Device for heat transport with two-phase fluid

Abstract

A heat transport device with a diphasic working fluid contained in a general closed circuit, includes an evaporator having a microporous body suitable for providing capillary pumping of liquid phase fluid; a condenser; a tank having an inner space, with a liquid portion and a gas portion; and an inlet/outlet arranged at the liquid portion, wherein the volume of the liquid portion can vary between a minimum volume and a maximum volume. The gas portion of the tank contains the vapor phase of the working fluid, at a first partial pressure, and a noncondensable auxiliary gas at a second partial pressure, wherein the second partial pressure is greater than the first partial pressure, at least when the liquid portion is at the minimum volume thereof.

Claims

1. A device for heat transfer, suited for extracting heat from a hot source and for returning this heat to a cold source via a two-phase working fluid contained in a general closed circuit, comprising: at least one evaporator, having an inlet and an outlet, at least one condenser, separate from and away from the evaporator, a tank with an internal volume , the internal volume including a liquid portion and a gas portion, and at least one inlet/outlet orifice laid out near the liquid portion, where the liquid portion has a volume that can vary between a minimum volume and a maximum volume, a first connection circuit for the working fluid in vapor phase, connecting the outlet of the evaporator to an inlet of the condenser; a second connection circuit, for the working fluid in liquid phase, connecting an outlet from the condenser to the tank and to the inlet of the evaporator; wherein the gas portion from the tank includes the vapor phase of the working fluid with a first partial pressure and a non-condensable auxiliary gas with a second partial pressure, where the second partial pressure is set to obtain a total pressure greater than or equal to a preset minimum operating pressure when the liquid portion in the entire general closed circuit is at the minimum volume, the device having no sensor and no active control, and hence operates in a purely passive operating mode, said device being configured to operate in a temperature range from -50 C. to +50 C., wherein: the internal volume of the tank is fixed and predefined during design for a given application; and the second partial pressure is at least several times greater than the first partial pressure when the liquid portion is at the minimum volume such that the minimum operating pressure is high enough to allow an instantaneous startup under a significant thermal load without preparation.

2. The device according to claim 1, wherein the non-condensable auxiliary gas is helium.

3. The device according to claim 1, wherein the working fluid is methanol.

4. The device according to claim 1, wherein the total volume of the tank is between 1.3 and 2.5 times said maximum volume of the liquid portion.

5. The device according to claim 1, mainly subject to terrestrial gravity, wherein the inlet/outlet orifice is arranged in an area of a low point of the tank.

6. The device according to claim 1, mainly subject to microgravity, wherein the tank comprises a porous mass laid out at least in an area of the inlet/outlet orifice.

7. The device according to claim 1, wherein the evaporator includes a micro-porous mass adapted for assuring capillary pumping of the liquid phase working fluid.

8. The device according to claim 1, subject mainly to gravity, wherein the evaporator is placed below the condenser and the tank, whereby gravity is used for moving the liquid phase working fluid towards the evaporator.

9. The device according to claim 1, wherein an anti-backflow valve is laid out at the inlet of the evaporator.

Description

(1) Other aspects, purposes and advantages of the invention will appear upon reading the following description of two embodiments of the invention, given as examples without limitation, with reference to the attached drawings in which:

(2) FIG. 1 shows a general view of a device according to an embodiment of the invention;

(3) FIG. 2 shows the fluids in a general phase transition diagram;

(4) FIGS. 3A and 3B show the tank with a respectively minimum and maximum liquid portion;

(5) FIG. 4 shows a second embodiment of the device;

(6) FIGS. 5A and 5B show diagrams of saturation pressure and temperature as a function of ambient temperature.

(7) In the various figures, the same references designate identical or similar items.

(8) FIG. 1 shows a device for heat transport with two-phase fluid loop. In the first embodiment, pumping is provided by drawing on the capillarity phenomenon. The device includes an evaporator 1, having an inlet 1a and an outlet 1b, and a microporous mass 10 adapted for providing capillary pumping. For this purpose, the microporous mass 10 surrounds a limited central longitudinal hollow 15 connected with the inlet 1a in order to receive the working fluid in the liquid state.

