A NUMERICAL INVESTIGATION OF NON-ISOTHERMAL EXTRUSION THROUGH ANNULAR DIES

Abstract

The process of non-isothermal extrusion of Newtonian fluids through annular dies is studied numerically using a finite element method; in particular the phenomenon of extrudate swell is considered. It is shown that swelling of the extrudate thickness increases with increasing Nahme–Griffith number, Na, but decreases with higher Péclet number, Pe, in qualitative agreement with an extrapolated interpretation of a previous theory. In particular a combination of high Na and low Pe values, or equivalently in the present context high sensitivity of the fluid viscosity to temperature change and narrow annular gap, can result in very large extrudate thickness swell; here a maximum swell value of about 108% relative to the annular gap size has occurred. The large swell values obtained are much higher than those reported in the literature, and show that while geometry and temperature have been recognised as having influence on extrudate swell, the extent to which their combination can affect this phenomenon can be much more than perhaps so far realised. Changes in the extrudate's outer and inner radii, on the other hand, are much smaller, with the outer radius generally following thickness swell's trend, whereas the inner radius varies in the opposite direction. The study also shows that a natural tendency for the extrudate is to swing outward. The very similar behaviour of the maximum pressure and maximum temperature of the flow field is also demonstrated.