COOLING SYSTEM AND ELECTRONIC APPARATUS USING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cooling system for an electronic apparatus in which an enclosure thereof accommodates a plurality of heat generating sources, such as CPUs, and an electronic apparatus using the cooling system.

2. Related Art

In recent years, in an electronic apparatus an representative example of which is a server, what is called a semiconductor device, such as a central processing unit (CPU), is mounted on a circuit substrate in multiple positions in order, for example, to improve the processing speed, and the circuit substrate along with a plurality of hard disk drives and other components is accommodated in a box-shaped rack at a high density.

A semiconductor device, such as a CPU, described above, when its temperature exceeds a predetermined value, cannot typically maintain its performance and may even be damaged in some cases. The temperature of the device therefore needs to be controlled, for example, by cooling the device, and a technology for efficiently cooling a recent semiconductor device that generates an increased amount of heat is strongly desired.

In view of the technical background described above, a cooling apparatus for cooling a semiconductor device (such as CPU) that generates an increased amount of heat requires a high-performance cooling capability that allows the semiconductor device to be efficiently cooled. An electronic apparatus, such as a server, has typically used an air-based cooling apparatus in many cases. Under the circumstances described above, however, an air-based cooling apparatus has already come close to a point where its cooling performance is not enough, and cooling systems based on new methods are therefore desired. A new system that has received attention is, for example, a cooling system using a coolant, such as water.

As related art of the invention, for example, JP-A-10-173115 discloses a configuration of cooling fins. Assuming that the low-boiling-point coolant in JP-A-10-173115 is water, the cooling fins are configured to have a fin height ranging from 0.1 to 1.0 mm and an inter-fin gap ranging from 0.06 to 0.6 mm, the latter calculated from the interval between the fins.

JP-A-2010-256000 discloses a heat pipe for cooling a CPU in a personal computer, and the heat pipe is configured to have an inter-fin gap ranging from 0.1 to 0.35 mm, a fin upper-portion diameter ranging from 0.09 to 0.3 mm, and a fin height ranging from 0.05 to 0.3 mm.

JP-A-2011-047616 discloses a configuration in which the fin upper-portion diameter is 0.2 mm.

Further, JP-A-2010-050326 discloses a configuration in which the inter-fin distance is twice or more the diameter of a departing air bubble and the fin height is 1 to 3.4 times the diameter of the departing air bubble.

SUMMARY OF THE INVENTION

In the related art described above, JP-A-10-173115 describes no fin upper-portion diameter, and in JP-A-2010-256000, the fin height ranges from 0.05 to 0.3 mm, which are small values and do not form an optimum shape of a boiling heat transfer surface or JP-A-10-173115 shows no consideration for the optimum shape. Further, none of JP-A-10-173115, JP-A-2010-256000, JP-A-2011-047616, and JP-A-2010-050326 describes an optimum shape (inter-fin gap, fin height, and fin upper-end diameter) of a boiling heat transfer surface for different coolants or considers an optimum shape of the boiling heat transfer surface.

To solve the problem described above, a cooling system according to the invention includes a boiler portion and a condenser portion, a steam pipe and a liquid pipe that connect the boiler portion and the condenser portion to each other, and a heat receiving jacket, and the shape of each fin formed on a boiling heat transfer surface of the heat receiving jacket is optimized in terms of a fin upper-hole diameter, an inter-fin gap, and a fin height.

Further, to solve the problem described above, an electronic apparatus according to the invention includes a cooling system including a boiler portion and a condenser portion and a steam pipe and a liquid pipe that connect the boiler portion and the condenser portion to each other, and a plurality of cooling fans that cool components in the electronic apparatus, and the condenser portion is cooled with the plurality of cooling fans.

According to the configuration of the invention, since boiling nucleus departure in a boiled coolant and smooth liquid flow in a liquid flowing-in process can be achieved, the heat transfer performance can be improved.

