THE STUDY OF THE RHEOLOGICAL PROPERTIES OF POLYMERIC SOLUTIONS IN STEADY SHEAR FLOW USING A SLIT RHEOMETER

ROBERT MICHAEL YBARRA, Purdue University

Abstract

In this research, an experimental and theoretical study of viscoelastic liquids in plane Poiseuille flow was conducted. This seldomly explored geometry has profound advantages in measuring both shear and normal stresses throughout the fundamentally and industrially important regions of shear. Newly developed theory (Davies et al., 1973) has been examined experimentally over a wide range of deformations for a number of rheologically different fluids (glycerol and aqueous solutions of polyethylene oxide). A functional dependence of exit pressure on wall shear stress was developed for viscoelastic fluids. In the limit of low Reynolds number, a quadratic dependence of exit pressure was postulated, that resulted in a simple expression for the primary normal stress difference in terms of exit pressure N(,1) = 3p('(HL)). Experimental data on four rheologically different fluids confirmed the quadratic dependence of p('(HL)) on (tau)(,w) at low shears. At high Reynolds numbers where inertial terms are dominant, N(,1) can be estimated using White and Metzner's (1963) momentum equations. For highly elastic fluids and fluids whose inertia was too low to produce a jet with measurable thrust, slit flow also permitted estimation of N(,1). Thus, the use of a slit rheometer offers potential for evaluating normal stresses over a much wider range of shear rates than previously thought. The upper limit on shear rate for the rheological measurements of polymeric liquids was fixed by the onset of an unstable flow--solution fracture. The inception of this phenomena was dependent to a smal extent on die geometry (plate spacing) and polymer concentration, but appears to occur for our polymer solutions at lower (tau)(,w) than has been previously reported (Petrie and Denn, 1975). The design of our rheometer allowed coverage of a wide range of shear rates in our experiments. The lower shear regions overlapped upper shear rate limits of the rotational devices, thereby allowing the possibility of future comparisons of shear and normal stress data taken on the slit rheometer with data obtained by rotational rheometers. The flush mounted transducers incorporated in our slit rheometer eliminated any hole errors and admitted a direct measurement of the viscosity function as well as the total transverse normal stress T(,22). For the viscoelastic fluids examined in this study, the total normal stress components T(,11) and T(,22) were found dependent on the rheometer's plate spacing. Exit pressures were also found for Newtonian fluids at low Reynolds numbers. The dependence of exit pressure for the Newtonian fluids was entirely different from that of polymeric liquids. This result implies that the mechanism responsible for extrudate swell of Newtonian fluids at low N(,Re) differs from the mechanism causing swell in macromolecular fluids. A solution to the energy equation for power law fluids undergoing viscous dissipation was provided to predict the onset of significant deviation from isothermality. This effect was found to be minimized by employing thin slits.

Degree

Ph.D.

Subject Area

Chemical engineering|Energy

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