Dough rheology and extrusion: Design and scaling by numerical simulation

Whole wheat dough and gluten dough rheology was characterized using three nonlinear differential viscoelastic models namely the Phan-Thien Tanner, the White-Metzner and the Giesekus models. The Phan-Thien Tanner model was found to be the most suitable for numerical simulations and also predicted transient shear and extensional properties of wheat flour doughs.; Numerical simulation was conducted using Newtonian and power law models to validate Polyflow results with the literature. Modeling studies were conducted using the viscosity model of Mackey and Ofoli (1990). The flow considered was non-isothermal which required the simultaneous solution of the energy equation and momentum balance. The temperature field was dependent upon the velocity gradients in the flow due to viscous dissipation. This is a high Peclet number problem, which signifies the importance of convective heat transport in the flow. Streamline Upwinding (SU) in the energy equation was used to circumvent this problem.; Numerical simulations were then conducted using the Phan-Thien Tanner model. The Weissenberg number in the flow was 5.22 while the Deborah number was 0.001. This signifies that the flow behavior will be close to Newtonian, but it is a numerically difficult problem due to the high Weissenberg number. Higher Deborah numbers could not be reached due to lack of robust numerical algorithms.; Residence time distribution (RTD) and specific mechanical energy (SME) were the two crucial design parameters identified for scaling and design that must remain constant during scaling. The impact of screw geometry on RTD and SME was studied by generating 20 screw geometries with different geometric variables such as helix angle, channel depth, screw diameter to channel depth ratio, screw length to screw diameter ratio, and the clearance between the screw flights and barrel. Numerical simulation using the Mackey and Ofoli (1990) model for wheat doughs was conducted on all the screw geometries. A trend chart for SME showing the dependencies with the screw parameters was developed. RTD curves were obtained for all the simulations. It was found that SME and RTD was maintained constant between extruders of two sizes when the smaller extruder with smaller screw diameter and channel depth, has a higher D/H ratio and helix angle compared to the larger extruder.