Patent Publication Number: US-9414525-B2

Title: Coolant-cooled heat sink configured for accelerating coolant flow

Description:
BACKGROUND 
     As is known, operating electronic components produce heat. This heat should be removed in an effective manner to maintain device junction temperatures within desirable limits, with failure to do so resulting in excessive component temperatures, potentially leading to thermal runaway conditions. Several trends in the electronics industry have combined to increase the importance of thermal management, including heat removal for electronic components, including technologies where thermal management has traditionally been less of a concern, such as CMOS. In particular, the need for faster and more densely packed circuits has had a direct impact on the importance of thermal management. First, power dissipation, and therefore heat production, increases as device operating frequencies increase. Second, increased operating frequencies may be possible at lower device junction temperatures. Further, as more and more devices or components are packed onto a single chip, heat flux (Watts/cm 2 ) increases, resulting in the need to dissipate more power from a given size chip or module. These trends have combined to create applications where it is no longer desirable to remove heat from modern devices solely by traditional air cooling methods, such as by using air cooled heat sinks with heat pipes or vapor chambers. Such air cooling techniques are inherently limited in their ability to extract heat from an electronic component with high power density. 
     The need to cool current and future high heat load, high heat flux electronic devices therefore mandates the development of aggressive thermal management techniques using, for instance, liquid cooling. 
     BRIEF SUMMARY 
     Provided herein in one or more aspects is a method which includes: providing a coolant-cooled heat sink configured to facilitate cooling at least one electronic component. The providing the coolant-cooled heat sink comprises: providing a thermally conductive structure with a coolant-carrying compartment comprising, at least in part, a varying cross-sectional coolant flow area through which coolant flows in a direction substantially parallel to a main heat transfer surface of the thermally conductive structure to couple to the at least one electronic component; providing a coolant inlet and a coolant outlet associated with the thermally conductive structure and in fluid communication with the coolant-carrying compartment of the thermally conductive structure to facilitate coolant flow therethrough; and wherein the cross-sectional coolant flow area of the coolant-carrying compartment decreases, at least in part, between the coolant inlet and the coolant outlet in a direction of coolant flow through the coolant-carrying compartment, the decreasing coolant flow area facilitating providing an increasing effective heat transfer coefficient between the main heat transfer surface and the coolant within the coolant-carrying compartment in a direction of coolant flow by, at least in part, accelerating the coolant flow within the coolant-carrying compartment of the coolant-cooled heat sink. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1 . depicts one embodiment of a conventional raised floor layout of an air-cooled data center; 
         FIG. 2  depicts one embodiment of a coolant distribution unit facilitating liquid-cooling of electronics racks of a data center, in accordance with one or more aspects of the present invention; 
         FIG. 3  is a plan view of one embodiment of an electronic system (or node) layout illustrating an air and liquid cooling apparatus for cooling components of the electronic system, in accordance with one or more aspects of the present invention; 
         FIG. 4  depicts one detailed embodiment of a partially assembled electronic system layout, wherein the electronic system includes eight heat-generating electronic components to be cooled, each having, in one embodiment, a respective cooling apparatus associated therewith, in accordance with one or more aspects of the present invention; 
         FIG. 5A  depicts one embodiment of a cooled electronic module comprising at least one electronic component and a cooling apparatus comprising a coolant-cooled heat sink, in accordance with one or more aspects of the present invention; 
         FIG. 5B  is an elevational view of the coolant-cooled heat sink of  FIG. 5A , taken along line  5 B- 5 B in the cross-sectional plan view of  FIG. 5C , in accordance with one or more aspects of the present invention; 
         FIG. 5C  is a cross-sectional plan view of the coolant-cooled heat sink of  FIGS. 5A &amp; 5B , taken along line  5 C- 5 C in the elevational view of  FIG. 5B , in accordance with one or more aspects of the present invention; 
         FIG. 5D  is a cross-sectional plan view of an alternate embodiment of the coolant-cooled heat sink of  FIGS. 5A-5C , in accordance with one or more aspects of the present invention; 
         FIG. 6A  is a cross-sectional plan view of another embodiment of a coolant-cooled heat sink, in accordance with one or more aspects of the present invention; and 
         FIG. 6B  is a cross-sectional elevational view of the coolant-cooled heat sink of  FIG. 6A , in accordance with one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the terms “electronics rack” and “rack unit” are used interchangeably, and unless otherwise specified include any housing, frame, rack, compartment, blade server system, etc., having one or more heat-generating components of a computer system, electronic system, or information technology equipment, and may be, for example, a stand-alone computer processor having high, mid or low end processing capability. In one embodiment, an electronics rack may comprise a portion of an electronic system, a single electronic system, or multiple electronic systems, for example, in one or more sub-housings, blades, books, drawers, nodes, compartments, etc., having one or more heat-generating electronic components disposed therein. An electronic system within an electronics rack may be movable or fixed relative to the electronics rack, with the rack-mounted electronic drawers and blades of a blade center system being two examples of systems of an electronics rack to be cooled. 
