Abstract:
Effective utilization of a parallel flow air-cooled microchannel array at the micro electro mechanical systems (MEMS) scale is prohibited by unfavorable flow patterns in simple rectangular arrays. The primary problem encountered is the inability of the flow stream to penetrate a sufficient depth into the fin core to achieve the desired fin efficiency. Embodiments of the present invention overcome this problem using a manifold with open nozzle discharge and integrated lateral exhaust along with a microchannel array cooler with micro spreading cavities for internal air distribution.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/238,096, entitled “Architecture for Gas Cooled Parallel Microchannel Array Cooler,” filed on Aug. 28, 2009 in the U.S. Patent and Trademark Office, the entire content of which is herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Aspects of embodiments of the present invention relate to cooling systems. More specifically, aspects of embodiments of the present invention relate to microchannel heat sink cooling systems. 
         [0004]    2. Description of Related Art 
         [0005]    Modern electronics, such as computer processor chips, are capable of generating enormous amounts of heat in a small amount of space. Cooling such components can present many design challenges. Environmental factors can further complicate design considerations. For instance, on military aircraft, where the size, weight, power consumption, and durability of any hardware (such as cooling systems) are factors, features such as small size, light weight, low power consumption, and rugged design are benefits. Air cooling is especially attractive on such platforms. Otherwise, fuel or another fluid that can serve as a coolant must be routed or configured in such a fashion as to remove the heat. 
         [0006]    Two examples of cooling systems for electronics and other heat sources include microchannel heat sinks and jet impingement cooling. With microchannel heat sinks, coolant (for example, air) is directed between microchannels, that is, narrow openings between closely spaced surfaces (for example, thin fins of metal) that are heated from the electronics. Microchannel heat sinks rely on the large surface area exposed to the coolant to dissipate the heat. Sufficient coolant pressure must be present to carry away heat from the surface area via the microchannel. Jet impingement cooling, on the other hand, uses narrow high-speed jets of coolant directed at the heat source, causing rapid cooling, limited primarily to the region receiving the direct impact of the jets. Microchannel heat sinks can suffer from insufficient coolant pressure to dissipate the heat from the large surface area while jet impingement cooling can suffer from the somewhat localized area of heat rejection. 
         [0007]    Effective air cooled microchannel arrays with parallel flow on a micro electro mechanical systems (MEMS) scale, however, is prohibited by unfavorable flow patterns in simple rectangular arrays of fins. The primary problem encountered is the inability of the flow stream to penetrate a sufficient depth into the fin core to achieve the desired fin efficiency. 
         [0008]    Thus, there is a need for a compact, air-cooled, microchannel heat sink with a favorable flow pattern that uses minimal air mover power and minimal flow rate. 
       SUMMARY 
       [0009]    Embodiments of the present invention reduce or overcome these problems using a separate manifold structure to deliver cool air and exhaust warm air, combined with a microchannel array cooler that uses micro-plenums (spreading cavities) integrated into the microchannel core, which allows segregated circular or slot nozzle delivered airflow jets to distribute into the microchannel core to achieve maximum core performance. This allows a compact design to reject a large amount of heat with minimal pressure drop. In addition, the separate manifold is isolated from direct contact with the cooler, leading to a lower flow resistance design that requires less air mover power without a reduction in thermal performance. 
         [0010]    Designing the cool air delivery passages (or channels) in the manifold in a complementary sawtooth pattern with the warm air exhaust passages improves both cool air delivery through nozzles in the manifold and warm air exhaustion through openings in the manifold. Power consumption can be kept low by using an array of small fans to provide the airflow. Such a system is capable of rejecting a large amount of heat in a small amount of space using minimal air mover electrical power. In addition, the system is capable of using ambient air as the coolant. Also, the manifold and cooler can be made of durable components that do not need to maintain physical contact, thus greatly reducing the risk of damage from shock or vibration. 
         [0011]    In an exemplary embodiment according to the present invention, a parallel microchannel array cooler is provided. The cooler is for cooling a heat source with a gas that is cooler than the heat source. The cooler includes a base and one or more rows in a widthwise direction. The base is for transferring heat from the heat source. Each of the one or more rows includes fins in a lengthwise direction and a micro-plenum in the lengthwise direction. The fins are separated by microchannels. The fins are for dissipating the heat from the base. The micro-plenum is for dispersing the gas to the fins via the microchannels. The gas is for transferring the heat from the fins. 
