Experimental investigation on friction factor in pipes with large roughness

• Deviation from classic theory for laminar flow in rough tubes was confirmed.
• Earlier transition between flow regimes was due to obstruction of roughness.
• Range of transitional flow reduces with an increase in surface roughness.
• Transitional flow disappeared in porous media with no limits between flow regimes.
• Linear law for laminar flow in porous media may be an approximate expression.

This paper experimentally and theoretically investigated the differences on friction factor in rough pipes with large surface roughness. Deviation from theoretical theory for laminar flow in rough tubes was confirmed again. The product of fRe was larger than 64 and can be predicted as a quadratic equation of relative roughness. Earlier transition between flow regimes was observed. The critical Reynolds number between flow regimes decreased as the relative roughness increased. The range of Reynolds number for transitional flow regime gradually reduces with an increase in surface roughness. When relative roughness reach peak (Δ/d = 1/2.4), transitional flow regime disappeared. Navier–Stokes equation was used to analyze the differences on flow behavior. It indicates flow obstruction of surface roughness caused curve flow for laminar flow in porous medium and linear law for laminar flow in porous media may be an approximate expression by neglecting the inertial forces at low Reynolds number.