HK1243859B - Systems and methods for cooling electronic devices in a data center - Google Patents

Description

System and method for cooling electronic equipment in a data center

Technical Field

This document relates to systems and methods for providing cooling to electronic devices, such as computer server racks and related devices in a computer data center.

Background Art

Computer users often focus on the speed of their computer's microprocessors (e.g., megahertz and gigahertz). Many forget that this speed often comes with a price—higher power consumption. This power consumption also generates heat. This is because, according to the simple laws of physics, all energy has to go somewhere, and somewhere it is ultimately converted into heat. A pair of microprocessors mounted on a single motherboard can draw hundreds of watts of power or more. Multiply this number by thousands (or tens of thousands) for the many computers in a large data center, and the amount of heat that can be generated can be easily understood. The impact of the power consumed by critical loads in a data center is often exacerbated when all the auxiliary devices required to support the critical loads are incorporated.

Many techniques can be used to cool electronic equipment (e.g., processors, memory, networking equipment, and other heat-generating devices) located on server or network rack trays. For example, forced convection can be created by providing a cooling airflow over the equipment. Fans located near the equipment, fans located in the computer server room, and/or fans located in a duct system that is in fluid communication with the air surrounding the electronic equipment can force cooling airflow over the trays housing the equipment. In some cases, one or more components or devices on a server tray may be located in an area of the tray that is difficult to cool; for example, an area where forced convection is less effective or unavailable.

The result of insufficient and/or inadequate cooling can be the failure of one or more electronic devices on a tray due to the device's temperature exceeding its maximum rated temperature. While some redundancy may be built into computer data centers, server racks, and even individual trays, equipment failure due to overheating can still be extremely costly in terms of speed, efficiency, and expense.

Summary of the Invention

In an exemplary embodiment, a data center cooling system includes a modular heat sink and a working fluid. The modular heat sink includes an evaporator configured to be in thermal contact with heat-generating electronic equipment to receive heat from the data center heat-generating electronic equipment; a condenser coupled to the evaporator and configured to transfer heat from the heat-generating electronic equipment to a cooling fluid; and a plurality of delivery tubes fluidically coupling the evaporator and the condenser, at least one of the plurality of delivery tubes including an open end disposed in the evaporator and a closed end disposed in the condenser. The working fluid vaporizes in the evaporator upon receiving heat from the heat-generating electronic equipment and circulates in a delivery member from the evaporator to the condenser in a vapor phase and from the condenser to the evaporator in a liquid phase.

In aspects that may be combined with example embodiments, the working fluid includes water and the evaporator includes copper.

In another aspect that may be combined with any of the previous aspects, the water comprises deionized or reverse osmosis (RO) water.

Another aspect that may be combined with any of the previous aspects further includes a fan configured to circulate the cooling fluid over the condenser.

In another aspect that may be combined with any of the previous aspects, the fan is mounted on a frame of a server board subassembly supporting heat generating electronic equipment.

Another aspect that may be combined with any of the previous aspects further includes a heat transfer surface disposed within the interior volume of the evaporator.

In another aspect that may be combined with any of the previous aspects, the heat transfer surface comprises copper fins formed integrally with the evaporator.

In another aspect that may be combined with any of the previous aspects, the copper fins extend upwardly from a bottom surface of the interior volume of the evaporator, and a height of the finned structure is less than an operating level of the working fluid in the evaporator.

In another aspect that may be combined with any of the previous aspects, at least a portion of the heat transfer surface is coated with a porous coating.

In another aspect that may be combined with any of the previous aspects, the porous coating comprises copper particles.

In another aspect that may be combined with any of the previous aspects, the condenser is mounted vertically above the evaporator.

In another aspect that may be combined with any of the previous aspects, the condenser is mounted to a frame of a server board subassembly supporting heat generating electronic equipment.

In another aspect that may be combined with any of the previous aspects, the plurality of transport tubes include thermal pipes, each of the thermal pipes including a wick structure.

In another aspect that may be combined with any of the previous aspects, the closed ends of the plurality of transfer tubes are disposed in respective regions of the condenser.

In another aspect that may be combined with any of the previous aspects, the respective regions of the condenser comprise different hot zones of the condenser.

In another example embodiment, a method for cooling data center electronic equipment includes: vaporizing at least a portion of a working fluid in an evaporator of a modular heat sink using heat from heat-generating electronic equipment in thermal contact with the evaporator; circulating the vapor phase of the working fluid from the evaporator to a condenser of the modular heat sink through corresponding open ends of a plurality of conveying tubes fluidly coupled to the evaporator, the corresponding open ends being disposed in the evaporator; condensing at least a portion of the vapor phase of the working fluid into a liquid phase of the working fluid in corresponding closed ends of the plurality of conveying tubes disposed in the condenser; and circulating the liquid phase of the working fluid through the plurality of conveying tubes to the evaporator.

In aspects that may be combined with example embodiments, the working fluid includes water and the evaporator includes copper.

In another aspect that may be combined with any of the previous aspects, the water comprises deionized or reverse osmosis (RO) water.

Another aspect that may be combined with any of the previous aspects also includes circulating a cooling air flow over the condenser.

In another aspect combinable with any of the previous aspects, circulating the cooling airflow comprises circulating the cooling airflow with a fan mounted on a frame supporting a server board subassembly of heat generating electronic equipment.

Another aspect that may be combined with any of the previous aspects also includes transferring heat from the heat generating electronic device to the liquid phase of the working fluid via a heat transfer surface disposed within the interior volume of the evaporator.

In another aspect that may be combined with any of the previous aspects, at least a portion of the heat transfer surface is coated with a porous coating comprising copper particles.

In another aspect that may be combined with any of the previous aspects, the condenser is mounted vertically above the evaporator.

In another aspect that may be combined with any of the previous aspects, the plurality of transport tubes include thermal pipes, each of the thermal pipes including a wick structure.

In another aspect that may be combined with any of the previous aspects, the closed ends of the plurality of transfer tubes are disposed in respective regions of the condenser.

In another aspect that may be combined with any of the previous aspects, the respective regions of the condenser comprise different hot zones of the condenser.

