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 a boiling cooler receives heat from an external heating element to boil the internal liquid-phase coolant, causing a phase change from liquid to vapor. The boiling heat transfer surface 1 is formed by bending the tips of multiple 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, thereby forming cavities 4 between adjacent fins 3. Multiple small holes 5 are formed at predetermined distances from the upper end of the cavities 4. The cavities 4 have an inclined surface on one side (inner wall 3a) and an inclined surface on the other side (inner wall 3b) that is tilted toward one side of the metal substrate 2, and a bottom surface 4a that is tilted upward from one side to the other.

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, electronic devices, etc., their heat generation density 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 heat transport and heat dissipation using a vaporization and condensation cycle of a refrigerant and does not require a driving part such as a pump is expected. This boiling cooler vaporizes a previously injected liquid-phase refrigerant according to the amount of heat generated to generate steam, and the moment when the liquid-phase refrigerant vaporizes into steam has the highest heat transfer efficiency. Therefore, by creating many situations where the liquid-phase refrigerant vaporizes, the cooling performance as a cooler can be improved.

[0003] A boiling heat transfer surface for vaporizing a liquid-phase refrigerant into steam is provided in the boiling cooler, and various boiling heat transfer surfaces for improving 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 raised from a metal substrate, the tip side of the fin is bent into an arc shape, the bent tip part 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 predetermined intervals 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 them, 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, thereby forming cavities between adjacent fins, a plurality of small holes are formed at predetermined distances apart at the upper end of the cavities, the inner walls on one side of the cavities and the inner walls on the other side are inclined toward one side of the metal substrate, and the bottom surface is formed as an inclined surface that gets higher from one side to the other.

[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 involves forming a bent portion on the tip side of each fin, and bending the tip side of the fin from the bent portion to make the height of the multiple small holes from the metal substrate equal.

[0014] Furthermore, the invention described in claim 4 further includes a projection that protrudes into the cavity on the bottom surface of the cavity corresponding to the small hole formed on the tip side of each fin.

[0015] Furthermore, the invention described in claim 5 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 step of moving the excavation tool to one side at a predetermined angle inclination with the metal substrate, excavating from the surface of the metal substrate with the cutting edge to form a plate-shaped fin upright, and forming a notch at the tip of the fin with the grooves formed on the cutting edge, and repeating the step of raising the excavation tool and moving it horizontally to one side, pressing the tip of the fin to one side of the perpendicular of the metal substrate to incline the fin at a predetermined angle, thereby forming a fin group with a plurality of parallel adjacent fins on the metal substrate, pressing the tip of a plurality of fins from above with a flattening tool to bend them, joining the bent tip of each to an adjacent fin, and forming a cavity between adjacent fins.

[0016] Furthermore, the invention described in claim 6 further comprises a first sliding contact surface formed at a small angle with respect to the bottom surface of the digging tool, and a second sliding contact surface formed at a larger angle continuously with respect to the other side of the first sliding contact surface, wherein the dimension of the first sliding contact surface in the direction of movement is the dimension from the base end of the fin to the bottom of the notch, and when the fin is erected by the digging tool, the tip side of the fin is bent at a predetermined angle at the boundary between the first sliding contact surface and the second sliding contact surface to form a bent portion.

[0017] Furthermore, the invention described in claim 7 is such that when the fin is erected using the excavation tool, 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 excavation tool. [Effects of the Invention]

[0018] 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 one 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 inside the cavity, can be quickly removed to prevent it from being covered with large bubbles, thereby improving cooling performance. In other words, when the bottom surface inside the cavity is heated by an external heating element, The liquid-phase refrigerant is vaporized into vapor, and this vapor forms bubbles, which sequentially generate 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, causing them to move towards the higher side of the inclined surface without detaching from the bottom surface. Therefore, the 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 with large bubbles, thus improving cooling performance. Furthermore, as the bubbles on the bottom surface move along the inclined surface, they do not merge with each other. After reaching a higher position on the inclined surface, they rise along 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 through multiple small holes formed at the upper end of the cavity. In this case as well, because the inner wall of the cavity is sloped, the behavior of the bubbles as they rise is corrected by the sloped surface, allowing them to be smoothly discharged from the small holes, and preventing the bubbles from merging during this rising process.

