Cooling device and centrifugal pump to be used in the same device

The present invention relates to a cooling device which cools heat-producing electronic components, such as a central processing unit (CPU), disposed in a housing by circulating coolant. It also relates to a centrifugal pump to be used in the cooling device.

A speed enhancement technology has been developed rapidly in the computer industry, so that a clock frequency of a CPU becomes substantially higher than a previous one. As a result, the CPU produces too much heat for a conventional heat sink to air-cool the CPU. Thus a cooling device of high power and high efficiency is vitally required. One of such cooling devices is disclosed in Japanese Patent Application Non-Examined Publication No. H07-142886. This cooling device circulates coolant through a substrate on which heat-producing electronic components are mounted, thereby cooling the substrate.

This conventional cooling device that circulates the coolant for cooling an electronic apparatus is described hereinafter. The electronic apparatus in this description refers to such an apparatus that carries out a process by loading a program to a CPU, in particular, a small size and portable apparatus such as a notebook-size computer. Besides the foregoing apparatus, the conventional cooling device can also cool an apparatus in which electronic components producing heat due to energizing are mounted.

A first conventional cooling device has a structure as shown in FIG. 9 . Housing 100 accommodates circuit board 102 , cooler 103 and radiator 104 . Heat-producing electronic component (hereinafter referred to simply as component) 101 is mounted on board 102 . Cooler 103 exchanges the heat between component 101 and coolant, thereby cooling component 101 . Radiator 104 liberates the heat from the coolant. Pump 105 circulates the coolant through pipe 106 , and fan 107 air-cools radiator 104 .

An operation of the first conventional cooling device is described hereinafter. Pump 105 discharges the coolant, which then travels through pipe 106 and arrives at cooler 103 , where the coolant collects heat off component 101 and its temperature thus rises. The coolant then travels to radiator 104 , where fan 107 air-cools forcibly the coolant, so that its temperature lowers. The coolant returned to pump 15 , and repeats the foregoing cycle. As discussed above, the coolant circulates through pipe 106 , thereby cooling component 101 .

The foregoing Non-Examined Publication also discloses a second conventional cooling device of which structure is shown in FIG. 10 . When a heat-producing member is mounted in a confined housing, this second cooling device efficiently transfers the heat generated from the heat-producing member to a wall of the metal housing which works as a radiator, thereby cooling the heat-producing member.

An electronic apparatus includes circuit board 108 , keyboard 109 , heat-producing semiconductor element (hereinafter referred to simply as element) 110 , disc device 111 and display device 112 accommodated in metal housing 116 . The second cooling device thermally couples heat-producing element 110 to housing 116 with a heat-transport device having a flexible structure. The heat transport device includes a liquid flow channel mounted to element 110 , and is formed of heat-receiving header 113 , heat-radiating header 114 and flexible tube 115 . Flat heat-receiving header 113 exchanges heat with element 110 . Heat-radiating header 114 includes the liquid flow channel and contact with a wall of housing 116 . Flexible tube 115 couples header 113 to header 114 . A liquid driving mechanism integrated in heat-radiating header 114 drives or circulates the liquid that is sealed in the mechanism between heat-receiving header 113 and heat-radiating header 114 . This liquid driving mechanism thus couples element 110 to housing 116 with ease regardless of the components arrangement, and transports the heat efficiently using the liquid movement. Since heat-radiating header 114 is thermally coupled to housing 116 , which has a high heat conduction rate, the heat diffuses extensively to housing 116 .

The first conventional cooling device, however, needs cooler 103 , radiator 104 , pump 105 and a refilling tank (not shown) for refilling pump 105 with the coolant. Those elements are assembled into the cooling device, so that the device becomes bulky and complicated. As a result, it is difficult to reduce the size of the device and the device becomes expensive. In other words, the first cooling device is basically fit for cooling a large size electronic apparatus, but is not suitable for a recent notebook-size computer which is compact, light-weight, slim, and carried in a variety of postures.

