Abstract:
A method for forming a pulsating air pattern at a surface of an electronic carrier assembly. The electronic carrier assembly includes a circuit card having an attached module or chip. An air circulation pattern, or flow pattern, is formed at the surface of an electronic carrier assembly by natural convection, or by a steady air flow generated by a fan, respectively. A suitably-positioned rotatable disk having one or more void regions, and in a state of rotation, interrupts the air circulation or flow pattern by causing a pulsation air pattern at the surface of the electronic carrier assembly. The effect of the pulsating air pattern is to increase the rate of heat transfer from the electronic carrier assembly by two mechanisms. First, the thickness of the boundary layer at the surface of the electronic carrier is reduced, thereby increasing the heat transfer coefficient at the boundary layer. Second, stagnant air trapped between nearby electronic components on the electronic carrier assembly is swept away. When no fan is present, the method generates a pulsating air pattern that enhances natural convection heat transfer. When a fan is present, the disk may be positioned between the fan and the electronic carrier assembly. Alternatively, the electronic carrier assembly may be positioned between the fan and the disk. Other configurations are possible, including those having two fans, two disks, and two or more electronic carrier assemblies.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method for forming a pulsating air pattern at a surface of an electronic carrier assembly. 
     2. Related Art 
     An electronic carrier assembly comprises an electronic carrier, such as a circuit board, having an attached electronic component such as a module or a chip. When electric current flows within an electronic carrier assembly, heat is generated and the heat must be dissipated. Natural convection provides an effective mechanism for heat removal where the required rate of heat removal is small. For situations in which the required rate of heat removal is large, a cooling fan is typically used, since a fan generates an air flow across a surface of an electronic carrier assembly which removes heat by forced convection. The heat transfer coefficient associated with forced convection, which is generally higher than the heat transfer coefficient associated with natural convection, increases as the velocity of air flow increases. A surface of an electronic carrier assembly includes the surface of the electronic carrier and the surfaces of electronic components attached to the electronic carrier. Under this definition, a surface of an electronic carrier assembly includes surfaces from which heat may be transferred from the electronic carrier assembly to the surrounding air, or other surrounding fluid. 
     The required rate of heat removal increases with increasing current flow, which is a consequence of increasing power input. The required rate of heat removal is generally higher in a closed system than in an open system. Under the assumption that the electronic carrier assembly is coupled to a housing, a closed system is a configuration in which one or more housing surfaces are located so as to impede air flow normal to the surfaces of the electronic carrier assembly. With an open system, housing surfaces do not impede air flow normal the surfaces of the electronic carrier assembly. Accordingly, an open system allows better heat transfer from an electronic carrier assembly than does a closed system when the primary mode of heat transfer is natural convection, especially when the electronic carrier assembly is oriented vertically. With forced convection and adequate venting, a closed system is the more efficient system for dissipating heat, because the housing enhances the air flow velocity at the surface of the electronic carrier assembly. A disadvantage of a closed system with forced convection heat transfer, however, is a generation of higher pressure drops, which in turn raises the level of acoustic noise. Another applicable system is a partially open system in which housing surfaces are located so as to impede air flow normal from one surface of the electronic carrier assembly, but not from another surface of the electronic carrier assembly. 
     While present systems remove heat from operating electronic carrier assemblies, it would be advantageous to remove such heat removal more efficiently. It would also be advantageous to remove heat from an electronic carrier assembly by natural convection where a cooling fan would otherwise be required. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for forming a pulsating air pattern at a surface of an electronic carrier assembly, comprising: providing a rotatable disk; and rotating the rotatable disk to form the pulsating air pattern at the surface of the electronic carrier assembly. 
     The present invention provides an electrical structure, comprising: an electronic carrier assembly; a rotatable disk; and a system for rotating the rotatable disk, to form a pulsating air pattern at a surface of the electronic carrier assembly. 
     The present invention has the advantage of improving the transfer of heat from an operating electronic carrier assembly for cases when a fan is present, and also for cases in which no fan is present. 
