HEAT EXCHANGER WITH A TWO-PHASE WORKING FLUID

Description

Technical field

[0001] The present invention relates to a two-phase working fluid heat transfer device. The present invention also relates to a method for reducing the temperature of the working fluid contained in the liquid zone of the evaporator of such a two-phase working fluid heat transfer device. Prior art

[0002] Two-phase working fluid heat transfer devices are known from the prior art, adapted to extract heat from a hot source and to restore this heat to a cold source. To do this, a two-phase working fluid generally circulates, between an evaporator and a condenser, in a generally closed circuit, by means of capillary pumping produced by a capillary structure of the evaporator.

[0003] Furthermore, there is also provided, in the working fluid circuit, an available volume acting as an expansion tank, called in the trade ' reservoir '. Two distinct configurations are often used, namely LHP And CPL ; in the configuration LHP ("Loop Heat Pipe"), the reservoir is physically associated with the evaporator, whereas the configuration CPL ("Capillary Pumped Loop"), the reservoir is physically separated from the evaporator.

[0004] In the remainder of this document, we focus on the CPL (Capillary Pumped Loop) configuration. In this configuration, the evaporator and the reservoir are not thermally coupled.

[0005] During operation, part of the thermal power of the heat source is transferred to the evaporator structure and the capillary structure, thus producing an increase in the temperature of the working fluid in the liquid phase, contained in a so-called liquid zone of the evaporator. The working fluid in the liquid zone can then be in a state close to saturation or even in a superheated liquid state. The increase in temperature of the working fluid in the liquid zone can therefore produce evaporation of the latter. As a result, vapor bubbles form in the liquid zone of the evaporator.

[0006] The presence of such vapor bubbles in the liquid zone of the evaporator can lead to the drying out of the capillary structure, which is no longer supplied with liquid, and can lead to loss of priming of the device.

[0007] US 7,004,240-B1 and EP 2,291,067-A1 describe examples of heat transfer devices. Summary

[0008] The present invention limits, or even eliminates, the formation of vapor bubbles in the liquid zone of the evaporator.

[0009] According to the invention, as defined by the appended independent claim 1, there is provided a two-phase working fluid heat transfer device, comprising: an evaporator comprising a liquid zone (eg formed in particular by a main zone and / or a plurality of channels) and a vapor zone (egcomprising in particular a plurality of channels), a condenser having at least one inlet and at least one outlet, a first communication circuit, for working fluid essentially in vapor phase, connecting the vapor zone of the evaporator to at least one inlet of the condenser, a second communication circuit, for working fluid essentially in liquid phase, connecting at least one outlet of the condenser to the liquid zone of the evaporator, a reservoir containing working fluid with a liquid phase and a vapor phase, a selectively controlled energy supply element, adapted to increase the pressure of the working fluid contained in the reservoir, the reservoir being connected only to the liquid zone of the evaporator by means of a third communication circuit for working fluid essentially in liquid phase, a portion of the third communication circuit being thermally coupled to a first cold source.

[0010] Thus, when the temperature of the working fluid in the liquid zone of the evaporator increases, a flow of working fluid having a lower temperature is conveyed into the liquid zone from the reservoir, advantageously allowing the working fluid in the liquid zone to be cooled and any vapor bubbles to condense therein. Such a device also makes it possible to avoid undercooling of the working fluid at the condenser, which requires oversizing the condenser and also reduces the performance of the heat transfer device.

[0011] Under the term "liquid zone" of the evaporator, one or more volumes internal to the evaporator (channels or other forms), in communication with each other, and which may be in contact with a capillary pumping member. Under the term " area"steam" of the evaporator, one or more volumes internal to the evaporator (channels or other forms), in communication with a collector in the direction of an outlet, and which may be in contact with a capillary pumping member, the capillary pumping member being interposed between the volumes of the liquid zone and the volumes of the vapor zone.

[0012] The reservoir can be arranged at a distance from the evaporator, in particular in a separate thermal environment. Thus, advantageously, it is possible to thermally regulate the working fluid in the reservoir, independently of the thermal environment of the evaporator. Furthermore, the saturation temperature of the working fluid in the heat transfer device is imposed by the temperature of the working fluid in the reservoir. In other words, the regulation temperature of the device is imposed by the temperature of the working fluid in the reservoir. Thus, this makes it possible to reduce and maintain a regulation temperature of the device lower than the temperature of the environment in which the device is located. This also makes it possible to arrange the reservoir and the evaporator independently to improve the size of the device or to reduce possible parasitic heat flows with the ambient environment.

