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Small Form Factor Cooling with Jet Air Mover Technology

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Overview: In this webinar, attendees will learn how Jet Cooling Technology has far reaching implications on small form factor design. These novel air movers increase the design space, flexibility and performance of a wide range of products, from consumer handhelds to LEDs to outdoor telecom.

This presentation covers both the fundamentals and applications of using PulseJets to cool electronics. We will progress from science, to technology, to products, to applications so that comprehensive understanding is developed. Attendees will learn about the structure of the PulseJet flow and why it is different from other active cooling. We will also cover the unique way that the flow is generated, and how it is superior to rotating machinery like fans and pumps. Lastly we will use examples of the technology in product form to illustrate the simple yet powerful design principles that engineers should follow in order to be successful.

Lee JonesLee Jones
Lee Jones joined Aavid-Thermalloy in the summer of 2014 with the acquisition of Nuventix Inc, the global leader in synthetic jet cooling. As part of the team at Nuventix, Lee led technical and product development efforts to create the world’s most compact and reliable turbulent air coolers. His role was an inside-outside one spanning SynJet engineering and customer product and advanced development teams to tailor SynJet solutions that would enable breakthrough products for customers. At Aavid he performs a similar role leading the product management efforts of SynJet development globally. Lee has a Bachelor’s Degree in Mechanical Engineering from Rochester Institute of Technology, and a Master’s Degree in Mechanical Engineering from The University of Texas at Austin. He has 19 years in Thermal Management, and his areas of expertise include New product development, technology commercialization and SynJets.

The following are questions presented to the speaker by the attendees during the webinar, along with answers to each.

Why is this device taking air near to heat sink, where the ambient is already at high temperature, and hence the air thrown on fins is not that cool?
Answer: This can happen – it all depends on the system design. If the air that is local to the PulseJet is hot then yes, hot air will blow cross the heat sink. However, with good system design (a service that Aavid can provide for you if you need it) we can often find ways to provide fresh air to the inlet area and achieve maximum thermal performance.

Also the outlet of jet is just a small cross section, how will it cover the full fin base
and fin cross section area?

Answer: Think of the jet outlet as a long thin cross section. The width of the cross section spans the width of the heat sink to make sure that air is injected into every fin channel. And while the jet outlet is small in the vertical direction, the jet spreads out at an angle and the secondary entrained flow that is dragged along with the Pulse fill the channel with flow.

You can prove this to yourself if you had a sample of one of our products and fog/smoke generator.

Generally inside an enclosure housing pcb, the overall air is heated. How does this device take air from outside the enclosure, to do more efficient cooling?
Answer: This is a great question – and it taps into what Aavid is good at – because it’s a systems engineering question. The key to getting fresh air into the solution is to place small vents near the interface between the PulseJet and the heat sink. This causes the secondary entrained air to be pulled from outside the chassis or enclosure, and into the heat sink. It’s not always possible – but it’s a best practice design principle.

Can this tech combine thermoacoustic refrigeration in future to actually deliver cool air?
Answer: It seems that the answer to all fluid and heat transfer questions is, “It depends,” and this one is no exception.

I am not an expert on thermoacoustic refrigeration, so please take everything I say with a grain of salt. An expert is always preferred! Thermoacoustic refrigerators come in two broad flavors – standing wave and traveling wave. The distinction of the wave physics results in a difference in thermodynamic cycles inside the device – the Brayton Cycle and Stirling Cycle respectively. (Interested readers can read all about this on Wikipedia – they have a nice simple summary that I am paraphrasing and links to companies and research groups that are working on this technology.) Both systems rely on resonance in a cavity to function.

It is theoretically possible that we could use a PulseJet actuator (transducer) to generate the wave in the resonant cavity of the TA. This could be coupled with flow generation from the backside of the actuator to enhance the heat transfer from the surface of the thermoacoustic refrigerator. While this is interesting, it probably isn’t practical without a lot of R&D thrown behind it. It gets ridiculously tricky because you may have different working fluids on the inside of the device (some people use helium) and it’s important to minimize transducer non-linearities for the sake of efficiency (in addition to the inherent first and second law losses you have to overcome) so very quickly you are working on a problem that requires deep understanding of the multi physics domains that are coupled.