     The present invention has the advantage of being inexpensive to implement. 
     The present invention has the advantage of consuming low power, since the primary power required over that of existing systems is the power input to the slowly rotating disk. 
     The present invention has the advantage of having modest space requirements, since the rotatable disk may be thin and is positioned at a side of the electronic carrier assembly where there typically is available space. 
     The present invention has the advantage of ease of retrofitting the disk to existing electronic configurations. 
     The present invention has the advantage of being easily reworkable, since reworking merely requires removing the rotatable disk. 
     The present invention has the advantage of enabling some electronic configurations to have heat removed by natural convection where a cooling fan would otherwise be required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a front view of an electrical structure, in accordance with a preferred embodiment of the present invention. 
     FIG. 2 depicts the disk in the electrical structure in of FIG. 1, as including a solid sector having a hole. 
     FIG. 3 depicts the disk in the electrical structure of FIG. 1, as including a solid polygon having hole. 
     FIG. 4 depicts the disk in the electrical structure of FIG. 1, as including alternating solid sectors. 
     FIG. 5 depicts the disk in the electrical structure in FIG. 1, as including an irregular shape. 
     FIG. 6 depicts the electrical structure of FIG. 1, with an illustration of the air flow pattern. 
     FIG. 7 depicts the electrical structure of FIG. 1, with a second electronic carrier assembly. 
     FIG. 8 depicts an open-system variant of the electrical structure of FIG.  1 . 
     FIG. 9 depicts a partially open-system variant of the electrical structure of FIG.  1 . 
     FIG. 10 depicts the electrical structure of FIG. 1, with a second rotatable disk. 
     FIG. 11 depicts the electrical structure of FIG. 1, oriented vertically. 
     FIG. 12 depicts the electrical structure of FIG. 11, with the disk repositioned. 
     FIG. 13 depicts the electrical structure of FIG. 11, with a second rotatable disk. 
     FIG. 14 depicts an open-system variant of the electrical structure of FIG.  11 . 
     FIG. 15 depicts a partial open-system variant of the electrical structure of FIG. 
     FIG. 16 depicts the electrical structure of FIG. 1, with a fan. 
     FIG. 17 depicts FIG. 16 with a change of electronic components, showing an air flow pattern at a first time in the cycle of fan rotation. 
     FIG. 18 depicts FIG. 16 with a change of electronic components, showing an air flow pattern at a second time in the cycle of fan rotation. 
     FIG. 19 depicts the electrical structure of FIG. 16, with the disk repositioned. 
     FIG. 20 depicts the electrical structure of FIG. 16, with a second rotatable disk. 
     FIG. 21 depicts the electrical structure of FIG. 16, with a second fan. 
     FIG. 22 depicts the electrical structure of FIG. 16, with a second fan and a second rotatable disk. 
     FIG. 23 depicts an open-system variant of the electrical structure of FIG.  16 . 
     FIG. 24 depicts FIG. 23 having a pull-type fan. 
     FIG. 25 depicts a partial open-system variant of the electrical structure of FIG.  16 . 
     FIG. 26 depicts a fan and a semicircular cover. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a front view of an electrical structure  10 , oriented horizontally, of a preferred embodiment of the present invention. The electrical structure  10  includes an electronic carrier assembly  20  and a rotatable disk  60 . The electrical structure  10  may also include a housing. If present, the housing may include an upper housing surface  44  or a lower housing surface  46 , or both. The electronic carrier assembly  20  includes an electronic carrier  22  having a top surface  31  and a bottom surface  33 , an electronic component  24  having a surface  34 , an electronic component  26  having a surface  32 , an electronic component  28  having a surface  29 , and an electronic component  30  having a surface  35 . The electronic carrier  22  may be any type of electronic carrier, such as a circuit board. Any type of electronic component may be represented by electronic components  24 ,  26 ,  28 , and  30 , such as a module or a chip. The top surface  40  of the electronic carrier assembly  20  includes the top surface  31  of the electronic carrier  22 , the surface  34  of the electronic component  24 , and the surface  32  of the electronic component  26 . The bottom surface  42  of the electronic carrier assembly  20  includes the bottom surface  33  of the electronic carrier  22 , the surface  29  of the electronic component  28 , and the surface  35  of the electronic component  30 . Although components  24  and  26  are shown coupled to top surface  31  of the electronic carrier  22 , and bottom components  28  and  30  are shown as coupled to bottom surface  33  of the electronic carrier  22 , it should be noted that electronic components may be coupled only to the top surface  31  of the electronic carrier  22  or coupled only to the bottom surface  33  of the electronic carrier  22 . Any number, including zero, of electronic components may be coupled to each of surfaces  31  and  33  of the electronic carrier  22 . 
