Computational studies of the FENE-CR model in a two-roll mill

Computational studies of polyacrylamide based Boger fluids for the viscoela stic flow past a sphere using the FENE-CR model, have suggested that a smal l value of the dumbbell extensibility parameter and a large value of the co ncentration parameter gives reasonable agreement with experimental observat ions. However, it is not clear to what extent the FENE-CR model with these same parameter values can predict other flows. In this paper, the viscoelas tic flow predicted by the FENE-CR model in a co-rotating two-roll mill is s tudied numerically for relatively small values of the dumbbell extensibilit y parameter and relatively large values of the concentration parameter. For the co-rotating two-roll mill, the kinematics of the flow are characterize d in terms of the magnitude of thr rate of strain, the flow-type parameter, and a rotation or symmetry angle, all measured at the central stagnation p oint. For L-2 > 75, it is shown the flow-type parameter at the stagnation p oint decreases relative to the Newtonian value, in qualitative agreement wi th experimental observations on dilute solutions of high molecular weight, nearly monodisperse, polystyrene-based Boger fluids. For L-2=25, on the oth er hand, the flow-type parameter at the stagnation point increases relative to the Newtonian value. The increase in the flow-type parameter relative t o the Newtonian value observed for L-2 = 25 is a clear signature of small e xtensibility which may be used for comparison with experiments for polyacry lamide based Boger fluids. It is hoped that the present study, coupled with future experiments using polyacrylamide based Boger fluids, will further o ur understanding of the behavior of these dilute, Boger-like fluids in exte nsion dominated flows with large residence times. (C) 2000 Elsevier Science B.V. All rights reserved.