Patent Publication Number: US-2011058334-A1

Title: Fan apparatus, electronic device, and fan-attached case

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fan apparatus to be attached to a case for an electronic device. 
     2. Description of the Related Art 
     A variety of fan apparatuses including a plurality of fans have been provided to be attached to cases designed to contain electronic devices. A heating element cooling apparatus 5 disclosed in FIG. 4 of JP-A 2002-368470, for example, includes a heat sink 2 having a plurality of straight fins 1, and two axial fans 4 placed on one side of the straight fins 1. In the heating element cooling apparatus 5, the two fans 4, placed side by side, are arranged to rotate in mutually opposite directions so that whirl flows of air between the two fans 4 may not cancel each other out but combine to cause a strong flow of air. This results in an increased heat transfer coefficient, leading to a reduced temperature of a heating element 3 placed in the vicinity of the heat sink 2. 
     JP-A 2008-140406 discloses another example of a conventional rack-mounted system 10 including fan/LAN tray slots 23. As shown in FIG. 8 of JP-A 2008-251067, each fan/LAN tray slot 23 is provided with eight air blowers. JP-A 2008-251067 discloses a power supply module 40 used for a disk array apparatus. In a fan portion 42 of the power supply module 40, two fans 43 having the same structure are arranged in tandem, one in front of the other, along an axial direction. 
     SUMMARY OF THE INVENTION 
     With improved performance and increased density of electronic devices, cases for such electronic devices are required to have improved ventilation efficiency. Preferred embodiments of the present invention improve efficiency in introducing, sending, or discharging air into, inside, or out of such cases. 
     A fan apparatus according to a preferred embodiment of the present invention includes an array of a plurality of axial fans arranged in at least two rows and at least two columns in parallel or substantially in parallel with a mounting surface provided in a case of an electronic device. Each of the plurality of axial fans has a rotation axis extending perpendicularly or substantially perpendicularly to the mounting surface. At least one of the plurality of axial fans is arranged to rotate in a first rotation direction to introduce air into the case or discharge air out of the case, while the other of the plurality of axial fans are arranged to rotate in a second rotation direction opposite to the first rotation direction to cause a flow of air in the same direction in which the at least one axial fan is arranged to cause a flow of air. 
     Preferred embodiments of the present invention make it possible to introduce, send, or discharge air into, inside, or out of a case of an electronic device while achieving a reduction in noise. 
     Fan apparatuses according to preferred embodiments of the present invention can be used as fan apparatuses attached to a variety of electronic devices. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a rear view of an electronic device according to a first preferred embodiment of the present invention. 
         FIG. 2  is a plan view of the electronic device. 
         FIG. 3  is a rear view of an electronic device according to a second preferred embodiment of the present invention. 
         FIG. 4  is a plan view of the electronic device according to the second preferred embodiment of the present invention. 
         FIG. 5  is a plan view of an electronic device according to a third preferred embodiment of the present invention. 
         FIG. 6  is a right side view of the electronic device according to the third preferred embodiment of the present invention. 
         FIG. 7  is a left side view of the electronic device according to the third preferred embodiment of the present invention. 
         FIG. 8  is a plan view of an electronic device according to a fourth preferred embodiment of the present invention. 
         FIG. 9  is a right side view of the electronic device according to the fourth preferred embodiment of the present invention. 
         FIG. 10  is a front view of a rack-type electronic system according to a fifth preferred embodiment of the present invention. 
         FIG. 11  is a right side view of the rack-type electronic system. 
         FIG. 12  is a plan view of a fan apparatus according to the fifth preferred embodiment of the present invention. 
         FIG. 13  is a plan view of a fan apparatus according to another preferred embodiment of the present invention. 
         FIG. 14  is a plan view of a fan apparatus according to still another preferred embodiment of the present invention. 
         FIG. 15  is a plan view of a fan apparatus according to still another preferred embodiment of the present invention. 
         FIG. 16  is a right side view of a rack-type electronic system according to a sixth preferred embodiment of the present invention. 
         FIG. 17  is a plan view of a second fan apparatus according to the sixth preferred embodiment of the present invention. 
         FIG. 18  is a right side view of a rack-type electronic system according to a seventh preferred embodiment of the present invention. 
         FIG. 19  is a partial cross-sectional view of a contra-rotating fan according to the seventh preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
       FIG. 1  is a rear view of an electronic device  10  according to a first preferred embodiment of the present invention.  FIG. 2  is a plan view of the electronic device  10 . A lower side of  FIG. 2  corresponds to a rear of the electronic device  10 , while an upper side of  FIG. 2  corresponds to a front of the electronic device  10 . The electronic device  10  is used, for example, as a digital versatile disc (DVD) drive apparatus, a desktop PC, or the like. The electronic device preferably  10  includes a fan-attached case  20  and a circuit board  22 . In  FIGS. 1 and 2 , the circuit board  22  and a case  21  of the fan-attached case  20  are represented by chain double-dashed lines. The same is true of  FIGS. 3 ,  4 ,  5 ,  6 ,  7 ,  8 , and  9  described below. 
