Heat transfer device using capillary pumping

A capillary-driven heat transfer device is adapted to extract heat from a heat source and release this heat to a cold source using a two-phase working fluid. The device includes an evaporator having a microporous mass performing capillary pumping of fluid in the liquid phase, a condenser, a reservoir having an inner chamber and an inlet and/or outlet port, a vapor communication circuit, connecting the outlet of the evaporator to the inlet of the condenser, a liquid communication circuit, and a non-return device arranged between the inner chamber of the reservoir and the microporous mass of the evaporator, and arranged to prevent liquid present in the evaporator from moving to the inner chamber of the reservoir.

The present invention relates to capillary-driven heat transfer devices, in particular two-phase fluid loop passive devices.

It is known from document FR-A-2949642 that such devices are used as a means to cool electrotechnical power converters.

However, it has appeared that the startup phases were especially subject to problems in the presence of high thermal power levels, drying-out of the capillary wick may occur resulting in startup failure.

There therefore appeared a need to increase the reliability of the startup and operation of such loops.

To this end, the invention relates to a capillary-driven heat transfer device, adapted to extract heat from a heat source and to release 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, and a microporous mass adapted to perform capillary pumping of fluid in the liquid phase

at least one condenser, having an inlet and an outlet,

a reservoir having an inner chamber and at least one inlet and/or outlet port,

a first communication circuit, for fluid mainly in the vapor phase, connecting the outlet of the evaporator to the inlet of the condenser,

a second communication circuit, for fluid mainly in the liquid phase, connecting the outlet of the condenser to the reservoir and to the inlet of the evaporator,

characterized in that it includes a non-return device arranged between the inner chamber of the reservoir and the microporous mass of the evaporator, and arranged to prevent liquid present in the evaporator from moving into the inner chamber of the reservoir, the device being mainly under the influence of gravity, the non-return device including a float returned by buoyancy thrust to a seating in the closed state.

Thanks to these arrangements, liquid is prevented from returning from the evaporator in the direction of the reservoir. In this way, startup under strong thermal load is made more reliable. Moreover, the float is able to let gas bubbles pass through thus avoiding the formation of a gas lock; furthermore, the non-return device is simple and reliable and in addition it can let vapor or gas bubbles pass through.

In various embodiments of the invention, one and/or the other of the following arrangements les optionally be applied:

the float presents a lower density than the density of the fluid in the liquid phase, and comprised between 60% and 90% of the density of the fluid in the liquid phase; whereby the non-return device does not hinder the capillary pumping;

the float is made of stainless steel; such that its durability is extremely good;

the non-return device is arranged in the second fluid communication circuit; such that it can be independent of the reservoir and of the evaporator;

the non-return device is arranged in the lower area of the reservoir; such that it can be combined with the reservoir;

the non-return device is arranged in the upper area of the evaporator; such that it can be combined with the evaporator;

the fluid communication circuit is a tubular conduit; such that its cost is moderate;

the inlet/outlet port is arranged in the lower area of the reservoir, preferably in the lower side area of the reservoir;

the second fluid communication circuit can be in the form of a single conduit with a T coupling or of two independent conduits;

the reservoir includes an input stream deflector near the inlet port; whereby a mixing effect due to the input stream can be avoided;

the reservoir includes a plurality of separate volumes remaining in fluid communication; whereby mixing of the volume of liquid contained in the reservoir is limited;

the reservoir includes a plurality of inner partitions forming compartments adapted to separate said multiple separate volumes;

the plurality of inner partitions forms a compartment structure in the form of a honeycomb; such that the cost-effectiveness ratio is optimised;

the heat transfer device preferentially is deprived of a mechanical pump; such that its reliability is increased;

the device includes in addition an energy-providing element at the reservoir to control the pressurisation of the loop during startup; such that the startup of the loop can be made more reliable.

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

FIG. 1shows a capillary-driven heat transfer device, with a two-phase fluid loop. The device includes an evaporator1, with an inlet1aand an outlet1b, and a microporous mass10adapted to perform capillary pumping. For this purpose, the microporous mass10surrounds a blind central longitudinal recess15communicating with inlet1ain order to receive working fluid9in a liquid state from a reservoir3.

The evaporator1is thermally coupled with a heat source11, such as for example an assembly comprising electronic power components or any other element generating heat, by Joule effect for example, or by any other means.

Under the effect of the supply of calories at the contact16of the microporous mass filled with liquid, fluid passes from the liquid state to the vapor state and is evacuated through the transfer chamber17and through a first communication circuit4which conveys said vapor to a condenser2which has an inlet2aand an outlet2b.

In the evaporator1, the evacuated vapor is replaced by the liquid drawn in by the microporous mass10from the aforementioned central recess15; this is the capillary pumping phenomenon as is well known per se.

Inside said condenser2, heat is released by the fluid in the vapor phase to a cold source12, which causes cooling of the vapor fluid and its phase change to the liquid phase, that is to say its condensation.

