Further reply from study author in response to my message stating that I had posted his reply to this forum, and pointed him to it:

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Thanks for your interest. With regard to filling the air gap region with conductive fluid, the problem is that frictional shearing losses become prohibitively large even at low rotation speeds. It’s not that it wouldn’t work at all, but that it wouldn’t work very well; thermal conductivity is important, but so is viscosity.

There a number of other issues that apparently have caused confusion as well. Maybe we can use this forum to provide clarification. Please feel free to post what follows.

It appears that many people were unclear about what I was attempting to convey with regard to the subject of dust fouling. I did not mean to imply that there is literally no dust fouling; some dust accumulation eventually becomes visible to the naked eye on the very leading edge of the blades. The point is that dust fouling is reduced to such a large extent that we are unable to detect any degradation of cooling performance operating the device in a relatively dirty environment over an extended period of time. Thus for all intents and purposes the dust fouling problem has been taken of the table. In contrast, with conventional CPU coolers, eventually the entire heat exchanger surface becomes entombed in dust.

Some people have expressed concern that such a rotating heat-sink-impeller would constitute a safety hazard. Any real world device would include a screen, grill, or other form of protective enclosure. Such protective measures are widely used on conventional fans. We photographed our device without an enclosure so that it would be easier for people to see what it looks like.

There seems to be confusion about where the potential for significant electricity savings resides. The vast majority of it is associated with applications such as air conditioning and refrigeration, not electronics cooling. But such energy sector applications will only materialize if air bearing heat exchanger technology proves amenable to size scaling. We are in the process of evaluating this question.

Many have expressed skepticism about the practicality of a 0.001” air gap. That’s certainly understandable, and it was one of the first things we investigated. After all, if the requirement for a small air gap precludes the possibility of low-cost manufacturing, reliability, etc. then I would be the first to agree that all of this is a pointless exercise. The end-of-project report has a discussion of why this is not the case. Another counter-intuitive point is that air bearings are extremely mechanically stiff, rugged and reliable. This is also discussed the report.

On a related subject, many were concerned about the manufacturability of the heat-sink-impeller. We are converging on cold forging as the best route to low cost fabrication. We understand that if we can’t drive the cost down that air bearing heat exchanger technology will have little impact.

Others have pointed out that intuitively it would seem like the last thing you’d want to do is intentionally introduce an air gap (rather than something like thermal grease) in the thermal conduction path between the CPU and heat exchanger. Qualitatively this sounds like a persuasive argument. But as discussed in the report, quantitatively, the numbers (gap distance, gap area, thermal conductivity of air, enhancement of conductivity by convection) work out quite well. For a 10 cm diameter device, an air gap resistance of 0.02 C/W is certainly feasible.

Another important point is that the version 1 prototype device discussed in this report is badly unoptimized. The main objective for version 1 was to test our hypotheses regarding the advantages of such a device architecture. If things continue to go well in lab, I suspect eventually we’ll end up at about 0.05 C/W for a 10 cm diameter device that’s of order 3 cm high, operates at three to four thousand rpm, and consumes about 5 watts of electrical power. But believe it when you see it. There’s always risk involved in try to solve tough problems. We’re giving it our best shot. We’re also working on alternative device geometries that may be capable of providing considerably better performance.

What else? A couple people stated that the thermal brick wall is at 4 GHz, not 3 GHz. Fair enough. My point is that if you introduce a drastic improvement in thermal management technology, whatever the number for thermal brick wall may be, it gets pushed a lot higher.

Other people had questions about why such a heat-sink-impeller is so quiet. What it boils down to is that you’ve got a lot more flexibility with regard to blade geometry than you do with a fan. That means you are free to design the blade geometry to smoothly split and smoothly rejoin the flow field at the impeller entrance and exit; the device architecture allows you to decouple the engineering constraints of adequate air flow and low noise.

A couple of people surmised that the air bearing heat exchanger requires a source of compressed air because we used a hydrostatic air bearing in the version 1 device. As described in the report, in a real-world device you’d use a hydrodynamic (or self pressurizing) air bearing. That’s what we’re using now in versions 2 and 3. The use of a hydrostatic bearing in version 1 was an experimental convenience.

I have to go to a 5:30 meeting, so I will leave it at that. Hopefully people will find portions of the above material informative.

Sincerely,
Jeff Koplow
Sandia National Labs
Livermore, CA