Abstract:
As a cooling structure, a cooling device has a configuration that comprises a mounting surface for a member to be cooled, a cooling fin provided on a rear surface side of the mounting surface, and a cooling passage for cooling the cooling fin with a liquid refrigerant, wherein the liquid refrigerant is caused to flow from a flow inlet to the cooling passage and is discharged from a discharge outlet. The cooling passage comprises a first surface provided with the cooling fin, a second surface opposing the first surface, and a wall portion provided between the first surface and the second surface and is configured into a channel longer than a channel connecting the flow inlet and the discharge outlet in a straight line. The wall portion has a first wall portion extending from the second surface to the first surface, and a second wall portion extending in parallel with the first wall portion from the first surface to a height exceeding a top portion of the first wall.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a cooling structure for cooling a target object with a liquid refrigerant. 
       BACKGROUND ART 
       [0002]    Electric devices such as inverters (power converters) for use on electric automobiles, fuel cell automobiles, etc., for example, are made up of various electric components (active components and passive components) such as switching elements, reactors, capacitors, etc. Since such electric components generate a considerable amount of heat when energized, the components need to be cooled sufficiently. 
         [0003]    Japanese Laid-Open Patent Publication No. 2007-207917 (hereinafter referred to as “JP2007-207917A”) discloses a structure for cooling an inverter with a liquid refrigerant that flows through a cooling passage having cooling fins, the inverter being disposed on an upper surface of the cooling passage. 
       SUMMARY OF INVENTION 
       [0004]    Certain types of cooling passages have a structure that extends in a straight direction from an inlet port to an outlet port for a liquid refrigerant, whereas other types of cooling passages have a meandering or bent structure depending on the layout and number of parts of an electric device that serves as the target object to be cooled. Cooling passages having such a meandering or bent structure provide a roundabout channel between the inlet port and the outlet port. More specifically, according to a structure in which a case, including a wall that is higher than the cooling fins, is covered with a cap to define a cooling passage, thereby providing a roundabout channel (see, for example, JP2007-207917A), a liquid refrigerant, which is introduced from an inlet port, flows directly from a gap between the top surface of the wall and the inner surface of the cap into the cooling passage near an outlet port. In this case, a potential exists for the cooling passage not to be supplied with a sufficient amount of liquid refrigerant. 
         [0005]      FIG. 7  of the accompanying drawings shows, by way of example, a cooling device  100  with a roundabout channel defined by a wall disposed in a cooling passage. The cooling device  100  shown in  FIG. 7  includes an outward passageway  102   a  through which a liquid refrigerant flows immediately after the liquid refrigerant has been introduced from an inlet port, and a homeward passageway  102   b  through which the liquid refrigerant flows after the liquid refrigerant has returned from a non-illustrated turnaround section. The passageways  102   a,    102   b  are mutually separated and lie parallel to each other, one on each side of a wall  106  disposed in a casing  104 . The passageways  102   a,    102   b  house cooling fins  110  therein for cooling an electric component  108  that is placed on the casing  104 . The cooling fins  110  extend along directions in which the liquid refrigerant flows, i.e., in directions perpendicular to the sheet of  FIG. 7 . The wall  106  includes a top surface  106   a,  which is held in contact with an inner surface of a casing cap  112  mounted on the casing  104 , thereby separating the passageways  102   a,    102   b  from each other. Sealing members  114  are provided, which form a seal between the casing  104  and the casing cap  112 . The casing  104  and the casing cap  112  are fastened to each other by bolts  116 . 
         [0006]    With the cooling device  100  constructed in the foregoing manner, if due to poor assembling and machining accuracy, the liquid refrigerant leaks from a gap G that is created between the top surface  106   a  of the wall  106  and the inner surface of the casing cap  112 , then as indicated by the arrow F 0  in  FIG. 7 , the liquid refrigerant flows directly from the passageway  102   a  that leads from the inlet port into the passageway  102   b  that leads to the outlet port. Therefore, the liquid refrigerant does not flow through the passageways  102   a,    102   b  sufficiently. In particular, when the casing cap  112  is bent under pressure from the liquid refrigerant, as shown in  FIG. 7 , the gap G widens and leakage of the refrigerant increases. As a result, the amount of liquid refrigerant that flows through the passageways  102   a,    102   b  is further reduced, thus making it difficult for the cooling device  100  to cool the target object to be cooled sufficiently. 
         [0007]    The present invention has been made in view of the aforementioned problems. An object of the present invention is to provide a cooling structure, which is capable of causing a liquid refrigerant to flow smoothly along a cooling passage, for thereby effectively cooling a target object. 
