Computational analysis of techniques to determine extensional viscosity from entrance flows

Recent computational analysis of entrance flows (Mitsoulis et al. 1998) suggests that the entrance pressure drop is insensitive to large changes in steady extensional viscosity-a result that directly contradicts a large body of experimental work in this area. A re-examination of entrance flows using numerical simulations is presented in this work which shows that entrance pressure drops do depend on the steady extensional viscosity, provided the extension rate in the entrance flow is large enough. Numerical simulations are presented using both the strain thinning and thickening versions of the Phan-Thien–Tanner (PTT) constitutive model. Several techniques for extracting extensional viscosity from entrance pressure are applied to the results of these simulations. The resulting predictions of extensional viscosity are compared to the steady extensional viscosity curves predicted by the PTT constitutive model used to generate the simulated pressure drop curves. The analytical techniques examined here are shown to provide reasonably accurate estimates of the steady extensional viscosity. This work also clearly demonstrates the advantage of using variable power-law coefficients for the rheological properties, used as inputs to the analyses, to capture the extensional behavior at deformation rates below the power law region more accurately. Received: 23 July 1999/Accepted: 24 November 1999