         [0012]    The cooler may further include a structural cap for protecting the fins from handling damage. 
         [0013]    The cooler may be monolithic. 
         [0014]    The fins may include notches such that adjacent rows of the one or more rows include V-grooves in the lengthwise direction between the adjacent rows. The V-grooves are for facilitating removing of the heated gas from the cooler. 
         [0015]    The one or more rows may include a plurality of rows. 
         [0016]    In another exemplary embodiment according to the present invention, a gas cooling device is provided. The gas cooling device is for cooling a heat source with a gas that is cooler than the heat source. The gas cooling device includes a parallel microchannel array cooler and a manifold. The cooler includes a base and one or more rows in a widthwise direction. The base is for transferring heat from the heat source. Each of the one or more rows includes fins in a lengthwise direction and a micro-plenum in the lengthwise direction. The fins are separated by microchannels. The fins are for dissipating the heat from the base. The micro-plenum is for dispersing the gas to the fins via the microchannels. The gas is for transferring the heat from the fins. The manifold includes nozzles and openings. The nozzles are for delivering jets of the gas to the cooler. Each of the nozzles is for delivering one of the jets to a corresponding receiving point in the micro-plenum of one of the one or more rows. The openings are for removing the gas delivered to the cooler. The manifold is separated from the cooler by a fixed distance. 
         [0017]    The jets may be substantially parallel to one another. 
         [0018]    The fixed distance may be between about 25 mils and about 100 mils. 
         [0019]    The gas may be air. 
         [0020]    The cooler further may further include a structural cap for protecting the fins from handling damage. 
         [0021]    The cooler may be monolithic. 
         [0022]    The fins may include notches such that adjacent rows of the one or more rows include V-grooves in the lengthwise direction between the adjacent rows. The V-grooves are for facilitating the removing of the heated gas from the cooler via the openings. 
         [0023]    The V-grooves may correspond to the openings. 
         [0024]    The gas cooling device may further include a plurality of coolers. 
         [0025]    The one or more rows may include a plurality of rows. 
         [0026]    The manifold may further include a first set of nozzles and a second set of nozzles. The first set of channels is for delivering the gas to the nozzles. The second set of channels is for removing the gas from the openings. The first set of channels and the second set of channels may be in a complementary sawtooth arrangement. 
         [0027]    In yet another exemplary embodiment according to the present invention, an air cooling system is provided. The system is for cooling a heat source with air. The system includes a parallel microchannel array cooler, manifold, and an air mover. The parallel microchannel array cooler includes a base and one or more rows in a widthwise direction. The base is for transferring heat from the heat source. Each of the one or more rows includes fins in a lengthwise direction and a micro-plenum in the lengthwise direction. The fins are separated by microchannels. The fins are for dissipating the heat from the base. The micro-plenum is for dispersing the air to the fins via the microchannels. The air is for transferring the heat from the fins. The manifold includes nozzles and openings. The nozzles are for delivering jets of air to the cooler. Each of the nozzles is for delivering one of the jets to a corresponding receiving point in the micro-plenum of one of the one or more rows. The openings are for removing the air delivered to the cooler. The air mover is for delivering the air to the manifold. The manifold is separated from the cooler by a fixed distance. 
         [0028]    The fixed distance may be between about 25 mils and about 100 mils. 
         [0029]    The air cooling system may further include a debris screen between the air system and the manifold. 
         [0030]    The debris screen may be configured to improve distribution of an airflow. 
         [0031]    The air mover may include a fan or an array of small fans. 
         [0032]    The cooler may further include a structural cap for protecting the fins from handling damage. 
         [0033]    The fins may include notches such that adjacent rows of the one or more rows include V-grooves in the lengthwise direction between the adjacent rows. The V-grooves are for facilitating the removing of the heated gas from the cooler via the openings. 