     “Electronic component” refers to any heat-generating electronic component of, for example, a computer system or other electronics unit requiring cooling. By way of example, an electronic component may comprise one or more integrated circuit die (or chips) and/or other electronic devices to be cooled, including one or more processor chips, memory chips and/or memory support chips. Further, the term “cold plate” refers to any thermally conductive structure having one or more compartments, channels, passageways, etc., formed therein for the flowing of coolant therethrough. In addition, “metallurgically bonded” refers generally herein to two components being welded, brazed or soldered together by any means. 
     As used herein, a “liquid-to-liquid heat exchanger” may comprise, for example, two or more coolant flow paths, formed of thermally conductive tubing (such as copper or other tubing) in thermal or mechanical contact with each other. Size, configuration and construction of the liquid-to-liquid heat exchanger can vary without departing from the scope of the invention disclosed herein. Further, “data center” refers to a computer installation containing one or more electronics racks to be cooled. As a specific example, a data center may include one or more rows of rack-mounted computing units, such as server units. 
     One example of the coolants discussed herein, such as the facility coolant or system coolant, is water. However, the cooling concepts disclosed herein are readily adapted to use with other types of coolant on the facility side and/or on the system side. For example, one or more of the coolants may comprise a brine, a fluorocarbon liquid, a hydrofluoroether liquid, a liquid metal, or other similar coolant, or refrigerant, while still maintaining the advantages and unique features of the present invention. 
     Reference is made below to the drawings, which are not drawn to scale to facilitate an understanding thereof, wherein the same reference numbers used throughout different figures designate the same or similar components. 
       FIG. 1  depicts one embodiment of a raised floor layout of an air cooled data center  100  typical in the prior art, wherein multiple electronics racks  110  are disposed in one or more rows. A data center such as depicted in  FIG. 1  may house several hundred, or even several thousand microprocessors. In the arrangement illustrated, chilled air enters the computer room via perforated floor tiles  160  from a supply air plenum  145  defined between the raised floor  140  and a base or sub-floor  165  of the room. Cooled air is taken in through louvered covers at air inlet sides  120  of the electronics racks and expelled through the back (i.e., air outlet sides  130 ) of the electronics racks. Each electronics rack  110  may have one or more air moving devices (e.g., fans or blowers) to provide forced inlet-to-outlet airflow to cool the electronic devices within the system(s) of the rack. The supply air plenum  145  provides conditioned and cooled air to the air-inlet sides of the electronics racks via perforated floor tiles  160  disposed in a “cold” aisle of the computer installation. The conditioned and cooled air is supplied to plenum  145  by one or more air conditioning units  150 , also disposed within the data center  100 . Room air is taken into each air conditioning unit  150  near an upper portion thereof. This room air may comprise in part exhausted air from the “hot” aisles of the computer installation defined, for example, by opposing air outlet sides  130  of the electronics racks  110 . 
     Due to the ever-increasing airflow requirements through electronics racks, and the limits of air distribution within the typical data center installation, liquid-based cooling may, for instance, be combined with the above-described conventional air-cooling.  FIGS. 2-4  illustrate one embodiment of a data center implementation employing a liquid-based cooling system with one or more cold plates coupled to high heat-generating electronic components disposed within an electronics rack. 
     In particular,  FIG. 2  depicts one embodiment of a coolant distribution unit  200  for a data center. The coolant distribution unit is conventionally a relatively large unit which occupies what would be considered a full electronics frame. Within coolant distribution unit  200  is a power/control element  212 , a reservoir/expansion tank  213 , a heat exchanger  214 , a pump  215  (often accompanied by a redundant second pump), facility water inlet  216  and outlet  217  supply pipes, a supply manifold  218  supplying water or system coolant to the electronics racks  210  via couplings  220  and lines  222 , and a return manifold  219  receiving water from the electronics racks  210 , via lines  223  and couplings  221 . Each electronics rack includes (in one example) a power/control unit  230  for the electronics rack, multiple electronic systems  240 , a system coolant supply manifold  250 , and a system coolant return manifold  260 . As shown, each electronics rack  210  is disposed on raised floor  140  of the data center with lines  222  providing system coolant to system coolant supply manifolds  250  and lines  223  facilitating return of system coolant from system coolant return manifolds  260  being disposed in the supply air plenum beneath the raised floor. 
