J. Non-Newtonian Fluid Mech. 111 (2003) 41–61 Entry flow of LDPE melts in a planar contraction Evan Mitsoulis a,∗ , Martin Schwetz b , Helmut Münstedt b a School of Mining Engineering and Metallurgy, National Technical University of Athens, Zografou, Athens 157-80, Greece b Institute of Polymer Materials, University Erlangen-Nürnberg, D-91058 Erlangen, Germany Received 10 September 2002; received in revised form 22 December 2002; accepted 3 January 2003 Abstract Experiments using laser–Doppler velocimetry (LDV) and numerical simulations have been undertaken for the creeping entry flow of two rheologically well-characterised low-density polyethylene (LDPE) melts in a 14:1 planar contraction. LDV measurements of the flow reveal a distinct behaviour between the two melts regarding vortex formation. The flow of the melts has been modelled using an integral constitutive equation of the K-BKZ type with a spectrum of relaxation times, modified to properly account for strain hardening in the planar extensional flow relevant for the geometry used. Numerical values for the constants appearing in the equation have been obtained from fitting dynamic data (G ′ and G ′′ ) as well as data for the viscosities and normal stresses as measured in shear. Elongational data were derived from transient and steady uniaxial elongation experiments. The experiments show that for one LDPE melt the vortex keeps small with increasing flow rate, while for the other there is a vortex growth in the same range of flow rates. The numerical simulations also predict the same trend in agreement with the experiments. The differences in flow behaviour of the two branched melts are found to be attributed to big differences in the average relaxation times, giving rise to different De numbers or dimensionless stress ratios S R , for the same range of flow rates. Equally important is the more pronounced extensional strain-hardening behaviour of the vortex-producing polymer, giving rise to high Trouton ratios T R . © 2003 Elsevier Science B.V. All rights reserved. Keywords: Planar entry flow; Polymer melts; Vortex growth; Extensional viscosity; Viscoelasticity; Integral constitutive equations 1. Introduction In the continuing effort to better understand the behaviour of polymer solutions and melts in flow problems and how this behaviour is related to rheology, multifaceted efforts have been undertaken by several researchers worldwide. Regarding flow experiments, flow channels have been constructed and ∗ Corresponding author. Fax: +30-210-772-2251. E-mail addresses: mitsouli@metal.ntua.gr (E. Mitsoulis), polymer@ww.uni-erlangen.de (M. Schwetz), polymer@ww.uni-erlangen.de (H. Münstedt). 0377-0257/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0257(03)00012-0