         [0034]    The manifold may further include a first set of channels and a second set of channels. The first set of channels is for delivering the air to the nozzles. The second set of channels is for removing the air from the openings. The first set of channels and the second set of channels may be in a complementary sawtooth arrangement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The accompanying drawings illustrate embodiments of the present invention and, together with the description, serve to explain the principles of embodiments of the present invention. 
           [0036]      FIG. 1  is a side view of a parallel microchannel array cooling device according to an exemplary embodiment of the present invention. 
           [0037]      FIG. 2  is an oblique view of a parallel microchannel array cooler according to an exemplary embodiment of the present invention. 
           [0038]      FIG. 3  is a cutaway view of half of a microchannel array cooler row according to an exemplary embodiment of the present invention. 
           [0039]      FIG. 4  depicts a pair of parallel microchannel array coolers according to another exemplary embodiment of the present invention. 
           [0040]      FIG. 5 , which includes  FIGS. 5A and 5B , shows the exemplary coolers of  FIG. 4  deployed on example heat sources. 
           [0041]      FIG. 6  depicts the operation of a microchannel air cooling system according to an exemplary embodiment of the present invention. 
           [0042]      FIG. 7  shows an example small fan array suitable for use with the air cooling system of  FIG. 6 . 
           [0043]      FIG. 8  illustrates an example configuration of components of the air cooling system of  FIG. 6 . 
           [0044]      FIG. 9  is a top view of a manifold according to an exemplary embodiment of the present invention. 
           [0045]      FIG. 10 , which includes  FIGS. 10A and 10B , shows the nozzle and opening layout, the airflow, and the tapering air channels of the manifold depicted in  FIG. 9 . 
           [0046]      FIG. 11  is an illustration of the exemplary coolers of  FIG. 4  shown with a manifold according to another exemplary embodiment of the present invention. 
           [0047]      FIG. 12  illustrates an operation of the exemplary manifold of  FIG. 11  with the example coolers and heat sources of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    Exemplary embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. In the drawings, like reference numerals refer to like elements throughout. 
         [0049]    Referring now to  FIG. 1 , in a side view (i.e., a widthwise view) of an exemplary embodiment of the present invention, an air-cooled parallel microchannel array cooling device  400  is shown attached to an example heat source, in this case a ball grid array (BGA)  30  mounted on a circuit board with chassis  40 . The heat source can be anything, electronics or otherwise. While this particular cooling device uses air with which to cool, the present invention is not limited thereto. For instance, any gas may serve as a cooling medium. For purposes of this disclosure, therefore, air will serve as an example cooling gas. 
         [0050]    The cooling device  400  includes a parallel microchannel array cooler  100  and a manifold  200 . The cooler  100  includes a base  110  attached to the heat source (i.e., the BGA  30  in  FIG. 1 ). The heat source transfers heat to the base  110  through conduction. The base  110  supports five rows (in this example) in the widthwise direction, each of the rows including densely packed thin fins  120  arranged in a lengthwise direction. Each pair of (lengthwise) adjacent fins  120  is separated by a microchannel (hence, a microchannel array configuration). Much of the heat transferred to the base then transfers to the fins  120  through conduction. The cooler  100  can be monolithic (single piece) in construction, or multiple coolers can be used. The microchannel array can be built, for example, by diffusion bonding precision etched thin metal sheets or foils. 
         [0051]    The manifold  200  includes nozzles  210  (five of which are visible in  FIG. 1 , at least from this side view) that are configured to divide up a flow rate of air into multiple parallel (or substantially parallel) high-speed flows (jets) of air to receiving points on the cooler  100 , as illustrated by the blue arrows  10 , representing cool air. For example, a 2-inch-by-2-inch manifold design might break up the airflow into 50 separate jets. The incoming airflow can come from any source, such as a fan array or an air scoop. It should be noted that the manifold  200  and cooler  100  can be of comparable surface areas in some embodiments, while in other embodiments, the manifold may exchange air with numerous coolers, which can be in a variety of arrangements (for example, in an array of contiguous coolers, a separated collection of coolers, or a combination of contiguous and separated coolers). 
