Going with the Flow : Computational Fluid Dynamics Simulation of Dough Mixers

Wheat flour dough is a rheologically complex viscoelastic material, whose unique time-dependent properties depend on the quality of the flour, the available moisture, and the extent to which the dough is mixed. The rheological properties developed in dough during mixing are governed by the rate, amount, and type of deformation applied. As a result, dough mixers have evolved into highly complex geometries that shear, stretch, and fold. In addition, they often have close wall clearances to ensure that there are no regions of ineffective mixing. Changing between mixer types, especially between batch and continuous mixers, is difficult because of the very different flow, shear, extension, and mixing profiles that characterize each one. These differences can have an effect on the mixing time as well as an effect on the gluten structure developed in the dough, which can lead to differences in the final product texture and quality. In industry, determining mixing times and designing mixer configurations is largely done on a trial and error basis. The farinograph-style twin sigma blade mixers and mixograph-style planetary pin mixers are two common devices used for assessing flour properties during mixing. Both mixers provide empirical measurements related to the rheological properties and energy input during mixing of flour and water dough. They are used to determine flour quality and moisture absorption, despite dissimilar geometries and mixing actions. The results are also frequently used as a guide for determining the mixing requirements of flour in fullsize industrial mixers. Because of their importance in the cereal industry, a complete understanding of the flow and mixing action of these particular mixer styles is needed. Computational fluid dynamics (CFD) simulation has provided a means to more fully develop that understanding.