JP5252230B2 - Natural circulation boiling cooling system - Google Patents

Description

  The present invention relates to a natural circulation boiling cooling device that cools a heating element using a refrigerant.

  The boiling cooling device is a device that cools a heating element using latent heat when a liquid refrigerant boils. Some boiling cooling devices naturally circulate a liquid refrigerant depending on its structure. The natural circulation boiling cooling device transmits the heat generated by the heating element to the liquid refrigerant, causes the liquid refrigerant to boil and evaporate, condenses the vaporized refrigerant vapor, and returns it to the liquid refrigerant. The cycle of receiving heat is naturally performed by using the rise of the refrigerant vapor or the weight of the liquid refrigerant. A natural circulation boiling cooling device is described in, for example, Japanese Patent Laid-Open No. 10-173115 (Patent Document 1). Hereinafter, in the specification, the natural circulation boiling cooling device is simply referred to as “boiling cooling device”.

JP-A-10-173115

  By the way, the boiling cooling apparatus is provided with the accommodating part which accommodates a liquid refrigerant. The inside of the accommodating part is partitioned by a partition wall extending vertically, for example, into a boiling passage in which the liquid refrigerant that receives heat from the heating element is accommodated and a circulation passage that supplies the liquid refrigerant to the boiling passage. The boiling passage communicates with the reflux passage at the lower portion of the housing portion. Thereby, when the liquid refrigerant in the boiling passage decreases due to boiling, the liquid refrigerant in the circulation passage flows into the boiling passage by its own weight, and natural circulation is performed.

  In the boiling cooling device using natural circulation, the flow of the liquid refrigerant from the circulation passage to the boiling passage is important. Here, when bubbles (refrigerant vapor) flow into the circulation passage from the boiling passage, this flow is hindered, and the supply of liquid refrigerant to the boiling passage is hindered, leading to a decrease in cooling performance. Therefore, it is necessary to prevent bubbles from flowing back from the boiling passage to the circulation passage.

  On the other hand, from the viewpoint of heat transfer efficiency, in the boiling passage, a separation distance between a heat transfer surface that transmits heat received from the heating element to the liquid refrigerant and a surface that faces the heat transfer surface (hereinafter referred to as an opposite surface) is set. There is a need to reduce it to some extent. However, when the separation distance is reduced, the possibility that the bubbles are pushed out below the boiling passage (the communication portion with the circulation passage) is increased, and there is a possibility that backflow is induced.

  The present invention has been made in view of such circumstances, and maintains the separation distance between the heat transfer surface and the facing surface at a predetermined distance, and also maintains the flow of the liquid refrigerant from the circulation passage to the boiling passage. It is an object of the present invention to provide a boiling cooling device capable of performing the above.

  The boiling cooling device of the present invention includes an accommodating portion that accommodates a liquid refrigerant therein, the accommodating portion extends in the vertical direction, and transfers heat in the accommodating portion space. A boiling wall passage facing the surface and a first wall portion partitioning into the reflux passage, the circulation passage being a natural circulation boiling cooling device communicating with the boiling passage at a lower portion of the boiling passage, The second wall portion is disposed between the first wall portion and the heat transfer surface in a state where the escape passage is formed between the first wall portion and the heat transfer surface.

  According to the present invention, the above-mentioned separation distance (distance between the heat transfer surface and the opposing surface) that can affect the heat transfer efficiency can be maintained at a predetermined distance by the second wall portion. Since the escape passage is formed, when the bubbles generated in the boiling passage advance downward, the bubbles flow into the escape passage before flowing into the circulation passage and flow out from the upper outlet. That is, it is possible to prevent bubbles from flowing from the boiling passage into the circulation passage (that is, reverse flow) and to prevent natural circulation from being inhibited by the bubbles. Thus, according to this invention, since the 2nd wall part which forms a relief channel | path is arrange | positioned between the 1st wall part and a heat-transfer surface, the separation distance of a heat-transfer surface and an opposing surface is predetermined. While maintaining the distance, the flow of the liquid refrigerant from the circulation passage to the boiling passage can be maintained.

  Here, it is preferable that a 2nd wall part has a communicating path which connects a boiling path and an escape path. Thereby, since the bubbles can escape through the communication passage and flow into the passage, the backflow of the bubbles can be further prevented.

