Flow characteristics of water in microtubes

Water flow through microtubes with diameters ranging from 50 to 254 μm was investigated experimentally. Microtubes of fused silica (FS) and stainless steel (SS) were used. Pressure drop and flow rates were measured to analyze the flow characteristics. The experimental results indicate significant departure of flow characteristics from the predictions of the conventional theory for microtubes with smaller diameters. For microtubes with large diameters, the experimental results are in rough agreement with the conventional theory. For lower Re, the required pressure drop is approximately the same as predicted by the Poiseuille flow theory. But, as Re increases, there is a significant increase in pressure gradient compared to that predicted by the Poiseuille flow theory. The friction factor therefore is higher than that given in the conventional theory. The results also indicate material dependence of the flow behavior. For the same flow rate and the same diameter, an FS microtube requires a higher pressure gradient than a stainless steel microtube. The measured high pressure gradient may be due to either an early transition from laminar flow to turbulent flow or the effects of surface roughness of the microtubes. These phenomena are discussed in this paper. A roughness-viscosity model is proposed to interpret the experimental data.