Abstract:
A cooling apparatus comprises a tank for keeping a coolant liquid therein and having an air layer, a heat receiving element in contact with a heat generating part to receive heat therefrom, a radiator for radiating heat absorbed by a coolant liquid, and a circulating mechanism for circulating the coolant liquid from the heat receiving element through the tank and through the radiator again to the heat receiving element. The tank has a conduit forming area for forming a coolant liquid circulating conduit to pass through a middle position of the tank, and a narrow gap which divides the conduit forming area at the middle position of the tank.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a cooling structure for an electronic device which is equipped with heat generating elements, and more particularly, to the structure of a tank for keeping a coolant liquid (hereinafter simply called the “reservoir tank”) in a circulation-type liquid cooling system.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, as the performance of electronic devices increases, parts mounted in such electronic devices have generated increasingly more amounts of heat. As a result, stricter requirements have been imposed on cooling technology for devices. In the field of computers, as a result of increasing the number of transistors on a processor (CPU) and raising the operating clock frequency of the CPU to improve the processing performance and processing speed of the CPU, electric power density of a chip has increased to such a degree that TDP (Thermal Design Power) exceeds 30 W even in mobile applications. As such, there is an urgent necessity to establish an effective cooling technology for removing heat generated within a housing.  
         [0005]     Conventionally, for cooling an electronic device such as a personal computer, a heat sink is connected to a heat generating element such as a CPU to spread heat which is then discharged to the outside of a housing by use of forced air cooling. Recently, however, device-cooling methods based on liquid cooling technology have been under investigation because of their higher heat radiation performance and quiet operation.  
         [0006]      FIG. 1  illustrates the structure of a liquid cooling system mainly used in personal computers.  
         [0007]     Liquid cooling system  1  comprises radiator  3   a , circulating pump  4   a , reservoir tank  5   a , and heat receiving element  2   a  such as a cold plate. Heat receiving element  2   a  is connected to heat generating element  6  such as a CPU and a GPU for absorbing heat therefrom, and transports the amount of received heat to radiator  3   a  through a coolant liquid which flows within heat receiving element  2   a . Radiator  3   a  exchanges heat with external air through natural air cooling or a combination of the natural air cooling with forced air cooling in order to radiate heat. The coolant liquid cooled by radiator  3   a  is transported again to heat receiving element  2   a  by circulating pump  4   a . The circulation-type liquid cooling system configured in the manner described above is provided with reservoir tank  5   a  for keeping an amount of coolant liquid that is required to compensate for a lost amount of the coolant liquid due to volatilization from component members (mainly from resin tube  7   a ).  
         [0008]      FIGS. 2 and 3  illustrate a modular structure of such a liquid cooling system.  FIG. 2  illustrates a general-purpose liquid cooling module for use in desk top personal computers (PCs) and the like, while  FIG. 3  illustrates a thin-type liquid cooling module for use in notebook-type personal computers and the like. While both modules employ common components, the shape of a radiator and the like may be modified in accordance with the characteristics of a particular housing in which the liquid cooling system is mounted.  
         [0009]     The aforementioned reservoir tank, which forms part of a liquid cooling system, is responsible for the following three functions: (1) keeping a required amount of coolant liquid during a device for a guaranteed period; (2) alleviating of variations in the pressure within a circulating system due to an expanded volume of the coolant liquid caused by received heat; and (3) trapping and removal of bubbles generated within the circulating system.  
         [0010]     A resin tube for interconnecting components of a liquid cooling system permits volatilization of moisture of the coolant liquid through its molecular interstices. For this reason, anti-freeze condenses over a long-term use, and variations in viscosity due to a change in concentration degrades the coolant circulating performance, resulting in lower cooling performance of the system. Therefore, for guarantee the operation for a specific period, it is necessary to keep a surplus amount of coolant liquid in anticipation that an amount of moisture will volatilize, so that a reservoir tank is provided for pooling the coolant liquid.  
         [0011]     Also, during the operation of an apparatus, coolant liquid, which has absorbed heat from heated devices, expands in volume as it is heated to increase the inner pressure of a circulating path. In this event, an air layer must be provided, as well, for alleviating the pressure within the circulating system in order to prevent the coolant liquid from leaking from a module junction. To meet this requirement, the liquid level (boundary between the liquid and air layer) is adjusted within the reservoir tank to ensure that there is a required amount of air capacity.  
