Category Archives: Liquid Cooling

3-D-printed Heat Exchangers provide flexibility in thermal management

Posted on June 7, 2019 by | 4 comments

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

Additive manufacturing technologies have expanded in many directions in recent years with applications ranging across numerous industries and applications, including into the thermal management of electronics. As metal 3-D printing techniques have improved and become commercially viable, engineers are using it to create innovative cooling solutions, particularly heat exchangers.

3-D Printed Heat Exchangers
Figure 1. 3-D developed heat exchangers can feature shapes not obtainable using traditional forming methods. [2]

Why are engineer turning to additive manufacturing?

One reason is that additive manufacturing allows for generous cost savings. Companies can reduce 15-20 existing part numbers and print them as a single component. A single part eliminates inventory, additional inspections, and assemblies that would have been necessary when components were produced individually.

As AdditiveManufacturing.com notes, “Some envision AM (additive manufacturing) as a complement to foundational subtractive manufacturing (removing material like drilling out material) and to a lesser degree forming (like forging). Regardless, AM may offer consumers and professionals alike, the accessibility to create, customize and/or repair product, and in the process, redefine current production technology.” [1]

Developed at the Massachusetts Institute of Technology (MIT), 3-D printing is the most common and well-known form of additive manufacturing. Three-dimensional objects are made by building up multiple layers of material. Thanks to the continued (and rapid) development of the technology and advanced research in materials science, the layers can be composed of metal, plastic, concrete, living tissue or other materials.

In industrial applications, 3-D printing has encouraged creativity. With additive manufacturing, designers can create complex geometric shapes that would not be possible with standard manufacturing processes. For example, shapes with a scooped out or hollow center can be produced as a single piece, without the need to weld or attach individual components together. One-piece shapes can provide extra strength, with few or no weak spots that can be compromised or stressed. [4]

Making 3-D Printed Heat Exchangers

Heat exchangers are integral to thermal management. Any time heat, cool air, or refrigeration are required, a heat exchanger has to be involved to dissipate the heat to the ambient. This can be as simple as a standard heat sink or a complex metal structure used in liquid cooling. It can be as small as a few millimeters or as large as a building. Heat exchange is a multi-billion-dollar industry touching everything from consumer goods to automotive and aerospace engineering.

Compact heat exchangers are typically composed of thin sheets of material that are welded together. The complexity of the designs, particularly the density of the fin field, makes production both challenging and time-consuming, while the material used for the welding process adds to the overall weight of the part. Heat exchangers produced through 3-D printing techniques (such as those pictured below) can be made quicker, lighter, and more efficiently.

Figure 2. 3-D developed heat exchanger had a 20% increase in efficiency. [2]

In 2016, a Department of Energy-funded consortium of researchers developed a miniaturized air-to-refrigerant heat exchanger that was more compact and energy-efficient than current market designs. CEEE and 3-D Systems teamed to increase the efficiency of a 1 kW heat exchanger by 20 percent while reducing weight and size. The manufacturing cycle for the heat exchanger was reduced from months to weeks. [4]

Figure 3. A 3-D printed milli-structured heat exchanger made from stainless steel with a gyroid design. [5]

Using direct metal printing (DMP), manufacturers delivered a 20-percent more efficient heat exchanger and an innovative design. It was produced in weeks not months and with significantly lower weight. The one-part, 3-D-printed heat exchanger required minimal secondary finishing operations.

Ohio-based Fabrisonic uses a hybrid metal 3-D printing process, called Ultrasonic Additive Manufacturing (UAM), to merge layers of metal foil together in a solid-state thanks to high frequency ultrasonic vibrations. [5]

Figure 4. Aluminum and copper heat exchanger printed using ultrasonic additive manufacturing. (Photo via Fabrisonic) [6]

Fabrisonic mounts its hybrid 3-D printing process on traditional CNC equipment – first, an object is built up with 3-D printing, and then smoothed down with CNC machining by milling to the required size and surface. No melting is required, as Fabrisonic’s 6 ft. x 6 ft. x 3 ft. UAM 3-D printer can scrub metal foil and build it up into the final net shape, and then machines down whatever else is needed at the end of the process.