         [0052]    The air jets cross a gap  300  of a distance (for example, a fixed distance, which in the embodiment of  FIG. 1 , is 50 mils) to reach the cooler  100 . The gap  300  can vary from this size, say from 25 mils to 100 mils, but should be small enough that the air jets  10  do not lose pressure or are deflected, yet not too small that the manifold  200  risks impacting the cooler  100 . From there, the air jets  10  reach a micro-plenum  130 , also known as a cavity or spreading cavity, which distributes the air (in the lengthwise direction) throughout the row. A structural cap  140 , extending in the lengthwise direction along both sides of the tops of the fins  120  as well as on both ends of the row, helps protect the fins  120  from handling damage and also helps direct the airflow through the micro-plenum  130  and out the sides of the row through the microchannels. The thickness of the structural cap  140  is significantly greater than that of the fins  120 . In some embodiments, the base  110  and the structural cap  140  may be one piece. 
         [0053]    In order for the microchannel cooler  100  to reach its peak efficiency, the air should be delivered to each individual microchannel. The micro-plenum  130  (and, to a lesser extent, the structural cap  140 ) allows the cooler  100  to achieve this goal by evenly distributing the cooling air, which is delivered as jets of cooling air from the manifold  200  to receiving points of the cooler corresponding to the micro-plenum  130 . The air escapes from the cooler  100  via the microchannels, which dissipates heat from the fins  120  in the process, and returns to openings  220  in the manifold  200  (as illustrated by the red arrows  20 , representing warm air). 
         [0054]    In the exemplary embodiment of  FIG. 1 , the fins  120  are notched at their sides to form V-grooves  150  at the edges of adjacent rows, which extend in the lengthwise direction. These V-grooves  150  facilitate air to escape to the openings  220  in the manifold  200 , especially in tight assemblies. The structural cap  140  helps further direct the airflow to the V-grooves  150 . The V-grooves  150  are optional. In other embodiments, they may not be included or may take on an alternate shape. 
         [0055]      FIG. 1  also shows example lengths and heights for some of the features. For instance, the fins  120  have a height of 0.2 inch, while the nozzles  210  have a length of 0.125 inch and the rest of the manifold has a height of 0.35 inch. The gap  300  between the nozzles and the cooler  100  is 0.05 inch (50 mils). 
         [0056]      FIG. 2  depicts an oblique view of a parallel microchannel array cooler  100  according to an exemplary embodiment of the present invention. The cooler  100  includes a base  110  configured to transfer heat from a heat source (for example, a high-power processor or other electronics). Attached to the base are a set of very thin fins  120  (for example, 2 mils thick) arranged in rows and densely spaced (for example, 80 to 100 fins per inch, in the lengthwise direction) to form corresponding microchannels between the fins (for example, microchannel widths of 8 to 10.5 mils). Adjacent microchannels (that is, sharing a same fin) are substantially parallel, as are there corresponding fins. Thinner fins can allow fin densities as high as 500 fins per inch to be realized. The fins  120  are configured to dissipate heat from the base  110  with the help of rapidly moving air flowing between the microchannels via a micro-plenum (see FIGS.  1  and  3 - 4 ). 
         [0057]    Structural cap  140  extends up the ends of the rows and along the tops of the rows of fins  120 . The structural cap  140  helps protect the fins  120  from damage during handling as well as helps direct the airflow along the micro-plenum, through the microchannels, and out the sides of the rows. V-grooves  150  at the edges of the rows further facilitate heat dissipation from the fins  120 . 
         [0058]      FIG. 3  shows a cutaway view of half of a microchannel array cooler row according to an exemplary embodiment of the present invention. Micro-plenum  130  can be seen fanned between the fins  120  and the base  110 . The top portion of the structural cap  140  can be seen extending along the tops of the fins  120 . Half of a V-groove  150  can be seen in the notched side of the row of fins  120 . Example dimensions are also provided. For example, the row length is 2 inches, while half the row width is 0.2 inches (i.e., 0.4 inches of width per row, or 5 rows every 2 inches), and the cooler height (fins  120  plus base  110 ) is 0.26 inches. 
