Efficient simulation of time-dependent flows: Application to a twin screw extruder

An efficient, versatile technique was developed for detailed simulation of flows in complex processing equipment with multiple moving parts. It uses a suitably modified finite-volume CFD simulation algorithm to deal with the problems posed by the relative motion of moving parts in processing equipment. A fixed grid is used to discretize the mass and momentum transport equations for the contents of the entire equipment volume. The motion of the solid moving parts within this domain is accounted for by assigning velocities to nodes of the grid representing the location of these parts at any moment of time. The technique was first validated for a single screw extruder by comparing the results predicted by conventional techniques involving body fitted mesh and a rotating frame of reference. Because of its inherent nonbody fitted mesh characteristics, the fixed grid method resulted in a rough, time-dependent boundary for the moving parts. The extent of error arising from this artifact was also evaluated. The benefits of this technique were demonstrated by simulating complex three-dimensional (3-D) flow in an industrially important processing equipment with multiple moving parts—an intermeshed counterrotating twin screw extruder. The time required for geometry generation and meshing using this approach was very small, and the relevant geometric parameters including those associated with the moving parts can be varied by simply modifying values in the analytical expressions used to define them. This technique does not require recreating or remeshing the entire geometry, thus making it easy to optimize the design of complex equipment via quick parametric studies.