Numerical investigation on the effect of roughness on the flow resistance for incompressible fluid flow in microtubes

Though predicted by the Moody chart that if lower than 5%, the effect of roughness on laminar flow characteristics can be ignored in macrotubes, it will no longer be neglected for fluid flow in microtubes, which was made clear recently by some experimental investigations. Study of the former literature reveals that the obvious conflict between results of present study and the conventional ones may be partly attributed to the unsoundness of previous investigation on laminar flow in macro coarse tubes. While microtubes are characterized by higher relative roughness, more compact distributed roughness elements and laminar internal flows, which make the study of laminar flow in rough tubes important. In this paper, internal circumferential fins were used to simulate the roughness element and numerical calculations were carried out to study the characteristics of resistance for incompressible laminar flow in rough tubes. It is concluded that the values of f.Re can be up to 23% higher than 64 if k/d=3%, which indicates that the effect of roughness can be no longer ignored. Frictional resistance also increases with increasing l/d and Reynolds number. The physical mechanism of the increasing frictional factors for laminar flow in rough tubes lies in the shape drag brought by the presence of roughness elements: they induce vortex wakes, entangle fluids and then produce higher friction factors. The veracity and validity of the present numerical investigation is well demonstrated by experimental works concerning the deionized water flow in rough microtube of 136.5 mu m.