Exploring Flow Dynamics: A Computational Investigation of Spiral Air Jet Mills*

This work focuses on analyzing and optimizing air jet mills, which are widely used for fine-grinding powders to sizes under 10 microns. Air jet mills typically have an efficiency of 2% to 4%, so even small improvements can save significant energy. Spiral air jet mills are particularly notable for their lack of moving parts, low-temperature rise during grinding, better temperature control, minimal maintenance, and ability to classify ground material, setting them apart from other machines.

We studied gas flow in an air jet mill using CFD with OpenFOAM. The spin number, the tangential to radial velocity ratio, determines particle cut size by balancing inward drag and outward centrifugal forces. Simulations with a simpler geometry of air jet mill - wherein the entire curved surface of the outer cylinder is an inlet instead of jets. Studies on various design and operational parameters were carried out. Results showed the emergence of toroidal recirculation zones formed similar to the Taylor-Couette flow.

In the base case, simulations with the chamber's curved surface as a rotational inlet, airflow reached a steady state. Recirculation zones strengthened with higher spin numbers, concentrating maximum velocity near chamber walls and causing misclassification. The spin number showed a quadratic relationship with zone size. While laminar and turbulent streamlines differed slightly, recirculation zone enlargement was evident in both, with distinct velocity profiles. Flow patterns with jets were complex due to many dead zones inside the mill. The variation of the complex flow with the parametric variation is also studied. The study is useful in optimizing the dead zone size to balance the throughput and product size tread-off, needed for a particular application.