Note: Descriptions are shown in the official language in which they were submitted.
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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)
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In another aspect, the invention proposes a device comprising an
equipment item containing a heat source having a maximum thermal
operating condition, a part colder than the equipment item and a member for
transmitting heat from the equipment item to the cold part to evacuate at
least
part of the heat generated within the equipment item and to avoid overheating
the equipment item, characterized in that the equipment item and the cold part
are separated by a gas screen, wherein the member comprises at least one
heat pipe crossing said gas screen and being in contact with the equipment
item at one end and in contact with the cold part at the other end, and in
that
the member causes a limitation on the transmitted heat under thermal
conditions that exceed a threshold below said maximum condition defined to
avoid degradation of the cold part.
In a further aspect, the present invention provides a device comprising
a fuel pump generating heat and having a predefined maximum thermal
operating condition, liquid fuel colder than the fuel pump and a member for
transmitting heat from the fuel pump to the liquid fuel to evacuate at least
part
of the heat generated within the fuel pump and to avoid exceeding the
predefined maximum thermal operating condition of the fuel pump,
characterized in that the fuel pump and the liquid fuel are separated by a gas
screen, wherein the member comprises at least one heat pipe crossing said
gas screen and being in contact with the fuel pump at one end and in contact
with the liquid fuel at an other end, and in that the member causes a
limitation
on the transmitted heat under thermal conditions that exceed a defined
threshold, said defined threshold being below a maximum condition defined to
avoid degradation of the liquid fuel.
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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
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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
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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 Tnominal
corresponding to normal operation.
Material 103 is a heat conductor, and its thermal resistance Rmaterial 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 Tmeltmg 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
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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. 1B, 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. 16), 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. 1C.
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 Tnominal clearly lower than a desired maximum temperature.
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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 Tmoting 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
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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 Tmeiting 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
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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. 36 (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.
= = =
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If the temperature continues to rise beyond the vaporization temperature
of bonding material 303, the phenomenon just described and illustrated in Fig.
3B
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.
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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
402
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
407
emerges from the fuel forming cold source 402 and finishes its travel at a
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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 P
= threshold (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 P
= threshold
(respectively as long as the temperature is below the temperature threshold).
However, heat pipes 503 are such that, when the thermal power passing
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through them is above this threshold P
= threshold (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.
