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Homework Help: Find diameter of pipe

  1. Apr 20, 2016 #1
    1. The problem statement, all variables and given/known data
    when we find the diameter of a pipe , we have to found the value of D and f by iteration , how to choose the start value of friction factor when we do the iteration ?

    2. Relevant equations

    3. The attempt at a solution

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  2. jcsd
  3. Apr 20, 2016 #2
    Have you looked at the graph of friction factor vs Reynolds number?
  4. Apr 20, 2016 #3
    you mean moody chart ?
  5. Apr 20, 2016 #4
    Yes. You can see that on the chart, the friction factor does not very by several orders of magnitude, at least not for turbulent flow. I would start out with the value at a Re of 105.
  6. Apr 20, 2016 #5
    how to determine the value to start with ? the Re and relative roughness is not given ?
  7. Apr 20, 2016 #6
    Like I said, start with the friction factor f for Re = 100000. As far as roughness is concerned, you would have to know that for the particular pipe from its specs or just assume a worst-case value.
  8. Apr 20, 2016 #7
    when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?
  9. Apr 20, 2016 #8
    As I said, the pipe may come with that kind of information when it is purchased. Or, you could choose what you would consider a worst case value. Or, if it a homework problem, they will typically tell you what roughness to use.
  10. Apr 20, 2016 #9
    yes , this is a homework -like problem , the roughness , e is given , but we dont know the D , how to get the relative roughness(e / D) so that we can choose the friction factor ?
  11. Apr 20, 2016 #10
    Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.
  12. Apr 20, 2016 #11
    But, we do not know D, how to assume the value of D?
  13. Apr 20, 2016 #12
    1. Guess value of D
    2. Calculate Re
    3. Calculated e/D
    4. Determine f
    5. Determine hf and compare with required value
    6. Determine difference
    7. Go back to step 1
    8. Make a graph of either hf vs D or (hf difference) vs D.
  14. Apr 20, 2016 #13
    so , we need to assume the value of D first ?
  15. Apr 20, 2016 #14
    That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.
  16. Apr 20, 2016 #15
    is there any tips to assume the value of D ?
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