Secondary Flow and Stress Birefringence Patterns in the Pressure Hole

Abstract

Measurements were taken of both flow patterns and stress birefringence patterns of viscoelastic polymeric melts in a slit channel having various sizes of transverse slots. Materials used for the experiment were polystyrene, high‐ and low‐density polyethylenes, and polycarbonate. The slit channel had two glass windows (optical grade quartz), which permitted us to take photographs of the stress birefringence patterns of the molten polymers with the aid of a polariscope. With the polariscope removed, we were able to take photographs of the flow patterns. It was observed that secondary flows occur inside the pressure holes, and that neither the isochromatic fringe patterns nor the flow streamlines are symmetric about the centerline of the pressure hole. Also, measurements were taken of flow patterns of both Newtonian liquid and viscoelastic polymer solutions in the flow channel having various sizes of circular holes. Two additional flow channels, one with a circular cross section (a cylindrical tube) and the other with a rectangular cross section (a slit flow channel), were constructed of Plexiglas. The transparency of the Plexiglas permitted us to take photographs of the flow patterns of test solutions in the flow channel as well as inside the pressure hole. Aqueous solutions of polyacrylamide at various concentrations were used as viscoelastic test fluids and glycerine was used as the Newtonian one. It was found that the fluid streamlines are asymmetric about the centerline of the pressure hole, secondary flows start inside the pressure holes at Reynolds numbers much lower than unity (NRe<1), and the severity of secondary flow increases with wall shear stress (or shear rate). In view of the fact that when the asymmetry of the stress and flow patterns about the centerline of the pressure hole prevails the existing theories may not be useful, doubt is cast on the validity of the experimental studies of Higashitani and Lodge and of Baird, claiming that their hole pressure measurements may be used for determining the normal stress differences of viscoelastic fluids. In other words, the Higashitani‐Prichard theory was not tested critically by the experimental studies of Higashitani and Lodge and of Baird.