FREQUENTLY ASKED QUESTIONS
Principles of Operation
Q: Is the thermal resistance of the air gap region large enough to be a problem?
A: No—For example, in our current prototype device the thermal resistance of the air gap region is 0.02 °C/W.
Q: How does a hydrodynamic air bearing work?
A: A video of a very rudimentary hydrodynamic air bearing is available at
Rayleigh Hydrodynamic Thrust Bearing - YouTube
Q: Does the stationary base plate simply consist of a solid piece of thermally conductive material?
A: As in a conventional forced-convection heat sink, the base plate acts as a heat spreader, and may comprise solid piece of thermally
conductive material or be implemented in the form of a heat pipe.
Q: Is the radial-air-flow architecture depicted here the only possible device geometry?
A: No—a wide variety of device geometries are possible, including axial-flow configurations, and devices in which more than one rotating
cooling fin/impeller operate in parallel to further lower thermal resistance.
Real-World Practicality
Q: Can the device be mounted in any orientation?
A: Yes—the air bearing assembly is held together by magnetic attraction (between the stator and permanent-magnet rotor).
Q: Is a an air bearing suspension mechanically stiff and rugged?
A: Yes—because the rate of change of the pressure lifting force with respect to gap distance is extremely large.
Q: What other types of equipment use air bearings?
A: Devices range from hard disk read–write heads to large CNC milling machine spindles.
Q: What if small (less than 0.001″) particulates are somehow introduced into the air gap region?
A: They are swept outward and ejected by centrifugal force.
Q: How do you prevent contact between the air bearing surfaces at low rpm?
A: Depending on the application, one of several simple, passive mechanisms may be implemented.
Manufacturability
Q: Does the 0.001″ air gap require tight manufacturing tolerances?
A: No—the hydrodynamic air bearing gap distance is passively self-regulating.
Q: Is the surface quality/flatness spec of a conventional heat sink mating surface adequate?
A: Yes—For example, conventional fabrication processes such as cold forging can be used.
Performance
Q: Based on laboratory testing of early prototype devices, what level of performance is expected for a CPU cooler based on
Sandia’s air bearing heat exchanger principle?
A: We expect to achieve 0.05 °C/W in package that is considerably smaller and quieter than conventional high-performance CPU coolers.
Because low-thermal-resistance CPU coolers are particularly susceptible to performance degradation due to cooling-fin fouling (a thin
layer of dust can easily increase thermal resistance by a factor of 2), the added benefit of immunity to fouling is also a crucial performance
specification. Further reductions in thermal resistance would likely involve engineering trade-offs against device size and dBa rating.