Gas-solid multiphase flows are often encountered in chemical engineering processes. For the analysis of the gas-solid multiphase flows, numerical simulation technique has been applied frequently. The coupled Discrete Element Method[1] with the Computational Fluid Dynamics (DEM-CFD) method[2], one of the Eulerian-Lagrangian approaches, is a powerful and appropriate method for the complex gas-solid multiphase flow simulation. Although the adequacy of the DEM-CFD method has been proved in the previous studies, a large-scale problem is still difficult to solve by using the existing method. This is because the number of calculation particles is substantially limited when the calculation should be finished within practical time. To solve this problem, the coarse grain model [3] was developed in DEM. In the coarse grain model, the motion of a group of original particles is calculated by using coarse graining particles whose diameter is larger than those of original particles. In the previous studies [3]–[5], adequacy of the coarse grain model has been proven thorough some verification and validation tests. The coarse grain model makes it possible to improve the calculation efficiency drastically because the smaller number of calculation particles is used in the coarse grain model than in the original particle system. To extend the applicability of the coarse grain model, the van der Waals force model was also taken into consideration in previous study [5]. In the previous study, adequacy of the coarse graining DEM considering the van der Waals force was proven through verification test. In order to improve the accuracy we propose a new van der Waals force model by introducing correction factor regarding coordinate number. In order to show adequacy of the new coarse graining DEM, verification tests are performed for the simulations of cohesive powder flows in a rotational rectangular tank and in a fluidized bed. The macroscopic properties of the systems such as powder bed shape, the bed dimension and pressure drop were well agreed between the coarse graining systems and the original system. Consequently, adequacy of the new coarse graining DEM is shown in cohesive powder systems.
References
[1] P. A. Cundall and O. D. L. Strack, “A discrete numerical model for granular assemblies,” Geotechnique, vol. 29, pp. 47–65, 1979.
[2] Y. Tsuji, T. Kawaguchi, and T. Tanaka, “Discrete particle simulation of two-dimensional fluidized bed,” Powder Technol., vol. 77, no. 77, pp. 79–87, 1993.
[3] M. Sakai and S. Koshizuka, “Large-scale discrete element modeling in pneumatic conveying,” Chem. Eng. Sci., vol. 64, no. 3, pp. 533–539, 2009.
[4] M. Sakai, M. Abe, Y. Shigeto, S. Mizutani, H. Takahashi, A. Viré, J. R. Percival, J. Xiang, and C. C. Pain, “Verification and validation of a coarse grain model of the DEM in a bubbling fluidized bed,” Chem. Eng. J., vol. 244, pp. 33–43, 2014.
[5] M. Sakai, H. Takahashi, C. C. Pain, J. P. Latham, and J. Xiang, “Study on a large-scale discrete element model for fine particles in a fluidized bed,” Adv. Powder Technol., vol. 23, no. 5, pp. 673–681, 2012.
