Coolers and cooling devices

Abstract

To provide a cooler and a cooling device capable of suppressing increase of an ebullition start point to causing ebullition at a low degree of superheating.SOLUTION: A boiling type cooler for cooling a heating element, comprises: a container containing working fluid; a cooling member provided in the container so as to face the surface of the heating element, and made of a porous body; and at least one thin metal wire or thin metal film provided between the surface of the heating element and the cooling member, and configured to be able to be heated.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a cooler and a cooling device. [Background technology]

[0002] Coolers using the boiling method, which cools a heat-generating element with an external working fluid such as water, are known. Boiling methods include pool boiling and forced convection boiling. The cooling mechanism for a heat-generating element using the pool boiling method is described below. Conventional pool boiling coolers generally include a container and a working fluid contained within the container, with the container having a contact point with the heat-generating element to be cooled. When heat is generated in the heat-generating element and transferred to the working fluid through the contact point, the working fluid near the contact point boils. When vapor is generated by boiling, the working fluid is supplied to the contact point due to the difference in density between the gas and liquid. This newly supplied working fluid further evaporates and removes heat from the heat-generating element. Pool boiling coolers do not require an external power source for circulating the liquid, as is the case with forced convection boiling coolers, and are therefore advantageous in terms of compactness and energy conservation.

[0003] However, when a large heat flux is applied to the contact area, the amount of evaporation of the working fluid increases, and the contact area begins to be covered with vapor. When the contact area becomes completely covered with vapor and becomes dry, and working fluid is no longer supplied to the contact area, the cooling capacity of the cooler deteriorates significantly. The heat flux in this state is called the "critical heat flux (CHF)."

[0004] To address this issue, Patent Document 1 provides a structure in which a porous body of a predetermined shape is placed between the heating element and the water in a cooling container, and water is supplied to the heating element by the capillary action of the porous body, while the steam generated by this is discharged into the water in the container, thereby dramatically improving the conventional critical heat flux with a simple structure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-139005 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even if the cooler has a structure in which a porous material is provided between the heating element and the working fluid in the cooling vessel, if there are no boiling nuclei near the surface of the heating element, boiling will not occur smoothly, and as shown by the boiling curve in Figure 1, the temperature at which boiling begins to occur on the surface of the heating element, that is, the boiling onset point (ONB: Onset of Nucleate Boiling), will rise dramatically. In such cases, the degree of superheat (ΔT sat ) becomes higher, which can cause problems such as breakage or malfunction depending on the type of heating element. For coolers using the conventional boiling method, active research and development has been conducted on improving the CHF mentioned above, but there have been few reports on technologies for lowering ONB.

[0007] In order to solve such problems, an object of the present invention is to provide a cooler and a cooling device that can cause boiling at a low degree of superheat by suppressing the rise in the boiling initiation point. [Means for solving the problem]

[0008] After extensive research, the inventors discovered that by providing at least one heatable thin metal wire or thin metal film between the surface of the heating element and a cooling member made of a porous material, the heated thin metal wire or thin metal film can seed boiling nuclei, thereby causing boiling to occur at a low degree of superheat.

