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Introduction to Thermal Management: Keeping Devices Cool

Boyd Blog — Mon, 01 May 2023 17:53:44 +0000

The latest electrification trends drive products to contain more powerful and compact electrical components, which drives additional thermal management challenges to maintain or improve performance, reliability, and lifespan. Product designers and manufacturers need to maintain optimized heat source temperatures to ensure these devices operate efficiently and remain reliable over time. One way to optimize device temperatures is through effective thermal management.

What is Thermal Management?

Thermal management is the process of managing heat within a system to ensure efficient and safe operation. By designing and implementing techniques that leverage conduction, convection, and radiation, thermal management helps regulate device or system temperature by removing or dissipating excess heat.

Thermal management is essential for many device and system types, including electronic devices, vehicles, power plants, imaging systems, and high-performance computing systems. Without proper thermal management these devices can overheat, leading to reduced performance, shortened lifespan, and potential damage to components.

Effective thermal management techniques, like heat sinks, fans, liquid cooling systems, and thermal interface materials, enable devices and systems to operate safely, perform at their best, and have a longer lifespan.

Thermal Management Techniques

Most systems use a combination of thermal management techniques, so these solutions are categorized based on the primary cooling method used in the system. Categorization is generally based on working fluid and the type of convection in the cooling system.

Air Cooling, Liquid Cooling, and Two-Phase Cooling

Thermal management solutions are classified into air cooling, liquid cooling, and two-phase cooling based on the working fluid used. While air cooling is often the simplest and most cost-effective solution, liquid cooling and two-phase cooling solutions provide more efficient heat transfer and are better suited for high-performance applications.

However, liquid cooling can have potential complications with compatibility and maintenance, and two-phase cooling solutions may require additional design considerations for managing the vapor-liquid interface. The best thermal management solution depends on specific application needs and the trade-offs between performance, cost, and complexity.

Conduction Cooling

Systems that do not use any fluid are considered conduction cooling solutions. These thermal management solutions rely on the thermal conductivity of materials to effectively transport heat away from a source. Most air, liquid, and two-phase systems have some level of heat conduction within its components, but rely more heavily on fluid movement to move heat. Natural Convection and Forced Convection

Based on convection type, a thermal management solution is classified into natural convection (passive solution) or forced convection (active solution). Natural convection relies on the natural flow of air or liquid around a heat source to dissipate heat and occurs due to the temperature differences between the heat source and the surrounding environment. Whereas, forced convection uses a fan or blower, pump, or other mechanical device to circulate air or liquid around the heat source, enhancing heat transfer and improving cooling efficiency

Air cooling and two phase cooling leverage both active and passive configurations, but liquid cooling solutions are primarily active solutions.

Why is Thermal Management Important?

Incorporating thermal management into the design process helps avoid costly redesigns or repairs. Industries such as medical, aerospace, defense, and eMobility have strict regulatory requirements for thermal management. Failure to comply with these requirements results in legal consequences, fines, or even loss of life.

Inefficient thermal management leads to increased energy consumption, which not only impacts the device’s performance and lifespan but also has a negative impact on the environment. This is a major design focus for industries like data centers and high power compute. By designing devices with thermal management in mind, energy consumption can be minimized, leading to a more sustainable product.

Touch temperature is an important factor in the user experience of a product. If a device or a component is too hot to touch, it poses risks of burns or other injuries to the end-user. This can also lead to negative media coverage, potential damage to brand value, and other legal liabilities.

Overall, thermal management should be an integral part of the design process to ensure reliable and safe operation, compliance with regulatory requirements, cost savings, environmental sustainability, and brand reputation.

The Boyd Difference for Thermal Management Solutions

Boyd has several decades of experience and expertise designing and manufacturing at scale custom thermal management solutions for various industries, including eMobility, cloud, medical and more. Leverage our extensive supplier network, material science experience, and engineering expertise for comprehensive thermal management solutions that meet your unique needs and challenges. To learn more about our thermal management solutions or to discuss your project needs, schedule a consultation with our experts.