At condenser2, the temperature of the working fluid9is lowered below its liquid-vapor equilibrium temperature, which is also known as subcooling, such that the fluid cannot revert to the vapor state without a significant heat input.

The vapor pressure pushes the liquid in the direction of outlet2bof the condenser2which opens onto a second communication circuit5, which is also connected to the reservoir3.

The reservoir exhibits at least one inlet and/or outlet port31, here in the case ofFIG. 1a separate inlet port31aand outlet port31b, and the reservoir3presents an inner chamber30, filled with the heat transfer fluid9. The working fluid9can be ammonia for example or any other appropriate fluid, but methanol is a preferential choice. The working fluid9is a two-phase fluid and is present partly in the liquid phase9aand partly in the vapor phase9b. In an environment where gravity is exerted (vertically according to Z), the gas phase part9bis situated above the liquid phase part9aand a separation surface19separates the two phases.

It is the temperature of this separation surface19which determines the pressure in the loop, this pressure corresponds to the saturation pressure of the fluid at the temperature prevailing at the separation surface19.

At the base of the reservoir34, the temperature of the liquid is generally lower than the temperature prevailing at the separation surface19.

For correct operation of the capillary-driven loop, it is necessary to avoid a rapid change in the temperature prevailing at the separation surface19, and to avoid in particular mixing of the liquid phase9awhich tends to draw cold liquid from the bottom of the reservoir to the top and therefore make the surface temperature decrease, and with it the pressure also.

The first and second fluid communication circuits4,5are preferably tubular conduits, but they could be other types of conduits or fluid communication channels.

Likewise, the second fluid communication circuit5can be in the form of two separate and independent conduits5a,5b(cf.FIG. 1) or a single conduit with a T coupling5c(cf.FIG. 2).

In all cases, the second fluid communication circuit5connects the condenser outlet2bto the evaporator inlet1a, either indirectly by passing through the reservoir (in the case of two independent conduits) or directly (in the case of a single conduit with a T coupling).

According to the invention, the device includes a non-return device6, arranged between the inner chamber30of the reservoir and the microporous mass10of the evaporator1, to prevent liquid present in the evaporator from moving back into the inner chamber30of the reservoir. This non-return device6allows to avoid the return of liquid from the evaporator in the direction of the reservoir. An even limited return of liquid from the evaporator in the direction of the reservoir could cause local drying-out of the microporous mass which can lead to depriming of the pumping action of the two-phase loop, which is prevented by said non-return device6. This phenomenon is all the more pronounced if the power at startup is high (several kW and/or several tens of Watts per cm2). The non-return device6thus allows to increase the performance of the system at startup.

The position of said non-return device6can be chosen from a number of particularly useful locations depending on the pursued goal and the optimization pursued.

InFIG. 1, the non-return device6is positioned on conduit5bconnecting the reservoir to the evaporator1. In this way, the non-return device6can be inserted into a two-phase loop where the evaporator and the reservoir are given components that it is difficult to modify.

Furthermore, said non-return device6can be positioned, as shown inFIG. 2, adjacent to the evaporator1, such that said non-return device6can be combined with the evaporator, which allows to optimize the footprint the system.

In addition, said non-return device6can be positioned, as shown inFIG. 3, adjacent to the reservoir, such that said non-return device6can be combined with the reservoir as will be described in detail hereafter, which allows to optimise the footprint of the system.

Preferentially, this non-return device6can include a float60with a density which is slightly lower than the density of the fluid in the liquid phase, the float coming fully onto a seat in order to close the passage of liquid, as will be explained hereafter.

However this non-return device6can also take the more classic form of a non-return valve (not represented in the figures), with a shutter, a valve seat and an elastic return spring tending to push said shutter towards the valve seat. However, the strength of the elastic return spring must only be moderate so as not to counter too strongly the aforementioned capillary pumping force.

When the non-return device6is presented as a float, and as shown inFIGS. 4aand 4b, a unit forming a float60is arranged inside a hollow body63wherein the float60can move at least in a so-called longitudinal direction. The longitudinal direction coincides here with the direction Z wherein buoyant force and gravity are exerted.

In the example shown, the hollow body and the float exhibit rotational symmetry around this Z axis, but this could however be otherwise.

The float comprises an annular bearing surface67which comes to press against a corresponding annular seating66forming a shoulder directed radially inwards in the hollow body63. When the float is pressing against the seat66, the upstream space64of the second communication circuit5is isolated from the downstream space65of the second communication circuit5, which corresponds to the closed state.

As shown inFIG. 4a, when the loop is in established operation, the capillary pumping exerts a suction effect which establishes a slightly lower pressure in the downstream space, and this suction effect S draws the float downwards. The passage of liquid at the level of the seat66is then open and liquid can flow from upstream64to downstream65.