[0009] The above problems are solved by the present invention, which is specified as follows. (1) A boiling type cooler for cooling a heat generating body, a vessel containing a working fluid; a cooling member made of a porous material and provided in the container so as to face a surface of the heat generating element; At least one thin metal wire or thin metal film that is provided between the surface of the heating element and the cooling member and configured to be heatable; A cooler comprising: (2) The cooler according to (1), wherein the thin metal wire or thin metal film is configured to be heatable by passing an electric current through it. (3) A cooler according to (1) or (2), in which a plurality of the thin metal wires or thin metal films are provided. (4) A cooler described in any one of (1) to (3), wherein the porous body is provided with a working fluid supply section that supplies the working fluid to the surface of the heat generating element by capillary action, and a steam discharge section that discharges steam generated on the surface of the heat generating element to the working fluid side. (5) The cooler according to (4), wherein the porous body has a honeycomb structure. (6) A cooler according to any one of (1) to (5), further comprising a working fluid introduction body that is stacked on the working fluid side of the porous body and introduces the working fluid into the porous body. (7) A cooler according to any one of (1) to (6), a condenser connected to the vessel of the cooler for liquefying the evaporated working fluid; A cooling device equipped with: [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a cooler and a cooling device that can cause boiling at a low degree of superheat by suppressing an increase in the boiling initiation point. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a graph showing a boiling curve of a conventional cooler. [Figure 2] 1 is a schematic diagram of a boiling type cooler 10 according to an embodiment of the present invention. [Figure 3] 1 is an enlarged schematic cross-sectional view of a heating element 13, thin metal wires 15, and a cooling member 14. FIG. [Figure 4] 1 is a schematic plan view showing the positional relationship between thin metal wires 15 and a cooling member 14. FIG. [Figure 5] FIG. 1 is a schematic diagram of a boiling type cooler 10 according to another embodiment of the present invention. [Figure 6] FIG. 2 is a plan view of a porous body having a honeycomb structure. [Figure 7] FIG. 1 is a schematic diagram of a boiling type cooler 10 according to another embodiment of the present invention. [Figure 8] 1 is a schematic diagram of a cooling device 20 including a cooler 10 according to an embodiment of the present invention. [Figure 9] FIG. 1 is a schematic diagram of a pool boiling experimental apparatus according to a test example. [Figure 10] FIG. 1 is a schematic diagram showing the design of an ITO film (heat generating element) according to a test example. [Figure 11] FIG. 10 is a schematic diagram showing the arrangement of thin stainless steel wires according to a test example. [Figure 12] 1 is a photograph showing the appearance of an NA honeycomb of a honeycomb porous body. [Figure 13] 1 is a graph showing boiling curves according to test examples. [Figure 14] 1 is a schematic plan view showing the positional relationship between a metal thin film 22 and a cooling member 14. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] Next, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes and improvements may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0013] <Cooler> 2 is a schematic diagram of a boiling-type cooler 10 according to an embodiment of the present invention. The cooler 10 includes a container 12 that contains a working fluid 11, and a cooling member 14 that is provided in the container 12 so as to be in contact with the working fluid 11 and to face the surface of a heating element 13.

[0014] The cooling member 14 is made of a porous material. By making the cooling member 14 of a porous material, the working fluid 11 can be supplied to the surface of the heating element 13 by capillary action. Furthermore, the pores of the porous material can serve as boiling nuclei near the surface of the heating element 13. The porous material can be made of ceramics such as cordierite, sintered metal, or electrolytically deposited metal. In particular, it is desirable to make the porous material of a porous material with good wettability, such as an oxide, or a porous material that has been processed to improve wettability, such as by plasma irradiation. Furthermore, the form of the porous material is not particularly limited, and for example, the porous material may be made of an aggregate of porous particles. Furthermore, the porous material may be made of a porous layer.

[0015] As the working fluid 11, for example, a liquid having surface tension such as water, a low-temperature fluid, a refrigerant, an organic solvent, etc. Furthermore, in order to cope with the increase in heat generation density of heat generating elements such as electronic elements in recent years, in addition to general electrical insulating fluids such as fluorocarbons (FCs), chlorofluorocarbons (CFCs), alternative chlorofluorocarbons (HCFCs, HFCs, etc.), pure water, and ultrapure water, electrical insulating fluids with very high wettability such as HFE7100 (manufactured by 3M Japan Ltd.) can also be suitably used.

[0016] The cooler 10 further includes at least one thin metal wire 15 that is provided between the surface of the heating element 13 and the cooling member 14 and is configured to be heatable. Fig. 3 shows an enlarged cross-sectional schematic diagram of the heating element 13, the thin metal wire 15, and the cooling member 14. Fig. 4(A) shows a schematic diagram showing the positional relationship between the thin metal wire 15 and the cooling member 14 when the configuration in Fig. 3 is viewed from above. In the embodiment shown in Figs. 3 and 4(A), one thin metal wire 15 is provided between the heating element 13 and the cooling member 14 and passes through the center of the heating element 13 and the cooling member 14 when viewed from above.