[0013] The second communication circuit can be connected to a first port of the liquid zone and the third communication circuit is connected to a second port of the liquid zone, the first port being located in the vicinity of a first end of the liquid zone and the second port being located in the vicinity of a second end of the liquid zone. In other words, advantageously, considering the evaporator, the first port and the second port are arranged at opposite positions of the liquid zone.

[0014] Thus, a supply of cooled working fluid, coming from the condenser on the one hand, and from the reservoir on the other hand, is formed in two opposite regions of the liquid zone. The cooling of the working fluid in the liquid zone is therefore more efficient if compared to a single connection configuration in the liquid zone. It is noted that under several operating conditions, the supply of cooled working fluid, coming from the reservoir through the third communication circuit, is at a temperature lower than Tsat.

[0015] The evaporator may comprise a capillary structure separating the liquid zone from the vapor zone, the capillary structure being adapted for capillary pumping of the working fluid in the liquid phase. Such a capillary structure allows for a passive pumping system, requiring little or no maintenance, and particularly suitable for space, aeronautical and railway applications.

[0016] The working fluid can be selected from methanol, acetone, water, ethanol, ethane, refrigerants, and ammonia. Such working fluids are well suited for heat transport in both gravity and weightlessness.

[0017] The energy supply element for the tank can be a heating element, for example an electrical resistor or a Peltier cell. Thus, such an energy supply element is easy to control, for example via an electrical controller.

[0018] The device may further comprise a first working fluid temperature sensor in the vicinity of a first end of the third communication circuit, the first end of the third communication circuit being in communication with the liquid zone of the evaporator. This enables reliable and autonomous closed-loop control.

[0019] The device may include a pressure sensor of the working fluid contained in the reservoir.

[0020] The device may further comprise a second temperature sensor of the working fluid contained in the second communication circuit, preferably close to the evaporator.

[0021] The device may further comprise a third working fluid temperature sensor adjacent a second end of the third communication circuit, the second end of the third communication circuit being in communication with the reservoir.

[0022] Such sensors allow for finer control, taking into account operating parameters at several positions of the device so as to take into account all possible scenarios.

[0023] The condenser is coupled to a cold source, referred to herein as a second cold source. In one aspect, the first cold source may be thermally independent of the second cold source.

[0024] According to another aspect, the first cold source may be thermally coupled to the second cold source. In other words, the first cold source may be thermally connected to the second cold source, for example via a thermal bridge.

[0025] According to another aspect, the condenser can be coupled to the first cold source. In this case, the first cold source and the second cold source are combined or closely linked. Thus, the first cold source allows both the condensation of the working fluid in the condenser and the cooling of the working fluid in the third communication circuit.

[0026] According to another aspect, there is described a method for decreasing the temperature of the working fluid contained in the liquid zone of the evaporator of the device according to any one of the preceding claims, the method comprising the steps: i. increasing the pressure of the working fluid contained in the reservoir by means of the energy supply element, to produce a flow of working fluid in the liquid phase in the third communication circuit, towards the liquid zone of the evaporator, ii. decreasing the temperature of the working fluid circulating in the third communication circuit towards the liquid zone of the evaporator, by heat exchange with the first cold source, wherein steps i and ii are preferably carried out simultaneously. Brief description of the drawings

[0027] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analyzing the attached drawings, in which: There Figure 1 represents a schematic general view of a heat transfer device; The Figure 2 represents a schematic general view of a variant of the heat transfer device of the Figure 1 ; There Figure 3 represents a schematic general view of the heat transfer device of the Figure 1 in which the energy supply element is in operation; The Figure 4 represents a schematic view of the control system of the heat transfer device of the figures 1 to 3 ; There Figure 5 represents a timing diagram illustrating the operation of the heat transfer device of the figures 1 to 3 . Description of the embodiments

