The fanless spinning heatsink: more efficient, and immune to dust and detritus(extremetech.com)
extremetech.com
The fanless spinning heatsink: more efficient, and immune to dust and detritus
http://www.extremetech.com/extreme/89710-the-fanless-spinning-heatsink-the-heatsink-is-the-fan
16 comments
[deleted]
"Fanless" in that it is a fan itself.
It is fanless in an important sense. The job of a fan is to move the air, for which small fans are inefficient. Then, the relative motion of the air vs the heatsink allows cooling. But in this device, as the report says, the relative motion of the air vs the fins is obtained directly - and much more efficiently. The fact that the device also serves to move the air - as a fan - is irrelevant.
If the device didn't also move air away, wouldn't the air it was standing in be much higher, reducing its efficiency?
Edit: Of course, in a computer case there would be other fans which would keep replacing the air. Silly me!
Edit: Of course, in a computer case there would be other fans which would keep replacing the air. Silly me!
Two points of confusion (for me, at least) which the article doesn't satisfactorily alleviate: first, how can something dissipate heat effectively when there is an air cushion between the base plate and the cooling vanes? And second, how is this device "immune to dust and detritus"? For instance, it seems like dust could easily enter the thin air cushion layer and cause all kinds of problems.
Can anyone help me overcome my ignorance and understand these points?
Can anyone help me overcome my ignorance and understand these points?
The report says that because the cross section of the air gap is much larger than its thickness, and the air in the gap is violently sheared , it has low thermal resistance.
I'm guessing the action of the impeller sucking air through itself would be enough to keep a reasonable amount of dust from collecting in the gap.
It'll probably fall apart in heavy dust environments, but most things do anyway.
To quote the pdf:
"""Rotation of the heat sink at several thousand rpm also provides a potent remedy to the longstanding problem of heat exchanger fouling. Consider for example the CPU cooler shown in Figure 1. The finned, metal heat sink cannot be seen because it’s covered in dust. But the fan blade, which operates in the same environment, is for all intents and purposes perfectly clean. This contrast in dust accumulation is at first startling, but in hindsight entirely expected. The air bearing heat exchanger therefore provides a complete solution to the problem of performance degradation due to heat sink fouling. In specialized applications involving extremely high particle loading, a straight-radial rather than backward-swept fin design would likely be used [Bleier, 1997]."""
It'll probably fall apart in heavy dust environments, but most things do anyway.
To quote the pdf:
"""Rotation of the heat sink at several thousand rpm also provides a potent remedy to the longstanding problem of heat exchanger fouling. Consider for example the CPU cooler shown in Figure 1. The finned, metal heat sink cannot be seen because it’s covered in dust. But the fan blade, which operates in the same environment, is for all intents and purposes perfectly clean. This contrast in dust accumulation is at first startling, but in hindsight entirely expected. The air bearing heat exchanger therefore provides a complete solution to the problem of performance degradation due to heat sink fouling. In specialized applications involving extremely high particle loading, a straight-radial rather than backward-swept fin design would likely be used [Bleier, 1997]."""
The air gap is 0.001 inches? Those are pretty huge for dust particles. Preventing those very large particles from fouling the air bearing will be easier than preventing much smaller particles from fouling a lubricated one.
You get build up on fans normally due to the boundary effect. Basically, even on fast moving objects, there's this layer of air above the surface where the air is very slow moving (which results in dust settling). By reducing the spacing to the degree they have, they 'cut' into the boundary layer and thus prevent dirt build up.
I'm not exactly sure how transmitting heat across the gap would work. I imagine it would just be by convection, but I don't know how efficient that would be.
That said, I imagine that this type of design would be problematic in laptops. With the spacing so tight, it seems to me that any shocks to the fan could result in the vanes touching the baseplate.
I'm not exactly sure how transmitting heat across the gap would work. I imagine it would just be by convection, but I don't know how efficient that would be.
That said, I imagine that this type of design would be problematic in laptops. With the spacing so tight, it seems to me that any shocks to the fan could result in the vanes touching the baseplate.
To the first, from the article:
As the metal blades spin, centrifugal force kicks up the air and throws it up and outwards, much like an impeller, creating a cooling effect.
I had to read that part twice too, but it sounds like the air cushion is only there initially, but gets sucked out when the fan spins. (That said, if it leaves a vacuum, that doesn't sound great for conductivity either...)
As the metal blades spin, centrifugal force kicks up the air and throws it up and outwards, much like an impeller, creating a cooling effect.
