A CFD model with free surface tracking: predicting fill level and residence time in a starve-fed single-screw extruder

Mixing in extrusion is a vital part of achieving consistent and high-quality extrudates, with residence time being an elucidative measure of the mixing performance. Recent studies around numerical modeling of residence time distributions in single-screw extruders appear to consider flooded extruders mainly. This paper introduces a new and general CFD model to characterize the extruder fill length and residence time distribution for a viscoplastic ceramic material in a starve-fed extruder, including free surface tracking. The CFD model simulates a pulse-injection test, where a fluid parcel is injected at the inlet, with subsequent outlet concentration measured over time. The study includes material characterization and model validation based on laboratory tests. Results quantify the impact of accounting for the partially filled extruder instead of assuming it to be flooded, addressing the potential error when only considering simple analytical approximations to calculate system average residence times. Results further show the ability to fit simulation results to more simple analytical models. This underlines the importance of including the entire extrusion system and forming the basis for further work toward enabling real-time model predictions in starve-fed extrusion systems.

Automated summary

Mixing in extrusion is crucial for achieving consistent and high-quality extrudates, and residence time is a key measure of mixing performance. This study introduces a new CFD model that characterizes the extruder fill length and residence time distribution in a starve-fed extruder, including free surface tracking. The model is validated through laboratory tests and shows the impact of considering the partially filled extruder, rather than assuming it to be flooded, on residence time distribution. The results demonstrate the ability to fit simulation results to simpler analytical models, highlighting the importance of including the entire extrusion system for accurate predictions in starve-fed extrusion systems.