Rheol Acta 35:494-507 (1996) © Steinkopff Verlag 1996 Jacques Guillet Pascale Revenu Yves B6reaux Jean-Robert Clermont Experimental and numerical study of entry flow of low-density polyethylene melts Received: 24 July 1995 Accepted: 12 July 1996 J. Guillet • P. Revenu Laboratoire de Rh6ologie de Matibres Plastiques Universit6 Jean-Monnet Facult6 des Sciences et Techniques 23, Rue Paul Michelon 42023 Saint-Etienne Cedex 02, France Y. B6reaux • Dr J.-R. Clermont (~) Laboratoire de Rhdologie UMR 5520 (CNRS), INPG Grenoble and Universit6 Joseph-Fourier Domaine Universitaire BP No. 53 38041 Grenoble Cedex 9, France Abstract The present work deals with experimental and numerical features of entry flows of two polyethylene melts, namely a linear low-density polyethylene (LLDPE) and a low-density polyethylene (LDPE) in an axisymmetric con- verging geometry. The study also involves rheological characterization of the polymers and determination of flow parameters, at 160°C. For both fluids, the data are fed into a viscoelastic integral Wagner con- stitutive equation. The numerical flow simulations are performed by using a stream-tube mapping analysis. Consideration of a sub-do- main of the total flow domain, the "peripheral stream tube", close to the wall of the converging duct per- mits to relate the results of the numerical simulation to experimen- tal flow characteristics as total and entrance pressure drops. The agree- ment is good for the total pressure losses, but, concerning LDPE, a lack of consistency remains for the entrance pressure drop. Key words Polyethylene melts - entry flows - pressure drops - stream-tube methods - Wagner integral model Introduction A main objective in rheology concerns the use of con- stitutive equations to describe processing operations by numerical flow simulation. In relation to this purpose, two major problems are to be considered, namely the possibility of a theological model selected to fit the fluid properties and the efficiency of the computer program to simulate the flow. In this context, it should be underlined that the parameters involved in the constitutive equations, obtained from specific shear and elongational experi- ments, are still assumed to describe the liquid behaviour in mixed flows. Moreover, such considerations require, in the use of a constitutive equation for modelling visco- elastic flow, accurate tests for rheological characteriza- tion of the material and determination of flow character- istics in test sections of the processing equipment. These approaches are of significant interest for the description and the prediction of flow phenomena occurring in pro- cessing rheology. The numerous papers in the literature attest to the in- terest and progress of research in this field, in relation to the accuracy of characterization of complex fluids and numerical predictions expected from computer simula- tions. Several studies involved either experimental and numerical works performed simultaneously (for example Papanastasiou, Scriven, Macosko, 1987), or specific numerical flow simulations (e.g. Luo and Tanner, 1988; Baer and Finalyson, 1992; Mitsoulis, 1993). Many experi- ments have concerned polymer melts or solutions in 4:1 planar or axisymmetric contractions (e.g. White and Baird, 1986; Boger, 1987; MacKinley et al., 1991; Arm- strong et al., 1992). Though it should be emphasized that the constitutive equation chosen for any process should reflect the needs of the worker concerned and be selected to provide the information required, particular rheologi- cal models, such as the differential Phan-Thien-Tanner