Chapter 3 - Study of the Dynamics and Size Distributions of Air Bubbles During Mixing in a Continuous Food Mixer

Abstract

Bubble formation and breakup during the mixing of viscoelastic food materials such as wheat flour dough and extrusion of food materials hold the key to the formation of a foam structure which delivers ultimately foam-like product. As a starting step, bubble breakup dynamics was studied in a Readco 2” continuous processor (a twin-screw mixer) during the continuous mixing of a viscous Newtonian fluid material. The critical Capillary Numbers (Ca) for bubble breakup were estimated for the shear and extensional flow regions in the mixer using the equations of Khakhar and Ottino (1986). Experimental bubble size distributions were obtained through image capture and analysis for three different paddle element configurations of the mixer. The Capillary Numbers and the Mixing Indices (flow type) in the mixing region were estimated from finite element (FEM) simulation of the mixing process. The trend in the size distributions of the bubbles agreed with the breakup criterion predicted by the estimation of capillary numbers in the mixer from the FEM simulation of the mixing. This study opened up a possibility of predicting the final bubble size distributions for a set of mixer parameters, from the knowledge of Capillary Number distributions along the mixer.