Abstract:
A nozzle useful for enhancing localized cooling of electronic elements and the like is provided. The nozzle is mountable on a substantially planar face of a substrate. It is conformed to intercept a fluid flowing across said face and redirect it toward the substrate. The nozzle comprises a hood, an output window, and between them, a transition region. The transition region is conformed to turn the flowing fluid toward the substrate and to contract the fluid flow.

Description:
FIELD OF THE INVENTION 
   This invention relates to structures for diverting flows of coolant fluid to deliver the fluid to local hot spots, and to systems using such structures. More specifically, the invention relates to structures for improving the efficacy of forced air cooling in a circuit stack. 
   ART BACKGROUND 
   Electronic circuitry for applications such as signal processing, computing, and telephony often includes circuit boards that are densely populated with heat-generating components. The passive or active removal of excess heat from the environment of such circuit boards is often necessary to assure the proper operation of the circuitry. 
   Forced convection is a widely used active method of heat removal. In typical applications, circuit boards are stacked in parallel fashion within an enclosure, and fans are used to force air to flow, in a direction parallel to the faces of the boards through the gaps between successive boards. 
   Such an approach is most useful when the heat sources are uniformly distributed across the face of each circuit board. However, it is becoming increasingly common to design circuit boards in which an isolated hot spot, corresponding for example to a high-speed processor or a high-power component, generates substantially more heat per unit area than is average for the circuit board on which it is mounted. 
   There remains a need for more effective targeted cooling of such isolated hot spots. 
   SUMMARY OF THE INVENTION 
   We have developed a method and apparatus for targeted cooling of individual hot spots mounted on a planar substrate such as a circuit board. Our method can be employed together with the cooling, by forced convection, of a circuit board or a plurality of stacked circuit boards without substantially interfering with the cooling of other boards or of other components on the same board. 
   In one aspect, our invention involves the use of a shaped nozzle which is conformed to intercept air or another coolant fluid flowing across the face of the substrate and redirect it toward the substrate. The nozzle comprises a hood, a transition region, and an output window. In air, for example, airflow entering the nozzle is first intercepted by the hood, which defines an input cross section for the entering airflow. The airflow is turned through at least 45 degrees in the transition region and concentrated such that it is output through the output window with an output cross section that is no more than the input cross section. In specific embodiments of the invention, the nozzle is conformed to concentrate the airflow, e.g. by a contraction ratio of 2 or more, in substantially the same manner as a straight, i.e., non-turning, nozzle having a polynomial profile. In specific embodiments of the invention, the polynomial profile is bilaterally symmetric. 
   In a second aspect, our invention involves cooling a heat-generating element by using a nozzle of the kind described above to deflect the flow of a coolant fluid toward the substrate on which the heat-generating element is mounted. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a simplified perspective drawing of a stack of circuit boards cooled by forced convection. Included as a detail in  FIG. 1  is a simplified elevational view of an individual element on one of the circuit boards together with a cooling nozzle according to the invention in an exemplary embodiment. 
       FIG. 2  is a perspective view of a nozzle according to the present invention in an exemplary embodiment. For convenience, the nozzle is shown in an inverted orientation, such that in use, the substrate on which the nozzle is mounted would lie above the nozzle as seen in the figure. 
       FIG. 3  is an elevational view of a portion of the nozzle of  FIG. 2  as seen by an observer looking along the direction of the entering airflow, with the nozzle inverted such that the exit window is facing upward. 
       FIG. 4  is a cross section of a nozzle having a cubic profile. 
       FIG. 5  is an elevational view of the nozzle of  FIG. 2  mounted on a circuit board, as seen by an observer looking along the direction of the entering airflow. The exit window is facing downward as seen in the figure, and the circuit board is visible at the bottom of the figure. 
       FIG. 6  shows experimental results obtained using a nozzle as described here to cool a hot spot on a circuit board. Plotted in the figure is the difference between local and ambient temperature, expressed as a percentage of ambient temperature. 
   

