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Hello. Thank you for joining today’s webinar on ruggedized and Compact: Thermal Solutions Hosted by ACT Advanced Cooling Technologies, located here in Lancaster Pennsylvania.
If you have a question during the webinar, please use the questions panel in the goto Webinar Program on your screen, it will be submitted to our team, and we’ll do our best to provide an answer during the last segment of today’s presentation.
Can also download some great resources that are applicable to our discussion in the same area, so be sure to check those out, too.
It’s my pleasure to introduce you to Greg Hoeschele, one of our lead engineers in the ACT Products Development Group.
Greg is perfectly positioned to speak during today’s Webinar, as he’s well versed in the demands of military and aerospace, and he’s worked in a wide range of related thermal management projects, And I am Adam Say, International Business Development Manager.
My role is to manage and grow our International Customer Partnerships.
But no matter where you’re located, our sales team is always happy to meet and discuss your project, Please be sure to reach out to us, no matter what stage of product development you might be working on.
Before I get started, I’ll provide a quick overview of ACT.
We provide custom thermal solutions based on our experience in advanced thermal systems and Two-Phase Heat Transfer Technology.
When our company was founded in 2003, we focused on thermal management research and development, but since then, we’ve grown significantly.
We have over 220 employees and 140,000 square feet are 13,000 square meters of total space, which is all governed under ISO 9001 and S 9100 D, quality management systems.
We are proud to be the recipient of many awards, including the 2020 Military and Aerospace Technology Innovators Award.
And ACT personnel have also been awarded more than 60 patents, and we’re also the authors of more than 350 scientific publications, although I haven’t read all of them.
As I mentioned, ACT offers a complete range of thermal engineering services. So, everything from initial design and concept generation, too detailed product design, and of course, high volume production.
Although our R&D team is working on new innovations every day, we have commercialized many of our thermal solutions. And we serve customers in a lot of different markets.
For example, … solutions have been deployed in a number of different military applications.
The military segment has a number of unique requirements that create a number of engineering complications. Thankfully, my colleagues here, Greg, and our engineering team, are all very well experienced in this field, and actually, we’re going to discuss a lot of those challenges during today’s webinar.
ACT also offers technologies that can manage high heat loads for space applications.
With our CCHP products, we have over 50 million space flight hours on orbit without failure, and we offer we’ve also helped resolve thermal challenges for other spacecraft, rovers and satellites.
In fact, the copper-water heat pipes, HiK(TM) plates that Greg will talk about in a few minutes, also flew on the International Space Station.
Very recently, ACT acquired the company Tekgard, which offers custom environmental control systems (ECU’s) for extreme conditions, and aside from our military and aerospace customers, we also offer electronics cooling solutions for our industrial market customers.
So, for today’s agenda, we will discuss some of the common environmental conditions for ruggedized solutions, as well as some of the current market trends relating to those environments.
In particular, we will discuss the details of two different groups of ACT solutions, including some interesting case studies.
These solutions are heat pipes and HiK(TM) or high conductivity plates and phase change material (PCM) heat sinks.
Keep in mind that almost all of our solutions are bespoke, or custom designed, for our customer’s applications.
And all of our approaches today, in the webinar, are focused on passive rugged electronics cooling.
Some of these applications might include avionics, radar, embedded computing, communication equipment, signal intelligence, electro-optics, or UAV, control electronics.
When we’re working with applications that are related to rugged defense, performance and durability are always top considerations for our solutions.
So to meet these military standards, we need to guarantee that our solutions are erosion resistant and protected against conditions like dust, grain, humidity, and salt/fog.
We’ve also added to design solutions that can withstand rough shock and vibration, g-force or acceleration, as well as certain implications relating to the altitude, where the application will be performing.
In most cases, we also need to build solutions that are very lightweight, but also meet rugged structural requirements and fit within very tight volume or geometric restrictions.
Most of the common thermal challenges that we face in rugged defense applications might include wide ranges, in ambient temperatures from maybe -25C to over +45 C.
When heat sink temperatures are applicable, they might have maximum allowed temperatures of +85 degrees C.
Of course, we also have to consider the maximum allowed temperatures for this system electronics.
These are usually provided by the electronic supplier, but we might see typical values of over +135 degrees C.
And if altitude considerations are applicable, we have to calculate solutions that will perform in low ambient temperatures and lower air densities.
one of the most common thermal problems that our customers face are conduction limitations.
