Transition to turbulence in a concentric annular pipe

In this study the pressure-drop, mean and rms axial velocity data are measured using a differential pressure transducer and a laser Doppler anemometer for the flow of Newtonian and non-Newtonian fluids in a concentric annular pipe (radius ratio =0.5) at various Reynolds numbers encompassing the laminar, transitional and turbulent regimes. Three different fluids are utilized; a semi-rigid shear-thinning polymer (a xanthan gum), a polymer known to exhibit a yield stress (carbopol) and a Newtonian fluid (glycerine-water mixture) as the reference fluid. A longer Reynolds number range for the transitional flow regime is observed for the more shear-thinning fluid as determined from the axial rms fluctuation level measured at fixed radial locations. Contrary to what is observed for the Newtonian fluid the higher shear stress on the inner wall compared to the outer wall does not lead to earlier transition for the shear thinning and the yield stress fluids, xanthan gum and carbopol, where more turbulent activity is observed in the time traces at the outer wall region. The mean axial velocity profiles show a slight shift (~5%) of the maximum velocity location towards the outer pipe wall within the transitional regime only for the Newtonian fluid.