   DETAILED DESCRIPTION 
   Localized cooling is useful, among other places, in stacks of circuit boards  10 - 16 , as shown in  FIG. 1 . The circuit boards of  FIG. 1  are mounted within an enclosure (not shown) and cooled by forced convection with a principal airflow direction as indicated by arrow  20 . 
   It should be noted that although the present invention will find application in air-cooled environments, the scope of the invention is not so limited, because its principles are readily applicable to environments cooled by helium, nitrogen, or other gases, or by water, liquid halocarbons, or other liquids. Moreover, although the nozzle to be described here will often be mounted on a circuit board and used to cool an electronic element, it can readily be mounted on any of various substrates having substantially planar surfaces and used to enhance the cooling of local hot spots of various kinds, including heat sinks. 
   As shown in the figure, a portion of the airflow, which initially runs parallel to the faces of the circuit boards, is intercepted by nozzle  25  and directed toward substrate  14  in the vicinity of element  30 , which is for example an electronic integrated circuit. Arrow  35  is included for illustrative purposes to provide a qualitative representation of the airflow diverted by nozzle  25 , but it is not meant to be a realistic or limiting representation thereof. 
   Nozzle  25  is shown in more detail in  FIG. 2 . As noted, the nozzle is shown in an inverted orientation. Airflow through the circuit enclosure is incident from the left as seen in the figure, and the circuit board or other mounting surface, not shown, lies above the nozzle as seen in the figure. Incident airflow is intercepted by a hood region, which is defined by ceiling  40  and walls  45 A and  45 B. The ceiling and walls form three sides of a rectangle. The area of the rectangle which is so-defined is referred to here as the “input area.” The input area is approximately the cross-sectional area of the airflow portion which is intercepted by the nozzle. 
   We found that the performance of the nozzle was sensitive to the design of the hood, because the airflow would stall if the depth of the hood, in the input airflow direction, was not set properly. A specific example of a properly dimensioned hood will be provided below. 
   Airflow which has been intercepted by the hood region is directed through a transition region defined by turning surface  60  and sidewalls  75 A and  75 B. The transition region turns the airflow before exhausting it through window  50 , which is formed as an aperture in floor  55 . We have successfully tested a prototype nozzle which turns the airflow through an angle of 90 degrees. However, we believe that even smaller turning angles will be useful for at least some applications. That is, we believe that useful nozzles can be made which turn the airflow toward the substrate by any angle from 45 degrees to 90 degrees. 
   As an aide to interpretation of the figure, we have designated by reference numeral  65  a portion of turning surface  60  which is substantially parallel to the substrate surface (i.e., in this example the mounting surface of the circuit board), and by reference numeral  70  a portion which is substantially perpendicular to the substrate surface. Between portions  65  and  70 , turning surface  60  executes a smooth arc, advantageously a circular arc, of 90 degrees. 
   As will be seen in the figure, exit window  50  has three sides, which correspond to three sides of a rectangle, which in our experimental prototype is almost square. The area of the rectangle which is so-defined is referred to here as the “output area.” The output area is the cross-sectional area of the airflow portion which is exhausted by the nozzle. 
   In our experimental prototype, the ratio of the input area to the output area, referred to as a “contraction ratio,” is about 4. We have found that the increased flow velocity that results when we contract by such a ratio provides an advantageous enhancement of the cooling rate. More generally, we believe that useful performance can be achieved, for at least some applications, with contraction ratios as small as 1:1 and as large as 8:1 or more. At least for air cooling, we believe that contraction ratios as low as 2 will often perform favorably relative to, e.g., heat sinks. 
   Sidewalls  75 A and  75 B are shown more clearly in  FIG. 3 . The shape of each sidewall may be thought of as an initially rectangular strip which is bent through 90 degrees and simultaneously twisted through 90 degrees. As a result of such bending and twisting, leading edge  80  of, e.g., sidewall  75 A is oriented vertically as seen in the figure, whereas trailing edge projects perpendicularly out of the plane of the figure toward the viewer. 
   Sidewalls  75 A and  75 B also converge toward each other in a symmetrical manner to concentrate the airflow while it is being turned. We have found it advantageous to conform sidewalls  75 A and  75 B according to a bilaterally symmetric, cubic profile. That is,  FIG. 4  shows a profile for an example of an equivalent straight, i.e., non-turning, nozzle. The sidewalls of such a nozzle follow, respectively, the upper and lower curves of  FIG. 4 . Each of these curves is a sigmoid curve defined by a polynomial equation or pair of polynomial equations. As is known in the art of wind-tunnel design, such polynomials are advantageously of at least third degree. It is advantageous for the width of the channel defined between sidewalls  75 A and  75 B to have a similar profile, the necessary changes being made to account for the turning of the channel. Accordingly, a transition region is herein considered to be equivalent to a channel having a bilaterally symmetric, polynomial profile if, in addition to turning through 90 degrees or some other turning angle, the channel sidewalls follow a symmetric pair of sigmoid curves that deviate by no more than 10% from polynomial curves. 
   Turning back to  FIG. 3 , also visible in the figure are chamfers  90 A and  90 B. The chamfers are undercuts in the body of the nozzle conformed to facilitate airflow, beneath the nozzle, in the direction parallel to the substrate surface. Appropriate shapes for the chamfered surfaces will be known to those of skill in the art and need not be described here in detail. 
   Turning now to  FIG. 5 , shown there is a view of nozzle  25  mounted on circuit board  14 . (Like reference numerals are used to indicate features shown in previous figures.) In the embodiment depicted, projections  95  and  100 , integral with the body of the nozzle, are used to facilitate mounting on the circuit board. Projection  100  engages mounting guide, rail, or clip  105  which projects upward from the circuit board. Projection  95  mounts directly on the circuit board. It will be understood from the figure that the mounting is carried out so as to leave a vertical standoff distance αbetween the upper surface of the element  30  which is to be cooled, and the bottom of the nozzle. We have found that cooling is facilitated by a standoff distance which permits some air to flow, parallel to the substrate surface, between the nozzle and the cooled element. The precise standoff distance that is optimal will depend on the height of element  30 , the airflow velocity, and the rate of heat generation by element  30 . A specific example of a standoff distance that we found useful will be provided below. 
   In empirical tests, we determined that it was most advantageous to position the back edge of the output window at the trailing edge of the element to be cooled. In general, however, the precise position that is most advantageous may depend on various factors such as the dimensions of the hot element, the airflow velocity, and the rate of heat generation by the element. 
   EXAMPLE 
   Our prototype nozzle was designed for a nominal airflow velocity of about 2 meters per second. The input window was 4.4 cm wide in the direction parallel to the substrate, and 0.9 cm high in the direction perpendicular to the substrate. The output window was 1.0 cm wide in the direction transverse to the input airflow, and 0.9 cm long in the direction parallel to the input airflow. Accordingly, the contraction ratio, or the ratio of input to output areas, was 4.4. 
   The depth of the hood, i.e. as measured parallel to the input airflow direction, was 0.85 cm. 
   The standoff distance αwas 2 mm, and the upper surface of the hot element stood 3.5 mm above the circuit board. 
   The length of the transition region, as measured in the direction of the input airflow (i.e., parallel to the circuit board) from the trailing end of the hood to the trailing end of the output window, was 1.3 cm. 
   The nozzle was designed according to the well-known rules for contraction nozzles published in T. Morel, “Design of Two-Dimensional Wind Tunnel Contractions,”  J. Fluids Eng.  99, vol. 2, 371-378 (1977). The nozzle profile had the equations: 
   