  Here, it is preferable that the upper wall surface which divides the communication path is inclined so that the escape passage side is upward rather than the boiling passage side. Thereby, it becomes easy for bubbles to escape through the communication passage and advance toward the passage, and the backflow of bubbles can be effectively prevented.

  Moreover, it is preferable that the lower end surface of the 2nd wall part inclines so that the escape passage side may become upper rather than the boiling passage side. Thereby, it becomes easy for bubbles to escape and flow into the passage, and the backflow of bubbles can be effectively prevented.

  Moreover, it is preferable that the lower end of the first wall portion is located below the lower entrance of the escape passage. Thereby, the bubble which reached the lower entrance of the escape passage flows into the escape passage without exceeding the first wall portion. Therefore, the backflow of bubbles can be effectively prevented.

  Here, the upper outlet of the escape passage is preferably open to the boiling passage. As a result, the bubbles flowing into the escape passage rise and flow out again into the boiling passage. Therefore, bubbles do not enter the circulation passage, and the flow of the entire natural circulation can be prevented from being hindered.

  Further, the boiling cooling device of the present invention is disposed above the accommodating portion, and is a condenser having a condensing chamber that communicates with the boiling passage and the circulation passage and condenses the boiling refrigerant flowing from the boiling passage and returns it to the liquid refrigerant. In the case of further including a section, the upper outlet of the escape passage may open to the condensing chamber. Thereby, like the above, it can also prevent that the flow of the whole natural circulation is inhibited. In addition, since the air bubbles flowing into the escape passage directly flow into the condensing chamber, the heat transfer efficiency is also improved.

  In addition, the 1st wall part and 2nd wall part in this invention may be formed integrally, and in this case, the 2nd wall part is the 1st wall part so that the escape passage extended in an up-down direction may be divided. Extends from. This also exhibits the same effect as described above.

  According to the boiling cooling device of the present invention, the separation distance between the heat transfer surface and the facing surface can be maintained at a predetermined distance, and the flow of the liquid refrigerant from the circulation passage to the boiling passage can be maintained.

It is a longitudinal cross-sectional view which shows the boiling cooling device which concerns on 1st embodiment. It is a front view which shows the boiling cooling device which concerns on 1st embodiment. It is a longitudinal cross-sectional view which shows the boiling cooling device which concerns on 2nd embodiment. It is a longitudinal cross-sectional view which shows the boiling cooling device which concerns on other embodiment. It is a longitudinal cross-sectional view which shows the 2nd wall part which concerns on other embodiment. It is a longitudinal cross-sectional view which shows the 2nd wall part which concerns on other embodiment. It is a cross-sectional view which shows the boiling cooling device which concerns on other embodiment. It is a cross-sectional view which shows the boiling cooling device which concerns on other embodiment. It is a front view which shows the 2nd wall part which concerns on other embodiment.

  Next, the present invention will be described in more detail with reference to embodiments.

<First embodiment>
A boiling cooling device 1 according to a first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a longitudinal sectional view showing a boiling cooling device 1. FIG. 2 is a front view showing the boiling cooling device 1.

  As shown in FIG. 1, the boiling cooling device 1 includes a storage unit 2 and a condensing unit 3. The accommodating portion 2 is a substantially rectangular parallelepiped metal container that accommodates a liquid refrigerant (for example, water, alcohol, chlorofluorocarbon, etc.) therein. The accommodating part 2 has the heat-transfer surface 21, the 1st wall part 22, and the 2nd wall part 23 inside. The heat transfer surface 21 is a part that extends in the vertical direction and transfers the heat of the heating element Z to the liquid refrigerant. The heat transfer surface 21 faces a boiling passage A, which will be described later, and is the inner side surface of the housing portion 2 corresponding to the position of the heating element Z. Here, considering that the heat of the heating element Z is transmitted through the side wall of the housing portion 2, a range widened at an angle of about 45 degrees from the heating element Z as the heat transfer surface 21 as shown by the dotted line in FIG. Yes. The heating element Z is a power module or the like that is attached to the housing portion 2 and includes, for example, a semiconductor element. As shown in FIG. 2, here, a plurality of heating elements Z are attached to the side wall of the housing portion 2. The liquid level of the liquid refrigerant is located in the upper part in the housing part 2 when the boiling cooling device 1 is stopped.