This 3-D printing process was recently given a stamp of approval by NASA after testing at the Jet Propulsion Laboratory (JPL). A report from NASA and Fabrisonic said, “UAM heat exchanger technology developed under NASA JPL funding has been quickly extended to numerous commercial production applications. Channel widths range from 0.020 inch to greater than one inch with parts sized up to four feet in length.” [6]

There are challenges involved, to be sure. In an article from Alex Richardson of Aquicore highlighting research done at the University of Maryland, researchers discuss the problems that 3-D printing still has competing on price against traditional manufacturing techniques and the difficulties involved with physically scaling a technology up.

In the article, Vikrant Aute of the University of Maryland Center for Environmental Energy Engineering noted that his research team was “considering modularization to overcome the latter issue: Instead of making the exchangers bigger, it might be possible to arrange lots of them together to accomplish the same task.” [7]

Research Continues to Improve 3-D Printing Process

While there have been numerous advancements in the technology of metal 3-D printing, research is continuing on campuses and in companies around the world to try and improve the process and make it easier to create increasingly complex heat exchangers.

For example, Australia-based additive manufacturing startup Conflux Technology received significant funding to develop its technology specifically for heat exchange and fluid flow applications. [8] Another example was the University of Wisconsin-Madison, which received a grant from the U.S. Department of Energy (DOE) Advanced Research Projects Agency-Energy (ARPA-E) to build heat exchangers with “internal projections to increase turbulence and facilitate heat transfer. Such intricate shapes are impossible with traditional manufacturing.” [9]

In 2018, U.K.-based Hieta Technologies partnered with British metrology company Renishaw to commercialize its 3-D-printed heat exchangers. Renishaw used its AM250 system to 3-D print walls of the heat exchanger as thin as 150 microns. The samples were heat treated and characterized to confirm that the laser powder bed fusion process was effective. The process took only 80 hours, was 30 percent lighter, and had 30 percent less volume, while still meeting the heat transfer and pressure drop requirements. [10, 11]

Last month, GE Research announced that it was leading a multi-million-dollar program with Oak Ridge National Laboratory (ORNL) and the University of Maryland to develop compact heat exchangers that can withstand temperatures as high as 900°C and pressures as high as 250 bar. This was also based on funding from ARPA-E, as part of its HITEMMP (High-Intensity Thermal Exchanger through Materials and Manufacturing Processes) program. [12]

3-D Printed Heat Exchangers
Fig. 5. GE Research is leading a project to design a new, high-temperature heat exchanger with 3-D printing. [12]

To build the new heat exchanger, GE engineers are using a novel nickel superalloy that is designed for high temperatures and is crack-resistant. University of Maryland researchers are working with GE to create biological shapes that will make the heat exchanger more efficient and ORNL researchers are providing corrosion resistance expertise to develop the materials for long-term use.

These are just some examples of the many ways that 3-D printing has impacted electronics cooling. Researchers at the Fraunhofer Institute for Laser Technology ILT in Germany have demonstrated the feasibility of 3-D printing copper [13], U.K. researchers 3-D printed “smart materials” for energy storage [14], a researcher at Penn State (soon to be at MIT) is developing methods for creating rough surfaces through additive manufacturing to enhance boiling heat transfer [15], and at Virginia Tech researchers developed a new process for 3-D printing piezoelectric materials [16].

The technology is growing by leaps and bounds each year and is enhancing the options for engineers in the thermal management industry.

References

  1. http://additivemanufacturing.com/basics/
  2. https://www.3-Dsystems.com/learning-center/case-studies/direct-metal-printing-dmp-enables-ceee-manufacture-lean-and-green-heat
  3. https://www.spilasers.com/application-additive-manufacturing/additive-manufacturing-a-definition/
  4. https://www.3-Dsystems.com/learning-center/case-studies/direct-metal-printing-dmp-enables-ceee-manufacture-lean-and-green-heat
  5. http://fabrisonic.com/ultrasonic-additive-manufacturing-overview/
  6. https://aquicore.com/blog/3-D-printing-heat-exchangers/
  7. https://cdn2.hubspot.net/hubfs/3985996/Articles%20-%20published/NASA%20HX%20White%20Paper%20EWI.pdf
  8. https://www.confluxtechnology.com
  9. https://www.engr.wisc.edu/researchers-bring-3d-printing-cool-industry/
  10. https://3dprint.com/198933/hieta-renishaw-heat-exchangers/
  11. https://www.youtube.com/watch?v=r42Dc_PKBEc
  12. https://www.ge.com/research/newsroom/ge-researchers-utilize-3d-printing-design-ultra-performing-heat-exchanger-more-efficient
  13. https://www.ilt.fraunhofer.de/en/press/press-releases/press-release-2017/press-release-2017-08-30.html
  14. https://www.qmul.ac.uk/media/news/2018/se/scientists-design-material-that-can-store-energy-like-an-eagles-grip.html
  15. https://news.psu.edu/story/574464/2019/05/15/academics/heat-transfer-additive-manufacturing-powers-nsf-graduate-research
  16. https://vtnews.vt.edu/articles/2019/01/3d_printing_discovery.html