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Fundamentals of Heat Transfer

Boyd Blog — Tue, 25 Sep 2018 12:53:00 +0000

The following is a brief overview of some fundamental heat transfer concepts. To learn more, the reader is encouraged to review the source publications and cited websites

Introduction to Thermodynamics

1st and 2nd Laws of Thermodynamics

The 1st Law of Thermodynamics involves the conservation of energy. It states that – within a closed system where no other energy material can enter or leave – energy can neither be created nor destroyed.^{1, 2} Although energy cannot be created or destroyed, it can be transferred to work as other forms of energy.

Transferring heat energy is subject to the 2nd Law of Thermodynamics.³ The 2nd Law (again applying to a closed system) says that – for a spontaneous process – there is a net increase in entropy⁴ (i.e., a measure of the disorder that exists in a system⁵).

Three alternate but equivalent ways to describe the 2nd Law are:

Heat flows spontaneously from a hot body to a cool one. (Example: A hot microprocessor or laser diode is cooled by flow of heat into heat sink or cold plate.)

It is impossible to convert heat completely into useful work. (Example: In a combustion engine, a certain heat component must always be exhausted without performing work.)

Every isolated system becomes disordered in time. (Example: In conduction when hot and cold bodies first contact each other, the system is somewhat ordered. Hotter molecules move faster than cooler molecules. But, once the entire system attains a uniform temperature, this order is lost.)

Expressed in mathematical terms, any of the above statements imply the other two.⁶

The 1st and 2nd Laws of Thermodynamics govern the various modes of heat transfer: conduction, convection and radiation.

Modes of Heat Transfer

Conduction

In conduction, heat flows from a higher temperature region to regions of lower temperature. This occurs within solid, liquid, or gaseous mediums or between different mediums that make direct physical contact with each other.⁷ “The transfer of the energy of motion between adjacent molecules conducts the heat. In a gas, the ‘hotter’ molecules, have greater energy and motions, and impart energy to adjacent molecules at lower energy levels. This type of transfer occurs to some extent in all solids, gases or liquids in which a temperature gradient exists. In conduction, energy can also be transferred by “free” electrons, which is important in metallic solids.”⁸ Examples of conduction are heat transfer through the surfaces of a cold plate or through the walls of a refrigerator.

Convection

In convection, the combined action of heat conduction, energy storage, and mixing motion serve to transport energy. “Convection is most important as the mechanism of energy transfer between a solid surface and a liquid or a gas.”⁹ “In forced-convection heat transfer, a pump, fan, or other mechanism forces a fluid to flow past a solid surface. In natural or free convection, warmer or cooler fluid next to the solid surface causes a circulation because of density differences resulting from the temperature differences in the fluid.”¹⁰ An example of free convection is the loss of heat into ambient air via the fins of a heat exchanger. If a fan is used to circulate the air over the heat exchanger fins, this becomes an example of forced convection.

Radiation

In radiation, heat flows from a higher temperature body to a lower temperature body when the bodies are separated in space, even across a vacuum.¹¹ “The same laws that govern the transfer of light, also govern the transfer of heat. Solids and liquids tend to absorb the radiation being transferred through it, hence radiation is important mainly in transfer through space or gases.”¹²

Examples of radiation include the transfer of heat from the sun to the earth, and from a quartz lamp to a cool object that requires warming.

Mathematical Representation and Calculation of Heat Transfer

Fourier’s Equation

“The basic relation for heat transfer by conduction, proposed by the French scientist J.B.J. Fourier in 1822, states:

The rate of heat flow by conduction in a material, qk , equals the product of the following three quantities:

k – Thermal conductivity of the material
A – Area of the section through which heat flows by conduction as measured perpendicularly to the direction of heat flow
dT/dx – Temperature gradient at the section, i.e., the rate of change of temperature T with respect to the difference in the direction of the heat flow x.

Writing the heat conduction equation in mathematical form requires a sign convention; i.e., the direction of increasing distance x is the direction of positive heat flow. According to the second law of thermodynamics, heat will automatically flow from points of higher temperature to points of lower temperature. Thus, heat flow will be positive when the temperature gradient is negative. The basic equation for one-dimensional conduction in the steady state is: qk = -kA (dT/dx)”¹³.