     In the embodiment illustrated, the system coolant supply manifold  250  provides system coolant to the cooling systems of the electronic systems (more particularly, to liquid-cooled cold plates thereof) via flexible hose connections  251 , which are disposed between the supply manifold and the respective electronic systems within the rack. Similarly, system coolant return manifold  260  is coupled to the electronic systems via flexible hose connections  261 . Quick connect couplings may be employed at the interface between flexible hoses  251 ,  261  and the individual electronic systems. By way of example, these quick connect couplings may comprise various types of commercially available couplings, such as those available from Colder Products Company, of St. Paul, Minn., USA, or Parker Hannifin, of Cleveland, Ohio, USA. 
     Although not shown, electronics rack  210  may also include an air-to-liquid heat exchanger disposed at an air outlet side thereof, which also receives system coolant from the system coolant supply manifold  250  and returns system coolant to the system coolant return manifold  260 . 
       FIG. 3  depicts one embodiment of an electronic system  313  component layout wherein one or more air moving devices  311  provide forced air flow  315  to cool multiple components  312  within electronic system  313 . Cool air is taken in through a front  331  and exhausted out a back  333  of the system. The multiple components to be cooled include multiple processor modules to which liquid-cooled cold plates  320  (of a liquid-based cooling system) are coupled, as well as multiple arrays of memory modules  330  (e.g., dual in-line memory modules (DIMMs)) and multiple rows of memory support modules  332  (e.g., DIMM control modules) to which air-cooled heat sinks are coupled. In the embodiment illustrated, memory modules  330  and the memory support modules  332  are partially arrayed near front  331  of electronic system  313 , and partially arrayed near back  333  of electronic system  313 . Also, in the embodiment of  FIG. 3 , memory modules  330  and the memory support modules  332  are cooled by air flow  315  across the electronic system. 
     The illustrated liquid-based cooling system further includes multiple coolant-carrying tubes connected to and in fluid communication with liquid-cooled cold plates  320 . The coolant-carrying tubes comprise sets of coolant-carrying tubes, with each set including (for example) a coolant supply tube  340 , a bridge tube  341  and a coolant return tube  342 . In this example, each set of tubes provides liquid coolant to a series-connected pair of cold plates  320  (coupled to a pair of processor modules). Coolant flows into a first cold plate of each pair via the coolant supply tube  340  and from the first cold plate to a second cold plate of the pair via bridge tube or line  341 , which may or may not be thermally conductive. From the second cold plate of the pair, coolant is returned through the respective coolant return tube  342 . Note that in an alternate implementation, each liquid-cooled cold plate  320  could be coupled directly to a respective coolant supply tube  340  and coolant return tube  342 , that is, without series connecting two or more of the liquid-cooled cold plates. 
       FIG. 4  depicts in greater detail an alternate electronic system layout comprising eight processor modules, each having a respective liquid-cooled cold plate of a liquid-based cooling system coupled thereto. The liquid-based cooling system is shown to further include associated coolant-carrying tubes for facilitating passage of liquid coolant through the liquid-cooled cold plates and a header subassembly to facilitate distribution of liquid coolant to and return of liquid coolant from the liquid-cooled cold plates. By way of specific example, the liquid coolant passing through the liquid-based cooling subsystem is cooled and conditioned (e.g., filtered) water. 
       FIG. 4  is an isometric view of one embodiment of an electronic system or drawer, and monolithic cooling system. The depicted planar server assembly includes a multi-layer printed circuit board to which memory DIMM sockets and various electronic components to be cooled are attached both physically and electrically. In the cooling system depicted, a supply header is provided to distribute liquid coolant from a single inlet to multiple parallel coolant flow paths and a return header collects exhausted coolant from the multiple parallel coolant flow paths into a single outlet. Each parallel coolant flow path includes one or more cold plates in series flow arrangement to facilitate cooling one or more electronic components to which the cold plates are mechanically and thermally coupled. The number of parallel paths and the number of series-connected liquid-cooled cold plates depends, for example, on the desired component temperature, available coolant temperature and coolant flow rate, and the total heat load being dissipated from each electronic component. 
     More particularly,  FIG. 4  depicts a partially assembled electronic system  413  and an assembled liquid-based cooling system  415  coupled to primary heat-generating components (e.g., including processor die) to be cooled. In this embodiment, the electronic system is configured for (or as) a node of an electronics rack, and includes, by way of example, a support substrate or planar board  405 , a plurality of memory module sockets  410  (with the memory modules (e.g., dual in-line memory modules) not shown), multiple rows of memory support modules  432  (each having coupled thereto an air-cooled heat sink  434 ), and multiple processor modules (not shown) disposed below the liquid-cooled cold plates  420  of the liquid-based cooling system  415 . 
