This article presents basic equations for liquid cooling and provides numerical examples on how to calculate the loads in a typical liquid cooling system. When exploring the use of liquid cooling for thermal management, calculations are needed to predict its performance. While it is often assumed that a liquid coolant itself dissipates heat from a component to the ambient, this is not the case. A closed loop liquid cooling system requires a liquid-to-air heat exchanger. Because of its structure, several equations must be calculated to fully understand the performance and behavior of a liquid cooled system.
Cold plates bring localized cooling by transferring heat to a liquid that dissipates into the ambient or a secondary liquid. ATS cold plates cool high-powered electronics, IGBT modules, lasers, motor devices, automotive components, medical equipment, and other applications where liquid cooling is needed. Their internal, mini-channel fin structure enhances the surface area to maximize heat transfer with low pressure drop characteristics and provides uniform surface temperature.
Traditionally the IGBT modules were cooled by forced air-cooled heat sinks. Air-cooled heat sinks are still good thermal management solutions for low-power and less temperature-restricting IGBT modules. However, the high-power IGBT modules are exclusively cooled by liquid-cooled heat sinks, also known as cold plates. Learn more about their application in this white paper (PDF, download, no registration needed): Download it here
A microchannel heat sink (also known as a cold plate) can be used to cool high power IGBT
By Norman Quesnel, Senior Member of Marketing Staff Advanced Thermal Solutions, Inc. (ATS)
Liquid cooling systems transfer heat up to four times better than an equal mass of air. This allows higher performance cooling to be provided with a smaller system. A liquid cooled cold plate can replace spaceconsuming heat sinks and fans and, while a liquid cold plate requires a pump, heat exchanger, tubing and plates, there are more placement choices for cold plates because they can be outside the airflow. [1]
One-time concerns over costs and leaking cold plates have greatly subsided with improved manufacturing capabilities. Today’s question isn’t “Should we use liquid cooling?” The question is “What kind of liquid should we use to help optimize performance?”
Figure 1. A Liquid Cooling System for a Desktop PC with Two Cold Plates. [2]
For liquid cold plates, the choice of working fluid is as important as choosing the hardware pieces. The wrong liquid can lead to poor heat transfer, clogging, and even system failure. A proper heat transfer fluid should provide compatibility with system’s metals, high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability. [3]
Today, despite many refinements in liquid cold plate designs, coolant options have stayed relatively limited. In many cases, regular water will do, but water-with-additives and other types of fluids are available and more appropriate for certain applications. Here is a look at these coolant choices and where they are best suited.
Basic Cooling Choices
While water provides superior cooling performance in a cold plate, it is not always practical to use because of its low freezing temperature. Additives such as glycol are often needed to change a coolant’s characteristics to better suit a cold plate’s operating environment.
In fact, temperature range requirements are the main consideration for a cold plate fluid. Some fluids freeze at lower temperatures than water, but have lower heat transfer capability. The selected fluid also must be compatible with the cold plate’s internal metals to limit any potential for corrosion.
Table 1 below shows how the most common cold plate fluids match up to the metals in different cold plate designs.
Table 1: Compatibility Match-ups of Common Cold Plate Metals and Cooling Fluids [1]
The choices of cold plate coolants will obviously have varied properties. Some of the differences between fluids are less relevant to optimizing cold plate performance, but many properties should be compared. Tables 2 and 3 show the properties of some common coolants.
Tables 2 and 3. Comparisons of Properties of Typical Electronic Coolants. [4]
An excellent review of common cold plate fluids is provided by Lytron, an OEM of cold plates and other cooling devices. The following condenses fluid descriptions taken from Lytron’s literature. [5]
The most commonly used coolants for liquid cooling applications today are:
Water
Deionized Water
Inhibited Glycol and Water Solutions
Dielectric Fluids
Water – Water has high heat capacity and thermal conductivity. It is compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Facility water or tap water is likely to contain impurities that can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance. If you determine that your facility water or tap water contains a larger percentage of minerals, salts, or other impurities, you can either filter the water or you can opt to purchase filtered or deionized water. [5,6]
Deionized Water – The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids, deionized water has a high resistivity. Deionized water is an excellent insulator, and is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended. [5, 7]
Inhibited Glycol and Water Solutions – The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces. [5, 8]
Dielectric Fluid – A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water. However, it has a much lower thermal conductivity and much higher price. PAO is a synthetic hydrocarbon used for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid-cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO-compatible recirculating chillers are available. Like perfluorinated carbons, PAO has much lower thermal conductivity than water. [5, 9]
Conclusion
Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications.
