Patent Description:
A loop heat pipe technology has been traditionally known which uses phase changes of a working fluid to achieve high-density heat transport. A heat transport system employing such a loop heat pipe has been used, for example, to cool an electronic device such as a computer or home electric appliance. In some loop heat pipes, the working fluid is circulated by means of capillary force and/or gravity.

A loop heat pipe includes a closed loop formed by an evaporator, a condenser, a vapor conduit leading from the evaporator to the condenser, and a liquid conduit leading from the condenser to the evaporator. The closed loop is charged with a working fluid. In the evaporator, the working fluid in a liquid phase is heated by heat transferred from a heat-generating element, and a part of the working fluid changes into a gas phase. The gas-liquid two-phase working fluid moves in the vapor conduit under the action of pressure difference and buoyancy and reaches the condenser. In the condenser, the working fluid is cooled into the liquid phase. The liquid-phase working fluid returns to the evaporator under the action of capillary force and/or gravity. In this manner, the loop heat pipe allows the working fluid to circulate in the two-phase closed loop and transport heat from the evaporator to the condenser, thereby cooling the heat-generating element thermally connected to the evaporator.

Patent Literatures <NUM> to <NUM> each propose an evaporator for use in the loop heat pipe as described above, and the evaporator is in the shape of a rectangular parallelepiped in which the front and back surfaces have the largest area. This evaporator includes heat-absorbing elements disposed on the front and back surfaces of the rectangular parallelepiped and fins projecting from the heat-absorbing elements into the evaporator. A working fluid inlet is located in a lower portion of the side surface of the evaporator that lies between the front and back surfaces, and a working fluid outlet is located in an upper portion of the side surface.

Electronic devices have become more and more sophisticated and miniaturized, and this has recently led to a growing demand for thermal management in transportation machines such as watercrafts, railcars, automobiles, and aircrafts which are equipped with a large number of the sophisticated, miniaturized devices. Some transportation machines incorporate a heat transport system including a loop heat pipe as described above which uses gravity for circulation of a working fluid, and such a transportation machine, the position of the body of which constantly changes, suffers a position change-induced decrease in the drive force for allowing the working fluid to circulate and a corresponding decrease in the heat transport rate.

In an evaporator of a loop heat pipe, the working fluid exists both in the liquid phase and in the gas phase, and the gas-phase or gas-liquid two-phase working fluid is above the liquid-phase working fluid. In the case of the evaporator of any of Patent Literatures <NUM> to <NUM>, tilting of the evaporator could lead to the working fluid outlet of the evaporator being below the liquid surface. If the working fluid outlet is below the liquid surface, the flow rate of the working fluid flowing out of the working fluid outlet is reduced, and the drive force for allowing the working fluid to circulate decreases, with the result that the heat transport rate of the loop heat pipe decreases.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an evaporator and a loop heat pipe including the evaporator, the evaporator and loop heat pipe being adapted to resist disruption of the flow of the working fluid flowing out of the evaporator despite changes in the position of the evaporator.

An evaporator according to an aspect of the present invention is for evaporating at least a part of a working fluid by heat absorbed from a heat source, and includes: a housing having a plurality of surfaces including a front surface and a back surface, at least one of the front and back surfaces having the largest area among the plurality of surfaces; and a heat-absorbing element disposed on at least one of the front and back surfaces and thermally connected to the heat source. The housing includes: at least one working fluid inlet located in a surface of the housing, the surface being other than a top surface of the housing; and at least one pair of working fluid outlets located respectively at diagonally opposite corners in opposite longitudinal end portions of the top surface.

When the above evaporator is tilted, one of the two working fluid outlets is at or above an imaginary liquid surface, although the other of the working fluid outlets is below the imaginary liquid surface. Thus, despite changes in the position of the evaporator, the gas-phase or two-phase working fluid can smoothly flow out of the working fluid outlet located at or above the imaginary liquid surface. This "imaginary liquid surface" is defined as a boundary plane at which the volume percentage of the gas (void percentage) in the gas-liquid two-phase flow present in the housing of the evaporator is a given value (e.g., <NUM>%).