  The first wall portion 22 is a plate-like member, and is disposed in the housing portion 2 in parallel with the side wall of the housing portion 2. The first wall portion 22 partitions the internal space of the housing portion 2 into a boiling passage A and a reflux passage B. The first wall portion 22 is joined to the inner side surface of the housing portion 2, but is not joined to the inner bottom surface of the housing portion 2. For this reason, the boiling passage A and the circulation passage B communicate with each other on the lower side.

  The boiling passage A is a space surrounded by the inner side surface, the inner bottom surface, and the first wall portion 22 of the accommodating portion 2, and is a region on the heat transfer surface 21 side (left side in FIG. 1). The boiling passage A extends in the vertical direction, communicates with the condensing chamber 31 of the condensing unit 3 at the upper end, and communicates with the reflux passage B at the lower end. The boiling passage A causes the liquid refrigerant to boil by receiving heat from the heat transfer surface 21, and causes the refrigerant vapor to flow out to the condensation chamber 31.

  The circulation passage B is a space surrounded by the inner side surface, the inner bottom surface, and the first wall portion 22 of the housing portion 2 and is a region on the side (the right side in FIG. 1) without the heat transfer surface 21. The circulation passage B extends in the vertical direction, communicates with the condensation chamber 31 at the upper end, and communicates with the boiling passage A at the lower end. In the circulation passage B, the liquid refrigerant condensed in the condensing unit 3 flows from the upper end, and supplies the liquid refrigerant to the boiling passage A from the lower end. That is, the liquid refrigerant flows from the circulation passage B toward the boiling passage A by its own weight.

  The second wall portion 23 is a plate-like member, and is disposed apart from each other between the heat transfer surface 21 and the first wall portion 22. The second wall portion 23 is disposed in parallel to the heat transfer surface 21 at a position facing the heat transfer surface 21. The length of the second wall portion 23 in the vertical direction is smaller than that of the first wall portion 22 and is equal to or slightly larger than that of the heat transfer surface 21. Similar to the first wall portion 22, the second wall portion 23 is joined to the inner surface of the housing portion 2. The second wall portion 23 defines a separation distance between the heat transfer surface 21 in the boiling passage A and its opposing surface. That is, the second wall portion 23 can maintain the separation distance at a predetermined distance. The separation distance here is set to about 1 to 3 mm from the viewpoint of heat transfer efficiency.

  Since the second wall portion 23 and the first wall portion 22 are arranged apart from each other, an escape passage C is formed between them. The escape passage C is a space surrounded by the first wall portion 22, the second wall portion 23, and the inner surface of the accommodating portion 2. That is, the escape passage C is formed in the boiling passage A. The escape passage C extends in the vertical direction, and a lower inlet which is a lower end and an upper outlet which is an upper end are open to the boiling passage A.

  The condensing part 3 is arrange | positioned above the accommodating part 2, and condenses the refrigerant | coolant vapor | steam which flowed in from the boiling channel | path A, and returns it to a liquid refrigerant. The condensing unit 3 includes a condensing chamber 31 that is an internal space and a cooling pipe 32. The condensation chamber 31 is a substantially rectangular parallelepiped space, and communicates with the boiling passage A and the circulation passage B, respectively. Refrigerant vapor flows from the boiling passage A into the condensing chamber 31.

  The cooling pipe 32 is a pipe through which the cooling fluid circulates, and is disposed through the condensing unit 3 as shown in FIG. The cooling fluid may be gas or liquid, and cold water is used here. The cooling pipe 32 is externally connected to a radiator, a circulation pump, or the like. The cooling pipe 32 cools the condensation chamber 31 and condenses the refrigerant vapor in the condensation chamber 31. The condensed liquid refrigerant flows into the circulation passage B and then circulates into the boiling passage A.

  Here, the effect of the boiling cooling device 1 is demonstrated. The liquid refrigerant received from the heat transfer surface 21 boils on the surface of the heat transfer surface 21 and becomes bubbles (refrigerant vapor). The bubbles rise through the boiling passage A and are led out to the condensation chamber 31. Here, for example, when a large amount of bubbles are generated due to a large amount of heat generation, the bubbles are concentrated between the heat transfer surface 21 and the second wall portion 23 and are not likely to rise. Conventionally, if bubbles continue to be generated in such a situation, the bubbles are pushed downward and may flow back to the circulation passage B. However, since the boiling cooling device 1 has the escape passage C, it is possible to prevent bubbles from flowing back to the circulation passage B even in the above situation.

