Two different modes of flow may be observed when bulk solids are discharged from a silo: mass flow and funnel flow.
In the case of mass flow, the whole contents of the silo are in motion when unloading. Mass flow is only possible if the hopper walls are sufficiently steep, and the bulk solids use the whole outlet opening when discharging.
In the case of funnel flow, the hopper walls are not steep enough. The bulk solids are in motion in the area more or less above the outlet and the rest of the particles remain at rest at a “dead” or “stagnant” zone.
Given the angle of wall friction and internal friction (determined from the direct shear test) the hopper angle can be determined using the following graph:
Figure: Recommended wall angles to ensure mass flow in a conical bottom hopper.