Experimental investigation of granular flow through an orifice

The characteristics of continuous, steady granular flow through a flat-plate orifice have been experimentally investigated. In particular, the normal stress exerted on the orifice plate has been measured by the normal stress gauge which consists of a strain gauge attached to a cantilever beam. The cantilever beam supports the orifice plate which is freely hanging, and thus normal stresses on the orifice plate have been measured by strains developed in the beam due to normal forces on the plate by particles. Discharge rates of granular particles through the orifice have, therefore, been studied as a function of the average normal stress on the orifice plate. The results show that granular flows through the orifice are characterized by three regimes. When the flow is not choked, the discharge rate increases with the increasing normal stress (Regime I). With the further increase of the normal stress, the discharge rate reaches a maximum, at which the flow appears to start choking. Once the flow becomes choked, the discharge rate starts decreasing (Regime II) for further increase of the normal stress and then becomes independent of the normal stress on the orifice plate (Regime III). The transitional Regime II where the discharge rate decreases with the increasing normal stress is observed to be unstable. The asymptotic discharge rates at Regime III for various orifice sizes and particle sizes are in good agreement with results available in the literature. The maximum discharge rates, which are observed when choking just starts, exceed the asymptotic discharge rates by approximately 20–30%.