On Surface Tention and The Barrier Layer
i have this same post over at the [H]ardforums (which incidentally also got stickied to the top over there - YAY) the post is even under the same name and is about word for word the same thing. incidentally this post is also over at the AMDmB forums too.
at any rate, it's been really cool to see the different responses and see the different questions come up. this one is from the [H]ardforums. someone asked about how surface tention and the boundry layer all came into play so thought i'd post it over here for general overall knowlege. i suspect someone woulda asked the question sooner or later anyhow, so perhaps i'm beating it to the punch.
enough babblin. here goes.
PS the friend i mention with the PhD in Chemical Engineering, isn't you Karsta. it's someone who started backchanneling me a little while ago and is also been of great help.
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Amadon said:
This is some of the best data I have seen posted on the forums.. ever.
Thanks a million bro.
Although I'm not one to go in for sub ambient cooling, mainly because of the maintinence and that I'm paranoid, my following topic applies to both watercooling without a phase change system or peltier units.. and to the opposite.
The thing that I haven't seen addressed is surface tension.
I know very little about it really. But I assume that it is not caused by air "touching" the water. I DO know that if the surface tension is broken.. say.. with soap, that the water flows more freely over a surface.
Couldn't one conclude then that a lack of surface tension, at least in thin (as in depth) water creates a more favorable viscosity?
I always thought this was probably how water wetter helped.
Tell me if I'm wrong. or onto something.
Amadon
okay this is what i've gathered. this is in part from an extremely helpful friend with a PhD in chemical engineering (whom i've been in correspondence with about this) and is a summary of what i've learned about surface tention and the boundry layer having been put into my own words more or less.
theres surface tention and theres the barrier layer. whats the boundry layer you ask?
when a fluid moves across a non moving surface, there is a point where the fluid comes into contact with that surface. (obviously). we'll call the surface the Block.
when liquid moves over the block, the surface of the liquid reacts against the friction of the Block where they contact one another and the motion on the surface of the liquid slows down or stops at that contact point. this is called the boundry layer.
as we know, moving liquid quickly across a surface is what provides us the best heat transfer performance. when any or all of that liquid stops moving and sits there due to the friction it's encountering, it's suddenly doing a less efficient job. the liquid now composing the boundry layer is relying on normal convection to warm itself up enough so it rises off the block (and thus out of the barrier layer) and to be swept back into the waterflow. it's only when it warms up enough to leave the boundry layer due to normal convection that it's replaced with something cooler and the cooling cycle can continue.
creating a small amount of turbulence on the surface of the block will prevent the boundry layer from forming and thus increase the Heat Transfer potential of your apparatus. this can be achieved by soldering little nubs on the surface of your water-passages or simply by roughing it up with small perpendicular grooves in the surface of the metal (such as with a file or rasp). you can also run a small wire or spring (if the channel is big enough) down the length of your tubing if you're relying on your tubing to provide you with some of your cooling ability. (anyone not using copper piping for this last technique should forget it as rubber/nylon/vinyl/silicone tubing isn't heating or cooling your liquid one way or the other and theres really no point - on a slightly different note, this technique might help you guys with radiator problems looking to grab another couple of degrees of cooling power from it. i haven't tested it, but in theory it should be a decent idea).
basically you just want to find a way to interrupt the water that crosses across the surface of the block to prevent the boundry layer from forming.
summary: the boundary layer impedes heat transfer and it becomes thicker with increasing fluid viscosities.
However, the surface tension is a little different in that it reflects how much a fluid spreads out on a solid surface (i.e. low surface tension) vs beading up on a solid surface (i.e. high surface tension) if you wait long enough (equilibrium). The viscosity will affect how fast(rate)the fluid will spread out or bead up. The surface tension is the property which reflects the fluid's ability to wet a surface.
Even if a fluid has a low surface tension and can potentially wet a surface very well, it is not good for heat transfer if the fluid is too viscous and slooows its speed to wet or be replaced by itself with new cooler fluid
(surface renewal). Thus, even though you need the surface to be wetted (low surface tension) you also need it to be easily replaced (low viscosity).
That's why glycol sucks, incidently, compared to water or water/methanol as a transfer fluid in cooling applications. Glycol finds it specific use when
you need to increase the boiling point as well as use it as a heat transfer medium as in autos.
basically by breaking up the surface tention you're increasing the heat transfer potential of your liquid by making it less sticky to itself. you're essentially making the water more fragile and easier to break up which allows for better turbulence on the surface against the block.
use a combination of the two methods to break up the boundry layer on the block
and decrease the surface tention of your liquid and you're rockin like dokken.
basically the boundry layer can be addressed by altering the surface of the block. the surface tention can be addressed by treating the liquid. and both techniques can and should be used in collaboration with one another to recieve the most performance out of your heat exchanger AND your coolant if at all possible.
hope that explains things a bit better.
