Boiling heat transfer surface of a boiling condenser and method for forming the same

Abstract

The present invention provides a boiling heat transfer surface for a boiling cooler and a method for forming the same, which can improve cooling performance by quickly removing large bubbles from the heat transfer surface so that they do not become covered on the boiling heat transfer surface. [Solution] The boiling heat transfer surface of the boiling cooler receives heat from an external heating element and boils the internal liquid-phase coolant, causing it to change from liquid to vapor. The boiling heat transfer surface 1 is formed by bending the tip ends of a plurality of plate-shaped fins 3 that are formed standing upright in parallel at a predetermined distance from a metal substrate 2 and joining them to adjacent fins 3 to form cavities 4 between the fins 3. A plurality of small holes 5 are formed at predetermined distances from the upper end of adjacent cavities 4. The inner wall 3a on one side and the inner wall 3b on the other side of the cavity 4 are inclined toward the other side of the metal substrate 2, and the bottom surface 4a is formed as an inclined surface that gets higher from one side to the other. The small holes 5 are formed by grooves 51 formed on the tip side of the fins 3, and the tip side of the fins 3 is bent from a virtual bending line BL that connects the bottoms 51a of the plurality of grooves 51.

Description

Technical Field

[0001] The present invention relates to a boiling heat transfer surface for boiling a liquid-phase refrigerant in a boiling cooler that efficiently cools heat generated from a heating element such as an electronic component.

Background Art

[0002] With the improvement in performance and miniaturization of semiconductor devices and electronic equipment, etc., the heat generation density thereof has also increased, so a highly efficient and small cooling device is required. To meet this need, various cooling devices have been proposed and put into practical use. As a cooling device, a boiling cooler that uses a boiling cooling method for transporting and radiating heat using a vaporization and condensation cycle of a refrigerant and does not require a driving part such as a pump is expected. In this boiling cooler, the liquid-phase refrigerant injected in advance is vaporized according to the amount of heat generated to generate steam, and the moment when the liquid-phase refrigerant vaporizes into steam is the most efficient in heat transfer. Therefore, by creating many situations where the liquid-phase refrigerant vaporizes, the cooling performance as a cooler can be improved.

[0003] In a boiling cooler, a boiling heat transfer surface for vaporizing a liquid-phase refrigerant into steam is provided, and various boiling heat transfer surfaces for improving the cooling performance have been proposed. As an example, in Japanese Patent Application Laid-Open No. 2014-75563 (Patent Document 1), a boiling heat transfer surface is formed at an internal part corresponding to a heating element, and on this boiling heat transfer surface, a plurality of fins are erected from a metal substrate, the tip side of the fin is bent into an arc shape, the bent tip of this fin is joined to the back surface of another adjacent fin, and a plurality of small holes for allowing bubbles generated by boiling of the liquid-phase refrigerant to pass through are formed at the top.

[0004] Furthermore, the method for forming the boiling heat transfer surface disclosed in Patent Document 1 is to form a plurality of concave grooves at a predetermined interval on the surface of a metal material forming the boiling heat transfer surface, tilt a cutting tool at a predetermined angle and move it to cut and raise from the surface of the metal substrate deeper than the concave grooves to form the plate-shaped fins standing up while curling, and form a plurality of small holes of cut groove strips formed by the concave grooves on the tip side. The tip of each fin is bent into an arc shape, and the bent tip of the fin is brought into contact with the back surface of another fin that has been formed in front of it. Subsequently, the fin is cut upright with a cutting tool, and the tip of this fin is joined to the back surface of the other fin in front of it. This process is repeated sequentially, thereby forming a boiling heat transfer surface with multiple plate-like fins, each with an arc-shaped bend at its tip. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2014-75563 [Disclosure of the Invention] [Problems that the invention aims to solve]

[0006] As described in Patent Document 1, the boiling heat transfer surface of a boiling cooler is bonded to a heating element, such as a semiconductor element, that requires cooling. When the temperature of the boiling heat transfer surface rises above the foaming point of the liquid-phase refrigerant in the cavity, the liquid-phase refrigerant is vaporized into steam. This steam forms bubbles, generating buoyancy, which eventually rises within the cavity and passes through small holes. In this boiling cooling method, the moment the liquid-phase refrigerant vaporizes into steam has the highest heat transfer efficiency. Therefore, creating more situations where the refrigerant vaporizes improves the cooling efficiency.