     In addition to liquid-cooled cold plates  420 , liquid-based cooling system  415  includes multiple coolant-carrying tubes, including coolant supply tubes  440  and coolant return tubes  442  in fluid communication with respective liquid-cooled cold plates  420 . The coolant-carrying tubes  440 ,  442  are also connected to a header (or manifold) subassembly  450  which facilitates distribution of liquid coolant to the coolant supply tubes and return of liquid coolant from the coolant return tubes  442 . In this embodiment, the air-cooled heat sinks  434  coupled to memory support modules  432  closer to front  431  of electronic system  413  are shorter in height than the air-cooled heat sinks  434 ′ coupled to memory support modules  432  near back  433  of electronic system  413 . This size difference is to accommodate the coolant-carrying tubes  440 ,  442  since, in this embodiment, the header subassembly  450  is at the front  431  of the electronics drawer and the multiple liquid-cooled cold plates  420  are in the middle of the drawer. 
     Liquid-based cooling system  415  comprises a pre-configured monolithic structure which includes multiple (pre-assembled) liquid-cooled cold plates  420  configured and disposed in spaced relation to engage respective heat-generating electronic components. Each liquid-cooled cold plate  420  includes, in this embodiment, a liquid coolant inlet and a liquid coolant outlet, as well as an attachment subassembly (i.e., a cold plate/load arm assembly). Each attachment subassembly is employed to couple its respective liquid-cooled cold plate  420  to the associated electronic component to form the cold plate and electronic component (or device) assemblies. Alignment openings (i.e., thru-holes) are provided on the sides of the cold plate to receive alignment pins or positioning dowels during the assembly process. Additionally, connectors (or guide pins) are included within attachment subassembly, which facilitate use of the attachment assembly. 
     As shown in  FIG. 4 , header subassembly  450  includes two liquid manifolds, i.e., a coolant supply header  452  and a coolant return header  454 , which in one embodiment, are coupled together via supporting brackets. In the monolithic cooling structure of  FIG. 4 , the coolant supply header  452  is metallurgically bonded in fluid communication to each coolant supply tube  440 , while the coolant return header  454  is metallurgically bonded in fluid communication to each coolant return tube  452 . A single coolant inlet  451  and a single coolant outlet  453  extend from the header subassembly for coupling to the electronics rack&#39;s coolant supply and return manifolds (not shown). 
       FIG. 4  also depicts one embodiment of the pre-configured, coolant-carrying tubes. In addition to coolant supply tubes  440  and coolant return tubes  442 , bridge tubes or lines  441  are provided for coupling, for example, a liquid coolant outlet of one liquid-cooled cold plate to the liquid coolant inlet of another liquid-cooled cold plate to connect in series fluid flow the cold plates, with the pair of cold plates receiving and returning liquid coolant via a respective set of coolant supply and return tubes. In one embodiment, the coolant supply tubes  440 , bridge tubes  441  and coolant return tubes  442  are each pre-configured, semi-rigid tubes formed of a thermally conductive material, such as copper or aluminum, and the tubes are respectively brazed, soldered or welded in a fluid-tight manner to the header subassembly and/or the liquid-cooled cold plates. The tubes are pre-configured for a particular electronics system to facilitate installation of the monolithic structure in engaging relation with the electronic system. 
     One issue with liquid-cooled cold plates, or more generally, coolant-cooled heat sinks, is that as the liquid coolant flows through the liquid-cooled structure, for instance, through one or more compartments, channels, fin arrays, tubes, etc., the temperature of the coolant rises due to the coolant absorbing heat from the wetted heat transfer surfaces of the coolant-cooled heat sink. This increase in coolant temperature means that the temperature of the heat transfer surface must increase to transfer the same amount of heat as the coolant flows through the heat sink, which can result in a reduced effective cooling capability in the downstream regions of the coolant-cooled heat sink. The phenomenon can lead to a non-uniform temperature profile at a main heat transfer surface of the coolant-cooled heat sink, such as the base of a liquid-cooled cold plate, which can result in sub-optimal cooling of the electronic component(s) to which the liquid-cooled heat sink is attached. Thus, disclosed hereinbelow are various coolant-cooled heat sink geometries designed to enhance cooling capability in the downstream regions of the coolant-cooled heat sink, and thereby enhance thermal performance of the coolant-cooled heat sink. 