[0017] As described above, when the cooling element 14 facing the heating element 13 is made of a porous material, the pores of the porous material can become boiling nuclei near the surface of the heating element 13. However, if boiling nuclei do not occur for some reason, boiling does not occur smoothly, resulting in a dramatic increase in the onset of boiling point (ONB). For example, data centers, which are expected to see a significant increase in demand in recent years, consume a significant amount of energy. Therefore, energy conservation efforts have a significant impact, but the heat generation density has also increased significantly, raising concerns about a significant increase in ONB. Furthermore, when cooling electronic devices using a boiling-type cooler, an electrically insulating fluid is sometimes used as the working fluid. However, if a highly wettable electrically insulating fluid such as HFE7100 (manufactured by 3M Japan Ltd.) is used, the pores of the porous material, which serve as boiling nuclei, will be completely wetted. In such cases, there are no boiling nuclei near the surface of the heating element 13, boiling does not occur smoothly, and the ONB will increase dramatically. The upper limit operating temperature to ensure safe operation of electronic elements is known to be, for example, 85 to 105°C for CPUs or FPGAs, around 150°C for Si power devices, and 300 to 400°C for SiC power devices. However, if the ONB rises in this way, the upper limit operating temperature will be exceeded immediately after startup, causing problems such as damage or failure of the electronic elements.

[0018] To address this issue, the cooler 10 according to the embodiment of the present invention includes thin metal wires 15 that are disposed between the surface of the heating element 13 and the cooling member 14 and are configured to be heatable. When heated, the thin metal wires 15 generate steam, which can seed boiling nuclei (gas bubbles) in the pores of the porous material near the surface of the heating element 13. The presence of boiling nuclei near the surface of the heating element 13 allows boiling to occur smoothly. As a result, boiling can occur at a low degree of superheat, suppressing an increase in ONB.

[0019] The thin metal wires 15 may be heated by electrically connecting an external power source at any position and passing an electric current through the wires. In this way, the thin metal wires 15 may be configured to be heatable by passing an electric current through them. The heating temperature of the thin metal wires 15 is not particularly limited as long as it is equal to or higher than the boiling point of the working fluid 11. The amount of electric current passing through the thin metal wires 15 can be adjusted as appropriate to boil the working fluid 11, but using a pulsed electric current is particularly preferable in terms of cost as it can significantly reduce the electric power required for heating. Alternatively, the thin metal wires 15 may be configured to be heatable by electromagnetic induction or the like.

[0020] There are no particular limitations on the material of the thin metal wires 15 as long as they are thin metal wires that can be heated, but materials with high electrical resistivity, such as stainless steel wires, platinum wires, nichrome wires, Kanthal wires, or molybdenum wires, are preferred because they can be heated with voltage, do not require a large current for heating, and enable miniaturization of the cooler 10. The electrical resistivity of the thin metal wires 15 is preferably 2 to 120 μΩ·cm.

[0021] The size of the thin metal wires 15 is not particularly limited. For example, even if the size of the thin metal wires 15 is very small and the amount of bubbles generated by heating is very small, boiling nuclei are provided near the surface of the heating element 13, and boiling spreads from the boiling nuclei along the surface direction of the heating element 13, resulting in smooth boiling. Furthermore, if the size of the thin metal wires 15 is very large, problems arise in terms of cost and ease of handling, but the object of the present invention of inducing boiling at a low degree of superheat and suppressing an increase in ONB can be achieved.

[0022] The thinner (smaller diameter) the thin metal wires 15 are, the less power is required to generate bubbles by heating them. Furthermore, if the thin metal wires 15 are thin enough to fit within the surface roughness range of the porous body, the porous body will come into contact with the surface of the heating element, which has the effect of accelerating the activation of cavities (fine scratches on the surface of the heating element). From this perspective, the diameter of the thin metal wires 15 is preferably 100 μm or less, and more preferably 50 μm or less. The diameter of the thin metal wires 15 used in the embodiments of the present invention is typically 10 to 50 μm.

[0023] The shorter the length of the thin metal wires 15, the less power is required to heat them and generate bubbles. From this perspective, the length of the thin metal wires 15 is preferably 100 mm or less, and more preferably 50 mm or less. The length of the thin metal wires 15 used in the embodiment of the present invention is typically 20 to 50 mm.