[0007] However, even if the rate of bubble generation due to boiling can be improved on the boiling heat transfer surface, adjacent bubbles on the boiling heat transfer surface will merge, causing the bubbles to enlarge. These enlarged bubbles cover the boiling heat transfer surface, preventing the liquid phase refrigerant from directly contacting the surface, thus inhibiting boiling and significantly reducing cooling performance. Furthermore, these bubbles will merge with the next bubbles that form, growing into even larger combined bubbles that will broadly cover the boiling heat transfer surface. As a result, the contact area between the large combined bubbles and the heat transfer surface begins to dry, causing the heat transfer surface temperature to rise rapidly, and the cooling effect to rapidly decrease. If this condition continues, the heat transfer surface covered with large bubbles will dry completely, eventually leading to burnout and the loss of its function as a boiling cooler. Therefore, it was necessary to quickly remove bubbles generated from the evaporation surface.

[0008] The boiling cooler described in Patent Document 1 has multiple fins that are erected vertically from a metal substrate, forming a tunnel-shaped cavity between adjacent fins. However, when the bottom surface of the cavity is heated, numerous bubbles are generated sequentially. Due to the structure of this cavity, the bubbles tend to accumulate at the bottom and grow into large aggregated bubbles. As a result, the bubbles cannot be quickly removed from the evaporation surface, and boiling is inhibited because the liquid phase refrigerant does not come into direct contact with these aggregated bubbles, leading to a significant decrease in cooling performance.

[0009] Furthermore, the boiling cooler formation method described in Patent Document 1 involves tilting a cutting tool at a predetermined angle and moving it to cut deeper than the groove from the surface of the metal substrate, thereby curling and uprighting a plate-shaped fin, bending the tip into an arc shape, and bringing the arc-shaped tip into contact with the back surface of other fins that had been formed in advance. However, the shape of the arc-shaped tip and the height of the fin tip vary depending on the cutting state of the cutting tool or the metal substrate used. Therefore, There is a problem in that the state of air bubbles being discharged from the small holes formed at the arc-shaped tip varies depending on the location.

[0010] The problem that the present invention aims to solve is to provide a boiling heat transfer surface for a boiling cooler and a method for forming the same, which can improve cooling performance by quickly removing large bubbles from the heat transfer surface so that they do not become covered on the boiling heat transfer surface. [Means for solving the problem]

[0011] To solve this problem, the invention described in claim 1 is a boiling heat transfer surface for a boiling cooler that receives heat from an external heating element to boil an internal liquid-phase coolant and change its phase from liquid to vapor, wherein the boiling heat transfer surface is formed by bending the tip sides of a plurality of plate-shaped fins that are formed to stand upright in parallel at a predetermined distance apart from a metal substrate and joining them to adjacent fins to form cavities between the fins, a plurality of small holes are formed at predetermined distances apart at the upper ends of adjacent cavities, the inner walls on one side and the other side of the cavity are inclined toward the other side of the metal substrate, the bottom surface is formed as an inclined surface that gets higher from one side to the other, the small holes are formed by grooves formed on the tip side of the fins, and the tip side of the fins is bent from a virtual bending line connecting the bottoms of the plurality of grooves.

[0012] Furthermore, the invention described in claim 2 is characterized in that the angle between the lowest part of the inclined surface formed on the bottom surface of the cavity and the lower part of the inner wall on one side is formed at an acute angle.

[0013] Furthermore, the invention described in claim 3 has a protrusion that projects into the cavity on the bottom surface of the cavity corresponding to the small hole formed on the tip side of each fin.

[0014] Furthermore, the invention described in claim 4 is a method for forming a boiling heat transfer surface of a boiling cooler, comprising a metal substrate and an excavation tool having a plurality of grooves formed at predetermined intervals on the cutting edge of the tip in the direction of movement, the excavation tool being moved to one side at a predetermined angle inclined with respect to the metal substrate, the cutting edge being used to excavate from the surface of the metal substrate to form plate-shaped fins upright, and the grooves formed on the cutting edge being used to form cut grooves at the tips of the fins, the process of which is repeated, the metal substrate being formed with a plurality of parallel adjacent fins, the tip sides of the plurality of fins being pressed from above the fin group with a flat-punching tool to bend them along a virtual bending line connecting the bottoms of the plurality of cut grooves, and the bent tip sides of each to be joined to the adjacent fins, thereby forming a cavity between the adjacent fins.

