Retardation and transitions of dilute and dense granular flows in a vertical pipe induced by electric fields

Abstract

Granular flow of nickel particles down a vertical pipe from a hopper is shown to be retarded by a horizontal ac electric field applied to a local region along the pipe. The particles are released from the hopper by pulling out a stopper in the hopper. Two sequences of experiments with different initial flow conditions are performed. In the first sequence, a dilute flow in the pipe is created after a fixed voltage V (⩽4.8 kV) is applied across two short, vertical copper electrodes. The steady-state flow rate Q remains practically constant for V<V1 (=2.6 kV). At V=V1, Q drops abruptly; the drop depends on the location of the electrodes. For V>V1, the flow becomes dense; Q decreases with a power law, Q∼V−1. In the second sequence of experiments, V is first set at 4.8 kV; the flow is allowed to start, and soon becomes a dense flow; then, V is reduced to the desired voltage. The new, steady-state Q vs V curve coincides with the previous Q(V) curve of the first sequence, except for V2<V<V1, where V2=1.0 kV. The voltage V2 is a continuous transition point at which Q changes from a dense flow (V>V2) to a dilute flow (V<V2). Our results show that a large enough ac electric field can decrease the flow rate of a dilute or dense flow; the critical voltage that can reduce a dense flow, V2, is less than that for the dilute flow, V1.