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L. YERRAMILLI and M. V. Karwe. Department of Food Science, Rutgers University, 65 Dudley Road, New Brunswick, NJ 08901

Co-rotating twin-screw extruders are used to make a variety of food products. The characteristic section of a co-rotating twin screw extruder is the kneading section where intense mixing and shearing of the ingredients occurs. The width and the stagger angle of these kneading elements determine the amount of backflow that is generated which in turn determine the nature of mixing, i.e., dispersive or distributive. Kneading blocks generate high and low-shear regions, which enhance mixing effectiveness of the extruder.

The objective of this research was to determine the velocity distribution in the kneading sections of a co-rotating, self-wiping, twin screw extruder so as to quantify the mixing effectiveness of kneading blocks.

An extruder with Plexiglas barrel was used to carry out velocity measurements. A laser Doppler anemometry system was used to measure two components of the instantaneous velocity vector at a point in the screw channel. Using this data the velocity profiles were generated. Velocity profiles in the translational region were measured at different screw speeds using a Newtonian and non-Newtonian fluid. These velocity profiles were compared with those in a forward conveying element of the same pitch. Effect of pitch and thickness of individual kneading disc was also investigated.

Near the screw root, axial and tangential velocity distributions were similar to those in a conveying element of the same pitch. However, near the barrel the axial and tangential velocity showed steeper gradients and more back flow. Near the barrel, the velocity distributions were different at different axial locations.

The results imply that in kneading blocks, average the residence time of a food particle would be more due to back flow and more mixing will occur. The geometry of kneading block would significantly affect the amount of mixing that occurs.