1 contains a structure capillary 37 in order to manage in weightlessness the localization heat transfer liquid vis-à-vis steam or gases non-condensable contained by the tank.

If non-condensable gas is generated in the loop, this will be collected by tank 1. Due to the partial pressure of non-condensable gas in the tank, its temperature must be maintained at a temperature below that of vaporization evaporators to maintain pressure equality between the tank and the rest of the loop.

A Peltier effect thermoelectric cell can also be applied to the auxiliary evaporator to cool the fluid line relative to the evaporator auxiliary. In this case the end of the link capillary connecting the auxiliary evaporator to the line of fluid is connected by the cell to the evaporator auxiliary. Cooling the fluid line as well obtained makes it possible to condense the steam produced by the flow heat supply to the auxiliary evaporator and limit the size of the non-condensable gas bubbles. A excessively large bubble size gas with respect to the speed of circulation of the fluid towards the tank could cause the draining line to drain fluid to the condenser and therefore interrupt the supply of liquid from the evaporator.

Claims (8)

1. A capillary pumped heat transfer loop comprising at least one evaporator, at least one condenser and a reservoir for storing a heat transfer fluid, said evaporator comprising an output connected by a vapour line to an input of the condenser, an output of the condenser being connected to the reservoir, said evaporator comprising an evaporator body and being provided with a porous material provided for producing a capillary pumping pressure inside the loop and applying that pressure on the heat transfer fluid starting from the surface of the material in contact with the evaporator body, said evaporator being also provided for evaporating the heat transfer fluid by heat absorption, characterised in that the reservoir and the evaporator are thermally isolated from each other and connected with each other by a conduit comprising a first part formed by a capillary link provided for pumping the heat transfer fluid from the reservoir towards the porous material and a second part provided for evacuating gas bubbles and/or vapour formed within the evaporator towards the reservoir, which reservoir is provided to be kept at a temperature inferior to the one of the evaporator.
2. A loop as claimed in claim 1, characterised in that within said conduit, which connects the evaporator to the reservoir, the first part comprises at least a first channel and the second part at least a second channel, the diameter of the first channel being smaller than the one of the second channel.
3. A loop as claimed in claim 1 or 2, characterised in that the conduit, which connects the evaporator to the reservoir extends along the central axis of the evaporator, said porous material of the evaporator being coaxially applied with respect to the conduit.
4. A loop as claimed in any one of the claims 1 to 3, characterised in that the reservoir is thermally linked to at least one of the evaporators by a thermo-electrical cell with Peltier effect, provided for regularising the temperature of the reservoir.
5. A loop as claimed in any one of the claims 1 to 4, characterised in that it comprises an auxiliary evaporator connected to a fluid line issuing the condenser.
6. A loop as claimed in claim 5, characterised in that said fluid line crosses the auxiliary evaporator.
7. A loop as claimed in claim 5, characterised in that said auxiliary evaporator is connected to the fluid line by a capillary link.
8. A loop as claimed in claim 7, characterised in that the extremity of the capillary link, which is in contact with the fluid line, is thermally linked to the auxiliary evaporator by a thermo-electrical cell with Peltier effect, provided for cooling the line with respect to the auxiliary evaporator.

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