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com. To register for Qpedia and to get access to its archives, visit 
https://www.qats.com/Qpedia-Thermal-eMagazine.

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Immersion Liquid Cooling of Servers in Data Centers

Posted on March 26, 2019 by | 1 comment

A data center is a large infrastructure used to house large quantities of electronic equipment, such as computer servers, telecommunications equipment, and data storage systems, etc. The data center requires non-interrupted power, communication and internet access to all equipment inside, it also has dedicated environment control system which provides appropriate working conditions for the electrical devices hosted inside.

Immersion Cooling

Traditional data centers use cold air generated by a room air conditioner system (CRAC) to cool the servers installed on the racks. Cooling the electrical devices by cold air generated by an air conditioner is an easy method to implement. However, it is not a very efficient method in terms of power consumption.

The inefficiency of the method can be contributed to several causes: generating and delivering cold air from a chiller to servers is a multiple heat transfer process, such as the mixing of warm and cool air in the room, which reduces the efficiency and power consumption of cooling hardware such as chillers, computer room air conditioners (CRACs), fans, blowers and pumps.

Data center designers and operators have invented many ways to improve the data center’s thermal efficiency, such as optimizing the rack layout and air conditioner location, separating cold aisles and hot aisles, optimizing the configuration of pipes and cables in under-floor plenum, introducing liquid cooling to high-power severs.

While the above methods can improve the data center heat load management, they cannot dramatically reduce the Power Usage Effectiveness (PUE), which is a measure of how efficiently a datacenter uses its power and is defined as the ratio of total datacenter power consumption to the IT equipment power consumption.

An ideal PUE is 1,0. A better way, proposed and used by some new data centers, is directly bringing the outside cold air to the servers. This method can eliminate the computer room air conditioners (CRACs). To achieve this, the data center has to be located in a specific area where cold air can be provided for all four seasons and the servers have to have higher operating environmental temperature.

Another dramatic solution proposed and used by some companies is liquid immersion cooling for entire servers. When compared with traditional liquid cooling techniques, the liquid immersion cooling uses dielectric fluid as a working agent and open bath design. This eliminates the need for hermetic connectors, pressure vessels, seals and clamshells. There are several different liquid immersion cooling methods.

This article will review the active single-phase immersion cooling technology proposed by Green Revolution Cooling (GRC) [1] and a passive two-phase immersion cooling technology proposed by the 3M Company [2].

Green Revolution Cooling has designed a liquid-filled rack to accommodate the traditional servers and developed dielectric mineral oil as the coolant. Figure 1 shows the liquid cooling racks with chiller and an inside view of a CarnotJet cooling rack from GRC. The racks are filled with 250 gallons of dielectric fluid, called GreenDEF™, which is a non-toxic, clear mineral oil with light viscosity.

Figure 1. Server racks and chiller (left) and inside view of the server rack. [1]

The servers are installed vertically into slots inside the rack and fully submerged in the liquid coolant. Pumps are used to circulate the cold coolant from the chiller to the rack. The coolant returns to the chiller, after removing heat from the servers. Because of its high heat capacity and thermal conductivity, the GreenDEF™ can cool the servers more efficiently than air.

The server racks are semi-open to the environment and the coolant level is constantly monitored by the system. Figure 2 shows a server motherboard is being submerged in the coolant liquid inside a server rack from GRC.

