Experimental investigations of local friction factors of laminar and turbulent gas flows in smooth micro-tubes

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Local friction factors of laminar and turbulent gas flows including choked flows through micro-tubes were experimentally investigated under atmospheric back pressure and variable inlet pressure. The experiments were carried out using two glass micro-tubes with D = 265.7 and 399.9 μm and a fused silica micro-tube with D = 531.2 μm. Two to three pressure tap holes were drilled into the micro-tube wall at intervals of 5 ~ 6 mm to measure local pressures. The local Mach numbers, temperatures and friction factors were obtained by using stagnation temperatures and pressures, local pressures and mass flow rates. The results for wide range of Reynolds numbers and Mach numbers were obtained including choked flows. Both the local Fanning and the local Darcy friction factors were obtained under the assumption of both Fanno flow (adiabatic wall) and isothermal flow, respectively. Since the measured values of the inner relative surface roughness of the micro-tubes were less than 0.02 %, only the effect of compressibility on friction factors was assessed. The difference between Fanning and Darcy friction factors was described and compared with the ff and fd correlation as a function of Mach number. The friction factor difference obtained under the assumption of Fanno and isothermal flows was also compared with the available literature and numerical results. In the turbulent flow region including the choked flow, the local Fanning friction factors under the assumption of Fanno flow nearly coincided with Blasius correlation.