     The exterior of the case  20  in this preferred embodiment is that of a rectangular parallelepiped, but any other desired shaped could be used. A rear surface of the case defines a mounting surface to which a fan apparatus  23  is attached. The rear surface will be hereinafter referred to as a “mounting surface  211 ”. The fan apparatus  23  for the fan-attached case  20  is arranged to discharge air out of the case  21 . On the mounting surface  211 , the fan apparatus  23  is arranged one-sidedly near a side  211   a  of the case  21  defining a corner boundary between the mounting surface  211  and a right side surface of the case  21  as shown in  FIG. 2 . The circuit board  22  is arranged inside the case  21 . A principal surface of the circuit board  22  is arranged perpendicularly to the side  211   a . Hereinafter, a vertical direction of the electronic device  10  will be referred to as a “Z direction”, and a front-to-rear direction of the electronic device  10 , which is perpendicular to the Z direction, will be referred to as a “Y direction”. A left-to-right direction of the electronic device  10 , which is perpendicular or substantially perpendicular to both the Y and Z directions, will be hereinafter referred to as an “X direction”. Note that upper and lower sides along the Z direction may not necessarily coincide with upper and lower sides, respectively, along the direction of gravity. The same is true of the other figures. 
     As illustrated in  FIG. 1 , the fan apparatus  23  includes two first axial fans  3   a  and two second axial fans  3   b . The first axial fans  3   a  are arranged side by side in a row in a longitudinal direction of the mounting surface  211  as shown in  FIG. 1 , i.e., in the X direction. The second axial fans  3   b  are arranged side by side in a row below the row of the first axial fans  3   a  in parallel or substantially in parallel with the mounting surface  211  and the row of the first axial fans  3   a . As described above, the first and second axial fans  3   a  and  3   b  are arranged in an array of two rows and two columns in parallel or substantially in parallel with the mounting surface  211 . 
     Each of the first axial fans  3   a  is arranged to rotate counterclockwise as indicated by arrow R 1  in  FIG. 1 . Each of the second axial fans  3   b  is arranged to rotate clockwise as indicated by arrow R 2  to cause a flow of air in the same direction in which the first axial fans  3   a  are arranged to cause a flow of air. Note that the expressions “first axial fan  3   a ” and “second axial fan  3   b ” are used simply for the sake of convenience to distinguish differences in rotation direction. The same holds true in descriptions of other preferred embodiments provided below. A rotation axis J 1  of each of the first and second axial fans  3   a  and  3   b  is arranged to extend in the Y direction, which is perpendicular to the mounting surface  211 . In  FIG. 1 , the reference symbol “J 1 ” is assigned to the rotation axis of only one of the first axial fans  3   a . In the fan apparatus  23 , a boundary  91  between the upper row of the two first axial fans  3   a  and the lower row of the two second axial fans  3   b  is perpendicular or substantially perpendicular to the side  211   a . Hereinafter, unless the first axial fans  3   a  and the second axial fans  3   b  need to be differentiated from each other, the first and second axial fans  3   a  and  3   b  will be referred to as “axial fans  3 ”. The same holds true in the descriptions of the other preferred embodiments provided below. 
     Each of the axial fans  3  includes a motor, an impeller  31 , and a housing  32 . The motor is a direct current brushless motor. The impeller  31  includes a central cup portion  311  and a plurality of blades  312 . The blades  312  are arranged to project radially outward from an outside surface of the cup portion  311 . The motor is placed inside the cup portion  311 . The housing  32  is arranged to surround an outer circumference of the impeller  31 . 
     Near the boundary  91 , top portions of the blades  312  in each axial fan  3  are arranged to move in the direction of the side  211   a  to cause a flow of air traveling in a positive X direction along the boundary  91 . As illustrated in  FIG. 2 , on an imaginary plane in the middle in the Z direction of the case  21 , flows of air traveling inside the case  21  toward the fan apparatus  23  are steeply slanted in a negative X direction when viewed from the fan apparatus  23 , which allows a sufficient amount of air in a wide range to be discharged out of the case  21 . This results in efficient cooling of the circuit board  22  placed in a center of the case  21 . As illustrated in  FIG. 1 , the circuit board  22  is arranged perpendicularly to the side  211   a  and along the boundary  91  to permit the air to be discharged out of the case  21  without the circuit board  22  blocking the passage of the air. 
     The first preferred embodiment has been described above. Referring to  FIG. 1 , the top portions of the blades  312  of all the axial fans  3  in the fan apparatus  23  are arranged to move in the same direction near the boundary  91 , so that whirl components of flows of air caused by the respective axial fans  3  are oriented in the same direction. This leads to an efficient discharge of air while achieving a reduction in noise, i.e., so-called wind noise, caused by the flows of air being discharged through the fan apparatus  23 . 
     In a modification of the electronic device  10 , the fan apparatus  23  may be arranged to introduce air into the case  21 . In the fan apparatus  23  according to this modification, the lower row of the axial fans are arranged to rotate counterclockwise when viewed from the rear to introduce air into the case  21 , while the upper row of the axial fans are arranged to rotate clockwise to introduce air into the case  21 . The top portions of the blades  312  of all the axial fans  3  are thus arranged to move away from the side  211   a  near the boundary  91  illustrated in  FIG. 1 , to achieve an efficient introduction of air into the case  21  while achieving a reduction in the noise caused by the flows of air. The flows of air introduced through the fan apparatus  23  into the case  21  illustrated in  FIG. 2  are steeply slanted in the negative X direction when viewed from the fan apparatus  23 , on an imaginary plane in the middle in the Z direction of the case  21 , so as to spread widely inside the case  21 . The air is introduced into the case  21  without the circuit board  22  blocking the passage of the air, because the circuit board  22  is arranged along the boundary  91 . 
