I remember reading the paper (or "a" paper) on the Sandia cooler a couple of years ago. The author seemed to indicate that the disks spinning so-fast-yet-so-close served to break the boundary layer AND keep the gap clean and clear of dust and grime.
I'd be interesting to understand just how precisely matched the surfaces have to be for this to work well. Machining a reasonably flat reference surface on a CNC lathe or mill isn't all that difficult. The question in my mind is more about how flat these surfaces have to be. The cutting tools will leave some grooves, even if almost imperceptible. Do the surfaces have to be lapped (sanded) and polished for this heat exchanger to work well? What are the tolerances? A good shell cutter on a high-quality milling machine can produce a mirror-like surface. It's one thing to do this in small quantities and quite another in mass production (which I now nothing about).
The gap is 1-thou (30um), this isn't challenging machining - you could make the gap 30nm ! (it's a bit more annoying with copper)
Actually some machining marks would probably be good, small surface irregularities will break the boundary layer - like sharks skin and make the air flow mix better.
Agreed. I've made "reference" flat surfaces on aluminum using our Haas VF3-SS vertical milling machine with a good quality shell mill with new inserts and, if you do everything right, you can see yourself on the mirror-like surface that results. This machine is probably the grade of machine you might expect to find in a high-volume production shop.
I'd be interesting to understand just how precisely matched the surfaces have to be for this to work well. Machining a reasonably flat reference surface on a CNC lathe or mill isn't all that difficult. The question in my mind is more about how flat these surfaces have to be. The cutting tools will leave some grooves, even if almost imperceptible. Do the surfaces have to be lapped (sanded) and polished for this heat exchanger to work well? What are the tolerances? A good shell cutter on a high-quality milling machine can produce a mirror-like surface. It's one thing to do this in small quantities and quite another in mass production (which I now nothing about).