Airborne flow dynamics near free-falling bulk materials: CFD analysis from analytical pressure field

Abstract

The dust generated and spread by free fall of bulk materials is analyzed through CFD simulations that allow visualization of the velocity vector field and the air masses close to the main material stream (core) that generate preferential paths for the formation and escape of dust. The air velocity field near a free-falling bulk material is obtained through a CFD solution from the analytical pressure field; the model was fed with properties described in empirical studies allowing comparison and validation. CFD results allow visualization of the boundary layer formed close to the bulk material stream. The results indicate the existence of a critical height that, when reached, transforms the pressure field inducing the emergence of a toroidal vortex around the mainstream. This causes the expansion of the boundary layer leading to an increase in particle absorption and formation of dust dispersion zones. A relationship between the critical height and the material stream diameter is proposed. The results indicate that the fluid viscosity inside the boundary layer is constant - before its complete expansion - and after increases linearly. A linear viscosity model, adapted from Einstein's proposal, is applied in the CFD model to establish the relationship to the induced airflow as a function only of the fall height and core diameter.