Elastic Effects in Die Entry Flows

Abstract

The kinematics of die entry flow is a complex phenomenon in which inertia, shear-thinning and elasticity play a significant role. A clear understanding of the effect of these three features upon the flow field on the basis of experimental results (e.g. Cable and Boger, 1978) is difficult because of the practical impossibility of separating these effects in general. The discovery by Boger and Nguyen (1978) of a viscous fluid which is highly elastic at low shear rates while not shear thinning and optically clear led to a series of experimental observations of die inlet flow at very low Reynolds numbers where the sole parameter is the elasticity of the flow. Dramatic results have been reported by Nguyen and Boger (1979). At low flow rates, a corner vortex upstream of the small tube entrance remains stationary; when the flow rate (or the elastic character of the flow) increases, the vortex grows and leads eventually to various modes of unstable flow. All test fluids (glucose syrup-Separan solutions) presented a domain of shear rate associated with second-order viscometric behavior. Nguyen and Boger (1979) made the observation that the initiation of the vortex growth regime occurs at shear rates just beyond the domain of second-order behavior.