A loop heat pipe according to another aspect of the present invention includes: an evaporator that changes at least a part of a working fluid from a liquid phase into a gas phase, the evaporator being as defined above; a condenser that changes the working fluid from the gas phase into the liquid phase; a vapor conduit connecting the working fluid outlet of the evaporator and an inlet of the condenser; and a liquid conduit connecting an outlet of the condenser and the working fluid inlet of the evaporator.

As previously stated, the evaporator can, despite changes in its position, allow the gas-phase or two-phase working fluid to smoothly flow out of the working fluid outlet located at or above the imaginary liquid surface. Thus, the loop heat pipe including such an evaporator can, despite changes in its position, avoid disruption of the circulation of the working fluid and ensure a stable heat transport rate.

The present invention can provide an evaporator and a loop heat pipe including the evaporator, the evaporator and loop heat pipe being adapted to resist disruption of the flow of a gas-phase or two-phase working fluid from the evaporator despite changes in the position of the evaporator.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. <FIG> illustrates a schematic configuration of a loop heat pipe <NUM> according to an exemplary embodiment of the present invention.

The loop heat pipe <NUM> of <FIG> includes an evaporator <NUM>, a vapor conduit <NUM>, a condenser <NUM>, and a liquid conduit <NUM>, which form a closed loop. The closed loop is degassed beforehand to remove a non-condensable gas such as air and then charged with a working fluid. The working fluid naturally circulates in the loop heat pipe <NUM> by making use of phase changes and gravity. The working fluid is not limited to particular fluids, and may be a condensable fluid commonly used as a working fluid in heat pipes. Examples of the condensable fluid include water, an alcohol, ammonia, a fluorocarbon, a hydrofluorocarbon, a hydrofluoroether, and a liquid mixture of these fluids.

The evaporator <NUM> is thermally connected to a heat-generating element <NUM> serving as a heat source. In this evaporator <NUM>, the working fluid in the liquid phase absorbs heat from the heat-generating element <NUM>, and a part of the working fluid boils and changes into the gas phase. The resulting two-phase working fluid moves in the vapor conduit <NUM> connecting the outlet of the evaporator <NUM> and the inlet of the condenser <NUM> under the action of pressure difference and buoyancy and reaches the condenser <NUM>.

The condenser <NUM> is located above the evaporator <NUM>. The condenser <NUM> is provided with a working fluid cooling path (not shown), and the two-phase working fluid releases heat and is cooled into the liquid phase during passage through the working fluid cooling path. The liquid-phase working fluid descends in the liquid conduit <NUM> connecting the outlet of the condenser <NUM> and the inlet of the evaporator <NUM> under the action of gravity and returns to the evaporator <NUM>.

The following describes first and second embodiments of the evaporator <NUM> included in the loop heat pipe <NUM> configured as described above.

<FIG> is a perspective view of a vertically placed evaporator 2A according to the first embodiment, <FIG> illustrates the internal structure of the evaporator 2A of <FIG>, and <FIG> illustrates the evaporator 2A of <FIG> which is in a tilted position. As shown in <FIG> and <FIG>, the evaporator 2A according to the first embodiment includes: a housing <NUM> in the shape of a rectangular parallelepiped in which at least one of the front and back surfaces <NUM> and <NUM> have the largest area; and a heat-absorbing element <NUM> disposed on at least one of the front and back surfaces <NUM> and <NUM> and thermally connected to the heat-generating element <NUM> serving as the heat source. A radiating fin <NUM> projects from the heat-absorbing element <NUM> into the housing <NUM>.