Further, according to the configuration of the invention, even in pool boiling in which the amount of generated heat is relatively large and the amount of sealed coolant liquid is so increased that the heat transfer surface is well immersed in the coolant liquid, smooth liquid flow in a liquid flowing-in process can be achieved, whereby the heat transfer performance can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an overall schematic structure of a cooling system using a thermosiphon according to an embodiment of the invention;

FIG. 2 is an enlarged perspective view including a partial cross-section showing a detailed structure of a heat receiving jacket that forms the cooling system using a thermosiphon according to the embodiment of the invention;

FIG. 3 is a cross-sectional perspective view of a vaporization accelerator plate that forms the cooling system using a thermosiphon in the invention;

FIG. 4 is a cross-sectional view showing a steam bubble generated in a cavity of the vaporization accelerator plate in the invention;

FIG. 5 is a cross-sectional view showing a steam bubble generated when the vaporization accelerator plate that forms the cooling system using a thermosiphon in the invention has a narrow inter-fin gap;

FIG. 6 is a cross-sectional view showing a steam bubble generated when the vaporization accelerator plate has a wide inter-fin gap;

FIG. 7 is graph showing changes in steam height or fin height versus the inter-fin gap in a case where the coolant in the invention is water and the inter-fin gap is narrow;

FIG. 8 is a perspective view showing an overall structure of servers accommodated in a rack as an example of an electronic apparatus using the cooling system using a thermosiphon according to the invention;

FIG. 9 is a perspective view showing a state in which lids of the rack are removed in order to show an example of the internal structure in a server enclosure; and

FIG. 10 is a top view for describing the arrangement of the cooling system in the server enclosure.

DETAILED DESCRIPTION

Embodiments of the invention will be described below in detail with reference to the drawings.

Embodiment 1

FIG. 1 shows an overall structure of a cooling system using a thermosiphon. In FIG. 1, a semiconductor device 200, such as a CPU, is mounted as a heat generating source on a surface of a circuit substrate 100. A heat receiving jacket 310, which forms part of a cooling system 300 using a thermosiphon according to the invention, is attached to the front surface of the semiconductor device 200. More specifically, what is called a heat conductive grease 210 is applied onto the front surface of the semiconductor device 200 in order to ensure a satisfactory thermal joint between the semiconductor device 200 and the heat receiving jacket 310, and the heat receiving jacket 310 is fixed with screws (not shown) or any other fixtures to the front surface of the semiconductor device 200 with the front surface of the semiconductor device 200 in contact with the bottom surface of the heat receiving jacket 310. The cooling system 300, the structure of which will be described below in detail, includes a condenser 320 with a radiator as well as the heat receiving jacket 310 described above. A pair of pipes 331 and 332 are provided between and attached to the heat receiving jacket 310 and the condenser 320, and the interior of each of the pipes is maintained at a reduced (low) pressure of about one-tenth the atmospheric pressure.

The heat receiving jacket 310 described above forms a boiler portion, and the condenser 320 described above forms a condenser portion, together forming what is called a thermosiphon, which can use a phase change of water, which is a liquid coolant, to circulate the coolant liquid without external power, such as that from an electric pump, as also will be described below.

That is, in the cooling system using the thermosiphon described above, heat generated by the semiconductor device 200, which is a heat generating source, is transferred to the heat receiving jacket 310, which is a boiler portion, via the heat conductive grease 210. As a result, in the boiler portion, the water (Wa), which is a liquid coolant, is boiled and evaporated by the transferred heat under the reduced pressure, and the produced steam (ST) goes out of the heat receiving jacket 310 and is guided through one of the pipes, the pipe 331, into the condenser 320. In the condenser portion, the coolant steam is cooled, for example, by air (AIR) delivered through a cooling fan 400 or any other component into a liquid (water), which then passes through the other pipe 332 under the gravity and returns back to the heat receiving jacket 310 described above, as shown in FIG. 1.