Thermal Conductivity

Thermal conductivity is a measurement of the rate at which a given material will transfer heat.¹⁴ “The thermal conductivity of a substance is the quantity of heat in cal/sec passing through a body 1 cm thick with a cross section of 1 sq. cm when the temperature difference between the hot and cold sides of the body is 1 deg. C.”¹⁵ This intrinsic property is independent of the materials size, shape, or orientation.

Thermal Resistance

Thermal resistance is the inverse of thermal conductivity and indicates how a material inhibits the conduction of heat.¹⁶ Materials with a high thermal conductivity have a low thermal resistance and have poor heat insulation qualities (e.g., copper and aluminum). Conversely, materials with a low thermal conductivity have a high thermal resistance, and have good heat insulation qualities (e.g., fiberglass insulation and corkboard).¹⁷

References

1. https://www.chemistry.ohio-state.edu/~woodward/ch121/ch5_law.html.

2. https://theory.uwinnipeg.ca/mod_tech/node78.html.

3. ibid.

4. http://learn.chem.vt.edu/tutorials/entropy/2ndlaw.html.

5. Microsoft Encarta World English Dictionary, St. Martin’s Press, 1999, Pp 596.

6. de Sorgo, Miksa, ibid.

7. de Sorgo, Miksa, “Understanding Phase Change Materials”, ElectronicsCooling Magazine, May. 2002

8. http://learn.chem.vt.edu/tutorials/entropy/2ndlaw.html.

9. Kreith, Frank, Principles of Heat Transfer, 2nd Edition, University of Colorado, International Textbook Co., Chapter 1, Pp 6.

10. Transport Processes and Unit Operations, 3rd Edition, Christie Geankopolis, University of Minn. Prentice Hall, Chapter 4, Pp 215.

11. Kreith, Frank, Principles of Heat Transfer, 2nd Edition, University of Colorado, International Textbook Co., Page 8.

12. Transport Processes and Unit Operations, 3rd Edition, Christie Geankopolis, University of Minn. Prentice Hall, Chapter 4, Pp 216.

13. Kreith, Frank, Principles of Heat Transfer, 2nd Edition, University of Colorado, International Textbook Co., Pp 7.

14. Transport Processes and Unit Operations, 3rd Edition, Christie Geankopolis, University of Minn. Prentice Hall, Chapter 4, Pp 216.

15. Kreith, Frank, Principles of Heat Transfer, 2nd Edition, University of Colorado, International Textbook Co., Pp 9.

16. Transport Processes and Unit Operations, 3rd Edition, Christie Geankopolis, University of Minn. Prentice Hall, Chapter 4, Pp 216.

17. http://theory.uwinnipeg.ca/mod_tech/node75.html.

18. http://www.lib.umich.edu/dentlib/dental_tables/thermcond.html.

19. http://www.xrefer.com/entry/619844.

20. http://theory.uwinnipeg.ca/mod_tech/node75.html.

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Ducted Versus Bypass Flow

Boyd Blog — Mon, 23 Apr 2018 12:53:44 +0000

Ducted Versus Bypass Flow

Heat sink performance is impacted directly by how air flows through or around the fins of the heat sink. Ducting helps focus air flow through the fins, maximizing how much of the fluid is in direct contact with the fin surface area. This directly impacts flow resistance, which is limited by the “strength” of the fan or blower generating the flow. If we let air flow around the heat sink, that is considered bypass flow. Let’s define the differences between ducted, bypass, and free flow.

Ducted Flow

When fluid is fed through a sealed channel or duct that forces all the fluid through the fins of the heat sink, that flow is considered ducted. The top of the air duct is in contact with the fin tips.

Bypass Flow

Bypass flow is a modification of ducted flow, where the walls of the sealed channel are not right up against the edges of the heat sink. This extra space between the heat sink and the channel walls allows the flow to bypass the fins, hence the name.

Typically we refer to vertical bypass as additional clearance added between the tip of the fins and the top of the duct. Fluid flow will be able to move more freely in the space above the fins.

If we add clearance to the sides of the heat sink, that is considered horizontal clearance. If the width of the horizontal bypass is greater than the fin gap, fluid can travel more freely in the bypass than it can in between the fins.

