Date of Seminar	Your Name
Department of Food Science	Thesis Advisor:
Final Ph. D. Seminar

TITLE

A major challenge in the food industry is to scale up or identify an alternative geometry for a dough mixer, while maintaining a well mixed final product with consistent rheological character. Of particular interest to the food industry is the change from a batch to a continuous dough mixing operation, which exceptionally challenging because of the extremely different flow and mixing patterns encountered. The change could be accomplished more easily with accurate estimates of mixing parameters that could be used to find similar mixing profiles. Numerical simulation using particle tracking provides a potential tool for understanding the mechanism and effectiveness of mixing for opaque materials in a given mixer. Therefore, the main objective of this work is to gain better understanding of the flow and mixing in model batch and continuous dough mixers using numerical simulation.

The flow simulations are done using finite element method (FEM) with Galerkin’s method for purely viscous models and mixed Galerkin formulations with the streamline upwind (SU) method (Marchal & Crochet, 1988) for differential viscoelastic models (Connelly & Kokini, 2000). The effects of shear thinning and viscoelasticity are explored using models in the simulations that include a constant viscosity Newtonian model, a shear thinning, Bird-Carreau model and a shear thinning, differential viscoelastic Phan-Thien Tanner model with parameters that give dough-like responses in the conditions modeled. The co-rotating twin screw mixer is modeled using a variation of FEM based on a modification of the fictitious domain re-meshing technique of Bertrand et. al (1997) that compensates for the paddle motions using the viscous Bird-Carreau dough flow model from Dhanasekharan et. al (1999). Using the calculated flow profiles, trajectories for material points with random initial positions are calculated. The mixing of the particles is analyzed statistically using the segregation scale and dispersion index. Efficiency of mixing of the particles is evaluated using the lamellar model of Ottino (1989) for analysis of mixing.

As reported in Connelly & Kokini (2001), “the main effect of the rheology on the mixing in the single screw mixer is to alter the period of the circulation in the secondary flow pattern. Dispersion of a centralized clump is improved by viscoelasticity due to asymmetry in the flow pattern, but the ability to distribute material between the upper and lower halves is impeded. The time averaged efficiency in the single screw mixer is near zero, with viscoelasticity actually causing more energy to be dissipated by shortening rather than lengthening material lines over time. The overall mixing ability of the twin screw mixer is much better than the single screw mixer due to the folding in the overlap zone, with the length of stretch increasing exponentially and leading to positive efficiency values over time. However, dispersion of points initially located in a centralized clump on the left side is poor, indicating a dead zone in the twin screw mixer. The results provide insight into the mixing mechanisms in mixers of complex geometry for highly viscous and viscoelastic materials such as dough, allowing more effective design of dough mixers.”

References:

Bertrand, F., Tanguy, P.A. and Thibault, F. 1997. A three-dimensional fictitious domain method for incompressible fluid flow problems. Int. J. Numer. Meth. Fluids 25:719-736.

Connelly, R.K. and Kokini, J.L. 2000. 2-D numerical simulations of the flow and mixing of viscoelastic fluids in a continuous mixer. Proc. XIII Euro. Int. Congr. on Rheology, Cambridge, U.K., (Aug., 2000), 2:158-160.

Connelly, R.K. and Kokini, J.L. 2001. Analysis of mixing in a model dough mixer using numerical simulation with particle tracking. Proceedings of the Seventh Conference of Food Engr., A Topical Conference of the AIChE Annual Meeting, Reno, NV, Nov. 4-9, 2001. Pgs. 579-585.