15D-6 |
Modification of a pilot scale mixing device for average shear rate approximation |
T. A. GLENN, III and C. R. Daubert. Dept. of Food Science, North Carolina State Univ., Box 7624, Raleigh, NC 27695-7624 Mixer viscometry allows food engineers to characterize the viscosity of a fluid material during a mixing process. Due to the complex nature of many mixing processes, where shear fields are difficult to measure, a defined mixer viscometry constant can approximate average shear rates within a mixing vessel. Average shear rates are calculated as the product of this mixing constant and the impeller speed. The objective of this investigation was to determine mixer constants for a pilot scale, 2-gallon, twin axial ribbon blender. The blender was modified with a torque transducer and optical speed sensor installed in line on the drive shaft as driven by a ¾ hp electrical motor. Torque responses and shaft speeds were collected with a data acquisition system coupled to the measurement device. Impeller speed ranged between (0.5 - 10.5) rad/s, and torque responses ranged between (10 - 200) in-lb. Three CMC solutions with weight percentages of 2.0, 3.0, 4.0% respectively were used to examine pseudoplasticity, and one corn syrup solution was used as a Newtonian standard. Rheological properties of the working fluids were determined using a laboratory rheometer. Following a mixer viscometry concept, the Slope Method, a relationship between torque response, fluid properties, and impeller speed was used to evaluate average shear rates, while mixing. Plots of average shear rate vs. impeller speed revealed mixer constants ranging between 1.59 - 6.85 1/rad. The results of this investigation can be used to evaluate the viscosity of a fluid during a mixing process for this particular mixer configuration.
Session 15D, Food Engineering: Processing Technologies
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