One, some, or all embodiments of modular heat sinks according to the present disclosure may include one or more of the following features. For example, while heat generation in CPUs, GPUs, and ASICs continues to increase, the surface area of the heat source continues to decrease. This results in a large heat flux concentrated on the chip surface to be cooled. Furthermore, the main die surface area is typically much smaller than the contact surface between the package lid and the heat sink. This leads to localized hot spots on the lid surface area and large temperature gradients across the device surface. All of these design challenges can increase the thermal resistance from the junction to the ambient of traditional heat sink solutions and limit cooling capabilities. Copper heat sinks are often a traditional solution for dissipating heat and addressing hot spot cooling. However, this solution can result in poor thermal performance of the heat sink. Embodiments of modular heat sinks according to the present disclosure can address one or more of these issues, as well as other issues associated with cooling electronic heat-generating devices. For example, compared to traditional heat transfer devices used to cool CPUs, GPUs, ASICs, and other electronic devices, embodiments of modular heat sinks according to the present disclosure can have lower thermal resistance due to, for example, reduced soldering and a monolithic copper design. Furthermore, in some embodiments, modular heat sinks can provide for more uniform heat transfer through multiple transfer tubes coupled between the evaporator and condenser. Compared to conventional technologies, the transfer tubes can be customized to a specific heat transfer capacity or heat removal rate to more efficiently remove heat from the heat-generating electronic device. Additionally, in some embodiments, the modular heat sink can more efficiently cool the heat-generating device by utilizing water as the working fluid, which can have higher conductivity properties than dielectric coolants. As another example, embodiments of the modular heat sink can be less sensitive to the orientation of the heat-generating electronic device, which typically affects the performance of conventional cooling devices (such as thermosiphons). Thus, the tray, motherboard, and devices mounted thereon can be arranged in a variety of orientations without affecting the operation of the modular heat sink.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

1 illustrates a schematic diagram of a server rack and server rack subassemblies configured to be mounted within a rack for use in a data center environment and cooled by a modular heat sink in accordance with the present disclosure.

2A-2B illustrate schematic side and top views, respectively, of an example embodiment of a modular heat sink for a server rack subassembly according to the present disclosure.

3 illustrates a schematic side view of another example embodiment of a modular heat sink for a server rack subassembly according to the present disclosure.

4A-4B illustrate schematic isometric views of portions of other example embodiments of modular heat sinks for server rack subassemblies according to the present disclosure.

5A-5B illustrate schematic top and side views, respectively, of another example embodiment of a modular heat sink for a server rack subassembly according to the present disclosure.

6A-6B illustrate schematic side and top cross-sectional views, respectively, of another example embodiment of a modular heat sink for a server rack subassembly according to the present disclosure.

DETAILED DESCRIPTION

This document discloses a modular heat sink operable to cool one or more electronic heat-generating devices, such as those located on server rack subassemblies (e.g., server trays) in a data center. In some embodiments, the modular heat sink can provide hotspot cooling and three-dimensional distribution of heat generated by these devices to handle high heat fluxes. In some embodiments, a modular heat sink according to the present disclosure includes an evaporator, a transfer tube, and a condenser.

In some embodiments of a modular heat sink according to the present disclosure, the evaporator includes a small copper block having a heat transfer surface (e.g., a fin structure). The fins in the fin structure can be machined or cut, or can be welded or brazed to the evaporator small copper block as separate parts. The fins can be plate fins or pin fins and can be coated with copper porous particles to increase the evaporation rate and reduce thermal resistance. In some aspects, the evaporator can be formed entirely of copper, which can allow for easier implementation of finned surfaces compared to other materials. In addition, the manufacture of sintered copper can be safer for use in modular heat sinks (e.g., compared to sintered aluminum). In some aspects, the use of copper can allow for a one-piece evaporator (e.g., without joints or connections that could cause leaks or weaknesses).

In some embodiments of a modular heat sink according to the present disclosure, as heat is transferred from the heat generating device to the evaporator, the working fluid vaporizes in the evaporator. The vaporized working fluid circulates (e.g., naturally or forcedly) through a delivery pipe to a condenser, where it condenses to release the heat transferred from the device to a cooling fluid (e.g., a cooling liquid or air flow). The condensed working fluid circulates (e.g., naturally or forcedly) back to the evaporator through a delivery pipe. In some embodiments of a modular heat sink according to the present disclosure, the working fluid is water (e.g., purified water, deionized water, or reverse osmosis water). In some aspects, the water level is higher than the fin height in the evaporator, so that pool boiling occurs to increase the two-phase heat transfer rate and also increase the maximum heat flux. For example, water may have a higher capacity to transfer heat (e.g., an order of magnitude higher) than non-aqueous coolants. Therefore, a layer of water in contact with the evaporator surface may more easily change phase to steam than a non-aqueous coolant, thereby transferring more heat to the water. Additionally, modular heat sinks using water can operate at much lower pressures than conventional non-aqueous coolant-based thermosyphons (e.g., due to the boiling point of water being 100° C.). For example, conventional thermosyphons cannot operate using water as a coolant because the vapor-hydraulic pressure differential of such devices may be insufficient to naturally circulate the working fluid (i.e., water) between the hot and cold sides of the thermosyphon.

In some embodiments of modular heat sinks according to the present disclosure, fins (e.g., coated) can primarily address hotspot cooling. For example, fins increase the heat transfer surface area, while the porous structure increases the heat transfer rate. The fin structure (e.g., shape, number, size, spacing) and porous structure (e.g., material, size, and porosity) can be modified for each design and precisely positioned on top of local hotspots to address various thermal profiles of heat-generating electronic devices. For example, if a device such as a microprocessor has a specific power profile, the fins can be modified to reduce the temperature gradient and thermal resistance of the heat sink.

In some embodiments of modular heat sinks according to the present disclosure, the three-dimensional nature of heat dissipation can also help handle high heat fluxes over smaller equipment areas. Pool boiling over the coated fin area results in effective thermal conductivity that is approximately 3-5 times that of conventional copper heat sinks and twice that of industrial vapor chambers (e.g., up to 1000 W/ ^cm2 ).

In some embodiments of the modular heat sink according to the present disclosure, the delivery tube can be coated with a porous structure. The porous coating can be optimized to maximize the capillary effect which can increase the maximum power capacity of the modular heat sink.

In some embodiments of modular heat sinks according to the present disclosure, the solder resistance found in traditional heat pipes and copper heat sinks or conventionally designed vapor chambers can be eliminated. For example, a transfer pipe can directly transfer the working fluid vapor from the evaporator to the condenser. This results in the lowest possible thermal resistance.

In some embodiments of modular heat sinks according to the present disclosure, the delivery tube may be composed of multiple heat pipes (e.g., an open tube with a core structure inside the tube). In some aspects, as shown in Figures 4A to 4B, the direct delivery of the working fluid vapor to the multiple heat pipes can result in a more uniform power distribution between the heat pipes. In contrast, when the heat pipes are soldered to the copper heat sink in the conventional technology, there may be a temperature gradient across the heat pipes, which may result in less power being transferred through certain heat pipes. In some embodiments of modular heat sinks according to the present disclosure, the delivery tube heat pipes may have the same effectiveness, resulting in less thermal resistance in the transfer path, and will also result in higher fin efficiency and reduced condenser resistance.

Furthermore, in some embodiments of modular heat sinks according to the present disclosure, if a heat pipe of a transport tube is exposed to a cooler heat sink in the condenser than other heat pipes (e.g., exposed to a cooler airflow), condensation in that heat pipe may be accelerated, thereby transferring more heat than other heat pipes. This feature may act as a self-balancing effect, further improving the performance of the modular heat sink.