It should be noted that, if non-condensable vapor or gas bubbles are found in said liquid in the downstream part65, they can escape in the opposite direction (from downstream to upstream) which allows to avoid blocking the feeding of the evaporator with fresh liquid: the float is therefore able to let gas bubbles pass and thus avoid the formation of a gas lock, this function can also be called a degassing function.

According to an advantageous aspect of the invention, the float exhibits a lower density than the density of the fluid in the liquid phase, and comprised between 60% and 90% of the density of the fluid in the liquid phase (at a maximum temperature in the order of 100° C. for example). In this way, the resultant of the weight and of the buoyant force give a pushing force P directed upwards.

The intensity of this pushing force P must however be moderated to be lower than the suction effect of the aforementioned capillary pumping action.

In a transitional configuration, in particular during an initial startup or in the case of a sudden increase in the thermal load to be evacuated, a sudden increase in the generation of vapor in the evaporator tends to push the liquid contained in the cavity15back in the direction of the reservoir. This must be avoided in order to prevent drying-out of the microporous mass (also known as wick) which would deprime the loop.

As shown inFIG. 4b, in the event of liquid flowing from the cavity15of the evaporator, a pressure force F directed upwards has the effect of pushing the float60fully against the seating66and of thus closing the passage of liquid. Consequently, any reflux of liquid in the direction of the interior30of the reservoir is avoided.

In a particularly advantageous configuration where the non-return device6is arranged in the lower area of the reservoir, the non-return device6is arranged in the base of the reservoir, at the level of the outlet port31b(cf.FIGS. 3 and 5). In this case, the body63includes a collar68which is solidly fixed to the base37of the reservoir by well-known attachment means. Moreover, the base37at the level of the port31bcan be used directly as a shutting seat66.

According to the invention, the float can be made of stainless steel such that its durability is extremely good. As shown inFIG. 6, the float60can be made in the form of two half-shells61,62welded to each other at the level of a diameter by means of a weld68; the two half-shells61,62thus define an inner chamber89filled with preferably inert air or gaz. The thickness of the walls of the two half-shells61,62as well as the size of the inner chamber89are chosen to obtain the desired density for the overall float assembly60.

In addition, with a view to avoiding mixing phenomena inside the reservoir which are conducive to the cold shock phenomenon, there can be provided inside the reservoir, and as shown inFIGS. 7a-7b, multiple separate volumes separated from each other, said separate volumes remaining in fluid communication. In particular, and more precisely, in the reservoir there can be arranged a plurality of inner partitions7configured in order to separate said multiple separate volumes.

Moreover, advantageously according to the invention, the reservoir can include an input stream deflector8near the inlet port31aor the inlet/outlet port31depending on the configuration of the second conduit.

This input stream deflector8prevents a rapid surge of liquid in the reservoir from creating a bubbling phenomenom or a stream current likely to favour mixing of the liquid. It can exhibit the form of a U section oriented downwards, or of a bowl or of any other shape creating a sufficient deviation of the trajectory of the input stream.

The compartment structure71can present vertical partitions7, i.e. oriented in the direction of gravity. It should be noted however that the partitions can just as well be slightly or substantially inclined, as illustrated for example inFIG. 7a.

Advantageously, it is possible to choose a honeycomb structure with a hexagonal mesh.

It should be noted that the reservoir can have any shape, and in particular be parallelepiped or cylindrical. Moreover, the compartment structure can be made of stainless steel.

According to one aspect of the present invention, said multiple separate volumes communicate through passages with a small cross-section, preferably less than 1/10 of the largest cross-section du reservoir.

According to another advantageous aspect of the invention, the compartment structure can comprise a phase change material providing thermal inertia to said structure which helps to limit abrupt temperature variations.

FIGS. 7aand 7bshow that it is possible in the context of the present invention to have several evaporators1in parallel with each other to increase their capacity to evacuate calories and/or to position the evaporators as closely as possible to the heat sources.

According to the configuration inFIG. 7a, each evaporator has a non-return device6in its specific liquid supply circuit, whereas according to the configuration inFIG. 7b, the non-return device6is positioned in the shared branch5dupstream from the distribution5e,5fto the evaporators, which allows to mutualise the non-return device6and thus optimise the cost of a system with several evaporators.

Furthermore, the device may further include an energy-providing element36, for example a heating element or a pressuriser element, located at the reservoir to control the pressurisation of the loop during startup. A “Ctrl” control system38manages, in the case of a heating element, the supply of calories on this heating element36, according to temperature information and/or pressure information delivered by sensors (not shown), this being in order to ensure startup of the two-phase loop. Moreover, this “Ctrl” control system can also prepare the two-phase loop for an imminent and significant arrival of calories on the evaporator, which allows to anticipate the reaction of the two-phase loop with regard to the need for thermal dissipation. Sizing of the loop can thus be optimised for large amounts of heat to be evacuated.

Advantageously according to the invention, the device does not require the use of a mechanical pump even though the invention does not exclude the presence of an auxiliary mechanical pump.