         [0008]    The cooling structure according to the present invention has an installation surface for placement of a target object to be cooled, a cooling fin disposed on a reverse side of the installation surface, and a cooling passage for cooling the cooling fin with a liquid refrigerant that flows from an inlet port into the cooling passage and is discharged from an outlet port. The cooling passage has a first surface on which the cooling fin is disposed, a second surface confronting the first surface, and a wall assembly disposed between the first surface and the second surface, the cooling passage being constructed as a channel having a length that is longer than a straight line interconnecting the inlet port and the outlet port. The wall assembly has a first wall that extends from the second surface toward the first surface, and a second wall that extends in parallel with the first wall from the first surface to a height beyond a top of the first wall. 
         [0009]    With the above arrangement, the wall assembly is disposed between the first surface, which has the cooling fins disposed thereon, and the second surface. The wall assembly has the first wall that extends from the second surface toward the first surface, and the second wall that extends in parallel with the first wall from the first surface to a height beyond a top of the first wall. Therefore, the wall assembly effectively prevents liquid refrigerant from flowing across and over the wall assembly by way of a short cut across a portion of the cooling passage from the inlet port to the outlet port as a result of a reduction in gap accuracy in the vicinity of the wall assembly, which may be caused due to poor assembling accuracy and machining accuracy of the cooling structure and the casing, or due to deformations caused by the pressure of the liquid refrigerant. Consequently, the liquid refrigerant flows smoothly through the cooling passage and cools the target object appropriately. 
         [0010]    If the first wall and the second wall lie parallel to the cooling fin, then the first wall and the second wall impose an effectively higher flow resistance at the wall surfaces of the wall assembly. Thus, liquid refrigerant is prevented from flowing across and over the wall assembly more reliably. 
         [0011]    The cooling structure may further include second cooling fins disposed on the second surface and positioned in the cooling passage in juxtaposed relation to the cooling fin, and a second installation surface disposed on a reverse side of the second surface for placement of a second target object to be cooled by the second cooling fins. The cooling structure, which is constructed in this manner, is capable of cooling different target objects more efficiently on upper and lower surfaces disposed on both sides of the cooling passage. The second cooling fins, which are disposed in confronting relation to the aforementioned cooling fins, are highly effective at preventing liquid refrigerant from flowing across and over the wall assembly. 
         [0012]    The second wall may have a recess, and the first wall may be inserted in the recess. This arrangement provides a complex and elongate labyrinth structure in the wall assembly, thereby preventing the liquid refrigerant from flowing across and over the wall assembly more reliably. 
         [0013]    The cooling passage may contain the liquid refrigerant therein, which flows in different directions respectively on one surface side and another surface side of the wall assembly. 
         [0014]    The cooling passage may have a turnaround section on a way from the inlet port to the outlet port, and the wall assembly may comprise a dividing wall assembly that divides the cooling passage, which is folded back by the turnaround section, into an outward passageway and a homeward passageway. The cooling passage is thus divided into two passageways by a simple structure. 
         [0015]    According to the present invention, the wall assembly is disposed between the second surface and the first surface having the cooling fins disposed thereon. The wall assembly further has the first wall, which extends from the second surface toward the first surface, and the second wall, which extends in parallel with the first wall from the first surface to a height beyond the top of the first wall. Therefore, the wall assembly is effective at preventing liquid refrigerant from flowing across and over the wall assembly, by way of a short cut across a portion of the cooling passage from the inlet port to the outlet port as a result of a reduction in gap accuracy in the vicinity of the wall assembly, which may be caused due to poor assembling accuracy and machining accuracy of the cooling structure and the casing, or due to deformations caused under the pressure of the liquid refrigerant. Consequently, the liquid refrigerant flows smoothly through the cooling passage and cools the target object appropriately. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a sectional plan view of a cooling device, which serves as a cooling structure according to an embodiment of the present invention; 
           [0017]      FIG. 2A  is a cross-sectional view taken along line II-II of  FIG. 1 ; 
           [0018]      FIG. 2B  is an exploded cross-sectional view of the cooling device shown in  FIG. 2A , illustrating a heat sink body and a heat sink cap detached from the heat sink body; 
           [0019]      FIG. 3  is an enlarged fragmentary cross-sectional view of a wall disposed in a cooling passage together with other nearby parts; 
           [0020]      FIG. 4A  is a fragmentary cross-sectional view showing a sealing member according to a first modification, which is disposed between the heat sink body and the heat sink cap; 
           [0021]      FIG. 4B  is a fragmentary cross-sectional view showing a sealing member according to a second modification, which is disposed between the heat sink body and the heat sink cap; 
           [0022]      FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 1 , showing second cooling fins disposed on the heat sink cap; 
           [0023]      FIG. 6A  is a fragmentary cross-sectional view showing a wall assembly according to another modification; 
           [0024]      FIG. 6B  is a fragmentary exploded cross-sectional view of the wall assembly shown in  FIG. 6A , with the heat sink cap being detached from the heat sink body; and 
           [0025]      FIG. 7  is a cross-sectional view of a cooling device including a roundabout channel defined by a wall disposed in a cooling passage. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    Cooling structures according to preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
         [0027]      FIG. 1  shows in sectional plan view a cooling device  10 , which serves as a cooling structure according to an embodiment of the present invention.  FIG. 2A  is a cross-sectional view taken along line II-II of  FIG. 1 , and  FIG. 2B  is an exploded cross-sectional view of the cooling device  10  shown in  FIG. 2A , illustrating a heat sink body  12  and a heat sink cap  14  detached from the heat sink body  12 . 