Since we haven’t done it, we can’t say we can do it, but it seems possible. The key question is, “Does it make any sense on the system level to use the actuator used to set up the wave for external heat removal?”

It would be fun and interesting to look at an existing TA refrigerator in a lab somewhere and see what the practical implications of generating flow from the existing transducer would be.

If someone wants to investigate this with us here at Aavid, I’d love to discuss it. One one hand, we are a manufacturing company that likes to ship lots of products – but on the other hand we are a community of excellent thermal engineers that love to innovate and invent the next big thing. We are probably the only company on the planet with such a high concentration of thermal engineers. We are heat transfer geeks and we are proud of it!

How do PulseJets hold up to harsh environments.
Answer: They love harsh environments. Aavid manufactures PulseJets with a variety of different materials for different applications. Some of our customers use them outdoors, some use them in factories. We even have Pulsejets installed in the strand of lights that spans the Brooklyn Bridge in New York City. And everyone knows this about New York – if you can make it there, you can make it anywhere.

What is the MTBF?
Answer: We quote the lifetime or reliability of the PulseJets in terms of L10 lifetime, and our toughest and longest lasting products, the SynJets, have an L10 lifetime of 200,000 hours at 60C with 90% confidence. This can be ten times longer than a cheap commodity fan!

Are the diaphragms susceptible to failure?
Answer: Everything that moves is at risk of fatigue failure. As we all know, how you move and how much you move is very important for long life in a moving component. Our engineers have designed the diaphragms to have nearly infinite fatigue life because of the very low strain energy that is dissipated inside them. We can’t talk about this quantitatively because it’s one of our deepest held trade secrets, but rest assured that our long lifetime (decades) is supported by the robustness of the diaphragms, and they can operate for hundreds of billions of cycles.

Do you have customers in the aerospace industry who are using PulseJets?
Answer: The short answer is yes, but unfortunately our confidentiality agreements prevent us from telling you who it is. Our customers use them in small avionics boxes that are in cramped spaces with no airflow. It’s kind of ironic that in a jet going 500 miles an hour, there isn’t even 1 meter per second of airflow.

Does the electric field that moves the diaphragm induce currents or noise in the surrounding electrical circuits?
Answer: Generally no. We operate at low frequencies (around 100Hz or less) and our magnetic fields are small and highly focused. We’ve had hundreds of customers ask us this question and they’ve never had a problem with this.

Hello! Has this technology been used in compact optical modules?
Answer: Hello to you! We have helped some people design small form factor optical devices before. They were small volume customers and purchased parts through a distributor, so we lost sight of how many they are shipping.

Can you describe the diaphragm mechanism in more detail and if you use piezo devices to move the diaphragm?
Answer: The diaphragms in different products are all a little different – and it would be too long an answer, and too full of sensitive information to put on a website. It is something best discussed face to face. Andyes, some of our products use Piezos. We are motor-topology agnostic though – we choose the right actuation technology depending on the target end use.

How big can you scale up this technology?
Answer: Theoretically there are no limits. We once built a demonstrator out of 18” drivers that you could use to float a beach ball in the air. We did this just for fun in our lab (that’s how our engineers are) and ended up showing it at a few trade shows (that’s how our marketing folks are!) Realistically, the goal of PulseJets is to make them smaller rather than larger. This is because people who have lots of space can use passive cooling or large blowers to move lots of air. We like to solve the thermal problems that fans and natural convection cannot solve (be different!) and to accomplish that we are always challenging the engineers to make things smaller.

What is the range of heat flux, your solution can dissipate?
Answer: For a single PulseJet we typically cool less than 100W. It isn’t much. For thermal problems that have more heat and require more cooling, we will usually use more than one device and break the thermal design up into smaller “parallel” thermal problems. It’s like the old riddle – “How do you eat an elephant? One bite at a time.”