The influence of the upper wall on the die entry flow of viscoelastic fluids

The influence of the upper wall on the die entry flow of viscoeleastic fluids was investigated experimentally. Aqueous solutions of Separan AP-30 were pushed out by a compressed gas pressure through a capillary attached to the bottom of a reservoir having a height-adjustable upper wall. The driving gas pressure, the flow rate and the pressure at the center of the upper wall were measured and the flow patterns in the entry region above the inlet section of the capillary were observed. Flow rate measurements under the condition of constant driving pressure reveal that the maximum height of the upper wall required to reduce the flow rate is much larger for viscoelastic fluids than for Newtonians, and that this effect of the upper wall becomes more conspicuous with increasing driving pressure. What is curious is that under some conditions the flow rate is larger for a moderate upper wall height than for an infinite. These phenomena are shown to be attributed to the increasing and the reducing effects of the entry pressure loss by the upper wall. The latter effect may be called a new type of pressure-loss reduction phenomenon of viscoelastic fluids. The observation of the flow and the measurement of the pressure at the upper wall center show that typical flow patterns of the die entry flow of viscoelastic fluids are responsible for the far greater pressure loss than for Newtonians, and that the entry pressure loss for a spiralling flow is a little smaller than for the other two types even at an equal driving pressure.