[0015] Furthermore, in the invention described in claim 5, the protrusion formed on the bottom surface of the cavity protrudes with the same cross-sectional shape as the groove formed on the blade of the excavating tool when the fin is erected using the excavating tool.

[0016] Furthermore, in the invention described in claim 6, after excavating the metal substrate with the excavating tool and tilting the metal substrate at a predetermined angle to form the plate-shaped fins upright, the excavating tool is retracted and raised, then moved horizontally to one side to press the fins, thereby increasing the inclination angle of the fins from the metal substrate. [Effects of the Invention]

[0017] According to the present invention, by forming the inner walls of one side and the other side of the cavity formed between adjacent fins as inclined surfaces tilted toward the other side of the metal substrate, and further forming the bottom surface of the cavity as an inclined surface that rises from one side to the other, the bottom surface, which is the boiling heat transfer surface within the cavity, can be quickly removed to prevent it from being covered by large bubbles, thereby improving cooling performance. Specifically, when the bottom surface of the cavity is heated by an external heating element, the liquid phase refrigerant is vaporized into vapor, and this vapor becomes bubbles, sequentially generating small bubbles on the bottom surface. As these small bubbles are heated, they gradually grow larger, generating buoyancy. However, because the bubbles have surface tension, the surface tension and buoyancy balance out, and the bubbles move toward the higher side of the inclined surface without detaching from the bottom surface. Therefore, bubbles on the bottom surface can be quickly removed. As a result, the bottom surface, which is the boiling heat transfer surface, is not covered by large bubbles, thus improving cooling performance. Furthermore, as the bubbles on the bottom surface move along the inclined surface, they reach a high point on the inclined surface without merging with each other, and then rise up the inner wall of the cavity. At this time, because the inner wall is inclined, the surface tension of the bubbles prevents them from detaching from the inner wall, and they rise while in contact with the inner wall, eventually being discharged from multiple small holes formed at the upper end of the cavity. Even at this stage, because the inner wall of the cavity is inclined, the behavior of the bubbles as they rise is corrected by the inclined surface, allowing them to be discharged smoothly from the small holes, and preventing the bubbles from merging during this rising process as well.

[0018] Furthermore, by bending from a virtual bending line connecting the bottoms of the grooves formed on the tip side of the fins and joining them to adjacent fins, multiple small holes can be easily formed at the upper end of the cavity. Also, since the bending occurs from a virtual bending line connecting the bottoms of the grooves, the bending position becomes constant, eliminating positional variations of the multiple small holes formed at the upper end of the cavity, thus allowing air bubbles to be smoothly discharged from the small holes.

[0019] Furthermore, according to the invention described in claim 2, since the angle between the lowest part of the inclined surface formed on the bottom surface of the cavity and the lower part of the inner wall on one side is formed as an acute angle, small bubbles can be sequentially generated concentrated from this acute angle. Because this acute angle is at the very bottom of the inclined surface formed on the bottom surface, the generated small bubbles can be sequentially moved by the inclined surface on the bottom surface. At this time, a small amount of bubbles are generated from the top of the bottom surface, but since these small bubbles sequentially move towards the higher direction of the inclined surface formed on the bottom surface, they can be quickly removed without merging.

[0020] Furthermore, according to the invention described in claim 3, if a protrusion is projected from the bottom surface of the cavity corresponding to the small hole formed on the tip side of each fin, a right-angle corner is formed between the bottom surface and the protrusion, and bubbles can be generated from this corner. In addition, the area of ​​the boiling heat transfer surface is increased, which can create more conditions for the liquid phase refrigerant to vaporize, thereby improving the cooling performance of the cooler.

[0021] Furthermore, according to the invention of the method for forming a boiling heat transfer surface described in claim 4, by digging out from the surface of the metal substrate with the digging tool tilted at a predetermined angle relative to the metal substrate and forming plate-shaped fins upright, an inclined surface for moving air bubbles on the bottom surface of the cavity can be formed. In addition, by forming multiple recesses on the blade of the digging tool, when forming the fins upright, small holes can be simultaneously formed on the tip side of the fins. Furthermore, since the line connecting the bottoms of the cut grooves is used as a virtual bending line, the width of the fins that are bent by these cut grooves is reduced, and when flattening is performed from above with a flattening tool on a group of fins consisting of multiple fins, the fins bend from the virtual bending line, so the bending position can be kept constant, and the group of fins can be formed on a plane of uniform height, and as a result, the height of the small holes from the metal substrate can be made equal.