         [0012]     Likewise, during operation of the apparatus, bubbles can be produced within the circulating path due to introduction of air from the outside of the system, cavitation, decomposition of the liquid, and the like. If the bubbles stay within the path and block it, the coolant liquid cannot be circulated, possibly causing a loss in the cooling capabilities of the system. Also, if the bubbles within the liquid enter a thin flow between a rotor and a main shaft of a circulating pump, a gap is blocked which results in a semi-dry lubrication or a solid lubrication state between the main shaft and the rotor, which causes the generation of sudden abrasive heating which would damage the pump&#39;s bearings. Consequently, the lifetime of the pump is reduced. Further, if the bubbles are deposited such that they cause the entire rotor to be surrounded by an air layer, the pump becomes incapable of pumping the coolant liquid, resulting in a failure to circulate the coolant liquid. For this reason, bubbles staying within the system are released into the air layer to alleviate the pressure within the reservoir tank.  
         [0013]     Exemplary applications of such a liquid cooling system are known from configurations described in JP-A-266474/1994, JP-A-366260/2002, JP-A-022148/2003, JP-A-209210/2003, JP-A-047842/2004, and the like.  FIGS. 4A and 4B  illustrate an example of a conventional liquid cooling system which is disclosed in JP-A-209210/2003. Heat receiving element  2   d  such as a cold plate is mounted on the main body of notebook type personal computer  10  to cool down CPU  11 , while a thin radiator comprised of radiator tube  13  and metal radiator plate  14  are mounted on display  12  to dispel heat.  
         [0014]     In this configuration, reservoir tank  5   d  that is used to store coolant liquid is fixed on the radiator to maintain a required amount of coolant liquid for a guaranteed period and to remove bubbles produced in the circulating system, thereby helping the circulating pump to operate normally.  
         [0015]     When such a liquid cooling system is employed as a cooling means for an electronic device that can be installed in different orientations, depending on the requirements of a particular user, the function of the aforementioned reservoir tank does not work well in some situations.  
         [0016]     Taking as an example a projector apparatus for projecting an image onto a screen, the apparatus may be installed on a floor for use as illustrated in  FIG. 5A , or the apparatus may be suspended from a ceiling for use in an inverted position as illustrated in  FIG. 5B . In addition, depending on certain conditions, it is contemplated that the apparatus may be installed in an upright posture for projection through reflection at right angles.  
         [0017]     However, the conventional liquid cooling system is designed on the assumption that the apparatus with which it is used operates in the same orientation at all times. Also, the reservoir tank is not designed to support any orientation in an apparatus in which the reservoir tank is installed at any angle over 360 degrees, rather, the reservoir tank is simply a closed case having an inlet port and an outlet port with a proper amount of coolant liquid  24   a  stored therein.  
         [0018]     The conventional reservoir tank in such a structure is highly likely to significantly lose cooling performance due to an air layer within the tank which readily blocks the conduit inlet and outlet ports, following a change in the position of the apparatus, which prevents the coolant liquid from circulating. In particular, in the reservoir tanks disclosed in JP-A-304086/2003 and JP-A-08958/2004, only the opening at the end of an outlet tube within the tank is positioned at the center of the tank, while the opening at the end of an inlet tube within the tank is set largely spaced away from the opening at the end of the outlet tube within the tank. In this structure, the reservoir tank does function without an air within the tank approaching to the opening at the end of the inlet tube within the tank as long as the position is changed within a particular range (from 0° to 90°), but the reservoir tank is not designed to accommodate any changes in positioning from 0° to 360°.  
         [0019]     JP-A-078271/2003 describes an exemplary reservoir tank which is capable of supporting a change in orientation from 0° to 360°. However, the disclosed reservoir tank has the open end of an inlet tube and the open end of an outlet tube arranged in parallel in the same direction at the center of the tank. In this arrangement, since the open ends of the inlet tube and outlet tube are exposed, a change in the orientation of the tank is likely to cause air in the reservoir tank to flow again into a conduit. In other words, even if bubbles in the path are trapped and kept in the air layer within the tank, the bubbles will flow back into the conduit when the orientation of the tank is changed, resulting in a high susceptibility to detrimental effects such as clogging of the conduit by bubbles, damage to a pump, and the like.  
       SUMMARY OF THE INVENTION  
       [0020]     It is an object of the present invention to provide a cooling apparatus for an electronic device utilizing a liquid cooling system, which supports any orientation in which the electronic device is installed in whichever direction, and prevents accumulated air from again flowing into a conduit in the event of a change in orientation of the electronic device.  
         [0021]     A cooling apparatus according to the present invention includes a tank for keeping a coolant liquid therein and having an air layer, heat receiving means in contact with a heat generating part to receive heat therefrom, radiating means for radiating heat absorbed by a coolant liquid, and a circulating mechanism for circulating the coolant liquid from the heat receiving means through the tank and through radiating means again to the heat receiving means. The tank has a conduit forming area for forming a coolant liquid circulating conduit that passes a middle position of the tank, and a narrow gap which divides the conduit forming area at the middle position of the tank.  