  The bubbles pushed downward in the boiling passage A flow into the lower inlet of the escape passage C before the circulating passage B because the lower entrance of the escape passage C opens to the boiling passage A. Bubbles that flow into the escape passage C rise through the escape passage C and flow into the boiling passage A again from the upper outlet. Accordingly, bubbles are prevented from flowing into the circulation passage B from the inlet of the circulation passage B (the communication portion between the boiling passage A and the circulation passage B), and the flow from the circulation passage B to the boiling passage A is maintained. .

  Furthermore, since the upper outlet of the escape passage C is open to the boiling passage A, the bubbles do not flow into the circulation passage B, and the flow of liquid refrigerant in the circulation passage B is obstructed from flowing downward. Absent. Since the upper outlet of the escape passage C is opened in addition to the circulation passage B, the flow of the liquid refrigerant in the circulation passage B (upper end → lower end) can also be maintained.

  Further, since the lower end of the first wall portion 22 is located below the lower entrance of the escape passage C (lower end position of the second wall portion 23), the bubbles are less likely to flow into the circulation passage B, and Backflow to the passage B can be prevented.

<Second embodiment>
The boiling cooling apparatus 10 of 2nd embodiment is demonstrated with reference to FIG. FIG. 3 is a longitudinal sectional view showing the boiling cooling device 10. In the second embodiment, parts having the same names as those in the first embodiment have the same functions, and therefore, the portions having different arrangements and shapes will be mainly described.

  The boiling cooling device 10 includes a storage unit 20 and a condensing unit 30. The accommodating part 20 is substantially T-shaped with the upper part widened. The accommodating portion 20 includes heat transfer surfaces 211 and 212, first wall portions 221 and 222, and second wall portions 231 and 232.

  The heat transfer surfaces 211 and 212 correspond to the heating elements Z1 and Z2, and are respectively disposed on the inner side surfaces (left and right in FIG. 3) that are opposite to each other in the housing portion 20. A plurality of heating elements Z1 and Z2 are attached to the side wall of the accommodating portion 20 as in the first embodiment. The side wall of the housing portion 20 is a plate member Y from which a portion including the heat transfer surfaces 211 and 212 can be removed. For this reason, a leakage preventing seal member X is embedded in the side walls around the heat transfer surfaces 211 and 212.

  The first wall portion 221 (222) partitions the boiling passage A1 (boiling passage A2) and the circulation passage B in the accommodating portion 20. The first wall portions 221 and 222 are refracted in the widening direction in the upper widened portion 20 a of the housing portion 20. That is, the upper portion of the first wall portion 221 is refracted leftward in FIG. 3, and the upper portion of the first wall portion 222 is refracted rightward in FIG. As a result, the upper part of the circulation passage B is widened toward the condensing unit 30, and the liquid refrigerant dripped from the condensing unit 30 can be efficiently introduced into the circulation passage B.

  The boiling passage A <b> 1 faces the heat transfer surface 211 and is a space surrounded by the inner side surface, the inner bottom surface, and the first wall portion 221 of the housing portion 2. The boiling passage A <b> 2 faces the heat transfer surface 212 and is a space surrounded by the inner side surface, the inner bottom surface, and the first wall portion 222 of the housing portion 2. The boiling passage A1 (A2) has a convex widened portion A1a (A2a) in which a position corresponding to the heat transfer surface 211 (212) is widened in a convex shape by embedding the sealing member X or the like. The circulation passage B is a space surrounded by the inner side surface, the inner bottom surface, and the first wall portions 221 and 222 of the housing portion 2. The boiling passages A <b> 1 and A <b> 2 and the circulation passage B are in communication with each other at the lower part of the housing portion 20.

  The 2nd wall part 231 (232) is mutually spaced apart and arrange | positioned between the heat-transfer surface 211 (212) and the 1st wall part 221 (222). The second wall portion 231 (232) is disposed parallel to the heat transfer surface 211 (212) in the convex widened portion A1a (A2a) of the boiling passage A1 (A2). The separation distance between the second wall portion 231 (232) and the heat transfer surface 211 (212) is about 2 to 3 mm. Thus, even when the boiling passage is uneven due to the seal member or the like, the separation distance between the heat transfer surface and the facing surface can be defined as a predetermined distance by the second wall portion.

