Numerical simulation of the transport processes arising in a single screw extruder for non-Newtonian fluids such as plastics and food is considered. The study is directed mainly at simulating the heat and mass transfer inside the screw channel, for a power-law fluid. Moisture is taken as the species for mass transfer, since it is of particular interest in food processing. Finite difference computations are carried out to solve the governing set of partial differential equations for the velocity, temperature and moisture fields, over a wide range of governing parameters. The variation of viscosity with temperature and moisture is taken into account. The basic physical approach for modeling the complicated heat and mass transfer processes for inelastic non-Newtonian materials is outlined. As an application of this analysis to starchbased food systems, the reactive nature of the food constituents is incorporated by including the rate of reaction (gelatinization) between moisture and food. Results are presented in terms of the velocity, temperature and moisture concentration contours. Strong viscous dissipation effects are found to arise for typical operating conditions. The moisture contours indicate that starch gelatinization typically occurs first at the screw root. The effect of the various governing parameters on pressure build-up within the extruder channel is determined and discussed in terms of the underlying physical processes. Experimental validation of some of the numerical results has been carried out, and the comparisons are quite good. Results are also presented for the residence time distribution (RTD), an important design parameter in extrusion. Though this study considers moisture transport, the basic approach presented may easily be extended to reactions and mass transfer of other species in polymers and in other non-Newtonian materials.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
- Mechanical Engineering
- Fluid Flow and Transfer Processes