(9) The evaporator 1 is thermally coupled to a hot source 11, like for example an assembly comprising electronic power components or any other element generating heat, for example by resistive heating or by any other process.

(10) Under the effect of adding calories to the contact 16 of the liquid filled microporous mass, the fluid changes from the liquid state to the vapor state and leaves by the transfer chamber 17 and by a first connection circuit 4 which routes said vapor towards condenser 2 with an inlet 2a and an outlet 2b, where the condenser 2 is distinct and not adjacent to the evaporator 1.

(11) In the evaporator 1, the cavities cleared by the evaporated gas are filled by liquid aspirated by the microporous mass 10 from the aforementioned central hollow 15; it involves the well-known phenomenon of capillary pumping. The heat flow Q collected from the hot source corresponds to the flow rate multiplied by the latent heat of evaporation L of the working fluid (Q=L.Math.dM/dt).

(12) Inside of the condenser 2, heat is given off to cold source 12 by the fluid in vapor phase, which causes a cooling of the vapor phase fluid and phase transition thereof to the liquid phase, in other words condensation thereof.

(13) Near the condenser 2, the temperature of the working fluid is lowered below the liquid-vapor equilibrium temperature thereof, which is also called sub-cooling, such that the fluid cannot return to the vapor state without subsequent addition of heat.

(14) Vapor pressure pushes the liquid in the direction of the outlet 2b of the condenser 2 which opens into a second connection circuit 5, connected to the inlet 1a of the evaporator 1. A circulation loop of the two-phase fluid thus results that is capable of extracting heat from the hot source 11 and releasing this heat to a cold source 12.

(15) The heat transported by the vapor phase in the first connection circuit can be written Q=VS, where represents the density of the vapor phase, V the travel speed of the vapor phase and S the cross-section of the connection circuit.

(16) The second connection circuit 5 is also connected to a tank 3. This tank serves as an expansion vessel for the working fluid and contains working fluid in both liquid and gas phase. Along with the first and second connection circuits 4, 5 and the evaporator 1 and condenser 2, said tank forms a general, closed circuit otherwise referred to as hermetic.

(17) The tank 3 has at least one inlet/outlet orifice 31, and some inside volume 30 generally set during design for a given application. This volume could be adjustable by a manually or automatically maneuvered mechanical device. The tank also comprises a filling orifice 36 which is used for an initial filling of the circuit, where this filling orifice is closed the remainder of the time. It should be noted that the tank 3 can have an arbitrary shape, and in particular parallelepiped, cylindrical or other.

(18) The heat transfer device is designed in order to be able to operate in a certain ambient temperature range; in the example shown, this temperature range can be: [50 C., +50 C.]. Additionally, it is desirable that the hot source 11 not exceed a specific preset maximum temperature whatever the heat flux to be removed. This preset maximum temperature can for example be 100 C. Of course these temperatures can depend on the type of application targeted: space applications in microgravity, terrestrial applications on board a vehicle or in a fixed location.

(19) The working fluid of the loop is chosen in order to always be potentially two-phased in the temperature and pressure range of the fluid of the two-phase loop, based on the aforementioned temperature range (see reference 14, FIG. 2).

(20) Thus the working fluid can be chosen among a list including in particular ammonia, acetone, methanol, water, dielectric fluids of the HFE 7200 type or any other appropriate fluid. In the detailed example below, methanol will be preferably selected.

(21) A liquid portion 6 essentially comprising the working fluid (here methanol) in liquid phase and a gas portion 7 comprising the fluid in vapor phase, but also, as will be seen in detail later, a non-condensable auxiliary gas 8 are located inside the tank 3. The non-condensable auxiliary gas 8 (noted NCG, Non-Condensable Gas) remains confined in the gas portion of the tank without directly participating in the thermal exchanges; the effect thereof is creating a minimum pressure in this gas portion. The partial pressure of this non-condensable auxiliary gas 8 is written P2. Over the temperature and pressure range of the application, this non-condensable auxiliary gas remains in the gaseous state as will be seen in FIG. 2, on the right.