[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 the bubbles merging.

[0020] Furthermore, according to the invention described in claim 3, a bent portion is formed on the tip side of each fin, and the tip side of the fin is bent from the bent portion, so that the height of the fins becomes equal, and moreover, the height of the multiple small holes at the tip from the metal substrate can be made equal. As a result, variations in the position of the small holes are eliminated, and air bubbles that have risen inside the cavity can be stably discharged.

[0021] Furthermore, according to the invention described in claim 4, 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.

[0022] Furthermore, according to the invention of the method for forming a boiling heat transfer surface described in claim 5, by excavating from the surface of the metal substrate with the excavation tool inclined 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. Furthermore, after forming the plate-shaped fins upright, the fins are pressed to one side with the excavation tool and the upright fins are inclined at a predetermined angle, so that inclined surfaces can be formed on the inner walls of one side and the inner walls of the cavity formed between adjacent fins. In addition, by flattening the fin group consisting of multiple fins from above with a flattening tool, the tip side of each fin is pressed and bent, and the tip side can be joined to the adjacent fin. Furthermore, by flattening, the tip side of the fin group is formed to a uniform height, so that the height of the small holes from the metal substrate can be made equal.

[0023] Furthermore, according to the invention described in claim 6, by forming a first sliding contact surface with a small angle and a second sliding contact surface with a large angle continuously following the first sliding contact surface on the blade part of the excavating tool, when forming the fins upright, a bending part with an accurate angle can be formed on the tip side of the fins at the boundary between the first sliding contact surface and the second sliding contact surface. Due to this bending part, the bending position on the tip side of the fins is fixed, and when the fins are pressed to one side by the excavating tool, the tip side can be accurately joined to the adjacent fins.

[0024] Furthermore, according to the invention described in claim 7, by the concave groove formed on the blade part of the excavating tool, a dike on the bottom surface of the cavity can be formed, so that the dike can be easily formed during the process of forming the fins upright.

Brief Description of the Drawings

[0025] [Figure 1] It is a partial perspective view showing the boiling heat transfer surface of the boiling cooler according to the present invention. [Figure 2] It is a partial cross-sectional view 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 of the boiling cooler [Figure 4] (A)-(C) are explanatory diagrams showing the behavior of bubbles on the boiling heat transfer surface. [Figure 5] (A)-(G) are process explanatory diagrams showing the process of forming the boiling heat transfer surface. [Figure 6] It is a perspective view showing the excavating tool. [Figure 7] It is a front view showing the state where the fins are formed upright by the excavating tool [Figure 8] It is a perspective view showing the state where the fins are formed upright by the excavating tool.

Best Mode for Carrying Out the Invention

[0026] 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 tips of a plurality of plate-shaped fins, which are formed standing upright in parallel at a predetermined distance from a metal substrate, and joining them to adjacent fins, thereby creating cavities between adjacent fins. The inner walls on one side and the other side of the cavities are inclined toward one side of the metal substrate, and the bottom surface is formed as an inclined surface that becomes higher from one side to the other.

[0027] Furthermore, the method for forming the boiling heat transfer surface of a boiling cooler includes a metal substrate for forming the boiling heat transfer surface and a drilling tool having a plurality of grooves formed at predetermined intervals on the cutting edge at the tip in the direction of movement. The drilling tool is moved to one side at a predetermined angle inclined with the metal substrate, and the cutting edge is used to drill out plate-shaped fins from the surface of the metal substrate to form upright fins, while the grooves formed on the cutting edge create notches at the tips of the fins. The drilling tool is then raised and moved horizontally to one side, and the tip of the fin is pressed to one side of the perpendicular to the metal substrate to tilt the fin at a predetermined angle. This process is repeated to form a group of fins on the metal substrate with a plurality of parallel adjacent fins. A flat-punching tool is used from above the group of fins to press and bend the tip of each of the fins, and the bent tip is joined to the adjacent fin to form a cavity between the adjacent fins.