         [0022]     In the foregoing configuration, the coolant liquid circulating conduit is formed within the tank such that the conduit passes the middle position of the tank, and the coolant liquid circulating conduit is opened at the middle position of the tank. Accordingly, an air layer provided in the tank for alleviating the pressure will not be introduced into the opening of the coolant liquid circulating conduit formed by dividing the conduit by the narrow gap at the middle position of the tank, even if the air layer changes its position in conformity to a change in the orientation of the apparatus. Further, when the orientation of the tank is changed, minute bubbles produced within the conduit can be efficiently removed via the narrow gap formed by dividing the conduit forming area at the middle position of the tank. Consequently, a stable coolant circulation can be ensured in any orientation taken by the apparatus.  
         [0023]     In particular, the openings of the coolant liquid circulating conduit, which are formed by dividing the conduit via the narrow gap, are opposite to each other at the middle position of the tank, and are hidden from the outside, so that air in the tank is unlikely to again flow into the conduit upon change in the orientation of the tank. In this respect, the reservoir tank in the present invention is advantageous over the reservoir tank described in JP-A-078271/2003.  
         [0024]     The cooling apparatus according to the present invention, when used, eliminates restrictions on the orientation in which a liquid cooling system is installed, so that liquid cooling technology can be applied as well to electronic devices which can be installed in various orientations, like a projector apparatus.  
         [0025]     The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a diagram schematically illustrating the structure of a conventional liquid cooling system;  
         [0027]      FIG. 2  is a perspective view illustrating the structure of a conventional general-purpose liquid cooling system;  
         [0028]      FIG. 3  is a perspective view illustrating the structure of a conventional thin-type liquid cooling system;  
         [0029]      FIGS. 4A and 4B  are diagrams illustrating the structure of an electronic device equipped with a liquid cooling system shown in JP-A-209210/2003;  
         [0030]      FIGS. 5A and 5B  are side views each illustrating an orientation in which a projector apparatus is installed;  
         [0031]      FIG. 6  is a diagram generally illustrating the structure of a cooling apparatus according to a first embodiment of the present invention and a reservoir tank associated therewith;  
         [0032]      FIG. 7  is a diagram illustrating the internal structure of the reservoir tank in  FIG. 6 ;  
         [0033]      FIG. 8  is a front view of the reservoir tank in  FIG. 6 ;  
         [0034]      FIG. 9  is a perspective cross-sectional view of the reservoir tank in  FIG. 6 ;  
         [0035]      FIG. 10  is a diagram illustrating the structure of the reservoir tank in  FIG. 6  when it is filled with a coolant;  
         [0036]      FIGS. 11A  to  11 D are diagrams illustrating in combination how an accumulated layer of air moves when the orientation of the reservoir tank in  FIG. 6  is changed about an X-axis;  
         [0037]      FIGS. 12A  to  12 D are diagrams illustrating in combination how an accumulated layer of air moves when the orientation of the reservoir tank in  FIG. 6  is changed about a Y-axis;  
         [0038]      FIGS. 13A  to  13 D are diagrams illustrating in combination how an accumulated layer of air moves when the orientation of the reservoir tank in  FIG. 6  is changed about a Z-axis;  
         [0039]      FIG. 14  is a diagram generally illustrating the structure of a cooling apparatus according to a second embodiment of the present invention and a reservoir tank associated therewith;  
         [0040]      FIG. 15  is a diagram illustrating the internal structure of the reservoir tank in  FIG. 14 ;  
         [0041]      FIG. 16  is a perspective cross-sectional view illustrating the structure of the reservoir tank in  FIG. 14 ;  
         [0042]      FIG. 17  is a perspective cross-sectional view of the reservoir tank in  FIG. 14 ;  
         [0043]      FIG. 18  is a diagram for describing the operation of the reservoir tank in  FIG. 14 ;  
         [0044]      FIG. 19  is a diagram generally illustrating the structure of a cooling apparatus according to a third embodiment of the present invention and a reservoir tank associated therewith;  
         [0045]      FIG. 20  is a diagram illustrating the internal structure of the reservoir tank in  FIG. 19 ;  
         [0046]      FIG. 21  is a top plan view of the reservoir tank in  FIG. 19 ;  
         [0047]      FIG. 22  is a perspective view illustrating the structure of the reservoir tank in  FIG. 19 ;  
         [0048]      FIG. 23  is a perspective cross-sectional view of the reservoir tank in  FIG. 16 ;  