In some embodiments of the modular heat sink according to the present disclosure, the modular heat sink can be used for heat-generating electronic devices of different sizes (e.g., heights). For example, the modular heat sink can be used for relatively short (e.g., 1 rack unit) servers or relatively tall (e.g., 2 rack units) servers.

In some embodiments of modular heat sinks according to the present disclosure, the modular heat sink may be modular and may take several forms. For example, the condenser may be adjacent to the evaporator or on top of the evaporator. The evaporation surface improvement may be a porous core, grooves, microfins, or any other features that improve bubble nucleation in the solid-liquid interface. The condenser may be air-cooled, a liquid-cooled plate, or any other cooling interface (e.g., a two-phase coolant, cold water, condensed water, a Peltier-type cooler). In some aspects, multiple evaporators may be connected to one condenser. In some aspects, each evaporator may be connected to more than one condenser. For example, a fin stack is on top of the evaporator and a fin stack is located at a distance.

FIG1 illustrates an example system 100 that includes a server rack 105 (e.g., a 13-inch or 19-inch server rack) and a plurality of server rack subassemblies 110 mounted within the rack 105. Although a single server rack 105 is shown, the server rack 105 may be one of many server racks within the system 100, which may include a server farm or a co-location facility containing various rack-mounted computer systems. Additionally, although a plurality of server rack subassemblies 110 are shown mounted within the rack 105, there may be only a single server rack subassembly. Generally, the server rack 105 defines a plurality of slots 107 that are arranged within the server rack 105 in an ordered and repeating manner, and each slot 107 is a space within the rack into which a corresponding server rack subassembly 110 can be placed and removed. For example, the server rack subassemblies can be supported on rails 112 extending from opposite sides of the rack 105, which can define the location of the slots 107.

The slots 107 and server rack subassemblies 110 can be oriented in the illustrated horizontal arrangement (relative to gravity). Alternatively, the slots 107 and server rack subassemblies 110 can be oriented vertically (relative to gravity), but this would require some reconfiguration of the evaporator and condenser structures as described below. With the slots oriented horizontally, they can be stacked vertically in the rack 105; with the slots oriented vertically, they can be stacked horizontally in the rack 105.

For example, as part of a large data center, server rack 105 may provide data processing and storage capabilities. In operation, the data center may be connected to a network and may receive and respond to various requests from the network to retrieve, process, and/or store data. For example, in operation, server rack 105 typically facilitates communication of information via the network with a user interface generated by a web browser application of a user requesting services from an application running on a computer in the data center. For example, server rack 105 may provide or facilitate access to websites on the Internet or the World Wide Web to a user using a web browser.

The server rack subassembly 110 can be one of a variety of structures that can be installed in a server rack. For example, in some implementations, the server rack subassembly 110 can be a "tray" or tray assembly that can be slidably inserted into the server rack 105. The terms "tray" and "tray" are not limited to any particular arrangement, but rather apply to a motherboard or other relatively flat structure attached to the motherboard that supports the motherboard in place within the rack structure. In some implementations, the server rack subassembly 110 can be a server cradle or a server container (e.g., a server box). In some implementations, the server rack subassembly 110 can be a hard drive cage.

2A-2B , the server rack subassembly 110 includes a frame or cassette 120, a printed circuit board 122 (e.g., a motherboard) supported on the frame 120, one or more heat-generating electronic devices 124 (e.g., a processor or memory) mounted on the printed circuit board 122, and a modular heat sink 130. One or more fans 126 may also be mounted on the frame 120.

The frame 120 may comprise or be a flat structure upon which the motherboard 122 may be placed and mounted, such that a technician can grasp the frame 120 to move the motherboard into position and secure it in place within the rack 105. For example, the server rack subassembly 110 may be mounted horizontally within the server rack 105, such as by sliding the frame 120 into the slots 107 and over a pair of rails in the rack 105 on opposite sides of the server rack subassembly 110 (much like sliding a lunch tray into a cafeteria rack). Although Figures 2A-2B illustrate the frame 120 extending below the motherboard 122, the frame may have other forms (e.g., implementing it as a peripheral frame surrounding the motherboard) or may be eliminated so that the motherboard itself is disposed within the rack 105 (e.g., slidably engaged with the rack 105). Additionally, although Figure 2A illustrates the frame 120 as a flat plate, the frame 120 may include one or more sidewalls extending upward from the edges of the plate, and the plate may be the bottom plate of a box or case with a closed or open top.

The illustrated server rack subassembly 110 includes a printed circuit board 122 (e.g., a motherboard) mounted with various components, including heat-generating electronics 124. Although one motherboard 122 is shown mounted on the frame 120, multiple motherboards may be mounted on the frame 120 as required by a particular application. In some embodiments, one or more fans 126 may be positioned on the frame 120 so that air enters the server rack subassembly 110 at the front edge (left-hand side in Figures 2A-2B) (which is closer to the front of the rack 105 when the subassembly 110 is installed in the rack 105), flows over (as shown) the motherboard 122 and some of the heat-generating components on the motherboard 122, and exits the server rack assembly 110 at the rear edge (right-hand side) (which is closer to the rear of the rack 105 when the subassembly 110 is installed in the rack 105). The one or more fans 126 may be secured to the frame 120 via brackets 127. Thus, the fans 126 can pull air from the area of the frame 120 and, after the air has warmed, push it out of the rack 105. The underside of the motherboard 122 can be separated from the frame 120 by a gap.

The modular heat sink 130 includes an evaporator 132, a condenser 134 mounted on a base 139, and a transfer member 136 connecting the evaporator 132 to the condenser 134. The evaporator 132 contacts the electronic device 124 so that heat is drawn from the electronic device 124 to the evaporator 132 by conductive heat transfer. For example, the evaporator 132 is in thermally conductive contact with the electronic device 124. Specifically, the bottom of the evaporator 132 contacts the top of the electronic device 124.

In operation, heat from the electronic device 124 causes the working fluid (e.g., water) in the evaporator 132 to evaporate. As illustrated in FIG2A , the working fluid, as a liquid 137, fills the evaporator 132 to a specific height within the capacity of the evaporator 132, with the working fluid vapor 141 (due to the transferred heat) above the liquid 137. In some aspects, the liquid 137 fills the evaporator 132 to a height above the finned surface (not shown here) within the evaporator 132. In this example, the evaporator 132 is constructed from a small copper block with some fin structures (not shown). The fins can be machined/cut, or can be welded/brazed to the evaporator copper block as separate parts. The fins can be plate fins or pin fins and can be coated with copper porous particles to increase the evaporation rate and reduce thermal resistance.

The vapor 141 then passes through the transport member 136 to the condenser 134. Heat radiates outward from the condenser 134 into, for example, the air surrounding the condenser 134 or into air blown or drawn across the condenser 134, the heat transfer surface 138 (e.g., a finned surface), or both by one or more fans 126, causing the working fluid to condense. As shown in FIG2A , the condenser 134 can be located between the one or more fans 126 and the evaporator 132, but can also be located on the opposite side of the one or more fans 126 (e.g., near the edge of the subassembly 110).