     Second Preferred Embodiment 
       FIG. 3  is a rear view of an electronic device  10  according to a second preferred embodiment of the present invention. In a fan-attached case  20  of the electronic device  10 , a fan apparatus  23  is arranged substantially in a center in the X direction of a rear surface, i.e., a mounting surface  211 , of a case  21 . The fan apparatus  23  includes four axial fans  3   a  and  3   b  arranged in an array of two rows and two columns. Specifically, two first axial fans  3   a  arranged to rotate counterclockwise when viewed from the rear are arranged in upper left-hand and lower right-hand corners in  FIG. 3 , while two second axial fans  3   b  arranged to rotate clockwise are arranged in upper right-hand and lower left-hand corners. That is, each axial fan  3  is arranged to rotate in an opposite direction to that in which the adjacent one of the axial fans  3  in each of row and column directions of the array, i.e., the X and Z directions, is arranged to rotate. In the other respects, the electronic device  10  according to the second preferred embodiment is similar in structure to the electronic device  10  illustrated in  FIG. 1 . In the following description, components that have their counterparts in the electronic device  10  illustrated in  FIG. 1  are assigned the same reference numerals as those of the counterparts. 
     In the fan apparatus  23 , top portions of blades  312  of each pair of adjacent axial fans  3  in either the X direction or the Z direction are arranged to move in the same direction near a boundary  92  between the pair. There are four such boundaries  92 . This arrangement contributes to reducing the noise caused by the flows of air. In  FIG. 3 , reference symbol “ 92   a ” is assigned to two of the four boundaries  92  which are oriented parallel to a longitudinal direction of the mounting surface  211 . 
     Near each of the boundaries  92   a , the top portions of the blades  312  of both the adjacent axial fans  3  each on one side of the boundary  92   a  are arranged to move in the direction of a center of the fan apparatus  23 , i.e., a center of the array of the axial fans  3 , to cause flows of air traveling along the boundaries  92   a  from the left toward the center and from the right toward the center in the figure. As a result, as illustrated in  FIG. 4 , which is a plan view of the electronic device  10 , air inside the case  21  flows toward the fan apparatus  23  with flows of air steeply slanted to both sides along the X direction on an imaginary plane in the middle in the Z direction of the case  21 , which allows a sufficient amount of air in a wide range to be efficiently discharged out of the case  21 . In addition, a circuit board  22  placed in the center inside the case  21  is efficiently cooled. The air is discharged out of the case  21  without the circuit board  22  blocking the passage of the air, because the circuit board  22  is arranged along the boundaries  92   a  as illustrated in  FIG. 3 . 
     In a modification of the second preferred embodiment, the fan apparatus  23  may be arranged to introduce air into the case  21 . In the fan apparatus  23  according to this modification, each axial fan is arranged to rotate in an opposite direction to cause a flow of air in an opposite direction, as compared to its counterpart in the second preferred embodiment. Near the boundaries  92   a , the top portions of the blades  312  of all the axial fans are arranged to move away from the center of the fan apparatus  23 . The air introduced through the fan apparatus  23  into the case  21  illustrated in  FIG. 4  will spread inside the case  21  efficiently and widely from the fan apparatus  23  to both sides along the X direction, on the imaginary plane in the middle in the Z direction of the case  21 . 
     Third Preferred Embodiment 
       FIG. 5  is a plan view of an electronic device  10   a  according to a third preferred embodiment of the present invention. An upper side of  FIG. 5  corresponds to a front of the electronic device  10   a , while a lower side of  FIG. 5  corresponds to a rear of the electronic device  10   a . In the electronic device  10   a , a first fan apparatus  23   a  and a second fan apparatus  23   b  are attached to a right side surface of a case  21 , which is shown on the left-hand portion of  FIG. 5 , and a left side surface of the case  21 , which is shown on the right-hand portion of  FIG. 5 , respectively. In the electronic device  10   a , the first and second fan apparatuses  23   a  and  23   b  are arranged to introduce air into the case  21 . Hereinafter, the right side surface and the left side surface of the case  21  will be referred to as a “first mounting surface  212 ” and a “second mounting surface  213 ”, respectively. In the other respects, the electronic device  10   a  is substantially similar in structure to the electronic device illustrated in  FIG. 1 . In the following description, components that have their counterparts in the electronic device illustrated in  FIG. 1  are assigned the same reference numerals as those of the counterparts. 
       FIG. 6  is a diagram illustrating the first fan apparatus  23   a . In the first fan apparatus  23   a , three first axial fans  3   a  are arranged in a row in the Y direction, while, below the row of the first axial fans  3   a , three second axial fans  3   b  are arranged in a row in the Y direction. Each of the first axial fans  3   a  is arranged to rotate counterclockwise when viewed from outside the case  21 , while each of the second axial fans  3   b  is arranged to rotate clockwise. 
       FIG. 7  is a diagram illustrating the second fan apparatus  23   b . In the second fan apparatus  23   b , three second axial fans  3   b  are arranged in a row in the Y direction, while, below the row of the second axial fans  3   b , three first axial fans  3   a  are arranged in a row in the Y direction. Also in the second fan apparatus  23   b , each of the first axial fans  3   a  is arranged to rotate counterclockwise when viewed from outside the case  21 , while each of the second axial fans  3   b  is arranged to rotate clockwise. That is, in the electronic device  10   a  illustrated in  FIG. 5 , the first and second fan apparatuses  23   a  and  23   b  are symmetric with respect to an imaginary plane  99  parallel or substantially parallel to both the first mounting surface  212  and the opposing second mounting surface  213  and positioned in the middle between the first and second mounting surfaces  212  and  213 . 