[0022] Furthermore, according to the invention described in claim 5, the groove formed on the blade of the excavation tool forms a protrusion on the bottom surface of the cavity, so the protrusion can be easily formed during the process of forming the fins upright.

[0023] Moreover, in the invention according to claim 6, after the metal substrate is lifted by the lifting tool to form the fins standing upright, the fins standing upright are pressed by the lifting tool to increase the inclination angle from the metal substrate. Therefore, when performing flat rolling from above thereafter, the fins can be bent from the virtual bending line. For this reason, the volume of the cavity can be made constant.

Brief Description of the Drawings

[0024] [Figure 1] It is a perspective view of the main part showing the boiling heat transfer surface of the boiling cooler according to the present invention. [Figure 2] It is a cross-sectional view of the main part of the boiling heat transfer surface shown in FIG. 1. [Figure 3] It is a perspective view showing the overall configuration of the boiling heat transfer surface [Figure 4] (A) to (C) are explanatory diagrams showing the behavior of bubbles on the boiling heat transfer surface. [Figure 5] (A) to (F) are process explanatory diagrams showing the process of forming the boiling heat transfer surface. [Figure 6] It is a perspective view showing the lifting tool. [Figure 7] It is a perspective view showing the state where the fins are formed standing upright by the lifting tool. [Figure 8] It is a front view of the main part showing the fins formed standing upright. [Figure 9] It is an explanatory diagram showing the state of bending the fins

Best Mode for Carrying Out the Invention

[0025] The boiling heat transfer surface of the boiling cooler receives heat from an external heating element, causing the internal liquid-phase coolant to boil and undergo a phase change from liquid to vapor. The boiling heat transfer surface is formed by bending the tip ends of a plurality of plate-shaped fins that are formed standing upright in parallel at a predetermined distance apart from a metal substrate and joining them to adjacent fins to form cavities between the fins. A plurality of small holes are formed at predetermined distances apart at the upper ends of adjacent cavities. The inner walls of one side of the cavity and the inner walls of the other side are inclined toward the other side of the metal substrate, and the bottom surface is formed as an inclined surface that gets higher from one side to the other. The small holes are formed by grooves formed on the tip side of the fins, and the tip side of the fins is bent from a virtual bending line connecting the bottoms of the plurality of grooves.

[0026] Furthermore, the method for forming the boiling heat transfer surface of the boiling cooler includes a metal substrate and an excavation tool having a plurality of grooves formed at predetermined intervals on the cutting edge of the tip in the direction of movement. The excavation tool is moved to one side at a predetermined angle inclined with the metal substrate, and the cutting edge excavates from the surface of the metal substrate to form plate-shaped fins upright. The grooves formed on the cutting edge form cutting grooves at the tips of the fins. This process is repeated to form a fin group on the metal substrate with a plurality of parallel adjacent fins. A flat-pressing tool is used from above the fin group to press the tips of the plurality of fins, causing them to bend along a virtual bending line connecting the bottoms of the plurality of cutting grooves. The bent tips are then joined to adjacent fins, forming cavities between adjacent fins.

[0027] The boiling heat transfer surface of the boiling cooler according to the present invention will be described in detail below with reference to the drawings. Figure 1 shows an example of the boiling heat transfer surface 1 according to the present invention. The boiling heat transfer surface 1 is formed of a metal substrate 2 such as aluminum, an aluminum alloy, or copper, which has good thermal conductivity and is machineable. Multiple plate-shaped fins 3 are formed on the surface of the metal substrate 2, standing upright in parallel at predetermined intervals. Between each fin 3, as shown in Figure 2, a cavity 4 is formed by bending the tip side of each fin 3 and joining it to the adjacent fin 3. At the upper end of this cavity 4, on the tip side of the bent fin 3, multiple small holes 5 are formed at predetermined intervals. In addition, one inner wall 3a and the other inner wall 3b are formed inside the cavity 4, and these two inner walls 3a and 3b are inclined at an angle θ1 to the other side with respect to the perpendicular to the metal substrate 2. Furthermore, the bottom surface 4a of the cavity 4 is formed as an inclined surface that gets higher from one side to the other. Furthermore, a protrusion 4c is projecting from the bottom surface 4a of the cavity 4.

