AVERAGE CONVECTIVE HEAT-TRANSFER COEFFICIENTS IN SINGLE SCREW EXTRUSION OF NON-NEWTONIAN FOOD MATERIALS

This study deals with solving the Graetz-Nusselt problem for single-screw extrusion of power law fluids with viscous dissipation along the channel. The momentum and energy equations were solved numerically for the temperature and velocity profiles, which were then used for determining the Nusselt number. Results are presented in terms of the Nusselt, Brinkman, and reciprocal Graetz number. The results indicate that the Nusselt number is relatively unaffected by the Brinkman number, for both heating and cooling. However, the Brinkman number significantly affects the bulk temperature. For Brinkman numbers less than minus 40, the rate of viscous heat generation is greater than the convective cooling capacity of the extruder. Increasing the screw compression ratio produced significant increase in bulk temperature and Nusselt number for the case of heating. The Nusselt number is less sensitive to the flow behavior index for the case of cooling, while somewhat sensitive for the case of heating. Data from the literature were compared to results from this analysis.