Two Dimensional Flow Simulation in Twin Screw Food Extruder Die

Abstract

Food extrusion process involves heat and flow distribution. An accurate knowledge of flow pattern in food extruder die is essential for optimum extrusion operation. Limited works are available on simulation and modeling of heat and flow in food extruder. No reported work is available for flow modeling in twin screw food extruder die. In this study, two-dimensional flow simulation in food extruder die for intermeshing co-rotating twin-screw extruder were performed by solving Navier-Stokes equation and continuity equation for non-Newtonian fluid using finite element computer package, ANSYS/FLOTRAN. The objective of the study is to determine the nature of flow, heat and pressure distribution in the die and to determine the effect of screw speed on process parameters such as temperature, pressure and flow rate in the die. Four different die geometries were used. The levels of temperature considered were 120, 140, 160 and 180 ⁰ C (393, 413, 433 and 453 K). The screw speeds (taken as inlet velocity for the die) were 0.375, 0.5, 0.625 and 0.75 m/s (120, 160, 200 and 240 rpm). The results are presented for the flow profile, pressure distribution, temperature distribution and flow rate. For all the velocities considered, temperature has no significant effect on the velocity vector. The concentration of the vectors increase as the cross-sectional area becomes smaller. The vectors are relatively linear and smooth in the transition section of the die and get concentrated towards the die exit. The flow rate increases with increase in inlet velocity. Extruder throughput has a significant effect on the flow rate as reflected in higher flow rate recorded for increased throughput. Dough temperature has no effect on flow rate within the temperature range of 120 ⁰C to 180 ⁰C. The temperature of the dough decreases as it flows through the die. The values of temperature obtained for die exit temperature by simulation and experiment compare favorably. This shows that the simulation procedure employed is reliable enough to accurately predict the temperature of the dough at any point in the die. With increased inlet velocity, minimum temperature in the die decreased. The temperature differences between maximum and minimum values obtained are 37 ⁰C, 28 ⁰C, 132 ⁰C and 134 ⁰C for dies 1, 2, 3 and 4 respectively. The pressure increases with screw speed. The pressure at the die exit is lower than the highest pressure obtained for all the experimental runs. As the dough gets to the die exit, the dough experienced pressure drop. A pressure drop of more than 0.8 MPa between the entrance and exit of the die was obtained.