         [0028]    As shown in  FIGS. 1 ,  2 A and  2 B, the cooling device  10  serves as a cooling structure for cooling a target object  20 , which is mounted on an installation surface (mount surface)  18  of a heat sink body  12 , by passing a liquid refrigerant through a cooling passage (refrigerant channel)  16 , which is defined in the heat sink body  12  and is closed by a heat sink cap  14 .  FIG. 1  is a cross-sectional view of the cooling device  10  taken along a plane parallel to the directions indicated by the arrows F 1  and F 2  in  FIG. 1 , along which the liquid refrigerant flows through the cooling passage  16 . 
         [0029]    The target object  20  to be cooled by the cooling device  10  may be an electric component such as an inverter (power converter) for use on electric automobiles, hybrid electric automobiles, fuel cell automobiles, etc., for example. Such an electric component is made up of various electric sub-components, including switching elements, reactors, capacitors, etc. The electric component generates considerable heat when energized. 
         [0030]    As shown in  FIGS. 1 ,  2 A and  2 B, the cooling device  10  includes the heat sink body (casing)  12  in the form of a flat box having the cooling passage  16  defined therein, and the heat sink cap  14 , which is intimately secured to a surface of the heat sink body  12  where the cooling passage  16  opens, thereby delimiting the cooling passage  16 . The heat sink body  12  and the heat sink cap  14  are fastened to each other by bolts  21 , with a sealing member  19  being sandwiched therebetween around the cooling passage  16  for sealing the cooling passage  16  in a liquid-tight fashion. 
         [0031]    As shown in  FIGS. 2A and 2B , the sealing member  19  comprises a liquid packing, and is disposed along an outer circumferential surface of the heat sink body  12 . However, as shown in  FIG. 4A , the sealing member  19  may alternatively comprise a sealing member  19   a  (see  FIG. 4A ) in the form of an O-ring. As shown in  FIG. 4B , the sealing member  19  may further alternatively comprise a sealing member  19   b  in the form of a liquid packing or a thin sheet packing, which is interposed between a stepped surface of the heat sink body  12  and an inner surface of the heat sink cap  14 . 
         [0032]    The installation surface  18  on which the target object  20  is mounted is provided on a surface (outer surface) of the heat sink body  12 , which is remote from the cooling passage  16 . The heat sink cap  14  has a surface (outer surface) that is remote from the cooling passage  16 , and which serves as a second installation surface (second mount surface)  24  for mounting a second target object  22  thereon as another target object to be cooled. The target object  20  and the second target object  22  may be installed directly on the installation surface  18  and the second installation surface  24 , respectively, although a heat radiating sheet made of copper, aluminum, or the like, or a liquid-phase heat radiating paste may also be interposed therebetween. 
         [0033]    As shown in  FIG. 1 , a liquid refrigerant circulates through the cooling passage  16  via an inlet port  26  and an outlet port  28  that are defined in the heat sink body  12 , thereby effectively cooling the target object  20  and the second target object  22 , which are installed directly on the installation surface  18  and the second installation surface  24 , respectively. A circulating pump, a radiator, etc., not shown, for example, are connected via piping to the inlet port  26  and the outlet port  28 . When the circulating pump is actuated, the liquid refrigerant is circulated through the cooling passage  16 . Heat, which is received by the liquid refrigerant from the target object  20  and the second target object  22 , is radiated out of the cooling device  10  by the radiator. 