Figure 2. A Server Motherboard Being Immersed in Liquid Coolant in A Server Rack. [1]

Intel has conducted a year-long test with immersion cooling equipment from Green Revolution Cooling in New Mexico [3]. They have found that the technology is highly efficient and safe for servers. In their tests, Intel tested two racks of identical servers – one using traditional air cooling and the other immersed in a Green Revolution enclosure. Over the course of a year, the submerged servers had a partial Power Usage Effectiveness (PUE) of 1.02 to 1.03, equaling some of the lowest efficiency ratings reported using that metric.

The 3M Company is also actively engaged in immersion cooling technology and has developed a passive two-phase immersion cooling system for servers. Figure 3 illustrates the concept of the immersion cooling system developed by 3M. In a specially designed server rack, servers are inserted vertically in the rack. The servers are immersed in 3M’s Novec engineered fluid, a non-conductive chemical with a low boiling point.

The elevated temperature of electronic components on the sever boards will cause the Novec engineered fluid to boil. The evaporation of the fluid will remove a large amount of heat from the heated components with small temperature difference. The evaporated fluid travels to the upper portion of the server rack, where it condenses to liquid on the surface of the heat exchanger cooled by the cold water. The condensed liquid flows back to the rack bath, driven by the force of gravity. In 3M’s server rack, the liquid bath is also semi-open to the outside environment.

Because the cooling method is passive, there is no pump needed in the system.

Figure 3. Passive Two-phase Immersion Cooling System from 3M. [2]

By utilizing the large latent heat of Novec engineered fluid during evaporation and condensation, the coolant can remove heat from servers and dissipate it to water heat exchanger with small a temperature gradient. To enhance the boiling on the component surfaces, 3M invented special coating for electronic chips inside the liquid bath. The boiling enhancement coating (BEC) is a 100 mm thick porous metallic material.

The application of the BEC is illustrated in Figure 4. The coating is directly applied to the integrated heat spreader (IHS) of the chip. Tuma [2] claimed that the coating can produce boiling heat transfer coefficients in excess of 100,000 W/m2-K, at heat fluxes exceeding 300,000 W/m2.

Figure 4. Application of boiling enhanced coating (BEC). [2]

In his paper, Tuma [2] discussed the economic and environmental merits of the passive two-phase immersion cooling technology for cooling data center equipment. He concluded that liquid immersion cooling can dramatically decrease the power consumption for cooling relative to traditional air-cooling methods. It can also simplify facility construction by reducing floor space requirements, eliminating the need for air cooling infrastructure such as plenum, air economizers, elevated ceilings etc.

Green Revolution Cooling and 3M have demonstrated the feasibility and applicability of using immersion cooling technology to cool the servers in data centers. The main advantages of immersion liquid cooling are saving overall cooling energy and maintaining the component temperature low and uniform. However, both immersion liquid cooling technologies require specially designed sever racks. Specially formulated coolants are needed for both cooling technologies, too, and they are not cheap. For the traditional air-cooled data center, the air is free, abundant and easy to deliver.

In both immersion cooling technologies, the servers have to be vertically installed inside the server rack, which will reduce the date center footprint usage efficiency. Because the liquid baths used in immersion cooling are open to the environment, coolant is gradually and inevitably lost to the ambient during long term service.

The environmental impact of the discharge of a large amount of coolant by data centers has to be evaluated, too. The effect of the coolant on the connectors and materials used on the PCB is not also very clear.

Immersion liquid cooling is a very promising technology for cooling high-power servers. But, there are still obstacles that need to be overcome before their large scale application is assured.   

References

  1. http://www.grcooling.com
  2. Tuma, E. P., “The Merits of Open Bath Immersion Cooling of Datacom Equipment,” 26th IEEE SEMI-THERM Symposium, Santa Clara, California, USA  2010.
  3. http://www.datacenterknowledge.com

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

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Technology Review: Liquid Cooling Solutions

Posted on September 14, 2018 by | Leave a comment

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications.

Liquid Cooling Solutions

The focus of this article is to highlight recent patents for liquid cooling devices. (Advanced Thermal Solutions, Inc.)

We are specifically focusing on patented technologies to show the breadth of development in thermal management product sectors. Please note that there are many patents within these areas. Limited by article space, we are presenting a small number to offer a representation of the entire field. You are encouraged to do your own patent investigation.

Further, if you have been awarded a patent and would like to have it included in these reviews, please send us your patent number or patent application.

In this issue our spotlight is on liquid-based cooling solutions.