(22) It should be remarked here that according to the known prior art, the presence of non-condensable gas in the working circuit is undesirable because if bubbles of non-condensable gas get into the area of the capillary evaporator, this reduces the thermal performance of vaporization and can even go so far as loss of priming of the capillary evaporator, which can be catastrophic in certain critical applications.

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

(24) In a microgravity environment (weightlessness), the liquid portion is contained in a porous material and the gas portion occupies the remainder of the volume of the tank; in this case as well there is a liquid-vapor interface 19, but it is not planar.

(25) The temperature of this separation surface 19 is related one-to-one to the partial pressure P1 of the working fluid in the gas portion; this pressure corresponds to the saturation pressure Psat of the fluid at the prevailing temperature Tsat at the separation surface 19, as can be seen in FIG. 2, on the left.

(26) In practice, the temperature of the liquid portion, the gas portion and the envelope of the tank are relatively homogeneous; there is little or no temperature gradient inside the tank. Additionally the temperature of the tank is not far from the ambient temperature in which it is located.

(27) According to an advantageous aspect of the present invention, the inlet/outlet orifice 31 is laid out in the area of the liquid portion, such that the gas portion is never directly connected with the liquid connection circuit 5. The configuration of the capillary link between the tank and the porous mass can be like that described in the European patent EP 0832411.

(28) According to a particular aspect, in particular in the case of use in microgravity (scenario not shown in the drawings) but not exclusively, a porous mass 9 can be provided laid out in the area of the inlet/outlet orifice 31, whose function is to retain the liquid and consequently form a barrier blocking gas phase components from being aspirated towards the liquid connection circuit 5.

(29) In terrestrial applications where gravity operates, the inlet/outlet orifice 31 is arranged in the area of a low point of the tank. It should be noted that there can be several low points in the tank.

(30) The volume of the liquid portion 6 in the tank can vary between a minimum volume (Vmin) shown in FIG. 3A which corresponds to a minimum total volume of liquid in the entire general circuit and a maximum volume (Vmax) shown in FIG. 3B which corresponds to a maximum total volume of liquid in the entire general circuit.

(31) The difference between Vmax and Vmin is at least equal to the sum of two volumes which are called respectively expansion volume V0c and purge volume Vpurge which represent respectively first the thermal expansion of liquid and second the drainage of the liquid displaced by the presence of vapor in the vapor conduit 4 and of a portion from the condenser 2 of the loop. In other words, when the two-phase loop is at rest for some time, there is no more vapor in the loop and the liquid occupies all the volume inside the loop which gives a small liquid portion volume in the tank; inversely, when the thermal flow is maximal (Q=Qmax), the first connection circuit 4 is completely filled by the vapor along with a portion of the condenser circuit 2 and because of that the liquid is pushed back into the tank where it occupies a large volume. The volume of the liquid portion is also influenced by the ambient temperature, which results in the expansion volume V0c.

(32) More precisely, the minimum volume Vmin corresponds to a minimum ambient temperature and a zero thermal flow (Q=0) into the evaporator; this situation is shown in FIGS. 5A-5B by the points 61. Note that the dominant pressure in the gas portion is essentially due to the presence of auxiliary gas 8 (pressure P2) and not to the partial pressure P1 of the working fluid, which is very low. The total pressure exerted in the tank is Ptank=P1+P2; it is also substantially the pressure exerted everywhere else in the two-phase loop.

(33) Still without addition of heat to the evaporator (zero thermal flow, Q=0), but with a maximum ambient temperature, an expansion of the liquid is observed which gives a liquid portion volume written V0c, greater than Vmin. This situation is shown in FIGS. 5A-5B by the points 62.

(34) Under circumstances where the ambient temperature is maximal and the thermal flow is itself also maximal Q=Qmax, the volume of the liquid portion is increased by the volume corresponding to the purge Vpurge, which leads to the case shown in FIG. 3B. This situation is shown in FIGS. 5A-5B by the points 64.

(35) It can therefore be seen that, when the liquid portion 6 is at the minimum volume (Vmin) thereof which corresponds to a minimum total volume of liquid in the entire general circuit, the second pressure P2 is such that it is possible to get a total pressure in the tank greater than or equal to a required preset minimum operating pressure (shown at 0.7 bar in FIG. 5B as a nonlimiting example; in fact this minimum value can be set according to the application considered).