The second cooling device can be used in a notebook-size computer; however, both of heat-receiving header 113 and heat-radiating header 114 are box-shaped and substantially thick, which prevents the notebook-size computer from being slimmed. To be more specific, in the second cooling device, a reciprocating pump (not shown) is prepared in header 114 . This pump has a rather narrower width than other pumps and works as the liquid driving mechanism; however, the thickness of header 114 is specified by this pump, so that the overall thickness cannot be reduced. As a result, the notebook-size computer cannot be further slimmed.

In the notebook size computer, it is difficult for heat-receiving header 113 to accommodate the reciprocating pump of the second cooling device. To be more specific, the thickness of the pump on top of the thickness of element 110 increases the height of the computer, and this configuration goes against the trend toward the slimmed-down design. Further, the reciprocating pump produces vibrations and noises, which influence element 110 on which the pump is placed. The vibrations and noises sometimes cause harsh grating noises. This configuration is thus difficult to be realized.

In the second cooling device, heat-radiating header 114 brought into contact with a wall of housing 116 has a limited heat-radiating area, so that it has a low heat conduction efficiency and a limited cooling power. Increasing a heat-radiating area is one of the ideas for boosting the cooling power; however, the larger area lengthens the liquid flow channel, and an amount of circulating coolant increases, which requires a greater power of the built-in reciprocating pump. The greater power of the pump needs, inconsistently, a greater thickness of header 114 . Thus, the reciprocating pump can be independently accommodated in housing 116 ; however, in this case, another space must be prepared for the pump although the notebook-size computer has been ultimately downsized, and the assembly work becomes cumbersome. As discussed above, the second cooling device limits further downsizing of the notebook-size computer. To be more specific, the performance of the CPU will continue to increase, thereby requiring greater cooling power, so that the second conventional cooling device having the foregoing problems will no longer be used.

A conventional pump having a heat exchanger function needs a cooling water channel in the pump for cooling the coolant with the cooling water supplied separately, so that the pump becomes bulky and complicated. The pump also needs a second pump for circulating the cooling water and a second heat exchanger for collecting the heat off the cooling water, so that the cooling system becomes complicated and is difficult to downsize. The pump thus needs a number of components and its assembly work becomes inefficient. The conventional cooling devices thus cannot be expected to have better thermal efficiency or to be less expensive.

A cooling device of the present invention includes a radiator and a centrifugal pump of contact heat exchanger model, both disposed in a closed circulating channel in which coolant circulates. Heat-producing electronic components are brought into contact with the centrifugal pump, so that the coolant in the pump collects the heat off the electronic components due to its heat exchanger function, and the radiator of the cooling device dissipates the heat. The centrifugal pump includes a pump-casing made of highly heat conductive material and an impeller. The pump casing has a heat-receiving plane formed on a side face along an interior pump room, and a sucking channel prepared between the heat-receiving plane and an inner wall of the pump room. On the inner wall of the pump room, a recess is provided. In this recess, protrusions extending toward the impeller or dimples are provided. According to the present invention, the cooling device of a simple structure, which allows downsizing and slimming-down, is obtainable while its cooling efficiency is improved.

FIG. 1 shows a structure of a cooling device in accordance with an exemplary embodiment of the present invention.

FIG. 2 shows a front view illustrating an appearance of a centrifugal pump of heat-exchanger model, i.e. an element of the cooling device shown in FIG. 1 .

FIG. 3 shows a sectional view of the centrifugal pump of heat-exchanger model shown in FIG. 2 .

FIG. 4 shows a front view of an impeller of the centrifugal pump shown in FIG. 2 .

FIG. 5 shows an appearance of a wall of a pump room disposed in the centrifugal pump shown in FIG. 2 .

FIG. 6 shows a front view illustrating an appearance of another centrifugal pump of heat exchanger model, an element of the cooling device in accordance with the exemplary embodiment of the present invention.

FIG. 7 shows a sectional view of the centrifugal pump shown in FIG. 6 .

FIG. 8 shows an appearance of an inner wall of an interior pump room in the centrifugal pump shown in FIG. 6 .

FIG. 9 shows a structure of a first conventional cooling device that cools an electronic apparatus.