Ducted Versus Free Flow

The flow is considered “free flow” if the heat sink doesn’t have a well defined duct or the duct is fairly far away from the edges of the heat sink. Free flow is common in natural convection applications, but there may be forced convection that may have a free flow situation. An example would be an enclosure that has a fan blowing air into the system, with grilles or perforation that allow air.

In most simulation software, there needs to be some sort of boundary in order to limit the amount of computation the program undergoes. Otherwise, you’ll be asking you’re computer to calculate more than you need it to.

Ducting in Genie

Genie has a few different options when it comes to defining flow for your thermal simulation. Genie defaults to having ducted flow for all three flow types in the Flow Definition portion of a project. One flow type, standard fixed flow, has the option of adding bypass flow around your heat sink.

Mimicking Free Flow in Genie

You can mimic free flow in Genie by increasing both the vertical and horizontal bypass. By adding all this extra space, you reduce the influence of any boundary layers generated by the duct walls and let the flow acting on the heat sink be unhindered. Just make sure you use a flow velocity instead of a flow rate. Flow rate is volumetric, and by increasing your flow area, you’re drastically cutting down on the actual velocity.

There you have it, ducted versus bypass flow. Try it out in Genie in the Flow Definition page of your project! Test out what works best for your custom heat sink design. If you need some assistance in determining the constraints of your flow, Contact Us to talk with experienced Boyd Design Engineers.

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Thermal Radiation: Key for Natural Convection Solutions

Boyd Blog — Tue, 05 Sep 2017 12:53:44 +0000

Thermal engineers are radiant about the effects of thermal radiation. Thermal radiation is one of three modes of heat transfer, along with convection and conduction, so the more we know about this mode of heat transfer, the more effective we can use it to optimize our thermal management solutions.

What is Thermal Radiation?

In the “Project Conditions” page of Boyd Genie, there is a box that asks you the “Working Envelope.” This term refers to the heat sink design volume.

Thermal Radiation refers to the electromagnetic waves that escape a given volume from its surface. While all of the molecules within a given volume give off radiation, internal molecules generally absorb the radiation from their internal neighbors. Thermal energy leaves from the volume in question when surface molecules radiate heat.

This is why in thermal management, we’re so concerned about the surface of our heat sinks and cooling solutions. Thermal radiation is a portion of the electromagnetic section that ranges between 0.1 um to 100 um in wavelength. This includes the high end of UV radiation, visible light, and infrared radiation. You’ve certainly seen radiation when you’ve seen someone working red hot metal or glass. That visible light you’re seeing is thermal radiation.

Black Bodies and Emissivity

Every surface with a temperature greater than absolute zero emits and absorbs heat. Planck’s Law defines that at any given temperature, there is an ideal surface that can absorb and emit the most possible thermal radiation. We refer to this surface as a black body.

What we’re most interested in with a black body surface is it’s ideal ability to emit heat, or its emissivity. Since nothing emits more than a black body as defined by Kirchoff’s Law of Thermal Radiation, so we give it an emissivity value of 1. Any other surface cannot emit as much heat as a black body, so we define that surface’s emissivity as a ratio of how much that surface emits compare to a black body at that temperature. The more a surface can emit thermal radiation, the closer it is to an emissivity to 1. For thermal management, we generally want higher emissivities on our external surfaces to radiate as much heat out of the system as possible.

Surface Finish and Material Effect on Emissivity

The primary determining factors of emissivity are material composition and surface finish. Metallic surfaces generally have a low emissivity, which includes our typical heat sink materials copper and aluminum. While we prefer these metals for their high thermal conductivities, they’re poor materials when it comes to emitting radiation. This is because these materials are highly reflective, and not just in the visible spectrum, but all electromagnetic waves. The implication of this inherent shininess of every particle is that the surface molecules are more likely to reflect any thermal radiation from internal molecules back into the volume.

Rougher surfaces are better at emitting thermal radiation. These surfaces are less likely to reflect thermal radiation back into the volume. So the less metallic and duller the material, the better emissivity a surface will have.

