Investigating Rheological Techniques to Model and Predict Operating Conditions of a Single Screw Extruder with Internal Restrictions
Understanding rheology of raw materials and the numerous transformations they undergo is an essential aspect of modeling and predicting extrusion conditions. Both off-line capillary rheometry and in-line extruder fed rheometry techniques have been used to model rheology of materials during extrusion. Investigations on use of an off-line capillary rheometer to model apparent viscosity of full fat soy flours revealed inability of the capillary rheometer to handle food/biological materials high in oil content (20%) at high temperature (80°C) and low moisture content (15% wet basis). The lack shear degradation in the capillary rheometer also resulted in over-prediction of corn flour viscosities at high temperatures (80 to 120°C), shear rates (1 to 100 s-1) and low moisture contents (35 to 40%). A novel two-opening die attached to a small scale extruder operating at 100 to 300 rpm was used to measure in-line viscosities of cornmeal at 32.5 to 37.5% moisture during extrusion. Comparison of off-line versus in-line cornmeal viscosities at similar conditions consistently indicated that for food/biological materials sensitive to shear degradation, in-line viscosities were lower. Pasting property measurement revealed differences in material transformation which could be used to explain differences between techniques. Mechanistic model for the small-scale extruder with screw restrictions was built by considering a modified plane Couette flow setup with varying gaps and heat fluxes leaving at both plates, for one screw, one restriction and a die setup. The model successfully predicts the effect of screw restriction gap, extrusion pressure, viscous dissipation and heat fluxes.
Okos, Purdue University.
Food Science|Agricultural engineering
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