As shown in FIG2A , the delivery member 136 can be angled slightly (non-zero) so that gravity causes the condensed working fluid to flow through the delivery member 136 back to the evaporator 132. Thus, in some embodiments, at least a portion of the delivery member 136 is not parallel to the major surface of the frame 120. For example, the condenser-side end of the delivery member 136 can be approximately 1-5 mm (e.g., 2 mm) higher than the evaporator-side end of the delivery member 136. However, the delivery member 136 can also be a horizontal tube, or even angled slightly negatively (although a positive angle has the advantage of utilizing gravity to improve the flow of liquid from the condenser to the evaporator). Because multiple heat-generating electronic devices can be present on a single motherboard, multiple evaporators can be present on the motherboard, with each evaporator corresponding to a single electronic device.

During operation, the top surface of the working fluid (as a liquid) inside the condenser 134 can be higher (e.g., 1 to 10 mm higher) than the top surface liquid level 137 of the working fluid in the evaporator 132. This effect can be more easily achieved using a conveying member 136 at a slight (positive, non-zero) angle, but can also be achieved with a horizontal or slightly negative angled conveying member 136 by appropriately selecting the thermal and mechanical properties of the working fluid (e.g., water) given the expected heat transfer requirements of the modular heat sink 130.

During operation, the liquid phase 137 of the working fluid can flow through the liquid conduits of the transport member 136, and the vapor phase 141 (or mixed vapor and liquid phases) of the working fluid can flow through the vapor conduits of the transport member 136. Additionally, in some aspects, the transport member 136 can include a wick structure that helps circulate the liquid 141 back to the evaporator 132 (and the vapor 141 to the condenser 134) via capillary forces.

In some alternative embodiments, the modular heat sink 130 may have multiple evaporators; each evaporator may contact a different electronic device, or multiple evaporators may contact the same electronic device (for example, if the electronic device is particularly large or has multiple heat generating areas). Multiple evaporators can be connected to the condenser 134 in series via a transport member 136, for example, a single transport member 136 connects the condenser 134 to a first evaporator and a second evaporator. Alternatively, some or all of the multiple evaporators can be connected to the condenser 134 in parallel via multiple transport members, for example, a first transport member connects the condenser to the first evaporator and a second transport member connects the condenser 134 to the second evaporator. The advantage of a series embodiment is that there are fewer tubes, while the advantage of parallel tubes is that the tube diameter can be smaller.

2A-2B illustrate a modular heat sink 130 in which a common transport member 136 is used for both the flow of condensate from the condenser 134 to the evaporator 132 and the flow of vapor (or a mixed phase) from the evaporator 132 to the condenser 134. Thus, in this embodiment, the fluid coupling between the evaporator 132 and the condenser 134 is provided by a combined condensate and vapor transport line 136. A potential advantage of a combined condensate and vapor transport line is that the transport member 136 can be connected to one side of the condenser, thereby reducing the vertical height of the system compared to systems with separate lines for vapor (since the vapor line is typically coupled to or near the top of the evaporator). The transport member 136 can be a flexible tube or pipe, such as copper or aluminum.

3 , another example embodiment of a modular heat sink 230 is illustrated using a server rack subassembly 110. As shown in FIG2A-2B , the server rack subassembly 110 includes a frame or cassette 120, a printed circuit board 122 (e.g., a motherboard) supported on the frame 120, one or more heat-generating electronic devices 124 (e.g., a processor or memory) mounted on the printed circuit board 122, and a modular heat sink 130. One or more fans 126 may also be mounted on the frame 120.

The modular heat sink 230 includes an evaporator 232, a condenser 234 mounted on top of the evaporator 232, and one or more transfer members 236 connecting the evaporator 132 to the condenser 134. The evaporator 132 contacts the electronic device 124 so that heat is drawn from the electronic device 124 to the evaporator 132 by conductive heat transfer. For example, the evaporator 132 is in thermally conductive contact with the electronic device 124. Specifically, the bottom of the evaporator 132 contacts the top of the electronic device 124.

As shown in FIG3 , the condenser 234 is vertically positioned directly above the evaporator 232, which in some aspects can allow for space saving on the frame 120. A heat transfer surface 238 (e.g., fins) is mounted to the top of the condenser 234. In operation, heat from the electronic device 124 causes the working fluid (e.g., water) in the evaporator 232 to evaporate. As shown in FIG3 , the working fluid, which is a liquid 237, fills the evaporator 232 to a specific height within the capacity of the evaporator 232, wherein the working fluid vapor 241 (due to the heat transferred) is above the liquid 237. In some aspects, the liquid 237 fills the evaporator 232 to a height above the finned surface (not shown here) within the evaporator 232. In this example, the evaporator 232 is constructed from a small copper block with some fin structures (not shown). The fins can be machined/cut, or can be welded/brazed to the evaporator copper block as separate parts. The fins may be plate fins or pin fins and may be coated with copper porous particles to increase the evaporation rate and reduce thermal resistance.

The vapor 241 then passes through the transport member 236 to the condenser 234. In some aspects, natural variations in the density of the water vapor 241 and the water liquid 237 in the evaporator 232 can cause the vapor 241 to circulate through the transport member 236 to the condenser 234. The condenser 234 radiates heat outwardly, for example, into the air surrounding the condenser 234 or into air blown or drawn across the condenser 234 by one or more fans 126, heat transfer surfaces 238 (e.g., finned surfaces), or both, causing the working fluid to condense.

During operation, the top surface of the working fluid (as a liquid) inside the condenser 134 is higher than the top surface liquid level 237 of the working fluid in the evaporator 232. During operation, in this example, the liquid phase 137 of the working fluid can flow upward through the transport member 136 as (e.g., simultaneously with) the vapor phase 141 (or mixed vapor-liquid phases) of the working fluid can flow downward through the transport member 136.

In some alternative embodiments, the modular heat sink 230 may have multiple evaporators; each evaporator may contact a different electronic device, or multiple evaporators may contact the same electronic device (for example, if the electronic device is particularly large or has multiple heat generating areas). The multiple evaporators may be connected to the condenser 134 or multiple condensers via a conveying member 236.

4A-4B illustrate schematic isometric views of portions of other example embodiments of a modular heat sink for a server rack subassembly. For example, FIG4A shows an evaporator 432 and a delivery member (including a plurality of delivery tubes 436) of a modular heat sink according to the present disclosure. The evaporator 432 includes a finned structure 450 (e.g., in the form of a wheel) mounted or formed into an inner surface 452 of the evaporator 432. As shown in FIG4A , there are four delivery tubes 436; in alternative embodiments, there may be two, three, five, or another number of delivery tubes 436. In some aspects, the number of delivery tubes 436 may be determined at least in part based on the required or desired cooling capacity of the modular heat sink and the specific cooling capacity of each delivery tube 436. Each delivery tube 436 may be open to the evaporator 432 (e.g., the volume within the evaporator 432 including the liquid and vapor phases of the working fluid) and include a flow opening 437.