Thermal Management: Using Radiation

In thermal management, we typically optimize radiation in natural convection solutions, or solutions where we’re not actively pushing fluid through the system. You’ll see many natural convection solutions with anodized or coated surfaces, but you’ll see more unfinished aluminum and copper parts in forced convection cooling solutions. This is because the effect of thermal radiation is relatively small compared to the heat transfer accomplished with effective convection. In some applications where devices must be cooled within a vacuum, radiation is the only option for cooling a heat source.

Radiation in Boyd Genie

Boyd Genie includes the effects of thermal radiation for natural convection solutions by default. In case you’d like to see the estimated effect on thermal performance without radiation, Genie has a check box on the Project Conditions page to disable radiation calculations.

Forced convection simulations in Genie do not account for the effects of heat transfer by thermal radiation because of the minimal impact it has on the final performance. In this way, results for these forced convection will be a tad conservative.

Try out your own natural convection heat sink in Genie and see how thermal radiation helps your solution. Or Contact the Boyd team to help with your thermal management solution!

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Buoyancy: The Driving Force of Natural Convection

Boyd Blog — Wed, 30 Aug 2017 17:53:44 +0000

What is Buoyancy?

You’re already well aware and experienced in the effects of buoyancy. Buoyancy is what allows boats to float and hot air balloons to fly. It is the reason that the water in the ocean is separated from the air in the sky. This is due to the variation in the densities of these different fluids.

We Like to Move it, Move it

Buoyancy is pressure generated by the kinetic energy of all the fluid molecules moving around and colliding with each other and any other object within the vicinity. While this kinetic energy pushes in all directions, its largest opposing force is gravity.

The Heavy Science behind the Force of Buoyancy

The amount of buoyancy is dependent on the density of the fluid. Denser or cooler fluids have less kinetic energy than more excited fluids so they exert less pressure against other surrounding molecules. Since there is less pressure generated by this fluid, the force of gravity has more effect on this fluid than warmer fluids. This is why cool air falls and warm air rises.

Why Does Buoyancy Matter in Thermal Management?

Natural Convection

Buoyancy is what drives natural convection, where the pressure differential between air heated by heat sources and surrounding ambient air drives the hot air upwards and away from the heat source. The air rises up with the energy it absorbed and thus removes that energy from the area around the heat sources. Cooler ambient air then moves in and replaces the heated air that moved up. This rising and replacement of hot and cool air generates a consistent flow without the need of any active mechanisms to drive it.

Benefits of Natural Convection

One of the key points of natural convection is enabling heat transfer without the extra cost and assembly time associated with adding a fan into the product. Fans or blowers can decrease the overall reliability of the device as they are driven by motors that wear down and can break over extended use of the product.

How to Make Buoyancy and Natural Convection Work for You

Since buoyancy enables fluids to rise against gravity, surfaces that dissipate heat should be oriented in the same direction as the gravity vector. Typically this surface is your heat sink. The longest dimension of the heat sink surfaces should be vertical, so as the cool air contacts the bottom of the heat sink it travels upwards and continues to be heated up and increases the pressure differential.

Natural Convection and the Chimney Effect

By optimizing the amount of air that can be pulled into the fin gaps and the length of the heat sink, you can easily use natural convection to cool devices connected to your heat sink surface. You’ll typically see good natural convection heat sinks have a fin gap of around 0.25″ or 6.35mm. This allows enough space between the fins to accommodate boundary layers that form on each of the fin surfaces and still allow upward flow of heated air.

Natural Convection in Genie

As thermal engineers, we’re buoyant about buoyancy and how it allows us to use natural convection as an effective method for heat transfer. In Boyd Genie, we enable you to simulate natural convection with your heat sink design. Before you simulate, you can select the orientation of the fins on your heat sink in relation to gravity.

Orienting a heat sink with the flow length of the fins in the same direction as the gravity vector is the ideal orientation.

Otherwise, you should at least point the fin tips upwards. Other orientations of your heat sink, such as your fin tips and flow length pointing sideways or even downwards, inhibit the natural convection flow that’s generated by buoyancy. Heated air between these fins tends to stagnate since it has a difficulty rising up and away since the fins or base of the heat sink are in the way.

