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Reynolds Number and Power Number Problem

  1. Nov 29, 2012 #1
    1. The problem statement, all variables and given/known data
    If the Reynolds number is 4000, what is the Power (P) required in units of watts at a shaft speed (n) of 0.03 Hz? Assume the system contains acetone, with a kinematic viscosity of 0.419 stokes. The density of acetone is 0.785 grams per cubed centimeter. Us Curve B in the graph (attached as an image) to determine your answer. (Hint: Use the Reynolds number of the system to first calculate the diameter, then find the power number, and then calculate the power.


    2. Relevant equations
    Re = D2n[itex]\rho[/itex] / [itex]\mu[/itex]

    Np = P / [itex]\rho[/itex]n3D5

    3. The attempt at a solution
    When trying to perform the calculation to determine the diameter, I could not get the grams to cancel out to get an answer in the proper units. While trying to continue to work through the problem I got an answer for the diameter to be 2.67 m. This doesn't seem right. How do I go about solving this problem?
     

    Attached Files:

  2. jcsd
  3. Nov 29, 2012 #2

    SteamKing

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    First, find out what units are used in the formulas.
    Second, check the info given in the problem to see if the units used match the units required by the formulas.
     
  4. Nov 29, 2012 #3
    How do you get the grams from the density to cancel out in the Reynolds number equation?
     
  5. Nov 29, 2012 #4

    SteamKing

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    What are the units of the quantities in the formula for Reynolds number?

    You won't be able to solve this problem successfully without coming to grips with the units of the quantities in the equations.
     
  6. Nov 29, 2012 #5
    Re=unitless
    D=meters
    n=Hz=s-1
    [itex]\rho[/itex]=kg/m3
    [itex]\mu[/itex]=m2/s

    Where do the kilograms cancel out in this equation?
    I was correct to convert cm to m and g to kg right?
     
  7. Nov 30, 2012 #6

    SteamKing

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    You don't have the right units for mu. Mu has units of kg/m/s.
     
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