     A system  64  for rotating the disk  60  may include any device, medium, or mechanism, such as an electric motor or a flowing air current, that can cause the disk  60  to rotate. The system  64  may, or may not, be mechanically coupled to the disk  60 . Although various rotatable disks in FIGS. 2-26 are shown without a system for rotating the various rotatable disks, it should be understood that a system is nevertheless present for rotating the various rotatable disks. 
     The disk  60  in FIG. 1 includes many possible configurations, including those shown in FIGS. 2,  3 ,  4 , and  5 . FIG. 2 illustrates a rotatable disk  66 , as illustrative of the disk  60  of FIG. 1, having at a solid sector  84  that has a hole  85 . Although the angular size of the solid sector  84  is shown as about 180 degrees, the angular sector size of the solid sector  84  may be less than 180 degrees or greater than 180 degrees. The hole  85  may have any shape, such as a circular, square, or elliptical shape. The hole  85  may have any size and be positioned anywhere on the solid sector  84 . The hole  85  may be omitted or alternatively represent one of a plurality of holes within the solid sector  84 . 
     FIG. 3 illustrates a rotatable disk  67 , as illustrative of the disk  60  of FIG. 1, including a solid polygon  86  that has a hole  87  of any cross sectional area less than the surface area of the solid polygon  86 . The hole  87  may have any shape, such as a circular, square, or elliptical shape. The hole  87  may have any size and may be positioned anywhere on the solid polygon  86 . The hole  87  is shown as one hole, but may alternatively represent one of a plurality of holes within the solid polygon  86 . The solid polygon  86  is shown as a square disk, but may have the shape of any polygon of at least 3 sides. The rotatable disk  67  may also approximate a circular disk that has a hole. Where the solid polygon  86  is a regular polygon of n sides, the solid polygon  86  approaches a circle as n approaches infinity. The rotatable disk  67  may also approximate a solid rectangular disk. Where the solid polygon  86  is a rectangle, the solid polygon  86  approaches a solid rectangular disk as the cross sectional area of the hole  87  approaches zero. 
     FIG. 4 illustrates a rotatable disk  68 , as illustrative of the disk  60  of FIG. 1, including a plurality of solid sectors  90  in an alternating pattern with void sectors  88 . Each pair of consecutive solid sectors  90  bounds a void sector  88 . Each solid sector of the plurality of solid sectors  90  may have any angular size such that the sum of the angular sizes of the solid sectors  90  and the void sectors  88  is 360 degrees. Each solid sector of the plurality of solid sectors  90  may alternatively and independently include a hole  91  having any of the characteristics of the hole  85  of FIG.  2 . 
     The preceding examples in FIGS. 2-5 are mere illustrations of the numerous possible configurations of the rotatable disk  60  of FIG.  1 . It should be noted that the thickness of the rotatable disk  60  need not be constant and may therefore vary with spatial location on a surface of the rotatable disk  60 . 
     The rotatable disk  60  of FIG. 1 may include any shape, such as the irregular shape  82  of the rotatable disk  65  in FIG.  5 . The hole  83  within the irregular shape  82  may have any of the characteristics of the hole  85  of FIG.  2 . Alternatively, the hole  83  may be omitted or represent one of a plurality of holes within the irregular shape  82 . 
