LDV measurements of viscoelastic flow transitions in abrupt axisymmetric contractions: Interaction of inertia and elasticity

Abstract

Laser Doppler velocimetry (LDV) measurements of the axial and radial velocity components are reported for the axisymmetric contraction flow of a viscoelastic solution of polyisobutylene dissolved in tetradecane. This fluid exhibits shear-thinning of both the viscosity and first normal stress difference; the viscosity is well described by the Carreau-Yasuda model. Shear-rate-dependent Reynolds and Deborah numbers are used to described the flow conditions. Measurements at low flow rates for contraction ratios of 2:1, 4:1, and 8:1 indicate that the flow near the contraction plane scales with the small tube radius. A viscoelastic corner vortex is detected at moderate flow rates, yet vortex growth is not observed with increased flow rate, because inertial forces press the vortex into the outer corner. At high flow rates, where both the Reynolds number and Deborah number are large, the flow near the contraction plane is divided into an accelerating core and an outer region where the flow retains its upstream profile. The magnitude of the local rate-of-strain ▪ is calculated from the axial and radial velocity profiles. In the core, the magnitude of ▪ >50s −1 and its value is dominated by the axial gradients ∂ v/∂ z and ∂ v r /∂ z. Large extension rates along the centerline suggest that extensional thinning occurs in this core region. Finite element calculations for a Carreau-Yasuda fluid with the same viscosity as the PIB/C14 solution and under the same flow conditions do not predict the two-regions of the flow.