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Question about Heat Pipes, Orientation of Liquid-Filled Pipe

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June 22, 2006 1:53:53 AM

Liquid-Filled Heat Pipes, Horizontal vs. Vertical

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i am writing to ask for your help making sure that i am using the heat-pipe technology correctly.

in my work as an engineer, when we used the term "heat pipe" it was always for any construction that acted as a conduit for heat energy, from the heat source, to a surface closer to the "cold-plate".

a lot of military avionics, electronics for airplanes, is cooled by conduction - the chip is bonded to a circuit board, the circuit board has 64 copper vias, below the circuit board is an aluminum housing which is clamped to an aluminum frame which has cold fluid pumped through it.

so then, a lot of the doo-dads and gizmachies (those were professional terms used by us engineers ... paid for by your tax dollars !!) that we designed to route heat from a specific chip to the cold plate, we called "heat pipes".

since we were using CAD tools to do the design & analysis, our hardware was HP UNIX workstations and Dell PC's, mostly dual Xeon & ATI-Fire GL machines.

now, with PC's, like this
http://xbitlabs.com/articles/coolers/display/noctua-coo...

Noctua heat sink, i believe that the heat pipes in it are filled with some liquid. for example, ammonia, since it boils at a lower temperature. this part is my imagination - the bubbles are lighter than the surrounding liquid so they "go up". when they get to a cooler part of the envelope to which they are restricted, they experience another phase change back to liquid.

in each of those phase changes, from liquid to bubble and then from bubble to liquid, there is a latent heat of evaporation & condensation that is very effective in "moving heat".

designed effectively, it has a low thermal resistance. this is the beginning of the "Holy Grail" when you're working as a design engineer for a defense contractor - we would sit in presentations and calculate the junction temperature. if we could show that we could show that the junction temperature was "within spec", we got brownie points, which were redeemable when pay raises were being handed out.

OK, so now that Wusy at Tom's Hardware has pointed out that the Zalman 9500 is way out of spec, doesn't meet spec, and now that i've found this
http://www.frostytech.com/articleview.cfm?articleid=192...
i'm looking at these Noctua heatsinks, especially the NH-12.

.14 degrees C per watt in free air, that is its thermal resistance. don't know what fan that was measured with.

OK but there's a big assumption in all of this - the ORIENTATION of the heat sink.

i haven't taken apart the heatsinks like the Zalman and the Thermal right and the Noctua. from the way they're pinched off, and from a lot of the posts about them, i get the conclusion that they are liquid filled and that the movement of the liquid inside the heatpipe genuinely helps heat transfer, whether or not the liquid turns into fizzy little bubbles in the process.

is the main part of this heat sink supposed to be vertical ?

this would mean that, when they are placed in a conventional tower workstation, the end of the heat pipe away from the CPU, is horizontal. so, how does the cooled-off fluid get returned to the CPU area of the heat pipe for another dose of heat ? if the heat pipe is horizontal, and it's a skinny heat pipe, it's not like pouring cold vodka into hot chocolate, in which case it mixes real quick.

so i'm wondering, since most of us computer folks are using tower cases of one kind or another, doesn't that leave these big beautiful heat sinks operating in a sub-optimal orientation, with horizontal heat pipes ?

have any of you experimented with your systems, like for example measuring CPU temperatures with the case mounted horizontally and vertically, to see if there's a difference ?

thanks for some good technical feedback. vodka-hot chocolate recipes are appreciated too. :D 
June 22, 2006 3:01:51 AM

Quote:
have any of you experimented with your systems, like for example measuring CPU temperatures with the case mounted horizontally and vertically, to see if there's a difference ?


I've only piddled with it a little, nothing well controlled. I've read publications that claimed the differences are minimal even for inverted systems. I'm pretty sure the heat pipe internal surface area must be high to work well out of vertical and assume that the liquid volume to pipe volume is critical.
Related resources
July 3, 2006 4:18:09 AM

Heat pipes dont work upside down!

Best example Lian Li PC V 1000 + P5N32 SLI

P5N32 SLI has 2 massive heat pipes one for SB one for NB

The liquid must come into contact with the heat source....

ie the cooling system works if I lay my lian li on its side but not when its normal upright position....
July 3, 2006 4:53:38 AM

Quote:
Heat pipes dont work upside down!

Best example Lian Li PC V 1000 + P5N32 SLI

P5N32 SLI has 2 massive heat pipes one for SB one for NB

The liquid must come into contact with the heat source....

ie the cooling system works if I lay my lian li on its side but not when its normal upright position....


If I can find the refs I've read that disagree, I'll post them, but again, some tech sources do disagree. But there are huge differences in the internal structure of heat pipes and that might influence how well they work out of vertical.
July 3, 2006 5:42:25 AM

Someone with more engineering experience than I can correct me if I am wrong, but from my limited understanding of fluid dynamics, it should not matter what the orientation of the heat pipes are. The fluid (in both liquid state or gaseous state) will circulate thru the pipe from the hottest point to the coolest point, and back again. I'm pretty sure all well-constructed heat pipes have a capillary tube of some sort that ensures the heated liquid evaporates into a gas, moves thru the tube to the cooler portion, and then cools into a liquid. The laws of thermodynamics dictactes the flow, not the orientation.

"In this house, we obey the laws of thermodynamics!" - Homer Sinmpson to Lisa when she build a perpetual motion machine.

8)
July 3, 2006 5:50:04 AM

Well I can cook an egg on my NB! when I have placed my P5N32 SLI in my lian li (in its original position up-side-down).

Lay the lian li on its side NB is perfectly cooled (quite cool to touch actually) ie liquid has flowed to the NB heat source.

I dont think there is a wick inside my heat pipe! or there just isnt enough liquid!!!! Cheap as ss Asus!

I understand the limitations of horizontal and upright but...... up-side-down????!!!!!

And my heat pipe is almost a foot long!!!!????
July 3, 2006 5:52:05 AM

Quote:
Someone with more engineering experience than I can correct me if I am wrong, but from my limited understanding of fluid dynamics, it should not matter what the orientation of the heat pipes are. The fluid (in both liquid state or gaseous state) will circulate thru the pipe from the hottest point to the coolest point, and back again. I'm pretty sure all well-constructed heat pipes have a capillary tube of some sort that ensures the heated liquid evaporates into a gas, moves thru the tube to the cooler portion, and then cools into a liquid. The laws of thermodynamics dictactes the flow, not the orientation.


Modern heat pipe designs include some closely guarded secrets but I do know that internal surface area modification is a key component. I've heard numerous designers and engineers say that the liquid volume is significantly lower in new heat pipes than back in the early days. In other words, the liquid is well-dispersed on a rough/fibrous/porous inner pipe surface as opposed to a pipe mostly full of liquid. Heat transfer at the liquid/vapor interface is much faster than within a bulk liquid, so you need a nearly continuous vapor channel over the length of the pipe.
July 3, 2006 6:05:31 AM

Wow, I just took a look at that motherboard. That northbridge thermal solution is indeed unusual. From my experience with laptops (where heat pipes are used more often because of the limited space and weight requirements) the longer the heatpipes are, the lower the performance. I believe you when you say you have problems with orientation on a pipe that long! I would guess that the fluid is probable cooling somewhere closer to the middle, instead of the end after the curve, where the fins are, and the fluid as a gas is not able to reach the fins. I would venture to guess that this particular ASUS motherboard was designed for -and performs better in- the old-style desktop case, rather than a tower case.

Since that board is a fanless design, I bet adding a fan to the area where the fins are might help, or better yet, yank the long heat pipe off and install a good performance NB heatsink+fan.

8)
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