A working fluid inlet <NUM> is located in a lower portion of one of the two side surfaces <NUM> and <NUM> of the housing <NUM>. The liquid conduit <NUM> is connected to the working fluid inlet <NUM>. The working fluid inlet <NUM> is not limited to this location, and it is sufficient that at least one working fluid inlet <NUM> be located in a surface of the housing <NUM> other than the top surface <NUM>. The top surface <NUM> of the housing <NUM> is provided with working fluid outlets <NUM> and <NUM>, which are located respectively in opposite longitudinal end portions of the top surface <NUM>. Each of the longitudinal end portions of the top surface <NUM> of the housing <NUM> is defined as a region extending from the longitudinal end of the top surface <NUM> of the housing <NUM> to <NUM>/<NUM> of the length of the top surface <NUM>.

The vapor conduit <NUM> is connected to the working fluid outlets <NUM> and <NUM>. In the first embodiment, a proximal end of a vapor conduit <NUM> is connected to the working fluid outlet <NUM>, while a proximal end of a vapor conduit <NUM> is connected to the working fluid outlet <NUM>, and these vapor conduits <NUM> and <NUM> join each other at the inlet of a vapor conduit <NUM> (the main section of the vapor conduit <NUM>). When the housing <NUM> of the evaporator 2A is placed on a horizontal surface, a junction <NUM> of the two vapor conduits <NUM> and <NUM> is located between a vertical line <NUM> passing through the center of the opening of the working fluid outlet <NUM> and a vertical line <NUM> passing through the center of the opening of the working fluid outlet <NUM>.

It is recommended that the two working fluid outlets <NUM> and <NUM> be located at diagonally opposite corners of the top surface <NUM> of the housing <NUM>, if the length of the short sides of the top surface <NUM> of the housing <NUM> is sufficiently greater than the sizes of the working fluid outlets <NUM> and <NUM>.

In the evaporator 2A configured as described above, heat absorbed by the heat-absorbing element <NUM> from the heat-generating element <NUM> is released to the working fluid through the radiating fin <NUM>. At least a part of the working fluid boils under the action of the heat, and the gas-phase or two-phase working fluid fills the inner region of the housing <NUM> that is above the heat-absorbing element <NUM>. It is assumed that an imaginary liquid surface L of the working fluid is located below the working fluid outlets <NUM> and <NUM> and above the upper end of the heat-absorbing element <NUM> when the top surface <NUM> of the housing <NUM> is positioned horizontally. In fact, any distinct liquid surface is not present in the evaporator 2A. However, given that the volume proportion of the gas present in the liquid increases upward in the inner region of the evaporator 2A that is at a higher level than the heat-absorbing element <NUM>, a boundary plane at which the volume percentage of the gas (void percentage) in the gas-liquid two-phase flow is a given value (e.g., <NUM>%) is defined as the imaginary liquid surface L.

As described above, the evaporator 2A according to the first embodiment includes: a housing <NUM> having a plurality of surfaces including a front surface <NUM> and a back surface <NUM>, at least one of the front and back surfaces <NUM> and <NUM> having the largest area among the plurality of surfaces; and a heat-absorbing element <NUM> disposed on at least one of the front and back surfaces <NUM> and <NUM> and thermally connected to a heat source. The housing <NUM> includes: at least one working fluid inlet <NUM> located in a surface of the housing <NUM>, the surface being other than a top surface of the housing <NUM>; and at least one pair of working fluid outlets <NUM> and <NUM> located respectively in opposite longitudinal end portions of the top surface <NUM>.

When the evaporator 2A configured as described above is tilted as shown in <FIG>, the working fluid outlet <NUM>, which is one of the two working fluid outlets <NUM> and <NUM>, is below the imaginary liquid surface L, while the other working fluid outlet, i.e., the working fluid outlet <NUM>, is at or above the imaginary liquid surface L (hereinafter, the phrase "above the imaginary liquid surface L" will be used to mean being at or above the imaginary liquid surface L). Thus, despite changes in the position of the evaporator 2A, the gas-phase or two-phase working fluid can smoothly flow out of the working fluid outlet <NUM> located above the imaginary liquid surface L.

In the evaporator 2A, as illustrated in the first embodiment, the two working fluid outlets <NUM> and <NUM> may be located at diagonally opposite corners of the top surface <NUM> of the housing <NUM>.

