Flow Regimes in a Dropshaft with Limited Air Supply

Abstract

A physical model study was conducted to investigate the flow regimes in a plunging-type dropshaft under the conditions of no air supply and limited air supply. When the top of the dropshaft is sealed, three flow regimes are observed with the increase of the water flow rate: (1) free-flow regime, where the falling water partially occupies the cross-section and air can move into the dropshaft from the bottom with pressure atmospheric inside the dropshaft; (2) water-plug regime, where a water plug is formed at the bottom of the dropshaft, blocking the air flow and causing negative pressure; and (3) choked regime, where the water plug rises and submerges the inflow pipe and the dropshaft operates under the full pipe flow condition. The flow regime changes are studied as a function of the dimensionless water flow rate (using the dropshaft diameter and gravitational acceleration). A large drop height leads to an early formation of the water-plug regime. When the top of the dropshaft is partially open to the outside atmosphere by a valve, the water-plug regime appears slightly later than the cases with the dropshaft top sealed. The flow regime has a direct impact on the energy dissipation in the dropshaft.