Bulent Yesilata

EFFECT OF VISCOUS DISSPITATION ON POLYMERIC FLOWS BETWEEN TWO ROTATING COAXIAL PARALLEL DISCS

The polymeric (viscoelastic) flow between two rotating parallel discs (plates) has been studied. In the first section, the current state of the art is discussed and the limitations resulting from the simple flow assumptions are reported. As an attempt of considering the effects of finiteness of the plates and viscous dissipation, the flow problem is fully formulated; that is no assumptions are made on three-dimensional and nonisothermal nature of the flow. One of the major contributions to the formulation is to use a nonisothermal constitutive equation, so called nonisothermal Oldroyd-B equation. In this case, the numerical solution scheme of resulting equations is too complex, even for Oldroyd-B model that is considered one of the simplest constitutive equations for polymeric flows. Instead, the problem is reduced to one-dimensional nonisothermal infinite plate problem and viscous dissipation effect is then investigated. It is obtained here that viscous dissipation effect causes in significant temperature rise in the flow field and may yield errors in prediction of material properties of highly viscous and elastic fluids

Non-isothermal viscoelastic flow through an axisymmetric sudden contraction

Abstract Nonlinear dynamics of non-isothermal viscoelastic flow in an axisymmetric sudden contraction geometry are studied for a highly elastic polyisobutylene (PIB) based polymer solution (referred to as PIB-Boger fluid). Temperature gradient in the flow is generated by the external heating or cooling imposed at the boundaries of the system. The critical conditions for the onset of the elastic instabilities at different wall temperatures are determined by accurate instantaneous measurements of pressure drop across the contraction plane and temperature measurements in the flow. These results along with flow images by streakline photography are presented for various values of wall-to-fluid temperature differences and Deborah numbers (dimensionless flow rates). It is shown that the thermal effects have profound consequences for the onset, development, and structure of the vortical flows in viscoelastic entry flows. Thermal instabilities are not detected in these flows for the range of flow rates and wall temperatures used in the present observations.

Pressure measurements in highly viscous and elastic fluids

Pressure measurements in flows of highly viscous and elastic fluids are of practical importance in polymer processing and rheology systems. Special problems arise during such pressure measurements. High fluid viscosity results in excessive dynamic response time (rise time) of the pressure measuring systems. This is true for systems that consist of manometers as well as pressure transducers attached to the base of a small hole at the wall. We model the dynamic response and examine related disturbing effects. These systematic errors in pressure measurements include hole-pressure effects, instabilities in cavity flow, and the time lag of the disturbance wave

Nonlinear dynamics of a highly viscous and elastic fluid in pipe flow

Nonlinear dynamics of the inertialess viscoelastic flow in pipe are studied for a highly elastic polyisobutylene (PIB) based polymer solution (referred to as PIB Boger fluid). It is shown that for flow rates above the criticality the steady pipe flow bifurcates to a time dependent flow. Instantaneous pressure measurements at various axial locations imply that there is a series of flow transitions. The critical conditions for the onset of these elastic instabilities and the temporal structure of the resulting secondary flows are determined for a large range Deborah number.

Temporal Nature of Polymeric Flows Near Circular Pipe-Exit

Polymeric (viscoelastic) fluids have nonlinear mechanical properties and therefore the flow of a sufficiently elastic polymer solution may become irregular even at very low Re. Experimental evidences of these flow irregularities occurred near pipe exit at Re << 1 are presented here for Poiseuille flow of a highly elastic polymer solution. Although the Re is extremely low, the measured instantaneous pressures show turbulent-like fluctuations induced by elasticity of the fluid. Temporal nature of these fluctuations is analyzed in terms of Deborah number (De) that provides an appropriate measure for degree of nonlinearity in a polymeric flow.

TEMPERATURE INDUCED VISCOSITY VARIATION IN A POLYMERIC FLOW

The object of this study is to explore the local viscosity variations induced by temperature gradient in a polymeric (viscoelastic) flow. The flow is in a circular contraction geometry and temperature gradients are due to imposed heating and cooling through the walls of the flow geometry. Constant wall temperature is maintained during the experiments by circulating heating/cooling water through the rectangular box surrounding the test section. The measurements are performed for various values of the wall temperature, T w = 285, 305, and 327 K. The inlet temperature of the test fluid (To = 296.5 K) is held constant during the experiments. Significantly large viscosity stratifications induced by temperatures near the contraction plane are obtained. At high flow rates, in the vicinity of re-entrant corner, time-periodic fluctuations in the local fluid temperature are observed