(419d) Complex Flow Patterns and Their Impact on the Performance of the Spiral Air Jet Mill

The focus of the present work is the analysis and optimization of an air jet mill, which is a versatile equipment used in many industries for fine-grinding powders to sizes less than around 10 microns. The efficiency of the grinding process in an air jet mill typically varies from 2% to 4%, depending on the design and operating parameters so even small efficiency improvements can result in significant energy savings [1]. Amongst many of the available machines, spiral air jet mills are attracting attention because of their advantages like absence of the moving parts and low-temperature rise during grinding, resulting in better temperature control with minimal maintenance. Spiral air jet mills boast an exciting ability: not only can they grind feed particles, but they can also classify the resulting material as a product. This distinctive feature truly sets these machines apart.

From the impetus of the literature, as a first step, we study the gas flow in the mill using Computational Fluid Dynamics (CFD) with OpenFOAM and actual machine behavior through experiments. The spin number is the ratio of tangential velocity to the radial velocity of the flow. A particle inside the jet mill experiences the drag toward the outlet in the radially inwards direction and centrifugal force in the radially outward direction. A balance of the two forces defines a cut size of the particle at which both the forces are equal and opposite in direction. As per the resultant equation, the cut size is inversely proportional to the spin number, assuming other variables to be constant. Thus, as the system is dilute of particles, getting the flow behavior of a single phase helps to predict the cut size variation.

Simulations with variable gas flow rate (Q), classifier height (ht), classifier radius (rt), rotation (RPM) of the peripheral surface, the chamber sizes (R), the chamber heights (H) of the machine, fluid kinematic viscosity and the machine scale-up. Only one parameter was varied at a time keeping others unchanged from a base case, for univariate analyses. We have found that the spin number decreases with a rise in input gas flow rate (Q) and with falling peripheral surface rotation (RPM), increases with higher chamber radial size and lower classifier radius, and the spin number is not affected significantly by chamber height and classifier height. As per the simulations, the machine spin number drops more steeply for higher fluid viscosities. On the experimental front, the runs with different chamber sizes were carried out. It was found that larger diameter chambers yield smaller product particles, the findings of which are by simulation results (higher spin number yielding smaller product cut sizes).

In base case simulations wherein the entire curved surface of the chamber is a rotational inlet, the airflow was observed at the steady state of the machine dynamics. It was observed that there is a presence of recirculation zones getting stronger with an increased spin number. This phenomenon led to the maximum velocity being concentrated near the walls of the chamber, potentially contributing to the misclassification occurring within the mill. Interestingly, the spin number exhibited a quadratic relationship with the size of the zone. The streamlines in laminar and turbulent conditions differ a little but the enlargement of the recirculation zone was evident in both regimes, while the velocity profiles are distinct. Flow patterns of simulations with jets are quite complex, because of the presence of many dead zones inside the flow area of the mill.

1. Mebtoul, M., Large, J., Guigon, P., 1996. High-velocity impact of particles on a target-an experimental study. Int. J. Miner. Process. 44, 77–91. https://doi.org/10.1016/0301-7516(95)00020-8.