Water–air interactions in channel-tunnel water conveyance systems with different inflow conditions

Affected by increasing water demand or extreme factors, channel-tunnel water conveyance systems face such as air-pocket entrapment and pressure fluctuations during transitions between free-surface flow to pressured flow, posing serious threats to both structural safety and water conveyance efficiency. This study analyzes the evolution of flow patterns, air-pocket transport characteristics, and the dynamic response of hydraulic parameters in tunnels under both steady and unsteady inflows. The results indicate that under steady inflow conditions, the flow regime is delineated by the dimensionless discharge Q*. Throughout the transition from free-surface flow to pressured flow, the air pressure initially rises then falls, whereas the flow velocity increases monotonically. Under unsteady inflow conditions, the tunnel filling process comprises four stages: rapid compression, slow venting, pressure oscillation, and stable recovery. The slow venting stage exhibits low air venting efficiency. The most severe pressure surges occur during the oscillation stage, with pressure fluctuation amplitudes reaching 10–25 times those under steady flow conditions. These pressure fluctuations diminish along the tunnel's length, with the upstream experiencing negative pressure, while the downstream is subject to positive pressure. Accounting for factors such as flow surge intensity and tunnel geometry, a dimensionless parameter S is proposed to characterize the water–air dynamics in tunnels. The air venting rate increases with S, though pressure oscillations are also intensified. Air-pocket migration modes and pressure oscillation levels are categorized into three S intervals: S < 0.180 (slug flow-dominated migration with weak pressure oscillations), 0.180 < S < 0.780 (alternating bubbly-slug flow migration with moderate-to-strong pressure oscillations), and S > 0.780 (rapid unidirectional air migration with strong pressure oscillations). These results serve as a guideline for the safe design and efficient operation of water conveyance tunnels.