     In the first fan apparatus  23   a  illustrated in  FIG. 6 , near a boundary  931  between the row of the first axial fans  3   a  and the row of the second axial fans  3   b , top portions of blades  312  of all the first and second axial fans  3   a  and  3   b  are arranged to move in a negative Y direction, i.e., in the direction of the rear of the electronic device  10   a , to cause a flow of air traveling along the boundary  931  in the negative Y direction. Flows of air introduced into the case  21  through the first fan apparatus  23   a  illustrated in  FIG. 5  are thus steeply slanted in the negative Y direction while traveling in the positive X direction, on an imaginary plane in the middle in the Z direction of the case  21 . 
     Similarly, in the second fan apparatus  23   b  illustrated in  FIG. 7 , near a boundary  932  between the row of the first axial fans  3   a  and the row of the second axial fans  3   b , top portions of blades  312  of all the first and second axial fans  3   a  and  3   b  are arranged to move in the negative Y direction to cause a flow of air traveling along the boundary  932  in the negative Y direction. As illustrated in  FIG. 5 , flows of air introduced into the case  21  through the second fan apparatus  23   b  are thus steeply slanted in the negative Y direction while traveling in the negative X direction, on an imaginary plane in the middle in the Z direction of the case  21 . 
     The flows of air caused by the first and second fan apparatuses  23   a  and  23   b , respectively, pass through a central area inside the case  21 , and are then discharged out of the case  21  through an air outlet  219  defined in a rear surface of the case  21 . A large amount of air thus flows in the central area inside the case  21  to efficiently cool a circuit board  22  placed in the middle in the Z direction of the case  21 . 
     In the third preferred embodiment, the top portions of the blades  312  of all the axial fans  3  in the first fan apparatus  23   a  are arranged to move in the same direction near the boundary  931  to achieve a reduction in the noise caused by the flows of air, while at the same time the top portions of the blades  312  of all the axial fans  3  in the second fan apparatus  23   b  are arranged to move in the same direction near the boundary  932  to achieve a reduction in the noise. 
     In a modification of the electronic device  10   a , each of the first and second fan apparatuses  23   a  and  23   b  may be arranged to discharge air out of the case  21 . In each of the first and second fan apparatuses  23   a  and  23   b  according to this modification, each axial fan is arranged to rotate in an opposite direction to cause a flow of air in an opposite direction, as compared to its counterpart in the third preferred embodiment. Flows of air traveling inside the case  21  illustrated in  FIG. 5  toward the first fan apparatus  23   a  are steeply slanted in a positive Y direction while traveling in the negative X direction. On the other hand, flows of air traveling inside the case  21  toward the second fan apparatus  23   b  are steeply slanted in the positive Y direction while traveling in the positive X direction. This allows air in a wide range inside the case  21  to be efficiently discharged out of the case  21 . 
     Fourth Preferred Embodiment 
       FIG. 8  is a plan view of an electronic device  10   a  according to a fourth preferred embodiment of the present invention. The electronic device  10   a  according to the fourth preferred embodiment is different from the electronic device illustrated in  FIG. 5  in that an additional fan apparatus  23   c  is attached to the rear surface of the case  21  extending between the first and second mounting surfaces  212  and  213 . The fan apparatus  23   c  is similar in structure to the fan apparatus  23  illustrated in  FIG. 3 . In the other respects, the electronic device  10   a  according to the fourth preferred embodiment is similar in structure to the electronic device  10   a  illustrated in  FIG. 5 . Hereinafter, the fan apparatus  23   c  will be referred to as a “third fan apparatus  23   c ”, and the rear surface of the case  21  will be referred to as a “third mounting surface  214 ”. In the electronic device  10   a , the first and second fan apparatuses  23   a  and  23   b  are arranged to introduce air into the case  21 , while the third fan apparatus  23   c  is arranged to discharge the air out of the case  21 . 
       FIG. 9  is a diagram illustrating a left side of  FIG. 8 . The boundary  931  between the upper row of the first axial fans  3   a  and the lower row of the second axial fans  3   b  in the first fan apparatus  23   a  is at the same level in the Z direction as a boundary  933  between the upper row of the axial fans  3  and the lower row of the axial fans  3  in the third fan apparatus  23   c . Similarly, the boundary  932  between the upper row of the second axial fans  3   b  and the lower row of the first axial fans  3   a  in the second fan apparatus  23   b  as illustrated in  FIG. 7  is at the same level in the Z direction as the boundary  933  in the third fan apparatus  23   c  illustrated in  FIG. 9 . Thus, the air introduced into the case  21  through each of the first and second fan apparatuses  23   a  and  23   b  and traveling toward the third mounting surface  214  will efficiently flow into the third fan apparatus  23   c  to be discharged out of the case  21 . Like the fan apparatus  23  illustrated in  FIG. 4 , the third fan apparatus  23   c  is arranged to discharge air from a wide range inside the case  21 , resulting in a further improvement in the efficiency in the air discharge. 
     In each of the first and second fan apparatuses  23   a  and  23   b  according to the fourth preferred embodiment, the top portions of the blades  312  of all the axial fans  3  are arranged to move in the direction of the third fan apparatus  23   c  near the boundary  931  or  932 . This contributes to a reduction in the noise caused by the flows of air. 