It is important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop.
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.
This Technology Review will focus on recent developments in spray cooling technology. (Wiklmedia Commons)
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 spray cooling for electronics thermal management.
There are several US 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.
Full
Coverage Spray and Drainage System and Method for Orientation-Independent
Removal of High Heat Flux
A cooling system and
method that significantly improves spray evaporative cooling by using arrays of
slot or plane sprays to create coverage of the entire heated surface to be
cooled without allowing interaction between plumes that are spraying from the
nozzles. The sprays are directed at an angle to the surface to take advantage
of the high droplet momentum possessed by the spray to direct a flow of coolant
fluid across the surface toward desired draining points, thereby enabling
drainage regardless of the orientation of the unit.
The present invention provides a spray cooling system and
method that significantly improves spray evaporative cooling by creating a
directed momentum flow of cooling fluid across a surface to be cooled. In
accordance with the present invention, a spray of cooling fluid is directed
directly onto the surface of a work piece to be cooled at an angle with respect
to the work piece surface so as to create a flow of cooling fluid in a
substantially single direction along the work piece surface. The spray of cooling
fluid preferably may be delivered via a plurality of generally fan shaped
sprays. The sprays are positioned and aligned to create cooling fluid coverage
of the entire heated surface to be cooled without allowing interaction between
the spray plumes in a manner that may cause areas of cooling fluid stagnation
on the surface.
A full coverage spray and drainage system in accordance with the present invention may be implemented in an otherwise conventional spray cooling system including a reservoir of an appropriate cooling fluid (e.g., Fluorinert-72 for the cooling of electronic circuitry, preferably saturated with a non-condensable inert gas, such as nitrogen), a pump for delivering the cooling fluid under pressure from the reservoir to a spray chamber to be sprayed therein from nozzles onto the work piece to be cooled, and appropriate filtering, metering, and control systems. Cooling fluid is returned from the spray chamber to the coolant reservoir via a drainage point or points in the spray chamber.
In accordance with the present invention, the drainage point or points in the spray chamber may be positioned with respect to the coolant spray such that the flow of cooling fluid directed in a substantially single direction along the work piece surface also is directed toward the drainage point or points. Thus, the cooling fluid momentum directs the fluid toward the drainage point, thereby assuring proper drainage of the cooling fluid despite changes in the orientation of the cooling system.
Directly
Injected Forced Convection Cooling for Electronics
Electronic circuitry includes a circuit board and at
least one component mounted on the circuit board, with the at least one
component generating heat while in use. The circuit board includes one or more
apertures aligned with one or more respective components, and the electronic
circuitry is configured to provide, while in use, a path for coolant fluid to
flow through each aperture and past the respective component.
By providing at least one aperture aligned with a
component that generates heat in use, improved cooling of the electronic
circuitry may be provided, as cooling effects can more efficiently be targeted
at those parts of the circuitry that generate or dissipate heat.
Each aperture may be, but is not necessarily positioned
at that point or within that region of the circuit board that is a minimum
distance from the component or a respective one of the components.
The central axis of each aperture may be, but is not
necessarily, perpendicular to the plane of the circuit board and at least one
component. Preferably each aperture is arranged such that a straight line
extending out of the aperture along the central axis of the aperture would pass
through the component with which the aperture is aligned. Preferably each
aperture is arranged such that, in use, coolant fluid exits the aperture
towards the component with which the aperture is aligned.
The coolant fluid may be liquid or gas. The coolant fluid
may be water. The coolant fluid may comprise a dielectric fluid, for example
poly-alpha-olefin (PAO), or an inert gas, for instance nitrogen. Preferably the
coolant fluid is air. In some circumstances, the coolant fluid may be supplied
from a pressurized source, for instance a pressurized gas cylinder.
The position of each aperture may be such that, in use,
coolant fluid passing through the aperture approaches the surface of the component
with which the aperture is aligned from a perpendicular direction.
Thereby a jet impingement effect may be provided such
that, preferably, the coolant fluid breaks through a respective thermal
boundary layer next to the or each at least one heat generating component. Such
thermal boundary layers are stable layers of air or other fluid which may build
up next to the or each component and which exhibit a temperature gradient away
from the component. The presence of such thermal boundary layers can reduce
convective cooling effects.