There are many U.S. patents in this area of technology, and those presented here are among the more recent. These patents show some of the salient features that are the focus of different inventors.

Liquid Cooling Devices

Embedded Microchannel Cooling Package for a Central Processing Unit

US 7515415 B2, Monfarad, A. and Yang, J.

An indirect cooling liquid embedded package design for use with a computer central processor unit is suitable for thermal management of high heat dissipation electronic components such as server processors. The indirect contact cooling liquid embedded packaged CPU has mechanical coupling and embedded plumbing that attaches to the board pumped liquid supply and indirect cooling of the heat-generating portion of the CPU with an embedded microchannel heat exchanger. The coolant package system for the CPU removes higher levels of heat indirectly from the core of the processors by convective cooling.

Cooling liquid flows into the microchannel piping in the CPU substrate. Cooling liquid continues to flow out of the microchannel piping into a silicon or metallic microchannel heat exchanger that is directly bonded to a silicon die for cooling of the heat-generating portion of the CPU. As a result, an embedded microchannel indirect contact cooling liquid package for a CPU can be utilized to remove substantially higher levels of heat from the core of the processors by forced convective liquid flow through the microchannel heat exchanger attached to the core of the CPU. Cooling liquid is introduced into the package of the server CPU by mechanically attaching the CPU to the board through a socket interconnect. Pins of the socket serve to provide electrical connection between the board and the CPU, while a few pins are designed for the purpose of providing an inlet and an outlet for cooling liquid into and out of the CPU package.

The cooling system of the present invention also uses the existing package-to-board practice of using sockets and therefore the entire cooling system is embedded into the processor-to-board assembly. From the end user’s point of view, there is a tremendous amount of simplification of board design as the bulky fan and heat sink assemblies are removed. The replacement, according to the present invention, is a central liquid cooling system that can be made redundant to substantially prevent any reliability issues in the field.

Planar Heat Pipe for Cooling

US 8305762 B2, Wits, W., Mannak, J. and Legtenberg, R.

The invention claimed is a circuit board for cooling of heat-dissipating components assembled thereon, including at least two panels at least one of which is populated With heat-dissipating components, both panels being metal clad on a side, at least one of the panels being formed from a printed circuit board laminate and comprising a plurality of grooves on its metal clad side, the panels being bonded together by an adhesive layer With their metal clad sides oriented face to face so as to form a circuit board containing a sealed cavity having a height defined by a thickness of the adhesive layer and the separation of the metal clad sides, the cavity being partly filled with a fluid, the fluid circulating by capillary action along the grooves towards zones exposed to heat where the fluid vaporizes.

Vapor may circulate back by pressure gradient effect through the cavity towards zones not exposed to heat where it condenses. In a mode of implementation, the heat pipe may be embedded in a circuit board formed by the panels for inherent cooling of heat-dissipating components.

Thus, key advantage provided by the present invention in any of its aspects is that it is based on most standard processes of multilayer PCB fabrication such as laminating, selective metal plating and etching. Therefore, it is a highly cost-effective solution. Furthermore, the invention provides a very flexible design solution enabling to adapt the cooling paths to the PCB layout, especially to the higher heat dissipation locations. Not requiring any supplementary materials, it is even considerably lighter than a tubular heat pipe-based solution.

Implemented as enhancement of a computer aided engineering tool, heat pipe cooling cavities could be designed concurrently with the layout of components placement and printing of circuits, ensuring optimized thermal management. This enables multilayer PCB assemblies, which are high density electronic devices, to benefit the most from the integrated heat pipe cooling function.

Cooling System for Electronic Equipment

US 7508666 B1, Henneberg, M. and Johnson, L.

A cooling solution includes a system providing thermal energy dissipation for electronic equipment located in support racks or cabinets of a facility. According to one embodiment, the system is integrated with a facility where the support cabinets are located. The system providing thermal energy dissipation includes a cooling loop, a fan unit for moving air across the cooling loop and one or more ducts forming a confined flow pathway for the moving air between the fan unit and cabinets for delivery to the electronic equipment.

More specifically, the cooling loop contains a supply of circulating heat absorbing fluid such that the heat absorbing fluid removes thermal energy from the air moved by the fan unit. Each cabinet is formed with an exhaust pathway such that the moving air enters the cabinet from the duct, flows across the electronic equipment to remove thermal energy therefrom, and exits the cabinet.

