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Experimental studies of visco-plastic lubrication Huen, Chun Kit

Abstract

A multi-layer flow of two purely viscous fluids is often unstable, at any Reynolds number, however small. The primary instability initiates at the interface through various mechanisms such as the Kelvin-Helmholtz instability. Here we demonstrate how interfacial instabilities of multi-layer flows can be eliminated by introducing a yield stress lubricating fluid. The basic flow geometry is that an inner core fluid flows in the central layer lubricated by a yield stress fluid adjacent to the wall of a pipe. Rheologies of the two fluids are such that the interface between fluid layers is surrounded by an unyielded layer of the lubricating fluid. In this thesis, first we model a typical multi-layer flow in a circular pipe geometry by solving the axisymmetric Navier-Stokes equations. A simple control model is derived for controlling the position of the interface by varying the flow rates of the two fluids. Second we present experimental results of a visco-plastic lubrication flow, where the lubricating fluid is a Carbopol solution and the inner core fluid is a Xanthan solution. After conducting over 100 experiments with 4 different fluid pairs over broad flow rate range, we can unequivocally state that stable multi-layer flows, of the visco-plastic lubrication type, can be achieved in a laboratory setting. The experiments have been designed with the aid of the control model which predicts the boundary between stable and unstable flows. Our experimental results do approximately identify this stability boundary. Results also strongly suggest that it would be possible to control the interfacial radius on-line, via regulation of the individual flow rates, in such flows.

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