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#21
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck,don't blow ! heatfins direction)
On 19/08/2012 05:18, John Larkin wrote:
On Sat, 18 Aug 2012 17:19:48 -0700 (PDT), Robert Macy wrote: On Aug 18, 12:59 pm, John Larkin wrote: I have this theory that the fins of a heat sink should reduce a fan's free-flow rate by 50% for optimum heat transfer. optimum heat transfer? not sure what the criteria would be, Minimum theta would do. but think instead about the air's thermal mass, thermal resistance form metal to bulk air. and you see you're left with characteristics of the heat sink, not the characteristics of the fan. As a mind argument enfisionone hell of a powerful fan. Now block that to half flow, what do you have? versus an 'underpowered' fan that is blocked to half flow. . If the heat sink doesn't reduce air flow at all, the air is going around the fins, not through them (as Skybuck suggests) and the air does no good. And if you block all the air flow, it does no good. So More to the point if the heatsink fins are not thick enough to conduct heat away from the thing being cooled it doesn't matter how easily you can push air through them. Equally it is no good if you get perfect laminar airflow since then only the air touching the surface warms up and the core air remains cool. So you have to have some turbulence and opposition to free flow but the tricky question is how much is enough? Something like this might be close : ====o ====o ====o ====o ====o ====o ====o ====o (slightly tighter together than ASCII art will allow) Airflow from left to right with a blob on the end to mix the air up. the amount of airflow restriction that results in the lowest theta must be somewhere between those two extremes. Dead center is a pretty good guess. But also very probably wrong. The volume of air going through the heat sink is proportional to the amount of cooling you get for a given design so there is a definite bias towards not blocking off half the free air flow. I would guess at something more like allowing 2/3 to 3/4 of free airflow as about the best depending on the exact heatsink geometry. It could easily be higher - easy enough to do the experiment. I suspect the perfect shape for an optimum heatsink is rather more complex than the typical fins we get but the designs used at present are good enough and much easier to engineer. Heat pipes have helped enormously with the latest generation of quiet heatsinks. It is a sobering thought that high performance CPUs often have a heat output per unit area that exceeds the tip of a soldering iron. Regards, Martin Brown |
#22
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)
"Jeff Liebermann" wrote in message ... On Sat, 18 Aug 2012 12:59:41 -0700, John Larkin wrote: I have this theory that the fins of a heat sink should reduce a fan's free-flow rate by 50% for optimum heat transfer. If the input and output temperatures were the same, that might be true. It might also be true if you consider the reduction in free flow rate caused by the back pressure due to the head sink obstructing the air flow. However, air expands when heated, causing an increase in air flow at the exhaust end. That's why the exhaust port for a heat removal system is larger than the intake. My guess(tm) is that the increased exhaust air flow caused by heating is much larger than the reduction in intake air flow caused by the fins getting in the way. On the original assertion, that it's better to suck than to blow, methinks that's wrong. You can demonstrate this with a dirty computer. Take a vacuum cleaner and try to remove the dust by sucking. Most of it will still be in the machine when you're done. Now, put the hose on the same vacuum cleaner exhaust and blow the dust out of the machine. Notice that remaining dust is effectively blown all over the room. It's dispersion versus concentration. When sucking, one pulls air from the sides and from all around the heat sink, including air that does not need cooling. This makes the fan work harder moving excess air, leaving less air flow for between the heatsink fins. Turn the fan around and blow air at the heat sink, and the entire air flow is involved in cooling the fins. Similarly, you can demonstrate the effect by comparing the CPU temperatures with the fan in the normal position (blowing air down towards the heat sink), versus flipping the fan over and sucking air out. I did this with a Pentium 4 dual core. I forgot the measured temperatures, but the difference was substantial. I tried that on an old AMD and got the same results; the CPU was much hotter with the air being pulled through than it was with it being blown through. One