I had to read that part twice too, but it sounds like the air cushion is only there initially, but gets sucked out when the fan spins. (That said, if it leaves a vacuum, that doesn't sound great for conductivity either...)
It wouldn't leave a vacuum, as the air on the outside of the fan would rush in to replace it. I think the point is that (if this works), the boundary layer itself will also be moving instead of sitting there stagnant.
I can't answer your questions, but I was wondering the same things. Also, what makes this design quieter than a traditional fan/heatsink? Isn't this still basically a fan?
[deleted]
Yeah but the fan is the heatsink... vs. a heatsink with a fan on top.
So like current tech = chip -> heatsink -> fan whereas this = chip -> magic spinning heatsink
So like current tech = chip -> heatsink -> fan whereas this = chip -> magic spinning heatsink
"The cooler consists of a static metal baseplate [...A HEATSINK...], which is connected to the CPU, GPU, or other hot object, and a finned, rotating heat exchanger [...A FAN...] that are cushioned by a thin (0.001-inch) layer of air."
OK, there's only 0.001-inch between them, but still a heatsink + a fan, no?
OK, there's only 0.001-inch between them, but still a heatsink + a fan, no?
The article should probably be re-titled "Metal Fans Replace Plastic Fans as Industry Standard Cooling Solutions".
How about "combining fan and heatsink yields efficiency gains"? It's clearly not (just) a metal fan, since it has thermal mass and will likely be optimised for surface area just like a heatsink would be.
"Moving beyond 3GHz". Really?
How many 4GHz chips have been produced that don't need watercooling to operate at load?
Pretty much every intel desktop chip of the last few years will do 4ghz+ with good air cooling. I have an i7 930 @ 4.2ghz on air, my flatmate has a 2600K @ 4.8ghz on air.
Lots of things can run faster than they are supposed to, but that doesn't mean they should or that people are going to like the solution. Often times the bleeding edge processors are clocked according to MTTF based on a reference implementation (which would include some standard of heat dissipation and a processor's TDP and probably not exotic copper heatsinks and high speed fans. If this fan can more efficiently transfer heat out of the processor and dissipate it with a lower total energy consumption, then it could be very useful, especially when you are talking about things like datacenters where total energy consumption and MTTF are things that really matter.
If you can improve the MTTF by leveraging this technology to make the entire server highly resistant to dust fouling, then there might be another tremendous benefit for datacenters.
The real issue is the power consumption, not the actual heating at 4GHz. But even if we don't count the OC, 3.1GHz+ is still beyond 3GHz by any decent math.
3.1GHz is also 3GHz to one digit of precision.
There are chips closer to 4Ghz than 3, though. The author should have aimed higher. http://www.intel.com/products/processor/corei7EE/index.htm
There are chips closer to 4Ghz than 3, though. The author should have aimed higher. http://www.intel.com/products/processor/corei7EE/index.htm
[deleted]
I wonder what the production cost of this is compared to a "traditional" heatsink. It looks like it would require more metal, but I'm not sure. I don't know if it would require more precision in the construction process, but this smells like it will be more expensive. It might not replace stock CPU coolers, just high-end ones. Certainly at first that'll be the case.
It looks like the benefit of this over other high-end coolers (i.e., water cooling) is that it can be a drop-in replacement for them. It doesn't seem to require special parts, knowledge, or tools to install. It could be installed as easily as a normal CPU cooler. The reduction in electricity costs is exciting for businesses (consumers don't care about a 7% electricity reduction). That it doesn't get clogged with dust is also very useful in the long run: less maintenance.
I'll be more excited when this is in production, even at a high cost. Let's hope it gets there.
It looks like the benefit of this over other high-end coolers (i.e., water cooling) is that it can be a drop-in replacement for them. It doesn't seem to require special parts, knowledge, or tools to install. It could be installed as easily as a normal CPU cooler. The reduction in electricity costs is exciting for businesses (consumers don't care about a 7% electricity reduction). That it doesn't get clogged with dust is also very useful in the long run: less maintenance.
I'll be more excited when this is in production, even at a high cost. Let's hope it gets there.
There is one existing technology that could perhaps do away with the problem of the air gap (if that's a significant problem at all): heatpipe.
The good old heatpipe could extend from the stationary baseplate, as an axle/shaft of the impeller, well into the spinning part and here flange out internally. The seal/bearing would have to be quite gas-tight, but if that's achieved, heat transfer could be great.
The good old heatpipe could extend from the stationary baseplate, as an axle/shaft of the impeller, well into the spinning part and here flange out internally. The seal/bearing would have to be quite gas-tight, but if that's achieved, heat transfer could be great.
