Approximation of friction factor for Laminar Flow

In summary, there are two formulas for calculating friction factor - ƒ = 16/Re and ƒ = 64/Re. The former is known as the Fanning friction factor and the latter is known as the Darcy friction factor. Both are derived from different values and dimensions but essentially have the same purpose. This can be confusing, but now you have a better understanding of the two formulas.
  • #1
Dong Aleta
29
0
Hi!

I read from Perry Green's ChE Handbook that the friction factor for Re ≤ 2,100 can be approximated by ƒ = 16/Re. But there was this question that I encountered (though I don't know the source) and according to it, ƒ = 64/Re for laminar flow. Can someone clarify which is which? Thank you!
 
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  • #2
The formula f=64/Re is called the Darcy friction factor and is derived from f=(Δp⋅2⋅D)/(L⋅ρ⋅u2)

The formula f=16/Re is called the Fanning friction factor and is derived from f=(Δp⋅R)/(L⋅ρ⋅u2)

Basically they are the same, but different values/dimensions are used to define them
 
  • #3
stockzahn said:
The formula f=64/Re is called the Darcy friction factor and is derived from f=(Δp⋅2⋅D)/(L⋅ρ⋅u2)

The formula f=16/Re is called the Fanning friction factor and is derived from f=(Δp⋅R)/(L⋅ρ⋅u2)

Basically they are the same, but different values/dimensions are used to define them
I see. That's really confusing. Anyway, thank you!
 

FAQ: Approximation of friction factor for Laminar Flow

1. What is the friction factor for laminar flow?

The friction factor for laminar flow is a dimensionless quantity that represents the resistance to flow caused by the presence of a boundary or surface. It is commonly denoted as f and is calculated using the ratio of the shear stress to the dynamic pressure.

2. How is the friction factor for laminar flow approximated?

The friction factor for laminar flow can be approximated using the well-known Hagen-Poiseuille equation, which states that f = 64/Re, where Re is the Reynolds number. This approximation is valid for flow through a circular pipe or tube with a constant cross-sectional area.

3. What is the range of validity for the approximation of friction factor for laminar flow?

The Hagen-Poiseuille equation is valid for laminar flow with a Reynolds number (based on the tube diameter) in the range of 0 < Re < 2300. Beyond this range, the flow becomes turbulent and the friction factor is no longer accurately approximated using this equation.

4. How accurate is the approximation of friction factor for laminar flow?

The Hagen-Poiseuille equation provides a relatively accurate approximation of the friction factor for laminar flow within its valid range. However, it should be noted that this equation assumes a fully developed flow profile and a constant tube diameter, which may not always be the case in practical applications.

5. Are there any other methods for approximating the friction factor for laminar flow?

Yes, there are other methods for approximating the friction factor for laminar flow, such as the Blasius equation and the Prandtl-Colebrook equation. These equations can provide more accurate results for laminar flow in certain scenarios, but they may also be more complex and require additional input parameters.

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