To recap, the best practices for natural convection heat sinks are to point the flow length of the fins vertically and give your fins enough space to effectively pull in air. Try out your own natural convection heat sink in Genie!

Happy Designing

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Solar Eclipse Thermals: Radiation’s Effect on Weather

Boyd Blog — Tue, 22 Aug 2017 17:53:44 +0000

We generally take regular events for granted. Sometimes, it’s only when our regular programming gets disrupted that we realize how much impact these everyday occurrences have on our lives. The big disruption in the US this summer is the total solar eclipse that reaches coast to coast on August 21st, 2017. One thing that doesn’t come immediately to mind is the solar eclipse thermals and how it’ll effect weather on a local scale.

Solar Eclipse Thermals: The Power of Solar Radiation

The only type of thermal energy we receive from the sun is radiation. When the moon moves in between the sun and the Earth, all of that solar radiation is blocked from the surface of the Earth. While this event is transient, the sheer amount of energy we receive from the sun is so monumental that even a few minutes without it can have drastic effects.

Imagine sitting at a campfire on a cold night. You’re enjoying the heat from the fire in front of you, but your friend stops to chat with you. Unfortunately, they stop between you and the fire. That friend is now absorbing all that radiation instead of you and you start to feel the chill of the evening. Now, replace the campfire with the sun and your friend with the moon. The scale of this momentary chill increases monumentally.

Solar Radiation Temperature Effect Illustration

Humans will experience a noticeable temperature drop when the umbra sweeps over them. Dr. Mitzi Adams recorded a 15°F (8.3°C) drop in temperature during the total solar eclipse in Lusaka, Zambia on June 21st, 2001. Humans can expect the same for the 2017 total eclipse in the US. In fact, NASA’s article details all the different ways that you can measure your local temperature drop when you observe the solar eclipse this year.

Solar Eclipse Thermals: The Resulting Convection

With local cooling, fluids are also bound to react to the solar eclipse on a larger scale than just temperature change. The biggest effect is natural convection generated from the induced pressure differential. With the penumbra and umbra area receiving less thermal energy, the air in those regions will cool and sink due to buoyancy forces. This will draw air in from higher up in the atmosphere from outside of the penumbra. This may bring in moisture that cools upon contact with this suddenly chilled air. When the moisture cools, it may cool enough to condense into clouds. Extra breeze and clouds are additional manifestations of solar eclipse thermals.

Solar Eclipse Driving Local Natural Convection Illustration

Have Your Own Solar Eclipse Fun!

For those of you who still need to prepare for the solar eclipse, the Astronomical Society of the Pacific has a great resource for solar eclipse preparation.

Share Your Solar Eclipse Thermals With Us!

Share your your solar eclipse thermal experiments with us

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Thermal Convection: Natural versus Forced Convection

Boyd Blog — Mon, 21 Aug 2017 17:53:44 +0000

Thermal Convection: A Pillar of Heat Transfer

In a majority of thermal management solutions, we use thermal convection as a means to remove heat away from our sensitive components and devices. In the rare case we don’t use convection, it’s because we have little to no fluid to work with. Particular applications, like ones in the aerospace industry, are devoid of fluid and cannot utilize thermal convection. Otherwise, it’s the most popular way to get heat out of products.

But in the industry, you’ve probably heard the terms “forced convection” and “natural convection” thrown about. While it may not seem a big deal between the two, it has a large impact on how your thermal management solution is designed. So let’s take a look at natural convection versus forced convection and get into the differences.

Convection: The Tale of Two Processes

The process of convection as we refer to it in thermal management is a actually a combination of two processes. The first process is technically conduction, where the heat from the heat sink surface transfers to any fluid that contacts that surface. The second process is considered advection, which is bulk flow of fluid warmed by the device away from the heat source. What we do instead of referring to both individually, we lump them together as one single term: convection.

It’s important to understand the two portions of convection when we’re trying to improve our thermal performance of our solutions. When we comprehend the parts of convection, we’re more able to break down and improve each of these parts to better our overall heat transfer.

Natural Convection versus Forced Convection

We classify the type of convective flow as either natural or forced. We make this designation since each has it’s own implications for the application and product as a whole. These different types of flow have different design constraints and concerns that need to be individually addressed.