         [0059]      FIG. 4  depicts a pair of parallel microchannel array coolers  100 ′ according to another exemplary embodiment of the present invention. Each cooler  100 ′ includes a base  110 ′ with a single row of fins  120 ′ between which are the parallel microchannels. The exemplary coolers  100 ′ are smaller than those depicted earlier. For instance, each cooler  100 ′ is 0.3 inches×0.3 inches×0.07 inches high, with 80 fins per inch in an example configuration. The fins  120  are constructed, for example, using a diffusion bonded foil process. 
         [0060]      FIG. 5 , which includes  FIGS. 5A and 5B , shows the exemplary coolers  100 ′ of  FIG. 4  deployed on example heat sources  50 , which in this example illustration are computer chips. Such computer chips  50  can include, for example, gallium arsenide chips or silicon germanium chips.  FIG. 5A  depicts the chips  50  before placement of the coolers  100 ′, while  FIG. 5B  depicts the chips  50  with the coolers  100 ′ deployed on them. 
         [0061]      FIG. 6  illustrates the operation of a microchannel air cooling system  700  according to an exemplary embodiment of the present invention. In addition to previously described cooler  100  (including base  110 , fins  120 , micro-plenums  130 , structural cap  140 , and V-grooves  150 ) and manifold  200  (with nozzles  210 ), air cooling system  700  also includes an air mover array  500  of small fans and a debris screen  600 . The air mover array  500  includes several small fans configured to deliver sufficient air to the manifold to realize the air-cooling potential of the array. 
         [0062]    For instance,  FIG. 7  shows an example five-fan array  500  designed to deliver 50 cubic feet of air per minute (CFM) using no more than 33 watts (W) of power for a 4 inch square cooling system. Each individual cylindrical-shaped fan  510  in  FIG. 7  has a diameter of 1.25 inches, allowing three such fans to fit across in 4 inches of space. Thus, in the offset pattern shown in  FIG. 7 , five such fans  510  can fit in a 4 inch by 4 inch area with sufficient space between each pair of fans. The system is capable of rejecting 1000 W of heat with no more than 0.05 degrees Celsius (C) per watt of heat dissipated (i.e., no more than 50° C. total rise in temperature to reject 1000 W of heat). This measurement of efficiency is also known as thermal resistance or C/W. Such efficiency is an order of magnitude better than conventional systems. The entire package fits in a 4-inch cube, as shown in  FIG. 8 , which depicts the cooler  100 , manifold  200 , and air mover  500  in an exemplary configuration of an air cooling system  700 . 
         [0063]    Another measurement of cooling performance is the system coefficient of performance, which compares the heat (in watts) dissipated compared to the power (in watts) used to dissipate the heat. In the above example, the system coefficient is 1000 W/33 W=30. 
         [0064]    The debris screen  600  (see  FIG. 6 ) helps prevent sufficiently large objects from entering the cooling device and potentially clogging the microchannels or other air passages of the cooler  100  or manifold  200 . In addition, the debris screen  600  can be configured to help smooth the airflow, directing it more efficiently to the nozzles  210 . As can be seen in  FIG. 6 , cool or ambient air  10  is delivered from the air mover array  500 . It flows through the manifold  200  and debris screen  600 , eventually being directed to nozzles  210  of sufficiently narrow diameter (or slot width in the case of rectangular or slot nozzles) to break up the airflow into numerous high-speed jets of air directed at specific points on the cooler  100 , depicted by the blue arrows  10 . More specifically, the jets are directed to the surface of the base  110 . 
         [0065]    The high-speed jets of air do some jet impingement cooling when they hit the base  110 . In addition, the micro-plenums  130  disperse the high-speed airflow from the specific points throughout the cooler  100  and into the microchannels between the fins  120 . This dissipates heat from the fins  120  via microchannel heat sink cooling. The pressure is maintained by the return airflow, carrying the warm air  20  back to separate openings in the manifold  200  configured for that purpose, depicted by the red arrows  20  in  FIG. 6 . V-grooves  150  help with the return airflow, providing channels for the warm air to escape from the cooler  100 . 