         [0049]      FIGS. 24A  to  24 D are diagrams illustrating in combination how an accumulated layer of air moves when the orientation of the reservoir tank in  FIG. 19  is changed about the X-axis;  
         [0050]      FIGS. 25A  to  25 D are diagrams illustrating in combination how an accumulated layer of air moves when the orientation of the reservoir tank in  FIG. 19  is changed about the Y-axis;  
         [0051]      FIGS. 26A  to  26 D are diagrams illustrating in combination how an accumulated layer of air moves when the orientation of the reservoir tank in  FIG. 19  is changed about the Z-axis;  
         [0052]      FIG. 27  is a is a diagram illustrating the internal structure of a reservoir tank associated with a cooling apparatus according to a fourth embodiment of the present invention;  
         [0053]      FIG. 28  is a perspective cross-sectional view of the reservoir tank in  FIG. 27 ;  
         [0054]      FIGS. 29A  to  29 C are diagrams illustrating in detail a conduit tube within the reservoir tank in  FIG. 27 ;  
         [0055]      FIGS. 30A and 30B  are diagrams for describing the operation of the reservoir tank in  FIG. 27 ;  
         [0056]      FIG. 31  is a diagram illustrating the internal structure of a reservoir tank associated with a cooling apparatus according to a fifth embodiment of the present invention;  
         [0057]      FIG. 32  is a horizontal sectional view of the reservoir tank in  FIG. 31 ;  
         [0058]      FIG. 33  is a vertical sectional view of the reservoir tank in  FIG. 31 ;  
         [0059]      FIGS. 34A and 34B  are diagrams for describing the operation of the reservoir tank in  FIG. 31 ;  
         [0060]      FIGS. 35A and 35B  are diagrams each illustrating the structure of a reservoir tank associated with a cooling apparatus according to a sixth embodiment of the present invention;  
         [0061]      FIG. 36  is a plan view illustrating the cooling apparatus of  FIG. 35 ;  
         [0062]      FIGS. 37A and 37B  are diagrams for describing the operation of the reservoir tank in  FIG. 35 ;  
         [0063]      FIGS. 38A  to  38 C are side sectional views illustrating a variety of exemplary structures for the reservoir tank. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0064]     FIGS.  6  to  10  illustrate the structure of a reservoir tank associated with a cooling apparatus according to a first embodiment of the present invention.  FIG. 6  generally illustrates the reservoir tank;  FIG. 7  illustrates the internal structure of the reservoir tank;  FIG. 8  is a front view of the reservoir tank;  FIG. 9  is a vertically sectioned perspective view of the reservoir tank; and  FIG. 10  illustrates the structure of the reservoir tank when it is filled with a coolant liquid.  
         [0065]     In FIGS.  6  to  10 , cooling apparatus  15   a  of this embodiment comprises radiator  3   d , radiation fan  16   c , circulating pump  4   e , reservoir tank  5   e , and heat receiving element  2   e  such as a cold plate. Reservoir tank  5   e  is joined to resin tube  7   d , which interconnects components of cooling apparatus  15   a , for keeping a coolant liquid, retaining an air layer for buffering thermal expansion of the cooling liquid, and trapping bubbles produced in a circulating system, as illustrated in  FIG. 6 .  
         [0066]     As illustrated in  FIG. 7 , reservoir tank  5   e  comprises tank body unit  18   a  and tank cover  17   a  which are joined together to make up reservoir tank  5   e . In some cases, tube joint  19   a , which is provided as a separate member, is hermetically connected to tank body unit  18   a.    
         [0067]     Bridge  20   a  is formed with tank body unit  18   a , on a center axis of the reservoir tank which connects an inlet port and an outlet port of the coolant liquid, as illustrated in  FIGS. 7 and 8 . As illustrated in  FIG. 9 , a through-hole which serves as coolant liquid circulating conduit  21   a  is formed in bridge  20   a . Also, narrow gap  22   a  (see  FIG. 8 ) is defined in bridge  20   a , which is a conduit forming member, at a middle position of tank body unit  18   a . Narrow gap  22   a  divides coolant liquid circulating conduit  21   a  to form fault  23   a  in bridge  20   a  for trapping air.  
         [0068]     The reservoir tank in the structure as described above is filled with a proper amount of coolant liquid  24 , as illustrated in  FIG. 10 , and an inlet end and an outlet end of resin tube  7   d  are connected to tube joint  19   a  to build a circulation-based liquid cooling system. The amount of coolant liquid filled in the reservoir tank in this case is adjusted to ensure that the accumulated layer of air  25   a  (see  FIG. 10 ) has a constant capacity in an upper portion of the tank. The accumulated layer of air  25   a , thus ensured, alleviates an increase in the internal pressure of the circulating system by receiving variations in volume of the coolant liquid due to thermal expansion by the accumulated layer of air  25   a  to prevent leakage of the coolant liquid and therefore guarantees the reliability of the apparatus.  
