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Thread: Understanding the actual mechanics of what goes on in flow in pipes

  1. #1
    Associate Engineer
    Join Date
    Apr 2013

    Understanding the actual mechanics of what goes on in flow in pipes


    So I'm really trying to understand the actual mechanics of what's going on in a pipe with fluid flow. Not all the equations and maths. So correct me if I'm wrong the floowing examples:

    1) Reduced from larger pipe to smaller, say 10m of 10mm down to 10m of 5mm pipe. This will reduce pressure and increase velocity. And this is due to the fact that the same volume of water is trying to get through a smaller hole, therefore the same number of molecules are in a more confined space, therefore more chance of bouncing off the pipe walls and causing friction, which reduces the pressure. Also the length of pipe increases the pressure drop due to increased friction over the entire length.

    2) Pipe with an orifice: Say 20m of 10mm pipe with a coin wedged in it with a hole in the middle of 5mm i.e. at the 10m mark. I'm assuming the pressure will reduce at the orifice and velocity increase temporarily, but by the end of the next 10m it would have returned to it's original pressure & flow

    3) How about several changes in diameter? Let's say we go from a small diameter pipe, to a larger one, then back to a smaller one, then down to an even smaller one. What's going on in terms of volume flow rate, pressure, and velocity in the pipe and at the source, and at the outlet?

    Many thanks
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  2. #2
    Associate Engineer
    Join Date
    Oct 2015
    You're basically right.

    Considering that the rate of flow is constant, and as the rate of flow is equal to the velocity multiplied by the cross section area (Q = v * A) so when the cross section decrease the velocity increase.
    Following Bernouilli's principle, in your case: P1+v1 = P2+v2 so you're right when the velocity increase the pressure decrease.

    Keep in mind that this model is not applicable in all cases.

    Also it's not because the fluid goes faster in smaller pipes that it will speed up your whole fluid transfer. For example in case 2), yes the water goes faster in the little "coin with a hole", but this little "coin with a hole" will reduce the max rate of flow and hance decrease the overall speed of your system. It's about the same as in a hourglass, if you look closely, the sand goes way faster when it comes closer to the little hole, but th elittle hole slows everything down because the maximum rate of flow is limited in this little hole.

    For more information you should read this I think:
    Last edited by Foudzing; 11-13-2015 at 09:37 AM.

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