Hydraulic performance of helical-step dropshaft

Abstract

Aiming to achieve energy dissipation and prevention of cavitation erosion, a kind of dropshaft in urban drainage systems called the helical-step dropshaft is introduced in this paper. It dissipates flow energy by means of step geometry and prevents cavitation erosion through air entrainment. To verify its availability, the hydraulic characteristics of the helical-step dropshaft were experimentally investigated, including the flow regimes, the efficiency of energy dissipation, characteristics of air entrainment and pressure distribution. The results demonstrate that, even for a large discharge, flow can be discharged smoothly and steadily, and a high energy-dissipation rate of over 87% is achieved. There are three distinct flow regimes observed in the dropshaft, namely nappe flow, mixed flow and skimming flow. Moreover, there is no less than 1.6% air concentration and a reasonable pressure distribution on the step surface. This study provides an attractive alternative for the design of drop structures. HIGHLIGHTS A novel helical-step dropshaft prototype was modelled and verified by experimentally studying its hydraulic characteristics.; Flow regimes in the dropshaft were specially observed and a novel mixed flow regime for the helical stepped chute was defined and described.; Air concentration near the step surface was measured to help understand the cavitation prevention in the helical-step dropshaft for the first time.;

Keywords

• air entrainment
• drop structure
• energy dissipation
• helical-step dropshaft
• urban drainage system