     In one aspect, disclosed below is a cooling apparatus which includes a coolant-cooled heat sink having a thermally conductive structure with a coolant-carrying compartment comprising, at least in part, a varying transverse cross-sectional coolant flow area through which coolant flows in a direction substantially parallel to a main heat transfer surface of the thermally conductive structure, for instance, to a main heat transfer surface to which one or more electronic components to be cooled are coupled, and across which heat is transferred from the electronic component(s) to the heat sink. The coolant-cooled heat sink further includes a coolant inlet and a coolant outlet associated with the thermally conductive structure, which are in fluid communication with the coolant-carrying compartment to facilitate coolant flow through the coolant-carrying compartment of the thermally conductive structure. The cross-sectional coolant flow area of the coolant-carrying compartment is designed to decrease (or partially converge), at least in part, between the coolant inlet and the coolant outlet in a direction of coolant flow through the coolant-carrying compartment. This decreasing coolant flow area facilitates increasing the effective heat transfer coefficient between the main heat transfer surface of the coolant-cooled heat sink and the coolant within the coolant-carrying compartment in the direction of coolant flow by, at least in part, accelerating the coolant flow within the coolant-carrying compartment of the coolant-cooled heat sink. 
     In one embodiment, the thermally conductive structure is further configured with an increasing wetted surface area within the coolant-carrying compartment in the direction of coolant flow; that is, is configured with an increasing surface area exposed to the coolant flow on which the increasing effective heat transfer coefficient may act. 
     In one implementation, the thermally conductive structure further includes multiple coolant flow regions serially coupled in fluid communication within the coolant flow compartment, wherein the cross-sectional coolant flow area varies between coolant flow regions of the multiple coolant flow regions of the coolant-carrying compartment of the thermally conductive structure. By way of example, the multiple coolant flow regions may comprise multiple thermally conductive fin regions, wherein one or more fin region characteristics or attributes may vary between different thermally conductive fin regions of the multiple thermally conductive fin regions. 
     For example, in one embodiment, a size of thermally conductive fins may increase from one thermally conductive fin region to another thermally conductive fin region of the multiple thermally conductive fin regions, which facilitates providing a reduced cross-sectional coolant flow area in the another thermally conductive fin region compared with the one thermally conductive fin region, wherein the one thermally conductive fin region is upstream of the another thermally conductive fin region in the direction of coolant flow through the coolant-carrying compartment. 
     In another example, a number of thermally conductive fins may increase from one thermally conductive fin region to another thermally conductive fin region of the multiple thermally conductive fin regions, which facilitates providing a reduced cross-sectional coolant flow area in the another thermally conductive fin region compared with the one thermally conductive fin region, wherein the one thermally conductive fin region is upstream of the another thermally conductive fin region in the direction of coolant flow through the coolant-carrying compartment. 
     As a specific example, a size of thermally conductive fins may increase from a first thermally conductive fin region to a second thermally conductive fin region of the multiple thermally conductive fin regions, which facilitates reducing the cross-sectional coolant flow area in the second thermally conductive fin region compared with the first thermally conductive fin region, and a number of thermally conductive fin regions may increase from the second thermally conductive fin region to a third thermally conductive fin region of the multiple thermally conductive fin regions, which further reduces the cross-sectional coolant flow area in the third thermally conductive fin region compared with the second thermally conductive fin region. In this example, the first thermally conductive fin region is upstream of the second thermally conductive fin region, and the second thermally conductive fin region is upstream of the third thermally conductive fin region in the direction of coolant flow through the coolant-carrying compartment of the thermally conductive structure. 
     In one implementation, the coolant-cooled heat sink includes a coolant inlet manifold region and a coolant outlet manifold region within the coolant-carrying compartment, the coolant inlet manifold region receiving coolant from the coolant inlet, and the coolant outlet manifold region exhausting coolant from the coolant outlet, wherein the multiple coolant flow regions are disposed between the coolant inlet manifold region and the coolant outlet manifold region. 
     In another embodiment, the multiple coolant flow regions may comprise multiple thermally conductive pin fin regions, and wherein one thermally conductive pin fin region of the multiple thermally conductive pin fin regions may comprise pin fins of different sizes, with smaller pin fins being interspersed among larger pin fins. Further, in an implementation where the thermally conductive fins comprise pin fins, density of the thermally conductive pin fins may increase from one thermally conductive fin region to another thermally conductive fin region, which facilitates providing a reduced transverse coolant flow area in the another thermally conductive fin region compared to the one thermally conductive fin region, wherein the one thermally conductive fin region is upstream of the another thermally conductive fin region in the direction of coolant flow through the coolant-carrying compartment. 
     In still another implementation, multiple coolant flow channels may be disposed within the coolant flow compartment of the thermally conductive structure. These multiple coolant flow channels may be configured to provide the decreasing cross-sectional coolant flow area. For instance, at least one of a height or a width of one or more of the coolant flow channels of the multiple coolant flow channels may decrease in the direction of coolant flow through the coolant-carrying compartment. In one specific example, both the height and the width of the coolant flow channels may decrease in the direction of coolant flow through the coolant-carrying compartment. 
