Experiments investigation of the effects of surface roughness on laminar flow in macro tubes

► Six organic glass tubes with relative roughness ranging from 1/118.75 to 1/6.66. ► Roughness has no effect on laminar flow when relative roughness is less than 5%. ► Deviation from the classical law was observed in large roughness tubes. ► The product of fRe increased with an increase in relative roughness. ► Earlier transition from laminar to turbulent flow was also confirmed. This paper experimentally investigated the effects of surface roughness on laminar flow in macro tubes (d=19mm). Head drop experiments were conducted in six organic glass tubes with relative roughness (Δ/d) ranging from 1/118.75 to 1/6.66. Water head drop data were used to characterize the friction factor over a Reynolds number ranging from 300 to 25000. The experimental results for tubes with relative roughness (Δ/d5%). The product of fRe was larger than 64 and increased with an increase in relative roughness. Earlier transition from laminar to turbulent flow was also confirmed. The critical Reynolds number linearly deceased as the relative roughness increased.