     In a modification of the electronic device  10   a  according to the fourth preferred embodiment, in each of the first, second, and third fan apparatuses  23   a ,  23   b , and  23   c , each axial fan may be arranged to rotate in an opposite direction to cause a flow of air in an opposite direction, as compared to its counterpart in the fourth preferred embodiment. In such a modification, the top portions of the blades  312  of all the axial fans  3  in each of the first and second fan apparatuses  23   a  and  23   b  are arranged to move in the direction away from the third fan apparatus  23   c  near the boundary  931  or  932 . This allows air introduced into the case  21  through the third fan apparatus  23   c  to be efficiently directed toward the first and second fan apparatuses  23   a  and  23   b  to be discharged out of the case  21  therethrough. 
     In each of the first, third, and fourth preferred embodiments described above, the top portions of the blades  312  of the paired axial fans are arranged to move in the same direction near the boundary therebetween to cause a flow of air along the boundary. This flow of air is employed to efficiently cool the inside of the case  21 . The circuit board to be cooled is preferably arranged along the boundary between the paired axial fans inside the case  21 . In this case, the flows of air caused by the axial fans will pass in the vicinity of the circuit board, leading to improved efficiency in cooling the circuit board. 
     Fifth Preferred Embodiment 
       FIG. 10  is a front view of a rack-type electronic system  4  according to a fifth preferred embodiment of the present invention.  FIG. 11  is a right side view of the rack-type electronic system  4 . The rack-type electronic system  4 , which is an electronic device, preferably includes a case  41 , a plurality of substantially plate-shaped blade servers  42 , a plurality of power supply units  43 , and four fan apparatuses  44 . Hereinafter, the blade servers  42  will be referred to simply as the “servers  42 ”. 
     The exterior of the case  41  is preferably in the shape of a rectangular parallelepiped, but any other desirable shape could be used. The case  41  contains the servers  42 , the power supply units  43 , and the fan apparatuses  44 . The case  41  has openings on top and bottom portions thereof. The servers  42  are arranged horizontally and in an upright orientation. Three arrays of the servers  42  are provided at different vertical levels. As illustrated in  FIG. 11 , the power supply units  43  are arranged behind the arrays of the servers  42 . Other components, such as a control device, a communication device, and the like, are arranged near the power supply units  43  as appropriate. The four fan apparatuses  44  are arranged above and below the arrays of the servers  42 . 
       FIG. 12  is a plan view of the fan apparatus  44 . A lower side of  FIG. 12  corresponds to a front of the case  41 , while an upper side of  FIG. 12  corresponds to a rear of the case  41 . The fan apparatus  44  is a so-called fan tray, and includes a plurality of axial fans  3  and a frame  441 . In  FIG. 12 , the axial fans  3  are denoted by reference symbols “ 3   a ” and “ 3   b ”. The housings  32  of the axial fans  3  are joined together. In the case where the frame  441  is regarded as a horizontal mounting surface for the axial fans  3  provided in the case  41 , the axial fans  3  are arranged in three columns parallel to the Y direction, with the rotation axis J 1  of each axial fan  3  extending perpendicularly to the mounting surface. Each column includes a row of four of the axial fans  3 . Hereinafter, the three columns parallel to the Y direction in the array of the axial fans  3  in the fan apparatus  44  will be referred to as a “first column  81 ”, a “second column  82 ”, and a “third column  83 ”, respectively, from left to right. 
     As illustrated in  FIG. 11 , the frame  441  is arranged to be capable of being horizontally inserted into and removed from the case  41  through the front of the case  41 . This enables easy removal of the axial fans  3  from the case  41  to repair any of the axial fans  3  or replace any of the axial fans  3  with another axial fan  3 . Note that  FIG. 11  illustrates a situation in which the second fan apparatus  44  from the bottom has been drawn slightly out of the case  41 . The same is true of other similar figures described below. 
     As illustrated in  FIG. 12 , in the first column  81  of the fan apparatus  44 , two first axial fans  3   a , each of which is arranged to rotate counterclockwise in a plan view, and two second axial fans  3   b , each of which is arranged to rotate clockwise in the plan view, are arranged alternately, with one of the first axial fans  3   a  at the top. The same is true of the third column  83 . In the second column  82 , two second axial fans  3   b  and two first axial fans  3   a  are arranged alternately, with one of the second axial fans  3   b  at the top. That is, in the fan apparatus  44 , each axial fan  3  is arranged to rotate in an opposite direction to that in which an adjacent one or ones of the axial fans  3  in each of the X and Y directions, i.e., row and column directions of the array, respectively, are arranged to rotate. Also in descriptions of other fan apparatuses of a fan tray type provided below, for the sake of convenience in explanation, those axial fans  3  which are arranged to rotate counterclockwise in a plan view will be referred to as “first axial fans  3   a ”, while those axial fans  3  which are arranged to rotate clockwise in the plan view will be referred to as “second axial fans  3   b”.    
     In the fifth preferred embodiment, top portions of blades  312  of each pair of adjacent first and second axial fans  3   a  and  3   b  are arranged to move in the same direction near a boundary between the pair. This contributes to reducing noise caused by flows of air produced in the fan apparatus  44 . 
       FIG. 13  is a plan view of an example modification of the fan apparatus  44 . Similar to the fan apparatus  44  illustrated in  FIG. 12 , the fan apparatus  44  according to this example modification includes an array of axial fans  3  arranged in four rows and three columns. In the first column  81 , two first axial fans  3   a  are arranged in an upper portion thereof as shown in  FIG. 13 , while two second axial fans  3   b  are arranged in a lower portion thereof. The same is true of the third column  83 . In the second column  82 , two second axial fans  3   b  are arranged in an upper portion thereof, while two first axial fans  3   a  are arranged in a lower portion thereof. In other words, each of the columns  81  to  83  includes a block  841  of two first axial fans  3   a  and a block  842  of two second axial fans  3   b . Hereinafter, the block  841  will be referred to as a “first fan block  841 ”, and the block  842  will be referred to as a “second fan block  842 ”. 