     To restate, disclosed herein are various cooling apparatuses which comprise a coolant-cooled heat sink configured to couple to, for instance, one or more electronic components, such as an electronics module, to be cooled. The heat sink includes an inlet, an outlet, and a coolant-carrying compartment configured to allow coolant flow to traverse the compartment in a direction, at least partially, parallel to a main heat transfer surface of the heat sink to be coupled to the electronic component to be cooled. The cross-sectional coolant flow area within the coolant-carrying compartment of the heat sink is configured to decrease, at least partially, in the direction of coolant flow through the compartment so that the cross-sectional coolant flow area of the compartment transverse to the direction of coolant flow is larger closer to the inlet than the outlet. Within this structure, the coolant flow accelerates, and in various embodiments, interacts with more wetted heat transfer surfaces within the compartment as the coolant traverses the coolant-carrying compartment of the heat sink. The heat sink is thus configured to result in the coolant more uniformly absorbing thermal energy from the heat sink structure, notwithstanding a rise in temperature of the coolant as it passes through the compartment. Increasing or accelerating the coolant flow rate within the coolant flow compartment advantageously increases the effective heat transfer coefficient between the main heat transfer surface of the heat sink and the coolant flow, resulting in a more uniform temperature profile across the heat transfer surface of the heat sink in the direction of coolant flow, and thus across the electronic component to be cooled since the temperature difference between the surface and fluid equals the heat rate over the product of the heat transfer coefficient and the wetted surface area. 
       FIGS. 5A-5C  depict one embodiment of a cooled electronic module, generally denoted  500 , in accordance with one or more aspects of the present invention. Referring collectively to  FIGS. 5A-5C , cooled electronic module  500  includes one or more electronic components  501  to be cooled and a coolant-cooled heat sink  510  coupled to the electronic component(s)  501  to facilitate transfer of heat from the component to, for instance, a liquid coolant passing through coolant-cooled heat sink  510 . In one example, the liquid coolant may comprises a system coolant distributed such as described above in connection with  FIGS. 2-4 . 
     Coolant-cooled heat sink  510  includes a thermally conductive structure  515 , such as a thermally conductive casing or housing, fabricated (for instance) of a metal, which includes a coolant-carrying compartment  520  (e.g., chamber, channel, tube, passageway, etc.) through which coolant flows in a direction  505  through the compartment from a coolant inlet  511  to a coolant outlet  512  of coolant-cooled heat sink  510 . In this example, thermally conductive structure  515  includes a main heat transfer surface  513  coupled to and in thermal communication with the electronic component(s)  501  to facilitate heat transfer from the component(s) to the heat sink, and hence, to the coolant flowing through the heat sink. As one example, main heat transfer surface  513  may comprise the base surface of a liquid-cooled cold plate configured as disclosed herein. 
     As illustrated in the cross-sectional plan view of  FIG. 5C , coolant-cooled heat sink  510  includes within coolant-carrying compartment  520 , a coolant inlet manifold region  521  and a coolant outlet manifold region  525  disposed adjacent to coolant inlet  511  and coolant outlet  512  (see  FIG. 5B ), respectively. Coolant inlet manifold region  521  receives coolant from the coolant inlet, and coolant outlet manifold region  525  exhausts coolant from the coolant-carrying compartment  520  through the coolant outlet. Disposed between the coolant inlet and outlet manifold regions  521 ,  525  are one or more coolant flow regions  522 ,  523 ,  524  in series-fluid communication. In this example, three coolant flow regions  522 ,  523 ,  524  are depicted by way of example only. As disclosed herein, the cross-sectional coolant flow area is advantageously configured to vary between the different coolant flow regions of the multiple coolant flow regions  522 ,  523 ,  524  of the coolant-carrying compartment  520  to facilitate or provide, for instance, an accelerating coolant flow within the coolant-carrying compartment of the coolant-cooled heat sink in the direction  505  of coolant flow through the coolant-carrying compartment. This accelerating coolant flow rate in turn provides an increasing effective heat transfer coefficient between the main heat transfer surface  513  of coolant-cooled heat sink  510  and the coolant flowing therethrough, notwithstanding an increase in temperature of the coolant as the coolant flows through the compartment. Additionally, in certain coolant-cooled heat sink embodiments disclosed herein, the thermally conductive structure  515  is configured with an increasing wetted surface area within the coolant-carrying compartment  520  in the direction  505  of coolant flow through the compartment, that is, an increasing thermally conductive surface area within the compartment exposed to the coolant flow on which the increasing effective heat transfer coefficient may act. Taken together, the heat sink may thus be configured with a substantially uniform effective heat transfer coefficient across, for instance, the main heat transfer surface of the heat sink, and hence, across the electronic components coupled thereto. 
