Viscoelastic flow in converging-diverging pipes

Point velocity measurement has been applied to analyze the flow field of dilute polymer solution in converging-diverging pipes with axial symmetry. Four geometries with various lengths of converging-diverging sections were considered. Special attention was given to stationarity and steadiness of investigated flows. It was observed that the amplitude of oscillating flow grows as the Deborah number increases and reaches the maximum at De ∼ 0.7, at which the vortex length has achieved its maximum value. Further increase in Deborah number leads to decrease in the vortex length and suppresses the amplitude of observed oscillation. Using time-averaged analysis, the velocity profiles at various cross sections of converging-diverging tubes are constructed and discussed. A comparison with the analytical solution for the pipe flow is also provided. For Deborah numbers in the range from 1 to 6, the fourth regime was observed for which the viscoelastic flow behaves like “purely divergent” without any vortex zone near the boundary wall in the converging section. The transition to this flow regime can be predicted by a modified version of extension rate distribution.