[0028] Multiple plate-shaped fins 3, which are formed to stand upright from the surface of the metal substrate 2, are formed by excavating the metal substrate 2 using an excavation tool, as will be described later. The thickness of each fin 3 is set according to the required performance of the boiling cooler or the efficiency of the boiling heat transfer surface 1, but is generally 0.1 mm to 1.0 mm and the height is generally 1 mm to 20 mm. In addition, each fin 3 is tilted toward the other side, and an inclined inner wall 3a on the other side and an inner wall 3b on the one side are formed inside the cavity 4. The angle of inclination θ1 is 20 to 60 degrees with respect to the metal substrate 2. The other side is the direction toward which the tip of each fin 3 is facing, as shown in the figure, and the one side is the direction toward the base end of each fin 3.

[0029] The cavities 4 formed between each fin 3 are created by bending the tip of each fin 3 and joining it to the adjacent fin 3. The cross-sectional shape of these cavities 4 is approximately a parallelogram, as shown in the figure. Furthermore, the position where each fin 3 is bent is a virtual bending line BL connecting the bottoms 51a of a plurality of grooves 51 that have been pre-formed to create small holes 5, as shown in Figure 8. The width W of these grooves 51 is the same as the width of the small holes 5, and the depth D is approximately the same as the length L of the small holes 5.

[0030] The reason for bending the tip of each fin 3 from the virtual bending line BL is that the bending strength decreases as the width of the fin 3 bent by the groove 51 is reduced, so when pressed by the flat-punching tool described later, it naturally bends from the virtual bending line BL. When the tip of each fin 3 is bent from the virtual bending line BL, the bending position becomes constant, and the tip of each fin 3 joins with the adjacent fin 3, so that the multiple small holes 5 formed at the upper end of the cavity 4 are formed with high precision. As a result, the cavity 4 formed between adjacent fins 3 is formed uniformly and with high precision. Furthermore, since the tip of each fin 3 is pressed by the flat-punching tool, the upper surface of the boiling heat transfer surface 1 is formed to be almost flat, as shown in Figure 1. The depth and width of the multiple small holes 5 formed at the upper end of the cavity 4 are appropriately set according to the size of the bubbles discharged from the small holes 5.

[0031] Furthermore, the bottom surface 4a of the cavity 4 formed between the fins 3 is formed as an inclined surface that rises from one side to the other. The angle of this inclined surface of the bottom surface 4a is formed at an angle θ2 of 10 to 30 degrees with respect to the surface of the metal substrate 2, as will be described later, so that bubbles on the inclined surface can move smoothly. In addition, an acute angle portion 4b is formed on the bottom surface 4a of the cavity 4, where the lowest point on one side of the inclined surface and the lower part of the inner wall 3a on the same side are at an acute angle. The angle of this acute angle portion 4b is generally between 5 and 15 degrees. Also, a protrusion 4c is formed on the inclined bottom surface 4a that projects into the cavity 4. This protrusion 4c is formed at a position corresponding to the small hole 5, its width W is approximately the same as the width W of the small hole 5, and the angle with the bottom surface 4a is formed at a right angle. By forming this protrusion 4c, many bubbles can be generated from the right angle.

[0032] Figure 3 is a perspective view showing the overall configuration of the boiling heat transfer surface 1, in which multiple plate-shaped fins 3 are formed in parallel at predetermined intervals in the center of the metal substrate 2. Above this boiling heat transfer surface 1, a boiling section container (not shown) is sealed and covered, similar to a well-known boiling cooler, into which a liquid-phase refrigerant is injected so as to immerse the multiple fins 3. In this boiling section container, vapor formed when the liquid-phase refrigerant boils and undergoes a phase change accumulates. A condensing section is connected to the boiling section container, and the vapor of the liquid-phase refrigerant is cooled, causing it to undergo a phase change back into a liquid and return to the boiling section container. In this way, the liquid-phase refrigerant is boiled by the boiling heat transfer surface 1 and undergoes a phase change to vapor, and the vapor undergoes a phase change to reduce it back into the liquid-phase refrigerant, allowing the cooling function to operate continuously for a long period of time.