FIG. 10 shows a structure of a second conventional cooling device that cools an electronic apparatus.

FIG. 1 shows a structure of a cooling device in accordance with an exemplary embodiment of the present invention, FIG. 2 shows a front view illustrating an appearance of a centrifugal pump of heat-exchanger model, i.e. an element of the cooling device. FIG. 3 shows a sectional view of the centrifugal pump of heat-exchanger model taken out along line 3 - 3 in FIG. 2 . FIG. 4 shows a front view of an impeller of the centrifugal pump shown in FIG. 2 , and FIG. 5 shows an appearance of an inner wall of a pump room.

Centrifugal pump 1 of heat exchanger model (hereinafter referred to simply as pump 1 ) is an element of the cooling device. Heat-producing component 2 (hereinafter referred to simply as component) such as a CPU is, in general, a chip component having a flat surface. Pump 1 and component 2 are extremely small and mounted in a compact portable electronic apparatus such as a notebook-size computer. Radiator 3 radiates the heat collected from component 2 by coolant 41 to the outside. Closed circulating channel 4 couples pump 1 to radiator 3 , thereby circulating coolant 41 , which is liquid and preferably a water solution of propylene glycol which is harmless and used as a food additive. In the case of using aluminum or copper as a material of the casing, an anti-corrosion additive is preferably added in order to increase resistance to corrosion caused by those materials.

Radiator 3 is formed of material having a high heat conductivity and an excellent heat dissipating property, such as a thin plate of copper or aluminum. A coolant path and a reserve tank are formed in radiator 3 . A fan can be prepared for forcibly air-cooling radiator 3 so that a better cooling effect can be expected. Circulating channel 4 is formed of flexible rubber having a low gas-permeability, such as butyl rubber tubing because this material allows a flexible piping layout as well as preventing bubbles from entering into the tube.

Next, an interior structure of pump 1 is described. Open impeller 11 has through hole 11 A and open blades 12 . Ring magnet 13 is inserted into impeller 11 . Stator 14 is disposed inside ring-magnet 13 . Pump casing 15 forms pump room 15 A which accommodates impeller 11 while casing 15 restores kinetic energy, which impeller 11 has given to coolant 41 , by a pressure recovery method and guides the energy to a discharge port. Pump room 15 A restores kinetic energy, which blades 12 have given to supplied coolant 41 , by the pressure recovery method and guides the energy to the discharge port.

Heat-receiving plane 15 B is formed on an outer wall face of a base wall of casing 15 along pump room 15 A, and brought into contact with component 2 , thereby collecting heat off component 2 . Heat-receiving plane 15 B is formed on the outer wall face of casing 15 substantially parallel with a revolution surface of impeller 11 . On radially outer wall surface 15 C of the pump room 15 A, a large number of dimples 21 are formed. A recess (recessed area) 15 E defines a radially inner wall surface on a bottom of the pump room 15 A that faces toward impeller 11 , and has a large number of protrusions 24 projected from the radially outer wall surface and toward impeller 11 . Recess 15 E, slope 27 , and radially outer wall surface 15 C together define an inner wall face 50 of casing 15 . Casing cover (cover wall) 16 accommodates impeller 11 , and forms pump room 15 A together with casing 15 . As shown in FIG. 3 , fixed shaft (rotary shaft) 17 is disposed to extend between the cover wall 16 and the base wall of casing 15 rotatively supports impeller 11 . Fixed shaft 17 is equipped with bearing 18 provided at the center of impeller 11 . Sucking channel 19 sucks coolant 41 , and discharging channel 20 discharges coolant 41 . Sucking channel 19 is disposed between heat-receiving plane 15 B and inner wall face 50 .

Sucking groove 26 is provided together with sucking channel 19 unitarily along a direction common to channel 19 and groove 26 , and extends toward the rotational center of impeller 11 . Sucking groove 26 is formed as a step down from recess 15 E and communicates with recess 15 E. Sucking groove 26 guides coolant 41 sucked through sucking channel 19 to near the rotational center of impeller 11 . Slope 27 gradually goes uphill to radially outer wall surface 15 C, which is a sloped step up from recess 15 E. O-ring 25 seals the engagement between cover 16 and casing 15 , and packing 25 A functions similar to O-ring 25 .