     As one example, the multiple coolant flow regions may comprise multiple thermally conductive fin regions, wherein the first thermally conductive fin region (for instance, flow region  522 ) may include a plurality of thermally conductive fins  530  spaced in parallel, opposing relation to define a plurality of coolant flow channels  534  therebetween, and the second thermally conductive fin region (e.g., flow region  523 ) may comprise a plurality of thermally conductive fins  531 , also spaced in parallel, opposing relation to define a plurality of coolant flow channels  535  therebetween. As illustrated in  FIG. 5C , in this configuration, thermally conductive fins  531  are sized larger (e.g., wider) than thermally conductive fins  530 , which results in reduced space for coolant flow channels  535  defined between thermally conductive fins  531  (and thereby, a reduced cross-sectional coolant flow area) compared with that of thermally conductive fins  530 . Note that this example and comparison assumes that there are an equal number of fins within the first and second coolant flow regions  522 ,  523 . As one example, the fins of the coolant-cooled heat sink of  FIGS. 5A-5C  may comprise rectangular-shaped or plate-type fins extending between, for instance, a base inner surface and an upper inner surface of the thermally conductive structure. 
     In the example of  FIG. 5C , the third thermally conductive fin region (e.g., flow region  524 ) is disposed downstream of the second thermally conductive fin region (i.e., flow region  523 ) in the direction of coolant flow  505  through the coolant-carrying compartment  520 , and includes a larger number of thermally conductive fins  532  and channels  536  therebetween than the number of thermally conductive fins  531  in the second thermally conductive fin region, or the number of thermally conductive fins  530  in the first thermally conductive fin region. For instance, twice the number of thermally conductive fins  532  may be provided within the third thermally conductive fin region in order to, for instance, further reduce the cross-sectional coolant flow area within the coolant-carrying compartment, thereby providing a structure with multiple flow regions that will accelerate coolant flow through the compartment closer to the coolant outlet manifold region  525 , resulting in an increasing effective heat transfer coefficient between, for instance, the main heat transfer surface  513  of the coolant-cooled heat sink  510  and the coolant within the coolant-carrying compartment in the direction of coolant flow through the compartment. 
     Note that the particular configuration of multiple coolant flow regions  522 ,  523 ,  524  of  FIG. 5C  is presented by way of example only. Numerous variations thereof will be apparent to one of ordinary skill in the art based on the teachings provided herein. For instance, the size of the thermally conductive fins  532  in the third thermally conductive fin region could alternatively increase in size further from that of the thermally conductive fins  531  in the second thermally conductive fin region, without changing the number of fins in the third region. In another example, the number of thermally conductive fins  531  in the second thermally conductive fin region could increase from the number of thermally conductive fins  530  in the first region of thermally conductive fins. In each implementation, however, a goal is to tailor the heat sink so that the transverse cross-sectional coolant flow area through which the coolant flows decreases, for instance, in a direction substantially parallel to a main heat transfer surface of the thermally conductive structure from inlet to outlet thereof. As disclosed herein, the heat sink is configured such that the cross-sectional coolant flow area, at least in part, decreases or partially converges in a direction of coolant flow through the coolant-carrying compartment of the heat sink. Note also that the number and different plate fin regions (or groupings) of  FIGS. 5A-5C  are presented by way of example only. In other embodiments, the compartment, flow regions and/or fins could comprise any desired shape configured to facilitate a desired cooling profile. 
       FIG. 5D  depicts an alternate embodiment of a coolant-cooled heat sink  500 ′, in accordance with one or more aspects of the present invention. This coolant-cooled heat sink  500 ′ is substantially identical to coolant-cooled heat sink  500  described above in connection with  FIGS. 5A-5C , with the exception that the multiple coolant flow regions are modified. In the embodiment of  FIG. 5D , the multiple coolant flow regions comprise multiple thermally conductive pin fin regions  542 ,  543 ,  544 ,  545 , with four regions being shown by way of example only. As in the heat sink embodiment of  FIG. 5C , in the example of  FIG. 5D , the largest cross-sectional coolant flow area within the pin fin regions is provided closest to the coolant inlet manifold region  521 , and the smallest cross-sectional coolant flow area is provided closest to the coolant outlet manifold region  525 . 
     Note that in this example, each thermally conductive pin fin region  542 ,  543 ,  544 ,  545 , has a different geometry yielding successively less cross-sectional coolant flow area for liquid coolant to flow within the coolant-carrying compartment  520 , and thus an accelerating coolant flow rate as the coolant passes through the compartment. In the first thermally conductive pin fin region  542 , a plurality of pin fins  552  are provided arranged in a square or rectangular array, while in the second thermally conductive pin fin region  543 , the thermally conductive pin fins  552  are arranged in the same square or rectangular array as in the first thermally conductive pin fin region  542 , but there are also pin fins  553  at the centers of each square or rectangle in the array. In the third thermally conductive pin fin region  544 , the thermally conductive pin fins  554  are arranged in the same square or rectangular pitch as in the first thermally conductive pin fin region  542  but, in this example, the diameters of the thermally conductive pin fins  554  are significantly larger than the diameters of the thermally conductive pin fins  552  in the first and second thermally conductive pin fin regions  542 ,  543 . In the fourth thermally conductive pin fin region  545 , pin fins  555 ,  556  are provided of different diameters, with the smaller diameter pin fins  556  being interspersed among the larger diameter pin fins  555 , each in a square or rectangular array, by way of example. 
