Tag Archives: liquid cooling

Recent Research Into Next-Generation Heat Exchangers for Electronics Thermal Management

Posted on January 24, 2019 by | 1 comment

Since it was published around one year ago, the “What is a Heat Exchanger” video (watch it below) has been one of the most watched on the ATS YouTube page. With the obvious interest in heat exchangers in particular (and liquid cooling in general), we are curating recent research into the technology and its applications in the thermal management of electronics.

Heat Exchangers
Heat Exchangers are a common component in liquid cooling solutions for electronics. Below is recent research into this growing technology. (Advanced Thermal Solutions, Inc.)

The following are three examples of papers written about heat exchangers including applications in the automotive space to developing microchannels to enhance thermal performance to optimizing heat exchangers for use with high-powered electronics.

We have posted several pieces of content on this blog about heat exchangers in the past. Examples include:

Since heat exchangers remain a popular topic for engineers, we will continue to add new pieces about the technology in the coming months.

Novel Power Electronics Three-Dimensional Heat Exchanger

Read the full paper at https://www.nrel.gov/docs/fy14osti/61041.pdf.

Abstract: Electric-drive systems, which include electric machines and power electronics, are a key enabling technology to meet increasing automotive fuel economy standards, improve energy security, address environmental concerns, and support economic development. Enabling cost-effective electric-drive systems requires reductions in inverter power semiconductor area, which increases challenges associated with heat removal. In this paper, we demonstrate an integrated approach to the design of thermal management systems for power semiconductors that matches the passive thermal resistance of the packaging with the active convective cooling performance of the heat exchanger. The heat exchanger concept builds on existing semiconductor thermal management improvements described in literature and patents, which include improved bonded interface materials, direct cooling of the semiconductor packages, and double-sided cooling. The key difference in the described concept is the achievement of high heat transfer performance with less aggressive cooling techniques by optimizing the passive and active heat transfer paths. An extruded aluminum design was selected because of its lower tooling cost, higher performance, and scalability in comparison to cast aluminum. Results demonstrated a 102% heat flux improvement and a package heat density improvement over 30%, which achieved the thermal performance targets.

Microchannel Heat Exchanger for Electronics Cooling Applications

Read the full paper at http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1636343.

Abstract: The power consumption of electronic devices, such as semiconductor diode laser bars, has continually increased in recent years while the heat transfer area for rejecting the associated thermal energy has decreased. As a result, the generated heat fluxes have become more intense making the thermal management of these systems more complicated. Air cooling methods are not adequate for many applications, while liquid cooled heat rejection methods can be sufficient. Significantly higher convection heat transfer coefficients and heat capacities associated with liquids, compared to gases, are largely accountable for higher heat rejection capabilities through the micro-scale systems. Forced convection in micro-scale systems, where heat transfer surface area to fluid volume ratio is much higher than similar macro-scale systems, is also a major contributor to the enhanced cooling capabilities of microchannels. There is a balance, however, in that more power is required by microchannels due to the large amount of pressure drop that are developed through such small channels. The objective of this study is to improve and enhance heat transfer through a microchannel heat exchanger using the computational fluid dynamics (CFD) method. A commercial software package was used to simulate fluid flow and heat transfer through the existing microchannels, as well as to improve its designs. Three alternate microchannel designs were explored, all with hydraulic diameters on the order of 300 microns. The resulting temperature profiles were analyzed for the three designs, and both the heat transfer and pressure drop performances were compared. The optimal microchannel cooler was found to have a thermal resistance of about 0.07 °C-cm2 /W and a pressure drop of less than half of a bar.

Thermal Analysis of the Heat Exchanger for Power Electronic Device with Higher Power Density

Read the full paper at http://pe.org.pl/articles/2012/12a/70.pdf. Abstract: Liquid cooling system has been used in high power electronic device systems to cool down the temperature of power electronic device. Heat exchanger is an important part of liquid cooling system to transfer the heat generated by power electronic device into air. In this paper, a Streamline-upwind/Petrov-Galerkin (SUPG) stabilized finite element analysis method was proposed to solve the water and air governing formulas including the mass conservation equation, the momentum conservation and the energy conservation equation. Furthermore, the thermal characteristic of a heat exchanger is simulated, and the result was compared with an experiment. The comparison shows that this method is effective.

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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ATS Unveils Heat Exchanger Selection Tool

Posted on March 7, 2018 by | 1 comment

In 2017, Advanced Thermal Solutions, Inc. (ATS) added to its liquid cooling products with a new line of tube-to-fin, liquid-to-air heat exchangers with the industry’s highest density fins, which maximize heat transfer and provide greater cooling than other heat exchangers on the market.

Heat Exchanger Selection Tool

ATS released a line of tube-to-fin, liquid-to-air heat exchangers with the industry’s highest density fins to optimize heat transfer. (Advanced Thermal Solutions, Inc.)

ATS heat exchangers are available in seven different sizes and can be ordered with or without fans depending on your specific design requirements. Altogether, ATS offers 49 different heat exchanger options (not including the customized options to meet customer needs).

To make the selection process easier for engineers, ATS has recently unveiled a new Heat Exchanger Selection Tool that will point engineers to the exact option that will meet the inputted criteria.

Heat Exchanger Selection Tool

The tool asks five questions (measurement type in parentheses):

  • Air temperature from inlet to heat exchanger (Tai°C)
  • Heat need to be extracted by heat exchanger (QtotalW)
  • Water exit temperature from heat exchanger (Tfo°C)
  • Water flow rate (GPM)
  • Fan voltage (V)

Plug answers in to these questions and hit the “Optimum Heat Exchanger” button to see which of the ATS heat exchangers fits your specific liquid cooling system needs. Once you have the right part number, you can now purchase the right heat exchanger from Digi-Key Electronics.

In addition to heat exchangers, ATS has an array of liquid cooling products, including cold plates that provide 30 percent better thermal performance than comparable products on the market.

To learn more about the full line of liquid cooling options from ATS, visit https://www.qats.com/Products/Liquid-Cooling.

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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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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Posted in Cold Plates, Liquid Cooling, Thermal Analysis

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