As shown, flow opening 437 includes a liquid flow portion 438 and a vapor flow portion 439. In some aspects, although there may not be a physical barrier separating liquid flow portion 438 and vapor flow portion 439, such portions are separated based on the phase of the working fluid flowing within delivery tube 436 (e.g., to evaporator 432 or to condenser), and more specifically, due to the density difference between the vapor working fluid and the liquid working fluid. For example, the liquid working fluid may flow from the condenser to the evaporator 432 within the bottom portion of delivery tube 436 including liquid flow portion 438. The vapor working fluid may flow from the evaporator to the evaporator 432 within the upper portion of delivery tube 436 including vapor flow portion 438. Thus, in example operation, the working fluid (e.g., water or coolant) may be vaporized into a vapor phase by the transfer of heat from the heat-generating electronic device thermally coupled to evaporator 432. The vaporized working fluid can flow through the vapor flow region 439 of the transfer tube 436 to the condenser, where the vapor phase is cooled and condensed into a liquid phase (e.g., within the transfer tube 436). The liquid phase can then flow back to the evaporator 432 through the liquid flow region 438 (e.g., to begin the vapor-liquid cycle again). In some aspects, one or more of the transfer tubes 436 can be a heat pipe (e.g., having a wick structure) that is closed at the condenser end and open at the evaporator end (as shown).

FIG4B illustrates an evaporator 482 and a delivery member 486 of a modular heat sink according to the present disclosure. The evaporator 482 includes a finned structure 490 (e.g., as parallel fins) mounted or formed into an interior surface 492 of the evaporator 482. As illustrated in FIG4B , there are four delivery tubes 486; in alternative embodiments, there may be two, three, five, or another number of delivery tubes 486. In some aspects, the number of delivery tubes 486 may be determined at least in part based on the required or desired cooling capacity of the modular heat sink and the specific cooling capacity of each delivery tube 486. Each delivery tube 486 may open to the evaporator 482 (e.g., the volume within the evaporator 482 that includes the liquid and vapor phases of the working fluid) and include a flow opening 489.

As illustrated, the flow opening 487 includes a liquid flow portion 488 and a vapor flow portion 489. In some aspects, although there may not be a physical barrier separating the liquid flow portion 488 and the vapor flow portion 489, such portions are separated based on the phase of the working fluid flowing within the delivery tube 486 (e.g., to the evaporator 482 or to the condenser), and more specifically, due to the density difference between the vapor working fluid and the liquid working fluid. For example, the liquid working fluid may flow from the condenser to the evaporator 482 within the bottom portion of the delivery tube 486 including the liquid flow portion 488. The vapor working fluid may flow from the evaporator to the evaporator 482 within the upper portion of the delivery tube 486 including the vapor flow portion 488. Thus, in example operation, the working fluid (e.g., water or coolant) can be vaporized into a vapor phase by the transfer of heat from the heat-generating electronic device thermally coupled to the evaporator 482. The vaporized working fluid can flow through the vapor flow region 489 of the transfer tube 486 to the condenser, where the vapor phase is cooled and condensed into a liquid phase (e.g., within the transfer tube 486). The liquid phase can then flow back to the evaporator 482 through the liquid flow region 488 (e.g., to begin the vapor-liquid cycle again). In some aspects, one or more of the transfer tubes 486 can be a heat pipe (e.g., having a wick structure) that is closed at the condenser end and open at the evaporator end (as shown).

As shown in these example portions of the modular heat sink, the delivery members 436 and 486 are composed of a plurality of heat pipes (e.g., having a core structure therein) or delivery tubes, as shown. For example, there may be a specific total cross-sectional flow area (e.g., flow areas 437 and 487) required for the working fluid liquid to flow from the condenser back to the evaporator (e.g., based on the amount of heat generated by the electronic equipment in thermal contact with the evaporator). The total cross-sectional area can be divided between the multiple heat pipes or flow tubes, as shown. This can add redundancy to the flow delivery of the modular heat sink while also ensuring that heat is evenly transferred to/from the working fluid within these components through appropriate spacing of the tubes in the evaporator and condenser. As a result, hot (or cold) spots in the working fluid can be reduced or eliminated.

In some aspects, a modular heat sink according to the present disclosure can be manufactured according to the following example process. The example steps can be performed sequentially, in parallel, or in an order different from that described herein. First, the evaporator, condenser, and transfer tube portions can be manufactured by a stamping or machining process (e.g., from copper or other materials). Next, the evaporator fins are formed by cutting or welding to the desired areas in the evaporator (e.g., within the inner volume of the evaporator, on the bottom surface of the volume). Next, a coating (e.g., a porous coating of copper particles) is applied to at least a portion of the transfer tube and the evaporator. Next, the transfer members (e.g., as a single heat pipe or flow tube or multiple heat pipes or flow tubes) are welded to the evaporator and condenser at both ends. Next, the modular heat sink is cleaned and evacuated and filled with a working fluid (e.g., water), and the filled tubes are then sealed.

5A-5B illustrate schematic top and side views, respectively, of another example embodiment of a modular heat sink 530 for a server rack subassembly. In some aspects, the modular heat sink 530 can include multiple (e.g., two or more) ducts fluidly connecting an evaporator 532 with a condenser 538 to transfer heat from one or more heat-generating electronic devices 524 to the surrounding environment within a working fluid. In the example modular heat sink 530, as illustrated, the condenser 538 is positioned adjacent to the evaporator 532.

As shown in Figures 5A-5B, the server rack subassembly 510 includes a frame or cassette 520, a printed circuit board 522 (e.g., a motherboard) supported on the frame 520, one or more heat-generating electronic devices 524 (e.g., a processor or memory) mounted on the printed circuit board 522, and a modular heat sink 530. One or more fans 526 may also be mounted on the frame 520. The frame 520 may include or be a flat structure on which the motherboard 522 may be placed and mounted, so that a technician can grasp the frame 520 to move the motherboard into position and secure it in place within the rack 505. For example, the server rack subassembly 510 may be mounted horizontally in the server rack 505, such as by sliding the frames 520 into slots 507 on opposite sides of the server rack subassembly 510 and over a pair of rails in the rack 505 (much like sliding a lunch tray into a cafeteria rack). Although Figures 5A-5B illustrate frame 520 extending below motherboard 522, the frame may have other forms (e.g., it may be implemented as a peripheral frame surrounding the motherboard), or it may be eliminated so that the motherboard itself is disposed within (e.g., slidably engaged with) chassis 505. Additionally, although Figure 5A illustrates frame 520 as a flat plate, frame 520 may include one or more sidewalls extending upward from the edges of the flat plate, and the flat plate may be the bottom plate of a box or case with a closed or open top.