These problems could be rooted from a number of different issues.
Perhaps there are localized hotspots on your device due to high heat flux electronics or maybe there are long distances required to reach your heat sink.
Sometimes the heat sink is even remotely located.
Aluminum or copper offer a relatively good heat conductivity, but on their own aluminum is only around 170 watts per meter kelvin and although copper has a watt per meter K of around 400, it adds significant weight to the design.
So, when I hand things over to Greg, he will discuss our HiK(TM) plate solution which can actually increase conductivity up to 800 watts per meter K.
Another thermal problem that often creates challenges for our customers relates to transient power.
Designing to peak power always adds volume, weight, and complexity to the design.
And as many systems, such as directed energy, or transmit-receive devices operate on duty cycles Actually Greg will share more details about PCM- phase, change material heat sinks, which are excellent solutions for these types of scenarios.
So, with that, I will hand it over to Greg.
Greg: Thanks, Adam. Let’s dive into how a heat pipe works so we can understand how to best utilize them in these environments.
The heat pipe is comprised of three components: The envelope, the wick and the working fluid because the heat pipe is a vacuum system. The envelope is needed to hermitically seal the heat pipe. It also provides structure for them. The wick acts as a capillary pump to pump liquid, to the hotspots in the heat pipe, and, finally, the working fluid.
This is the two-phase fluid that actually transfers the heat from one end of the heat pipe to the other. Let’s look at the image at the top. The left-hand side is the evaporator, or heat input area, of the heat pipe. The liquid in the wick in this area is heated up due to the heat being put into the heat pipe. This causes the liquid to evaporate. The vapor then travels down the middle of the heat pipe and condenses the colder section on the far right. The capillary action of the wick then pumps the fluid back to the evaporator end of the heat pipe because of this pumping action.
Because of this, you can imagine that the amount of power, that the amount of fluid is being transferred is relative to the amount of power that is being put into the heat pipe, therefore at some point of capillary action, the heat pipe is not strong enough to pump enough fluid back to the evaporator. We call that the dry out of the heat pipe.
If a heat pipe is working against gravity, or against a G-load, that can reduce the capacity of the heat pipe basically limiting the amount of fluid that can be transferred back to the evaporator. So that’s one thing we want to keep in mind when we’re designing with heat pipes.
We also want to think about the evaporator and condenser sizes.
We want to maximize those two, Improve the overall assembly delta T, We’ll do an example of this in a couple of slides, Next up is a HiK(TM) or high conductivity plate.
HiK(TM) plates are basically aluminum plates with heat pipes integrated into them to increase the conductivity from the basic aluminum at 167 watts per meter Kelvin for 60/61.
This can go somewhere between 500 to one thousand watts per meter Kelvin, depending upon the geometry.
Because most of the frame is still made of aluminum. It has a similar weight and similar strength to aluminum.
And we can normally route our heat pipes around critical features so that all of your helicoils and dowel pins can be maintained.
We also offer a bunch of value add capabilities including coating, Helicoils, and dowel pins to make this really a turnkey solution for you.
Let’s take a look at this case study here.
Here we have a 4.5 U conduction current, with 4 90 watt heat loads located in the middle of the card.
We have rail temperatures of 60 degrees C in blue, on the outside of the cards.
We have an overall thickness of 5.5 million, so relatively thin because we don’t know how this is being oriented in the field, we want to design it to operate in any orientation.
So the first thing we’d like to do is go to the heat pipe calculator on our website link above.
This asks for four inputs, starting with the heat pipe length.
We’ll make a guess here, but we can figure that out from the geometry pretty easily, starting with a 4.25-inch line.
Next up is the evaporator and condenser lengths.
The evaporator will be the length of the heat input area at about one inch. The condenser will also be a bit of a guess because we’re not really sure of the layout, but let’s assume one inch from that.
Because this heat pipe will probably have some bends in it.
Overall, the height against gravity that the heat pipe has to pump in the worst-case orientation is going to be a little bit less than the overall length.
So let’s call it three inches for now.
With those inputs, the calculator outputs this curve.
The X-axis of this chart shows the operating temperature of the heat pipe, and the Y-axis is the maximum power it can carry before drying.
The different lines represent heat pipes of different diameters, ranging from three mm up to a quarter inch.