     Regarding the fan apparatus  44 , near a boundary between each pair of first and second fan blocks  841  and  842  which are adjacent to each other in either the X direction or the Y direction, the top portions of the blades  312  of the first and second axial fans  3   a  and  3   b  are arranged to move in the same direction. This contributes to reducing the noise caused by the flows of air. 
     Moreover, since the fan apparatus  44  includes the first and second fan blocks  841  and  842 , it is possible to assemble each individual block of axial fans  3  beforehand easily. This facilitates assemblage of the entire fan apparatus  44 . 
     As described above, in each of the fan apparatuses  44  illustrated in  FIGS. 12 and 13 , half the axial fans  3  are first axial fans  3   a  while the remaining half are second axial fans  3   b , and in addition, the first axial fans  3   a  and the second axial fans  3   b  are distributed evenly across the whole array of the axial fans  3 . This enables air to be introduced into the case  41  such that the air introduced will spread across a wide range inside the case  41 , or enables air in a wide range inside the case  41  to be discharged out of the case  41 , while achieving a reduction in noise. Note that, in other modifications of the fifth preferred embodiment, the number of first axial fans  3   a  and the number of second axial fans  3   b  may not necessarily be equal to each other, but may each be only nearly half the total number of axial fans  3 . Preferably, the difference between the number of first axial fans  3   a  and the number of second axial fans  3   b  is equal to or smaller than a third of the total number of axial fans  3 . Note that the wording “at least one axial fan arranged to rotate in a first rotation direction is half or nearly half a total number of the plurality of axial fans” as used herein means that the number of axial fans arranged to rotate in the first rotation direction and the number of axial fans arranged to rotate in a second rotation direction are equal or nearly equal to each other. 
       FIG. 14  is a plan view of still another example modification of the fan apparatus  44 . In the fan apparatus  44  according to this example modification, a row of first axial fans  3   a  is arranged in the first column  81 , a row of second axial fans  3   b  is arranged in the second column  82 , and a row of first axial fans  3   a  is arranged in the third column  83 . That is, the axial fans  3  in each column are arranged to rotate in an opposite direction to that in which the axial fans  3  in the adjacent column(s) are arranged to rotate. Thus, the top portions of the blades  312  of all the axial fans  3  in the first and second columns  81  and  82  are arranged to move in the same direction near a boundary between the first and second columns  81  and  82 , while at the same time the top portions of the blades  312  of all the axial fans  3  in the second and third columns  82  and  83  are arranged to move in the same direction near a boundary between the second and third columns  82  and  83 , so that a reduction in the noise caused by the flows of air is accomplished. 
       FIG. 15  is a plan view of still another example modification of the fan apparatus  44 . The fan apparatus  44  according to this example modification is similar in structure to the fan apparatus  44  illustrated in  FIG. 12  except that all the axial fans  3  share a common single-piece housing  32   a . Use of the common single-piece housing  32   a  eliminates the need for an operation of joining the respective housings of the axial fans  3  together. Note that the technique of using a common single-piece housing  32   a  shared by a plurality of axial fans  3  may be adopted in modifications of any other preferred embodiment. 
     Sixth Preferred Embodiment 
       FIG. 16  is a right side view of a rack-type electronic system  4   a  according to a sixth preferred embodiment of the present invention, and corresponds to  FIG. 11 . The rack-type electronic system  4   a  is provided with a pair of first and second fan apparatuses  44   a  and  44   b  in place of each fan apparatus  44  in the rack-type electronic system  4  illustrated in  FIG. 11 . The first fan apparatus  44   a  is similar in structure to the fan apparatus  44  illustrated in  FIG. 12 . 
       FIG. 17  is a plan view of the second fan apparatus  44   b . The positions of the first and second axial fans  3   a  and  3   b  in the second fan apparatus  44   b  are reversed as compared to those in the fan apparatus  44  illustrated in  FIG. 12 . In the other respects, the rack-type electronic system  4   a  illustrated in  FIG. 16  is similar in structure to the rack-type electronic system  4  illustrated in  FIG. 11 . The first and second fan apparatuses  44   a  and  44   b  in the rack-type electronic system  4   a  are driven to cause an upward flow of air. 
     As with the housings  32  of the axial fans  3  in the fan apparatus  44  illustrated in  FIG. 12 , the housings  32  of the axial fans  3  in the first fan apparatus  44   a  are joined together. In the case where the frame  441  is regarded as a horizontal mounting surface for the axial fans  3  provided in the case  41 , the axial fans  3  are arranged in an array of four rows and three columns in parallel to the mounting surface. As illustrated in  FIG. 16 , the frame  441  is arranged to be capable of being horizontally inserted into and removed from the case  41  through the front of the case  41 . 
     Also in the second fan apparatus  44   b  illustrated in  FIG. 17 , the housings  32  of the axial fans  3  are joined together. In the case where a frame  442  is regarded as a horizontal mounting surface for the axial fans  3  provided in the case  41 , the axial fans  3  are arranged in an array of four rows and three columns in parallel to the mounting surface. In addition, as illustrated in  FIG. 16 , the frame  442  is arranged to be capable of being horizontally inserted into and removed from the case  41  through the front of the case  41 . 