     The rotatable disk  60 , as illustrated by the examples in FIGS. 2-5, serves to introduce a pulsating disturbance in the air circulation pattern in the vicinity of the electronic carrier assembly  20 , for the purpose of increasing the rate of heat dissipation from the electronic carrier assembly  20 . To be consistent with this purpose, the rotatable disk  60  should have sufficient void area so as not to duly impede hot air from flowing through or around the disk  60 . Thus, a solid circular disk with a tiny pinhole, such that the disk occupies most or all of the flow area on a side of the electronic carrier assembly  20 , might decrease, rather than an increase, the rate of heat transfer from the electronic carrier assembly  20 . In contrast, a solid disk having holes with sufficient void area for circulation purposes is a candidate for enhancing the rate of heat transfer. The required or optimum void area associated with the disk  60  depends on several factors, including the geometry of the electrical structure  10  (surface geometry of the electronic carrier assembly  20 , housing geometry, etc.) and the heat dissipation requirements for a given application. 
     Returning to FIG. 1, power input to the electronic carrier assembly  20  generates heat in electronic components  24 ,  26 ,  28 , and  30 , resulting in a natural convection boundary layer  50 , defined by bounding surface  51 . Thus the boundary layer  50  is along the top surface  40  of the electronic carrier assembly  20 . The boundary layer  50  at the top surface  40  is characterized by a thickness t. The boundary layer thickness t may vary with location on the top surface  40  of the electronic carrier assembly  20 , depending on a variety of factors including local geometric characteristics of the top surface  40  and local rates of heat generation along the top surface  40 . There is a corresponding boundary layer (not shown) associated with the bottom surface  42  of the electronic carrier assembly  20 . The thickness t of the boundary layer  50  impacts the rate of heat dissipation from the top surface  40  of the electronic carrier assembly  20 , since the rate of heat dissipation increases as the boundary layer thickness t decreases. Heat dissipation is degraded by trapped air in the stagnation zone  56 . A stagnation zone is, generally, the space between two successive electronic components. In FIG. 1, the stagnation zone  56  is the space between successive electronic components  24  and  26 . 
     FIG. 6 illustrates the electrical structure of FIG. 1 with an air circulation pattern  70  at a given instant of time when power is supplied to the electronic structure  10 . The rotatable disk  60  may be rotated at a frequency f d , by use of any suitable powering device such as an electric motor. When rotated, the disk  60  causes the air circulation pattern  70  to oscillate in time at a frequency equal to fd or at a harmonic thereof for the disk configuration of FIG.  3 . If the disk of FIG. 3 contains N equal-sized alternating solid sectors, the natural convection air circulation pattern  70  will pulsate at a frequency of Nf d . The pulsation reduces the boundary layer thickness t shown in FIG. 1, which increases the rate of heat dissipation. It is possible that stagnant air may not be removed from the stagnation zone  56  (see supra discussion of FIG. 1 for a definition of a stagnation zone) by the pulsation, because the air circulation pattern  70  lacks a steady flow component that would assist the pulsation in sweeping away the stagnant air in the stagnation zone  56 . Thus, the use of a rotating disk to improve natural circulation heat transfer is most useful in applications where natural circulation alone is inadequate and where the improvement in heat transfer by the rotating disk avoids more costly and complicated heat removal methods, such as forced convection. Note that heat is transferred less effectively from the bottom surface  42  of the electronic carrier assembly  20  than from the top surface  40  of the electronic carrier assembly  20 , because of the tendency of hot air to rise toward the bottom surface  42  and away from the top surface  40 . 
     FIG. 7 illustrates the electrical structure of FIG. 1 with a second electronic carrier assembly  21 , which includes an electronic carrier  23  having a top surface  36 , a bottom surface  37 , and electronic components  25  and  27 . Although not shown, electronic components may be coupled to the top surface  36 , with or without electronic components coupled to the bottom surface  37 . Although FIG. 7 shows only two electronic carrier assemblies, namely  20  and  21 , the electrical structure  10  may include any number of electronic carrier assemblies. 
