Transition from gas-dominated transport to liquid-dominated slugging controlled by outlet clearance and gas-phase Froude number in a confined dropshaft

Confined dropshafts equipped with suspended internal gas-recirculation pipes can experience pressure rebound and increased air demand when the continuous gas pathway breaks down near the pipe outlet. Yet, the coupled roles of outlet clearance, gas-phase inertia, pipe diameter, and downstream backpressure remain unclear. Here, visual observations and time-averaged measurements of pressure, external air intake, and circulating-gas flow were integrated in a 1:4-scale physical model and reorganized in a dimensionless Cg–Frg framework, where Cg quantifies outlet clearance and Frg represents measured gas-transport capacity. The results reveal a transition from stable gas-dominated transport to blockage-prone liquid intrusion and liquid-dominated operation. Among the tested conditions, Cg ≈ 1.6 with Frg close to unity marked the most favorable balance, maintaining continuous gas transport and maximizing pressure-reduction efficiency. Further reducing Cg and driving Frg below unity promoted bottom-outflow intrusion into the pipe outlet, intermittent obstruction of the gas pathway, circulation weakening, and hydraulic deterioration. At Q* = 1.032, increasing L* from 0.709 to 0.877 increased the overall pressure difference by 68.39% and the maximum negative pressure by 69.28%, while external air intake exceeded the reference shaft by 16.76%. Pipe diameter regulated the transition through competing effects of internal gas-transport enhancement and external annular-passage confinement, with d* = 0.466 producing the strongest stable circulation. Downstream baffle height modulated post-transition pressure and air-intake responses through local backpressure rather than shifting the transition condition. These findings establish outlet-clearance restriction and insufficient gas-phase inertia as coupled controls on gas-transport breakdown, linking local blockage to system-scale pressure and air-demand deterioration.