It would have to be gas tight, and resist a pressure differential. (HSF heatpipes contain water in a partial vacuum)
It must also not leak at all over the course of several years at elevated temperature, while spinning at several thousand RPM, or else the heat pipe will stop working.
It also must be cheap enough to compete commercially with a solid piece of metal.
The design constraints are... loosely possible. But it sure as hell won't be cheap.
It must also not leak at all over the course of several years at elevated temperature, while spinning at several thousand RPM, or else the heat pipe will stop working.
It also must be cheap enough to compete commercially with a solid piece of metal.
The design constraints are... loosely possible. But it sure as hell won't be cheap.
…if these heat exchangers can find windespread adoption in computers and air conditioning units, Koplow estimates that the total US electricity consumption could drop by 7%.
Wikipedia says 11% of US electricity goes to air conditioning and 5% to all electronics combined. That about half of that is cooling fans does not survive the sniff test.
Wikipedia says 11% of US electricity goes to air conditioning and 5% to all electronics combined. That about half of that is cooling fans does not survive the sniff test.
Assuming you got your stats from here:
http://en.wikipedia.org/wiki/Energy_in_the_United_States#Cur...
Which says, for Commercial energy use: 25% lighting 13% heating 11% cooling 6% refrigeration 6% water heating 6% ventilation 6% electronics
You could find gains in ventilation, electronics, refrigeration, cooling, and probably even heating if this technology was widely used and performs well over all uses, so the 7% isn't out of the question, even if it might be very optimistic.
http://en.wikipedia.org/wiki/Energy_in_the_United_States#Cur...
Which says, for Commercial energy use: 25% lighting 13% heating 11% cooling 6% refrigeration 6% water heating 6% ventilation 6% electronics
You could find gains in ventilation, electronics, refrigeration, cooling, and probably even heating if this technology was widely used and performs well over all uses, so the 7% isn't out of the question, even if it might be very optimistic.
I don't disagree with you that the numbers may be off, but consider the fact that the compressors in an A/C unit don't have to work as hard if the heat exchange is more efficient. That's saving electricity from the cooling fan AND the compressor.
Furthermore, put a very large one in place of the fan in the A/C condensor and save more.
I hope to see this in systems soon.. It would be really nice not to risk hearing loss every time I walk into the data center!
Isn't most of the noise in a datacenter the compressors in the aircon units?
No, it is all of the fans sucking air in the front and dumping it out the back. Fire up a 2U server in an otherwise quiet room and it will sound like someone just fired up an vacuum cleaner...
I have two Dell PowerEdges in my living room and they make a sound not unlike an airplane revving up for takeoff.
What about submerging it in mineral oil?
The specific heat of mineral oil (1.67 kJ/kg.K) [1] isn't much better than air (1.0 kJ/kg.K) [2] when considering the difficulty of using a liquid. If you are going to use a liquid - water is a much better heat transfer medium (3.93 kJ/kg.K) [3].
1: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151... (search for "mineral")
2: http://www.engineeringtoolbox.com/air-specific-heat-capacity...
3: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html
1: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151... (search for "mineral")
2: http://www.engineeringtoolbox.com/air-specific-heat-capacity...
3: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html
The difference in specific heat is only a factor of 2. The big difference is in the density, which is about a factor of 1000 larger for liquids over air at atmospheric pressure.
(However, mineral oil has much higher viscosity than water, which I'm not sure is what you want as it suppresses convection. I guess it's a question of whether you take that over the non-conductivity.)
(However, mineral oil has much higher viscosity than water, which I'm not sure is what you want as it suppresses convection. I guess it's a question of whether you take that over the non-conductivity.)
Water may be better at transferring heat, but there is the slightly undesirable effect that it shorts out your electronics and causes corrosion.
Don't try this using water: http://youtu.be/PtufuXLvOok?t=2m10s
Don't try this using water: http://youtu.be/PtufuXLvOok?t=2m10s
Although I can't watch the video at work - I assume it's one of the many videos where the entire motherboard is submerged in mineral oil. Certainly that's the case, and it's why overclockers have been trying this for over a decade. It's just a problem with moving the heat away from a very concentrated area of the die. As lutorm observed, the viscosity (combined with the inability to transfer heat quickly) ends up preventing an efficient mineral oil based cooling solution.
Do away with the air bearing. Just put the whole computer in the base of the rotating heatsink/fan. Get power and data on/off of the thing using brushes. Implement an emergency mode where the CPU slows the clock if the motor fails, so that the heatsink still provides enough cooling without rotating.
So where do I buy them?
I've read the paper now, so I can chime in on an informed basis.