We want to make sure that our operating point is below these lines to ensure that the heat pipe can carry the required power.
Assuming that 60 degrees is the operating temperature for the heat pipe, we can see that one heat pipe will not carry the required 90 watts above all of the lines.
So we split the power between two heat pipes, which gets us a power of 45 watts and gets us to the operating point of the red star.
This gives us three options for heat pipes, 5 and 6 mm high heat pipe, and the quarter-inch heat pipe.
Given that our card thickness is only 5.5 mm will probably select the five mm pipe for this application, Which gets us to this design.
We integrate two heat pipes under each heat load, and transfer those
To the rails.
You can see, we’ve tried to extend the heat pipes along the rails to maximize the condenser lines, to increase the overall performance.
So we can then take that general layout and import it into an FBA Modeling Software.
So, the image on the left shows the results of this card without any heat pipes.
This gives a maximum temperature under those heat loads of about 170 degrees C, So it’s pretty hot.
Once we implement those, each eight heat pipes into this 4U card, that reduces the temperature down to 93 degrees F, So, a significant improvement in thermal performance, just by adding 8 heat pipes.
That kind of shows you a quick example of how heat pipes can be integrated into a card, and the performance benefits you can get by doing so.
Let’s change over to Phase Change Material (PCM).
Before we dive into the details of designing a PCM heat sink, let’s take a look at the basics, starting with the phase change material itself.
This material is any material that has a high latent heat value, as it changes phase, that can either be the solid to liquid transition, or the liquid to gas transition.
For most applications, we’d like to use the solid to liquid transition, because the changes in density between those two phases are significant significantly less than the liquid-gas transition. So it’s easier to keep it in one compartment.
Things to keep in mind when you select a PCM or the range. Obviously, you want that to be in a good temperature range for your application.
Compatibility with the base metal.
Normally, that’s aluminum but can vary, The type of PCM, the purity and the capacity.
This table shows attributes of a couple of common PCM materials starting with hydrated salts.
These can have very high thermal conductivity, which is good and have high latent heat values as well.
They can be corrosive to most metals and we unstable over repeated thermal thermal cycles.
So while they have good properties, they can be hard to actually implement into a rugged and long-life device.
Next up are metallics.
These also have high thermal conductivity but don’t offer as high latency as some of these especially in the higher temperature range can be corrosive.
So we try to avoid these because they don’t provide as high thermal performance.
Lastly is paraffin wax.
So these have very high latent heat values and are noncorrosive, unstable components, but have very low thermal conductivity. That can mean that designing with them is a bit challenging, but it’s normally worth it because of their high thermal performance.
So for a phase change material heat sink. As Adam was mentioning, the real use of this is storage for transient heat loads, so that you don’t have to oversize your heat sink.
Challenges, as we’re just talking about is the poor thermal conductivity of paraffin, which can create large gradients during the melt process.
So what we need to do when we design a PCM heat sink is properly designed the thin pitch and film thickness, the inside of a heat sink.
This beta is basically a trade study between the delta T going into the PCM and the PCM volume.
You can imagine if you’re your fence structure is too big, or there are too many things.
You can have really good heat transfer into the PCM, but you’ve actually removed a good portion of the PCM volume, which reduces the overall performance of the heat sink.
So we always have to trade off those two things when we do it.
A PCM, heat sink design.
Let’s take a look at this case study.
So here we have the design requirements. We have 10 watts being applied for 110 minutes over this four-inch bright orange area.
We have a starting temperature of 60 degrees.
And I file a max temperature on the surface of 75.
For this, we have three possible PCM materials.
And looking at these values here, comparing the density is latent heat and melt ranges.
It can be difficult to figure out which one is going to be the best selection for it. The first one has the highest density, meaning you can pack the most in there, but the lowest latency.
Where’s the next one is the opposite, the lowest density, but the highest late.
The last one is kind of in the middle.
So when we get something like this, we always like to start with our PCM calculator.
This gives us a good starting point to figure out which PCM is going to be best.
So we input the things, the inputs from the previous slide into this calculator.
This gets us a curve that looks like this.
Each one of these, representing a different PCM and it’s a little hard to see in this chart, but each one of these has a different volume and mass associated with it, so we can compare which one of these provides the best mass and best volume solution.
In this example, we actually then ran an FEA analysis using an FEA transient analysis for this design.