         [0069]     Here, the capacity of the accumulated layer of air  25   a  depends on the area of the wetted perimeter of resin tube  7   a , the total amount of the coolant liquid, and the withstanding pressure of the system, so that the respective factors are correlated to one another. Specifically, the total amount of coolant liquid depends on the length of a circulating conduit of the system; the area of the wetted perimeter of the resin tube depends on the length of the circulating conduit; and the area of the wetted perimeter of the resin tube affects the amount of coolant that is dissipated through volatilization, so that the amount of coolant liquid that fills in the reservoir tank is determined by a trade-off between the foregoing design parameters and a guaranteed period of an electronic device which is equipped with the cooling apparatus. In this event, since the amount of change in volume due to thermal expansion is determined by the amount of coolant liquid that is provided, the volume of the accumulated air that is necessary is determined so that it can accommodated a specified withstanding pressure of the circulating system within the liquid cooling system. However, when the volume of the accumulated air occupies one-half or more of the tank volume, the conduit opening will be disrupted by the accumulated air which moves when the tank&#39;s orientation is changed, even in the configuration in which the opening of the coolant liquid circulating conduit tube is positioned at the center of the tank. From the foregoing, the volume of the accumulated air is preferably one-third or less of the tank volume.  
         [0070]     As illustrated in FIGS.  7  to  9 , in front and at the back of air trapping fault  23   a  in bridge  20   a  of reservoir tank  5   e , cut groove  26   a  is formed to define a space for moving the accumulated air. Specifically, when reservoir tank  5   e  is placed upside down, resulting from a change in the posture of an installed electronic device which is equipped with the cooling apparatus, cut groove  26   a  is defined such that air layer  25   a  passes through the space in cut groove  26   a  when air layer  25   a  moves to a different staying position, thereby avoiding the air from flowing back from air trapping fault  23   a  to coolant circulating conduit  21   a.    
         [0071]      FIGS. 11A  to  11 D,  12 A to  12 D, and  13 A to  13 D illustrate bubble trapping operations of reservoir tank  5   e  of the embodiment described above in accordance with different orientations in which the apparatus containing the reservoir tank is installed.  
         [0072]      FIGS. 11B  to  11 D illustrate how the accumulated layer of air  25   a  in the reservoir tank moves in relation to a rotation of the reservoir tank about the coolant liquid circulating conduit (about an X-axis in  FIG. 11A );  FIGS. 12B  to  12 D illustrate how the accumulated layer of air  25   a  in the reservoir tank moves in relation to a rotation of the reservoir tank about an axis orthogonal to the coolant liquid circulating conduit (about a Y-axis in  FIG. 12A ); and  FIGS. 13B  to  13 D illustrate how the accumulated layer of air  25   a  in the reservoir tank moves in relation to a rotation about a vertical axis (about a Z-axis in  FIG. 13A ).  FIGS. 11C, 12C ,  13 C each illustrate a state that is rotated by 90° from the state illustrated in  FIGS. 11B, 12B ,  13 B, respectively, and  FIGS. 11D, 12D ,  13 D each illustrate a state that is rotated by 90° from the state illustrated in  FIGS. 11C, 12C ,  13 C, respectively. These figures also illustrate how minute bubbles  43  within the circulating conduit are trapped by air trapping fault  23   a  and combined with the accumulated layer of air  25   a.    
         [0073]     In this way, by providing fault  23   a  of coolant liquid circulating conduit  21  at the center of the reservoir tank, the accumulated layer of air  25   a  will not interfere with the position of the conduit opening (fault for trapping air) as a result of any change in the tank&#39;s orientation, so that the coolant liquid can be stably pumped out without sending air into the circulating conduit. Also, bubbles  43  produced during the operation can be effectively removed from the conduit through fault  23   a  which is used for trapping air. Further, since cut groove  26   a  is defined to facilitate movements of the air layer in the event of a change in orientation, through the space of cut groove  26   a  formed in bridge  20   a  within the tank body, bubbles will barely flow back toward the conduit opening.  
         [0074]     With the provision of the reservoir tank configured as described above, the resulting liquid cooling system can operate in any orientation in which the apparatus that contains the tank is installed.  
       Second Embodiment  
       [0075]     FIGS.  14  to  18  illustrate the structure of a reservoir tank that is contained within a cooling apparatus according to a second embodiment of the present invention.  FIG. 14  generally illustrates the reservoir tank;  FIG. 15  illustrates the internal structure of the reservoir tank;  FIG. 16  is a vertically sectioned, exploded perspective view of the reservoir tank;  FIG. 17  is a vertically sectioned perspective view generally illustrating the reservoir tank; and  FIG. 18  is a diagram for describing the operation of the reservoir tank.  
         [0076]     Reservoir tank  5   f  of the second embodiment illustrated in FIGS.  14  to  18  further facilitates the movement of the accumulated air through the gap defined by the cut groove formed in the bridge within the reservoir tank disclosed in the first embodiment, and is characterized by a pair of axially symmetric members which are joined to form the reservoir tank.  