The illustrated server rack subassembly 510 includes a printed circuit board 122 (e.g., a motherboard) mounted with various components, including heat-generating electronics 524. Although one motherboard 522 is shown mounted on the frame 520, multiple motherboards may be mounted on the frame 520 as required by a particular application. In some embodiments, one or more fans 526 may be positioned on the frame 520 so that air enters the server rack subassembly 510 at the front edge (left-hand side in Figures 5A-5B) (which is closer to the front of the rack 505 when the subassembly 510 is installed in the rack 505), flows through (as illustrated) the motherboard 522 and some of the heat-generating components on the motherboard 522, and exits the server rack assembly 510 at the rear edge (right-hand side) (which is closer to the rear of the rack 505 when the subassembly 510 is installed in the rack 505). The one or more fans 526 may be secured to the frame 520 via brackets. Thus, the fans 526 can pull air from the frame 520 area and, after the air is warmed, push it out of the rack 505. The underside of the motherboard 522 can be separated from the frame 520 by a gap.

The modular heat sink 530 includes an evaporator 532, a condenser 534 mounted on a base 539, and a transfer member 536 connecting the evaporator 532 to the condenser 534. The evaporator 532 contacts the electronic device 524 so that heat is drawn from the electronic device 524 to the evaporator 532 by conductive heat transfer. For example, the evaporator 532 is in thermally conductive contact with the electronic device 524. Specifically, the bottom of the evaporator 532 contacts the top of the electronic device 524.

As shown in this example, the transport member 536 includes a plurality of transport tubes 560 connecting the evaporator 532 and the condenser 534. In this example, there are six transport tubes 560, however, there may be fewer (e.g., two to five) or more (e.g., more than six) transport tubes 560. As illustrated, one or more of the transport tubes 560 include an open end 562 that is open to the vapor phase 541 of the working fluid in the evaporator 532 and a closed end 564 that is disposed in a specific area of the condenser 534. In some examples, each transport tube 560 can be cooled independently. For example, each transport tube 560 can be placed in a different condenser (e.g., a different condenser in the plurality of condensers 534) having a different geometry or having a different cooling medium. In another example, each closed end 564 is disposed in an area of the condenser 534 that is thermally different from other areas of the condenser 534 (e.g., there is little conductive heat transfer between adjacent areas of the condenser 534). Thus, in this example, the vapor phase 541 can fluidly enter the open end 562 of the transfer tube 560 and come to the closed end 564 located in the condenser 534 (e.g., via a pressure or thermal gradient).

In operation, heat from the electronic device 524 causes the liquid phase 537 of the working fluid (e.g., water or coolant) in the evaporator 532 to evaporate. As illustrated in FIG5A , the working fluid in its liquid phase 537 fills the evaporator 532 to a specific height within the volume of the evaporator 532, with working fluid vapor 541 (due to the transferred heat) above the liquid 537. In some aspects, the liquid 537 fills the evaporator 532 to a height above a finned surface (not shown) within the evaporator 532. In this example, the evaporator 532 is constructed from a small copper block with several fin structures (not shown). The fins can be machined/cut or welded/brazed to the evaporator copper block as separate parts. The fins can be plate fins or pin fins and can be coated with copper porous particles to increase the evaporation rate and reduce thermal resistance.

The vapor 541 then passes through the open end 562 of the duct 560 to the condenser 534. Heat radiates outward from the condenser 534, for example, into the air surrounding the condenser 534 or into air blown or drawn across the condenser 534 by one or more fans 526, one or more heat transfer surfaces 538 (e.g., finned surfaces), or both, causing the vapor phase 541 of the working fluid to condense within the duct 560 (e.g., within the closed end 564). As shown in FIG5A , the condenser 534 can be located between the one or more fans 526 and the evaporator 532, but can also be located on the opposite side of the one or more fans 526 (e.g., near the edge of the subassembly 510).

As shown in FIG5A , the delivery member 536 can be angled slightly (non-zero) so that gravity causes the condensed working fluid (e.g., liquid phase 537) to flow from the closed end 564 through the delivery tube 560 back to the evaporator 532. Thus, in some embodiments, at least a portion of the delivery tube 560 is not parallel to the major surface of the frame 520. For example, the condenser-side end of the delivery tube 560 can be approximately 1-5 mm (e.g., 2 mm) higher than the evaporator-side end of the delivery tube 560. However, the delivery tube 560 can also be horizontal or even angled slightly negatively (although a positive angle has the advantage of utilizing gravity to improve the flow of liquid from the condenser to the evaporator). Because multiple heat-generating electronic devices can be present on a single motherboard, multiple evaporators can be present on the motherboard, with each evaporator corresponding to a single electronic device.

In the illustrated embodiment, the liquid phase 537 of the working fluid can flow through a portion (e.g., the bottom or lower half) of the delivery tube 560, and the vapor phase 541 (or a mixed vapor-liquid phase) of the working fluid can flow through another portion (e.g., the top or upper half) of the delivery tube 560. Additionally, in some aspects, the delivery tube 560 can include a corresponding wick structure that helps circulate the liquid 541 back to the evaporator 532 (and the vapor 541 back to the condenser 534) via capillary forces.

In some alternative embodiments, the modular heat sink 530 may have multiple evaporators; each evaporator may contact a different electronic device, or multiple evaporators may contact the same electronic device (for example, if the electronic device is particularly large or has multiple heat generating areas). Multiple evaporators can be connected to the condenser 534 in series via ducts 560, for example, all ducts 560 connecting the condenser 534 to a first evaporator and a second evaporator. Alternatively, each of the multiple evaporators can be connected to the condenser 534 in parallel via a subset of ducts 560, for example, a first subset of ducts 560 connecting the condenser to the first evaporator, and a second subset of ducts 560 connecting the condenser 534 to the second evaporator. The advantage of the series embodiment is that there are fewer tubes, while the advantage of the parallel tubes is that the tube diameter can be smaller.

As shown in Figures 5A-5B, the closed ends 560 of the ducts 560 are positioned in (and the ducts 560 terminate in) different areas of the condenser 534. As shown in Figure 5B, the closed ends 560 can be positioned in a common plane within the condenser 534, but at different distances from the fan 526. Thus, in some aspects, the ducts 560 having closed ends 560 farther from the fan 526 can receive a cooler cooling airflow 590 from the fan 526 than the cooling airflow 590 received by the ducts 560 having closed ends 560 positioned closer to the fan 526. In some examples, the specific location of each closed end 560 of the ducts 560 can be selected or designed to maximize heat transfer from the vapor phase 541 of the working fluid in the ducts 560 to the cooling airflow 590. For example, it may be preferable or desirable for each duct 560 to have equal or substantially equal heat transfer capacity. Thus, due to, for example, different lengths of the ducts 560 (e.g., due to different locations of the closed ends 564 of the various ducts 560), various cooling fluid temperatures (e.g., based on the distance between the fan 526 and the respective closed ends 564 of the ducts 560) can balance the heat transfer capacity across the ducts 560.

