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Homework Help: Calculate flow

  1. Apr 27, 2017 #1
    I am trying to calculate the flow in a system from an underground tank up through a pump into a vessel.

    But cannot find or derive a formula that yields a good result, I don't think I can use the same head calculations as Velocity will have changed.

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

    ρ = 1000 kg m–3
    μ = 0.001 Pa s
    P1 = 1bar
    P2 = 2bar
    Area/Diameter of pipe mm = 0.00636/90
    Power = 2749 Watts

    2. Relevant equations

    Image 1

    3. The attempt at a solution

    I have attempted using ##Re= \frac {UcdP} {p},##

    (assuming streamlined)##2000= \frac {Uc*0.09*1000} {0.001},##
    ## Uc= \frac {2000} {90000},##
    ##Uc = 0.02 ms^{-1}##

    ## Flow = Velocity * Area ##
    = 0.02 * 0.00636
    ## = 0.00013m^3s^{-1}##

    However I haven't used power here so I cannot see it being the correct way to calculate the flow.

    Attached Files:

  2. jcsd
  3. Apr 27, 2017 #2
    Why did you choose 2000 for Re?
  4. Apr 28, 2017 #3
    From my notes this equation looked to give a reasonable answer. It however finds the Critical Velocity, which I now see is the velocity in which the flow changes into turbulent flow.
    I used 2000, as I know from a previous question using the exact same system but given a flow, different Viscosity and density, that the pipework is laminar.

    Is there a way to transpose the old Viscosity and density with the new one to find flow or velocity?
    I am told the solution may require an iterative solution.

  5. Apr 28, 2017 #4
    Suppose, instead of knowing the power supplied to the pump, you knew in advance the velocity of the flow in the pipe. Could you then solve for the power supplied to the pump?
  6. Apr 28, 2017 #5
    The question states using the same pump as before. So I think I need to use the power previously stated and work backwards to velocity and hence flow.
  7. Apr 28, 2017 #6
    That wasn't my question. I'm not asking about your specific problem. Read my question carefully. If I told you that the flow in the pipe is 3 m/s, would you be able to calculate the power required from the pump to achieve this flow? Yes or no.
  8. Apr 28, 2017 #7
    Given Fluid information and heads calculated from Velocity. Using Darcy Formula
  9. Apr 28, 2017 #8
    Good. Do it. Let's see what you get for the power.
  10. Apr 28, 2017 #9
    So I calculated Flow to be ##0.0019m^3s##
    Hm = 0.023m
    Hf = 1.97m
    Therefore using Darcys Equation is ended up with Hp to be 29.18 (very similar to my previous fluid)

    Which gave me a Power of 858 kW.

    From this I assume that Hp won't change much in this given system.

    Hence from my first question. ##Q= \frac {2749} {1000*9.81*29.19} ##

    Last edited: Apr 28, 2017
  11. Apr 28, 2017 #10
    Do you really mean kW?
  12. Apr 28, 2017 #11
    In your equation, what does hm stand for?
  13. Apr 28, 2017 #12
    Is there a diagram of the system?
  14. Apr 28, 2017 #13
    I apologise I inputted Velocity not flow. The answer should be 5441 Watts or 5.4 Kw.

    Hm stands for the frictional losses caused by fittings. Though I'm now looking thinking my overall process for Hf is incorrect.

    I now think it should be 5069 Watts

    Attached Files:

  15. Apr 28, 2017 #14
    Can you please provide the entire problem statement. Thanks.
  16. Apr 28, 2017 #15
    Initial parts

    Screen Shot 2017-04-28 at 17.42.43.png Screen Shot 2017-04-28 at 17.42.54.png

    That was the original
  17. Apr 28, 2017 #16
    Issue we started with.

    Screen Shot 2017-04-28 at 17.53.09.png
  18. Apr 28, 2017 #17
    For a velocity of 3 m/s, I get a volumetric flow rate (assuming a diameter of 0.09 m) of 0.0191m^3/s. Is this what you got?
    I get a mass flow rate of 19.1 kg/sec.
    I get a Reynolds number of 270000. Is this what you get?
    For a smooth pipe, I get a Darcy Weisbach friction factor of about 0.15. Is this what you got?
    I get a velocity head of 0.459 m.
    I get a frictional head of about 24.5 m. Is this what you get?
  19. Apr 28, 2017 #18
    The pressure head and the gravitational head are both independent of flow rate.

    Pressure head = 1 bar = 10 m
    Gravitational head = 16 m

    Is this your assessment?
  20. Apr 28, 2017 #19
    Thanks for you patience with me

    I got the Flow rate, mass flow rate, Reynolds and velocity head. I don't believe i have studied finding the Darcy Weisback friction factor.
    I think my frictional head loss is 23.29 as I was going from the water surface not the whole leg i.e. 17m height.
  21. Apr 28, 2017 #20
    Then you must be using Fanning. Darcy Weisback friction factor is 4 x Fanning, and, in the DW equation, you only use L/D, compared with than 4L/D with Fanning.
    The frictional head should be based on the total length of pipe, 32 meters.
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