         [0077]     Specifically, as illustrated in FIGS.  15  to  17 , reservoir tank  5   f  comprises a pair, of tank body units  18   b ,  18   c  which have bridge  20   b  including coolant liquid circulating conduit  21   b ,  21   c  on a center axis thereof, and are axially symmetric to each other about the position of air trapping fault  23   b . Tank body units  18   b ,  18   c  are vertically fitted to each other to complete reservoir tank  5   f . Cut grooves  26   b ,  26   c  formed in bridge  20   b  are also axially and symmetrically arranged when tank body units  18   b ,  18   c  are fitted. Thus, as illustrated in  FIG. 18 , gaps  27   a ,  27   b , through which the accumulated air moves in association with a change in the orientation in which an apparatus that holds the reservoir tank is installed, are vertically and symmetrically arranged as well, so that the accumulated layer of air  25   b  within the tank can stably move in whichever orientation at which the apparatus that contains the reservoir tank is installed.  
       Third Embodiment  
       [0078]     FIGS.  19  to  23  illustrate the configuration of a reservoir tank that is contained within a cooling apparatus according to a third embodiment of the present invention.  FIG. 19  generally illustrates the reservoir tank;  FIG. 20  illustrates the internal structure of the reservoir tank;  FIG. 21  is a horizontally sectioned top plan view of the reservoir tank;  FIG. 22  is a vertically sectioned, exploded perspective view of the reservoir tank; and  FIG. 23  is a vertically sectioned perspective view generally illustrating the reservoir tank.  
         [0079]     Cooling apparatus  15   c  of the third embodiment illustrated in FIGS.  19  to  23  makes the circulating system compact by positioning the coolant liquid inlet port and the coolant liquid outlet port of the reservoir tank in the first embodiment on the same side, with the intention of facilitating the mounting of cooling apparatus  15   c  to an electronic device.  
         [0080]     Specifically, in FIGS.  19  to  23 , cooling apparatus  15   c  of this embodiment comprises radiator  3   d , radiation fan  16   c , circulating pump  4   e , reservoir tank  5   g , and heat receiving element  2   e  such as a cold plate. As illustrated in  FIG. 19 , reservoir tank  5   g , which has an inlet port and an outlet port, connected to resin tube  7   d  and positioned to face in the same direction, keeps a coolant liquid, retains an air layer for buffering the thermal expansion of the coolant liquid caused by received heat, and traps bubbles produced in the circulating system.  
         [0081]     As illustrated in  FIG. 20 , reservoir tank  5   g  comprises a pair of tank body units  18   d ,  18   e  arranged in mirror symmetry, and tube joints  19   c ,  19   d  which are coupled to tank body units  18   d ,  18   e , respectively. As illustrated in  FIG. 21 , tank body units  18   d ,  18   e  contain U-shaped bridge  20   c  integrally formed therewith, and conduit grooves  28  are formed on joining surfaces of U-shaped bridge  20   c , such that conduit grooves  28  are formed into a U-shaped coolant liquid circulating conduit when a pair of upper and lower body tank units  18   d ,  18   e  are joined to each other.  
         [0082]     Also, air trapping fault  23   c  is formed at the bent portion of U-shaped bridge  20   c  at which narrow gap  22   b  divides conduit groove  28 , as can be seen in  FIGS. 21 and 22 . Further, restriction  29  having a width smaller than that of conduit groove  28  is formed at a location of conduit groove  28  open to air trapping fault  23   c , as can be seen in  FIGS. 21 and 23 .  
         [0083]      FIGS. 24A-24D ,  25 A- 25 D,  26 A- 26 D illustrate bubble trapping operations of reservoir tank  5   g  of the embodiment described above in accordance with different orientations in which the apparatus containing the reservoir tank is installed.  
         [0084]      FIGS. 24B  to  24 D illustrate how the accumulated layer of air  25   c  in the reservoir tank moves in relation to a rotation of the reservoir tank about the coolant liquid circulating conduit (about an X-axis in  FIG. 24A );  FIGS. 25B  to  25 D illustrate how the accumulated layer of air  25   c  in the reservoir tank moves in relation to a rotation of the reservoir tank about an axis orthogonal to the coolant liquid circulating conduit (about a Y-axis in  FIG. 25A ); and  FIGS. 26B  to  26 D illustrate how the accumulated layer of air  25   c  in the reservoir tank moves in relation to a rotation about a vertical axis (about a Z-axis in  FIG. 26A ).  FIGS. 24C, 25C ,  26 C each illustrate that is a state rotated by 90° from the state illustrated in  FIGS. 24B, 25B ,  26 B, respectively,  FIGS. 24D, 25D  each illustrate a state that is further rotated by 90° from the state illustrated in  FIGS. 24C, 25C , respectively, and  FIG. 26D  illustrates a state that is further rotated by 180° from the state illustrated in  FIG. 26C . These figures also illustrate how minute bubbles  43  within the circulating conduit are trapped by air trapping fault  23   c  and combined with the accumulated layer of air  25   c.    
