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Flow through pipe

  1. Mar 13, 2012 #1
    I need help!!

    I was given the pressure and flow rate needed through a manifold and I need to find out if this is producing the require flow rate based on the inlet/outlet diameter and total length.

    I have been out of school for years and need help on this! thanks
  2. jcsd
  3. Mar 15, 2012 #2
    If I understand your problem correctly, you'll need a little more information to find out what you need.

    Do you have the available pressure or fluid velocity?

    Dynamic pressure in fluid flow is

    P = ρ . v^2 / 2
    P = Dynamic pressure (Pa)
    v = Velocity (m/s)
    ρ = Fluid Density (kg/m3)

    Using this formula we can solve for velocity.
    v = (2*P/ρ)^0.5

    Now we can apply the flow rate formula:

    Q = v.A

    Q = Volumetric Flow Rate (m3/sec)
    v = velocity (m/s)
    A = Area (m2)

    This will give volumetric flow rate at given pressure. It won't matter what area you use (inlet, outlet, or in between) as flow rate is constant under constant pressure (how I'm imagining this problem). Velocity will change with cross sectional area but it doesn't sound like you're particularly interested in velocity.

    The length won't affect flow rate if you ignore friction. Even taking friction into account will only reduce the flow rate by a significant amount on a very low manifold (how long are we talking?).

    Also keep in mind the first equation is for laminar flow (I think.. it's late here and brain's switched off for the night). Google 'laminar flow' if you're unfamiliar with the concept. I'd guess you'd be passing either water (or similar viscosity) or air through this manifold and I'd expect it to be in the turbulent region, not laminar flow. I dont' know how to accurately calculate this information for non-laminar flow.

    Hope this helps.
    Last edited: Mar 15, 2012
  4. Mar 15, 2012 #3


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    Hi vision, welcome to the board. You'll need a bit more than Bernoulli's equation to determine irreversible pressure drop in pipe. In the thread below you'll find an attachment (post #2) that gives you the Darcy Weisbach equation for irreversible pressure drop through pipe which is what you need. The standard text in industry is the Crane paper TP410 and can be purchased online here.
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