| Rheometers for real time measurement of the rheological properties of the fluids can greatly improve the process control and product quality control during processing. In this study, a process rheometer was designed and tested to measure the shear (viscous and elastic) and extensional rheological material functions of food doughs and polymer melts (for comparison) mostly by attaching the rheometer in-line to a laboratory model single screw extruder. A sidestream valve, to vary the flow rate through the rheometer at a fixed screw speed during the extrusion of the food dough, was used to obtain reliable rheological data. Slit and cylindrical geometries were examined to obtain the flow curves of the fluids. Exit and hole pressure methods were employed to estimate the first normal stress difference. Entrance pressure drop method to estimate the extensional viscosity using the analyses given by Cogswell, Binding and Gibson were examined.;Commercial low density polyethylenes (180;Good predictions of the shear viscosity were obtained using slit and cylindrical geometries. The hole pressure method gave better predictions of the first normal stress difference than the exit pressure method. Estimations of experimental errors in the hole pressure method showed that the errors were less than 10%. The accuracy of the prediction of the extensional viscosity by the entrance pressure drop method could not be determined, even though they seemed to give good qualitative predictions. Cogswell's analysis is the simplest and best. Complex changes occurring during extrusion of foods did not permit an easy interpretation of the effect of microstructural changes on the rheological measurements. |