         [0085]     By thus providing the U-shaped coolant liquid circulating conduit within the reservoir tank, the inlet port and outlet port of the coolant liquid are oriented in the same direction, so that the liquid cooling system can be connected in a compact manner, thus facilitating mounting to an electronic device.  
         [0086]     Further, by defining air trapping fault  23   c  at a position that changes the flow direction in the U-shaped coolant liquid circulating conduit, a conduit opening is arranged at a middle position of the tank. Thus, like the first embodiment, since the accumulated layer of air  25   c  does not interfere with the conduit opening in whichever direction the orientation is changed, the coolant liquid can be stably pumped out with stability. Also, with restriction  29  formed at the opening of U-shaped conduit groove  28 , bubbles are unlikely to flow back into the conduit when air remaining in the tank moves in response to a change in the orientation.  
       Fourth Embodiment  
       [0087]     FIGS.  27  to  30  illustrate the configuration of a reservoir tank that is contained within a cooling apparatus according to a fourth embodiment of the present invention.  FIG. 27  illustrates the internal structure of the reservoir tank;  FIG. 28  is a vertically sectioned perspective view of the reservoir tank;  FIGS. 29A  to  29 C illustrate in detail a conduit tube within the reservoir tank; and  FIG. 30  is a diagram for describing the operation of the reservoir tank.  
         [0088]     Reservoir tank  5   h  of the fourth embodiment illustrated in FIGS.  27  to  30  is used to improve the productivity of the reservoir tank illustrated in the first embodiment. Specifically, the bridge and coolant liquid circulating conduit in the first embodiment are made of a single circular tube which is separated from the tank body units.  
         [0089]     As illustrated in  FIG. 27 , reservoir tank  5   h  of this embodiment comprises tank body unit  18   f ; single coolant liquid circulating conduit tube  30  extending through tank body unit  18   f  along the center axis thereof; and tank cover  17   b  joined to tank body unit  18   f . As illustrated in  FIGS. 29A  to  29 C, cross-shaped air trapping through-hole  34   a  is formed through coolant liquid circulating conduit tube  30  at a middle position of the tank in a form perpendicular to the conduit. Here, air trapping through-hole  34   a  extends vertically and horizontally through coolant liquid circulating conduit tube  30  because the reservoir tank can generally support the any orientation at which the apparatus, which holds the tank, is installed. Based on this design, the conduit opening for trapping bubbles can be specified at the center of the tank, as illustrated in  FIGS. 30A and 30B , so that the accumulated layer of air  25   d  can be prevented from interfering with the conduit in any orientation, and a gap for moving the air layer can be defined in a space outside of cooling liquid circulating conduit tube  30 .  
       Fifth Embodiment  
       [0090]     FIGS.  31  to  33 ,  34 A and  34 B illustrate the structure of a reservoir tank contained within a cooling apparatus according to a fifth embodiment of the present invention.  FIG. 31  illustrates the internal structure of the reservoir tank;  FIG. 32  is a horizontally sectioned perspective view of the reservoir tank;  FIG. 33  is a vertically sectioned perspective view of the reservoir tank; and  FIGS. 34A and 34B  are diagrams for describing the operation of the reservoir tank.  
         [0091]     Reservoir tank  5   i  of the fifth embodiment illustrated in FIGS.  31  to  33 ,  34 A and  34 B is used for improving the workability of the reservoir tank illustrated in the third embodiment. Specifically, the U-shaped bridge and U-shaped coolant liquid circulating conduit in the third embodiment are made of a single bent circular tube which is separated from the tank body units. In particular, in this embodiment, a flexible tube is employed for the circular tubes, as represented by a resin tube.  
         [0092]     As illustrated in  FIG. 31 , a pair of joint tubes  32   a ,  32   b  are arranged in tank body unit  18   h  to handle an inflow coolant liquid and an outflow coolant liquid, respectively, in the same direction. Open ends of two joint tubes  32   a ,  32   b  on the inside of the tank body are connected to resin-made conduit tube  33  curved in a U-shape to constitute a circulating conduit. At a position that changes the flow direction in conduit tube  33  at the center of the tank, air trapping through-hole  34   b , as illustrated in  FIG. 29  of the fourth embodiment, is defined in the horizontal and vertical directions (see  FIGS. 32 and 33 ).  