         [0034]    The cooling passage  16  includes a straight first passageway (outward passageway)  16   a  through which a liquid refrigerant, such as water, a coolant, or the like introduced from the inlet port  26  initially flows, a turnaround section  16   b  for reversing the direction of the liquid refrigerant that has passed through the first passageway  16   a,  and a second passageway (homeward passageway)  16   c  through which the liquid refrigerant flows after having passed through the turnaround section  16   b.  The first passageway  16   a  and the second passageway  16   c  are separated from each other by a wall assembly  30 . The liquid refrigerant, which has passed through the second passageway  16   c,  is discharged out of the cooling device  10  from the outlet port  28 . After the heat from the discharged liquid refrigerant is radiated by the radiator, the liquid refrigerant is reintroduced into the inlet port  26 . 
         [0035]    As can be seen from  FIG. 1 , the cooling passage  16  is constructed as a roundabout channel, which is longer than a straight line interconnecting the inlet port  26  and the outlet port  28 . The roundabout channel is made up of the first passageway  16   a,  the turnaround section  16   b,  and the second passageway  16   c,  as a roundabout channel between the inlet port  26  and the outlet port  28 . The first passageway  16   a  and the second passageway  16   c  are separated from each other by the wall surfaces of the wall assembly  30 . Such a structure makes the overall dimensions of the cooling device  10  as small as possible, while making it possible to provide a desired channel length. 
         [0036]    As shown in  FIGS. 1 ,  2 A and  2 B, the cooling passage  16  houses therein a plurality of cooling fins  32  in the form of thin plates extending along the direction in which the liquid refrigerant flows. The cooling fins  32  project from a first surface  12   a,  which is the inner surface of the heat sink body  12 , and terminate short of a second surface  14   a,  which is the inner surface of the heat sink cap  14  that confronts the first surface  12   a  (see  FIG. 2A ). 
         [0037]    The cooling fins  32 , which are made of a material that exhibits high thermal conductivity, e.g., copper, aluminum, or the like, are capable of transmitting heat from the target object  20  highly efficiently to the liquid refrigerant. The heat sink body  12  and the heat sink cap  14  may be made of the same material as the cooling fins  32 . The heat sink body  12  and the cooling fins  32  have an integral structure produced by aluminum die casting. 
         [0038]    According to the present embodiment, each of the first passageway  16   a  and the second passageway  16   c,  which constitute straight sections of the cooling passage  16 , accommodates therein three sets of four straight cooling fins  32 , which extend along the direction in which the liquid refrigerant flows. As viewed in plan in  FIG. 1 , the four straight cooling fins  32  lie parallel to each other in widthwise directions of the cooling passage  16 , i.e., in directions perpendicular to the direction in which the liquid refrigerant flows. Each of respective sections formed upstream and downstream of the first passageway  16   a,  i.e., curved sections of the cooling passage  16 , houses therein a set of three curved cooling fins  32 , which line parallel to each other in the widthwise direction of the cooling passage  16 . Each of respective sections formed upstream and downstream of the second passageway  16   c  houses therein a similar set of three curved cooling fins  32 . The number of installed cooling fins  32  and the intervals at which the cooling fins  32  are installed can be changed appropriately depending on the specifications and application of the cooling device  10  and the type of target object  20 . 
         [0039]    As shown in  FIG. 3 , the wall assembly  30 , by which the first passageway  16   a  and the second passageway  16   c  are separated from each other, has a first wall  34  that extends from the second surface  14   a  toward the first surface  12   a,  and second walls  36  that extend from the first surface  12   a  in parallel with the first wall  34 , to a height beyond the top (top surface)  34   a  of the first wall  34 . The height h 2  of the second walls  36 , which project from the first surface  12   a,  is greater than the distance h 1  from the top  34   a  of the first wall  34  to the first surface  12   a  (h 1 &lt;h 2 ), so that the tops of the first wall  34  and the second walls  36  are staggered and positioned adjacent to each other in overlapping relation. 
         [0040]    According to the present embodiment, the second walls  36  are provided as a pair in sandwiching relation to the first wall  34 . The wall assembly  30  thus provides a labyrinth structure, with the first wall  34  being inserted in a recess  36   a  defined between the pair of second walls  36 . 
