Hydraulic test study on the pressure slope section at the inlet of the outlet tunnel of shaft spillway

Abstract

Shaft spillway tunnels are widely used flood discharge and energy dissipation structures in pumped storage power stations. The pressure slope connection section, as the core component linking the stilling well and the tailrace tunnel, plays a critical role in controlling the flow regime of the entire system. Under high-head and large-discharge conditions, problems including severe flow turbulence, unstable outflow and even flow impact on the tunnel crown often occur, threatening the safety of the tunnel lining structure. Taking the drop-flow shaft spillway tunnel at the lower reservoir of a 1800 MW pumped storage power station as the research object, this paper tests the flow regime, flow velocity and time-averaged pressure distribution characteristics of the original design scheme through hydraulic model tests. To address the flow crown impact problem, three optimization schemes are comparatively studied: deepening the stilling well, modifying the geometry of the pressure slope section, and installing a middle partition wall in the pressure slope section. The results show that neither simply deepening the stilling well nor adjusting the slope ratio and outlet size of the pressure slope section can effectively solve the flow crown impact problem. The installation of a middle partition wall downstream of the pressure slope section splits the large-scale vortices in the outflow from the stilling well, suppresses transverse turbulence, greatly reduces the water surface fluctuation at the outlet of the pressure slope section, and completely eliminates the flow crown impact. The proposed optimization scheme features a simple structure and convenient construction, providing a basis and engineering reference for the pressure slope design of similar high-head and large-discharge shaft spillway tunnels.