[0028] There Figure 1represents a heat transfer device 1 with a two-phase working fluid. The working fluid may, for example, be ammonia, methanol, acetone, water, ethane, ethanol or any other suitable heat transfer fluid. The device generally comprises an evaporator 2 having an inlet and an outlet. The evaporator 2 further comprises a capillary structure 20 adapted to provide capillary pumping. The capillary structure 20 separates the evaporator 2 between a liquid zone 21 in communication with the inlet of the evaporator and a vapor zone 22 in communication with the outlet of the evaporator. Here, the inlet of the evaporator 2 forms a first orifice 24 of the liquid zone 21 of the evaporator. According to a particular exemplary embodiment, the liquid zone 21 may form a blind central recess surrounded by the capillary structure 20.According to another particular embodiment, the liquid zone 21 may in particular form a main zone and / or a plurality of channels. The vapor zone may form a plurality of channels separated by partitions of capillary structure or metal or glue. The capillary structure 20 may be a porous wick or a porous mass.

[0029] The evaporator 2 is thermally coupled to a heat source 23, for example an assembly comprising power electronic components or any other element generating heat Qh, in particular, by the Joule effect, or by any other process. The heat input Qh may, for example, be of the order of 6 to 7 kW.

[0030] Under the effect of the heat input Qh in contact with the capillary structure 20 filled with liquid, the working fluid passes from the liquid state to the vapor state and is evacuated through the vapor zone 22 (i.e. its channels). In the capillary structure 20 of the evaporator 2, the energy input in the form of heat Qh, coming from the hot source, is transferred to the vapor formed. A dissipative surface of the capillary structure is also continuously supplied with working fluid in the liquid phase; this is the phenomenon of capillary pumping.

[0031] The working fluid in the vapor phase is then conveyed, from the vapor zone 22, to a condenser 3 through a first communication circuit 11 comprising one or more lines. In the example shown, the first communication circuit 11 comprises a single line. The condenser 3 comprises at least one inlet and at least one outlet. The first communication circuit 11 then connects the outlet of the evaporator 2 to at least one inlet of the condenser 3. In the example shown, the condenser 3 comprises an inlet and an outlet.

[0032] Inside condenser 3, heat Qc is released by the working fluid in the vapor phase. This causes the working fluid to cool and the working fluid to change state from the vapor phase to the liquid phase. In other words, it causes the working fluid to condense.

[0033] The pressure of the vapor-phase working fluid pushes the liquid-phase working fluid in the condenser 3 toward a second communication circuit 12. The second communication circuit 12 connects the outlet of the condenser 3 to the inlet of the evaporator 2. The second communication circuit 12 essentially contains liquid-phase working fluid. The liquid-phase working fluid is thus conveyed to the liquid zone 21 of the evaporator 2. The device 1 thus forms a closed two-phase loop.

[0034] The device further comprises a reservoir 4. The reservoir 4 has an interior volume containing working fluid. The working fluid contained in the reservoir 4 has a vapor phase and a liquid phase. The temperature of the separation surface 41 between the liquid phase and the vapor phase in the reservoir 4, called the saturation temperature T SAT , determines the pressure in the two-phase loop. This pressure corresponds to the saturation pressure P SAT of the working fluid at the temperature prevailing at the separation surface by virtue of the one-to-one relationship which links the saturation pressure and the saturation temperature in saturation conditions.

[0035] The reservoir 4 is connected to the liquid zone 21 of the evaporator 2 via a third communication circuit 13. Remarkably, the reservoir 4 is only connected to the liquid zone 21 by the third communication circuit 13. In other words, the reservoir 4 is in fluid communication only with the evaporator 4, in its liquid zone 21. The third communication circuit 13 communicates with the liquid zone 21 of the evaporator via a second orifice 25 of the liquid zone 21. The third communication circuit is suitable for a flow of working fluid, in particular in the liquid phase, from the reservoir 4 to the liquid zone 21 of the evaporator 2. Such an arrangement allows flexibility in the design and integration of the device. Indeed, the reservoir 4 can be positioned freely in the device.In other words, the positioning of the reservoir 4 is not constrained by the positioning of the other elements of the device, nor by the orientation of the device and the presence or absence of gravity. In particular, the reservoir 4 can be positioned independently relative to the evaporator 2, in particular the reservoir 4 can be in a region thermally different from the region of the evaporator 2.