[0033] Next, the operation of the boiling heat transfer surface 1 according to the present invention described above will be explained with reference to Figure 4. A heat-generating element (not shown), such as a heat-generating semiconductor that requires cooling, is bonded to the lower surface of the metal substrate 2 of the boiling heat transfer surface 1. In addition, a liquid-phase coolant is injected above the metal substrate 2, and a plurality of fins 3 are immersed in it. When the heat from the heat-generating element is transferred to the metal substrate 2 and the bottom surface 4a is heated and reaches the boiling point of the liquid-phase coolant, the liquid-phase coolant vaporizes on the bottom surface 4a and turns into steam, generating bubbles 6 from this steam. It is known that these bubbles 6 are generated using minute depressions as nuclei. Since the bottom surface 4a of the cavity 4 has an acute-angled portion 4b formed at a sharp angle, this acute-angled portion 4b is a minute depression and becomes a foaming point, which is the nucleus for bubbles 6, so bubbles 6 are generated one after another with the acute-angled portion 4b as a specific bubble nucleus. Furthermore, since there are few minute depressions on the bottom surface 4a that can serve as nuclei for bubbles, the generation of bubbles 6 from the bottom surface 4a is minute. However, even a small amount of air bubbles, if allowed to remain on the bottom surface 4a of cavity 4, will hinder the cooling effect, so they must be removed promptly.

[0034] As shown in Figure 4(A), tiny bubbles 6 are generated with the sharp corners 4b, which are minute indentations, as nuclei. Subsequently, heating causes the bubbles 6 to grow and generate buoyancy, but because the bubbles 6 have surface tension, they do not detach from the bottom surface 4a. As the bubbles 6 grow larger, their buoyancy gradually increases, but due to their high surface tension, they adhere to the bottom surface 4a of the cavity 4. However, since the bottom surface 4a is formed as an inclined surface that rises from one side to the other, the upward force due to buoyancy and the attractive force due to surface tension balance out, causing the bubbles 6 to move to the other side where the inclined surface is higher. As the bubbles 6 grow and move along the inclined surface, they eventually reach the inner wall 3b on the other side and rise along the inner wall 3b on the other side due to buoyancy. During this rise, because the inner wall 3b on the other side is inclined, the bubbles 6 do not detach due to the surface tension and continue to rise along the inner wall 3b on the other side, eventually reaching the small holes 5 and being discharged from the cavity 4.

[0035] In this way, tiny bubbles 6, nucleated by the sharp corners 4b, move along the inclined surface of the bottom surface 4a as they grow, preventing the bottom surface 4a from being covered by bubbles 6, and also preventing bubbles 6 from merging with each other. As a result, in a boiling cooler, it is possible to create many conditions in which the liquid-phase refrigerant, which is important for increasing heat transfer efficiency, vaporizes into steam. In addition, bubbles 6 that reach the inner wall 3b on the other side of the cavity 4 rise quickly while in contact with the inclined inner wall 3b on the other side and are discharged from the small holes 5 formed at the upper end of the cavity 4. This prevents bubbles 6 from merging with each other, and also solves the conventional problem where large bubbles 6 cover the bottom surface 4a, drying out the heat transfer surface and causing the boiling cooler to lose its function. In this way, by quickly discharging bubbles 6 from the small holes 5, the liquid-phase refrigerant equivalent to the volume of the discharged bubbles 6 flows into the cavity 4 from other small holes 5 and covers the bottom surface 4a again, thus allowing the boiling cooling action that generates bubbles 6 to continue.

[0036] The angle of the inclined surface formed on the bottom surface 4a of the cavity 4 is set to an angle of 10 to 30 degrees relative to the surface of the metal substrate 2. However, it was confirmed that if the angle of the inclined surface is less than 10 degrees, the balance between the buoyancy and surface tension of the air bubbles 6 is disrupted, causing the air bubbles 6 to remain stationary on the bottom surface 4a without moving along the inclined surface. Furthermore, it was confirmed that if the angle of the inclined surface is 30 degrees or more, the air bubbles 6 move along the inclined surface, but there is no change in the behavior of the air bubbles 6 during movement, and it also reduces the volume of the cavity 4, which is undesirable.

[0037] Figure 4(B) shows a state in which a minute bubble 6, which is generated around the sharp corner 4b in the cavity 4, rises via the inner wall 3b on one side. Bubbles 6 generated from the sharp corner 4b may rise immediately. In this case, the bubble 6 that attempts to rise adheres directly to the inner wall 3a on one side due to surface tension, and then rises as the bubble 6 grows, increasing its buoyancy, and eventually being discharged from the small hole 5. In this way, the bubble 6 rises along the inclined surface of the inner wall 3a on one side due to buoyancy and surface tension.

