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Simple Fluid Flow

  1. Oct 17, 2009 #1
    Doing some review here:

    1. The problem statement, all variables and given/known data

    A fire hose has an inner diameter of 5 inches and water is flowing at 600 gal/min. The flow exits through a nozzle contraction with
    diameter dn. For steady flow, what should dn be in inches in order for the exit velocity to be 25 m/s.

    My only question here is really do I even need the given inlet diameter = 5 in ?

    As far as I can see. Since density is constant, the volume flow rates at inlet and exit should be equal. That is,

    [tex]Q_1=Q_2[/tex]

    [tex]\Rightarrow Q_1 = \frac{500\,\text{gal}}{\text{min}} = A_2V_2[/tex]

    [tex](\frac{\pi}{4}d_n^2)V_2=\frac{500\,\text{gal}}{\text{min}}[/tex]

    Now assuming I convert my units properly, this should be correct? It's just that given inlet diameter that's bugging me.

    Thanks.
     
  2. jcsd
  3. Oct 17, 2009 #2

    rl.bhat

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    Can you convert gal/min to A*V?
     
  4. Oct 17, 2009 #3
    I would have to say yes.

    [tex]\frac{gal}{min}\equiv\frac{Volume}{time}[/tex]

    [tex]Area*V\equiv\frac{dx}{dt}*A\equiv\frac{Volume}{time}[/tex]
     
  5. Oct 17, 2009 #4

    rl.bhat

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    Inner diameter is given. Find the velocity of inflow. Velocity of the outflow is given. Find dn
     
  6. Oct 17, 2009 #5
    I am sorry rl, I still do not see what I am supposed to use the inner diameter for. I thought that we just said.

    Q1=Q2=A2V2

    I am given Q1, why should I bother finding V1?
     
  7. Oct 17, 2009 #6

    rl.bhat

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    Yes. You are right.
     
  8. Oct 17, 2009 #7
    Okay. Thank you. :smile:
     
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