[0028] 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. The tip of each fin 3 is bent and joined to the adjacent fin 3, and a cavity 4 is formed between each fin 3, as shown in Figure 2. 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 formed to be inclined to one side with respect to the perpendicular of the metal substrate 2. Furthermore, the bottom surface 4a of the cavity 4 is formed as an inclined surface that becomes higher from one side to the other. Furthermore, a cavity 4 protrusion 4c is projected from the bottom surface 4a of cavity 4.

[0029] Multiple plate-shaped fins 3, which are formed to stand upright from the surface of the metal substrate 2, are formed by excavating the surface of 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 one side, and an inclined inner wall 3a on one side and an inner wall 3b on the other side are formed inside the cavity 4. The angle of inclination θ1 is 10 to 60 degrees with respect to the metal substrate 2. Note that "one side" is the direction in which the tip of each fin 3 is facing, as shown in the figure, and "the other side" is the direction in which the base end of each fin 3 is facing.

[0030] 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. A bent portion 3c is formed on the tip of each fin 3, and the fin bends from this bent portion 3c. The bent portion 3c is formed by a cutting tool in the formation process described later, and the tip is bent in a V-shape from the bent portion 3c. At the end of the formation process, the tips of multiple fins 3 are pressed and bent with a flat pressing tool, thereby joining the tips of each fin 3 to the adjacent fin 3. In this way, the tips of the fins 3 can be accurately bent by the bent portion 3c, so that the cavities 4 formed between adjacent fins 3 can be formed accurately and uniformly. In this way, by bending the tips of multiple plate-shaped fins 3 that are formed standing upright in parallel at a predetermined distance apart from the metal substrate 2, and joining the tips to the adjacent fins 3, the upper surface of the boiling heat transfer surface 1 is formed to be almost flat, as shown in Figure 1.

[0031] Furthermore, the small holes 5 formed at a predetermined distance apart at the upper end of the cavity 4, on the tip side of the bent fin 3, have a depth equal to the length from the bent portion 3c to the tip, and their width is appropriately set according to the size of the bubbles discharged from the small holes 5.

[0032] Furthermore, the bottom surface 4a of the cavity 4 formed between the fins 3 is formed as an inclined surface that gets higher 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, so that bubbles on the inclined surface can move smoothly, as will be described later. 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 is approximately the same as the width of the small hole 5, and its corner 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.

[0033] 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 become vapor, and the vapor undergoes a phase change back into the liquid-phase refrigerant, allowing the cooling function to operate continuously for a long period of time.

[0034] 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 into vapor on the bottom surface 4a, and bubbles 6 made of this vapor are generated. 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, so bubbles 6 are generated one after another with the acute-angled portion 4b as a specific bubble nucleus. Also, since there are few nuclei on the bottom surface 4a, the generation of bubbles 6 is minute.

[0035] 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 gets higher 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. Even during this ascent, the bubble 6 has surface tension, so it does not detach from the inner wall 3b on the other side. Furthermore, since the upper side of the inner wall 3b on the other side is inclined to the one side, the free ascent of the bubble 6 is prevented, and it eventually reaches the small hole 5 and is discharged from the cavity 4.

[0036] In this way, tiny bubbles 6, nucleated by the acute angle 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 the 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 quickly rise 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, so that the bubbles 6 do not merge with each other, and the conventional problem of large bubbles 6 covering the bottom surface 4a and drying out the heat transfer surface, thereby rendering the boiling cooler inoperable, is resolved. In this way, by quickly discharging the 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, covering the bottom surface 4a again and performing the boiling cooling action that generates bubbles 6.

[0037] 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.

[0038] Figure 4(B) shows a state in which a minute bubble 6, which is generated around the acute angle 4b in the cavity 4, rises via the inner wall 3b on the other side. Bubbles 6 generated from the acute angle 4b may rise immediately. In this case, the bubble 6 that attempts to rise is directly adsorbed to the inner wall 3a on one side by surface tension, and then rises as it grows, increasing its buoyancy and being discharged from the small hole 5. In this case, the bubble 6 follows a path that rises along the inclined surface of the inner wall 3a on one side due to surface tension. Also, since the small hole 5 corresponds to the vertical direction of the inner wall 3a on one side, the bubble 6 is smoothly discharged from the small hole 5.

