5 ms·

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 statio

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.
- thrownawayfan 14y ago
  Wouldn't the boundary layer thickness be affected by the centrifugal force that presses the air against the fins? I was under the impression that that was the main advantage of this design.
  - robomartin 14y ago
    I seriously doubt it. That said, I'll claim ignorance here because I have not studied that part of things in depth. I know that it can take a lot to break-up the boundary layer in any meaningful way. In aircraft design there are reasons having nothing to do with heat transfer to want to break-up the boundary layer. As I understand it this can be difficult to achieve across a large surface (I'm assuming at sub-sonic speeds, don't know). They even resort to such devices as drilling small holes and literally sucking the boundary layer from the surface. This problem is tougher than it might seem. I am not an aero guy, I've learned a lot from thousands of hours dedicated to finding solutions to our specific problems, but I am far from an expert. Maybe an aero expert on HN can fill-in some of the holes. At some level, think about it this way: At some point, microscopic as it might be, some of the air molecules touching the fins have to "stick" to the surface of the fins. At that point air molecule velocity with respect to the fin surface is zero. Then there are molecules that stick to these molecules one layer above, and so on. After a certain thickness the greater airflow will win out and molecules will move at the average velocity of the bulk air mass moving across the fins. What you have is a velocity profile from zero to the average air mass velocity. That's your boundary layer. You can do things to make it thinner, but eliminating it is very difficult. Techniques like impingement cooling do this to varying degrees of efficiency.
  - devenson 14y ago
    Yes, spinning the heatsink is the whole point, to take advantage of the reduced boundary effect from the centrifugal force. I imagine the force of the air pressing against the fins in this case is considerable at such high RPMS. The other clever aspects of the design like the air bearing are simply to overcome problems that result from having a heatsink that's moving with respect to the heat source.
- 14y ago

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.

thrownawayfan 14y ago
Wouldn't the boundary layer thickness be affected by the centrifugal force that presses the air against the fins? I was under the impression that that was the main advantage of this design.
- robomartin 14y ago
  I seriously doubt it. That said, I'll claim ignorance here because I have not studied that part of things in depth. I know that it can take a lot to break-up the boundary layer in any meaningful way. In aircraft design there are reasons having nothing to do with heat transfer to want to break-up the boundary layer. As I understand it this can be difficult to achieve across a large surface (I'm assuming at sub-sonic speeds, don't know). They even resort to such devices as drilling small holes and literally sucking the boundary layer from the surface. This problem is tougher than it might seem. I am not an aero guy, I've learned a lot from thousands of hours dedicated to finding solutions to our specific problems, but I am far from an expert. Maybe an aero expert on HN can fill-in some of the holes. At some level, think about it this way: At some point, microscopic as it might be, some of the air molecules touching the fins have to "stick" to the surface of the fins. At that point air molecule velocity with respect to the fin surface is zero. Then there are molecules that stick to these molecules one layer above, and so on. After a certain thickness the greater airflow will win out and molecules will move at the average velocity of the bulk air mass moving across the fins. What you have is a velocity profile from zero to the average air mass velocity. That's your boundary layer. You can do things to make it thinner, but eliminating it is very difficult. Techniques like impingement cooling do this to varying degrees of efficiency.
- devenson 14y ago
  Yes, spinning the heatsink is the whole point, to take advantage of the reduced boundary effect from the centrifugal force. I imagine the force of the air pressing against the fins in this case is considerable at such high RPMS. The other clever aspects of the design like the air bearing are simply to overcome problems that result from having a heatsink that's moving with respect to the heat source.
14y ago