         [0066]      FIG. 9  is a top view of a manifold  200  according to an exemplary embodiment of the present invention. The manifold  200  has 50 nozzles  210  for directing jets of air at the cooler. The nozzles  210  are arranged in five rows of 10 evenly spaced equal-sized nozzles apiece. In addition, the manifold has 14 openings  220  for returning warm air that has passed through the cooler. The openings  220  are arranged in four rows of three differently sized openings between the rows of nozzles, and a single opening at each side of the manifold  200 . The openings thus correspond to the V-grooves of the cooler while the nozzles correspond to the micro-plenums when the manifold  200  and the cooler are brought together to function as a cooling device. In addition,  FIG. 9  depicts a counter-tapering (“sawtooth”) design, where nozzle  210  paths taper one direction while opening  220  paths taper the opposite direction, which helps maintain an even pressure and flow distribution that is beneficial to the cooler performance. 
         [0067]      FIG. 10 , which includes  FIGS. 10A and 10B , shows the nozzle and opening layout, the airflow, and the tapering air channels of the manifold  200  depicted in  FIG. 9 .  FIG. 10A  shows the layout of nozzles  210 , while  FIG. 10B  adds the openings  220  and illustrates the corresponding airflows. As can be seen in the exemplary design of  FIG. 10A ,  25  cooler elements or “cells,” each of dimensions 0.4 inch by 0.4 inch, are arranged in a 5×5 array, which makes up a 2 inch by 2 inch cooler. Each cooling element has two manifold nozzles directing air at it, for a total of 50 nozzles in the manifold  200 . 
         [0068]    As shown in the exemplary design of  FIG. 10B , cool air (blue arrows)  10  enters from one side (the left side) and is directed to the nozzles  210  to form jets of cool air directed at the cooler. Warm air (red arrows)  20  returns from the cooler and is exhausted through the openings  220  in the manifold  200  to the other side (the right side). Cool air channels (which include the nozzles  210 ) in the manifold  200  taper inward (to help deliver the cool air to the cooler more efficiently) while warm air channels (which include the openings  220 ) taper outward (to help exhaust the warm air from the cooler more efficiently). 
         [0069]    The manifold can be manufactured in a number of ways (for example, injection molded or machined) and from a number of materials (e.g., metal or plastic). 
         [0070]      FIG. 11  is an illustration of the exemplary coolers  100 ′ of  FIG. 4  shown with a manifold  200 ′ according to another exemplary embodiment of the present invention. In the illustrated example of  FIG. 11 , coolers  100 ′ are arranged in staggered rows, with spaces in between. The coolers  100 ′ are separated from the manifold  200 ′ by, for example, 0.05 inches. 
         [0071]    The manifold  200 ′ includes cool air channels (with nozzles  210 ′) for delivering jets of cool air to the coolers  100 ′. The configuration of the nozzles  210 ′ corresponds to the receiving points (exposed fins) of the coolers  100 ′. The manifold  200 ′ also includes warm air channels (with openings  220 ′) for exhausting the warm air exiting the sides of the coolers  100 ′. The configuration of the openings  220 ′ corresponds to the spaces between the rows of coolers  100 ′. 
         [0072]      FIG. 12  illustrates an operation of the exemplary manifold  200 ′ of  FIG. 11  with the example coolers  100 ′ and heat sources  50  of  FIG. 5 .  FIG. 12  is a widthwise view of six rows of computer chips  50 , each with a cooler  100 ′ deployed on them. The computer chips  50  are mounted on a circuit board  40 ′. Cool air  10  is delivered under sufficient pressure in the cool air channels to exit nozzles  210 ′ (corresponding to the receiving points of the coolers  100 ′) as high speed jets of cool air. The coolers  100 ′ thus function as both microchannel heat sinks as well as allowing jet impingement cooling to take place at the receiving points in the coolers  100 ′. 
         [0073]    In addition, warm air  20  is exhausted through openings  220 ′ in the manifold  200 ′ corresponding to the space between rows of coolers  100 ′. The warm air  20  proceeds through the warm air channels of the manifold  200 ′ before exiting out of the top of the manifold  200 ′. The cool air warms through the jet impingement cooling and heat dissipation with the fins of the coolers  100 ′ (via their corresponding microchannels) before exiting out the sides of the coolers  100 ′. 
         [0074]    Although certain exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims, and equivalents thereof.