They claim a 0.2°C/W, which would be really something. You can get down to 0.37°C/W[1] with air cooling, using heroic measures, and blisteringly awful efficiency. Doing 0.2°C/W would be a real step up.
I don't really think they've done it, here. Their experimental setup used six 1"x1" 10 watt heating elements. This is because their heatsink needs a very large cross-section to overcome the lousy thermal conductivity of the air gap between the impeller and the base plate.[2] Total area, 38.7cm^2, total power, 60W.
The Intel Core i7's heatspreader has a surface area of ~20.25cm^2, and a thermal design power of 130W. 52% smaller, 216% times the heat output. That's about four times more heat per square centimetre.
The smaller the heat source, the longer the average thermal path between the source and the air/heatsink boundry, the worse the C/W, and the less effective the heatsink will be. If they had actually used a computer processor, rather than a bunch of heating elements, then my WAG is that they would have done 0.35°C/W.
It's a beautiful idea, but their setup looks nothing like reality.
1: http://www.dansdata.com/quickshot012.htm
2: They say that, due to the high sheer speed, there's no boundary layer, which "increases thermal conductivity several-fold". Well that's a cool story, bro, but air (0.025 W/(mxk)) is still sixteen thousand times less thermally conductive than copper! (401.0 W/mxk)) If you increased the thermal conductivity of air by 6.4 times, then it would be as conductive as... rubber, something which is not world renowned as a good conductor of heat!
They claim a 0.2°C/W, which would be really something. You can get down to 0.37°C/W[1] with air cooling, using heroic measures, and blisteringly awful efficiency. Doing 0.2°C/W would be a real step up.
I don't really think they've done it, here. Their experimental setup used six 1"x1" 10 watt heating elements. This is because their heatsink needs a very large cross-section to overcome the lousy thermal conductivity of the air gap between the impeller and the base plate.[2] Total area, 38.7cm^2, total power, 60W.
The Intel Core i7's heatspreader has a surface area of ~20.25cm^2, and a thermal design power of 130W. 52% smaller, 216% times the heat output. That's about four times more heat per square centimetre.
The smaller the heat source, the longer the average thermal path between the source and the air/heatsink boundry, the worse the C/W, and the less effective the heatsink will be. If they had actually used a computer processor, rather than a bunch of heating elements, then my WAG is that they would have done 0.35°C/W.
It's a beautiful idea, but their setup looks nothing like reality.
1: http://www.dansdata.com/quickshot012.htm
2: They say that, due to the high sheer speed, there's no boundary layer, which "increases thermal conductivity several-fold". Well that's a cool story, bro, but air (0.025 W/(mxk)) is still sixteen thousand times less thermally conductive than copper! (401.0 W/mxk)) If you increased the thermal conductivity of air by 6.4 times, then it would be as conductive as... rubber, something which is not world renowned as a good conductor of heat!
Dan's article is very old (2003). At that time is was not easy to mount a big cooler on a motherboard. The heatpipes were not widely used.
These days you can find huge coolers with (or at least claimed[1]) less than 0.2°C/W. Most of the lower power CPUs (40-50W) can run fanless at decent temperatures with half a kilo of metal fins + some heatpipes, and you can mount that kind of heat spreader easily with bolt-through screws.
Also, that metal fan is dangerous! If you ever touched even a slow-spinning plastic fan's blades, you know how painful (or bloody) it can be. And that was only a very light plastic blade, probably less than 30 grams. Now think about the momentum of a 200 gram metal thingie spinning with more than 1000rpm (as I understand, they tested it to 7-8000 rpm). You really want to be sure you will not touch it while spinning. But if you enclose it, even in a wire cage, the performance will decrease.
1: http://www.scythe-usa.com/support/cpu/006/scmn1000_scnj1000p...
These days you can find huge coolers with (or at least claimed[1]) less than 0.2°C/W. Most of the lower power CPUs (40-50W) can run fanless at decent temperatures with half a kilo of metal fins + some heatpipes, and you can mount that kind of heat spreader easily with bolt-through screws.
Also, that metal fan is dangerous! If you ever touched even a slow-spinning plastic fan's blades, you know how painful (or bloody) it can be. And that was only a very light plastic blade, probably less than 30 grams. Now think about the momentum of a 200 gram metal thingie spinning with more than 1000rpm (as I understand, they tested it to 7-8000 rpm). You really want to be sure you will not touch it while spinning. But if you enclose it, even in a wire cage, the performance will decrease.
1: http://www.scythe-usa.com/support/cpu/006/scmn1000_scnj1000p...