     The first and second fan apparatuses  44   a  and  44   b  in the rack-type electronic system  4   a  are placed one above the other in the Z direction. As a result, each of the first and second axial fans  3   a  and  3   b  in the second fan apparatus  44   b  illustrated in  FIG. 17  is arranged in contact or close proximity with a separate one of the second and first axial fans  3   b  and  3   a  in the first fan apparatus  44   a , which is similar to the fan apparatus  44  illustrated in  FIG. 12 , with the rotation axis J 1  thereof coinciding with the rotation axis J 1  of the separate one of the second and first axial fans  3   b  and  3   a  in the first fan apparatus  44   a . That is, in the rack-type electronic system  4   a  illustrated in  FIG. 16 , each of the axial fans  3  in the second fan apparatus  44   b  and a separate one of the axial fans  3  in the first fan apparatus  44   a  which is arranged in contact or close proximity therewith along the rotation axes J 1  are arranged to rotate in mutually opposite directions to each cause a flow of air in the same direction. This leads to an improvement in air static pressure-air volume characteristics. 
     This makes it possible to efficiently introduce, send, and discharge air into, inside, and out of the case  41 , which contains the electronic components arranged densely. Note that the array of the axial fans  3  in the first fan apparatus  44   a  and the array of the axial fans  3  in the second fan apparatus  44   b  may be spaced from each other by a greater distance than in the case illustrated in  FIG. 16 , as long as a rotational velocity component of an air flow from each axial fan  3  on an inlet side serves to increase efficiency of the associated axial fan  3  on an outlet side in sending out the air. 
     Also in the sixth preferred embodiment, the top portions of the blades  312  of each pair of adjacent first and second axial fans  3   a  and  3   b  in the first fan apparatus  44   a  are arranged to move in the same direction near the boundary between the pair. This contributes to reducing the noise caused by the flows of air. The same is true of the second fan apparatus  44   b . In modifications of the rack-type electronic system  4   a , the array of the first and second axial fans  3   a  and  3   b  in the first fan apparatus  44   a  may be similar to that illustrated in  FIG. 13  or  14 . In such modifications, the positions of the first and second axial fans  3   a  and  3   b  in the second fan apparatus  44   b  are reversed as compared to those illustrated in  FIG. 13  or  14 . 
     Seventh Preferred Embodiment 
       FIG. 18  is a right side view of a rack-type electronic system  4   b  according to a seventh preferred embodiment of the present invention. Fan apparatuses  45  in the rack-type electronic system  4   b  are each, in essence, a combination of the first and second fan apparatuses  44   a  and  44   b  as illustrated in  FIG. 16  joined together, and each include a plurality of contra-rotating fans  5 . The contra-rotating fans  5  are arranged in a horizontal two-dimensional array in the X and Y directions. Thus, the collection of the contra-rotating fans  5  corresponds to a combination of a two-dimensional array of a plurality of axial fans and another two-dimensional array of a plurality of axial fans which are arranged in contact or close proximity with each other in an axial direction. 
     In the other respects, the rack-type electronic system  4   b  is similar in structure to the rack-type electronic system  4   a  illustrated in  FIG. 16 . As with the axial fans  3  illustrated in  FIG. 12 , the contra-rotating fans  5  are arranged in an array of four rows and three columns, with rotation axes thereof extending perpendicularly or substantially perpendicularly to a frame  451  defining a horizontal mounting surface provided in the case  41 . Each fan apparatus  45  in the rack-type electronic system  4   b  is arranged to be capable of being horizontally, i.e., in parallel or substantially in parallel to the frame  451 , inserted into and removed from the case  41 . 
       FIG. 19  is a partial cross-sectional view of one of the contra-rotating fans  5 . Note, however, that the collection of the contra-rotating fans  5  include those contra-rotating fans  5  whose rotation directions are opposite to those of the contra-rotating fan  5  illustrated in  FIG. 19 . The contra-rotating fan  5  includes an upper fan portion  51  and a lower fan portion  52  arranged below the upper fan portion  51  and joined to the upper fan portion  51  along the rotation axis J 1 . The upper fan portion  51  includes an impeller  511 , a motor  512 , a housing  513 , and a rib  514 . The rib  514  is arranged to support the motor  512  in relation to the housing  513 . The lower fan portion  52  includes an impeller  521 , a motor  522 , a housing  523 , and a rib  524 . The rib  524  is arranged to support the motor  522  of the lower fan portion  52  in relation to the housing  523 . The relative vertical positions of the impeller and the motor are reversed between the upper and lower fan portions  51  and  52 . That is, the impeller  521  is arranged below the motor  522  in the lower fan portion  52 , while the impeller  511  is arranged above the motor  512  in the upper fan portion  51 . 
     The housing  513  of the upper fan portion  51  is joined to the housing  523  of the lower fan portion  52  in a vertical direction. In  FIG. 19 , a mechanism to join the housings  513  and  523  to each other is omitted from illustration. In addition, the rib  514  of the upper fan portion  51  and the rib  524  of the lower fan portion  52  are arranged in contact with each other in the vertical direction. Note that the ribs  514  and  524  may be arranged only in close proximity with each other. 
     In the contra-rotating fan  5 , the impeller  511  of the upper fan portion  51  and the impeller  521  of the lower fan portion  52  are arranged to rotate in mutually opposite directions, so that an improvement in the air static pressure-air volume characteristics is achieved. It is possible to manufacture each fan apparatus  45  in the rack-type electronic system  4   b  at a low cost, by using known contra-rotating fans. 
