Airflow Allocation and Driving Mechanisms in the Multi-Level Ventilation System of the Baihetan Spillway Tunnels: A Prototype-Scale CFD Study

To clarify airflow allocation and its driving mechanisms in a complex ventilation system during multi-tunnel flood discharge, a prototype-scale three-dimensional air–water two-phase flow model was developed for the three spillway tunnels and the complete ventilation system of the Baihetan Hydropower Station. The model included the upstream reservoir, three spillway tunnels, multi-level ventilation passages, and downstream channel, and was evaluated against prototype air-discharge measurements and outlet surface-velocity observations. At the normal pool level with all three spillway tunnels fully open, the Y-shaped ventilation tunnels, upper-section ventilation adit, and steep-slope-section ventilation tunnels accounted for 17.1%, 37.0%, and 45.9% of the total air inflow, respectively. Although individual ventilation components showed different inter-tunnel allocation patterns, their combined effects produced similar total air discharges among the three tunnels. Flow separation and vortices in the Y-shaped ventilation tunnels reduced the effective flow area, producing local peak air velocities approximately twice the cross-sectional mean. In the upper-section ventilation adit, axial momentum, branch-turning resistance, and the terminal boundary jointly produced sequential flow diversion that increased from Spillway Tunnel No. 3 to No. 1. Air discharge through each branch in the steep-slope section was jointly controlled by the effective pressure difference and passage resistance. Compared with external independent air supply, intake from the tunnel-roof airspace reduced aerator air discharge and shortened aeration cavities. Although the results are specific to the Baihetan configuration, they highlight the need to evaluate complex spillway-tunnel ventilation systems using their complete connected geometry rather than isolated ventilation branches.