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 with 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 flow field. In our numerical simulation of this experiment, we use a separable Rivlin-Sawyers integral constitutive equation with a spectrum of nine relaxation times to model the fluid. We assume that the ratio of second normal stress difference to first normal stress difference is a nonzero constant. The material is well-characterized with both shear 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 experimentalist. We then repeat the experimentalist's detailed analysis of the flow field at a single flow rate using particle tracking. Specifically, particles are tracked along several streamlines and we compute the shear and elongational rates, as well as the relative shear strain and stretch ratios close to the die entry. The detailed experimental data used for comparison were obtained from the measured velocity field. Comparisons of experimental and numerical results show good qualitative and, in some cases, quantitative agreement. From our numerical particle tracking, we also compute the shear stress, the normal stress differences, and the invariants along streamlines. Finally, the shear and elongational contributions to the energy dissipation and the entrance 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 addition, in regions in front of the die entry, elongational effects dominate, although shear effects are not negligible, even at high flow rates.