- #1

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## 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 ?

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- Thread starter foo9008
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- #1

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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

Chestermiller

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Have you looked at the graph of friction factor vs Reynolds number?

- #3

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you mean moody chart ?

- #4

Chestermiller

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- #5

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how to determine the value to start with ? the Re and relative roughness is not given ?^{5}.

- #6

Chestermiller

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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.how to determine the value to start with ? the Re and relative roughness is not given ?

- #7

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when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?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

Chestermiller

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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.when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?

- #9

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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 ?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

Chestermiller

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Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.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

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But, we do not know D, how to assume the value of D?Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.

- #12

Chestermiller

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1. Guess value of DBut, we do not know D, how to assume the value of D?

2. Calculate Re

3. Calculated e/D

4. Determine f

5. Determine h

6. Determine difference

7. Go back to step 1

8. Make a graph of either h

- #13

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so , we need to assume the value of D first ?1. Guess value of D

2. Calculate Re

3. Calculated e/D

4. Determine f

5. Determine h_{f}and compare with required value

6. Determine difference

7. Go back to step 1

8. Make a graph of either h_{f}vs D or (h_{f }difference) vs D.

- #14

Chestermiller

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That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.so , we need to assume the value of D first ?

- #15

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is there any tips to assume the value of D ?That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.

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