Narrow
Gap Spray Cooling in a Globally Cooled Enclosure
Electronic circuit boards are arranged as respective
parallel pairs defining a narrow gap there between. One or more such pairs of
boards are supported within a hermitically sealable housing and cooled by way
of spraying an atomized liquid coolant from a plurality of nozzles into each
narrow gap. Transfer of heat from the circuit boards results in vaporization of
at least some of the atomized liquid within the narrow gap. The housing further
serves to guide a circulation of vapors out of each narrow gap, back toward the
nozzles, and back into each narrow gap. A heat exchanger exhausts heat from the
housing and overall system, wherein vapor is condensed back to liquid phase
during contact and heat transfer therewith. Condensed liquid is collected and
re-pressurized for delivery back to the nozzles such that a sustained cooling
operation is performed.
One embodiment provides for a system, including a first
entity and a second entity that are respectively disposed such that they define
a narrow gap between them. The system also includes at least one nozzle,
wherein the nozzle is configured to spray an atomized liquid so that a flow of
the atomized liquid and a vapor is induced through the narrow gap. The system
also includes a heat exchanger that is configured to condense some of the vapor
to liquid, the condensed vapor defining a condensate. The system further
includes a housing configured to guide a circulation of at least some of the
vapor, which is flowing out of the narrow gap, away from the heat exchanger and
into proximity with the at least one nozzle.
Another embodiment provides for a system, the system
comprising a housing configured to selectively open-ably enclose a plurality of
electronic circuit boards. The system further includes a plurality of
electronic circuit boards supported in the housing, wherein at least some of
the electronic circuit boards are arranged to define respective pairs of
boards. At least one pair of boards defines a narrow gap there between. The
system also includes at least one nozzle associated with each narrow gap, each
nozzle being configured to spray an atomized liquid into the narrow gap defined
by the associated pair of boards. The housing is also configured to guide a
circulation of a vapor exiting each narrow gap into proximity with the at least
one nozzle associated with the at least one narrow gap.
Still another embodiment provides an apparatus. The
apparatus includes a nozzle configured to spray an atomized liquid in a
generally conical distribution pattern. The apparatus further includes a
re-shaper that is configured to reform the spray of atomized liquid into a
generally planar distribution pattern.
Enhanced
Spray Cooling Technique for Wedge Cooling
The
present invention relates to apparatus and methods for heat removal and, more
particularly, apparatus and methods for spray cooling a wedge of a generator
rotor.
In
generators, electromagnetic losses occur in the magnetic iron and the copper.
These losses result in production of heat which must be removed to maintain
overall temperature below that allowable for the copper coating and the
insulation used in the construction of the generators. The rotor core, which is
made of magnetic iron, can be conduction cooled by flowing fluid through the
rotor shaft. However, the removal of heat from copper is better managed if oil
is passed through the hollow wedges. Due to lower thermal resistance, the flow
of fluid in the vicinity of copper is much more effective in removing heat from
the copper and in keeping the overall temperature below the allowable limit.
This is done with conduction mode of heat removal.
Since
the heat transfer coefficient (HTC) depends upon the velocity of the fluid, the
removal of heat is not very efficient, and a very high flow is needed to create
a reasonable HTC for conduction cooling. Further, because the rotor is a
rotating component, having a large amount of fluid at a radius away from the
rotor shaft is not desirable, especially for high powered larger diameter and high-speed
machines.
In one
aspect of the present invention, a spray cooling manifold comprises a manifold
ferrule adapted to circumscribe a shaft of a rotating machine; a manifold pipe
having a bend of about 90 degrees having a first end attached to the manifold
ferrule and a second, opposite end; a cooling fluid channel running from an
inside surface of the manifold ferrule to the second, opposite end of the
manifold pipe; and a pipe extending from the second, opposite end of the
manifold pipe, the pipe adapted to extend into a wedge of the rotating machine,
the pipe having a plurality of holes formed there along.
In another aspect of the present invention, a rotating machine rotor comprises a shaft; a plurality of coils disposed on the shaft; a plurality of wedges disposed between the coils; bands securing the wedges on the rotor; and a manifold comprising a manifold ferrule adapted to circumscribe the shaft; a plurality of manifold pipes, each having a bend of about 90 degrees, each having a first end attached to the manifold ferrule and a second, opposite end attached to a wedge pipe extending into the wedges; a cooling fluid channel running from an inside surface of the manifold ferrule to the wedge pipe; and a plurality of holes disposed along the wedge pipe.
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.
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]
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.