In this case, when the evaporator 2A is tilted in the forward/backward direction, one of the two working fluid outlets <NUM> and <NUM> is above the imaginary liquid surface L, although the other of the working fluid outlets <NUM> and <NUM> is below the imaginary liquid surface L. Thus, despite changes in the position of the evaporator 2A, the gas-phase or two-phase working fluid can smoothly flow out of the working fluid outlet <NUM> or <NUM> located above the imaginary liquid surface L.

In the evaporator 2A, as illustrated in the first embodiment, the working fluid outlets <NUM> and <NUM> may include a first working fluid outlet <NUM> to which a first vapor conduit <NUM> is connected and a second working fluid outlet <NUM> to which a second vapor conduit <NUM> is connected, and when the housing <NUM> is placed on a horizontal surface, the first and second vapor conduits <NUM> and <NUM> may join each other at a junction located between a first vertical line <NUM> passing through a center of an opening of the first working fluid outlet <NUM> and a second vertical line <NUM> passing through a center of an opening of the second working fluid outlet <NUM>.

In this case, even when the evaporator 2A is tilted so that the working fluid outlet <NUM>, which is one of the two working fluid outlets <NUM> and <NUM>, is below the imaginary liquid surface L and the vapor conduit <NUM> connected to the working fluid outlet <NUM> is closed by the liquid, the junction <NUM> of the first and second vapor conduits <NUM> and <NUM> can avoid being closed by the liquid, and the gas flow path can be maintained in the vapor conduit <NUM>. Thus, despite changes in the position of the evaporator 2A, the gas-phase or two-phase working fluid can smoothly flow out of the working fluid outlet <NUM> located above the imaginary liquid surface L.

The loop heat pipe <NUM> according to an exemplary embodiment includes: the evaporator 2A that changes at least a part of a working fluid from a liquid phase into a gas phase; a condenser <NUM> that changes the working fluid from the gas phase into the liquid phase; a vapor conduit <NUM> connecting the working fluid outlets <NUM> and <NUM> of the evaporator 2A and an inlet of the condenser <NUM>; and a liquid conduit <NUM> connecting an outlet of the condenser <NUM> and the working fluid inlet <NUM> of the evaporator 2A.

As previously stated, the evaporator 2A can, despite changes in its position, allow the gas-phase or two-phase working fluid to smoothly flow out of the working fluid outlet <NUM> or <NUM> located above the imaginary liquid surface L. Thus, the loop heat pipe <NUM> including such an evaporator 2A can, despite changes in its position, avoid disruption of the flow of the working fluid from the working fluid inlet <NUM> of the evaporator 2A to the condenser <NUM> and maintain the circulation of the working fluid to continue the heat transport.

Next, an example evaporator 2B that does not fall within the scope of the claims will be described. <FIG> illustrates an aircraft <NUM> which is an example of a transportation machine incorporating a vertically placed evaporator. The example evaporator 2B is incorporated into a transportation machine whose allowable tilt angle is α° and which is permitted to tilt from the horizontal to the allowable tilt angle during normal operation. Examples of the transportation machine include watercrafts (including submersibles), railcars, automobiles, and aircrafts.

<FIG> partially shows a fuselage <NUM> and main wing <NUM> of the aircraft <NUM>. The fuselage <NUM> has a multilayer structure including an outer panel <NUM> and an inner wall <NUM> located closer to the cabin than the outer panel <NUM>. Between the outer panel <NUM> and inner wall <NUM> is defined a cooling chamber <NUM>. The temperature inside the cooling chamber <NUM> is low because of cold energy transferred from the outer panel <NUM> which during flight is exposed to outside air having a considerably lower temperature than that near the ground. Alternatively, the outer panel <NUM> may be provided with an air inlet and air outlet communicating with the cooling chamber <NUM>, and the outside air may be introduced into the cooling chamber <NUM> during flight.