[0036] A portion of the third communication circuit 13 is thermally coupled to a first cold source 6. The first cold source 6 has a temperature at least 10°C lower than the saturation temperature of the working fluid. The first cold source can operate continuously. Here, the first cold source is thermally independent of a second cold source 30 of the condenser. The second cold source 30 of the condenser 3 is the main cold source for extracting the energy transferred to the working fluid by the hot source. In other words, the first cold source 6 and the second cold source 30 are distinct here. Thus, the cold sources can be respectively arranged in regions that are thermally different from each other. Also, the first cold source 6 can have a temperature different from the temperature of the second cold source 30.The first cold source 6 can, for example, be a cold plate.

[0037] Alternatively, the first cold source 6 and the second cold source 30 may be thermally coupled, for example via a thermal bridge between the first cold source and the second cold source.

[0038] Alternatively again, as shown in the Figure 2 , the condenser 3 and the third communication circuit 13 are thermally coupled to a single cold source 8. The size of the device can thus be reduced.

[0039] The device further comprises a selectively controllable energy supply element 5. The energy supply element 5 is adapted to increase the pressure of the working fluid contained in the reservoir 4. The energy supply element 5 is here a heating element, for example an electrical resistor or a Peltier cell.

[0040] As illustrated in the Figure 3, when the energy supply element 5 is in operation, the temperature of the working fluid contained in the reservoir 4 is increased by an energy supply in the form of heat Ores. The increase in the temperature of the working fluid contained in the reservoir 4 causes an increase in the pressure of the latter. The increase in pressure in the reservoir 4 then produces a flow of working fluid in the liquid phase in the third communication circuit 13, from the reservoir 4 to the liquid zone 21 of the evaporator 2. In particular, the energy supply element 5 is adapted to convey liquid, initially in the reservoir 4, into the liquid zone 21 of the evaporator. To do this, Qres can, for example, be taken, in comparison with Qh, in a range of values ​​between 1% and 6% of the value of Qh, advantageously between 2% and 3% of the value of Oh.

[0041] Alternatively, the energy supply element 5 may be a mechanical element adapted to increase the pressure of the fluid in the reservoir 4 in order to produce the same effect. For example, such an element may be a piston mechanism or a diaphragm mechanism, notably comprising a flexible membrane.

[0042] The working fluid circulating in the third communication circuit 13 is then cooled by a heat transfer Qrl with the first cold source 6, when it circulates in the portion of the third communication circuit 13 coupled to the first cold source 6. The working fluid is in particular cooled to have a temperature lower than the saturation temperature of the working fluid, preferably a temperature significantly lower than the saturation temperature.

[0043] Thus, working fluid having a temperature lower than the saturation temperature is conveyed into the liquid zone 21 from the reservoir 4. In this way, when the temperature of the working fluid in the liquid zone 21 is in a state close to saturation or even in a superheated liquid state, it is then cooled by the supply of cooled working fluid from the reservoir 4 and any vapor bubbles in the liquid zone 21 are condensed.

[0044] Also, as described above, the evaporator and the reservoir may be located in thermally different regions. For example, the evaporator may be in an environment having a temperature of 100°C while the reservoir may be in an environment having a temperature of 60°C. Thus, it is possible to impose saturation conditions of the working fluid in the evaporator at a temperature of 60°C while the environment in which the evaporator is located has a temperature of 100°C. In other words, it is possible to regulate the heat transfer device at a temperature of 60°C while the environment in which the evaporator is located has a temperature of 100°C. In addition, the supply of cooled working fluid from the reservoir, in the liquid zone of the evaporator, makes it possible to maintain these saturation conditions at 60°C in the evaporator.In fact, without the supply of cooled fluid, it would not be possible to maintain such regulation conditions in the evaporator because it is located in an environment with a temperature higher than the saturation temperature, in this example equal to 100°C.

[0045] Such a supply of cooled working fluid, coming from the reservoir, also makes it possible to avoid resorting to sub-cooling of the working fluid at the condenser 3 ("sub-cooling" in English) to cool the working fluid in the liquid zone. This has an advantageous effect because to obtain such sub-cooling, the second cold source must be capable of extracting more energy than the energy transported by the fluid to allow the temperature of the working fluid to be reduced below the saturation temperature. However, such sub-cooling at the condenser requires oversizing of the condenser, on the one hand, and a single-phase heat exchange in addition to a two-phase heat exchange, on the other hand, this reducing the efficiency of the two-phase loop.