     As liquid coolant flows from coolant inlet manifold region  521  to coolant outlet manifold region  525 , the coolant encounters a decreasing or partially converging cross-sectional coolant flow area within the different regions of the compartment. This decreasing coolant flow area in the direction of coolant flow through the coolant-carrying compartment results in an increasing effective heat transfer coefficient between, for instance, a main heat transfer surface of the thermally conductive structure  515  and the coolant by, at least in part, accelerating the coolant flow within the coolant-carrying compartment. Additionally, in the embodiments of  FIGS. 5C &amp; 5D , the thermally conductive structure comprises thermally conductive fin regions which have increasing wetted surface areas within the coolant-carrying compartment in the direction of coolant flow through the compartment. Such increasing wetted surface area exposed to the coolant flow on which the increasing effective heat transfer coefficient acts further facilitates heat transfer closer to the coolant outlet manifold region, which allows for a more uniform heat transfer profile across the heat sink, and thus across the electronic compartment to be cooled. 
       FIGS. 6A-6B  depict another embodiment of a cooling apparatus, generally denoted  600 , comprising a coolant-cooled heat sink  610 , in accordance with one or more aspects of the present invention. As illustrated, coolant-cooled heat sink  610  includes a thermally conductive structure  615 , such as a thermally conductive casing or housing, which defines a coolant-carrying compartment (or chamber, passageway, etc.)  620  through which coolant flows from a coolant inlet  611  to a coolant outlet  612 . The coolant-carrying compartment  620  includes a varying cross-sectional coolant flow area transverse to, at least in part, a direction  605  of coolant flow substantially parallel to a main heat transfer surface  613  of the thermally conductive structure  615 . In one example, the coolant-cooled heat sink  610  is configured as a liquid-cooled cold plate, and the main heat transfer surface  613  is a base surface of the liquid-cooled cold plate. In the embodiment of  FIGS. 6A &amp; 6B , a plurality of thermally conductive fins  630  are provided, defining a plurality of coolant flow channels  635 . By way of example, the plurality of thermally conductive fins  630  may each have a decreasing height and a decreasing width in the direction  605  of coolant flow through coolant-carrying compartment  620  from a coolant inlet manifold region  621  to a coolant outlet manifold region  622  thereof. 
     By way of specific example, the thermally conductive fins  630  may comprise in cross-section, trapezoidal-shaped fins of varying height and width. A baffle  640  may be provided over the plurality of thermally conductive fins  630  to ensure that coolant flow within compartment  620  is through the coolant-carrying channels  635  defined between the thermally conductive fins  630 . In one embodiment, baffle section  640  may be affixed to or integrated with an upper plate  617  of the coolant-carrying heat sink  610 . Note that the trapezoidal-shaped (in plan view) thermally conductive fins  630  have wider fin widths or thicknesses downstream in the direction of coolant flow through the compartment, closer to the coolant outlet manifold region  622 , and thus the area for liquid flow through channels  635  is reduced along the coolant-carrying channels. In addition to this width reduction, the thermally conductive fins may also be sloped, with a reducing height in the direction of coolant flow, thus leading to even greater reduction in transverse cross-sectional flow area through the channels. Together, these configurational changes result in a tapering or decreasing coolant flow area within the coolant-carrying compartment in the region of the plurality of thermally conductive fins. 
     Those skilled in the art should note that a variety of manufacturing techniques may be employed in constructing the coolant-cooled heat sinks of the cooling apparatuses disclosed herein. For instance, a higher fin density region of an accelerating coolant flow heat sink may be fabricated by providing different arrays of fins associated with an upper plate and lower plate of the heat sink, and then joining the plates together such that the fins intersperse or interleave. In one example, this manufacturing approach could be readily employed to create a fin region with twice the fin density as another fin region by providing a similar fin pattern in the second fin region associated with the upper plate as the bottom plate, but not in the first fin region. Different fin arrays associated with the upper plate and lower plate may be provided in different regions of the coolant-cooled heat sink to create any desired geometry. In one implementation, thermally conductive joints are also established so that, for instance, thermally conductive fins extending downwards from the upper plate may perform nearly as well as the thermally conductive fins extending upwards from the base plate. Solder, brazing or welding techniques may be employed to ensure the presence of good thermally conductive joints. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention.