CA2657777A1 - Heat-flow device - Google Patents

Abstract

A device comprises equipment (501) with a heat source having a maximum thermal operation condition, a cold part (502) relative to the equipment and an element (503) capable of transmitting the heat from the equipment to the cold part. The element (503) is capable of causing the transmitted heat to be limited for thermal conditions above a specified threshold below said maximum condition.

Description

Heat-flow device The invention relates to a heat-flow device.
In such a device it is sought to evacuate the thermal energy (or heat) dissipated in an equipment item by a heat source of any kind (such as an electrical circuit or an electronic component).
This is traditionally achieved by connecting the equipment item, by means of a heat-conducting member, to a relatively colder part, which acts as a cold source.
Thus an amount of heat flows across the conductive member, with a power inversely proportional to the thermal resistance thereof, thus making it possible to evacuate at least part of the heat generated within the equipment item and thus to avoid overheating it.
US Patent Application 2003/0196787, for example, uses this technique and also proposes, for reasons related to the operation of the equipment item, to reduce such evacuation of heat at low temperature.
The inventors have noted that these solutions could present risks in practice, especially when the part constituting the cold source is not adapted to all conditions of temperature and/or of dissipated thermal power, as is the case, for example, when this cold part is formed from a material that is combustible or sensitive to temperature elevations.
In order to avoid such problems, the invention proposes a device comprising an equipment item with a heat source having a maximum thermal operating condition, a part relatively colder than the equipment item and a member capable of transmitting the heat from the equipment item to the cold part, characterized in that the member is capable of causing a limitation of the transmitted heat under thermal conditions that exceed a defined threshold below the said maximum condition.

SUBSTITUTE SHEET (RULE 26) In this way the heat generated within the equipment is no longer totally transmitted (or even almost no longer transmitted) to the cold part when these thermal conditions are encountered (or in other words, for example, when the temperature or the thermal power transmitted across the member exceeds the said threshold), and so overheating of the said cold part is avoided.
The thermal conditions correspond, for example, to a thermal power transmitted across the member. In this case, the member may limit the transmitted thermal power to the value of the said defined threshold.
The equipment item and the cold part may additionally be separated substantially by a gas screen, at least under the said thermal conditions, in order that the transmission of electrical phenomena (such as electrical arcs), especially the propagation of electrical arcs from the equipment item to the cold source, can also be avoided under these conditions.

The equipment item and the cold part are, for example, separated by the said screen regardless of the thermal conditions, and the member may then comprise a heat pipe passing through the said screen.

In this context, an advantage is taken of the limitation, beyond a certain threshold, of the thermal power that the heat pipes can transmit, in order to limit the thermal power transmitted by the member to this threshold.

According to another possible solution, the member comprises at least one component whose change of state (for example from the liquid state to the gas state) under the said thermal conditions causes an increase of the thermal resistance, also making it possible to limit the amount of heat transmitted.
In this case, advantage is taken of the increase in thermal resistance generally associated with such a change of state. The component may then form the said screen after the said change of state, which is a practical way of obtaining this screen.

According to another conceivable embodiment, the member is configured to lose contact with the equipment item or the cold part under the said thermal conditions. In this case it is the breaking of contact between the different components that causes the interruption of the heat path between the equipment item and the cold part and consequently the limitation of the transmission of heat.

The member in this case comprises, for example, at least one component whose change of state under the said thermal conditions causes the said loss of contact.

In this context it is possible to provide that the said component participates in conduction from the equipment item to the cold part outside the said thermal conditions, and disappears due to its change of state under the said thermal conditions, thus substantially insulating the equipment item and the cold part.

According to another approach, which may be combined if applicable with the foregoing, the change of a mechanical property of the component during its change of state may lead to a movement of part of the member, thus causing the said loss of contact.

In this case also, the member may be configured in such a way that the change of state of the component makes it possible to form the said gas screen.
The change of state then makes it possible not only to interrupt the thermal path but also to prevent the propagation of electrical phenomena.

In this context the change of state may be a transition from the solid state to the liquid state or a transition from the liquid state to the gas state.

The equipment may be a fuel pump and the cold part a liquid fuel, for example in an aircraft; the invention is particularly interesting in this context, although it naturally has numerous other applications, such as protection against overheating of members of heat sinks that are sensitive to temperature elevations, such as carbon structures.

The arrangements proposed hereinabove, some of which are optional, thus make it possible in particular to evacuate the heat produced by the equipment items, such as electronic components as in the case of fuel pumps, while avoiding overheating of the heat sink (such as the fuel), by virtue of the limitation of the transmitted heat, as well as propagation of electrical arcs from the equipment items to this sink.

The invention also proposes an aircraft equipped with such a device.
Other characteristics and advantages of the invention will become evident in light of the description hereinafter with reference to the attached drawings, wherein:

- Figs. 1A to 1C represent a first exemplary embodiment of the invention;

- Figs. 2A to 2C represent a second exemplary embodiment of the invention;

- Figs. 2D to 2F represent a variant of the second example presented in Figs. 2A to 2C;

- Figs. 3A to 3C represent a third exemplary embodiment of the invention;
- Figs. 4A to 4C represent a fourth exemplary embodiment of the invention;

- Figs. 5A and 5B represent a fifth exemplary embodiment of the invention.
Fig. 1A represents a first exemplary embodiment of the invention under normal operating conditions.

In this example, a hot plate 101 comprising a heat source (not illustrated) is connected to a cold plate 102 (such as a structural part of the device) by means of a material 103 that is solid at the nominal temperature TnOm;nai corresponding to normal operation.

Material 103 is a heat conductor, and its thermal resistance Rmarer;ai is therefore relatively low. Thus the heat generated by the heat source within hot plate 101 is evacuated under normal operating conditions across material 103 to cold plate 102, which acts as a heat sink or cold source.

Material 103 is also chosen such that its melting temperature Tmeit;n9 is lower than or equal to the desired maximum operating temperature Tmax. Such a maximum temperature may be desired, for example, to avoid degradation of cold plate 102 or other negative consequences, such as, for example, a risk of fire when the cold plate is made in the form of a combustible material, such as the fuel of an aircraft.

Thus, as represented in Fig. 1 B, when the temperature T of material 103 attains the melting temperature Tmeiting of material 103, for example due to a departure from normal operating conditions, the said material changes state:
material 103 passes from the solid state to the liquid state (represented by reference 103' in Fig. 1 B), which leads to its disappearance (in this case its flow via appropriate means) from its initial position in contact with hot plate 101 and cold plate 102.

Because of this fact, when the temperature between plates 101, 102 is higher than the desired maximum temperature Tmax, hot plate 101 and cold plate 102 are no longer connected by the material but are separated by an air screen 106, whose thermal resistance Rair is very much greater than that of the material Rmaterial, as represented in Fig. 1 C.

Cold plate 102 is then thermally insulated from hot plate 101 by virtue of air screen 106 separating them; this screen also acts as an electrical insulator, which also makes it possible to prevent transmission of electrical energy (for example, in the form of electrical arcs) from the hot plate to cold plate 102.
This latter advantage is particularly interesting in the case in which hot plate 101 is provided with an electrical or electronic equipment item whose potential malfunctions could prove dangerous to cold plate 102, especially when this has attained a temperature above the desired maximum temperature Tmax.

Wax is used, for example, as material 103, since its thermal properties permit heat conduction clearly greater than that permitted by the thermal resistance of air 106.

Fig. 2A represents a second exemplary embodiment of the invention under normal operating conditions, that is, for example, at an operating temperature Tnom;na, clearly lower than a desired maximum temperature.

In this example, an equipment item 201 comprising a heat source is situated at a distance from a cold plate 202 and is consequently separated from it by an air screen 206. Furthermore, equipment item 201 is connected to cold plate 202 by means of a heat drain 203 formed in a material that is a good heat conductor (that is having low thermal resistance) and that therefore extends partly into the space formed by air screen 206.
Heat drain 203 is maintained in contact with cold plate 202 by interposition of a bonding material 204 in solid state between a part of equipment item 201 and conducting drain 203. Furthermore, a compression spring 205 is interposed between drain 203 and cold plate 202, spring 205 being compressed when drain 203 is in contact with cold plate 202.

Drain 203 is connected to equipment 201, on the one hand across bonding material 204 and on the other hand directly at parts of equipment item 201 other than those receiving bonding material 204, for example at a side wall 208 of equipment item 201.

When the temperature in bonding material 204 rises beyond the normal operating conditions and attains the melting temperature Tmeit;n9 of bonding material 204, the latter passes from the solid state to the liquid state (as represented in Fig. 2B, in which the bonding material in liquid state is represented by reference 204'), and flows away from the device via appropriate means.

Because of this fact, drain 203 is no longer maintained in contact with cold plate 202 but instead is moved away under the action of spring 205. Because of the displacement of drain 203 and its loss of contact with cold plate 202, equipment item 201 and cold plate 202 are separated by the thickness (or screen) of air 206, except for spring 205, whose thermal conductivity is negligible, and these two members are therefore substantially insulated by means of air screen 206, as represented in Fig. 2C.

Fig. 2D represents a variant, under normal operating conditions, of the second example just described.
As for the second example described in the foregoing, an equipment item 211 comprising a heat source is situated at a distance from a cold plate 212 and consequently separated therefrom by an air screen 216. Furthermore, equipment item 211 is connected to cold plate 212 by means of a heat drain 213 formed in a material that has low thermal resistance and that therefore extends partly into the space formed by air screen 216.