  An escape passage C1 (C2) is formed between the second wall portion 231 (232) and the first wall portion 221 (222). The escape passage C1 (C2) extends in the vertical direction, the lower inlet opens to the boiling passage A1 (A2), and the upper outlet also opens to the boiling passage A1 (A2). The lower entrance of the escape passage C1 (C2) is located above the lower end of the first wall portion 221 (222).

  In addition, the upper part of the 2nd wall part 231 is chamfered, and is the inclination guide part 231a which inclined so that the escape passage C1 side might become upward from the heat-transfer surface 211 side. Thereby, it becomes easy for a bubble to face the boiling passage A from the convex widened portion A1a. A similar inclined guide portion 232 a is also formed on the upper portion of the second wall portion 232. The inclined guide portions 231a and 232a may be curved surfaces or flat surfaces. The escape passage C1 (C2) has a boiling passage A1 (A2) in the vertical upper direction, and can return to the boiling passage A1 (A2) without hindering the bubbles.

  The condensing unit 30 is disposed above the housing unit 20 and includes a condensing chamber 31 and a plurality of cooling pipes 32. The condensation chamber 31 communicates with the boiling passages A1 and A2 and the reflux passage B.

  Here, the effect of the boiling cooling device 10 is demonstrated. The liquid refrigerant in the boiling passage A1 (A2) boils by receiving heat from the heating element Z1 (Z2), rises as bubbles, and flows into the condensing chamber 31 of the condensing unit 30. This refrigerant vapor is condensed in the condensing chamber 31, returns to the liquid refrigerant, and drops into the widened reflux passage B. The reflux passage B supplies the liquid refrigerant to the boiling passages A1 and A2 that communicate with each other below. Here, as in the first embodiment, when bubbles are pushed downward in the boiling passage A1 (A2), the bubbles flow into the escape passage C1 (C2) in front of the reflux passage B without entering the reflux passage B. Ascend in the escape passage C1 (C2) and return to the boiling passage A1 (A2) from the upper outlet again. Accordingly, the bubbles pushed downward do not flow back into the reflux passage B and do not hinder the flow of the liquid refrigerant from the reflux passage B to the boiling passages A1 and A2. That is, natural circulation can be effectively maintained.

  Moreover, in 2nd embodiment, since a convex wide part is in a boiling channel | path, although it becomes easy for a bubble to go to a reflux channel below a boiling channel | path, a bubble is escaped by an escape channel | path. That is, the present invention is particularly effective when the boiling passage has a convex widened portion.

<Other embodiments>
As shown in FIG. 4, the upper outlet of the escape passage C may open to the condensation chamber 31. Thereby, the air bubbles flowing into the escape passage C can be directly led out to the condensing chamber 31, and natural circulation can be maintained and heat transfer efficiency can be improved.

  Further, as shown in FIG. 5, taking the second wall portion 231 of the second embodiment as an example, the second wall portion 231 communicates from the boiling passage A1 facing the heat transfer surface 211 to the escape passage C1. D may be included. Thereby, bubbles near the heat transfer surface 211 escape through the communication path D and flow into the path C1 before being pushed downward. Therefore, the backflow of bubbles to the circulation path B can be prevented.

  Furthermore, as shown in FIG. 5, the upper wall surface that defines the communication path D may be inclined such that the escape passage C1 side is higher than the boiling passage A1 side. As a result, the air bubbles flowing into the communication passage D can easily escape from the boiling passage A1 and go to the passage C1, and further prevent the backflow. In addition, it is more preferable that the position of the communicating path D in the vertical direction is on the upper side than on the lower side. By being positioned on the upper side, the bubbles in the vicinity of the heat transfer surface 211 are encouraged to rise, and by preventing the bubbles from being pushed downward, the backflow can be prevented. A plurality of communication paths D may be formed in the second wall portion 231. Further, the communication path D may be a hole.

  Moreover, as shown in FIG. 6, the lower end surface of the 2nd wall part 231 may incline so that the escape channel | path C side may escape from the boiling channel | path A side. As a result, the bubbles pushed downward are encouraged to flow into the escape passage C, and the backflow can be further prevented. Note that the second wall portions 23 and 232 can be similarly applied.