         [0041]    As shown in  FIG. 1 , the wall assembly  30  extends parallel to the flow directions of the refrigerant in the first passageway  16   a  and the second passageway  16   c,  from a position corresponding to the inlet port  26  and the outlet port  28  to an inner circumferential surface of the turnaround section  16   b.  The first passageway  16   a  is defined on one surface side of the wall assembly  30 , and the second passageway  16   c  is defined on the other surface side of the wall assembly  30 . 
         [0042]    Operations and advantages of the cooling device  10  which is constructed in the foregoing manner will be described below. 
         [0043]    If the cooling device  10  is used to cool an inverter mounted on an electric automobile, for example, the switching elements thereof, for example, are installed as a target object  20  on the installation surface  18 , whereas the reactor, for example, is installed as the second target object  22  on the second installation surface  24 . Under the control of a controller such as an ECU or the like of the electric automobile, liquid refrigerant is circulated from the inlet port  26  to the outlet port  28  by the circulating pump and the radiator. Therefore, the target object  20  is cooled by means of heat exchange between the liquid refrigerant that flows through the cooling passage  16  and the cooling fins  32  and the first surface  12   a.  Similarly, the second target object  22  is cooled by means of heat exchange between the liquid refrigerant and the second surface  14   a.    
         [0044]    With the cooling device  10  according to the present embodiment, the cooling passage  16  is constructed as a channel, which is longer than a straight line interconnecting the inlet port  26  and the outlet port  28 . The wall assembly  30  is disposed between the first surface  12   a  of the heat sink body  12  and the second surface  14   a  of the heat sink cap  14 , thereby dividing the cooling passage  16  into the first passageway  16   a  and the second passageway  16   c.  The wall assembly  30  includes the first wall  34  that extends from the second surface  14   a  toward the first surface  12   a,  and the second walls  36  that extend from the first surface  12   a  in parallel with the first wall  34  to a height beyond the top  34   a  of the first wall  34 . The first wall  34  and the second walls  36  have confronting wall surfaces lying parallel to each other. The expression that the first wall  34  and the second walls  36  lie parallel to each other covers not only the wall surfaces thereof lying completely in parallel with each other, but also that the wall surfaces lie substantially parallel to each other while being slightly inclined, such that the wall surfaces are spaced from each other by distances with slight clearances, and that the wall surfaces are held in contact with each other due to face-to-face contact therebetween, insofar as the first wall  34  and the second walls  36  are juxtaposed to such an extent that the first wall  34  and the second walls  36  are effective to prevent liquid refrigerant from flowing easily from the first passageway  16   a  over the wall assembly  30  and into the second passageway  16   c.    
         [0045]    Therefore, even if a flow F 3  (see  FIGS. 1 and 3 ) is generated in the liquid refrigerant from the first passageway  16   a  into the second passageway  16   c  in a direction directly from the inlet port  26  toward the outlet port  28 , under the pressure of the liquid refrigerant that flows in from the inlet port  26  and the pressure of the liquid refrigerant that flows through the cooling passage  16 , the labyrinth structure, in which the first wall  34  and the second walls  36  are disposed in overlapping relation to each other, enables the wall assembly  30  to impose a very high flow resistance, thereby preventing the flow F 3  from flowing by way of a short cut across and over the wall surfaces of the wall assembly  30 . 
         [0046]    More specifically, since the second walls  36  extend to a height beyond the top  34   a  of the first wall  34 , the wall assembly  30  reliably maintains a flow blocking function (refrigerant leakage blocking function), even if the gap accuracy in the vicinity of the wall assembly  30  is low as a result of poor assembly and machining accuracy of the heat sink body  12  and the heat sink cap  14 , and due to curvature of the heat sink cap  14  caused by the pressure of the liquid refrigerant. Consequently, the liquid refrigerant flows smoothly through the cooling passage  16 , thereby appropriately cooling the target object  20  and the second target object  22 . Further, since the cooling device  10  has a complex and elongate labyrinth structure in which the first wall  34  is inserted in the recess  36   a  defined between the pair of second walls  36 , the flow blocking function is rendered more effective. 
         [0047]    As described above, the cooling device  10  includes the wall assembly  30  disposed in the cooling passage  16  to prevent the liquid refrigerant from flowing directly from the first passageway  16   a  into the second passageway  16   c.  Therefore, the sealing member  19  ( 19   a,    19   b) , which is interposed between the heat sink body  12  and the heat sink cap  14 , may be disposed only in one position (e.g., the position indicated by the two-dot-and-dash lines S shown in  FIG. 1 ) along the outer circumferential surface of the heat sink body  12  (cooling passage  16 ). Stated otherwise, since there is no need for a sealing member to be disposed in a position corresponding to the wall assembly  30  that separates the first passageway  16   a  and the second passageway  16   c  from each other, the cooling device  10  is simple in structure, and can be manufactured with increased efficiency at a reduced cost. 
