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Find diameter of pipe

  • Thread starter foo9008
  • Start date
  • #1
678
4

Homework Statement


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 ?

Homework Equations




The Attempt at a Solution

 

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Answers and Replies

  • #2
19,918
4,094
Have you looked at the graph of friction factor vs Reynolds number?
 
  • #3
678
4
you mean moody chart ?
 
  • #4
19,918
4,094
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.
 
  • #5
678
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.
how to determine the value to start with ? the Re and relative roughness is not given ?
 
  • #6
19,918
4,094
how to determine the value to start with ? the Re and relative roughness is not given ?
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.
 
  • #7
678
4
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.
when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?
 
  • #8
19,918
4,094
when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?
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.
 
  • #9
678
4
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.
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 ?
 
  • #10
19,918
4,094
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 ?
Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.
 
  • #11
678
4
Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.
But, we do not know D, how to assume the value of D?
 
  • #12
19,918
4,094
But, we do not know D, how to assume the value of D?
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.
 
  • #13
678
4
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.
so , we need to assume the value of D first ?
 
  • #14
19,918
4,094
so , we need to assume the value of D first ?
That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.
 
  • #15
678
4
That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.
is there any tips to assume the value of D ?
 

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