THE FLOW OF A LDPE MELT THROUGH AN AXISYMMETRICAL CONTRACTION - A NUMERICAL STUDY AND COMPARISON TO EXPERIMENTAL RESULTS

The flow of a LDPE melt in an abrupt 10:1 axisymmetric contraction is simulated using a finite element program, and comparisons are made wit h experimental results reported by another researcher. The researcher performed his die entry experiment at a temperature of 150-degrees-C, and he used Laser Doppler Anemometry to measure the velocity field at several flow rates. He thus obtained detailed information about the fl ow field. In our numerical simulation of this experiment, we use a sep arable Rivlin-Sawyers integral constitutive equation with a spectrum o f nine relaxation times to model the fluid. We assume that the ratio o f second normal stress difference to first normal stress difference is a nonzero constant. The material is well-characterized with both shea r and simple elongational data from which we deter-mine the parameters in the constitutive equation. The general performance of our model is determined by comparing the vortex growth and entrance pressure loss for various flow rates with the experimental results reported by the e xperimentalist. We then repeat the experimentalist's detailed analysis of the flow field at a single flow rate using particle tracking. Spec ifically, particles are tracked along several streamlines and we compu te the shear and elongational rates, as well as the relative shear str ain and stretch ratios close to the die entry. The detailed experiment al data used for comparison were obtained from the measured velocity f ield. Comparisons of experimental and numerical results show good qual itative and, in some cases, quantitative agreement. From our numerical particle tracking, we also compute the shear stress, the normal stres s differences, and the invariants along streamlines. Finally, the shea r and elongational contributions to the energy dissipation and the ent rance pressure loss is determined throughout the entire domain and in various regions. We find that the majority of the contribution to the entrance pressure loss comes from regions close to the die entry. In a ddition, in regions in front of the die entry, elongational effects do minate, although shear effects are not negligible, even at high flow r ates.