FIG. 2, which is one of the accompanying drawings, shows a detailed structure of the heat receiving jacket 310 described above. The heat receiving jacket 310 is formed, for example, by placing a lid 312, which is made of copper, stainless steel, or any other suitable metal and formed in a drawing process into a bowl shape, on an upper portion of a rectangular bottom plate 311, which is made of copper or any other material that excels in thermal conductivity, and bonding the peripheries of the bottom plate 311 and the lid 312 to each other, for example, in a pressurized welding process, as shown in FIG. 2. As clearly seen from FIG. 2, a vaporization accelerator plate 313 having a rectangular plate-like shape is attached to the upper surface of the bottom plate 311 described above, and through holes are formed through the upper portion and a sidewall of the lid 312 and connected to the pair of pipes 331 and 332 described above, respectively.

The vaporization accelerator plate 313, which has a porous structural surface, shows stable evaporation performance (vaporization performance) unless the liquid coolant is run out. When the amount of input heat is small, the holes of the porous structural surface are impregnated and filled with the liquid coolant, whereas when the amount of input heat is large, the liquid coolant with which the holes are filled is evaporated and the amount of the liquid coolant in the holes decreases. In the latter case, the region in the porous structural surface where a thin coolant liquid film is present increases, which facilitates more evaporation, resulting in an increases in heat dissipation performance and hence an increase in the amount of transported heat. That is, since an increase in the amount of input heat not only facilitates the evaporation in accordance with the temperature but also facilitates the evaporation in accordance with the increase in the amount of steam, a greater amount of input heat results in a significantly greater amount of transported heat and hence improvement in the heat transportation efficiency.

The thus configured vaporization accelerator plate 313 is attached to the inner wall of the bottom plate 311, which forms the heat receiving jacket 310 described above, for example, in a welding process. The invention is, however, not necessarily configured this way, and the porous structural surface described above can alternatively be directly formed on the inner wall surface of the copper plate that forms the bottom plate 311 described above.

FIG. 3 is a cross-sectional perspective view of the vaporization accelerator plate 313. The vaporization accelerator plate 313 has a large number of cavities 13, which are formed between fins having a geometric shape characterized by an inter-fin gap 10, a fin height 11, and fin upper-end holes 12. A steam bubble produced at a base portion 14 in each of the inter-fin gaps 10 of the vaporization accelerator plate 313 moves in the vertical direction corresponding to the fin height 11 and exits through one of the fin upper-end holes 12. Further, since the pressure in the cavity 13 decreases after the steam bubble exits out thereof, the coolant flows into the cavity 13 through the other fin upper-end holes 12. When the series of actions described above are repeated, stable heat transfer performance can be provided and hence high heat transfer performance can be maintained. When a computer or any other apparatus stops operating and does not generate heat any more, the series of actions stop. In this case, since the fin upper-end holes 12 allow the cavity 13 to hold steam bubbles, boiling smoothly occurs when the computer or any other apparatus starts operating next time and heat is generated, whereby an abrupt temperature increase that is likely to occur at the start of boiling can be avoided. The steam bubble generation in each of the cavities 13 will next be described.

FIG. 4 is a cross-sectional view showing a steam bubble 15 generated in one of the cavities 13 of the vaporization accelerator plate 313. The steam bubble 15 is produced based on the surface tension between the steam bubble 15 and the heat transfer surface with a certain contact angle 16 therebetween. The radius of the steam bubble 15 in this case is r, whereas the radius of the steam bubble 15 to the interface between the steam bubble 15 and the heat transfer surface is r′.

