3 ms·

"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

"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." You misunderstood the physics involved. When they refer to thinning the boundary air, they are not talking about within the planar air gap binding. They are talking about the air within each of the impeller's channels. The air in an air gap binding experiences positive pressurization. The proof of this is how the impeller lifts above the bearing surface. The pressure overcomes gravity in this case once the sheer within the gap is sufficient. While there is some mass exchange in and out of the gap, there is no significant volume moving through it. If you think about it, this makes sense, because the volume of flow would be limited by the surface area of the perimeter. Given that the gap is so slight, not much of a window for any flow. It is more like a captured air lubricant, and heat transfer through it is more conductive than convective. Rather, they are talking about the boundary around the impeller itself, particularly with each vane channel. The air within these channels experiences a centrifugal force. This shrinks the boundary layer thickness over the impeller surface as a whole. If you think about it, the impeller has a lot more surface area than the gap. This is why getting a thinner boundary layer has a big benefit. To simulate this you'll need not FEA but state of the art in CFD simulation that is aware of boundary layer effects and vortex flows. Liquid cooling is very neat, especially for very large systems or high head loads. I think an important secondary benefit is that warm liquid is easier to use as an industrial energy input than a mass of warm air. But air only cooling devices have a big cost advantage, so if you can live with the performance constraints they offer, you'll probably be lower total cost. The goal here is cost effectiveness, not a race to the highest score as a watt number. "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." The impeller is doing double duty: radiating heat and moving air. While I believe you about the value of fins in a radiating surface, here they're optimizing against two effects, so I think it's plausible that taller fins result in more flow per vane. "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 ex-changer that uses the Sandia heatsink to move heat into the surrounding air, and, thereby, cool the CPU." I think once you've suffered the costs of putting the heat into a liquid, a passive radiator is going to be better at getting it to the air per dollar, because you're no longer constrained to compact geometry.

robomartin 14y ago
I'll have to play with this. I don't doubt what you are saying at all. I've been lazy, I said FEA when I meant FEA and CFD, we have both available and yes, boundary layer effects and vortex flows are taken into account. Fun stuff to play with. For some problems you could spend your entire life simulating and never reach a conclusion! I'll repeat my warning that I am not an aero engineer and what I've learned has lots of holes here and there. As I understand it, boundary layer control is difficult. I remember going through NASA papers that talk about techniques used in centrifugal superchargers to control the boundary layer. These are impellers running at speeds way beyond that of the proposed heatsink. If I remember correctly, one of the problems with various techniques is that of flow separation. This would cause large portions of the blade surface to, effectively, not exchange any heat to speak of with the separated flow. Airfoil choice is important here. That said, the proposed heatsink does not, as its primary design intent, have the requirement to be a good pump. The primary design requirement is to opimize heat transfer to the surrounding air. Things can and probably do change when you are optimizing for that. What I would really like to see is the performance of a complete design. One that includes a casing with suitable inlet and outlets as well as the required safety devices. In my limited experience, that's when things can start to change. For example, the nature of the intake airflow can greatly affect what happens when the airflow hits the vane leading edge and beyond. Also, noise levels can go up. It'd be interesting if someone in aerodynamics could pitch-in and talk about some of these effects and how things can change outside of simulations and free-air environment prototypes.
- jasonwatkinspdx 14y ago
  That's a really good point, I hadn't thought of how sensitive the effect might be to turbulence caused by cables and whatnot around it in a case.

robomartin 14y ago

I'll have to play with this. I don't doubt what you are saying at all. I've been lazy, I said FEA when I meant FEA and CFD, we have both available and yes, boundary layer effects and vortex flows are taken into account. Fun stuff to play with. For some problems you could spend your entire life simulating and never reach a conclusion! I'll repeat my warning that I am not an aero engineer and what I've learned has lots of holes here and there. As I understand it, boundary layer control is difficult. I remember going through NASA papers that talk about techniques used in centrifugal superchargers to control the boundary layer. These are impellers running at speeds way beyond that of the proposed heatsink. If I remember correctly, one of the problems with various techniques is that of flow separation. This would cause large portions of the blade surface to, effectively, not exchange any heat to speak of with the separated flow. Airfoil choice is important here. That said, the proposed heatsink does not, as its primary design intent, have the requirement to be a good pump. The primary design requirement is to opimize heat transfer to the surrounding air. Things can and probably do change when you are optimizing for that. What I would really like to see is the performance of a complete design. One that includes a casing with suitable inlet and outlets as well as the required safety devices. In my limited experience, that's when things can start to change. For example, the nature of the intake airflow can greatly affect what happens when the airflow hits the vane leading edge and beyond. Also, noise levels can go up. It'd be interesting if someone in aerodynamics could pitch-in and talk about some of these effects and how things can change outside of simulations and free-air environment prototypes.

jasonwatkinspdx 14y ago
That's a really good point, I hadn't thought of how sensitive the effect might be to turbulence caused by cables and whatnot around it in a case.