IC 29 regulates an output voltage applied to the windings of stator 14 and is mounted to control board 28 . Screw holes 31 are provided for screwing cover 16 to casing 15 . Springs or screws are inserted into fixing holes 32 for bringing pump 1 into contact with component 2 , urging pump 1 toward component 2 , or fixing pump 1 to component 2 .

Cover 16 and casing 15 are assembled into the outer shell of pump 11 . Of the cover 16 and the casing 15 , at least the casing 15 , is formed of material having a high thermal conductivity and an excellent heat dissipating property, e.g. copper or aluminum, of which thermal conductivity is 380-400 W/m·k and ca. 230 W/m·K respectively. It is preferable to use the material having at least the foregoing specific thermal conductivity.

Pump 1 has the following specifications: thickness is 5-50 mm; representative dimension of radial direction is 10-100 mm; flow rate is 0.2-5.0 L/minute; head is 0.2-4 m. More typical values are listed below: thickness along the axial direction of rotation is 7-40 mm; representative dimension of radial direction is 20-50 mm; flow rate is 0.3-3.0 L/minute; head is 0.35-2 m; and rpm is 2500-4000. If the specification of this pump is expressed in terms of specific speed, it can be 30-250 m·m 3 /minute rpm. This value proves that pump 1 is extraordinarily smaller and slimmer than the conventional pumps.

In pump 1 , blades 12 of impeller 11 are disposed facing component 2 , and heat-receiving plane 15 B is formed to have a surface complementary to the upper surface of component 2 , so that pump room 15 A receives the heat via heat-receiving plane 15 B. FIG. 3 shows that pump 1 is disposed above component 2 ; however pump 1 can be placed below component 2 , or those elements can be laterally placed depending on the placement of component 2 .

Stator 14 is press-fitted into cover 16 so that the outer rim of stator 14 faces the inner rim of ring-magnet 13 . Cover 16 is placed between stator 14 and ring-magnet 13 as a partition for isolating those two elements, so that stator 14 is completely isolated from coolant 41 flowing through pump room 15 A. Ring-magnet 13 is a magnet of which at least one face of its circumference is covered with soft magnetic material shaped like a ring. Impeller 11 can be unitarily formed with ring-magnet 13 ; however, it is separately formed from magnet 13 in this embodiment. In the case of a unitary design, impeller 11 is made of magnet material and a section thereof facing stator 14 is magnetized. Ring magnet 13 is rotated by the rotary magnetic field of stator 14 , thereby rotating impeller 11 . Then negative pressure occurs around the center of impeller 11 , so that coolant 41 is sucked through sucking channel 19 communicating with impeller 11 . Coolant 14 is discharged outside by kinetic momentum due to impeller 11 . A discharge port (not shown) is provided to casing 15 at a section facing the outer rim of impeller 11 , so that coolant 41 is discharged to circulating channel 4 via discharging channel 20 .

Bearing 18 made of resin having abrasion resistance and low friction is press-fitted at the center of impeller 11 , and fixed shaft 17 made of stainless steel is prepared inside bearing 18 with its two ends fixed at casing 15 and cover 16 . Bearing 18 can be insert-molded with impeller 11 instead of being press-fitted. Fixed shaft 17 can also be insert-molded with cover 15 or press-fitted. As shown in FIG. 4 , bearing 18 is provided with a D-cut on its outer wall-face, so that a space is prepared between a press-fit hole (not shown) of impeller 11 and bearing 18 per se. As a result, pump room 15 A communicates with the backside of impeller 11 . This space is used as through hole 11 A deviated from the rotation center. Through hole 11 A guides parts of coolant 41 pushed out due to the centrifugal force of impeller 11 to the backside of impeller 11 , then the parts of coolant 41 pass through hole 11 A and flow out to sucking inlet 19 A of impeller 11 with negative pressure. In other words, the parts of coolant 41 flow back. Coolant 41 flowing back is mixed at sucking inlet 19 A, so that flow-back coolant 41 is always replaced with another flow-back and never stays at one place.

