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 Post subject: Heat pipes DO work upside down......how?
PostPosted: Fri May 11, 2007 5:27 am 
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A few years ago, cooling devices started showing up on the market utilizing heat pipes for heat transfer. When reading the laymen's description (found in many product reviews at the time), I always came away with the impression that gravity was involved in transferring the condensed/cooled liquid away from the cool end, and back to the hot end. So I always wondered how effective they could be upside-down.

Well, now I'm using the Scythe Ninja in a P180, and as you can imagin, only 3 of the 12 heat pipes comes anywhere close to the proper orientation. So I tried laying the case on it's side so that all 12 pipes were facing the ceiling.....and my temps did not improve. I so looked it up on the net and found that while heat pipes "can" use gravity, I don't believe our consumer level products need it, they use capillary action instead.

Quote:
In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action.


So, I saw someone else basing a comment on the same assumption, and it prompted me to post this thread. Now you know. 8)

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 Post subject: Re: Heat pipes DO work upside down......how?
PostPosted: Fri May 11, 2007 6:09 am 
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miahallen wrote:
A few years ago, cooling devices started showing up on the market utilizing heat pipes for heat transfer. When reading the laymen's description (found in many product reviews at the time), I always came away with the impression that gravity was involved in transferring the condensed/cooled liquid away from the cool end, and back to the hot end. So I always wondered how effective they could be upside-down.

Well, now I'm using the Scythe Ninja in a P180, and as you can imagin, only 3 of the 12 heat pipes comes anywhere close to the proper orientation. So I tried laying the case on it's side so that all 12 pipes were facing the ceiling.....and my temps did not improve. I so looked it up on the net and found that while heat pipes "can" use gravity, I don't believe our consumer level products need it, they use capillary action instead.

Quote:
In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action.


So, I saw someone else basing a comment on the same assumption, and it prompted me to post this thread. Now you know. 8)


a good example is people with long jeans that touch the floor, if they stand in a puddle, the water will seep into the jeans upwards against gravity ;)


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 Post subject: Re: Heat pipes DO work upside down......how?
PostPosted: Fri May 11, 2007 4:46 pm 
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Tainek wrote:
a good example is people with long jeans that touch the floor, if they stand in a puddle, the water will seep into the jeans upwards against gravity ;)

So you're saying theres a guy wearing jeans inside me heat pipes :shock: just joking :lol:

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 Post subject:
PostPosted: Fri May 11, 2007 7:53 pm 
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capillary action!

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 Post subject:
PostPosted: Tue May 15, 2007 3:42 am 
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Heat pipes are a lot more efficient if the hot end is below the cold end, but they still do something if they are upside down.
A 6mm HP will conduct 20W, so if 3 HPs are the right way up, the other HPs are not needed.


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 Post subject:
PostPosted: Tue May 15, 2007 5:06 am 
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I don't think the orientational effect is that big. If I remember my fluid dynamics lectures right, capillary forces are many times higher than gravity (best case). So, if we assume the engineers did a proper job with that, the gravitational effect should be within the margin of error of our home measurings.


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 Post subject:
PostPosted: Thu May 17, 2007 2:38 am 
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If we go back to the original example

a good example is people with long jeans that touch the floor, if they stand in a puddle, the water will seep into the jeans upwards against gravity

The water will rise a short way up the jeans, but will not reach the top. If the water only needs to rise a few cm, then the HP will work with only a small loss of performance. If there is a large height difference the HP will not work.


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 Post subject:
PostPosted: Thu May 17, 2007 4:51 am 
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efcoins2 wrote:
...If the water only needs to rise a few cm, then the HP will work with only a small loss of performance. If there is a large height difference the HP will not work.

That depends on the size of the tubes inside the pipe. Really, really small tubes could suck water up a meter or more. Plus the liquid in a heat pipe is probably not water, so if it has a higher surface tension or lower density than water it could rise further.

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 Post subject:
PostPosted: Thu May 17, 2007 6:35 am 
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I don't think anyone mentioned that what makes the capillary action work is a wick inside the heat pipe. If I remember right, a heat pipe can be more efficient without the wick, but then it is dependent on gravity to return the fluid.