It’s Natural (Flow)

Natural convection is when the natural buoyancy drives the advective flow. You’ve probably heard the terms “plume” or “chimney effect” to describe natural convection. Essentially, as the fluid inside or near the heat source and heat sink gets hotter than ambient temperature, it has less pressure. Here on Earth, we have gravity, so less pressure means more buoyancy. This pressure differential generates movement of the hotter air upwards, away from the source of gravity. The cooler surrounding fluid then fills the place the hot air is leaving from, thus generating a flow inwards and then upwards.

Go with the Flow: Natural Thermal Convection Design Considerations

Super Reliability for Natural Thermal Convection Solutions

In applications where reliability is critical, natural convection is the preferred type of flow within a thermal management solution. By relying on natural forces to apply movement to your fluid, key components like fans or pumps aren’t required. These components, while heavily engineered and tested, will still wear down over time. As long as you have frictional parts, like the motors in fans or pumps, you’ll be concerned about the reliability of your fluid movers.

Fluid Options for Natural Thermal Convection

Natural convection tends to be easier in air cooled applications as opposed to liquid cooled systems. Liquid needs to be contained and unless the system is submerged, and most electronics don’t go well with liquids, the whole route of the liquid needs to be planned out and contained. This implies more engineering time especially during the design and validation portions of product development. On the other hand, we’re surrounded by air and any movement of air away from a system will be quickly replaced by other ambient air.

Natural Thermal Convection Fin Spacing

When you’re talking about natural convection versus forced convection heat sinks, you’ll see a difference in the overall structure of the heat sink. No matter the fluid, we want to optimize our heat sink to maximize the chimney effect. This means there is enough room between heat sink fins for them to “breathe”. You need enough room to heat up next to the fins within their boundary layer on each side of the gap, as well as some extra room in the middle for air to flow upwards. You’ll see the looser fin spacing on the thermal contours below on the left allow cooler air to get much further up the fin gaps than the heat sink on the right. That’s why you’ll notice some heat sinks have much larger fin gaps than others. The ones with fin gaps of about 1/4″ and larger are generally designed for natural convection.

Forcing the Subject of Forced Thermal Convection

When a mechanism besides natural buoyancy generates this advective flow, we call it forced convection. In these cases, we’re typically using something like a fan or pump to drive the flow of fluid. Forced convection can also be generated by things such as someone blowing on their skin to cool down a burn, or palm frond wielding servants. The point is that there is some sort of mechanism besides physics driving the flow, it’s considered forced convection.

Design Implications for Forced Convection

Force Out the Heat!

The big positive attribute of forced convection versus natural convection is the increased amount of heat transfer. By being able to move more fluid through a system in the same period of time, more heat absorbed by the fluid can be forced away from your heat source. This keeps the heat from lingering and building up and in thermal management, that is the last thing we want.

What Forced Convection Means for Reliability

Unfortunately, the drawback of having something force flow through your system is that it might give out. Frictional parts in our pumps and fans wear out, the minor burn victim gets light headed from all that blowing, or the servants go to eat or sleep. These things cannot run indefinitely. That’s where design engineers need to consider the reliability of their components and make sure that the end product is serviceable enough to replace broken parts or the parts are able to live longer than the expected lifetime of the final product. This is especially true for critical devices that support life or safety.

Moving Parts and Noise

Since forced convection requires moving parts to make fluid flow faster, it also produces sound. Fan or pump motors generate more noise compared to natural convection. For some applications, this can be a real drawback. I mean, it really takes you out of your immersive experience with video games or a movie when a fan jumps into high gear and starts humming loudly. You still need the fan, since you want to play games and watch movies for years to come. But that fan might kick in during those intense moments of your audio/visual experience.

Design Implications for Forced Convection

When it comes to your design and your final product needs, you and your end customer are the experts. You should be able to determine your preferred flow type based off your reliability and end user requirements. But remember, you’re not alone. Genie can help walk through the process of looking at natural convection versus forced convection for your application. If you find you need more help, Boyd Design Engineers have developed solutions for tough, high power natural convection situations or made forced convection solutions meet hard reliability requirements. Either way your application goes, whether it be natural or forced, Boyd can help you out with what you need.

