Experimental Study on Discharge and Energy Dissipation of Baffle-Drop Shaft in Deep Tunnel Drainage System

In order to study the transition characteristics with the high-speed air-water flow in the drop shaft of a deep tunnel drainage system, a hydraulic model test was conducted to observe the flow patterns in the process of drop shaft discharge, analyze the relationship between the maximum discharge and the baffle spacing, and calculate the energy dissipation rates of the drop shaft under different conditions. On this basis, the energy dissipation mechanism in the process of drop shaft discharge was revealed. Results show that there are three kinds of flow regimes in the discharge process of drop shaft, i.e., wall-impact confined flow, critical flow, and Free-drop flow. Hydraulic jump is the primary cause of energy dissipation of water on the baffle, and the collision of the water flow with the bottom-drop shaft fluid in the reverse direction and the resulted breakage achieve the ultimate purpose of energy dissipation. The maximum discharge of drop shaft is between 8.7 × 10⁻³ and 14.7 × 10⁻³ m³/s when the shaft diameter D = 0.4 m and the baffle spacing d ranges from 16.02 to 24.56 cm, and there is a linear relationship between the baffle spacing and the maximum discharge (Q_m). The formula of energy dissipation rate is deduced according to the law of conservation of energy, from which the optimal energy dissipation rate of drop shaft is achieved at d = 19.4 cm and inclination angle θ = 10°. The aperture diameter (Ф) of the cover-plate has a great influence on the internal pressure of the drop shaft. When Ф ≥ 4 cm, the internal pressure is nearly 0; the baffle with a certain inclination angle is conducive to accelerating the discharge process of the drop shaft. The impact forces on the upper, middle and lower baffles (denoted by F_u, F_m and F_d, respectively) present a relation of F_u > F_m > F_d; the maximum surface loads of the upper, middle and lower baffles are 42.8, 30.7 and 22.8 kN/m², respectively. The experimental results about the maximum discharge and optimal energy dissipation rate of baffle-drop shaft could provide a reference for the design and operation of the baffle-drop shaft of deep tunnel drainage systems.