Experimental Investigation of Helical-step Dropshaft Hydraulics

With the rapid urbanization in China, large-scale urban storm water management projects have been planned. Some of them are of the depth of 50-60 m underground, the length of over 15 km through a city, and the tunnel diameter of about 10 m. Generally, the project consists of gathering rainfall system on the ground, several dropshafts and underground tunnel. Due to large flow discharge and high speed flow, energy dissipation and cavitation damage prevention are a serious concern in the design of dropshafts.

In the present work, a kind of helical-step dropshaft was developed, which consists of an air vent pipe at the center of the dropshaft and a continuous helical stepped chute attached to the wall. It dissipates the energy of the flow by means of stepped geometry and avoids the cavitation damage through air entrainment resulting from both vortex between the two steps and the flow surface.

Aiming at the large-scale shaft of 40 m in depth and 10 m in diameter, physical helical-step dropshaft models of scale 1:20 were designed according to Froude number criteria. The experiments were therefore conducted on the hydraulic characteristics of helical-step dropshaft, including the observation of flow regimes, the measurements of energy dissipation ratio and air concentration in vertical and horizontal surfaces in the steps. And the results were discussed and compared with existing dropshaft designs.

Experimental results demonstrate that (1) water can be conveyed by the helical-step dropshaft smoothly and steadily, with the prototype discharge up to Q = 45 m³/s in the present experiment; (2) the energy dissipation of the helical-step dropshaft decreases with increasing discharges, but still exceeds 85.0 % for the maximum discharge; and (3) the proposed dropshaft enjoys few cavitation risk on the basis of air concentration of the flow.

In general, the design of helical-step dropshaft stands out for (1) no requirements for specially-designed inlet precondition geometry and desirable downstream flow condition; (2) more efficient energy dissipation than helicoidal ramp dropshafts; (3) better maximum discharge capability compared to baffle type dropshafts of the same size and (4) good cavitation erosion prevention performance.