         [0048]    In the above embodiment, the cooling fins  32  are mounted only on the first surface  12   a,  which is on the opposite side of the installation surface  18  on which the target object  20  is installed. However, second cooling fins  38  (see the two-dot-and-dash lines in  FIGS. 2A and 2B ) may be mounted on the second surface  14   a,  which is on the opposite side from the second installation surface  24  on which the second target object  22  is installed. The second cooling fins  38  may also be positioned in gaps formed between adjacent ones of the cooling fins  32 . 
         [0049]    The second cooling fins  38 , which are included in this manner, are effective to increase the cooling performance on the second target object  22 . Furthermore, the second cooling fins  38  increase the rigidity of the heat sink cap  14 , thereby preventing the heat sink cap  14  from becoming curved under the pressure of the liquid refrigerant, and preventing the liquid refrigerant from leaking across and over the wall assembly  30  more reliably. Since the heat sink cap  14  can be reduced in thickness while maintaining a sufficient level of rigidity, the cooling device  10  can be reduced in size and weight. Further, only the second cooling fins  38  may be provided on the heat sink cap  14 , whereas the cooling fins  32  on the heat sink body  12  may be dispensed with. One or three or more target objects may be cooled by the cooling device  10 . 
         [0050]      FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 1 , showing the second cooling fins  38  disposed on the heat sink cap  14 , in a curved section located downstream from the second passageway  16   c.  As shown in  FIG. 5 , the second cooling fins  38 , which are disposed on the heat sink cap  14 , may be positioned in curved sections of the cooling passage  16 . If the second cooling fins  38  are disposed on the heat sink cap  14  in curved sections of the cooling passage  16 , in and particular, in curved sections disposed near the inlet port  26  and the outlet port  28 , then the second cooling fins  38  overlap alternately with the cooling fins  32 , thereby producing a wall surface effect that minimizes seepage of liquid refrigerant onto the contacting surfaces of the heat sink body  12  and the heat sink cap  14 . Therefore, the flow blocking function of the wall assembly  30  is rendered more effective. 
         [0051]    In the above embodiment, the wall assembly  30  that is disposed in the cooling passage  16  includes the first wall  34  and the second walls  36 , which extend vertically between the first surface  12   a  and the second surface  14   a  in parallel with the cooling fins  32 . However, the wall assembly may be of a structure other than that shown in  FIG. 3 , insofar as the structure is capable of preventing liquid refrigerant from flowing directly by way of a short cut from the first passageway  16   a  into the second passageway  16   c.    
         [0052]    For example, as shown in  FIGS. 6A and 6B , the wall assembly  30  may be replaced with a wall assembly  44  having a tapered first wall  40 , and second walls  42 , which are disposed in overlapping relation to the first wall  40  and have wall surfaces that lie parallel or substantially parallel to the wall surface of the first wall  40 . The wall assembly  44  also includes a recess  42   a  defined between the second walls  42 , which are provided as a pair, the recess  42   a  becoming progressively wider toward the open end thereof. Therefore, when the heat sink body  12  and the heat sink cap  14  are assembled together, the top  40   a  of the first wall  40  is easily inserted into the recess  42   a.  The wall assembly  44  can thus be assembled with utmost ease. 
         [0053]    The above-described wall assembly  30  has the first wall  34  and the second walls  36 , which lie parallel to the cooling fins  32 . Since the first wall  34  and the second walls  36  extend perpendicularly from the second surface  14   a  and the first surface  12   a,  respectively, the wall assembly  30  imposes a higher flow resistance on the liquid refrigerant than on the wall assembly  44 , and functions more highly to block the flow of the liquid refrigerant. 
         [0054]    A U-shaped cooling passage  16 , which includes the first passageway  16   a  and the second passageway  16   c  with the turnaround section  16   b  being interposed therebetween, has been illustrated above. However, the cooling passage may have another shape, e.g., a V shape, an S shape, or the like. Even if the cooling passage is formed in a V-shape or an S-shape, the cooling passage is effective at preventing liquid refrigerant from flowing directly from the inlet port into the outlet port, with a wall assembly that is similar to the wall assembly  30 . 
         [0055]    The present invention is not limited to the above embodiments, but various arrangements may be adopted therein without departing from the scope of the invention.