A description will now be made of acritical diameter of a steam bubble produced in an overheated liquid and the diameter of an air bubble departing from a heat transfer surface. First, the critical diameter of a steam bubble produced in an overheated liquid can be expressed as follows by using a surface tension σ, a saturation temperature Tsat, a saturation steam density ρv, an evaporation latent heat Llv (l stands for liquid and the same holds true for the following description), and an overheat degree ΔTsat.

r=2·σ·Tsat/(ρv·Llv·ΔTsat)

In the case of water, provided that the parameters described above are assumed as follows: the surface tension σ is 69.4 mN/m; the saturation temperature Tsat is 318 K; the saturation steam density ρv is 65.6×10⁻³kg/m³; the evaporation latent heat Llv is 2392 kJ/kg; and the overheat degree ATsat is 3K, r=94 μm or the diameter is about 200 μm.

On the other hand, in the case of HFE7000 (trade mark) manufactured by Sumitomo 3M Limited as a representative example of an inert coolant, provided that the parameters described above are assumed as follows: the surface tension σ is 12.4 mN/m; the saturation temperature Tsat is 318 K; the saturation steam density ρv is 14 kg/m³; the evaporation latent heat Llv is 142 kJ/kg; and the overheat degree ΔTsat is 3K, r=1.3 μm or the diameter is about 3 μm.

When the diameter of each of the steam bubbles described above is approximately equal to the diameter of the fin upper-end holes 12 shown in FIG. 4, the steam bubbles can smoothly exit through the holes, whereas the steam bubbles (boiling nuclei) can be reliably held when the heat generation stops. In consideration of manufacture of the fins on the boiling heat transfer surface, however, a reasonable diameter of the fin upper-end holes is about 100 μm, and the diameter of the fin upper-end holes is 100 μm or smaller for HFE7000 (product name) manufactured by Sumitomo 3M Limited. Further, in the case of water, an optimum diameter of the fin upper-end holes ranges from about 0.15 to 0.25 mm in consideration of the fin manufacturability.

Further, the diameter of an air bubble departing from a heat transfer surface can be expressed as follows by using the contact angle θ, the surface tension σ, the gravitational acceleration g, the steam density ρv, and a liquid density ρl (l stands for liquid and the same holds true for the following description).

db=0.0209·θ(σ/(g·(ρl−ρv)))^1/2

In the case of water, provided that the parameters described above are assumed as follows: the contact angle θ is 38°; the surface tension σ is 69.4 mN/m; the gravitational acceleration g is 9.8 m/s²; the steam density ρv is 65.6×10⁻³kg/m³; and the liquid density ρl is 1×10³kg/m³, db is about 2.2 mm.

On the other hand, as in the case described above, in the case of HFE7000 (product name) manufactured by Sumitomo 3M Limited as a representative example of an inert coolant, provided that the parameters described above are assumed as follows: the contact angle θ is 1°; the surface tension σ is 12.4 mN/m; the gravitational acceleration g is 9.8 m/s²; the steam density ρv is 14 kg/m³; and the liquid density ρl is 1400 kg/m³, db is about 0.02 mm.

When the diameter of each of the steam bubbles described above is approximately equal to the inter-fin gap 10 shown in FIG. 4, the steam bubbles can smoothly pass through the cavity, but the diameter of the steam bubbles after departing from the heat transfer surface changes to the critical diameter of a steam bubble produced in an overheated liquid described above. In view of the fact described above, to reliably hold the steam bubble 15 in the inter-fin cavity, the inter-fin gap 10 shown in FIG. 4 is set at a value about twice the diameter of the fin upper-end holes. That is, when the coolant is water, an optimum inter-fin gap 10 ranges from 0.3 to 0.5 mm, whereas when the coolant is HFE7000 (product name) manufactured by Sumitomo 3M Limited, an optimum inter-fin gap 10 ranges from 0.2 mm or smaller.

Embodiment 2

The fin height of a boiling heat transfer surface will further be described. The shape of a departing steam bubble 15 changes in accordance with the size of the inter-fin gap 10, as shown in FIGS. 5 and 6. FIG. 5 is a cross-sectional view showing a steam bubble generated when the vaporization accelerator plate, which forms the cooling system using a thermosiphon, has a narrow inter-fin gap. In this case, the steam bubble 15 is in contact with the fins and hence has an elongated shape along the space between the fins.

