DEM ANALYSIS OF FACTORS INFLUENCING THE GRANULAR CAPILLARITY

Granular capillarity refers to the phenomenon that when a narrow tube is vertically inserted into a container filled with particles and then set into vertical vibration, the particles rise up along the tube and eventually reach a certain height. This provides a potential technical method for the transportation of granular materials against gravity. To explore the factors influencing the granular capillarity, the processes of granular capillarity were numerically investigated using the discrete element method (DEM). On this basis, the evolutions of vertical velocities of particles at different tube diameters were shown, and the dependences of the final capillary height of the particles on the tube diameter at different container widths and vibrational parameters were examined. The results obtained under the conditions with the container-width-to-particle-diameter ratio of 40, vibration-amplitude-to-particle-diameter ratio of 14.33 and vibration frequency of 12 Hz show that at the tube-to-particle diameter ratio D/d = 3.33 severe jamming occurs for the particles in the tube, which makes the particles rise slowly and leads to a discontinuous granular column within the tube. At D/d = 8.33, the granular capillary height rises rapidly at the beginning, and then the increasing rate of the capillary height decreases gradually. In this case, there is almost no particle velocity gradient along the tube radius. However, at D/d=15, as the granular column height within the tube increases, the particles in the tube separated into two layers. In the upper layer, almost no particle velocity gradient along the tube radius is observed, whereas in the lower layer obvious velocity gradient can be found. It is also found that for the tube diameter range in which the granular capillarity can occur, there exists a critical tube diameter that corresponds to the maximal final capillary height. When the tube diameter is less than the critical tube diameter, the final capillary height increases with the tube diameter, whereas when the tube size is greater than the critical tube diameter, the final capillary height tends to decrease with the tube diameter. Moreover, increasing the container width leads to an increase in the critical tube diameter, while increasing the vibration amplitude of the tube and appropriately increasing the vibration frequency can effectively promote the increase of the critical tube diameter.