Two points of confusion (for me, at least) which the article doesn't satisfactorily alleviate: first, how can something dissipate heat effectively when there is an air cushion between the base plate and the cooling vanes? And second, how is this device "immune to dust and detritus"? For instance, it seems like dust could easily enter the thin air cushion layer and cause all kinds of problems.
Can anyone help me overcome my ignorance and understand these points?
The report says that because the cross section of the air gap is much larger than its thickness, and the air in the gap is violently sheared , it has low thermal resistance.
I'm guessing the action of the impeller sucking air through itself would be enough to keep a reasonable amount of dust from collecting in the gap.
It'll probably fall apart in heavy dust environments, but most things do anyway.
To quote the pdf:
"""Rotation of the heat sink at several thousand rpm also provides a potent remedy to the
longstanding problem of heat exchanger fouling. Consider for example the CPU cooler
shown in Figure 1. The finned, metal heat sink cannot be seen because it’s covered in dust.
But the fan blade, which operates in the same environment, is for all intents and purposes
perfectly clean. This contrast in dust accumulation is at first startling, but in hindsight
entirely expected. The air bearing heat exchanger therefore provides a complete solution to
the problem of performance degradation due to heat sink fouling. In specialized applications
involving extremely high particle loading, a straight-radial rather than backward-swept fin
design would likely be used [Bleier, 1997]."""
The air gap is 0.001 inches? Those are pretty huge for dust particles. Preventing those very large particles from fouling the air bearing will be easier than preventing much smaller particles from fouling a lubricated one.
You get build up on fans normally due to the boundary effect. Basically, even on fast moving objects, there's this layer of air above the surface where the air is very slow moving (which results in dust settling). By reducing the spacing to the degree they have, they 'cut' into the boundary layer and thus prevent dirt build up.
I'm not exactly sure how transmitting heat across the gap would work. I imagine it would just be by convection, but I don't know how efficient that would be.
That said, I imagine that this type of design would be problematic in laptops. With the spacing so tight, it seems to me that any shocks to the fan could result in the vanes touching the baseplate.
As the metal blades spin, centrifugal force kicks up the air and throws it up and outwards, much like an impeller, creating a cooling effect.
I had to read that part twice too, but it sounds like the air cushion is only there initially, but gets sucked out when the fan spins. (That said, if it leaves a vacuum, that doesn't sound great for conductivity either...)
It wouldn't leave a vacuum, as the air on the outside of the fan would rush in to replace it. I think the point is that (if this works), the boundary layer itself will also be moving instead of sitting there stagnant.
I can't answer your questions, but I was wondering the same things. Also, what makes this design quieter than a traditional fan/heatsink? Isn't this still basically a fan?
Can anyone help me overcome my ignorance and understand these points?