Numerical investigations of flow and heat transfer of polymer melt in underwater extrusion pelletizers

•3D CFD calculations of non-Newtonian flow of a polymer in an extruder are conducted.•The turbulent flow of heating oil and conjugate heat transfer through die are modeled.•Model is validated by comparing to measurements from an industrial-scale pelletizer.•Studies demonstrate the utility of flow characteristics in predicting freeze-off. The thermoplastics compounding industry extensively utilizes underwater die-face pelletizers, where, the polymer melt is extruded through a capillary, then cut, stress-relaxed and cooled by a medium, which all have a significant impact on the final pellet quality. Some of the common practical problems in these applications are referred to as (1) “die-hole freeze-off”, due to the low temperatures and (2) “pellet marriages”, due to the polymer being too soft. In order to address some of these issues, three-dimensional (3D) computational fluid dynamics (CFD) calculations of non-Newtonian flow of a polymer in an extruder, along with the turbulent flow of heating oil and the conjugate heat transfer through the die are carried out using ANSYS Fluent. The computational model is validated by comparing predictions of a probed temperature in the die plate, and pressure drop within the polymer, to experimental measurements in the industrial-scale pelletizer, for five different operating conditions, with the maximum resulting error being 225∘C and mass flow rates 12 kg/hr.