[0046] Furthermore, the first orifice 24 is located in the vicinity of a first end of the liquid zone 21 and the second orifice 25 is located in the vicinity of a second end of the liquid zone 21. Thus, the working fluid coming from the condenser 3 and opening into the liquid zone 21 via the second communication circuit 12, opens into a first region of the liquid zone 21 located in the vicinity of the first end of the liquid zone 21. Similarly, the cooled working fluid coming from the third communication circuit 13 opens into a second region of the liquid zone 21 located in the vicinity of the second end of the liquid zone. The first region and the second region of the liquid zone are here opposite.The cooling of the working fluid in the liquid zone is thus improved compared to a configuration in which the working fluid from the reservoir and the working fluid from the condenser flow into the liquid zone through the same connection.

[0047] Alternatively, the first orifice 24 and the second orifice 25 may be located on the same side of the liquid zone 21 of the evaporator 2. In particular, the first and second orifices 24 and 25 may be adjacent.

[0048] The operation of the energy supply element 5 is intermittent. In other words, the energy supply element 5 is activated temporarily to temporarily cool the working fluid in the liquid zone 21. Indeed, it is a question of controlling the activation of the energy supply element 5 to cool the working fluid in the liquid zone 21 while maintaining the required performance of the transfer device.

[0049] There Figure 4 represents a diagram of the control system of the heat transfer device allowing the energy supply element 5 to be controlled. For this purpose, the device firstly comprises a first temperature sensor T1 of the working fluid in the vicinity of a first end of the third communication circuit 13, the first end of the third communication circuit 13 being in communication with the liquid zone 21 of the evaporator. Thus, the temperature recorded by the temperature sensor T1 is close to, or even equal to, the temperature of the working fluid located in the vicinity of the second orifice 25 in the liquid zone 21.

[0050] The device further comprises an electronic circuit 7. The first temperature sensor T1 is connected to the electronic circuit 7. The electronic circuit 7 is also connected to the energy supply element 5. The electronic circuit comprises a logic for controlling the activation of the energy supply element 5 according to the temperature detected by the first temperature sensor T1.

[0051] There Figure 5represents a timing diagram of such a control logic. When the temperature detected by the first temperature sensor T1 is greater than or equal to a maximum threshold temperature Tseuil_max, then the energy supply element 5 is activated. From then on, a quantity of heat Qres is transmitted to the working fluid contained in the reservoir 4, thus forming a flow of working fluid in the third communication circuit 13 towards the liquid zone 21 of the evaporator 2. The flow of working fluid in the third cooling circuit has a flow rate D. The working fluid opening into the liquid zone 21 then cools the working fluid contained in the working zone, resulting in a reduction in the temperature detected by the first temperature sensor T1 in response to the formation of the flow rate D.When the temperature detected by the first temperature sensor T1 is less than or equal to a minimum threshold temperature Tseuil_min, then the energy supply element is deactivated. The energy supply element 5 is activated for a first time period τ1.

[0052] The temperature of the working fluid in the liquid zone 21 can increase again. The energy supply element 5 is then activated again for a second time period τ2. The second time period τ2 can be different from or equal to the first time period τ1.

[0053] The device may further comprise other sensors to improve the activation control logic of the energy supply element 5, in particular to determine more precisely the activation and deactivation times of the energy supply element. In this way, the reduction in the performance of the device due to the increase in pressure and temperature of the working fluid in the tank is limited or even prevented.

[0054] For this purpose, the device may comprise a pressure sensor P of the working fluid contained in the reservoir 4. It is thus possible to determine and control the flow rate D of the flow of working fluid produced in the third communication circuit 13. Furthermore, the pressure indication inside the reservoir 4 also makes it possible to obtain an indication of the temperature inside the reservoir 4 due to the one-to-one relationship which links the pressure and the temperature of the working fluid in saturation conditions. An indication of the saturation pressure and the saturation temperature then makes it possible to determine the operating conditions of the device. Alternatively, a temperature sensor may be provided inside the reservoir 4.

