8 ms·

From the article: "This centrifugal force is what gives the Sandia Cooler such massive efficiency, too." That is not correct. The efficiency boost comes from

From the article: "This centrifugal force is what gives the Sandia Cooler such massive efficiency, too." That is not correct. The efficiency boost comes from the very efficient transfer of heat from the CPU to the spinning fan/heat exchanger by, effectively, using the very thin air-gap under the impeller. The rotation of the impeller breaks the boundary layer in the airgap and you get good heat transfer across a very thin gap. The airflow over the machined aluminum blades and, in general, the rotation of the entire impeller assembly, serve to keep the heat exchanger free from dust accumulation (which restricts heat flow). Of course, as many have pointed out, it remains to be seen how easily this concept translates to a mass-manufactured low-cost solution in terms of performance and reliability under varying conditions. I've done a ton of heat flow FEA work when working on various approaches to cool a custom high-power LED array (1,500W power-in). We could get reasonable results with complex forced-air solutions and relatively expensive custom machined heatsinks as well as carefully modeled airflow controls. In these cases the solutions were always very large (volume). When we switched to fluid-based cooling things changed dramatically. One of the design challenges was to maintain a narrow delta-T across the LED array. This is because thermal uniformity was required in order to have uniform performance across the array. The fluid solution, with some tricks, could easily achieve ten times better thermal uniformity than the air-cooled approach. And, in addition to this, cool the entire array to a much lower final temperature. A fluid cooling system was constructed using only a small fluid pump and no air-moving fans at all. A passive natural convection radiator could easily handle the heat-load in a normal air-conditioned office environment. While I have not looked at the specific case of cooling a CPU, based on my experience I have to say that far greater gains can be had by rapidly moving heat from the CPU surface using fluid-based cooling. This, effectively, creates the opportunity for much greater surface extension than can reasonably be applied to the small surface area of a CPU. Again, I have never studied CPU cooling, but I am not sure that this 150W cooling limit applies to fluid-based cooling. I can see building a systems that can very easily move 150W, or even double that, using a relatively simple fluidic cooler. At some level it is a matter of how many molecules of the fluid you can move across the CPU-side heat exchanger per unit time. The answer to that is "a lot". I can't see the Sandia or any other pure air-based cooling system used for CPU cooling at the extremes. The assembly would have to be very precisely manufactured and lots of work would have to be done in order to ensure that vibrations and harmonics of the motor drive system itself don't cause damage to the circuit board. If the system needs to have an impeller spinning at 2K RPM or more, lots of work needs to go into making it safe for servicing as a metal impeller like that can shred fingers in an instant. Finally, there's the question of the mass of the spinning impeller. In order to transfer heat into the impeller blades you are limited to certain geometry. If the spinning base and/or the blades get too thin you simply won't be able to move the heat out no matter how well it can move from the stationary plate up to the revolving disk. This is critical and it means that there are certain minimum geometry constraints that are likely to make the impeller somewhat massive. From my FEA work on heat transfer I know that you can only go so thin on blades before they become useless past a few millimeters above the heatsink base-plate. The same is the case here. What I can see is the use of this concept to create a fluid based solution that uses a liquid to quickly move heat from a CPU to a much larger heat exchanger that uses the Sandia heatsink to move heat into the surrounding air, and, thereby, cool the CPU. Even at that, I'd like to see data comparing conventional forced-air convection cooling of the external heat exchanger and even a comparison to a natural convection solution.

