Integration of a new shape-dependent particle–fluid drag coefficient law in the multiphase Eulerian–Lagrangian code MFIX-DEM

Abstract

A new shape-dependent fluid–particle drag law has been added into the open source fluid dynamic software MFIX-DEM (MFIX-Discrete Element Method), which is the Eulerian–Lagrangian version of the classic MFIX Eulerian–Eulerian multiphase flow model. The drag law had been obtained by previous settling experiments of volcanic pumices in a motionless Newtonian liquid (water or alcohol). Pumices are characterized by a highly irregular shape, which is much different from a sphere and drastically influences fluid drag. The new drag law defines the particle–fluid drag coefficient as a function of both the fluid regime and particle characteristics, of which the shape factor is a compact descriptor that quantifies how the particle shape differs from a simple sphere. As a validation of the integration of the new drag law in the simulation software MFIX-DEM, the code has been used to replicate the experiment results. The comparison with simulations performed with other formulas demonstrates that, by means of the new drag law, a significant improvement in the capability of the MFIX-DEM code to predict the terminal velocities of irregularly shaped particles is obtained. Thanks to this implementation, MFIX-DEM should be used, from now on, for simulating fluid–particle flows in which the particles are significantly different from simple spheres, as is usually the case of environmental flows like explosive eruptions or ash and pollutant dispersal. Based on the results of this research, in the future an improved version of MFIX-DEM will be also presented, with a drag law useful also in the case of mixtures to be treated with a Eulerian–Eulerian multiphase model.