[0024] The thin metal wire 15 may be provided in a single or multiple number. FIG. 4(A) shows an example in which a single thin metal wire 15 is provided so as to pass through the center of the heating element 13 and the cooling member 14 in a plan view and protrude beyond the heating element 13 and the cooling member 14, but this is not limiting. For example, as shown in FIG. 4(B), a single thin metal wire 15 may be provided so as to pass through the center of the heating element 13 and the cooling member 14 in a plan view and not protrude beyond the heating element 13 and the cooling member 14. As shown in FIG. 4(C), two thin metal wires 15 may be provided so as to extend parallel to each other across the center of the heating element 13 and the cooling member 14 in a plan view. As shown in FIG. 4(D), six thin metal wires 15 may be provided with a predetermined interval between them. As shown in FIG. 4(E), two thin metal wires 15 may be provided so as to intersect each other. The number of thin metal wires 15 is not limited to one or two, and may be three or more. Furthermore, a plurality of thin metal wires 15 may be connected by a thin non-metallic wire made of resin or the like. When a plurality of thin metal wires 15 are provided, it is preferable to configure all of the thin metal wires 15 to be heatable by electrically connecting them to an external power source by wiring or the like.

[0025] Even if there is only one thin metal wire 15, it generates bubbles when heated, providing boiling nuclei near the surface of the heating element 13, and boiling spreads from the boiling nuclei along the surface direction of the heating element 13, resulting in smooth boiling. Therefore, from the viewpoint of reducing the amount of power required to energize the thin metal wire 15, it is preferable to use a single thin metal wire 15. On the other hand, if multiple thin metal wires 15 are provided, boiling nuclei (bubbles) can be more reliably seeded in the pores of the porous body near the surface of the heating element 13. This has the advantage that the bubbles generated by the thin metal wire 15 are not wasted and can be reliably seeded in the targeted pores of the porous body.

[0026] Furthermore, as shown in Fig. 4(F), a single thin metal wire 15 may be provided in a circular shape. When viewed from above, the thin metal wire 15 is not limited to a circular shape as shown in Fig. 4(F), but may also be formed in a triangular, rectangular, or other polygonal shape. Furthermore, the thin metal wire 15 may also be formed in a knotted shape such as a ribbon.

[0027] Alternatively, a metal thin film 22 may be provided instead of the thin metal wires 15. That is, the metal thin film 22 is provided between the surface of the heating element 13 and the cooling member 14 and configured to be heatable. With this configuration, the heated metal thin film 22 generates steam, which can seed boiling nuclei (gas bubbles) in the pores of the porous material near the surface of the heating element 13. The presence of boiling nuclei near the surface of the heating element 13 allows boiling to occur smoothly. As a result, boiling can be caused at a low degree of superheat, and an increase in ONB can be suppressed.

[0028] The metal thin film 22 may be heated by electrically connecting an arbitrary position to an external power source with wiring or the like and passing an electric current through the electrical connection. In this way, the metal thin film 22 may be configured to be heatable by passing an electric current. The heating temperature of the metal thin film 22 is not particularly limited as long as it is equal to or higher than the boiling point of the working fluid 11. The amount of electric current passing through the metal thin film 22 can be adjusted appropriately so as to boil the metal thin film 22, but using a pulsed electric current is particularly preferable in terms of cost as it can significantly reduce the electric power required for heating. Alternatively, the metal thin film 22 may be configured to be heatable by electromagnetic induction or the like.

[0029] There are no particular limitations on the material of the thin metal film 22 as long as it is made of a heatable metal, but materials with high electrical resistivity, such as stainless steel wire, platinum wire, nichrome wire, Kanthal wire, or molybdenum wire, are preferred because they can be heated with voltage, do not require a large current for heating, and enable miniaturization of the cooler 10. The electrical resistivity of the thin metal film 22 is preferably 2 to 120 μΩ·cm.

[0030] The metal thin film 22 may be triangular, rectangular, other polygonal, circular, elliptical, or irregular in shape when viewed from above. The smaller the metal thin film 22, the less power is required to heat it and generate bubbles. From this perspective, the metal thin film 22 should have an area of ​​0.01 to 1 cm in plan view. 2 The metal thin film 22 used in the embodiment of the present invention typically has an area of ​​0.01 to 0.1 cm in plan view. 2 , and the thickness is 10 to 20 μm.

