Material characterization of agricultural and industrial solutions and melts in elongational processes.

摘要

Many manufacturing and agriculture processes involve elongational flow of viscoelastic fluids. To model these processes, one must know the properties of the fluid (e.g. viscosity, relaxation time, retardation time etc.) in elongational flows. The research of this dissertation exploits a free surface rheometer that measures the free surface profiles of a filament with resolution sufficient to compute the first and second numerical derivatives of the profile, and that measures the normal stress difference at the nozzle. This rheometer allows for variation of flow rate, windup speed, and filament length. The measurements from the rheometer are combined with analysis to characterize the elongational response of the test fluid. In this dissertation a single test fluid is investigated under a range of flow conditions.;Two characterization procedures are studied in this dissertation: (i) In the first characterization procedure, seven viscoelastic constitutive models are selected, namely, single mode Oldroyd fluid-B model, single mode Giesekus model, single mode FENE-P model, modified single mode FENE-P model, two-mode Oldroyd fluid-B model, and modified two-mode FENE-P model, and three-mode Oldroyd fluid-B model. The experiments for fourteen different take-up flow conditions are performed; to examine reproducibility, one of these flows is duplicated in separate experiments. For each constitutive form, optimal material constants are determined through comparison with the experimental measurements. The dependence of optimal coefficients in the seven viscoelastic forms on the flow conditions is investigated. (ii) In the second characterization procedure, the uniaxial response of the test fluid is characterized by computing the paths of many different experiments in stress/strain/strain-rate space, and then fitting these paths to a surface in this space; thereby deducing the constitutive functional form of the elongational viscosity of the test fluid and the coefficients in this form.