The described features can be used to design cooling systems with significantly lower thermal resistance (e.g., relative to conventional technologies), which can also result in uniform temperatures across heat-generating devices (e.g., CPUs, etc.). For example, the heat transfer capacity ( _Qmax ) of a duct as described herein can be proportional to the inverse of the effective length ( _Leff ) of the duct (e.g., _Qmax = 3000/ _Leff ). Furthermore, the thermal resistance (R) between the surface of the heat-generating device and the cooling airflow at each duct contact location can be equal to the corresponding temperature difference ( _TD ) divided by the heat capacity of the duct: R = _TD / _Qmax . The thermal resistance itself is a function of the duct length and (if a wick is included) the wick structure. Thus, while each duct may experience a different cooling fluid temperature, its length and (if a wick is included) the wick structure can be selected (e.g., using the above equation) to achieve a uniform temperature across the surface of the heat-generating device. In some aspects, the reduction in surface temperature due to the described features can be approximately 5°C, or approximately 30% of the thermal resistance.

6A-6B illustrate schematic side and top views, respectively, of another example embodiment of a modular heat sink for a server rack subassembly according to the present disclosure. In some aspects, the modular heat sink 630 may include multiple (e.g., two or more) ducts fluidly connecting an evaporator 632 with a condenser 638 to transfer heat from one or more heat-generating electronic devices 624 to the surrounding environment via a working fluid. In the example modular heat sink 630, as shown, the condenser 638 is vertically disposed above the evaporator 632.

6A-6B , a modular heat sink 630 is illustrated with a server rack subassembly 610. The server rack subassembly 610 includes a frame or cassette 620, a printed circuit board 622 (e.g., a motherboard) supported on the frame 620, one or more heat-generating electronic devices 624 (e.g., a processor or memory) mounted on the printed circuit board 622, and the modular heat sink 630. One or more fans 626 may also be mounted on the frame 620.

The modular heat sink 630 includes an evaporator 632, a condenser 634 mounted on top of the evaporator 632, and a plurality of transfer tubes 636 connecting the evaporator 632 to the condenser 634. The evaporator 632 contacts the electronic device 624 so that heat is drawn from the electronic device 624 to the evaporator 632 via conductive heat transfer. For example, the evaporator 632 is in thermally conductive contact with the electronic device 624. Specifically, the bottom of the evaporator 632 contacts the top of the electronic device 624 (e.g., directly or through a heat transfer surface such as a phase change material).

As shown in Figures 6A and 6B, the condenser 634 is vertically positioned directly above the evaporator 632, which in some aspects can allow for space savings on the frame 620. A heat transfer surface 638 (e.g., fins) is mounted to the top of the condenser 634. As shown in this example, a plurality of delivery tubes 636 connect the evaporator 532 and the condenser 534. In this example, there are six delivery tubes 636, however, there may be fewer (e.g., two to five) or more (e.g., more than six) delivery tubes 636. As shown, one or more of the delivery tubes 636 include an open end 642 that opens to the vapor phase 639 of the working fluid in the evaporator 632 and a closed end 640 positioned in a specific area of the condenser 634. Thus, in this example, the vapor phase 639 can fluidly enter the open end 642 of the delivery tube 636 and reach the closed end 640 located in the condenser 634 (e.g., via a pressure or thermal gradient). In some examples, each closed end 640 is disposed in a region of the condenser 634 that is thermally distinct from other regions of the condenser 634 (e.g., there is little conductive heat transfer between adjacent regions of the condenser 634). Thus, in this example, the vapor phase 639 can fluidly enter the open end 642 of the transfer tube 636 and pass to the closed end 640 located in the condenser 634 (e.g., via a pressure or thermal gradient).

In operation, heat from the electronic device 624 causes the liquid phase 637 of the working fluid (e.g., water or coolant) in the evaporator 632 to evaporate. As illustrated in FIG6A , the working fluid, in liquid phase 637, fills the evaporator 632 to a specific height within the volume of the evaporator 632, with working fluid vapor 639 (due to the transferred heat) above the liquid 637. In some aspects, the liquid 637 fills the evaporator 632 to a height above a finned surface (not shown) within the evaporator 632. In this example, the evaporator 632 is constructed from a small copper block with several fin structures (not shown). The fins can be machined/cut or welded/brazed to the evaporator copper block as separate parts. The fins can be plate fins or pin fins and can be coated with copper porous particles to increase the evaporation rate and reduce thermal resistance.

The vapor 639 then passes through the open end 642 of the transfer tube 636 to the condenser 634. Heat radiates outward from the condenser 634, for example, into the air surrounding the condenser 634 or into air blown or drawn across the condenser 634 by one or more fans 626, one or more heat transfer surfaces 638 (e.g., finned surfaces), or both, causing the vapor phase 639 of the working fluid to condense within the transfer tube 636 (e.g., within the closed end 640). The condensed liquid phase 641 can then circulate (e.g., fall) down the transfer tube 636 to the evaporator 632.

In the illustrated embodiment, the liquid phase 637 of the working fluid can flow through a portion of the delivery tube 636, and the vapor phase 641 (or a mixed vapor-liquid phase) of the working fluid can flow through another portion of the delivery tube 636. Additionally, in some aspects, the delivery tube 636 can include a corresponding wick structure that helps circulate the liquid 641 back to the evaporator 632 (and the vapor 641 to the condenser 634) via capillary forces.

In some alternative embodiments, the modular heat sink 630 may have multiple evaporators; each evaporator may contact a different electronic device, or multiple evaporators may contact the same electronic device (for example, if the electronic device is particularly large or has multiple heat generating areas). The multiple evaporators may be connected to the condenser 634 in series via the ducts 636, for example, all of the ducts 636 connect the condenser 634 to the first evaporator and the second evaporator. Alternatively, each of the multiple evaporators may be connected to the condenser 634 in parallel via a subset of the ducts 636, for example, a first subset of the ducts 636 connects the condenser to the first evaporator, and a second subset of the ducts 636 connects the condenser 634 to the second evaporator. The advantage of the series embodiment is that there are fewer tubes, while the advantage of the parallel tubes is that the tube diameter can be smaller.

As shown in Figures 6A and 6B, the closed ends 640 of the ducts 636 are positioned in different areas of the condenser 634, and the ducts 636 terminate in different areas of the condenser 634. As shown in Figure 6B, the closed ends 640 may be positioned in a common plane within the condenser 634, but at different distances from the fan 626. Thus, in some aspects, the ducts 636 with closed ends 640 positioned farther from the fan 626 may receive a cooler cooling airflow 690 from the fan 626 than the cooling airflow 690 received by the ducts 636 with closed ends 640 positioned closer to the fan 626. In some examples, the specific location of each closed end 636 of the ducts 636 may be selected or designed to maximize heat transfer from the vapor phase 641 of the working fluid in the ducts 636 to the cooling airflow 690. For example, it may be preferable or desirable for each duct 636 to have equal or substantially equal heat transfer capacity. Thus, due to, for example, different lengths of the ducts 636 (e.g., due to different locations of the closed ends 664 of the various ducts 636), various cooling fluid temperatures (e.g., based on the distance between the fan 626 and the respective closed ends 664 of the ducts 636) can balance the heat transfer capacity across the ducts 636.