Embodiment 3

On the other hand, FIG. 6 is a cross-sectional view showing a steam bubble generated when the vaporization accelerator plate has a wide inter-fin gap. In this case, the steam bubble 15 departs without touching the fins. In the invention, to improve the heat transfer performance by increasing the fin area, the inter-fin gap 10 is narrowed. That is, Embodiment 3 will be described with reference to the case shown in FIG. 5, in which the inter-fin gap is narrow. The buoyancy of a steam bubble can be expressed as follows in consideration of the balance between the buoyancy and the surface tension of the steam bubble by using a steam bubble volume V, the gravitational acceleration g, the steam density ρv, the liquid density ρl, the radius r′ of the steam bubble 15 to the interface between the steam bubble 15 and the heat transfer surface, the surface tension σ, the contact angle θ, and a surface roughness coefficient K.

V·g·(ρl−σv)=2·π·r′·σ·sin θ·K

where the surface roughness coefficient K is 0.3 for a cut surface produced in a lancing formation process, an extrusion formation process, or a drawing formation process, whereas being 1 for a ground, mirror-finished surface. In the case of fins, the former value 0.3 is used.

Embodiment 4

FIG. 7 shows changes in the steam height or the fin height versus the inter-fin gap in a case where the coolant is water and the inter-fin gap is narrow. When the coolant is water and the inter-fin gap has an optimum value ranging from 0.3 to 0.5 mm as described above, an optimum fin height ranges from about 0.75 to 1.2 mm. On the other hand, when the coolant is HFE7000 (product name) manufactured by Sumitomo 3M Limited, although not shown in a figure unlike the case of water coolant shown in FIG. 7, the fin height may be smaller because the air bubble departure diameter is smaller. In consideration of the manufacturability of the boiling heat transfer surface, an optimum fin height is 0.2 mm or smaller.

Embodiment 5

An example of an electronic apparatus using the cooling system using a thermosiphon described above will subsequently be described below in detail with reference to accompanying figures, FIGS. 8 to 10.

First, FIG. 8, which is one of the accompanying drawings, is a perspective view showing the exterior appearance of a server, particularly a plurality of servers incorporated in a rack as a representative example of the electronic apparatus using the cooling system using a thermosiphon according to the invention. In FIG. 8, a rack 1 includes a housing 2 and lids 3 and 4 (3 represents front door and 4 represents rear door) and removably accommodates a plurality of server enclosures 5 having a predetermined shape and dimensions.

Each of the plurality of server enclosures 5 accommodates not only a plurality of (three in present embodiment) hard disk drives 51, each of which is a large-capacity recording device, on one side (front side corresponding to right side in FIG. 9 in present embodiment) in consideration of maintenance thereof but also a plurality of (four in present embodiment) cooling fans 52 for cooling the hard disk drives, which are heat generating sources in the enclosure as in the above embodiments, in a region behind the hard disk drives. A block 54, which accommodates a power source, a LAN circuit that is a communication means interface, and other components, is provided along with a cooling fan 53 in a space between the hard disk drives and the other side of the server housing 5 (that is, in rear space). The circuit substrate 100, on which a plurality of (two in present embodiment) CPUs 200, which are heat generating sources, are mounted, is further disposed in the remaining space. FIG. 9, which is a perspective view, shows a state in which the lids are removed.

Each of the CPUs 200 is provided with the cooling system 300 using a thermosiphon according to the invention described above, as clearly seen from FIGS. 9 and 10. That is, the heat receiving jacket 310 described above is so disposed that the bottom surface thereof is in contact with the front surface of each of the CPUs 200 with a heat conductive grease applied to the front and bottom surfaces for a satisfactory thermal joint. According to the invention, the condenser 320, which includes offset fins and forms the cooling system 300, is disposed in multiple positions behind the four cooling fans 52 for cooling the hard disk drives described above. That is, the condensers 320, which form the cooling system, are arranged in series along the passage of air (cooling air) externally supplied through the cooling fans 52. That is, the condensers 320, each of which includes offset fins, are attached in series in parallel to the row of the cooling fans 52 described above.