excuse-me 14y ago
I would have though the efficiency comes from spinning the heatsink and so having a very efficient metal-air heat transfer compared to blowing air over a stationary heatsink It's weakpoint is getting the heat from the CPU across the air gap into the spinning part
- robomartin 14y ago
  I don't see the super-thin air-gap as an issue. In fact, it is probably an asset. As an example, when we build the 1500W LED array we used a very thin insulating film to transfer heat from the LEDs to an aluminum clad circuit board. If you have surface area (the stationary disk in their case) a thin gap can be surprisingly efficient at transferring heat from one element to the other. Ad to this breaking up the boundary layer within that gap and now air becomes a "better" conductor. There is no difference between spinning the heatsink and moving air at the same velocity across stationary heatsink fins. It's about molecules of air at a lower temperature going across a surface with at a higher temperature. N molecules per unit time "absorb" some of the heat and come of of the process at a higher temperature. The surface temperature drops by a similar amount. What kills you in both cases is a very thin boundary layer of air that sticks to the surface and has a velocity profile that goes from zero at the surface to whatever the air velocity might be. This boundary layer isn't good for heat transfer from the surface to the moving air molecules, which takes a toll on the overall efficiency of the process. There are techniques to help break-up the boundary layer in stationary fin heatsinks. Some break-up airflow to introduce turbulence. Others use raw velocity to break it up (air impingement cooling). I don't know how well the rotating aluminum impeller breaks-up the boundary layer surrounding the fins. It won't eliminate it completely but it certainly could be much improved from the case of stationary fins with low-speed air going across them. If you had a heatsink with stationary fins and air moving at the same speed as in the case of the rotating impeller (by using fans, of course) you'd probably get similar boundary layer effects. What I learned is that there really is very little magic in heat transfer. I have seen some really wacky ideas brought to market that always seem interesting but have never proven to beat the simple solutions. Funny enough, other than materials, most of the focus is always around dealing with the boundary layer, an issue that disappears (in terms of its significance) once you move into fluid-based cooling. Caveat: I have not studied rotating aluminum impeller/heat-sinks, so I could very well be missing something that I simply don't know. My opinions are based on thousands of hours of research and FEA work in looking for solutions for the aforementioned project.

excuse-me 14y ago

I would have though the efficiency comes from spinning the heatsink and so having a very efficient metal-air heat transfer compared to blowing air over a stationary heatsink It's weakpoint is getting the heat from the CPU across the air gap into the spinning part

robomartin 14y ago
I don't see the super-thin air-gap as an issue. In fact, it is probably an asset. As an example, when we build the 1500W LED array we used a very thin insulating film to transfer heat from the LEDs to an aluminum clad circuit board. If you have surface area (the stationary disk in their case) a thin gap can be surprisingly efficient at transferring heat from one element to the other. Ad to this breaking up the boundary layer within that gap and now air becomes a "better" conductor. There is no difference between spinning the heatsink and moving air at the same velocity across stationary heatsink fins. It's about molecules of air at a lower temperature going across a surface with at a higher temperature. N molecules per unit time "absorb" some of the heat and come of of the process at a higher temperature. The surface temperature drops by a similar amount. What kills you in both cases is a very thin boundary layer of air that sticks to the surface and has a velocity profile that goes from zero at the surface to whatever the air velocity might be. This boundary layer isn't good for heat transfer from the surface to the moving air molecules, which takes a toll on the overall efficiency of the process. There are techniques to help break-up the boundary layer in stationary fin heatsinks. Some break-up airflow to introduce turbulence. Others use raw velocity to break it up (air impingement cooling). I don't know how well the rotating aluminum impeller breaks-up the boundary layer surrounding the fins. It won't eliminate it completely but it certainly could be much improved from the case of stationary fins with low-speed air going across them. If you had a heatsink with stationary fins and air moving at the same speed as in the case of the rotating impeller (by using fans, of course) you'd probably get similar boundary layer effects. What I learned is that there really is very little magic in heat transfer. I have seen some really wacky ideas brought to market that always seem interesting but have never proven to beat the simple solutions. Funny enough, other than materials, most of the focus is always around dealing with the boundary layer, an issue that disappears (in terms of its significance) once you move into fluid-based cooling. Caveat: I have not studied rotating aluminum impeller/heat-sinks, so I could very well be missing something that I simply don't know. My opinions are based on thousands of hours of research and FEA work in looking for solutions for the aforementioned project.