[0031] The metal thin film 22 may be provided in multiple bodies rather than as a single body. FIG. 14(A) shows an example in which an integral rectangular metal thin film 22 is provided so as to pass through the center of the heating element 13 and the cooling member 14 in a plan view and extend beyond the heating element 13 and the cooling member 14, but this is not limiting. For example, as shown in FIG. 14(B), an integral rectangular metal thin film 22 may be provided in the center of the heating element 13 and the cooling member 14 in a plan view so as not to extend beyond the heating element 13 and the cooling member 14. As shown in FIG. 14(C), two metal thin films 22 may be provided in parallel, sandwiching the center of the heating element 13 and the cooling member 14 in a plan view. As shown in FIG. 14(D), an integral circular metal thin film 22 may be provided in the center of the heating element 13 and the cooling member 14 in a plan view. The metal thin films 22 are not limited to one or two bodies, but may be three or more bodies. Furthermore, the metal thin films 22 may be formed by connecting multiple bodies with thin nonmetallic wires made of resin or the like. When a plurality of thin metal films 22 are provided, it is preferable to configure all of the thin metal films 22 to be heatable by electrically connecting them to an external power source via wiring or the like.

[0032] As shown in the schematic diagram of Fig. 5, the porous body of cooling member 14 preferably includes a working fluid supply section 16 that supplies working fluid 11 to the surface of heating element 13 by capillary action, and a vapor discharge section 17 that discharges vapor generated on the surface of heating element 13 to the working fluid side. An example of a porous body of cooling member 14 that includes such a working fluid supply section 16 and vapor discharge section 17 is a porous body having a honeycomb structure, as shown in the plan view of Fig. 6.

[0033] The working fluid supply unit 16 supplies the working fluid 11 to the surface of the heating element 13 by capillary action. The vapor discharge unit 17 discharges the steam generated by heat from the heating element 13 from the surface of the heating element 13 to the working fluid side. In this embodiment, the lattice-shaped porous layer surrounding the rectangular holes of the honeycomb structure of the porous body functions as the working fluid supply unit 16, which supplies the working fluid 11 to the surface of the heating element 13 by capillary action, and the rectangular holes function as the vapor discharge unit 17, which discharges the steam generated on the surface of the heating element 13 to the working fluid side. By supplying the working fluid 11 and discharging the steam using separate paths in this way, it is possible to prevent the steam from covering the contact area and limiting the critical heat flux. As a result, the critical heat flux of the cooler 10 is improved.

[0034] The pore radius of a porous body may be the radius of the pores originally present in each porous body, or the radius of the pores formed in each porous body. Here, the shape of the pores in the porous body can be various shapes such as polygonal, circular, elliptical, etc., and the "pore radius" refers to the radius of the circumscribed circle of such various pore shapes.

[0035] As for the shape of the porous body, since the contact area of ​​the porous body with the contact part is large, the size of the pores for releasing the steam generated at the contact part into the water is preferably small, and for example, the pore radius can be set to 100 to 2000 μm. Furthermore, since this can reduce the pressure loss when passing through the porous bottom, the spacing between the pores for releasing the steam generated at the contact part into the water is preferably small, and for example, can be set to 100 to 1000 μm.

[0036] In Figure 5, the working fluid supply section 16 and the vapor discharge section 17 are shown as being perpendicular to the surface of the heating element 13 below and the working fluid side above, but the working fluid supply section 16 and the vapor discharge section 17 may be configured to be, for example, curved or bent, rather than perpendicular, as long as they provide a path between the surface facing the surface of the heating element 13 and the surface on the working fluid side, respectively.