     
       
         
           
             
               
                 H 
                 - 
                 
                   H 
                   2 
                 
               
               
                 
                   H 
                   1 
                 
                 - 
                 
                   H 
                   2 
                 
               
             
             = 
             
               1 
               - 
               
                 
                   x 
                   3 
                 
                 
                   
                     X 
                     2 
                   
                   ⁢ 
                   
                     L 
                     3 
                   
                 
               
             
           
           , 
           
             
               
                 x 
                 L 
               
               ≤ 
               X 
             
             ; 
           
         
       
     
     
       
         
           
             
               
                 H 
                 - 
                 
                   H 
                   2 
                 
               
               
                 
                   H 
                   1 
                 
                 - 
                 
                   H 
                   2 
                 
               
             
             = 
             
               
                 
                   ( 
                   
                     1 
                     - 
                     
                       x 
                       L 
                     
                   
                   ) 
                 
                 3 
               
               
                 
                   ( 
                   
                     1 
                     - 
                     X 
                   
                   ) 
                 
                 2 
               
             
           
           , 
           
             
               x 
               L 
             
             &gt; 
             
               X 
               . 
             
           
         
       
     
   
   In the preceding equations, H represents width of the nozzle (the height is assumed constant), and x represents axial position. The parameters H 1  and H 2  are respectively, the pertinent dimensions of the input and output windows, and L is the length of the nozzle. The parameter X is the location of a pressure minimum in the nozzle, normalized to the length of the nozzle. It is the match point between the two curves specified in the above pair of equations. In our design, the parameters had the following values: 
   
     
       
             
             
             
             
           
         
             
                 
             
           
           
             
               H 1  = 4.445 cm 
               H 2  = 0.9525 cm 
               X = 0.610174 
               L = 6.136358 cm 
             
             
                 
             
           
        
       
     
   
   It should be noted that our exemplary embodiment of the invention using cubic contraction-nozzle profiles is not meant to be limiting. Instead, we believe that other profiles effective for contraction nozzles, including profiles based on even higher-order polynomials, will also be effective in the present context and thus fall within the scope and spirit of the present invention. 
     FIG. 6  shows experimental results obtained using a nozzle of the kind described above to cool a hot spot on a circuit board. Plotted in the figure is the difference between local and ambient temperature, expressed as a percentage of ambient temperature. This so-called “percent differential temperature” is plotted versus position along a circuit pack in the downwind direction. Curve A represents temperature deviations near the hot spot when no means for enhancing the cooling of the circuit pack are employed. Curve B represents the results when a heat sink is used at the hot spot. Curve C represents the results when a nozzle according to the present invention is used. It will be understood from the figure that the nozzle outperforms the heat sink not only at the hot spot, but also at locations downstream of the hot spot, where the heat sink offers no improvement.