[0038] Figure 4(C) shows the behavior of a tiny bubble 6 that originates from a fine depression in the bottom surface 4a of the cavity 4. The behavior of the bubble 6 originating from the bottom surface 4a is almost the same as that of the bubble 6 originating from the sharp corner 4b described above. Since the bubble 6 on the bottom surface 4a has surface tension, it gradually increases its buoyancy as it grows without detaching, and the bubble 6 moves to the other side where the inclined surface is higher as the upward force due to buoyancy balances with the attractive force due to surface tension. The grown bubble 6 eventually reaches the inner wall 3b on the other side, rises along the inner wall 3b on the other side due to buoyancy, and is then discharged from the small hole 5.

[0039] Next, the method for forming the boiling heat transfer surface of the boiling cooler will be explained with reference to Figure 5. As mentioned above, the boiling heat transfer surface 1 is formed from a flat metal plate that has good thermal conductivity and can be cut. Plate-shaped fins 3 are formed on the surface of the metal substrate 2 by a cutting tool 10. As shown in Figure 6, the cutting tool 10 has a plurality of grooves 102 formed at predetermined intervals on its bottom surface, and the cutting edges of these grooves 102 also have blades for cutting out the fins 3. The spacing and width of these grooves 102 are the same as those of the small holes 5 mentioned above. In addition, the blade portion 101 of the cutting tool 10 has an angle θ3 with respect to the bottom surface from 5 degrees to 20 degrees. This angle θ3 of the blade portion 101 is the same as the angle of the acute angle portion 4b mentioned above.

[0040] The boiling heat transfer surface 1 is formed by the formation process shown in Figure 5. The excavation tool 10 is attached to a drive device (not shown) at an angle θ4 inclined relative to the upper surface of the metal substrate 2, with its rear end higher than the upper surface. This inclination angle θ4 is set to 10 to 30 degrees, the same inclination angle θ2 as the inclined surface of the bottom surface 4a of the cavity 4 described above.

[0041] First, as shown in Figure 5(A), the blade portion 101 of the digging tool 10 is brought into contact with the surface of the metal substrate 2. Furthermore, the digging tool 10 is moved in the direction indicated by the arrow while tilted by the drive device, and the blade portion 101 of the digging tool 10 is driven into the surface of the metal substrate 2. As a result, as shown in Figure 5(B), the fins 3 are dug out to a predetermined length while sliding along the sliding contact surface 103.

[0042] In this way, during the process of forming the fin 3 upright, the multiple grooves 102 formed on the bottom surface of the excavation tool 10 create grooves 51 at the tip of the fin 3, which later become small holes 5, as shown in Figures 7 and 8. This is because the grooves 102 formed on the excavation tool 10 are set back from the tip of the blade portion 101, causing them to be excavated later, resulting in the formation of grooves 51 that are recessed more than the tip of the fin 3. In addition, the grooves 102 of the excavation tool 10 form a protrusion 4c, as shown in Figure 7. The width and height of this protrusion 4c are the same shape as the grooves 102, and are also approximately the same as the width of the small holes 5. The corners of the protrusion 4c with the bottom surface 4a are formed at right angles. If the grooves 102 are formed with arcs at the corners instead of rectangles as shown, the cross-sectional shape of the protrusion 4c and the shape of the small holes 5 will also be the same as the grooves 102.

[0043] Furthermore, by excavating the fins 3 with the excavation tool 10, an acute-angled portion 4b is formed at the lowest point on one side of the inclined surface formed on the bottom surface 4a. This acute-angled portion 4b is the same angle θ3 as the cutting edge of the excavation tool 10 that forms the fins 3 upright. Since the cutting edge of the excavation tool 10 is formed into a sharp blade for excavating the metal substrate 2, the depth of the acute-angled portion 4b is a minute depression, which becomes a foaming point, a nucleus for bubbles.

[0044] After forming the fin 3 with its bent tip using the excavation tool 10, the tip of the fin 3 is pressed in a direction to one side of the perpendicular to the metal substrate 2, thereby increasing the inclination angle θ1 of the fin 3 to 10 to 60 degrees with respect to the perpendicular to the metal substrate 2.

[0045] As described above, by repeatedly performing the steps of forming fins 3 upright on the surface of the metal substrate 2 and pressing them to tilt them to one side, a fin group 30 is formed on the surface of the metal substrate 2 by multiple parallel and adjacent fins 3, as shown in Figure 5(D). In this state, the upper ends of adjacent fins 3 are spaced apart.