[0037] The cooling member 14 may have a laminated structure in which a first porous body facing the surface of the heating element 13 and a second porous body on the working fluid side are stacked. In this case, the second porous body includes a working fluid supply section that supplies the working fluid 11 to the first porous body and a vapor discharge section that discharges the vapor discharged from the first porous body to the working fluid side. The cooling member 14 may also have a third porous body on the working fluid side of the second porous body, resulting in a total of three layers. In this case, the third porous body includes a working fluid supply section that supplies the working fluid 11 to the second porous body and a vapor discharge section that discharges the vapor discharged from the second porous body to the working fluid side. Similarly, the porous body may be configured in a total of four or more layers by stacking multiple porous bodies on the working fluid side of the second porous body. In this way, when the porous material of the cooling member 14 has a structure in which multiple layers are stacked, the amount of working fluid 11 supplied to the surface of the heating element 13 and the amount of steam discharged from the heating element 13 are both increased, thereby more effectively preventing the critical heat flux from being limited.

[0038] As shown in the schematic diagram of FIG. 7, the cooler 10 preferably further includes a working fluid introduction body 18, which is stacked on the working fluid side of the porous body of the cooling element 14 and has a working fluid introduction section 19 that introduces the working fluid 11 into the porous body. From the perspective of the capillary limit mechanism, a thinner porous body of the cooling element 14 is preferable. However, if the thickness is thinner than the thickness of the macro liquid film, liquid depletion easily occurs within the porous body, resulting in a problem of a reduced critical heat flux. In response to this problem, as shown in FIG. 7, by providing a working fluid introduction body 18 on top of the porous body (on the working fluid side) that introduces the working fluid 11 into the porous body, the working fluid introduction body 18 exists between the porous body and the vapor mass above it, amply supplying the working fluid 11 toward the porous body and retaining the working fluid 11 above the porous body. Therefore, even if the thickness of the porous body is thin, liquid depletion is suppressed, preventing a decrease in the critical heat flux. Furthermore, since a larger liquid supply rate through the working fluid introduction body 18 is preferable, the thickness of the working fluid introduction body 18 is also preferably large. Specifically, for example, when the thickness of the porous body is thinned to about 100 μm, the thickness of the working fluid introducing body 18 is preferably set to about 1 mm or more.

[0039] The working fluid introducing body 18 may have a plurality of holes penetrating in the height direction, and the plurality of holes may constitute the working fluid introducing section 19. Furthermore, the plurality of holes constituting the working fluid introducing section 19 may have a circular or polygonal cross section.

[0040] The material constituting the working fluid introducer 18 may be a porous material or a non-porous material. The working fluid introducer 18 may be made of a metal such as stainless steel or Teflon (registered trademark), or a resin. In particular, by forming the working fluid introducer 18 from a metal, the wettability of the working fluid introducer 18 is improved, and the hydrophilicity is improved, so that a larger amount of the working fluid 11 can be taken in and supplied to the surface of the heating element 13.

[0041] In another embodiment of the present invention, cooling can be performed by immersing the entire heating element 13 in the working fluid 11, or by immersing a portion of the heating element 13 below the liquid surface of the working fluid 11. In this case, the heating element 13 may take various forms depending on the situation, such as floating or placed on the bottom of the container 12, but the key is to attach a cooling member 14 made of a porous material to the portion immersed in the working fluid 11, and cooling can be performed in the same manner as in the above example.

[0042] <Cooling device> 8 is a schematic diagram of a cooling device 20 including a cooler 10 according to an embodiment of the present invention. The cooling device 20 includes the cooler 10 and a condenser 21 connected to a container 12. In the condenser 21, the evaporated working fluid 11 is liquefied and returned to the container 12. The cooling device 20 does not require an external power source such as a pump, and is excellent in compactness and energy saving as an entire device.

[0043] <Application> The cooler 10 and cooling device 20 of the present invention can be applied to various electronic devices and other thermal devices with high heat density. Examples include improving the performance of capillary pump loops, semiconductor lasers, cooling data center servers, chlorofluorocarbon-cooled chopper control devices, and power electronic devices. They can also be used as water-cooled jackets installed on the sides or bottom of fire-resistant walls of large waste incinerators and other facilities to cool them from the outside and reduce damage. [Example]

[0044] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0045] <Test Example 1> Experimental equipment A pool boiling experimental apparatus with the configuration shown in Figure 9 was prepared as the experimental equipment. The pool boiling experimental apparatus shown in Figure 9 used a borosilicate glass tube as the cooler container, an electrically insulating fluid HFE7100 (manufactured by 3M Japan Ltd.) as the working fluid, and a rectangular ITO (indium tin oxide) film as the heating element. The ITO film (heating element) was located at the bottom of the cooler container and was installed so that the working fluid in the container came into contact with its surface.

