The effect of polymer solutions on reverse flow in a pipe & vortex rings

Abstract

This thesis is focused on the effect of polymer solutions on two caricature flows, namely the instability in a pipe with reverse flow and vortex rings. These caricature flows are intended to be representative of the wall and bulk regions, respectively, of a turbulent flow in a pipe in the presence of dissolved polymers. Dissolved polymers have been observed to reduce the turbulent drag in a pipe, with the measured reduction in drag being close to 80 %. This phenomenon of turbulent polymer drag reduction has great practical utility, and is currently being used to reduce the pumping effort required to pump oil over large distances. Two competing theories, namely the viscous and elastic theories have been proposed to explain the mechanism of drag reduction. Both these theories have been shown to have merit in the prediction of drag reduction, although the underlying physics is completely different. A single theory to explain the complicated phenomenon of drag is yet to be formulated. The two caricature flows are obtained by generating a known impulse using a piston cylinder mechanism. The piston-cylinder mechanism is driven by a servo motor with a closed loop control system. A trapezoidal velocity profile is generated by the piston-cylinder mecha nism. The polymer solutions studied are aqueous solutions of polyethylene oxide (PEO) and hydrolysed polyacrylamide (PAMH). The rheological properties are varied by increasing the concentration of the polymer for the PEO solutions (PEO50, PEO100, PEO200, PEO400), and by adding a known quantity of salt for the PAMH solutions (PAMH25, PAMH2510). The shear rate dependent viscosity of these polymer solutions are measured using an Anton Paar MCR 302 rheometer mounted with the cone plate and double gap geometries. The PEO solutions show a steady increase in shear thinning behaviour with the increase in concen tration. The PAMH solutions show a drastic reduction in shear viscosity on the addition of salt (the low shear rate of viscosities of PAMH25 are an order of magnitude larger than those of the PEO solutions and PAMH2510). The relaxation time of the polymer solutions is extracted by using the Carreau-Yasuda model, and it is found that all PEO solutions and PAMH2510 have similar values of relaxation time, while PAMH25 has a relaxation time three orders of magnitude larger than the other polymer solutions