     In the rack-type electronic system  4   b , the upper fan portions  51  in each pair of adjacent contra-rotating fans  5  are arranged to rotate in mutually opposite directions, while at the same time the lower fan portions  52  in each pair of adjacent contra-rotating fans  5  are arranged to rotate in mutually opposite directions. This contributes to reducing the noise caused by the flows of air. In modifications of the seventh preferred embodiment, the upper fan portions  51  in the array of the contra-rotating fans  5  may be respectively arranged to rotate in the directions in which the axial fans  3  in the fan apparatus  44  illustrated in  FIG. 13  or the fan apparatus  44  illustrated in  FIG. 14  are arranged to rotate. In such modifications, the lower fan portions  52  are respectively arranged to rotate in opposite directions to those in which the axial fans  3  illustrated in  FIG. 13  or  14  are arranged to rotate. 
     While preferred embodiments of the present invention have been described above, it will be understood that the present invention is not limited to the above-described preferred embodiments, but that a variety of modifications are possible. In each of the first to sixth preferred embodiments described above, at least one of the axial fans  3  in the fan apparatus is preferably arranged to rotate in an opposite direction to that in which all the other axial fans  3  are arranged to rotate, to achieve a reduction in noise in the air introduction or discharge. In the seventh preferred embodiment, the upper and lower fan portions  51  and  52  in at least one of the contra-rotating fans  5  preferably are arranged to rotate in opposite directions to those in which the upper and lower fan portions  51  and  52 , respectively, in each of the other contra-rotating fans  5  are arranged to rotate, to achieve a reduction in noise. 
     In each of the first and second preferred embodiments described above, the fan apparatus  23  is preferably attached to the rear surface of the case  21  of the electronic device  10 . Note, however, that in modifications thereof, the fan apparatus  23  may be attached to any other principal surface of the case  21 , such as a side surface of the case  21 . Also note that in modifications thereof, the fan apparatus  23  may include three or more rows of axial fans  3 . In such modifications thereof, the axial fans  3  in each row are preferably arranged to rotate in an opposite direction to that in which the axial fans  3  in the adjacent row(s) are arranged to rotate. Alternatively, each pair of axial fans  3  which are adjacent to each other in either the row direction or the column direction of the array may preferably be arranged to rotate in mutually opposite directions. In modifications of each of the first and second preferred embodiments, any number of axial fans  3  may be used in the fan apparatus  23 , as long as the axial fans  3  are arranged in an array of at least two rows and at least two columns. The same is true of both the third and fourth preferred embodiments. 
     In modifications of each of the fifth to seventh preferred embodiments described above, any number of fans may be used in the fan apparatus  44 ,  44   a ,  44   b , or  45 , as long as the fans, i.e., the axial fans  3  or the contra-rotating fans  5 , are arranged in an array of at least two rows and at least two columns. 
     In modifications of the fan apparatus  44  illustrated in  FIG. 13 , four or more columns of axial fans  3  may be arranged. In such modifications, a collection of axial fans  3  arranged in an array of two or more rows and two or more columns and arranged to rotate in the same direction may be regarded as a fan block. Various modifications of the structure and arrangement of the first and second fan blocks are possible, as long as each of a plurality of first fan blocks is a collection of adjacent axial fans arranged to rotate in the same direction, each of a plurality of second fan blocks is a collection of adjacent axial fans arranged to rotate in an opposite direction to that in which the axial fans in the first fan blocks are arranged to rotate, and each first fan block is arranged adjacent to one or more of the second fan blocks. 
     In modifications of each of the first to fourth preferred embodiments described above, more than one fan apparatus may be attached to the same mounting surface. In modifications of each of the fifth to seventh preferred embodiments described above, a fan apparatus similar to the fan apparatus according to any of the first to fourth preferred embodiments, or a fan apparatus similar to the fan apparatus according to any of the fifth to seventh preferred embodiments, may be attached to a side surface, a rear surface, or the like of the case  41 . 
     In modifications of the fourth preferred embodiment, the structure of the third fan apparatus  23   c  is not limited to the 2×2 array, as long as an imaginary plane which is parallel or substantially parallel to the X-Y plane and includes the boundary  931  in the first fan apparatus  23   a  illustrated in  FIG. 9  and the boundary  932  in the second fan apparatus  23   b  is arranged to intersect with the third fan apparatus  23   c . For example, only two axial fans  3  arranged to rotate in mutually opposite directions may be arranged in the third fan apparatus  23   c.    
     In modifications of the contra-rotating fan  5  illustrated in  FIG. 19 , the housing  513  of the upper fan portion and the housing  523  of the lower fan portion  52  may be replaced with a single-piece housing shared by the upper and lower fan portions  51  and  52 . Moreover, in modifications of the fan apparatus  45  according to the seventh preferred embodiment, the housings  513  and  523  of all the contra-rotating fans  5  arranged in the fan apparatus  45  may be replaced with a single-piece housing shared by all the contra-rotating fans  5 . In modifications of each of the fifth to seventh preferred embodiments described above, the joining of the housings in directions perpendicular or substantially perpendicular to the rotation axes J 1  may be accomplished, instead of through the frame, directly through the housings themselves by using a joining member or the like. Also in such modifications thereof, the collection of the axial fans  3  or of the contra-rotating fans  5  are arranged to be capable of being inserted into and removed from the case  41  in a direction parallel or substantially in parallel to the predetermined mounting surface provided in the case  41 . An electronic device according to a preferred embodiment of the present invention may be either an electronic device as a single complete product, or a component of an electronic device system, such as a power supply or a light source. Examples of such single complete products include, for example, a computer, a server system, a large router, and a home appliance product, such as audio-visual (AV) equipment. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.