Start-up flow visualization of viscoplastic materials in horizontal pipes using particle image velocimetry

Abstract

The flow of viscoplastic materials is widely found in various industrial sectors, such as polymer processes, food pumping, paraffin oil industry, among others. In shutdown situations due to emergency or maintenance, the material at rest exhibits a gel-like structure after a period of time. In order to restart the flow high pressures are applied which result in a overestimation of the dimensions of the pipeline and the production costs. Therefore, an accurate prediction of the minimum pressure to restart the flow is necessary as an operational requirement. The objective of this experimental study is to investigate and visualize the start-up flow of yield stress materials in pipes. For a better understanding of the flow dynamics of these materials, a test section of length 0.980 m with a circular cross-section of inner radius 0.011 m attached to two reservoirs to control the inlet and outlet pressure and Two-dimensional Particle Image Velocimetry (PIV) visualization system is used as an experimental setup. Two aqueous solutions of Carbopol® (25 and 50 wt%) are used as viscoplastic fluids. The velocity profiles, the presence of plug structure and the solid-fluid transition are analyzed before the fluid begins to yield. A relationship between the minimum start-up flow pressure drop and velocity field evolution is observed for the different solutions. Moreover, the time required to restart the jammed line depends mainly on the applied pressure drop and the viscosity that decreases with time. Finally, it was observed that the slip velocity increases in an exponential way with the wall shear stress and velocity gradients.