[0046] Subsequently, the fin group 30 formed on the metal substrate 2 is subjected to the flattening process shown in Figure 5(E). In this flattening process, the fin group 30 is pressed from above with a flattening tool 11 whose lower surface is formed flat. As a result of this pressing, as shown in Figure 9, the tip of each fin 3 bends from the virtual bending line BL to the other side, and the tip of each fin 3 joins with the adjacent fin 3. As a result, a cavity 4 is formed between adjacent fins 3. In this way, when a fin group 30 consisting of multiple fins 3 is pressed from above with a flattening tool 11, the fins 3 are precisely bent from the easily bendable virtual bending line BL. Furthermore, since the upper surface of the fin group 30 is formed to be almost flat by the pressing, the upper surface as a boiling heat transfer surface 1 is also formed to be almost flat.

[0047] In this way, when the fin group 30 is pressed from above with the flat-punching tool 11, the tips of the fins 3 join with adjacent fins 3, forming a cavity 4 between the core fins 3, and a number of small holes 5 are formed at predetermined intervals at the upper end of this cavity 4.

[0048] The present invention described above is not limited to these embodiments and can be modified in various ways without departing from the present invention. In the embodiments described above, a plurality of small holes are formed at equal intervals at the upper end of the cavity, but these small holes may be formed at unequal intervals to correspond to the size and location of the heating element joined to the boiling heat transfer surface. The width of the small holes may also be varied depending on the location. [Explanation of Symbols]

[0049] 1. Boiling heat transfer surface 2 Metal substrate 3 fins 3a One side of the inner wall 3b Inner wall on the other side 4 cavities 4a Bottom 4b Acute angle 4c Pier 5 small holes 6 bubbles 10 Excavation Tools 101 Blade part 102 Recessed groove 11 Flat-forging tool 51 Grooves 51a bottom BL virtual curve line

Claims

1. A boiling heat transfer surface of a boiling cooler that receives heat from an external heating element to boil an internal liquid-phase coolant, causing it to change from liquid to vapor, The boiling heat transfer surface is formed by bending the tip ends of a plurality of plate-shaped fins that are formed in parallel at predetermined intervals from a metal substrate and joining them to adjacent fins, thereby creating cavities between the fins, and a plurality of small holes are formed at predetermined intervals at the upper ends of adjacent cavities. The inner walls on one side and the other side of the cavity are inclined toward the other side of the metal substrate, and the bottom surface is formed as an inclined surface that gets higher from one side to the other. The aforementioned small holes are formed by grooves formed on the tip side of the fins. The boiling heat transfer surface of a boiling cooler is characterized in that the tip of the fin is bent from a virtual bending line connecting the bottoms of the multiple grooves.

2. The boiling heat transfer surface of the boiling cooler according to claim 1, wherein the angle between the lowest part of the inclined surface formed on the bottom surface of the cavity and the lower part of the inner wall on one side is formed at an acute angle.

3. The boiling heat transfer surface of a boiling cooler according to claim 1, wherein a protrusion projecting into the cavity is formed on the bottom surface of the cavity corresponding to the small hole formed on the tip side of each of the fins.

4. The aforementioned metal substrate, The digging tool has multiple grooves formed at predetermined intervals on the cutting edge at the tip in the direction of movement, The excavation tool is moved to one side at a predetermined angle relative to the metal substrate, and the blade portion excavates from the surface of the metal substrate to form a plate-shaped fin, and the groove formed on the blade portion creates a cutting groove at the tip of the fin, and this process is repeated, thereby forming a group of fins on the metal substrate with a plurality of parallel adjacent fins. A method for forming a boiling heat transfer surface of a boiling cooler, characterized by pressing the tip ends of a plurality of fins from above the fin group with a flat-punching tool to bend them along a virtual bending line connecting the bottom ends of a plurality of grooves, joining the bent tip ends of each fin to adjacent fins, thereby forming a cavity between adjacent fins.

5. The method for forming a boiling heat transfer surface of a boiling cooler according to claim 4, wherein the protrusion formed on the bottom surface of the cavity protrudes with the same cross-sectional shape as the groove formed on the blade of the excavating tool when the fin is erected using the excavating tool.

6. A method for forming a boiling heat transfer surface of a boiling cooler according to claim 4, wherein the metal substrate is excavated with the excavating tool to form a plate-shaped fin upright by tilting it at a predetermined angle with the metal substrate, the excavating tool is then retracted and raised, then moved horizontally to one side to press the fin, thereby increasing the tilt angle of the fin from the metal substrate.

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