(585w) Evaluation of Mixing Using Extension Rate Distribution in a Co-Rotating Twin Screw Mixer

Many food processes involve mixing to incorporate ingredients, introduce air and form desirable structural attributes. When examining mixing efficiencies, one is concerned with both distributive and dispersive mixing. One common measure of dispersive mixing is the Manas-Zloczower mixing index (λ). Although this incorporates elongational flow, there is additional information that is not captured in this parameter. Another way of examining mixing is by mapping extension rate distribution.

This work examined a nine-paddle mixer geometry with a fluid inflow rate of 55.31 cc/s and a mixer speed of 100 rpm. The material was modeled as a power law fluid using parameters previously obtained for a 2% aqueous CMC (carboxymethylcellulose) solution. Numerical simulation was performed using Polyflow (ANSYS, Inc.). Five xy planes were chosen to compare extension rate as axial distance from the mixer entrance increased.

The initial plane had significant extension rate peaks slightly left of the mixer center due to the vertical position of the left-hand paddle. Additional features seen in later planes were not evident in this location due to its position relative to the mixer entrance. Beyond the initial plane, remaining planes had similar maximum extension rate values. Greatest extension rate values were seen in the mixer center and in the region of the vertical left-hand paddle, with peaks at the upper and lower paddle tips. No gradual increase was seen along the paddle length. Maximum extension rate values were seen in areas with high shear, between the two paddles and between paddle tips and the barrel wall.

The highest extension rate values were seen between λ values of 0.4 and 0.75, a much wider range than the predicted value for efficient dispersive mixing at λ=0.7. More elongational flow did not indicate more material stretching above a certain λ value. A more equal division of rotational and elongational flow produced more stretching.