[0046] More specifically, a silicone sheet and an ITO film were placed on a Teflon flange and pressed down to seal. A circuit was then constructed by soldering wires to the electrodes of the ITO film (heating element) described below. Heating was performed using a Stabitron DC power supply (SCIZ-2B15, manufactured by Nippon Stabilizer Industrial Co., Ltd.). For the experimental subject (3) described below, a thin stainless steel wire was placed on top of the ITO film and heated intermittently using a DC power supply (ZX-S-800LAN, manufactured by Takasago Seisakusho Co., Ltd.). An 87 mm inner diameter borosilicate glass tube was secured at both ends with flanges to form a pool vessel. The working fluid was maintained at saturation temperature using a preheater, and its temperature was measured using a calibrated thermocouple. The working fluid height was 100 mm, and a condenser was attached to the top of the pool vessel to prevent changes in the liquid volume due to boiling. The temperature at the bottom of the ITO film was measured using an infrared camera (FLIR A6700sc) through a mirror (gold mirror: Suruga Seiki S03-25-1 / 10). The reflector was copper coated with blackbody spray (ε = 0.94).

[0047] ITO film (heating element) design Figure 10 shows the details of the ITO film design. Cr (30 nm) and Au (200 nm) were deposited on both ends of a sapphire substrate (40 mm x 40 mm, 1 mm thick) as electrodes, and an indium-tin oxide (ITO) film (250 nm) and TiO2 (100 nm) were deposited in the central area (20 mm x 10 mm) as the heating element. The ITO film acts as a resistance when heated by electrical current, and the TiO2 film is deposited on top of the ITO film to improve wettability. The ITO film is a conductive film that is transparent to visible light but opaque to infrared light with wavelengths of 3 to 5 μm. Sapphire is also transparent to both visible light and infrared light with wavelengths of 3 to 5 μm. Therefore, the temperature at the bottom of the ITO film can be measured by taking advantage of the fact that the ITO film is opaque to infrared light with wavelengths of 3 to 5 μm and the sapphire is transparent.

[0048] ·Fine stainless steel wire Figure 11 shows the details of the placement of the stainless steel thin wire. The stainless steel thin wire (diameter 50 μm, length 4 cm) was placed between the surface of the heating element and the honeycomb porous body, so as not to come into contact with the electrodes. During the experiment, a DC power supply was used to apply electrical heating intermittently at 37 W, taking care not to cause thermal damage to the surface of the heating element.

[0049] Honeycomb porous body The porous honeycomb used in (2) and (3) below was a commercially available NA honeycomb (NA-180CR) used for automobile exhaust gas treatment, as shown in Figure 12. The NA honeycomb had a rectangular outer shape measuring 20 mm × 20 mm, a cell width of 5.0 mm, and a wall thickness of 1.0 mm. The NA honeycomb's components were 30–50 mass% calcium aluminate (CaO·Al2O3), 40–60 mass% fused silica (fused SiO2), and 5–20 mass% titanium dioxide (TiO2), and its effective thermal conductivity was 4 W / (m·K). Furthermore, the logarithmic differential pore volume distribution of the NA honeycomb revealed a median pore radius of 0.129 μm, an average pore radius of 0.0372 μm, and a porosity of 24.8%. The logarithmic differential pore volume distribution revealed that the pores in the honeycomb porous body used in this experiment were relatively uniform, and that the median pore radius was also on the submicron order, making it an extremely dense porous body.The honeycomb porous body was fixed onto the surface of the heating element by pulling it vertically downward with equal force using two 0.3 mm diameter stainless steel wires against the surface of the heating element.

[0050] ·experiment Three types of experimental surfaces were prepared: (1) a bare surface (nothing on the ITO film), (2) a honeycomb porous body only, and (3) a thin stainless steel wire sandwiched between the honeycomb porous body and the ITO film. For each of the experimental devices (1) to (3), the working fluid was boiled for 1 hour using a preheater to degas the fluid. Next, the ITO film was heated by passing electricity from a DC power source at atmospheric pressure and at the saturation temperature to generate heat.