I would hesitate twice and then hesitate one more time before placing any confidence at all in a manufacturer supplied number for a statistic like C/W, which can be so easily fudged, as we have seen.
But yeah, I've been out of the overclocking scene for a couple years, now. I have no idea how well heatpipe heatsinks perform, other than "better".
As for the danger of a large piece of metal spinning at kRPM, I agree. This is bad enough that there probably would have to be an safety interlock on computer case access panels that reverses the impeller to a halt when the enclosure is opened.
But yeah, I've been out of the overclocking scene for a couple years, now. I have no idea how well heatpipe heatsinks perform, other than "better".
As for the danger of a large piece of metal spinning at kRPM, I agree. This is bad enough that there probably would have to be an safety interlock on computer case access panels that reverses the impeller to a halt when the enclosure is opened.
SPCR is a review site pretty serious about their methology; their newest heatsink reviews don't have °C/W figures due to difficulty measuring the power output of socket 1366 CPUs, but here's a mid-2009 review of a huge heatpipe/tower heatsink: http://www.silentpcreview.com/scythe-mugen2
Page 6 states °C/W values of 0.29 down to 0.14 depending on the amount of air pushed with the fans. SPCR's test fans are pretty low-flow to begin with (47 CFM/1080 RPM at 12 V), so you could probably go slightly lower by using real screamers.
Page 6 states °C/W values of 0.29 down to 0.14 depending on the amount of air pushed with the fans. SPCR's test fans are pretty low-flow to begin with (47 CFM/1080 RPM at 12 V), so you could probably go slightly lower by using real screamers.
Miahi isn't kidding - I caught my finger in an xbox fan, it did more damage than you'd think!
Could diamond-like carbon (which is very slick) be used instead of the air bearing, so that the spinning heatsink remained in contact with its heat source?
EDIT: Apparently, DLC is more conductive than copper (http://en.wikipedia.org/wiki/List_of_thermal_conductivities), and by a lot. Copper is around 400 W/mk, DLC is 1100-3500 W/mk ...
[edited to reflect acquired info, pre-comments]
EDIT: Apparently, DLC is more conductive than copper (http://en.wikipedia.org/wiki/List_of_thermal_conductivities), and by a lot. Copper is around 400 W/mk, DLC is 1100-3500 W/mk ...
[edited to reflect acquired info, pre-comments]
Reply from study author:
You raise a very good point. The philosophy thus far is to see how far we can get with conventional, inexpensive materials, and periodically reevaluate where problems remain. Low-cost manufacturability is critically important. For example, in the case of the rotating heat-sink-impeller we are converging towards cold-forging as the fabrication process. Having said that, in a manufacturing setting many seemingly exotic coating materials are used on a routine basis in small quantities without incurring significant additional cost (e.g. CVD coatings for tungsten carbide end mills). With regard to CVD diamond coating, the main questions I have concern the current state of affairs in this technology area. How expensive are such coatings, and what are typical values for coefficient of friction?
With regard to the thermal resistance of the air gap, unfortunately it is not possible to reduce the air gap much below ~10 microns without incurring large frictional losses; the power dissipation associated with shearing of the “fluid” layer in the air gap regions scales as 1/h, were h is the air gap distance. The approach you suggest would be tractable at very low pressure, but at very low pressure convection cooling is ineffective.
You raise a very good point. The philosophy thus far is to see how far we can get with conventional, inexpensive materials, and periodically reevaluate where problems remain. Low-cost manufacturability is critically important. For example, in the case of the rotating heat-sink-impeller we are converging towards cold-forging as the fabrication process. Having said that, in a manufacturing setting many seemingly exotic coating materials are used on a routine basis in small quantities without incurring significant additional cost (e.g. CVD coatings for tungsten carbide end mills). With regard to CVD diamond coating, the main questions I have concern the current state of affairs in this technology area. How expensive are such coatings, and what are typical values for coefficient of friction?
With regard to the thermal resistance of the air gap, unfortunately it is not possible to reduce the air gap much below ~10 microns without incurring large frictional losses; the power dissipation associated with shearing of the “fluid” layer in the air gap regions scales as 1/h, were h is the air gap distance. The approach you suggest would be tractable at very low pressure, but at very low pressure convection cooling is ineffective.
Further reply from study author in response to my message stating that I had posted his reply to this forum, and pointed him to it:
===
Thanks for your interest. With regard to filling the air gap region with conductive fluid, the problem is that frictional shearing losses become prohibitively large even at low rotation speeds. It’s not that it wouldn’t work at all, but that it wouldn’t work very well; thermal conductivity is important, but so is viscosity.