     The magnitude of the heat transfer improvement by use of the rotating disk  60  in FIG. 1 depends on factors such as geometry and availability of open space for air circulation. FIG. 8 illustrates the electrical structure  10  of FIG. 1 with the upper housing surface  44  removed and the lower housing surface  46  removed. The configuration of FIG. 8, with a representative air circulation patterns  72  and  73 , is an example of an open system, allowing more space for air circulation than does the closed system of FIG. 1. A consequence of the increased space for air circulation is an increase in the rate of heat transfer from the electronic carrier assembly  20 . 
     FIG. 9 illustrates the electrical structure  10  of FIG. 1 with the upper housing surface  44  removed. The configuration of FIG. 9, with a representative air circulation pattern  74  in the vicinity of top surface  40  of the electronic carrier assembly  20 , is an example of a partially open system, allowing more space for air circulation than does the closed system of FIG. 1. A consequence of the increased space for air circulation in FIG. 9 is an increase in the rate of heat transfer from the top surface  40  of the electronic carrier assembly  20 , as compared the corresponding heat transfer rate in FIG.  1 . 
     FIG. 10 illustrates the electrical structure  10  of FIG. 1 with a second rotatable disk  61 . The electronic carrier assembly  20  is interposed between the disk  60  and the second rotatable disk  61 . The second rotatable disk  61  has any of the features available to the disk  60 . The second rotatable disk  61  is not necessarily the same as the disk  60  for a given electrical structure  10 . For example, the disk  60  may include one solid sector having an angular extent of 120 degrees, while the second rotatable disk  61  may include a solid octagon having 10 randomly spaced holes. With the disk  60  alone in operation, the pulsating air circulation pattern diminishes in intensity with increasing horizontal distance from the disk  60 . The second rotatable disk  61  serves to enhance the pulsating air circulation pattern in the most distant locations from the disk  60 , so as to maximize the overall improvement in heat transfer from the entire top and bottom surfaces,  40  and  42  respectively, of the electronic carrier assembly  20 . 
     FIG. 11 illustrates the electrical structure  10  of FIG. 1 rotated 90 degrees, so as to orient the electrical structure  10  in a vertical direction. With the vertical orientation, the natural convection air circulation pattern  77  has a steady upward component due to a thermally-induced air-density gradient in the downward direction. When rotated at a frequency f d , the disk  60  induces a pulsating air circulation component of frequency f d , or of Nf d  for positive integers N if the disk includes N alternating equal-sized solid sectors (see discussion associated with FIG. 6 supra concerning generation of harmonics of f d  in the pulsating flow pattern). As explained for FIG. 6, the pulsating the air circulation pattern  77  in FIG. 11 reduces the boundary layer thickness along the top surface  40  of the electronic carrier  20 , which in turn increases the rate of heat dissipation from the top surface  40  of the electronic carrier  20 . Additionally, the steady upward air flow component of the air circulation pattern  77 , combined with the pulsating flow component indued by rotation of the disk  60 , facilitates local air circulation  78  that sweeps stagnant air out of the stagnation zones  57 . Thus, the vertical orientation improves heat transfer by both reducing the boundary layer thickness and convecting stagnant air out of stagnation zones. As a result, the rotating disk is potentially more effective in dissipating heat in the vertical orientation than in the horizontal orientation for a given electrical structure  10 . It should be noted that the electrical structure  10  may be oriented horizontally, vertically, or at any angle with respect to the horizontal direction. 
     FIG. 12 illustrates the electrical structure of FIG. 11 with the disk  62  representing the disk  60  of FIG. 11 after the disk  62  is positioned above the electronic carrier assembly  20 . The configuration of FIG. 12, while effective to some extent, may not be as effective as the configuration of FIG. I 1  in improving heat transfer, because the disk  62  in FIG. 12 interacts with the steady vertical flow after the flow passes the electronic carrier assembly  20 , while the disk  60  in FIG. 11 interacts with the steady vertical flow before the flow passes the electronic carrier  22 . 