of the electrical engineers at work explained it this way: If the air is being pulled through, most of the air is moving through the fin area closest to the fan, with the lower fins (closest to the CPU) getting the least amount of flow. Therefore the heat has to transfer through the fins before it gets to an area where there is enough air flow to actually aid in heat removal. But, with the air being blown down, through the fins, there is enough back-pressure to allow the air to travel almost equally across all fin surfaces before exiting, carrying a larger amount of heat with it. Don't know if that's exactly how it works, but it made sense to me, and would explain why most newer heatsinks have the air blown through rather than pulled through the fins. -- SC Tom |
#23
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck,don't blow ! heatfins direction)
On 19/08/2012 10:50 AM, Flasherly wrote:
On Aug 18, 7:57 pm, Rheilly Phoull wrote: Umm, you would not perchance be employed in a government position (spin doctor) ?? If I had my druthers, I'd as rather gainfully, that is contractually and under government auspices, to be on your tax dollar, sic, whereby to impose mandatory interpolation of required observances, forthwith said forthrightly, such that as an agreeable conscientious citizen, I'm sure you are, there could be no other possible meaning given you to mistake my greater schemes. -- 'Within a judicial system, the only worse case scenario, other than a divorce, is a lawyer in full possession and faculty of doctorates both in law and philosophy.' -Socrates, A Known Drinker of Hemlock Yeah, right couldn't have said it better myself. |
#24
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck,don't blow ! heatfins direction)
On Aug 19, 12:12 am, John Larkin
wrote: Right. I've bought about every heatsink or fan imaginable, given and within, as another mentioned, an axiomatic engineering construct concept -- 'if it's done right [in the first place], it's time to [attempt] an improvement, [if and while not in need of entirely new construction concepts]';- time simply follows, technologically speaking, to march upon and then past any R&D Dept., in failure aptly to communicate, if at least not uniquely, then an underlying implementation of adequacy pertaining to key hard and software, constructional elements coming in, daily, across so broad a field as computers. Where, specifically, I see you for a fit is at a coincident juncture of heatsink fins and augmented airflow, both being common terms to common acceptance for pragmatic practice. The argument you have placed to ally yourself as well follows true to the same axiom: that being one, effectively, of a quest for perpetual motion: for so long as the key component of design efficiency is established, widely employed to a common basis of underlying industrial acceptance, the cost factor, then, is effectively one which requires of me nothing more, out of my pocket, than to advance a better sense of encouragement that such benefit, you have chosen to propose, indeed, is of worthier consideration. Stop by anytime if you'd like to talk, John. My office is at the top of the stairs. |
#25
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck,don't blow ! heatfins direction)
On Aug 18, 9:18*pm, John Larkin
wrote: On Sat, 18 Aug 2012 17:19:48 -0700 (PDT), Robert Macy wrote: On Aug 18, 12:59*pm, John Larkin wrote: I have this theory that the fins of a heat sink should reduce a fan's free-flow rate by 50% for optimum heat transfer. optimum heat transfer? not sure what the criteria would be, Minimum theta would do. *but think instead about the air's thermal mass, thermal resistance form metal to bulk air. and you see you're left with characteristics of the heat sink, not the characteristics of the fan. As a mind argument enfisionone hell of a powerful fan. Now block that to half flow, what do you have? *versus an 'underpowered' fan that is blocked to half flow. * . If the heat sink doesn't reduce air flow at all, the air is going around the fins, not through them (as Skybuck suggests) and the air does no good. And if you block all the air flow, it does no good. So the amount of airflow restriction that results in the lowest theta must be somewhere between those two extremes. Dead center is a pretty good guess. -- John Larkin * * * * * * * * *Highland Technology Incwww..highlandtechnology.com* jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME *analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators Doesn't lowest theta occur at MAXIMUM airflow? Think of it as a stiction layer of air. All the heat from the metal must travel through that stiction layer [which by definition is an insulative sheet of air on the heat sink.] Now as air moves rapidly over the HS's surface; that stiction layer becomes thin and voila! lowers the theta. That's why all the HS curves of temp ris for given wattage always asymptotically approach some value as the air speed increases. It's because the stiction layer can be thinned, but not removed. |