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 Post subject:
PostPosted: Fri May 18, 2007 1:52 am 
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Almost all HPs do use water.
Yes the tubes can be made really small, but then very little water will be sucked through them. 0.2mm tubes will raise water by 14cm, but very little water can flow through a 0.2mm tube. Very little water flow means the HP provides very little cooling.

http://en.wikipedia.org/wiki/Capillary_action


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 Post subject:
PostPosted: Fri May 18, 2007 2:12 am 
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I was under the impression that more than a few heatpipes used any number of vapors, not limited to water. Water+ethanol, water+ammonia and water+methanol were common, too.

I'm also pretty sure that it's import that the vapor be at a slight vacuum, or at least very low pressure, so that there's a force other than heat to maintain a vaporative state.

I'm also not a chemical/ thermal engineer. I wish that dude who murdered that innocent Ninja did so with a more scientific (if Mengelian) approach.

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 Post subject:
PostPosted: Fri May 18, 2007 2:13 am 
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Quote:
Very little water flow means the HP provides very little cooling.



with water’s large latent heat of vaporization,even a small amount of water could provide a surprisingly large amount of cooling.


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 Post subject:
PostPosted: Fri May 18, 2007 4:30 am 
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Mr Evil wrote:
efcoins2 wrote:
...If the water only needs to rise a few cm, then the HP will work with only a small loss of performance. If there is a large height difference the HP will not work.

That depends on the size of the tubes inside the pipe. Really, really small tubes could suck water up a meter or more. Plus the liquid in a heat pipe is probably not water, so if it has a higher surface tension or lower density than water it could rise further.


Trees are an example of capillary action sucking water up a lot higher than just a couple meters.

Water is vaporized in the leaves, and capillary action draws more water from the roots.

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 Post subject:
PostPosted: Fri May 18, 2007 7:26 am 
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lm wrote:
Trees are an example of capillary action sucking water up a lot higher than just a couple meters.

Water is vaporized in the leaves, and capillary action draws more water from the roots.

According to Wikipedia, capillary action is not the important mechanism for transporting water up trees. I have been searching Google for an answer to how high capillary action can work, but can't find anything definitive. The formula for calculating the height would say that the height can approach infinity as the diameter of the tube approches zero, but obviously that can't happen as the tube can't possibly be smaller than a water molecule, and surface tension would stop working properly at sizes well above that. That's why I was conservative in my previous reply :P

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 Post subject:
PostPosted: Sun Jun 03, 2007 7:26 pm 
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Basically you have very little liquid substance in the heat pipes. Water for instance expands 1600 times when it vaporizes at 1 atmosphere ( sea level ). So if a heat pipe can hold 20 atmsophere you could fill 20/1600 of the pipe with liquid.

When one end of the pipe is heated, the working fluid inside the pipe evaporates and increases the vapour pressure inside the heat pipe. The latent heat ( the amount of energy in the form of heat released or absorbed by a substance during a change of phase ) of evaporation reduces the heat at the hot end of the pipe.

Because of the pressures inside the pipe the vapour travels to the cool end of the pipe and then condenses, releasing its latent heat and warms the cool end of the pipe. The fins on the heat sink then conduct the heat away from the heat pipe and then air removes the heat from the fins.

The capillary force is not to move heat away from the hot end but to speed up the return of the condensed liquid back to the hot end of the pipe.

So inside a heat pipe, "hot" vapor flows in one direction, condenses to the liquid phase, and migrates back in the other direction to evaporate again and repeat the cycle. Gravity has very little to no effect on it.

I don't know what is commonly used in Computer Heat Pipes but I would imagine that they would use ethanol, ammonia or water ( or a mixture ).


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 Post subject:
PostPosted: Sun Jun 03, 2007 7:43 pm 
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Khrono Devil wrote:
I don't know what is commonly used in Computer Heat Pipes but I would imagine that they would use ethanol, ammonia or water ( or a mixture ).

Generally, computer heat pipes use water in a partial vacuum.

As for trees, yes it's true that capillary action is not the main agent. The reason is easy to understand: it would be hard for a tree to be over 30 feet tall if the water were being "sucked up" as opposed to "pumped up".

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