(36) It can also be seen that, in an illustrative example, when the liquid portion 6 is at the minimum volume (Vmin) thereof, the second partial pressure P2 (NCG) is greater than the first partial pressure P1. This condition continues to be satisfied over a major portion of the ambient temperature range at Q=0 and even over the cold temperatures when Q=Qmax.

(37) It can thus be observed that when the liquid portion is at the minimum volume (Vmin) thereof, the second partial pressure P2 (NCG) can be several times, for example 5 or 10 times, greater than the first partial pressure P1 (see point 61).

(38) The minimum pressure related to the presence of the non-condensable auxiliary gas in the tank (0.7 bar in the example from FIG. 5B) serves to get a high saturation temperature in the second connection circuit (50 C. in the example from FIG. 5A), which makes it possible to get a minimum density p of the vapor phase of the working fluid, and given that the heat transport capacity of the loop is proportional to the vapor phase density (Q=VS), instantly upon cold startup of the loop a heat transport capacity can be obtained sufficient to avoid loss of priming of the evaporator and to get a good loop yield.

(39) In order to keep satisfactory thermal evacuation performance in the most constraining thermal case (maximum ambient temperature and maximum thermal flux), shown by the points 64, it is necessary to provide a volume of the gas portion 7 sufficiently above the volume of the liquid portion Vmax.

(40) Advantageously, it can be provided that the total volume 30 of the tank is included 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 C. and a maximum flux Qmax, remains below 90 C., which allows continued collection of calories at the hot source 11.

(41) As for the selection of non-condensable auxiliary gas 8, this gas must remain in the vapor phase over the full operating range of the loop and in particular for the pressure and temperature conditions in the tank and it must have a very low boiling point; additionally the coefficient of diffusion thereof into liquids and the Oswald coefficient thereof must also be very low in order to avoid infiltration of this auxiliary gas inside the liquid portion 6 of the tank and into the remainder of the loop.

(42) Advantageously, helium can be selected as auxiliary gas. Helium is chemically neutral and its industrial availability is satisfactory. However, using other gases like nitrogen, argon or neon is not excluded.

(43) FIG. 4 shows a second embodiment of thermosiphon type, in which the condenser 2 is placed above the evaporator 1 such that gravity naturally drives the liquid in the direction of the evaporator; under these conditions the role of the porous material in the evaporator is to promote thermal exchanges and vaporization instead of performing the capillary pumping function itself. Apart from the source of liquid movement and the relative position of the elements which vary, all the rest and in particular the operation is identical to the first embodiment described above and will therefore not be repeated.

(44) Because of the pressurization exerted by the presence of the auxiliary gas 8, it is possible to do away with the presence of a heating element for conditioning the two-phase loop before effective thermal startup.

(45) It also needs to be remarked that such a two-phase loop can do without active regulation, which is a decisive advantage for reliability.

(46) Advantageously according to the invention, the device does not have any mechanical pump although the invention does not exclude the presence of a supplemental mechanical pump.

(47) It needs to be noted that the proportions of the elements in the drawings are not necessarily representative of the proportions or relative dimensions of the various members.

(48) The first and second fluid connection circuits 4, 5 are preferably tubular conduits, but it could be a matter of other types of fluid connection conduits or channels (e.g. rectangular conduits, flexible tubes, etc.). Similarly, the inlet/outlet orifice 31 could have the form of a distinct inlet and outlet.

(49) The two-phase loop could be advantageously equipped with an anti-backflow valve 18 located at the entry of each evaporator so as to increase the maximum startup power. In fact, the anti-backflow valve 18 blocks liquid backflow in the direction opposite to the normal circulation direction, and thus blocks drying of the evaporator on start up under heavy load.

(50) In an application subject to gravity, the anti-backflow valve can be formed by a floating element restored by a buoyancy force against a gate for closing the passage and thus blocking liquid backflow.

(51) It is remarked that advantageously according to the invention the two-phase fluid system presented here is completely self-adapting and does not require any command law or any sensor. The result of this is a particularly simple design, particularly simple manufacturing, an absence of maintenance needs and an incomparable reliability.