Claims include a cooling system for a facility housing electronic equipment, the facility having a support surface on which a cabinet holding the electronic equipment is located, the system comprising: a cooling loop located beneath the support surface and containing a supply of circulating heat absorbing fluid; a fan unit located beneath the support surface and configured to move air across the cooling loop such that the heat adsorbing fluid removes thermal energy from the moving air; at least one duct forming a confined flow pathway for the moving air between the fan unit and the cabinet, wherein the cabinet is formed with a substantially lateral exhaust pathway such that the moving air enters via a back region of the cabinet, flows across the electronic equipment, housed by the cabinets, to remove thermal energy therefrom, and exits a front region of the cabinet; and a chilling plate positioned downstream of the flow of moving air exiting the cabinet such that the chilling plate is located outside of the front region of the cabinet, the chilling plate is coupled to a secondary cooling loop containing a supply of heat absorbing fluid.

Claims also include a method for providing thermal energy dissipation for network-based electronic equipment housed within a plurality of cabinets located on a support surface of a facility, each of the plurality of cabinets having an interior formed with a through passageway extending from an entrance at a first side of each of the plurality of cabinets to an exit at a second side of each of the plurality of cabinets, the method comprising: providing a cooling loop containing a supply of circulating heat adsorbing fluid; providing a fan unit configured to move air across the cooling loop such that the heat adsorbing fluid removes thermal energy from the moving air; directing the moving air from the fan unit to the through passageway of each of the plurality of cabinets such that the moving air flows across the electronic equipment to remove thermal energy therefrom, and exits the plurality of cabinets; and providing a plurality of chilling plates positioned downstream of the flow of moving air exiting the plurality of cabinets such that one of the plurality of chilling plates is located outside of each of the plurality of cabinets, respectively, each of the plurality of chilling plates is coupled to a secondary cooling loop containing a supply of heat absorbing fluid.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

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ATS Releases New Liquid Cooling eBook

Posted on June 8, 2018 by | Leave a comment

Advanced Thermal Solutions, Inc. (ATS) has created an new Engineering eBook on Liquid Cooling and its applications in the thermal management of electronics. Click here to download this free, 16-page eBook loaded with technical content.

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How to Achieve Localized Cooling with Cold Plates

Posted on January 16, 2018 by | 6 comments

Many applications in electronics cooling require a cold plate to remove heat from discrete components laid out on a board. In these circumstances, it is more efficient that the liquid does not completely fill the cold plate, but is only transferred to areas that need to be cooled.

With this kind of design, the required volumetric flow rate of the coolant will be significantly lower than if the entire cold plate was filled with liquid. The schematic for a typical example of this scenario is given in Figure 1. [1]

Localized Cooling With Cold Plates

Figure 1. Schematic of a Board with a Localized Area of Heat Dissipation . [1]

In Figure 1, areas A, B, C and D must be cooled for the components dissipating from 5 to 15 W/cm2. The other areas, designated as open, have components that interfere with the cold plate and must be avoided in the design. Two designs were considered for this case: a drilled hole and a press-fit tube. Figure 2 shows the drilled hole concept.

As can be seen, there are multiple small holes around the heat dissipating components under the cold plate surface. Large holes are machined to interconnect the smaller holes. A technique called gun drilling was used for machining the long holes. The entire cold plate was made from a copper block.

Cold Plates

Figure 2. Schematic of the Drilled Hole Cold Plate Design. [1]

Figure 3 shows the press-fit tube design. In this approach, a copper tube with high thermal conductivity is routed through the areas of heat transfer and either brazed or epoxied to the aluminum cold plate base. This design is considerably lighter and cheaper than the drilled hole design.

Cold Plates

Figure 3. Schematic of the Press-Fit Tube Cold Plate Design. [1]

To analyze the performance of this cold plate configuration, simple analytical tools can be used for a standard cold plate design. A brief summary of the equations is described here. To analyze the problem, we first have to calculate how much flow is going through the cold plate, and evaluate the pressure drop of the flowing fluid.

Pressure drop is calculated from:

Where
Um = bulk mean fluid velocity (m/s)
f = fanning friction factor
Awet = wetted surface area of the tube
Ac = cross section of the tube
K = loss coefficients related to turns, sudden expansion and contraction, etc.