Impeller 11 receives negative pressure around its center due to the centrifugal force of itself, so that cavitation tends to occur there. However, since pump 1 has specific speed of 30-250 m·m 3 /minute·rpm, the cavitation hardly occurs, and even if it occurs, it is mixed by the foregoing flow-back and discharged. The cavitation possibly generated will not stay around the center of impeller 11 because coolant 41 circulates so as to replace itself with incoming coolant at the backside of impeller 11 and sucking inlet 19 A. If air is mixed somewhere in the cooling device, and if the air is sucked into pump 1 , the bubbles will not stay around the center of impeller 11 due to the circulation, and the bubbles are discharged gradually. Therefore, there is little noise caused by the cavitation, and no gas space is formed, so that the flow becomes turbulent, which increases an amount of heat transferred.

In pump 1 , recess 15 E is provided around the center of inner wall face 50 of the pump room of casing 15 , and cylindrical protrusions (surface-area-increasing parts) 24 extend from the bottom face of recess 15 E toward impeller 11 . This structure reduces the overall thickness of pump 1 by the depth of recess 15 E, so that the heat transferred from the electronic component to casing 15 can be collected by coolant 41 with ease. As discussed above, the thickness of pump 1 is reduced as much as possible while the performance of pump 1 is maintained, so that a high heat-collecting efficiency is achieved.

In this embodiment, blades 12 are disposed radially outside the area having protrusions 24 instead of being disposed within the area, so that impeller 11 is placed nearer to casing 15 . This structure reduces the overall thickness of the pump, and yet, the height of protrusions 24 becomes greater. If the thickness of the pump can increase in some degree, blades 12 can be provided within the area of protrusions 24 , i.e. above protrusions 24 .

Sucking channel 19 does not have a hole-shape all the way to the center of impeller 11 , but its upper thickness is removed to form sucking groove 26 , so that recess 15 E and protrusions 24 can be formed. This structure allows pump 1 to reduce its overall size while its performance is maintained.

Further, as shown in FIG. 3 , a number of dimples 21 are formed on at least a part of a section, where impeller 11 slides, of radially outer wall surface 15 C (backside of heat-receiving plane 15 B). In this structure, coolant 41 flowing due to the rotation of impeller 11 peels a laminar boundary layer formed on radially outer wall surface 15 C, so that coolant 41 flows turbulently. As a result, an amount of heat transferred from heat-receiving plane 15 B to coolant 41 becomes greater. The presence of dimples 21 at the sliding section of impeller 12 facilitates peeling of the laminar boundary layer of coolant 41 off radially outer wall surface 15 C, so that the amount of the heat transferred from heat-receiving plane 15 B to coolant 41 becomes even greater. In this case, the torque necessary for spinning impeller 11 increases a little, so that the placement and the number of dimples should be designed appropriately to the torque tolerance. Radially outer wall surface 15 C can undergo a shot peening process or a sand blasting process to form peaks and valleys thereon or roughening itself. Such a process can also increase the heat collecting efficiency based on the same principle though the effect is rather low. Although it is not shown in the accompanying drawings, a spiral groove can be formed on radially outer wall surface 15 C for coolant 41 to flow turbulently, so that the amount of heat transferred can be increased.

Slope 27 disposed around recess 15 E reduces flow resistance as much as possible when coolant 41 passes from recess 15 E to radially outer wall surface 15 C stepped-up, in other words, step 27 helps maintain the pump performance.

O-ring 25 is sandwiched in a radial direction between cover 16 and casing 15 when those two elements are engaged with each other, and seals the two elements off from each other. Packing 25 A is compressed by cover 16 and casing 15 when those two elements are tightened up together with screws (not shown), thereby sealing the two elements off from each other. Thus pump room 15 A is doubly sealed. This structure allows coolant 41 to vaporize in substantially less quantity than in the case of a single seal, and increases the safety against liquid-leakage from the cooling device. O-ring 25 and packing 25 A can be unitarily formed of rubber for double sealing.

