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Question about water flow through a PC watercooling system

  1. May 17, 2009 #1
    So I decided that I want to machine my own water cooling system from scratch. I know it's going to take a lot of time for virtually no money saved, but I want some experience machining and it'd be really cool to make something that I'd actually use.

    So anyway, I can't figure out why all CPU waterblocks have the in port and out port on the same face. It seems to me like if they were on opposite faces (so the flow is parallel to the motherboard) that it would flow easier. Better flow means better cooling, right?

    Right now I'm still in the design stage, and my first thought was to have a larger cavity than most waterblocks (approx 5/8x5/8" for 1/2" tubing) and have grooves in the copper base for increased surface area. I am not taking fluids until this semester, so I don't completely understand everything that's going on with the water. Would a design such as this cool better than a typical waterblock with the in port and out port on the same plane?

    Thanks for any help/advice!
     
  2. jcsd
  3. May 17, 2009 #2

    mgb_phys

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    No ,better flow means LESS cooling.
    Imagine the perfect case of a flow that went through a high speed not touching the sides, the water would exit at the same temperature it went in at and there would be no cooling.
    The point of having lots of pipes in a radiator or cooler is for the fluid to touch the sides as much as possible and reach the temperature of the block.
    Having the in-out ports on the same side simply makes the connections easier
     
  4. May 17, 2009 #3
    Really? That's quite counterintuitive.

    You'd think that, while the water would exit at a lower temperature if the flow were faster, there would be more water replacing it in the reservoir to take its place in absorbing heat.

    If heat flows more quickly for a larger temperature difference, which I imagine it does, it would seem that this scenario - faster moving water, where any segment of water absorbs proportionally less heat but you have proportionally more segments absorbing it - would be better.

    It seems, in fact, like there should be an optimal speed depending on the temperature to which you can cool the water in the cooling portion of the apparatus.

    I'm not sure I really believe the idea that slower is better... in the limit, you just put a pool of water with no flow at all, and this is clearly not going to work. Intuition would seem to indicate that, if the water were moving infinitely fast, it would be the same physical situation as the pool of water without the water raising in temperature (not because it doesn't absorb heat, mind you, but because the cooling portion removes it as fast as it is put in). The static situation would be the same, except the temperature of the water would raise until it reached thermal equilibrium with the thing it was cooling.

    If you're right, that's a very interesting phenomenon. Why is it like this? I don't by the idea of "water moving so fast it doesn't touch the sides".
     
  5. May 17, 2009 #4
    I think he means that if I have a waterblock where the water is moving very quickly, it means there is no turbulence in the water. If there is no turbulence, then there are going to be hot spots near the sides of my waterblock. It's not necessarily that less flow creates a better heatsink, but that there needs to be some contact with the sides of the pipe (or in this case, a waterblock) in order for it to work well.

    I'm not really sure if that's what he meant, but that's what I thought of after reading his post.

    Edit: But I completely agree with everything you say, and the way MGB worded his post definitely does confuse me for the same reason it does you.
     
  6. May 17, 2009 #5
    Oh no, I agree that increasing surface area of the water/heat source interface is good, and keeping hot water away from the interface seems better. I just don't see how less flow achieves this better than more flow.
     
  7. May 17, 2009 #6
    I could be wrong, but I *think* what he was getting at is that with lack of flow comes turbulence. Without turbulence there will be hot spots.

    This is the part that I don't agree with:

    "Imagine the perfect case of a flow that went through a high speed not touching the sides, the water would exit at the same temperature it went in at and there would be no cooling."

    Edit: Or maybe he misunderstood? Because the water will be touching the sides...
     
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