[0039] 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.

[0040] 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 Figures 6 and 7, the cutting tool 10 has a plurality of grooves 102 formed at predetermined intervals on its bottom surface. These grooves 102 form recesses in the blade portion 101, as shown in Figure 6, and blades for cutting out the fins 3 are also formed in these recesses. The spacing and width of these grooves 102 are the same as those of the small holes 5 mentioned above. The depth D of the grooves 102 is approximately the same as the depth of the small holes 5. Furthermore, the blade portion 101 of the digging tool 10 has a first sliding contact surface 103 formed at a small angle θ3 with respect to the bottom surface of the digging tool 10, and a second sliding contact surface 104 formed at a larger angle θ4, which is continuous with the other side of the first sliding contact surface 103 and above. The angle θ3 of the first sliding contact surface 103 is set to 5 to 20 degrees with respect to the bottom surface of the digging tool 10. The length L of the first sliding contact surface 103 is set to the length from the base end of the fin 3 to the lower end of the small hole 5, but it may be shorter than this length. The first sliding contact surface 103 and the second sliding contact surface 104 are bent. The angle θ4 of the second sliding contact surface 104 is set to 50 to 80 degrees with respect to the bottom surface of the digging tool 10.

[0041] 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 θ5 inclined relative to the upper surface of the metal substrate 2, with its rear end higher than the upper surface. This inclination angle θ5 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.

[0042] First, as shown in Figure 5(A), the blade portion 101 of the excavation tool 10 is brought into contact with the surface of the metal substrate 2. Furthermore, the excavation tool 10 is moved in the direction indicated by the arrow while tilted by the drive device, causing the blade portion 101 of the excavation tool 10 to bite into the surface of the metal substrate 2, thereby excavating the fin 3 while sliding on the first sliding surface 103. As the excavation tool 10 moves, the fin 3 slides on the first sliding surface 103 as shown in Figure 5(B), and eventually reaches the second sliding surface 104. Then, as shown in Figure 5(C), the tip of the fin 3 is bent and its orientation is changed by the second sliding surface 104, and the tip slides further on the second sliding surface 104, and the formation of the fin 3 is completed when the movement of the excavation tool 10 stops. In the erection process shown in Figure 5(C), the tip of the fin 3 is bent by the second sliding contact surface 104, and this boundary line becomes the bent portion 3c of the fin 3. The bent portion 3c is approximately the same as the inner end of the small hole 5. Therefore, when the tip of the fin 3 is bent by the second sliding contact surface 104, the load is reduced. The position of the bend line can be arbitrarily set depending on the size of the small hole 5.

[0043] In this way, during the process of forming the fin 3 upright, a notch 51, which will later become a small hole 5, is formed at the tip of the fin 3 by a plurality of grooves 102 formed on the bottom surface of the excavation tool 10. 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 a notch 51 that is 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 8. 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 hole 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 in the figure, the cross-sectional shape of the protrusion 4c and the shape of the small hole 5 will also be the same as the grooves 102.

[0044] 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 as the cutting edge of the first sliding contact surface 103 of the excavation tool 10 that forms the fins 3 upright. Since the cutting edge of this first sliding contact surface 103 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 air bubbles.

[0045] After forming the fin 3 with its bent tip using the excavation tool 10, the process moves to a step where the tip of the fin 3 is pressed in a direction to one side of the perpendicular to the metal substrate 2 to tilt the fin 3 at a predetermined angle. The fin 3 formed upright by the excavation tool 10 on the metal substrate 2 is tilted to the other side, as shown in Figure 5(C). From this state, as shown in Figure 5(D), the excavation tool 10 is moved back to the other side, and then the excavation tool 10 is moved horizontally to one side as indicated by the arrow, and as shown in Figure 5(E), the fin 3 is pressed to tilt it to one side at a predetermined angle. This tilt angle is set to an angle θ1 of 10 to 60 degrees with respect to the perpendicular to the metal substrate 2, as shown in Figure 2.

