(303a) Investigating the Applicable Range of the Coarse-Grained Method for Granular Shear Flow

The discrete element method (DEM) has been extensively utilized for modeling and simulating powder handling processes. In a DEM, unsteady motion of each individual solid particle consisting of bulk powder is computed based on Newton’s second law. This results in extremely high computational load, limiting the number of particles that can be handled. Number of the particles that can be handled by DEM is approximately from 1 million to 10 million even with the latest workstations. whereas it is necessary to handle more than 1 billion particles for simulation of actual powder handling processes. To address this issue, the coarse-grained (CG) DEM is attracting much attention. The CG-DEM calculates the powder dynamics by replacing the group of original particles with artificially up-scaled single particles (CG particles), thereby reducing the number of particles and computational load. In the CG-DEM, an appropriate scaling law for the CG particles is critical for accurate simulation of the motion of the original particles.

Various scaling laws have been developed for various powder handling processes. However, most of them were based on processes in which the fluid–particle interaction is dominant, and few CG models had been developed for powder mixing processes in which contact and collision between particles are dominant. We have developed a coarse-grained method for granular shear flow (CGSF)^1-3). In the CGSF, original scaling laws that match not only kinetic energy but also friction damping energy, elastic energy, and viscous damping energy between the group of original particles and CG particles were derived. The CGSF also developed a unique method to express the granular shear flow within the group of original particles by scaling the angular velocity of the CG particle. Validity of the CGSF was demonstrated so far^1-3), however, issues remain concerning the applicable range of the CGSF. Because the CGSF assumed the ideal granular shear flow for the group of original particles when deriving its scaling laws, there is a certain applicable range. However, it is unclear.

In this study, we investigated the applicable range of the CGSF. We employed a drum mixer, which is common powder mixer, as the simulation system. We performed DEM simulations of powder flow in a drum mixer with various particle velocity gradients (i.e., various intensities of the granular shear flow) by changing friction coefficients. We investigated accuracy of the CGSF by comparing the CG-DEM results with the original DEM results, and examined the range of granular shear flow to which the CGSF is applicable.

【Reference】

1) H. Nakamura, et al., Coarse-grained discrete element method for granular shear flow, Chem. Eng. J. Adv. 4 (2020) 100050, https://doi.org/10.1016/j.ceja.2020.100050.

2) N. Kishida, et al., " Coarse-grained discrete element simulation of particle flow and mixing in a vertical high-shear mixer ", Powder Technol., (2021), 390, 1-10, https://doi.org/10.1016/j.powtec.2021.05.028.

3) M. Saruwatari, et al., " Coarse-grained discrete element method of particle behavior and heat transfer in a rotary kiln ", Chem. Eng. J., (2022), 428, 130969, https://doi.org/10.1016/j.cej.2021.130969.