According to this variant, however, heat drain 213 is maintained braced against cold plate 212 by means of a solid block 214 interposed between conducting drain 213 and a structural part 210. Furthermore, as in the second example, a compression spring 215 is interposed between drain 213 and cold plate 212, spring 215 being compressed when drain 213 is in contact with cold plate 212 because of the presence of solid block 214.

Thus, according to the present variant, solid block 214 does not necessarily participate in the flow of heat.

When the temperature in solid block 214 rises beyond the normal operating conditions and attains the melting temperature Tme,t;n9 of the material constituting block 214, this passes from the solid state to the liquid state (as represented in Fig. 2E, in which the molten block is represented by reference 214'), and flows away from the device via appropriate means.

Because of this fact, drain 213 is no longer maintained in contact with cold plate 212 but instead is moved away under the action of spring 215. Because of the displacement of drain 213 and its loss of contact with cold plate 212, equipment item 211 and cold plate 212 are separated by the thickness (or screen) of air 216, except for spring 215, whose thermal conductivity is negligible, and these two members are therefore substantially insulated by means of air screen 216.

According to the embodiment represented in Fig. 2F, the displacement of drain 213 then continues until it comes into contact with structural part 210, which then in this case could in turn act as a heat sink.
Fig. 3A represents a third exemplary embodiment of the invention under normal operating conditions.

According to this example, heat-generating equipment item 301 and cold part 302 acting as cold source are situated respectively in the upper part and the lower part of a chamber 305.

A space formed in the chamber between equipment item 301 and cold part 302 is filled with a bonding material 303 in liquid form having low thermal resistance, and which forms a heat-conduction path between equipment 301 and cold part 302.

Chamber 305 hermetically houses equipment item 301, bonding material 303 and cold part 302. Only a safety valve 304 penetrating into the chamber in the space filled with bonding material 303 makes it possible, if necessary, to evacuate liquid when the pressure exceeds a threshold, as explained hereinafter.

Bonding material 303 is such that its vaporization temperature corresponds approximately (and preferably is slightly lower) to a desired maximum temperature in cold part 302.

Because of this fact, when the temperature of the bonding material exceeds the vaporization temperature (and therefore attains the desired maximum temperature), for example by reason of a malfunction of equipment item 301, bonding material 303 passes from the liquid state to the gas state during a phase represented in Fig. 3B (the material in gaseous form 303' naturally appearing in the upper part of the space of chamber 305 previously occupied by the liquid, in contact with equipment item 301).

The change of state in hermetic chamber 305 causes a pressure rise therein until the pressure attains the trip threshold of safety valve 304, and the liquid part of bonding material 303 consequently begins to escape, as represented in Fig. 3B.

If the temperature continues to rise beyond the vaporization temperature of bonding material 303, the phenomenon just described and illustrated in Fig.

continues until the space of chamber 305 situated between equipment item 301 and cold part 302 is completely filled with gas phase 303' of the bonding material.
The heat path initially formed by bonding material 303 in liquid form is therefore interrupted, and by virtue of this fact cold part 302 is thermally insulated from equipment item 301, since the thermal resistance of the bonding material in gaseous form is much greater than that of the bonding material in liquid form.

It is noted that the change of phase (or in other words the transition from the liquid state to the gas state) of the bonding material has also made it possible to replace the heat path by a gas screen, which makes it possible in particular to prevent the formation of electrical arcs between equipment item 301 and cold part 302.
Fig. 4A represents a fourth exemplary embodiment of the invention under normal operating conditions, or in other words for temperatures (including the normal operating temperature) clearly lower than a permitted maximum temperature.

In this exemplary embodiment, a chamber 405 is formed in the lower prolongation of a hot plate 401 (which constitutes, for example, part of an equipment item containing a heat source, such as a fuel pump with which the aircraft are equipped).

Chamber 405 is hermetic and its lower part contains, under normal operating conditions, a liquid component 403.
Part of a heat drain 404 is also accommodated inside chamber 405: an upper part 406 (substantially horizontal in this case) extends over the entire surface (horizontal in this case) of chamber 405, in such a way as to form a piston separating an upper part of chamber 405, filled with air, for example, from a lower part of chamber 405, filled with liquid component 403 under normal operating conditions.