As described above, in the structure of the electronic apparatus described above, the cooling fans 52, which are means for cooling devices other than the condensers 320 and accommodated in each of the enclosures 5 in the electronic apparatus, are used (or shared) as cooling means (radiators) for cooling the condensers 320, which form the cooling system 300 using a thermosiphon according to the invention. According to the configuration described above, the CPUs 200, which are heat generating sources in the enclosures 5, can be efficiently and reliably cooled without provision of dedicated cooling fans, in other words, by using the cooling system that is relatively simple and inexpensive, requires no pumping power for driving a liquid, and excels in energy conservation. Further, using the cooling system 300 using a thermosiphon according to the invention allows electronic apparatus, such as servers required to be implemented at a high density, to be arranged in a highly flexible manner in the relatively simple structure having relatively high heat exchange efficiency.

Further, each of the condensers 320, which form the cooling system 300, is so disposed that it covers the air outlet surfaces of a plurality of (two in present embodiment) the cooling fans, as clearly seen from FIGS. 9 and 10. According to the configuration of the invention, even when one of the cooling fans stops operating due to failure, the condenser 320 can continue the cooling operation by using the cooling air produced by the other cooling fan, that is, redundancy of the cooling system is ensured, whereby the configuration described above is suitable for the structure of a cooling system for an electronic apparatus. Further, as indicated by the encircled portion in FIG. 10, in particular, when the positions where the steam pipes 331, which guide the coolant steam produced in the heat receiving jackets 310 to the condenser 320, are attached to heads of the condensers are shifted toward the cooling fan, which is a radiator, having a small area that faces the condensers (one of four cooling fans 52 arranged vertically in FIG. 10, second one counted from below), the redundancy of the cooling system can be further improved in case that any of the cooling fans stops operating due to failure.

In the present embodiment, three cooling fans are allocated for condenser portions of the two thermosiphon, or one and a half cooling fans correspond to one condenser portion. When one of the one and a half cooling fans stops operating in this configuration, only the remaining one-half fan carries out the cooling operation, which is equal to a situation in which two-third of the radiators for the condenser portion of each of the thermosiphons cannot dissipate heat. In a server system, since a certain amount of period is required to allow the system to normally stop operating in the event of emergency, the cooling capability needs to be available during the period. In a water-based cooling radiator of related art, in which a coolant uniformly flows throughout the radiator, a decrease in an effective heat dissipating area by two-third means that the cooling performance of the coolant decreases accordingly, and the decrease in the cooling performance directly contributes to an increase in the temperature of a CPU. In contrast, in a thermosiphon-based system, in which the steam will not condense in a portion where the radiator does not dissipate heat, the steam is consequently concentrated to the remaining portion where the cooling operation is carried out. The steam concentrated to part of the cooling system has a high flow rate and hence pushes out the liquid film in a flat-shaped pipe, which contributes to improvement in the condensing performance. Further, in each of the thermosiphons of the present embodiment, a large amount of steam is likely to flow through a flat-shaped pipe 323, which is located in a position close to the pipe 331, which supplies the condenser portion with steam. When the positions where the steam pipes 331 are attached to the heads of the condensers are shifted toward the cooling fan, which is a radiator, having a small area that faces the condensers based on the characteristic described above, a decrease in heat dissipation performance can be reduced when one of the cooling fans stops operating. Using the thermosiphons therefore ensures the redundancy of the cooling system with a decreased number of fans.

COOLING SYSTEM AND ELECTRONIC APPARATUS USING THE SAME

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