[0055] The device may also comprise a second temperature sensor T2 of the working fluid contained in the second communication circuit 12. Such a second temperature sensor T2 makes it possible in particular to compare the working fluid in liquid phase having been cooled by the condenser and the temperature of the working fluid recorded by the first temperature sensor T1. In this way, it is possible to evaluate the capacity of the cooled working fluid coming from the condenser to reduce the temperature of the working fluid in the liquid zone.

[0056] The device may also comprise a third temperature sensor T3 of the working fluid in the vicinity of a second end of the third communication circuit 13, the second end of the third communication circuit being in communication with the reservoir.

[0057] Furthermore, the description and figures are devoid of directional indications in that the device is operable regardless of the orientation of gravity or in the absence of gravity.

Claims

1. Two-phase working fluid heat transfer device (1), comprising: - an evaporator (2) including a liquid zone (21) and a steam zone (22), - a condenser (3) having at least one inlet and at least one outlet, - a first communication circuit (11), for working fluid essentially in the vapour phase, connecting the vapour zone (22) of the evaporator (2) to at least one inlet of the condenser (3), - a second communication circuit (12), for working fluid essentially in the liquid phase, connecting at least one outlet of the condenser (3) to the liquid zone (21) of the evaporator (2), - a reservoir (4) containing working fluid with a liquid phase and a vapour phase, - an energy supply element (5), with selective control, adapted to increase the pressure of the working fluid contained in the reservoir (4), the reservoir being connected only to the evaporator (2), in its liquid zone (21), by means of a third communication circuit (13) for working fluid essentially in liquid phase, a portion of the third communication circuit being thermally coupled to a first cold source (6).

2. Device according to claim 1, wherein the second communication circuit (12) is connected to a first orifice (24) of the liquid zone (21) and the third communication circuit (13) is connected to a second orifice (25) of the liquid zone (21), the first orifice (24) being located in the vicinity of a first end of the liquid zone (21) and the second orifice (25) being located in the vicinity of a second end of the liquid zone (21).

3. Device according to either one of claims 1 or 2, wherein the evaporator (2) includes a capillary structure (20) separating the liquid zone (21) from the vapour zone (22), the capillary structure (20) being adapted for capillary pumping of the working fluid in the liquid phase.

4. Device according to any one of the preceding claims, wherein the working fluid is selected from methanol, acetone, water, ethanol, ethane, a refrigerant fluid, and ammonia.

5. Device according to any one of the preceding claims, wherein the energy supply element (5) is a heating element, for example an electrical resistor or a Peltier cell.

6. Device according to any one of the preceding claims, furthermore comprising a first sensor (T1) for the temperature of the working fluid in the vicinity of a first end of the third communication circuit (13), the first end of the third communication circuit (13) being in communication with the liquid zone (21) of the evaporator (2).

7. Device according to claim 6, the device furthermore comprising at least one of: - a pressure sensor (P) of the working fluid contained in the reservoir (4), - a second sensor (T2) for the temperature of the working fluid contained in the second communication circuit (12), - a third sensor (T3) for the temperature of the working fluid in the vicinity of a second end of the third communication circuit (13), the second end of the third communication circuit (13) being in communication with the reservoir (4).

8. Device according to any one of the preceding claims, wherein the condenser (3) is coupled to a second cold source (30), the first cold source (6) being thermally independent of the second cold source (30).

9. Device according to any one of claims 1 to 7, wherein the condenser (3) is coupled to a second cold source (30), the first cold source (6) being thermally coupled to the second cold source (30).

10. Method for lowering the temperature of the working fluid contained in the liquid zone (21) of the evaporator (2) of the device according to any one of the preceding claims, the method comprising the steps of: i. increasing the pressure of the working fluid contained in the reservoir (4) by means of the energy supply element (5), to produce a flow of liquid-phase working fluid in the third communication circuit (13), toward the liquid zone (21) of the evaporator (2), ii. decreasing the temperature of the working fluid circulating in the third communication circuit (13) toward the liquid zone (21) of the evaporator (2), by heat exchange with the first cold source (6), wherein steps i and ii are preferably performed simultaneously.