A temperature of IC 29 rises as high as nearly 100° C. or more during the pump operation; however if IC 29 is mounted such that it is brought into contact with casing 15 , the heat of IC 29 is collected by casing 15 . In other words, pump 1 also collects the heat off IC 29 , thereby lowering the temperature of IC 29 during the operation. As a result, IC 29 works always steadily. Further, this structure allows IC 29 to extend its life because IC 29 prolongs its service life at a lower temperature. FIG. 3 shows that board 28 is placed such that IC 29 is directly contact with casing 15 . However, a free-face of board 28 (the face where electronic components such as IC 29 are not mounted, i.e. the face behind the face having IC 29 ) can be brought into contact with casing 15 for dissipating the heat of IC 29 .

Next, an operation of pump 1 is demonstrated hereinafter. IC 29 and board 28 apply a control voltage to the windings of stator 14 , thereby generating rotary magnetic field in stator 14 . The rotary magnetic field spins ring-magnet 13 , namely, impeller 11 .

Magnetic flux flows from ring-magnet 13 toward stator 14 ; however some flux leaks in other directions, so that the rotation of impeller 11 extends the shifting magnetic field leaked from ring-magnet 13 to metal casing 15 (conductive material), and an eddy current occurs in casing 15 . This eddy current makes resistance against the rotation of ring-magnet 13 . Thus if the minimum distance between ring-magnet 13 and casing 15 can be kept at not less than 2 mm in every direction, the resistance can be substantially reduced. In the case of disposing stator 14 within ring-magnet 13 in a radius direction, ring-magnet 13 is magnetized to produce magnetic force inward in the radius direction. This preparation allows the leakage magnetic field to occur mainly at the outside as far as within the height in an axial direction of ring-magnet 13 . Thus if metal casing 15 (conductive material) is placed at a distance not less than 2 mm from ring-magnet 13 and in the same level as the height of ring-magnet 13 in the axial direction, the resistance against the rotation of ring-magnet 13 can be substantially reduced. In other words, the outer rim of ring-magnet 13 is preferably off casing 15 by not less than 2 mm along the radius direction of the rotating axis of impeller 11 . If a yoke is further provided at the outer rim of ring-magnet 13 , the leakage magnetic field can be almost prevented. If ring-magnet 13 tapers inward and is magnetized at its inner rim, and both of ring-magnet 13 and stator 14 are placed in the radial direction, the resistance against the rotation caused by the eddy current due to the shifting magnetic field can be reduced.

Coolant 41 is sucked through sucking channel 19 by spinning blades 12 , and discharged from discharging channel 20 . During this operation, in pump room 15 A, coolant 41 sucked from channel 19 is transported to recess 15 E of casing 15 and further flows along sucking groove 26 to near the rotation center of impeller 11 . Then coolant 41 hits against bearing 18 to diffuse in every direction, and hits a number of protrusions 24 before entering between blades 12 . On the other hand, the heat generated from component 2 travels to casing 15 and is transferred to protrusions 24 projected from recess 15 E, so that coolant 41 collects the heat when it hits against protrusions 24 . The flow of coolant 41 through spaces between protrusions 24 produces eddies, which peel thermal boundary layers off protrusions 24 . In other words, coolant 41 peels liquid film clung to protrusions 24 off, thereby collecting the heat off protrusions 24 . Instead of protrusions 24 , dimples 40 can be provided on the bottom face of recess 15 E. This preparation also peels the thermal boundary layer and can collect the heat off casing 15 with ease.

The heat transferred from component 2 to casing 15 travels everywhere on radially outer wall surface 15 C. On the other hand, the laminar boundary layer, which is formed when coolant 41 passes through pump room 15 A in such a manner as the layer clings to radially outer wall surface 15 C, is peeled off by dimples 21 and impeller 11 or blades 12 sliding there. In other words, the layer which stores the heat transferred to casing 15 , namely, the thermal boundary layer, is peeled off. As a result, much more heat transferred to casing 15 can be collected by coolant 41 . As discussed above, the heat generated from component 2 , i.e. the heat stored in protrusions 24 and casing 15 , is collected by coolant 41 , and discharged through discharging channel 20 together with coolant 41 by spinning blades 12 . Coolant 41 then flows through circulating channel 4 to radiator 3 , which cools coolant 41 , and is sucked into pump 1 again with a lower temperature. This circulation is repeated, thereby cooling component 2 .

