15D-11 |
Use of CFD for design and scale-up of food extrusion |
M. DHANASEKHARAN and J. L. Kokini. Dept. of Food Science, Rutgers, The State Univ. of New Jersey, 65 Dudley Rd., New Brunswick, NJ 08901 Scaling of screw extrusion has remained largely an unsolved problem. If all dimensions of the extruder were increased by a factor a, flow and power requirements would theoretically increase by a factor of a3. However the heat transfer for heating the barrel and also the viscous dissipation of mechanical energy will increase as a2, because they are related to surface area. The objective of this work is to develop scale-up and design principles for the extrusion of foods in a single screw extruder using numerical methods. Using the numerical techniques of computational fluid dynamics (CFD) it is possible to derive fundamental dependencies between screw design parameters and operating characteristics such as residence time distribution (RTD) and specific mechanical energy (SME). The viscosity model for wheat dough as described by Mackey and Ofoli (1990) is used for the analysis. A commercial CFD code Polyflow (Fluent Inc., New Hampshire) is used to solve the fundamental flow and heat transfer equations. 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. 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 is 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. This CFD methodology can easily be extended to study a variety of food products by incorporating the corresponding viscosity models. Twin screw extruders can also be designed and scaled up by the same approach.
Session 15D, Food Engineering: Processing Technologies
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