Abstract:
A fluid flow management system for enhancing cooling of a circuit pack. The system includes a bar extending transverse to the direction of fluid flow. The bar breaks up the otherwise laminar flow into a turbulent flow.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an improvement in fluid flow management for heat producing circuit packs of an electronic system. 
     Modern electronic equipment, such as for telecommunications purposes, is typically constructed with modular circuit packs having electronic components mounted to a circuit board. The circuit packs are installed in racks to make up the overall system. Many of the electronic components generate heat, and it is therefore necessary to remove the heat from the equipment to prevent damaging heat sensitive components. A typical method for cooling electronic equipment is by forcing a fluid, usually air, over the heat generating electronic components. The rate at which heat is removed from those components is based on the heat transfer coefficient, the surface area of the components in contact with the fluid, and the temperature difference between the fluid and the components. A problem with this method of cooling is that the rate at which fluid is moving over the components causes the flow to be laminar. This means that the heat transfer coefficient is low and therefore the cooling of the electronic devices is limited, unless a very expensive and power hungry fan system is employed. This type of system is particularly expensive when only one circuit pack, out of up to twenty in a typical rack, needs the increased amount of fluid flow. Accordingly, a need exists for an inexpensive and efficient way of improving the cooling of a circuit pack. 
     SUMMARY OF THE INVENTION 
     According to the present invention, additional fluid flow is not required for enhanced cooling. Instead, the laminar flow is changed into turbulent flow. Thus, according to the present invention, there is provided a fluid flow management system for a circuit pack having electronic components mounted to a circuit board, wherein fluid flow across the circuit pack is in a defined direction. The inventive system comprises a turbulator having mounting members. The mounting members are adapted to secure the turbulator to the circuit board. 
     In accordance with an aspect of this invention, the turbulator includes an elongated cross bar. 
     In accordance with another aspect of this invention, the turbulator cross bar has a chamfer on an upstream-facing side to direct the fluid flow toward the circuit board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings in which like elements in different figures thereof are identified by the same reference numeral and wherein: 
     FIG. 1 schematically depicts fluid flow across a circuit board without the inventive system being installed, and shows the boundary layer of the laminar flow; 
     FIG. 2 is a perspective view of a first embodiment of an inventive turbulator; 
     FIG. 2A is a cross sectional view taken along the line  2 A— 2 A in FIG. 2; 
     FIG. 3 is a perspective view of a circuit board having the first embodiment of the inventive fluid flow management system installed thereon; 
     FIG. 4 is a view similar to FIG. 1 showing the fluid flow across a circuit board having the inventive fluid flow management system installed thereon; 
     FIG. 5 is a graph showing boundary layer height as a function of fluid flow velocity along a flat plate and is used to illustrate how the turbulator dimensions are selected; 
     FIG. 6 is a perspective view of a second embodiment of an inventive turbulator; 
     FIG. 7 is an exploded perspective view of the turbulator shown in FIG. 6; and 
     FIGS. 8-13 show various mounting configurations for the turbulator shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a circuit board  10  with electronic components  12  mounted thereon. The line  14  represents the boundary layer of fluid flowing from right to left over the circuit board  10 . Note that the boundary layer  14  is smooth over the electronic components  12 . This indicates that the flow between the boundary layer  14  and the circuit board  10  is laminar. As previously discussed, such laminar fluid flow results in a low heat transfer coefficient. 
     FIG. 2 illustrates a first embodiment of an inventive turbulator, designated generally by the reference numeral  20 , designed for mounting to the circuit board  10  so as to distrupt the fluid flow boundary layer  14  and create turbulent, rather than laminar, fluid flow which increases the heat transfer coefficient. The turbulator  20  has an elongated cross bar  22  and mounting structure  24 ,  26  spaced along the bar  22 . Illustratively, the turbulator  20  is molded of plastic as a single unitary piece with the mounting structure  24 ,  26  at opposite ends of the bar  22 . The bar  22  has a chamfer  28  on one face extending from approximately the horizontal centerline of that face to the bottom of the lower edge  30 . Illustratively, the angle θ of the chamfer  28  is about 37°. 
     To install the turbulator  20  onto the circuit board  10 , as shown in FIG. 3 an elongated rail  32  is mounted to the circuit board  10  parallel to the direction of fluid flow, which is in the direction shown by the arrow  34 . Spaced from the rail  32 , and across the electronic components which are being cooled, an array of through-holes  36  are formed in the circuit board  10 . The through-holes  36  extend along a line which is parallel to the rail  32  and at a distance equal to the length of the turbulator  20 . The mounting structure  24  includes a block-like member which is bifurcated to have two depending legs  38 ,  40 . The distal ends of the legs  38 ,  40  are formed with camming surfaces  42 ,  44  respectively, terminated by shoulders  46 ,  48  to leave a central opening  50  complemental to the rail  32 . Thus, as the mounting structure  24  is moved downwardly over the rail  32 , the legs  38 ,  40  are spread apart by the action of the camming surfaces  42 ,  44  and the rail  32  passes into the opening  50 . As the shoulders  46 ,  48  pass the lower edge of the rail  32 , the legs  38 ,  40  move toward each other, capturing the rail  32  within the opening  50  in a snap fit manner. At the other end of the cross bar  22 , the mounting structure  26  is formed as a post with a split snap fit feature  52  at its distal end which is adapted for receipt in one of the through-holes  36 . Thus, the turbulator  20  can be installed on the circuit board  10  at a selected position along the fluid flow path  34 . When the turbulator  20  is so installed, it is oriented so that the chamfer  28  is facing the upstream direction of the fluid flow. This creates a downward vector of the fluid flow toward the circuit board  10 . 