As described above, pump 1 includes casing 15 made from material having a high thermal conductivity, and open impeller 11 having blades 12 . Inner wall face 15 C has recess 15 E, and protrusions 24 extend from the bottom face of recess 15 E toward impeller 11 . This structure allows reducing the thickness of pump 1 and increasing the surface area of interior wall of the pump room, so that a heat collecting efficiency increases. The shape of heat-receiving plane 15 B and the shape of an upper surface of component 2 complement each other three-dimensionally, so that sucking channel 19 does not extend over component 2 . This structure allows heat-receiving plane 15 B and the upper surface of component 2 to solidly contact with each other, so that the heat can be transferred efficiently.

Next, another structure of the centrifugal pump in accordance with the embodiment is described hereinafter. FIG. 6 shows a front view illustrating an appearance of this another centrifugal pump of heat exchanger model, i.e. an element of the cooling device in accordance with this exemplary embodiment of the present invention. FIG. 7 shows a sectional view of the centrifugal pump taken along line 7 - 7 in FIG. 6 . FIG. 8 shows an appearance of an interior wall of a pump room disposed in the centrifugal pump shown in FIG. 6 . Centrifugal pump 1 A differs from pump 1 in the following point: A sucking channel delivered from the outside of pump 1 A crosses a part of recessed section in the pump room and extends to near the center of the impeller with itself being kept in a tubular form. Similar elements to those of pump 1 shown in FIG. 2-FIG . 5 have the same reference marks, and the descriptions thereof are omitted here.

Recess 15 F of the pump room is disposed one step down from radially outer wall surface 15 C of casing 15 . Cylindrical protrusions 24 extend from a bottom face of recess 15 F toward impeller 11 . Recess transversal sections 30 , 30 A, 30 B cross recess 15 F and extend from sucking channel 19 toward the center of casing 15 . Protrusions 24 A extend from the upper faces of transverse sections 30 , 30 A, 30 B.

Transverse section 30 has sucking channel 19 therein, and guides channel 19 to near the center of casing 15 . The upper face of transverse section 30 has approximately the same height as radially outer wall surface 15 C. In other words, sucking channel 19 crosses parts of recess 15 F. During the operation of pump 1 A, coolant 41 sucked is drawn by impeller 11 and flows on recess 15 F to the outside of impeller 11 . In this case, transverse section 30 works as resistance against the flow of coolant 41 and floats impeller 11 . In order to keep the flotation balance of impeller 11 , transverse sections 30 A, 30 B having a similar shape viewed from pump room 15 A to transverse section 30 are provided at the same intervals. This structure allows impeller 11 to rotate free from slanting because impeller 11 receives floating force well balanced. If there is only transverse section 30 , impeller 11 tends to rotate slantingly with respect to radially outer wall surface 15 C. In this case, fixed shaft 17 and bearing 18 shorten their service lives, or impeller 11 rotates contacting with wall face 15 C, so that the performance of the pump lowers, and in the worst case, the pump stops. In this embodiment, three transverse sections 30 , 30 A, 30 B are provided at the same intervals (120 degrees). If they are provided radially from shaft 17 at the same intervals, it is preferable to provide a number of transverse sections.

Transverse sections 30 , 30 A, 30 B have protrusions 24 A on their faces toward pump room 15 A, and blades 12 are not provided in the area of protrusions 24 A. This structure allows increasing the surface area of radially outer wall surface 15 C, so that the overall thickness of the pump can be reduced while the heat collecting efficiency increases. In the case of increasing the pump performance instead of the heat collecting performance, protrusions 24 A are omitted, and blades 12 can be provided at the places toward the protrusion-omitted places. If a user does not stick to a slim pump, protrusions 24 A are provided, and blades 12 are also provided above the protruded area such that blades 12 do not touch protrusions 24 A. In this case, both of the heat-collecting performance and the pump performance can increase.