There a number of other issues that apparently have caused confusion as well. Maybe we can use this forum to provide clarification. Please feel free to post what follows.
It appears that many people were unclear about what I was attempting to convey with regard to the subject of dust fouling. I did not mean to imply that there is literally no dust fouling; some dust accumulation eventually becomes visible to the naked eye on the very leading edge of the blades. The point is that dust fouling is reduced to such a large extent that we are unable to detect any degradation of cooling performance operating the device in a relatively dirty environment over an extended period of time. Thus for all intents and purposes the dust fouling problem has been taken of the table. In contrast, with conventional CPU coolers, eventually the entire heat exchanger surface becomes entombed in dust.
Some people have expressed concern that such a rotating heat-sink-impeller would constitute a safety hazard. Any real world device would include a screen, grill, or other form of protective enclosure. Such protective measures are widely used on conventional fans. We photographed our device without an enclosure so that it would be easier for people to see what it looks like.
There seems to be confusion about where the potential for significant electricity savings resides. The vast majority of it is associated with applications such as air conditioning and refrigeration, not electronics cooling. But such energy sector applications will only materialize if air bearing heat exchanger technology proves amenable to size scaling. We are in the process of evaluating this question.
Many have expressed skepticism about the practicality of a 0.001” air gap. That’s certainly understandable, and it was one of the first things we investigated. After all, if the requirement for a small air gap precludes the possibility of low-cost manufacturing, reliability, etc. then I would be the first to agree that all of this is a pointless exercise. The end-of-project report has a discussion of why this is not the case. Another counter-intuitive point is that air bearings are extremely mechanically stiff, rugged and reliable. This is also discussed the report.
On a related subject, many were concerned about the manufacturability of the heat-sink-impeller. We are converging on cold forging as the best route to low cost fabrication. We understand that if we can’t drive the cost down that air bearing heat exchanger technology will have little impact.
Others have pointed out that intuitively it would seem like the last thing you’d want to do is intentionally introduce an air gap (rather than something like thermal grease) in the thermal conduction path between the CPU and heat exchanger. Qualitatively this sounds like a persuasive argument. But as discussed in the report, quantitatively, the numbers (gap distance, gap area, thermal conductivity of air, enhancement of conductivity by convection) work out quite well. For a 10 cm diameter device, an air gap resistance of 0.02 C/W is certainly feasible.
Another important point is that the version 1 prototype device discussed in this report is badly unoptimized. The main objective for version 1 was to test our hypotheses regarding the advantages of such a device architecture. If things continue to go well in lab, I suspect eventually we’ll end up at about 0.05 C/W for a 10 cm diameter device that’s of order 3 cm high, operates at three to four thousand rpm, and consumes about 5 watts of electrical power. But believe it when you see it. There’s always risk involved in try to solve tough problems. We’re giving it our best shot. We’re also working on alternative device geometries that may be capable of providing considerably better performance.
What else? A couple people stated that the thermal brick wall is at 4 GHz, not 3 GHz. Fair enough. My point is that if you introduce a drastic improvement in thermal management technology, whatever the number for thermal brick wall may be, it gets pushed a lot higher.
Other people had questions about why such a heat-sink-impeller is so quiet. What it boils down to is that you’ve got a lot more flexibility with regard to blade geometry than you do with a fan. That means you are free to design the blade geometry to smoothly split and smoothly rejoin the flow field at the impeller entrance and exit; the device architecture allows you to decouple the engineering constraints of adequate air flow and low noise.
A couple of people surmised that the air bearing heat exchanger requires a source of compressed air because we used a hydrostatic air bearing in the version 1 device. As described in the report, in a real-world device you’d use a hydrodynamic (or self pressurizing) air bearing. That’s what we’re using now in versions 2 and 3. The use of a hydrostatic bearing in version 1 was an experimental convenience.
I have to go to a 5:30 meeting, so I will leave it at that. Hopefully people will find portions of the above material informative.
Sincerely, Jeff Koplow Sandia National Labs Livermore, CA
===
Thanks for your interest. With regard to filling the air gap region with conductive fluid, the problem is that frictional shearing losses become prohibitively large even at low rotation speeds. It’s not that it wouldn’t work at all, but that it wouldn’t work very well; thermal conductivity is important, but so is viscosity.
There a number of other issues that apparently have caused confusion as well. Maybe we can use this forum to provide clarification. Please feel free to post what follows.
