DEM study of the influences of the geometric and operational factors on the mechanical responses of an underwater mixing process

Abstract

Mixing granular materials plays versatile roles in many engineering fields. In this study, we focus on an underwater mixing process as a novel application for offshore mining. We apply the discrete element method (DEM), which was augmented by a lubrication model, to evaluate the mechanical responses of the mixing process under water. Further, a non-dimensional parameter is introduced to evaluate the mixing resistance. The DEM simulation is first calibrated by a mixing experiment involving colored sand immersed in water. A parametric study is then conducted to evaluate the geometric and operational factors involved in the mixing process, including the blade angle, penetration depth, filling depth, container-blade aspect ratio, model scale, and rotation speed. Their influences on the mechanical responses of the mixing process are evaluated, i.e., the resultant torques on the mixer head, the resultant forces on the ground and side wall of the container, the effective particle masses mobilized by mixing, and the proposed mixing resistance parameter. It was found that the influences range from insignificant to highly nonlinear, and vary considerably among the evaluated factors and steady-state responses. Especially, an optimal blade angle range is observed in terms of minimal mixing resistance. These findings can contribute to the optimization of the mixer geometry, selection of the rotation speed, estimation of the motor power, and economic design for numerical and laboratory experiments with scaled-down models.