Flow of viscoelastic fluids through a rectangular duct

Abstract

A study is carried out on the flow of polymer melts in a rectangular duct. As the theoretical study a three‐constant Oldroyd model is used to derive expressions which correlate the rheological properties of materials with the distributions of wall shear rates and wall normal stresses in the rectangular duct. As the experimental study, a die of rectangular cross section having an aspect ratio of 6 is designed, and then melt extrusion experiments are performed with high density polyethylene. In the experiments wall normal stresses are measured along two adjacent walls of the rectangular duct as a function of the axial position. The measurements permit one to obtain the normal stress differences at the duct exit, and then to calculate the distributions of shear rates at two adjacent walls of the rectangular duct, by use of the theoretically derived expressions. Also measured is the extrudate swell, showing trat more pronounced extrudate swell occurs at the long side of the rectangular duct than at the short side. This behavior of extrudate swell correlates with the exit pressure measurements at two adjacent walls of the rectangular duct.

On intrinsic errors in pressure‐hole measurements in flow of polymer melts

“…In most cases, such pressure measuring devices are connected to small pressure-holes. These holes are drilled through the wall normal to the surface mainly because the sensing device is much larger than the capillary diameter /' 10 ( 2 ) Tanner and Pipkin ( 3 ) then presented an analysis using the second-order approximate constitutive equations which supports their experimental results. It should be noted here that the analysis of Tanner and Pipkin is strictly restricted to flows so slow that the second-order approximate constitutive equations are valid.…”