The aircraft <NUM> incorporates the heat-generating element <NUM> and the loop heat pipe <NUM> which uses the heat-generating element <NUM> as the heat source for the evaporator <NUM>. Examples of the heat-generating element <NUM> include, but are not limited to: an electronic device including heat-generating parts, such as a control board, an engine control unit (ECU), or a computer; a friction heat-generating mechanical part such as a bearing; and a battery. Air inside the cabin may be used as the heat source instead of the heat-generating element <NUM>.

The evaporator <NUM> is thermally connected to the heat-generating element <NUM>, and the condenser <NUM> is located in the cooling chamber <NUM>. In the cooling chamber <NUM> is disposed a fan <NUM> for forcing a gas flow to pass the condenser <NUM>. The condenser <NUM> condenses the working fluid using cold energy from the outside air. During flight of the aircraft <NUM>, the temperature of the outside air is considerably lower than normal temperatures, and thus a large temperature difference is created between the heat source to which the evaporator <NUM> is thermally connected and the medium (the gas in the cooling chamber <NUM>) to which the condenser <NUM> provides heat. In consequence, highly efficient heat transport is achieved by the loop heat pipe <NUM>.

<FIG> is a perspective view of the example evaporator 2B, <FIG> illustrates the internal structure of the example evaporator 2B of <FIG>, and <FIG> illustrates the example evaporator 2B of <FIG> which is in a tilted position. For the example evaporator 2B, the elements which are the same as or similar to those of the first embodiment described above are denoted by the same reference sings in the figures and will not be described below.

As shown in <FIG> and <FIG>, the example evaporator 2B includes: a housing <NUM> having a plurality of surfaces including a front surface <NUM> and a back surface <NUM>, at least one of the front and back surfaces <NUM> and <NUM> having the largest area among the plurality of surfaces; and a heat-absorbing element <NUM> disposed on at least one of the front and back surfaces <NUM> and <NUM> and thermally connected to the heat-generating element <NUM> serving as the heat source. A radiating fin <NUM> projects from the heat-absorbing element <NUM> into the housing <NUM>.

A working fluid inlet <NUM> is located in a lower portion of one of the two side surfaces <NUM> and <NUM> of the housing <NUM>. The liquid conduit <NUM> is connected to the working fluid inlet <NUM>. The working fluid inlet <NUM> is not limited to this location, and it is sufficient that at least one working fluid inlet <NUM> be located in a surface of the housing <NUM> other than the top surface <NUM>.

At least one working fluid outlet <NUM> is located in a central portion of the top surface <NUM> of the housing <NUM>. The vapor conduit <NUM> is connected to the working fluid outlet <NUM>. More specifically, as shown in <FIG>, in the housing <NUM> placed on a horizontal surface and viewed from the front surface <NUM>, an intersection between a horizontal plane L1 passing through the upper end of the heat-absorbing element <NUM> or radiating fin <NUM> and a vertical line H passing through the center of the top surface <NUM> of the housing <NUM> is defined as a reference point O. In the housing <NUM> placed on a horizontal surface and viewed from the front surface <NUM>, a straight line A drawn from a central point 20c of the opening of the working fluid outlet <NUM> to the reference point O forms an angle θ of α° to <NUM>° with the horizontal plane.

The angle α° represents the allowable tilt angle up to which the transportation machine incorporating the example evaporator 2B can tilt during normal operation. The "normal operation" of the transportation machine refers, for example, to operation performed taking into consideration economic efficiency and safety. The angle α is not limited to the allowable tilt angle, and may be any one of a maximum value, a recommended value, and an average value of the angle of tilt of the transportation machine with respect to the horizontal. The range of tilting movement during normal operation varies for different transportation machines. For aircrafts, the allowable limit of the pitch angle may be about <NUM>°, and the allowable limit of the roll angle (bank angle) may be about <NUM>°. For railcars, the allowable tilt angle with respect to a horizontal plane may be from about <NUM> to <NUM>°. For watercrafts other than submersibles, the allowable limit of the pitch angle may be about <NUM>°, and the allowable limit of the roll angle may be about <NUM>°. For roads on which automobiles travel, the maximum tilt angle is about <NUM>° (about <NUM>%). Since, as described above, the range of tilting movement varies for different transportation machines, a suitable value of the angle α may be chosen depending on the type of the transportation machine. Given that the allowable tilt angle during normal operation is <NUM>° or less for most transportation machines, the value of the angle α may be set to <NUM> irrespective of the range of tilting movement of the transportation machine. The angle α is not limited to the allowable tilt angle, and may be any one of a maximum value, a recommended value, and an average value of the angle of tilt of the transportation machine with respect to the horizontal.