Many embodiments have been described. However, it will be understood that various modifications can be made without departing from the spirit and scope of what has been described. Accordingly, other implementations are within the scope of the following claims.

Claims (31)

1. A data center cooling system, comprising: Modular heat sink, the modular heat sink comprising: An evaporator configured to be in thermal contact with a heat-generating electronic device to receive heat from the heat-generating electronic device; A condenser, coupled to the evaporator and configured to transfer the heat from the heat-generating electronics to a cooling fluid; and A plurality of delivery pipes fluidly couple the evaporator and the condenser, the plurality of delivery pipes extending through the outer casing of the condenser and into the internal capacity of the condenser, such that the plurality of delivery pipes are structurally independent of the structure of the condenser, each of the plurality of delivery pipes including an open end disposed in the evaporator and each of the plurality of delivery pipes including a closed portion extending from a location at the outer casing of the condenser and terminating at a closed end disposed in the internal capacity of the condenser, the closed ends of the plurality of delivery pipes being disposed in corresponding regions of the condenser including different thermal regions of the condenser, such that at least a portion of the plurality of delivery pipes has different effective lengths measured from a corresponding open end to a corresponding closed end of that portion of the plurality of delivery pipes; and A working fluid, which vaporizes in the evaporator based on heat received from the heat-generating electronic device, and circulates in the delivery pipe from the evaporator to the condenser in a vapor phase and from the condenser to the evaporator in a liquid phase. The working fluid in each closed section of the delivery pipe located within the internal capacity of the condenser is fluidly separated from the working fluid in the other closed sections of the delivery pipe located within the internal capacity of the condenser. 2. The data center cooling system of claim 1, wherein the working fluid comprises water, and the evaporator comprises copper. 3. The data center cooling system according to claim 2, wherein the water comprises deionized or reverse osmosis (RO) water. 4. The data center cooling system of claim 1 further includes a fan configured to circulate cooling fluid above the condenser. 5. The data center cooling system of claim 4, wherein the fan is mounted on a frame supporting the server board assembly of the heat-generating electronic equipment. 6. The data center cooling system according to claim 1 further includes a heat transfer surface disposed within the contents of the evaporator. 7. The data center cooling system of claim 6, wherein the heat transfer surface comprises copper fins integrated with the evaporator. 8. The data center cooling system of claim 7, wherein the copper fins extend upward from the bottom surface of the contents of the evaporator, and the height of the finned structure included in the evaporator is less than the operating level of the working fluid in the evaporator. 9. The data center cooling system of claim 6, wherein at least a portion of the heat transfer surface is coated with a porous coating. 10. The data center cooling system of claim 9, wherein the porous coating comprises copper particles. 11. The data center cooling system of claim 1, wherein the condenser is vertically mounted on top of the evaporator. 12. The data center cooling system of claim 1, wherein the condenser is mounted to a frame supporting the server board assembly of the heat-generating electronic equipment. 13. The data center cooling system of claim 1, wherein the plurality of delivery pipes include heat pipes, and each of the heat pipes includes a core structure. 14. The data center cooling system of claim 4, wherein the respective closed ends of the portion of the plurality of delivery pipes are disposed within a common plane in the condenser. 15. The data center cooling system of claim 14, wherein the corresponding closed end of this portion of the plurality of delivery pipes is closer to the fan than the corresponding closed end of another portion of the plurality of delivery pipes. 16. The data center cooling system of claim 1, wherein adjacent distinct thermal zones of the condenser have substantially no conductive heat transfer between them. 17. The data center cooling system of claim 1, wherein the working fluid in each delivery pipe is fluidly separated from the working fluid in other delivery pipes between the respective open ends and the respective closed ends of the plurality of delivery pipes. 18. The data center cooling system of claim 1, wherein the effective length of each delivery pipe is proportional to the thermal resistance between the surface of the heat-generating electronic device and the cooling fluid adjacent to the closed end of the delivery pipe. 19. The data center cooling system of claim 1, wherein each of the plurality of delivery pipes includes a cross-sectional area as part of the total cross-sectional flow region of the liquid phase of the working fluid flowing from the condenser to the evaporator. 20. A method for cooling data center electronic equipment, comprising: In a modular heat sink evaporator, heat from a heat-generating electronic device that is in thermal contact with the evaporator is used to vaporize at least a portion of the working fluid. The vapor phase of the working fluid is circulated from the evaporator through the respective open ends of a plurality of delivery pipes fluidly coupled to the evaporator to the condenser of the modular heat sink, the respective open ends being located in the evaporator; At least a portion of the vapor phase of the working fluid is condensed into the liquid phase of the working fluid in corresponding closed portions of the plurality of delivery pipes, the plurality of delivery pipes extending from the outer shell of the condenser and into the internal capacity of the condenser, such that the plurality of delivery pipes are structurally independent of the structure of the condenser pipes, the plurality of delivery pipes terminating in closed ends within the internal capacity of the condenser, the closed ends of the plurality of delivery pipes being located in corresponding regions of the condenser including different thermal regions of the condenser, such that the plurality of delivery pipes have different effective lengths measured from corresponding open ends of that portion of the plurality of delivery pipes to corresponding closed ends of that portion of the plurality of delivery pipes; and The liquid phase of the working fluid is circulated to the evaporator through the plurality of delivery pipes such that the working fluid in each of the delivery pipes is fluidly separated from the working fluid in the other delivery pipes. 21. The method of claim 20, wherein the working fluid comprises water, and the evaporator comprises copper. 22. The method of claim 21, wherein the water comprises deionized or reverse osmosis (RO) water. 23. The method of claim 20, further comprising circulating the cooling airflow over the condenser. 24. The method of claim 23, wherein circulating the cooling airflow includes using a fan mounted on the frame supporting the server board assembly of the heat-generating electronics to circulate the cooling airflow. 25. The method of claim 20, further comprising transferring heat from the heat-generating electronic device to the liquid phase of the working fluid via a heat transfer surface disposed within the contents of the evaporator. 26. The method of claim 25, wherein at least a portion of the heat transfer surface is coated with a porous coating comprising copper particles. 27. The method of claim 20, wherein the condenser is mounted vertically on top of the evaporator. 28. The method of claim 20, wherein the plurality of delivery pipes comprise heat pipes, each of the heat pipes comprising a core structure. 29. The method of claim 23, wherein the respective closed ends of the portions of the plurality of delivery pipes are disposed within a common plane in the condenser. 30. The method of claim 29, wherein the corresponding closed end of this portion of the plurality of delivery pipes is closer to the fan than the corresponding closed end of another portion of the plurality of delivery pipes. 31. The method of claim 20, wherein adjacent distinct thermal regions of the condenser have substantially no conductive heat transfer between them.