A PIV investigation of laminar and turbulent viscoplastic fluid flow on axisymmetric abrupt contraction

Abstract

The current work is devoted to analyzing viscoplastic fluid flow in a circular axisymmetric abrupt contraction with ratio D/d=1.85. The velocity fields and static pressure along a hydraulic loop were measured employing the Particle Image Velocimetry technique and a pressure transducers system, respectively. Four aqueous solutions of Carbopol® 940 with different rheological parameters were used as viscoplastic fluids. Laminar and turbulent flow conditions are evaluated and compared with the generalized Hagen-Poiseuille law and the Malin friction factor correlations supported by turbulent fluctuations measurements. The formation of a plug core region is observed around the centerline of the pipe. The relationships between the velocity profiles and the shear stress distribution with the plug core were investigated supported via shear rate calculations. The results show that the yield stress highly influences the shape of the velocity profiles and the appearance of unyielded regions at the contraction edges. The shear stress maps show a layered distribution across the pipe diameter with higher values located at the contraction entrance. Instead, a constant shear stress distribution is found in the unyielded regions. The size of these unyielded regions increases with the yield stress but is reduced at regions with high shear rates due to increased inertial forces. Pressure drop measurements contrasted with the shear stress and velocity gradients maps illustrated that the pressure losses along the pipe are affected by the yield stress and the Reynolds number. However, a non-dependence of the pressure loss coefficient at the contraction plane with the rheological parameters is evidenced, as presented in earlier experimental results. This differs from many numerical studies since the contraction's pressure loss coefficient for viscoplastic fluids is directly affected by the increase of inertial forces at this region.