[0051] To confirm durability, after the above-mentioned boiling experiment, (1) was left at room temperature for 0, 1, and 4 hours, (2) for 9, 20, and 24 hours, and (3) for 6, 9, 24, and 46 hours, and then each was heated again and a boiling experiment was performed. Therefore, with condensation when returning to room temperature, the pores on the surface of the heating element and in the honeycomb porous body gradually become all wet, making it difficult for boiling to occur when reheated and prone to overheating. The boiling curves obtained from these experiments for the test subjects (1) to (3) are shown in FIG.

[0052] According to Figure 13, for ONB, in the case of bare surface (1), the degree of superheat (ΔT sat ) was approximately 21 K. In the case of (2) where only the honeycomb porous body was provided, the superheat dropped to approximately 14 K 9 hours after the honeycomb porous body was immersed in the working fluid, but after 24 hours it rose to 23 K, almost the same superheat as that of the bare surface. This is because the pores in the honeycomb porous body acted as bubble nuclei immediately after immersion, promoting foaming, but as time passed, the highly wettable working fluid filled the pores and the bubble nuclei disappeared.

[0053] (3) When a thin stainless steel wire was placed between the honeycomb porous body and the ITO film and heated, the superheat level dropped to about 13 K after 6 hours, and boiling could be initiated at a stable low superheat level of about 12 K even after 46 hours.

[0054] From the above, the allowable temperature of the electronic element is ΔT sat= 24.3K (approximately 80°C), which means that when only the bare surface or the honeycomb porous body was provided, the surface temperature of the heating element rose to a temperature that would damage the electronic elements, but by using the honeycomb porous body and heated thin metal wire, it was possible to stably reduce the degree of superheat regardless of the passage of time. This is thought to be due to the effect of the honeycomb porous body mentioned above, as well as the fact that the heated thin metal wire was heated by electrical current, causing bumping on the thin metal wire, which then generated bubbles that seeded bubble nuclei in the existing pores of the honeycomb porous body, promoting foaming. The above test example was conducted using the working fluid (HFE7100) that most easily wets the pores of the honeycomb porous body, but it is believed that the same effect will be obtained with all other working fluids. In addition, in this example, heating was performed using thin metal wires to seed boiling nuclei (gas bubbles) in the pores of the porous body near the surface of the heating element, but it is believed that a similar effect can be obtained even if heating is performed using a thin metal film instead, as bubbles will be generated by heating. [Explanation of symbols]

[0055] 10 Cooler 11 Working fluid 12 containers 13 Heating element 14 Cooling element 15 Fine metal wire 16 Working fluid supply section 17 Steam exhaust section 18 Working fluid introduction body 19 Working fluid inlet 20 Cooling device 21 Capacitor 22 Metallic thin film

Claims

1. A boiling type cooler for cooling a heat generating body, a vessel containing a working fluid; a cooling member made of a porous material and provided in the container so as to face a surface of the heat generating element; At least one thin metal wire or thin metal film that is provided between the surface of the heating element and the cooling member and configured to be heatable; A cooler comprising:

2. 2. The cooler according to claim 1, wherein the thin metal wire or thin metal film is configured to be heatable by passing an electric current through it.

3. The cooler according to claim 1 or 2, wherein a plurality of the thin metal wires or thin metal films are provided.

4. The cooler according to any one of claims 1 to 3, wherein the porous body comprises a working fluid supply section that supplies the working fluid to the surface of the heating element by capillary action, and a steam discharge section that discharges steam generated on the surface of the heating element to the working fluid side.

5. 5. The cooler according to claim 4, wherein the porous body has a honeycomb structure.

6. A cooler as described in any one of claims 1 to 5, further comprising a working fluid introduction body that is stacked on the working fluid side of the porous body and introduces the working fluid into the porous body.

7. The cooler according to any one of claims 1 to 6, a condenser connected to the vessel of the cooler for liquefying the evaporated working fluid; A cooling device equipped with:

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