  FIG. 7 is a cross-sectional view showing a boil cooling device according to another embodiment. FIG. 7 is a cross-sectional view as viewed from above, and illustrates a case where the number of heating elements Z is one. As shown in FIG. 7, the first wall portion 22 and the second wall portion 23 are integrally formed, and the second wall portion 23 defines the escape passage C extending in the vertical direction. It may extend from the wall 22. The escape passage C is a passage that penetrates the integrally formed wall portion in the vertical direction. Further, as shown in FIG. 8, a plurality of escape passages C may be formed in the wall portion. Also by these, the same effect as the first and second embodiments is exhibited.

  Further, the escape passage C may be opened not only at the upper part and the lower part but over the entire periphery of the second wall part 23. For example, as shown in FIG. 9, the width of the second wall portion 23 is made smaller than that of the first wall portion 22, and a gap is formed between the side surface of the second wall portion 23 and the inner side surface of the housing portion 2. May be. In this case, the holding of the second wall portion 23 forms, for example, a convex portion (see the dotted line) protruding toward the first wall portion 22 on the surface of the second wall portion 23 facing the first wall portion 22 side. And it can implement | achieve by joining the front-end | tip of the said convex-shaped part, and the 1st wall part 22. FIG. Thereby, the 2nd wall part 23 is hold | maintained at the 1st wall part 22, and the escape channel | path C has an opening also in the side surface direction of the 2nd wall part 23 in addition to a lower entrance and an upper exit. As a result, the same effect as described above is exhibited, and bubbles can also flow in from the side surface direction. The holding of the second wall portion 23 is not limited to the above, and may be realized by, for example, joining the convex portion protruding from the side surface of the second wall portion 23 and the inner side surface of the accommodating portion 2.

  As described above, these other embodiments may be combined, and can be applied to both the first and second embodiments. The condensing units 3 and 30 may be any one that condenses the refrigerant vapor, and may be, for example, an air-cooled type that directly cools the condensing chamber using fins or a radiator.

1, 10: Boiling cooling device,
2, 20: accommodating portion, 21, 211, 212: heat transfer surface,
22, 221 and 222: the first wall, 23, 231 and 232: the second wall,
3, 30: condensing part, 31: condensing chamber, 32: cooling pipe,
A, A1, A2: Boiling passage, B: Circulation passage, C, C1, C2: Escape passage,
D: Communication path, Z, Z1, Z2: Heating element

Claims (8)

It has an accommodating part for accommodating a liquid refrigerant inside,
The accommodating portion is
A heat transfer surface that extends in the vertical direction and transfers the heat of the heating element to the liquid refrigerant;
A first wall that divides the internal space of the housing into a boiling passage and a reflux passage facing the heat transfer surface;
Have
The recirculation passage is a natural circulation boiling cooling device in communication with the boiling passage at a lower portion of the boiling passage,
A natural circulation boiling method, wherein a second wall portion is disposed between the first wall portion and the heat transfer surface in a state in which an escape passage is formed between the first wall portion and the first wall portion. Cooling system.   2. The natural circulation boiling cooling device according to claim 1, wherein the second wall portion includes a communication passage that communicates the boiling passage and the escape passage.   The natural circulation boiling cooling device according to claim 2, wherein an upper wall surface defining the communication passage is inclined so that the escape passage side is higher than the boiling passage side.   The natural circulation boiling cooling device according to any one of claims 1 to 3, wherein a lower end surface of the second wall portion is inclined such that the escape passage side is higher than the boiling passage side.   The natural circulation boiling cooling device according to any one of claims 1 to 4, wherein a lower end of the first wall portion is positioned below a lower entrance of the escape passage.   The natural circulation boiling cooling device according to any one of claims 1 to 5, wherein an upper outlet of the escape passage is open to the boiling passage. Further comprising a condensing part that is disposed above the housing part, communicates with the boiling passage and the circulation passage, and has a condensing chamber for condensing the refrigerant vapor flowing from the boiling passage and returning it to the liquid refrigerant,
The natural circulation boiling cooling device according to any one of claims 1 to 5, wherein an upper outlet of the escape passage is open to the condensing chamber. The first wall portion and the second wall portion are integrally formed,
2. The natural circulation boiling cooling device according to claim 1, wherein the second wall portion extends from the first wall portion so as to define a relief passage extending in a vertical direction.

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