     FIG. 13 illustrates the electrical structure of FIG. 1 with a second rotatable disk  59  at a location above the electronic carrier assembly  20 . The electronic carrier assembly  20  is interposed between the disk  60  and the second rotatable disk  59 . The second rotatable disk  59  has any of the features available to the disk  60 . 
     FIG. 14 illustrates FIG. 11 after removal of housing surfaces  44  and  46 , which were respectively referred to supra in the horizontally-oriented configuration of FIG. 1 as upper housing surface  44  and lower housing surface  46 . Thus, FIG. 14 constitutes an open system in which the upward-flowing air circulation pattern  97  due to natural convection is the result of combining the bottom air pattern  95  with the peripheral air pattern  96 . 
     FIG. 15 illustrates FIG. 11 after removal of housing surface  46 , resulting in a partially open system having housing surface  44 . As a consequence, the electrical structure  10  in FIG. 15 includes an air circulation pattern similar to that in FIG. 11 between the housing surface  44  and the electronic carrier assembly  20 , and an air circulation pattern similar to that in FIG. 14 on the open side (i.e., side lacking a housing surface) of the electronic carrier assembly  20 . 
     FIG. 16 illustrates FIG. 1 with the addition of a fan  110 . The disk  60  is interposed between the fan  110  and the electronic carrier assembly  20 . An operating fan forms a steady flow component of air circulation along a surface, such as top surface  40  of the electronic carrier assembly  20 , while operating at an operating frequency. Any type of fan may be used, including a conventional fan that has a rotatable curved blade that rotates at the fan operating frequency. The fan operating frequency may be greater than, equal to, or less than the disk rotational frequency f d . For some applications, it may be preferred to have the fan operating frequency substantially exceed f d , such as by at least about an order of magnitude, since a low-frequency pulsation may provide an acceptable improvement in the rate of heat dissipation from the electronic carrier assembly  20 . The disk  60  may exist as mechanically separated from the fan  110 , or may be physically attached to the fan  110 . An example of the latter situation is where the disk  60  serves as a cover for the fan  110 , as illustrated in FIG.  26 . FIG. 26 shows a semicircular rotatable disk  69  as a cover to the fan  110 . The fan  110  may be either a push-type pan or a pull-type fan. A push-type fan pushes air along the top surface  40  and bottom surface  42  of the electronic carrier assembly  20 , by directing the flow of air from the fan  110  toward the electronic carrier assembly  20  in the direction  200 . A pull-type fan pulls air along the top surface  40  and bottom surface  42  of the electronic carrier assembly  20 , by directing the flow of air toward the fan in the direction  210 . 
     Although the fan  110  is used in FIG. 16, any device capable of generating a steady air flow may be used. For example, a device that establishes and maintains a pressure gradient, such as a pump in a closed loop, is capable of generating a steady air flow. 
     Although FIG. 16 shows a horizontally-oriented configuration, the electrical structure  10  of FIG. 16 will operate similarly in any angular orientation, because the theory of operation is based on forced convection. With forced convection, gravitational effects are negligible. In contrast, gravitational effects control natural convection such that air circulation is significantly different for the horizontal and vertical orientations of the electrical structure  10 , as explained supra in the discussion of FIG.  11 . Since each configuration in FIGS. 16-25, to be discussed infra, includes at least one fan, forced convection dominates the configurations of FIGS. 16-25. Thus, the flow patterns in each of FIGS. 16-25 are insensitive to the angular orientation of the electrical structure  10 . 