Happy Designing!

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Everyday Thermals: Boiling Pot of Water

Boyd Blog — Mon, 17 Apr 2017 17:53:44 +0000

Something we do all the time is eat. And to eat we need to cook. Cooking is just a fun and artful method of heat transfer into edible products to make them more digestible and palatable. We’re going to train our thermal vision onto a pot of boiling water. Boiling water is one of the essentials in cooking, though not all of us have a good handle on it.

Let’s find the three modes of heat transfer (conduction, convection, and radiation) in this everyday thermal task!

Calibrating our Thermal Vision

Here is our empty aluminum pot that we’re going to fill with water. It’s sitting on a glass top electric stove in the kitchen. Nothing to special when you think about it. It’s only when we point our thermal camera that things get interesting. Thermal imaging camera’s output a thermal contour image with different colors representing different temperatures. In this scale, we have dark blues as the coolest temperatures, to greens, yellows, orange, reds and up to white as the hottest temperatures.

Notice we have a few hot spots before we even get started. Since thermal camera’s typically adjust their color range to whichever temperatures it sees, there isn’t much temperature difference between these objects. Unfortunately, the images from our thermal camera don’t save with the whole scale, but we’ve put some temperature probes on later images to show some actual temperatures.

So what’s going on with these hot spots when we haven’t even turned on the stove?

The hottest spot we see is the LED display on the stove. As the stove is displaying the time, it’s emitting both light and heat, which is what we see with the camera. On the top right we see another hotter area. This is an electrical outlet. The resistance of electricity running through the wires generates a little heat and since the drywall doesn’t hide it, the thermal camera sees it.

Radiation

So we come to the two bright spots on the pot that supposedly has no heat on it yet. The bright yellow spot is actually a reflection of light and heat coming in from the kitchen window. And the yellow green spot? That’s my heat reflection. Since thermal energy can radiate, it acts just like light. If there are shiny surfaces, like my shiny pot, it will reflect the heat radiation.

Full disclaimer, I already turned on the stove in this next picture. You can actually see in this image where on the left I’m just standing in front of the pot and on the right I’m hiding behind a cloth covered ironing board. You can see that green smudge in the middle is missing on the right hand pot image.

Conduction

Let’s Turn on the Heat! Time to add some water and turn on the stove. I filled the pot about half full with water (yes, we’re optimists around here). Again in this image we see the LED of the stove and the electrical outlet on the top of the image. Notice how they’re much more subdued in this image than the last one. The relationship between the temperature of the stove and those things has dramatically changed and the colors of the thermal image updated to display that. We can also tell, based off the gradual change from mid green to blue and darker blue, there’s conduction bringing heat up along the sides of the pot.

We see the heat generated by the electric stove and how some of it’s conducting up and away from the bottom of the pot. We can also see some of the thermal radiation being reflected out from the inside of the pot in the green area. As the stove top gets hotter, dark splotches start showing up on the hottest areas. I learned that this is an artifact of the thermal camera since the temperature in those areas are too hot for the camera to measure. For our camera that limit is 302.4°F or 150°C.

Convection

We’ve seen radiation and conduction, so let’s find some convection. If we watch the pot closely, first, it will boil despite it being watched, and second, we can see some vapor forming before it starts to boil. Watching the vapor roll around in the pot above the water was mesmerizing, and the gif doesn’t quite do it justice. As a side note, those blue dots in the pot are where the handles are riveted into the pot. Those are a little cooler than the rest of pot since those handles essentially act like heat sinks, pulling heat away from the source.

A Rolling Boil

Once the pot starts boiling, more hotter vapor stops rolling inside the pot and starts rising up and out of the pot. There’s generally a central plume of hot air rising, but it’s behavior is fairly unpredictable. Natural convection is heavily dependent on surrounding conditions. Small variations in the ambient air can have a drastic effect on how the central plume of vapor moves. Again, mesmerizing.

Here’s a time lapse of the whole heating process with a few temperature probes to give a bit of scale.

So there’s the first of hopefully a long series! Everyday Thermals: Boiling Pot of Water. Didn’t think something so mundane would look this exciting, right?

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