It can therefore be considered that the drain floats on liquid component 403 during normal operation.
Heat drain 404 also comprises a rod (substantially vertical in this case), a lower part 407 of which is in contact, during normal operation as illustrated in Fig.
4A, with a cold part forming a heat sink, in this case composed of liquid fuel of the aircraft. Lower part 407 in this case is precisely immersed in fuel 402 as represented in Fig. 4A.
In the normal operating configuration shown in Fig. 4A (in other words, especially at nominal operating temperature), a heat path is therefore formed between equipment item 401 and cold part 402 by means of materials having relatively low thermal resistance, namely in this case the walls of chamber 405, liquid component 403 and heat drain 404.
When the temperature in chamber 405 rises above the nominal operating temperature (for example, because of a malfunction of equipment item 401) and attains the vaporization temperature of liquid component 403 (preferably chosen to be lower than a permitted maximum temperature inside chamber 405, which corresponds, for example, to a temperature beyond which risks exist due to the presence of fuel 402), a gas phase 403' is formed in the lower part of chamber 405, and the pressure exerted thereby tends to displace upward heat drain 404, whose upper part 406 it is recalled, forms a piston, as represented in Fig.
4B.
Thus the movement of heat drain 404 produced under the effect of pressure, itself caused by the change of state of liquid component 403, drives the vertical part of the heat drain at least partly beyond cold part 402, thus limiting the transfer of heat to this cold part and preventing overheating thereof.
If the temperature nevertheless happens to rise further beyond the vaporization temperature of liquid component 403, this entire component is transformed to gas and the pressure exerted in the lower part of chamber 405 rises in such a way that drain 404 is driven upward so far that its lower part emerges from the fuel forming cold source 402 and finishes its travel at a distance from it.
In this final position, the space situated between lower part 407 of drain 404 and the surface of liquid fuel 402 is filled with a thermally and electrically insulating gas screen (such as air, for example), so that equipment item 401 and liquid fuel 402 forming a cold source are sufficiently insulated thermally and electrically to avoid any risk of fire from fuel 402.
Fig. 5A represents a fifth exemplary embodiment of the invention.
According to this fifth example, an equipment item comprising a heat source (or hot plate) 501 is separated from a cold plate 502 (for example, a structural member of an aircraft) by means of an air screen 504, in order to prevent propagation of electrical arcs between equipment item 501 and cold plate 502.
A plurality of heat conduits (or heat tubes, closer to the English term "heat pipe") 503 (two in the case of Fig. 5A) pass through air screen 504, each heat pipe 503 being in contact at one end with equipment item 501 and in contact at the other end with cold plate 502. Alternatively, it is possible to use a single heat pipe when the dimensioning of the heat fluxes in the device so permits.

The heat pipes, constructed in the form of two-phase tubes, for example, make it possible to evacuate the heat generated inside equipment item 501 toward cold plate 502, and this during normal operation, or in other words when the power transmitted by the heat pipes (or alternatively the temperature thereof) does not exceed a power threshold Pthreshold (respectively a temperature threshold). (By temperature threshold there is understood here either an absolute value of temperature or a relative value, for example relative to the temperature outside the heat pipe.) The thermal resistance Rth of heat pipes 503 is therefore relatively low as long as the thermal power passing through them is below the threshold Pthreshold (respectively as long as the temperature is below the temperature threshold).

However, heat pipes 503 are such that, when the thermal power passing through them is above this threshold Pthreshold (respectively when the temperature is above the threshold temperature), their thermal resistance Rth increases rapidly, as illustrated in Fig. 5B.
Under these thermal conditions (which correspond to heat-pipe operating conditions different from the usual conditions), or in other words when this threshold of transmitted thermal power is attained (departure from normal operation of the heat pipe), the power transmitted by the heat pipe is limited to this threshold value.
Thus, even if the equipment item generates a thermal power above the power threshold of the heat pipe, the latter becomes saturated and therefore transmits only a limited thermal power to the cold plate, which prevents overheating thereof. Thus evacuation of the heat is continued in part, without, however, leading to a risk for the cold plate.
The foregoing exemplary embodiments are merely possible examples of implementation of the invention, which is not limited thereto.

Claims (9)

1. A device comprising an equipment item containing a heat source having a maximum thermal operating condition, a part relatively colder than the equipment item and a member capable of transmitting the heat from the equipment item to the cold part, characterized in that the member is capable of causing a limitation on the transmitted heat under thermal conditions that exceed a defined threshold below the said maximum condition.

2. A device according to claim 1, wherein the thermal conditions correspond to a thermal power transmitted across the member.

3. A device according to claim 2, wherein the member is capable of limiting the transmitted thermal power to the value of the said defined threshold.

4. A device according to one of claims 1 to 3, wherein the equipment item and the cold part are separated substantially by a gas screen, at least under the said thermal conditions.

5. A device according to claim 4, wherein the equipment item and the cold part are separated by the said screen regardless of the thermal conditions, and wherein the member comprises at least one heat pipe passing through the said screen.

6. A device according to one of claims 1 to 5, wherein the equipment item is a fuel pump.

7. A device according to one of claims 1 to 6, wherein the cold part is a liquid fuel.

8. A device according to one of claims 1 to 6, wherein the cold part is a member that is sensitive to temperature elevations.

9. An aircraft equipped with a device according to any one of claims 1 to 8.