It appears that many people were unclear about what I was attempting to convey with regard to the subject of dust fouling. I did not mean to imply that there is literally no dust fouling; some dust accumulation eventually becomes visible to the naked eye on the very leading edge of the blades. The point is that dust fouling is reduced to such a large extent that we are unable to detect any degradation of cooling performance operating the device in a relatively dirty environment over an extended period of time. Thus for all intents and purposes the dust fouling problem has been taken of the table. In contrast, with conventional CPU coolers, eventually the entire heat exchanger surface becomes entombed in dust.
Some people have expressed concern that such a rotating heat-sink-impeller would constitute a safety hazard. Any real world device would include a screen, grill, or other form of protective enclosure. Such protective measures are widely used on conventional fans. We photographed our device without an enclosure so that it would be easier for people to see what it looks like.
There seems to be confusion about where the potential for significant electricity savings resides. The vast majority of it is associated with applications such as air conditioning and refrigeration, not electronics cooling. But such energy sector applications will only materialize if air bearing heat exchanger technology proves amenable to size scaling. We are in the process of evaluating this question.
Many have expressed skepticism about the practicality of a 0.001” air gap. That’s certainly understandable, and it was one of the first things we investigated. After all, if the requirement for a small air gap precludes the possibility of low-cost manufacturing, reliability, etc. then I would be the first to agree that all of this is a pointless exercise. The end-of-project report has a discussion of why this is not the case. Another counter-intuitive point is that air bearings are extremely mechanically stiff, rugged and reliable. This is also discussed the report.
On a related subject, many were concerned about the manufacturability of the heat-sink-impeller. We are converging on cold forging as the best route to low cost fabrication. We understand that if we can’t drive the cost down that air bearing heat exchanger technology will have little impact.
Others have pointed out that intuitively it would seem like the last thing you’d want to do is intentionally introduce an air gap (rather than something like thermal grease) in the thermal conduction path between the CPU and heat exchanger. Qualitatively this sounds like a persuasive argument. But as discussed in the report, quantitatively, the numbers (gap distance, gap area, thermal conductivity of air, enhancement of conductivity by convection) work out quite well. For a 10 cm diameter device, an air gap resistance of 0.02 C/W is certainly feasible.
Another important point is that the version 1 prototype device discussed in this report is badly unoptimized. The main objective for version 1 was to test our hypotheses regarding the advantages of such a device architecture. If things continue to go well in lab, I suspect eventually we’ll end up at about 0.05 C/W for a 10 cm diameter device that’s of order 3 cm high, operates at three to four thousand rpm, and consumes about 5 watts of electrical power. But believe it when you see it. There’s always risk involved in try to solve tough problems. We’re giving it our best shot. We’re also working on alternative device geometries that may be capable of providing considerably better performance.
What else? A couple people stated that the thermal brick wall is at 4 GHz, not 3 GHz. Fair enough. My point is that if you introduce a drastic improvement in thermal management technology, whatever the number for thermal brick wall may be, it gets pushed a lot higher.
Other people had questions about why such a heat-sink-impeller is so quiet. What it boils down to is that you’ve got a lot more flexibility with regard to blade geometry than you do with a fan. That means you are free to design the blade geometry to smoothly split and smoothly rejoin the flow field at the impeller entrance and exit; the device architecture allows you to decouple the engineering constraints of adequate air flow and low noise.
A couple of people surmised that the air bearing heat exchanger requires a source of compressed air because we used a hydrostatic air bearing in the version 1 device. As described in the report, in a real-world device you’d use a hydrodynamic (or self pressurizing) air bearing. That’s what we’re using now in versions 2 and 3. The use of a hydrostatic bearing in version 1 was an experimental convenience.
I have to go to a 5:30 meeting, so I will leave it at that. Hopefully people will find portions of the above material informative.
Sincerely, Jeff Koplow Sandia National Labs Livermore, CA
What I especially like about this reply is how it clearly shows how easily he can answer the questions and objections raised. It acknowledges they are reasonable questions and as such they have already been considered. There is a current answer, that may change if new ideas/evidence come to light.
Readers of this kind of news often seem to underestimate the kind of thought that went into an idea/invention/prototype. They go "well, that won't work, because ..." and they come up with some reason that may sound reasonable (or simply is reasonable), but just isn't important enough. I think it would be good if more scientists replied in this kind of matter to comments to news of their ideas/inventions. It would, I hope, increase the respect people have for the work of scientists.
Readers of this kind of news often seem to underestimate the kind of thought that went into an idea/invention/prototype. They go "well, that won't work, because ..." and they come up with some reason that may sound reasonable (or simply is reasonable), but just isn't important enough. I think it would be good if more scientists replied in this kind of matter to comments to news of their ideas/inventions. It would, I hope, increase the respect people have for the work of scientists.