     FIGS. 17 and 18 illustrate the air flow patterns of FIG. 16 at two different times in the cycle of fan rotation. The hardware configuration of FIGS. 17 and 18 is essentially the same as that of FIG. 16 with the exception of a different arrangement of electronic components on the electronic carrier  22 . FIG. 17 shows a stronger air flow pattern  131  in the upper half  130  of the electronic structure  10  than the relatively weaker air flow pattern  136  in the lower half  135  of the electronic structure  10 . FIG. 18, which provides a snapshot at a different time in the cycle of fan rotation from that of FIG. 17, shows a weaker air flow pattern  132  in the upper half  130  of the electronic structure  10  than the relatively stronger air flow pattern  137  in the lower half  135  of the electronic structure  10 . The difference in flow patterns in FIGS. 17 and 18 is due to the difference in locations of the solid portion(s) of the disk  60  at the different snapshot times associated with FIGS. 17 and 18. With the steady flow of air generated by the operating fan  110 , combined with the pulsating air pattern induced by the rotating disk  60 , the rate of heat transfer from the electronic carrier assembly  20  is increased in several ways. One way is by reducing the thickness of the boundary layer along the top surface  40  and bottom surface  42  of the electronic carrier assembly  20 . Another way is by sweeping stagnant air out of the stagnation zones, such as stagnation zone  56 , between successive pairs of electronic components. 
     FIGS. 19-22 constitute modifications of FIG. 16 which illustrate various configurations of fans and rotatable disks. FIG. 19 shows FIG. 16 with rotatable disk  60  replaced by rotatable disk  160  such that the electronic carrier assembly  20  is interposed between the fan  110  and the disk  160 . 
     FIG. 20 shows FIG. 16 with a second rotatable disk  161  positioned such that the electronic carrier assembly  20  is interposed between the rotatable disk  60  and the second rotatable disk  161 . The second rotatable disk  161  has any of the features available to the disk  60 . FIG. 21 shows FIG. 16 with a second fan  170  positioned such that the electronic carrier assembly  20  is interposed between the rotatable disk  60  and the fan  170 . The second fan  170  has any of the features available to fan  110 . The second fan  170  is for generating a second steady flow component of air circulation along a surface, such as the top surface  40 , of the electronic air carrier assembly  20 . If the fan  110  is a push-type fan pushing air in the direction  300 , then the second fan  170  should be a pull-type fan pulling air in the direction  300 . If the fan  110  is a pull-type fan pulling air in the direction  310 , then the second fan  170  should be a push-type fan pushing air in the direction  310 . 
     FIG. 22 shows FIG. 16 with a second fan  171  and a second rotatable disk  162 . The electronic carrier assembly  20  is interposed between the rotatable disk  60  and the second rotatable disk  162 . The second rotatable disk  162  is interposed between the electronic carrier assembly  20  and the second fan  171 . The second rotatable disk  162  has any of the features available to the disk  60 . The second fan  171  has any of the features available to the fan  110 . If the fan  110  is a push-type fan pushing air in the direction  400 , then the second fan  171  should be a pull-type fan pulling air in the direction  400 . If the fan  110  is a pull-type fan pulling air in the direction  410 , then the second fan  171  should be a push-type fan pushing air in the direction  410 . FIG. 23 shows FIG. 16 with lower housing surface  44  and upper housing surface  46  removed. Thus the electrical structure  10  of FIG. 23 is an open system. The fan  110  should be a push-type fan pushing air in the direction  500 , since a pull-type fan would draw peripheral air from locations external to the electrical structure  10 . For example, FIG. 24 illustrates FIG. 23 when the fan  110  is a pull-type fan pulling air in the direction  510 . FIG. 24 shows the consequent peripheral air flow pattern  180 . Note that the peripheral air flow pattern  180  is not drawn along the electrical carrier assembly  20  and is therefore not very effective in dissipating heat from the electrical carrier assembly  20 . In contrast, the push-type fan  110  in FIG. 23 is capable of directing air along the electrical carrier assembly  20 . 
     FIG. 25 shows FIG. 16 with upper housing surface  44  removed, such that lower housing surface  46  remains. Thus the electrical structure  10  of FIG. 23 is a partially open system. With electronic components  24  and  26  coupled to the top surface  40  of the electrical carrier assembly  20 , the fan  110  should be a push-type fan pushing air in the direction  600 , in order to provide effective cooling along the top surface  40  of the electrical carrier assembly  20 . 
     While preferred and particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.