Next, an operation of pump 1 A is demonstrated. IC 29 and board 28 apply a control voltage to the windings of stator 14 , thereby generating rotary magnetic field in stator 14 . The rotary magnetic field spins ring-magnet 13 , namely, impeller 11 . Coolant 41 is sucked through sucking channel 19 by spinning impeller 11 , namely blades 12 , and discharged from discharging channel 20 . During this operation, in pump room 15 A, coolant 41 sucked from channel 19 crosses recess 15 F of casing 15 passing through transverse section 30 and further flows to near the center of pump room 15 A. Then coolant 41 hits against bearing 18 to diffuse in every direction, and hits a number of protrusions 24 before entering between blades 12 . On the other hand, the heat generated from component 2 travels to casing 15 and is transferred to protrusions 24 , 24 A projected from recess 15 F, so that coolant 41 collects the heat when it hits against protrusions 24 . The flow of coolant 41 through spaces between protrusions 24 produces eddies, which peel thermal boundary layers off protrusions 24 . In other words, coolant 41 peels liquid film clung to protrusions 24 off, thereby collecting the heat off protrusions 24 . Instead of protrusions 24 , dimples 40 can be provided on the bottom face of recess 15 F. This preparation also peels the thermal boundary layer and can collect the heat off casing 15 with ease.

The heat transferred from component 2 to casing 15 travels everywhere on radially outer wall surface 15 C. On the other hand, the laminar boundary layer, which is formed when coolant 41 passes through pump room 15 A in such a manner as the layer clings to radially outer wall surface 15 C, is peeled off by dimples 21 and impeller 11 or blades 12 sliding there. In other words, the layer which stores the heat transferred to casing 15 , namely, the thermal boundary layer, is peeled off. As a result, much more heat transferred to casing 15 can be collected by coolant 41 . As discussed above, the heat generated from component 2 , i.e. the heat stored in protrusions 24 and casing 15 , is collected by coolant 41 , and discharged through discharging channel 20 together with coolant 41 by spinning blades 12 . Coolant 41 then flows through circulating channel 4 to radiator 3 , which cools coolant 41 , and is sucked into pump 1 again with a lower temperature. This circulation is repeated, thereby cooling component 2 .

As described above, pump 1 A includes casing 15 made of material having a high thermal conductivity, and open impeller 11 having blades 12 . Radially outer wall surface 15 C surrounds recess 15 F, and protrusions 24 extend from the bottom face of recess 15 F toward impeller 11 . This structure allows reducing the thickness of the pump and increasing the surface area of interior wall of the pump room, so that a heat collecting efficiency increases. Further, transverse sections 30 , 30 A and 30 B positively guide coolant 41 sucked to near the center of recess 15 F. Protrusions 24 thus positively transferred the heat to coolant 41 , so that the heat collecting efficiency further increases. The shape of heat-receiving plane 15 B and the shape of an upper surface of component 2 complement each other three-dimensionally, so that sucking channel 19 does not extend over component 2 . This structure allows heat-receiving plane 15 B and the upper surface of component 2 to solidly contact with each other, so that the heat can be transferred efficiently.

In a closed circulating channel which circulates coolant, a radiator and a centrifugal pump of contact heat-exchanger model are provided. A heat-producing electronic component is brought into contact with the centrifugal pump, and the coolant in the pump collects the heat off the electronic component due to the heat exchanging function. Then the heat is dissipated from the radiator. A cooling device is thus obtainable. The centrifugal pump includes a pump-casing and an impeller both made of highly heat conductive material. In the pump casing, a heat-receiving plane is formed on a side face along a pump room disposed in the casing, and a sucking channel is prepared between the heat-receiving plane and an inner wall of the pump room. On the inner wall of the pump room, a recess is provided. In this recess, protrusions extending toward the impeller, or dimples are provided. The cooling device as structured above has a simple structure and can be downsized and slimmed while its cooling efficiency is improved.