Adding DLC doesn't change the problem of connecting to the heat source. There is still an air gap.
A conductive fluid bearing might work better than an air gap. DLC vs copper makes little difference since that's not the problem.
I see you just edited it to say that DLC is very slick - that doesn't help as much as you think. It's a fact of heatsinks that two solid surfaces only touch in three places. The rest is filled with a tiny air gap. You typically fill in this gap with a fluid.
A conductive fluid bearing might work better than an air gap. DLC vs copper makes little difference since that's not the problem.
I see you just edited it to say that DLC is very slick - that doesn't help as much as you think. It's a fact of heatsinks that two solid surfaces only touch in three places. The rest is filled with a tiny air gap. You typically fill in this gap with a fluid.
> It's a fact of heatsinks that two solid surfaces only touch in three places.
Is it?
Is it?
If you have two completely rigid objects, then as soon as they touch on two points, those two points define an axis. Rotate the objects on this axis until they touch at a third point. Now they are fixed relative to each other, and you can't move them to touch at more points. If you slide the objects on each other, as soon as they touch at a "new" point, one of the other three will be slightly further away, and won't touch any more. Unless you use a flexible material, or somehow make a perfectly flat surface, they will only touch at three points.
Also suspicious (for me) is:
(page 18 of PDF) "The prototype device is configured as a static (externally pressurized) thrust bearing. In realworld thermal management applications such an externally pressurized air bearing would be replaced by a hydrodynamic (self-pressurizing) air bearing, which uses a minute fraction of the mechanical power supplied by the brushless motor to generate the required lifting force."
So the external pump at the moment keeps the big rotor above the surface? They haven't demonstrated how the setup would otherwise work, but maybe for "self-pressurizing" variant the RPM would have to be really high, which would again mean not quiet at all?
(page 18 of PDF) "The prototype device is configured as a static (externally pressurized) thrust bearing. In realworld thermal management applications such an externally pressurized air bearing would be replaced by a hydrodynamic (self-pressurizing) air bearing, which uses a minute fraction of the mechanical power supplied by the brushless motor to generate the required lifting force."
So the external pump at the moment keeps the big rotor above the surface? They haven't demonstrated how the setup would otherwise work, but maybe for "self-pressurizing" variant the RPM would have to be really high, which would again mean not quiet at all?
Great analysis. A couple of questions through.
So lets say their device can handle 60W when it is 38.7cm^2 (or a bit more than 1.5W per square cm. Now you want to cool a 150W processor. That means 100cm^2 of cooling surface. If I did the math right 100cm^2 is sqrt(100/pi) * 2 diameter circle. Or 12cm diameter circle. So lets call it a 4 3/4" diameter circle with one of these gizmos on it.
That isn't a huge stretch to imagine. I'm thinking a copper slug which sits on the processor and makes this 4.75" diameter 'landing pad' for their cooler. Doesn't work well if the processor isn't oriented horizontally though.
In data centers this might up reliability and density if the net solution was 'shorter' than the current passive sinks.
So lets say their device can handle 60W when it is 38.7cm^2 (or a bit more than 1.5W per square cm. Now you want to cool a 150W processor. That means 100cm^2 of cooling surface. If I did the math right 100cm^2 is sqrt(100/pi) * 2 diameter circle. Or 12cm diameter circle. So lets call it a 4 3/4" diameter circle with one of these gizmos on it.
That isn't a huge stretch to imagine. I'm thinking a copper slug which sits on the processor and makes this 4.75" diameter 'landing pad' for their cooler. Doesn't work well if the processor isn't oriented horizontally though.
In data centers this might up reliability and density if the net solution was 'shorter' than the current passive sinks.
It's common in high-end CPU HSFs to mount the heatsink above the copper slug on the CPU, connected with heatpipes - a setup that would work well for a cooler of those dimensions.
This gives me a thought. . . could Dyson Air Multiplier technology be scaled down to be used in computers?
http://www.dyson.co.uk/technology/airmultiplier.asp#HowItWor...
http://www.dyson.co.uk/technology/airmultiplier.asp#HowItWor...
If it can, I'm not sure what the benefits might be for use in computer cooling.
Another thought. . . could you take advantage of the thermoelectric effect to cool a system and make it more efficient in the process?
Yes, to the first part. The computer applications are usually known as "Peltier coolers". Due to a variety of factors, current implementations are a fair bit less suitable to cooling most computer components than traditional heatsinks/fans. I've mostly seen them used in extreme overclocking situations.