In the example evaporator 2B configured as described above, the imaginary liquid surface L of the working fluid is below the working fluid outlet <NUM> and above the heat-absorbing element <NUM> when the top surface <NUM> of the housing <NUM> is positioned horizontally. Heat absorbed by the heat-absorbing element <NUM> from the heat-generating element <NUM> is released to the working fluid through the radiating fin <NUM>. At least a part of the working fluid boils under the action of the heat, and the housing <NUM> is filled with the gas-phase or two-phase working fluid.

As described above, the example evaporator 2B is incorporated into a transportation machine whose allowable tilt angle is α° to evaporate at least a part of a working fluid by heat absorbed from a heat source, and includes: a housing <NUM> having a plurality of surfaces including a front surface <NUM> and a back surface <NUM>, at least one of the front and back surfaces <NUM> and <NUM> having the largest area among the plurality of surfaces; and a heat-absorbing element <NUM> disposed on at least one of the front and back surfaces <NUM> and <NUM> and thermally connected to the heat source. The housing <NUM> includes: at least one working fluid inlet <NUM> located in a surface of the housing <NUM>, the surface being other than a top surface of the housing; and at least one working fluid outlet <NUM> located in the top surface <NUM>. When, in the housing <NUM> placed on a horizontal surface and viewed from the front surface <NUM>, an intersection between a horizontal plane L1 passing through an upper end of the heat-absorbing element <NUM> or a radiating fin <NUM> and a vertical line H passing through a center of the top surface of the housing <NUM> is defined as a reference point O, a straight line drawn from a central point 20c of an opening of the working fluid outlet <NUM> to the reference point O forms an angle of α° to <NUM>° with the horizontal plane L1.

As shown in <FIG>, in the event that the example evaporator 2B is tilted by an angle equal to or smaller than the allowable tilt angle of the transportation machine, the working fluid outlet <NUM> is not below the imaginary liquid surface L. Thus, despite changes in the position of the example evaporator 2B, the gas-phase or two-phase working fluid can smoothly flow out of the working fluid outlet <NUM> located above the imaginary liquid surface L.

The example evaporator 2B can constitute a part of the loop heat pipe <NUM> like the evaporator 2A according to the first embodiment described above. Thus, as with the case of the above embodiment, it is possible to provide the loop heat pipe <NUM> that can, despite changes in position, avoid disruption of the flow of the working fluid from the working fluid inlet <NUM> of the example evaporator 2B to the condenser <NUM> and maintain a sufficient amount of working fluid circulation.

Although the foregoing has described a preferred embodiment of the present invention, the scope of the present invention embraces modifications made to the details of the structures and/or functions of the above embodiments provided they are within the scope of the appended claims.

Claim 1:
An evaporator (2A) that evaporates at least a part of a working fluid by heat absorbed from a heat source (<NUM>), comprising:
a housing (<NUM>) having a plurality of surfaces including a front surface (<NUM>) and a back surface (<NUM>), at least one of the front and back surfaces having the largest area among the plurality of surfaces; and
a heat-absorbing element (<NUM>) disposed on at least one of the front and back surfaces and thermally connected to the heat source, wherein
the housing includes: at least one working fluid inlet (<NUM>) located in a surface of the housing, the surface being other than a top surface (<NUM>) of the housing; characterized by at least one pair of working fluid outlets (<NUM>, <NUM>) located respectively at diagonally opposite corners in opposite longitudinal end portions of the top surface.