Using the same inputs as you can see this is the yellow line in this table yell as Oh boy.
For this design, we then ran an actual FEA thermal transient model for this.
For this case study, we actually ran a transient thermal model for this.
For this heat sink, the yellow line represents the temperature of the heat sink, as the heat load is applied.
As you can see, once the heat load is first applied, the temperature ramps up rapidly, before it hits, before it hits the melt temperature of the PCM.
That goes through the long latent heat transition period and then raises it up to the maximum temperature of 75 degrees right near the end.
The blue line represents actual experimental results for the same heat sink.
As you can see, we’re a little bit conservative in our modeling approach, which gives us about a 10% increase in the actual performance compared to the model. That’s a good place to be and shows that our modeling is relatively accurate.
Lastly, what I want to talk about is how heat pipes and PCM can be combined for the ultimate thermal management solution.
If you have a high heat load, or concentrated load, or, if you’re PCM volume, is somewhat separated from your available, or from the heat load, you might need to combine heat pipes into a PCM heat sink.
In this case, the heat load is along the bottom edge of the assembly. The heat pipes are then transferring that heat up and into the PCM assembly where it dissipates the heat into the phase change material.
So four takeaways for these.
It’s important to understand your thermal requirements and technology options.
HiK(TM) plates are great for spreading and transferring heat, using heat pipes and PCM is great for storing transient loads in a low mass and volume.
Using these two-phase technologies can greatly improve your thermal performance with minimal impact on your assembly.
Adam: Ok Greg, so the first question is: If you have a design temperature requirement such as -25 degrees to 55 degrees C, would a heat pipe be able to meet the full requirement range?
Well, that’s a good one Adam.
So there’s a couple of different ways that we can solve this problem.
Kind of depends on the application. One of them would be to use methanol as the working fluid.
That will operate under this entire range, so there won’t be any problems there, But most of the time, we actually prefer to use water, Copper-water heat pipes still, and basically, because the ambient is so low, even though the water inside of the heat pipe freezes, there’s normally enough conduction gradient through the aluminum that’s left in the HiK(TM) card, that it will still meet the system requirements at those low temperatures.
Adam: OK, excellent.
The next question, We have heat pipes, then used in fighter jet application.
If so, how do you overcome significant G-loading?
Greg: We do have heat pipes in fighter jets. Again, there are a couple of different ways that we approached the solution, depending upon the exact requirements. one of the ways is, we can shorten up the heat pipes with the heat pipe short.
It means you have less distance against that G-loading, that you have to pump the liquid. So, we can overcome some pretty high G-forces by making the heat pipe short.
The other way is that because the G-loading is normally either not sustained or and only in one direction, we can design the heat pipe layout so that it can account for either short durations of those heat pipes not operating, or we have spreading in multiple directions.
So, that we ensure that only one heat pipe is not operating at a time, or at least enough heat pipes are still operating to meet the performance requirements.
Adam: OK, great.
The next question is, How many cycles can a PCM heat sink survive before you see performance degradation.
Greg: That’s an interesting one. We’ve done testing of our PCM heat sinks, up to a couple of hundred cycles without any performance degradation.
We’ve also done testing of the PCM, itself on a much smaller scale.
We’ve done testing up to a couple I think it was tens of thousands of cycles for a couple of different programs.
There is a very minor degradation at that point, but it’s really pretty insignificant, and that’s a lot of cycles at that point.
So in our experience, we really haven’t seen much significant degradation up to tens of thousands of cycles.
Adam: The next question is PCM ever integrated with Liquid cooling?
Greg: We do integrate PCM into liquid cooling systems. That can either be done right next to the component.
And then in that case, in case the liquid cooling system breaks down the PCM can absorb the heat. While the liquid cooling system is building backup. On the other, times we’ve done it where we’re trying to absorb some transient loads some really high transient loads. Typically that’s like a direct energy weapons.
So some really high short duration loads and we can then integrate what we normally do is integrate that PCM into a liquid-cooled PCM, heat sink so that the liquid cooling is still directly cooling the energy weapons. But we’re helping the rest of the cooling system so that it doesn’t, you know, require a very large heat sink there.
So we’re basically reducing the size of the liquid cooling system.
Well I think that is, actually our allotted time is just about up.
So thanks again for your time today, and we look forward to meeting you all soon. Cheers.