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Homework Help: Fluid Mechanics

  1. Aug 16, 2006 #1

    i am trying to brush up on my fluid mechanics for when i do it next year at uni.

    do any of you guys have any idea how to solve the follow question?

    'A suction pump moves water from a reservoir to a holding tank. The system is designed in such a way that the suction pump is inclined at an angle of 10degres from the horizontal. The operating envolope specifies that the pump cannot transport water if any gases are present iun the suction pipe. Gases are released from the water when the pressure falls below 30% of atmospheric pressure. If tyhe water is transported at a velocity of 1.8m/s in the suction pipe, and assuming the water in resiviour is at rest, determine the length of pipeline from the reservoir to the holding take'

    hope some one can help. this is a tricking one for me.

  2. jcsd
  3. Aug 16, 2006 #2


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    In a real world application, this is more like a calculation of NPSHa in a pump system. This has to do with the pressure dropping below the liquid's vapor pressure.

    In your case, the Bernoulli equation is what you are looking for. You know pretty much everything except the final elevation. Since you know that the pipe is at a 10° inlcline, you could calculate the delta Z and back out the line length from that.
  4. Aug 17, 2006 #3
    how would this be done? i have the forumula for bernoulli;s in front of me but cant really see how to use it for this question?


  5. Aug 17, 2006 #4


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    The two locations would be the reservoir and the end of the pipe at the tank you are transferring to.

    V1 =0 (given in the problem statement)
    V2 = 1.8 m/s
    P1 = 14.7 psia (atmospheric pressure assuming the reservoir is open to atm)
    P2 = .7*P1
    Z1 = 0 (use as a referernce)

    Give that a try. The rest will be basic trig.
  6. Aug 18, 2006 #5
    what if the resivior is closed to atm?

    would it then be a different figure?
  7. Aug 18, 2006 #6

    would it be

    0 + 14.7/w + 0squared/2g = Z2 + 10.29/w + 1.8squared/2g + loss of head

    woudl this be it?
  8. Aug 18, 2006 #7
    is 'w'

    pg? therefore 1000 x 9.81?

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