         [0093]     By using the reservoir tank in the structure as described above, the inlet port and outlet port of the circulating system are oriented in the same direction in the reservoir tank, so that the liquid cooling system can be made compact, thus facilitating the mounting to an electronic device. Further, as illustrated in  FIGS. 34A, 34B , air trapping through-hole  34   b  is defined at a position that changes the flow direction in U-shaped conduit tube  33  to provide a conduit opening at the center of the tank, and the space outside conduit tube  33  is used as a gap for moving the accumulated layer of air  25   e , thereby making it possible to provide a reservoir tank which can have any orientation. Further, by using a flexible tube for the U-shaped conduit tube, the resulting reservoir tank provides the ease of work and assembly.  
       Sixth Embodiment  
       [0094]      FIG. 35A  is a diagram generally illustrating the structure of a reservoir tank that is included within a cooling apparatus according to a sixth embodiment of the present invention, and  FIG. 35B  is a partially enlarged view of  FIG. 35A .  FIG. 36  is a plan view illustrating the cooling apparatus of this embodiment, and  FIG. 37  is a diagram for describing the operation of the reservoir tank.  FIGS. 38A  to  38 C are side views illustrating a variety of exemplary structures for the reservoir tank.  
         [0095]     Cooling apparatus  15   d  of this embodiment illustrated in  FIGS. 35A and 35B  comprises radiator  35   a , radiation fan  16   d , resin tube  7   e , circulating pump  4   f , a reservoir tank, and heat receiving element  2   f  such as a cold plate. Radiator  35   a  employs a thin structure as formed by joining two thin plates each formed with a conduit groove for circulating a coolant liquid. The reservoir tank is mounted on radiator  35   a  together with circulating pump  4   f  and radiation fan  16   d  to make up a liquid cooling module.  
         [0096]     The cooling apparatus in this embodiment is characterized by comprising the reservoir tank mounted on the thin radiator which supports any orientation at which the apparatus, which holds the cooling system, is installed. Specifically, the reservoir tank is assembled by joining tank cover  17   c  to radiator  35   a . Radiator conduit  36   a , which extends to the tank cover joint of radiator  35   a , is narrowed down in width at the middle position of the reservoir tank, and is divided by narrow gap  22   c , as illustrated in  FIG. 35B . On each of left and right radiator surfaces that are opposite to each other across opening  38  formed by the break, recessed cavity  37   a  is defined within the range in which it is covered by tank cover  17   c  (see  FIG. 36 ). In opening  38  of radiator conduit  36   a , pivot-shaped protrusion  39   a  is formed, as illustrated in  FIG. 35B .  
         [0097]     Next, the operation of the reservoir tank according to this embodiment will be described with reference to FIGS.  36  to  38 A.  
         [0098]      FIG. 36  is a plan view illustrating the cooling apparatus in this embodiment;  FIG. 37  illustrates the air trapping operation on a cross-section taken along line B-B in  FIG. 36 .  FIG. 37A  assumes that the cooling apparatus is installed on a floor, while  FIG. 37B  assumes that the cooling apparatus is installed upside down.  FIG. 38A  illustrates the structure of the reservoir tank viewed on a cross-section taken along line C-C in  FIG. 36 .  
         [0099]     In this embodiment, recessed cavities  37   a  are formed by being cut down in the thickness direction of radiator  35   a . In this way, when tank cover  17   c  is coupled to radiator  35   a , the stored coolant liquid and an air retention space can be provided at both the top and the bottom within tank cover  17   c . Then, an air trapping fault is defined in a radiator conduit opening formed by narrow gap  22   c  at the center of the tank to trap bubbles. In this event, even if the orientation is reversed (see  FIG. 37B ), each recessed cavity  37   a  formed on the radiator surface retains the accumulated layer of air  25   f  on the ceiling side, so that the cooling apparatus can support the reversed installation of the cooling apparatus. Further, as illustrated in  FIG. 37B , pivot-shaped protrusion  39   a , formed in radiator conduit opening  38  disturbs the circulating flow to agitate bubbles, thus facilitating the introduction of bubbles into recessed cavities  37   a  defined to the left and right of conduit opening  38 .  
         [0100]     Now,  FIGS. 38B and 38C  illustrate other exemplary structures for the reservoir tank of this embodiment.  
         [0101]     In the reservoir tank illustrated in  FIG. 38B , radiator  35   a  is pressed to form recessed cavities  37   a  and protrusion  39   a  instead of the recessed cavities formed by cutting out a portion of the upper surface of radiator  35   a , as illustrated in  FIG. 38A . This exemplary structure is suited when a radiator is made of thin plates.  
         [0102]     In the reservoir tank illustrated in  FIG. 38C , through-hole  40  is formed through radiator  35   a , and separate radiator cover  44  is joined from an opposite side of tank cover  17   c  to form recessed cavities  37   a . This exemplary structure may be applied to the exemplary structure of  FIG. 38B  when there is not enough volume to form the recessed cavities by pressing.  
         [0103]     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.