The friction factor was obtained from the following equation which is in satisfactory agreements for the laminar, turbulent and transition regimes [2]

Where

Where ν is the kinematic viscosity of the fluid (m2/s) and P is the wetted perimeter of the tube.

For the heat transfer calculation, the Nusselt number can be calculated from standard correlations in the literature for fully developed flow. The Nusselt number is related to the heat transfer coefficient as:

Where
Kf = fluid conductivity

For thermally developing flow the following correlation can be used: [3]

Where
Num = mean Nusselt number
Nu∞ = fully developed Nusselt number
L = duct length

Then the convective resistance can be calculated as:

Where
hm = mean heat transfer coefficient

For the tube fitted design the overall thermal resistance is made of four components: convection, tube conduction resistance, epoxy conduction resistance and the cold plate. It is stated as:

Where
Rh = convection resistance
Rtube = conduction resistance of tube walls
Repoxy = conduction resistance of the epoxy
Rcoldplate = conduction resistance of the cold plate

For the drilled design the overall thermal resistance can be written as:

If the heat transfer coefficient is based on the local fluid temperature, then a caloric resistance must be added based on the fluid mass flow rate. The effective heat transfer coefficient is then:

Where
ṁ = mass flow rate (kg/s)
Cp = fluid heat capacitance (kJ/kg·K)

Figure 3 shows the pressure drop of the two designs as a function of water flow rate. It can be seen that with a water flow rate up to 1.89 l/min (0.5 GPM) the pressure drop between the two designs is almost the same, but at higher flow rates the drilled design’s pressure drop exceeds the tube design. The sharp 90-degree turn of the drilled holes, which lead to a higher loss coefficient, is the major contributor to the higher pressure drop.

Figure 4. Total Pressure Drop of the Drilled Design and the Tube Design as a Function of the Volumetric Flow Rate. [1]

Figures 5 and 6 show the effective heat transfer coefficient of the two designs as a function of flow rate. The bend and sharp increase of the curves around 0.95 l/min (0.25 GPM) is due to the flow transitioning from laminar to turbulent. The drilled hole design shows effective convection heat transfer between 7,000 and 27,000 W/m2K for the range of flow between 0 and 7.56 l/m (2.6 GPM). The press-fit tube design on the other hand shows a lower effective heat transfer coefficient of between 6,000 and 17,000 W/m2K.

This is mostly due to the interfacial resistance and tube wall conduction. In the drilled design example, these two resistances do not exist. In a real application, the pumping of fluid is constrained by the pump and its characteristic curve. Even though the drilled hole shows a higher heat transfer coefficient for the same flow rate, the extra pressure drop caused by the drilled design may have a lower flow rate hence lowering the heat transfer coefficient.

Figure 5. Effective Heat Transfer Coefficient of the Tube Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 6. Effective Heat Transfer Coefficient of the Drilled Hole Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 7 shows another cold plate design, this one by Lytron. [4] In this design, the extended-surface cold plate material and micro-channel aluminum extrusion are sandwiched between aluminum sheets. The entire assembly is welded using vacuum brazing. It is all aluminum, which makes it very light weight. The flexibility of this design allows the placement of cooling channels in different positions to enable localized cooling.

Figure 7. Lytron Vacuum Brazing of a Cold Plate for Localized Cooling. [4]

The above analytics show that the performance of a cold plate for localized cooling can be calculated using a simple analytical tool. The designer then has to consider such factors as weight, manufacturing, cost and thermal performance to decide the best option for his or her design. The characteristic of the pump has a paramount effect on the design and cannot be neglected.

References:

1. Seaho, S., Moran, K. and Rearick, D. (IBM Corporation) and Lee, S. (Aavid Engineering), Thermal Performance Modeling and Measurements of Localized Water Cooled Cold Plate, http://www.aavidthermalloy.com/technical/papers/pdfs/water.pdf
2. Churchill, S., Comprehensive Correlating Equations for Heat, Mass and Momentum Transfer in Fully Developed Flow in Smooth Tubes, Ind. Eng. Chem. Fundam., Vol.16, 1977.
3. Al-Arabi, M., Turbulent Heat Transfer in the Entrance Region of a Tube, Heat Transfer Eng., Vol. 3, 1982.
4. http://www.lytron.com

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

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