#26
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)
On Sun, 19 Aug 2012 08:47:26 -0700 (PDT), Flasherly
wrote: On Aug 19, 12:12 am, John Larkin wrote: Right. I've bought about every heatsink or fan imaginable, given and within, as another mentioned, an axiomatic engineering construct concept -- (...) http://phaser.gfxile.net/ligen/technobabble.php -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#27
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)
On Sun, 19 Aug 2012 08:56:05 -0700 (PDT), Robert Macy
wrote: On Aug 18, 9:18*pm, John Larkin wrote: On Sat, 18 Aug 2012 17:19:48 -0700 (PDT), Robert Macy wrote: On Aug 18, 12:59*pm, John Larkin wrote: I have this theory that the fins of a heat sink should reduce a fan's free-flow rate by 50% for optimum heat transfer. optimum heat transfer? not sure what the criteria would be, Minimum theta would do. *but think instead about the air's thermal mass, thermal resistance form metal to bulk air. and you see you're left with characteristics of the heat sink, not the characteristics of the fan. As a mind argument enfisionone hell of a powerful fan. Now block that to half flow, what do you have? *versus an 'underpowered' fan that is blocked to half flow. * . If the heat sink doesn't reduce air flow at all, the air is going around the fins, not through them (as Skybuck suggests) and the air does no good. And if you block all the air flow, it does no good. So the amount of airflow restriction that results in the lowest theta must be somewhere between those two extremes. Dead center is a pretty good guess. -- John Larkin * * * * * * * * *Highland Technology Incwww.highlandtechnology.com* jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME *analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators Doesn't lowest theta occur at MAXIMUM airflow? Think of it as a stiction layer of air. All the heat from the metal must travel through that stiction layer [which by definition is an insulative sheet of air on the heat sink.] Now as air moves rapidly over the HS's surface; that stiction layer becomes thin and voila! lowers the theta. That's why all the HS curves of temp ris for given wattage always asymptotically approach some value as the air speed increases. It's because the stiction layer can be thinned, but not removed. Volume is good, sure, but everything else equal,that takes a bigger fan. The question is, given a fan optimize the resistance. It's an impedance matching sort of question. Constrict the air too much, with heatsink blades and the airflow goes down. Open it up and there is no contact between the heatsink and air. As others have mentioned, the point isn't to reduce the boundary layer by increasing velocity, rather to upset the boundary layer with a turbulent flow. ...just enough turbulence to upset the boundary layer but not so much as to restrict flow. Just enough heatsink material to transfer heat and not so much as to restrict flow. It's not just a single variable equation. The heat-transfer people at IBM (sat down the hall from me, moons ago) used our electronics simulation programs to design these things. Their sim models were just as large as ours and took just as much CPU time (hours). |
#28
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)
On Sun, 19 Aug 2012 08:56:05 -0700 (PDT), Robert Macy
wrote: On Aug 18, 9:18*pm, John Larkin wrote: On Sat, 18 Aug 2012 17:19:48 -0700 (PDT), Robert Macy wrote: On Aug 18, 12:59*pm, John Larkin wrote: I have this theory that the fins of a heat sink should reduce a fan's free-flow rate by 50% for optimum heat transfer. optimum heat transfer? not sure what the criteria would be, Minimum theta would do. *but think instead about the air's thermal mass, thermal resistance form metal to bulk air. and you see you're left with characteristics of the heat sink, not the characteristics of the fan. As a mind argument enfisionone hell of a powerful fan. Now block that to half flow, what do you have? *versus an 'underpowered' fan that is blocked to half flow. * . If the heat sink doesn't reduce air flow at all, the air is going around the fins, not through them (as Skybuck suggests) and the air does no good. And if you block all the air flow, it does no good. So the amount of airflow restriction that results in the lowest theta must be somewhere between those two extremes. Dead center is a pretty good guess. -- John Larkin * * * * * * * * *Highland Technology Incwww.highlandtechnology.com* jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME *analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators Doesn't lowest theta occur at MAXIMUM airflow? If you have a veriable speed fan, sure, more air flow cools better. That's not what I'm talking about. If you have some fan running full blast, with some flow-backpressure curve, and you blow it at/through a heat sink, you could vary fin density, fin shape, ducting, stuff like that. Maximum air flow implies zero interaction with the heat sink fins, which will make for zero cooling. 