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Homework Help: Velocity of the water flowing through the pipe

  1. Apr 29, 2006 #1
    Hi,

    i have a rotameter reading, flow rate (lit/min), Head Loss(mm) and diameter of a pipe.

    from this i need to find out the Velocity of the water flowing through the pipe.

    iam confused as to what formula to use to find this out, i have searched through 2 textbooks and cannot find anything.

    can anyone help me in the right direction as to which formula i should be using.

    thanks
     
    Last edited: Apr 29, 2006
  2. jcsd
  3. Apr 29, 2006 #2

    Doc Al

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    Read this: http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Pressure/VolumeFlowRate.html [Broken]
     
    Last edited by a moderator: May 2, 2017
  4. Apr 29, 2006 #3
    i know those formulas but i struggle to see where my data fits into the equation.
    i think i must be lost with the meaning of the rotameter and flow rates given.

    *** edit thought about it longer

    sorry hang on is it saying

    Flow rate = cross sectional area x velocity??????
     
  5. Apr 29, 2006 #4

    Doc Al

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    That's the one.
     
  6. Apr 29, 2006 #5
    do you mind if i ask, if my flow rate is 0.3 lit/min, and my cross sectional area is A = pi/4(19.23) = 15.103 mm^2

    V = av
    v = a/V
    v = 15.103/0.3 = 50.34?

    or was it ment to be v = V/a?
    my basic math is shocking :blushing:

    *** never mind im 90% sure its v = V/a so the answer would be 0.02m/s or 2 cm/s which makes more sense than 50.34 m
     
    Last edited: Apr 29, 2006
  7. Apr 29, 2006 #6

    Doc Al

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    I'm suspicious of your area formula: what data are you actually given?

    Before using: Flow rate = (Area)*(speed), or speed = (Flow rate)/(area), be sure to convert everything into standard units:

    Area in m^2; flow rate in m^3/sec; speed in m/sec.

    To convert from liters/minute, realize that:

    1 (liter)/(minute) = (10^-3 m^3)/(60 seconds)

    (or you can look up a unit conversion chart)
     
  8. Apr 29, 2006 #7
    we did some tests using rotameters. The rotameters would return results such as 50mm on rotameter 10 (i forget what the sizes mean) we then had a graph for the size rotameter we used and we would look at the graph and find 50mm to have a flow rate of 0.3 (lit/min).

    so eg for a size 18 rotameter with a reading of 151mm the flow rate was found to be on the graph 5.75 (lit/min) giving me the velocity of 0.381 m/s

    i have the diameter of the tube inside which is 19.23mm. by memory i thought that the cross sectional area of a tube to be A = pi/4 x (Diameter)
     
    Last edited: Apr 29, 2006
  9. Apr 29, 2006 #8
    or is the area supposed to be A = pi x diameter x diameter. ?
     
  10. Apr 29, 2006 #9

    Doc Al

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    The area of a circle is:
    Area = pi*radius^2 = (pi/4)*(Diameter)^2

    where (Diameter)^2 = (Diameter)x(Diameter)
     
  11. Apr 29, 2006 #10
    ah ok so its...

    A = pi/4*19.23^2 = 290.435 ???

    so for a rate of flow of 0.3 lit/min

    the answer would be

    0.3 = 290.435*v
    0.3/290.435 = v
    v = 0.00103m/s
     
    Last edited: Apr 29, 2006
  12. Apr 29, 2006 #11

    Doc Al

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    Put you data into standard units: length in meters, not mm; area in m^2, not mm^2. (Once you've used the formula to find the answer, you can convert the answer to any units you like.)
     
  13. Apr 29, 2006 #12
    the only data i have is 19.23mm(0.1923 m) diameter and the flow rate which is lit/min.

    to convert this into m^3/sec is totally baffaling me.
    We got a rotameter reading in mm, looked at a graph which gave us the flow rate in lit/min.
    why does this have to be changed?
    unless it doesnt i had got

    v = 0.3/0.02904
    v = 10.39 m/s

    thanks
     
    Last edited: Apr 29, 2006
  14. Apr 29, 2006 #13

    Doc Al

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    If you don't change to standard units, your expression for speed will have units of:

    (lit/min)/(m^2) :yuck: , which is certainly not equivalent to m/s.

    But if you expressed the flow rate in m^3/s, your speed will have units of:

    (m^3/s)/(m^2) = m/s o:)
     
  15. Apr 29, 2006 #14
    Argghhhhh i dont know how to do it. my heads aching lol.
    its very late maybe i should sleep on it.
    any other info or help on how to convert it would be much appreciated.
    thanks!

    thanks Doc Al, youve been alot of help :smile:
     
  16. Apr 29, 2006 #15

    Doc Al

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    I thought I showed how to do the conversion in post #6? Do the indicated arithmetic! (Or you can just Google it. Try it.)
     
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