Experimental investigation of instabilities in Newtonian and viscoelastic Taylor-Couette flows

This dissertation reports on the stability of Newtonian and viscoelastic fluids in the model problem of flow between concentric, rotating cylinders (Taylor-Couette flow, or TC flow). In particular, the focus of this research has been on the effects of viscous heating on the flow stability. Viscous heating is present in polymer processing applications due to the high viscosity of polymer melts.; We have discovered a new instability in the TC flow of Newtonian fluids, consisting of axisymmetric toroidal vortices that are oscillatory and travel slowly down the vertical axis of the TC cell. This instability is caused by a coupling of centrifugal destabilization and viscous-heating-induced gradients in viscosity. The time for onset of instability is dependent on the Peclet number and the onset times observed are consistent with the thermal time scale; i.e., the time for a fluid under shear to attain a steady temperature profile under the influence of viscous heating.; We also report on the effects of viscous heating on the hydrodynamic stability of viscoelastic fluids in TC flow. For viscoelastic fluids, the onset times for instability were found to be roughly consistent with the thermal time scale. Ramp tests were used to explore the critical conditions for a wide variety of flow conditions. These tests reveal nonaxisymmetric, oscillatory disturbance flows at low Peclet number with critical Deborah numbers of ∼10.6 that are insensitive to changes in Pe. The form of the disturbance flow is consistent with isothermal linear stability predictions for an Oldroyd-B fluid.; The work described in this thesis has considerable implications regarding the stability of flows of polymeric liquids because significant viscous heat generation occurs in the processing of polymeric materials due to the high viscosity of polymer melts and the high shear rates present in processing equipment. The experimental results demonstrate that viscous dissipation is a key factor in the hydrodynamic stability of highly viscous fluids and it is extremely important to consider this factor in hydrodynamic stability calculations and experiments. (Abstract shortened by UMI.)...