     FIG. 4 illustrates the fluid flow over the printed circuit board  10  after installation of the turbulator  20 . As shown, the line  54  represents the boundary layer of the fluid flow upstream from the turbulator  20  and it is seen to be a laminar flow. However, the turbulator  20  disrupts the laminar flow and causes it to be turbulent downstream from the turbulator  20 , as shown by the wave-like design  56 . This turbulent flow increases the heat transfer coefficient to enhance the cooling of the electronic components  12  mounted to the circuit board  10 . 
     FIG. 5 is a graph showing boundary layer height as a function of distance along the circuit board  10  for the configuration shown in FIG. 1 without the turbulator  20  being installed. Each of the four curves  58 ,  60 ,  62 ,  64  shows the boundary layer for a different respective fluid flow velocity, with the curve  58  being for the lowest fluid flow velocity and the curve  64  being for the highest fluid flow velocity. The broken circle  66  shows the placement and dimensions for a turbulator which will intersect the boundary layers over the range of fluid flows which can be expected to be encountered. 
     The aforedescribed turbulator increases the heat transfer coefficient, which lowers the device temperature and thus decreases the effects of one of the factors in component reliability (i.e., high temperature), without requiring additional fluid flow. It should be noted that the inventive turbulator is designed for use where forced convective cooling is in use. A positive aspect of the inventive turbulator is that the flow enhancing device is added on the circuit pack only when required, and its cost is only pennies per circuit pack. This low cost should be compared to the cost of ten dollars to twenty dollars per circuit pack if an additional fluid moving system is employed. (This assumes that all twenty circuit packs in a rack need the additional fluid flow.) 
     FIGS. 6-13 show a second embodiment of an inventive turbulator, designated generally by the reference numeral  70 . As shown in FIG. 7, the turbulator  70  includes a pair of mounting posts  72 ,  74 , each topped by a respective ball socket  76 ,  78 , and each terminated at its lower end by a respective pin  80 ,  82  adapted for receipt in a through-hole of the circuit board  10 . Alternatively, instead of the pins  80 ,  82 , the mounting posts  72 ,  74  can be terminated by a snap-fit feature similar to the snap-fit feature  52  (FIG. 2) of the turbulator  20 . 
     The turbulator  70  further includes a pair of rods  84 ,  86  each terminated at one end by a respective ball  88 ,  90  adapted for receipt in one of the sockets  76 ,  78 . The rod  86  is terminated at its end opposite the ball  88  by a coaxial smaller diameter rod  92  and the rod  84  is hollow so that it can receive therein in a telescoping manner the rod  92 , so as to selectively vary the distance between the balls  88 ,  90 . The turbulator  70  also includes a cross bar  94  having a chamfer  96  on a first face  98  and a circular slot  100  on an opposite face  102 . The chamfer  96  is similar in design and purpose to the chamfer  28  of the turbulator  20 . The slot  100  is dimensioned so that the cross bar  94  can be snap-fit over the rods  84 ,  86 , which are of equal diameter. This arrangement allows the cross bar  94  to be rotated about the longitudinal axis defined by the rods  84 ,  86 . Preferably, the dimensions of the slot  100  and the rods  84 ,  86  are such that there is frictional engagement between the cross bar  94  and the rods  84 ,  86  to maintain the cross bar  94  at whatever rotational angle it has assumed. 
     The turbulator  70  is advantageous in that it can be configured for particular fluid flow patterns. As shown in FIGS. 8-13, different patterns of through-holes can be provided in the circuit board  10  so that the turbulator  70  is rotatable about an axis perpendicular to the circuit board  10  to direct the fluid flow to a specific area on the circuit board  10  where a lot of heat is generated. Thus, as shown in FIGS. 8-10, with the through-holes  104  arrayed along a circular arc centered at the through-hole  106 , a fixed length turbulator  70  can be rotated about an axis passing through the through-hole  106 . As shown in FIGS. 11-13, with the through-holes  108  in a linear array, the turbulator  70  can be rotated about an axis extending through the through-hole  110  by changing the length of the turbulator  70 . Further, by increasing the length of the post  112  (FIG.  13 ), the turbulator  70  can be rotated about an axis parallel to the circuit board  10 . 
     By rotating the cross bar  98  about the longitudinal axis of the rods  84 ,  86 , this changes the angle of the chamfer  96  and such rotation can be utilized to reduce “congestion” in conditions of low fluid flow or situations where there are “tall” electronic components. By having the turbulator  70  of variable length, this can accommodate different widths of the channel through which fluid flow is directed. In the event a longer length turbulator  70  is required, an additional cross bar  94  can be snapped onto the rods  84 ,  86 . 
     It has also been found that if a plurality of turbulators  20 ,  70  are mounted to the circuit board  10  in a spaced array along the direction of fluid flow, enhanced cooling is effected. 
     Accordingly, there has been disclosed an improved fluid flow management system for a circuit pack. While exemplary embodiments of the present invention have been disclosed herein, it will be appreciated that various adaptations and modifications to the disclosed embodiments are possible and it is intended that this invention be limited only by the scope of the appended claims.