[0046] 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 the fins 3 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(E). In this state, the upper ends of adjacent fins 3 are spaced apart.

[0047] Subsequently, the fin group 30 formed on the metal substrate 2 is subjected to the flattening process shown in Figure 5(F). In this flattening process, the fin group 30 is pressed from above with a flattening tool 11 whose lower surface is formed flat. This pressing causes the fins 3 to bend from the bent portion 3c formed on each fin 3, joining the tip of each fin 3 to the adjacent fin 3 and forming a cavity 4 between adjacent fins 3. In this way, when the tip of multiple fins 3 is pressed and bent by the flattening tool 11, the tip of each fin 3 is bent precisely because the bent portion 3 is formed on the fin 3. In addition, 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.

[0048] The process involves moving the excavation tool 10 horizontally in one direction to press the fins 3 and tilt them to one side, and then pressing the fin group 30 from above with the flattening tool 11. This process forms a cavity 4 between adjacent fins 3, and multiple small holes 5 are formed at the upper end of this cavity 4. Furthermore, inside the cavity 4, one inner wall 3a and the other inner wall 3b are formed, tilted to one side with respect to the perpendicular of the metal substrate 2. The bottom surface 4a of the cavity 4 is formed as an inclined surface that rises from one side to the other. In addition, the corners of the bottom surface 4a and the other inner wall 3b are formed in an arc shape when pressed by the excavation tool 10 and when pressed by the flattening tool 11. By making the corners in an arc shape in this way, air bubbles 6 that have moved along the inclined bottom surface 4a of the cavity 4 can be smoothly transferred to the other inner wall 3b.

[0049] 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]

[0050] 1. Boiling heat transfer surface 2 Metal substrate 3 fins 3a One side of the inner wall 3b Inner wall on the other side 3c Bend part 4 cavities 4a Bottom 4b Acute angle 4c Pier 5 small hole 6 bubbles 10 Excavation Tools 11 Flat-forging tool 51 Notch

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 adjacent fins, and forming a plurality of small holes at predetermined intervals at the upper end of the cavities. The boiling heat transfer surface of a boiling cooler is characterized in that one inner wall and the other inner wall within the cavity are inclined toward one side of the metal substrate, and the bottom surface is formed as an inclined surface that gets higher from one side to the other.

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 bent portion is formed on the tip side of each fin, and the tip side of the fin is bent from the bent portion so that the height of the plurality of small holes from the metal substrate is equal.

4. 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 fin.

5. 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 process involves moving the excavation tool to one side at a predetermined angle relative to the metal substrate, using the blade portion to excavate from the surface of the metal substrate to form a plate-shaped fin, and forming a notch at the tip of the fin by forming a groove in the blade portion, After raising the excavation tool, it is moved horizontally to one side, and the tip of the fin is pressed to one side of the perpendicular to the metal substrate, thereby tilting the fin at a predetermined angle. This process is repeated, and a group of fins is formed on the metal substrate by multiple fins arranged in parallel and adjacent to each other. The process involves pressing and bending the tip ends of multiple fins from above the fin group using a flat-punching tool, joining the bent tip ends to adjacent fins, and forming a cavity between adjacent fins. A method for forming a boiling heat transfer surface of a boiling cooler, characterized by having the following features.

6. The blade portion of the excavation tool has a first sliding contact surface formed at a small angle with respect to the bottom surface of the excavation tool, and a second sliding contact surface formed at a larger angle, which is continuous with respect to the other side of the first sliding contact surface, and the dimension of the first sliding contact surface in the direction of movement is the dimension from the base end of the fin to the bottom of the notch. The method for forming a boiling heat transfer surface of a boiling cooler according to claim 5, wherein when the fins are formed upright using the excavation tool, the tip side of the fins is bent at a predetermined angle at the boundary between the first sliding surface and the second sliding surface to form a bent portion.

7. The method for forming a boiling heat transfer surface of a boiling cooler according to claim 5, 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.

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