Tips for Choosing the Value of D in Pipe Diameter Iteration

[foo9008]

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

 

[Chestermiller]

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

 

[foo9008]

you mean moody chart ?

 

[Chestermiller]

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.

 

[foo9008]

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 ?

 

[Chestermiller]

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.

 

[foo9008]

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?

 

[Chestermiller]

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.

 

[foo9008]

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 don't know the D , how to get the relative roughness(e / D) so that we can choose the friction factor ?

 

[Chestermiller]

yes , this is a homework -like problem , the roughness , e is given , but we don't 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.

 

[foo9008]

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?

 

[Chestermiller]

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 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.

 

[foo9008]

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.

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

 

[Chestermiller]

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.

 

[foo9008]

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 ?