Fanning friction factor

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The Fanning friction factor, named after John Thomas Fanning (1837–1911), is a dimensionless number used in fluid flow calculations. It is related to the shear stress at the wall as:


\tau = \frac{ f \rho v^2}{2}

where:

  • \tau is the shear stress at the wall
  • f is the Fanning friction factor of the pipe
  • v is the fluid velocity in the pipe
  • \rho is the density of the fluid

The wall shear stress can, in turn, be related to the pressure loss by multiplying the wall shear stress by the wall area (2 \pi R L for a pipe) and dividing by the cross-sectional flow area ( \pi  R^2 for a pipe).

The friction head can be related to the pressure loss due to friction by dividing the pressure loss by the product of the acceleration due to gravity and the density of the fluid. Accordingly, the relationship between the friction head and the Fanning friction factor is:


h_f = \frac{ 4fv^2L}{2gD}

where:

  • h_f is the friction loss (in head) of the pipe.
  • f is the Fanning friction factor of the pipe.
  • v is the fluid velocity in the pipe.
  • L is the length of pipe.
  • g is the local acceleration of gravity.
  • D is the pipe diameter.

Fanning friction factor formulæ[edit]

This friction factor is one-fourth of the Darcy friction factor, so attention must be paid to note which one of these is meant in the "friction factor" chart or equation consulted. Of the two, the Fanning friction factor is the more commonly used by Chemical Engineers and those following the British convention.

The formulae below may be used to obtain the Fanning friction factor for common applications.

The friction factor for laminar flow in round tubes is often taken to be:

f= \frac{16}{Re}

where Re is the Reynolds number of the flow.

For a square channel the value used is:

f = \frac{ 14.227}{Re}

The Darcy friction factor can also be expressed as[1]

f = \frac{8 \tau_w}{\rho V_{avg} ^ 2}

where:

  • \tau_w is the shear stress at the wall
  • \rho is the density of the fluid
  • V_{avg} is the average fluid velocity

For the turbulent flow regime, the relationship between the Fanning friction factor and the Reynolds number is more complex and is governed by the Colebrook equation [2] which is implicit in f:

{1 \over \sqrt{\mathit{f}}}= -4.0 \log_{10} \left(\frac{\frac{\epsilon}{d}}{3.7} + {\frac{1.256}{Re \sqrt{\mathit{f} } } } \right)  , \text{turbulent flow}

Various explicit approximations of the related Darcy friction factor have been developed for turbulent flow.

Stuart W. Churchill[3] developed a formula that covers the friction factor for both laminar and turbulent flow. This was originally produced to describe the Moody chart, which plots the Darcy-Weisbach Friction factor against Reynolds number. The Darcy Weisbach Formula  f_D is 4 times the Fanning friction factor  f and so a factor of  \frac{1}{4} has been applied to produce the formula given below.

  • Re, Reynolds number (unitless);
  • ε, roughness of the inner surface of the pipe (dimension of length);
  • D, inner pipe diameter;
 f = 8 \left( 
                  \left( \frac {8} {Re} \right) ^ {12}
                + \left( A+B \right) ^ {-1.5} 
               \right) ^ {\frac {1} {12} }
A = \left( 2.457 \ln \left( \left( \left( \frac {7} {Re} \right) ^ {0.9} + 0.27 \frac {\epsilon} {D} \right)^ {-1}\right) \right) ^ {16}
B = \left( \frac {37530} {Re} \right) ^ {16}

References[edit]

  1. ^ Yunus, Cengel. Heat and Mass Transfer. New York: Mc Graw Hull, 2007.
  2. ^ Colebrook, C.F. and White, C.M. 1937, "Experiments with Fluid friction roughened pipes.", Proc. R.Soc.(A), 161
  3. ^ Churchill, S.W., 1977, "Friction factor equation spans all fluid-flow regimes", Chem. Eng., 91