100% blocking by fins is zero air flow, also no cooling attributable to the fan. Think of it as a stiction layer of air. All the heat from the metal must travel through that stiction layer [which by definition is an insulative sheet of air on the heat sink.] Now as air moves rapidly over the HS's surface; that stiction layer becomes thin and voila! lowers the theta. That's why all the HS curves of temp ris for given wattage always asymptotically approach some value as the air speed increases. It's because the stiction layer can be thinned, but not removed. http://tinyurl.com/9xu8rvx The other thing going on is that the thermal resistance of the heatsink itself, resistance along the fins or pins, starts to isolate the extremes of the heatsink at high heat flows. You could attach the tips of the fins to a zero-theta block and still have the thermal resistance of the fins themselves. In real life, the baseplate lateral theta gets to be a problem too; you get a hot spot at the chip or mosfet or whatever, but the sink is cooler farther away. Heat sink extrusions are apparently specified with uniform heating of the baseplate. For a given chip, there would be a finite theta if it were bolted to the face of a half-the-universe-sized block of copper. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators |
#29
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)
On Sun, 19 Aug 2012 08:47:26 -0700 (PDT), Flasherly
wrote: On Aug 19, 12:12 am, John Larkin wrote: Right. I've bought about every heatsink or fan imaginable, given and within, as another mentioned, an axiomatic engineering construct concept -- 'if it's done right [in the first place], it's time to [attempt] an improvement, [if and while not in need of entirely new construction concepts]';- time simply follows, technologically speaking, to march upon and then past any R&D Dept., in failure aptly to communicate, if at least not uniquely, then an underlying implementation of adequacy pertaining to key hard and software, constructional elements coming in, daily, across so broad a field as computers. Where, specifically, I see you for a fit is at a coincident juncture of heatsink fins and augmented airflow, both being common terms to common acceptance for pragmatic practice. The argument you have placed to ally yourself as well follows true to the same axiom: that being one, effectively, of a quest for perpetual motion: for so long as the key component of design efficiency is established, widely employed to a common basis of underlying industrial acceptance, the cost factor, then, is effectively one which requires of me nothing more, out of my pocket, than to advance a better sense of encouragement that such benefit, you have chosen to propose, indeed, is of worthier consideration. Stop by anytime if you'd like to talk, John. My office is at the top of the stairs. Are you really John Fields? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators |
#30
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(NVIDIA) Fan-Based-Heatsink Designs are probably wrong. (suck, don't blow ! heatfins direction)
On Sat, 18 Aug 2012 22:05:11 -0700, Jeff Liebermann
wrote: On Sat, 18 Aug 2012 12:59:41 -0700, John Larkin wrote: I have this theory that the fins of a heat sink should reduce a fan's free-flow rate by 50% for optimum heat transfer. If the input and output temperatures were the same, that might be true. It might also be true if you consider the reduction in free flow rate caused by the back pressure due to the head sink obstructing the air flow. However, air expands when heated, causing an increase in air flow at the exhaust end. Not enough to matter. The expansion goes as absolute temperature. That's why the exhaust port for a heat removal system is larger than the intake. There's no advantage to making an inlet port bigger than the fan that site there. Disadvantage, if it would let air leak out. But an outlet port restricts flow, so make it big. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators |
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