Experimental investigate of upstream air venting dynamics in mixed air–water flow transients

Air–water mixed flow frequently develops in urban drainage and hydropower tailrace systems, necessitating experimental clarification of vent-controlled pressure transients and two-phase flow dynamics. A visualized experimental system incorporating a transparent vertical vent shaft was established to systematically investigate the effects of vent orifice size on transient pressure response and air–water two-phase flow behavior during pipeline filling. The experimental facility consists of a 20 m long inclined pipe with an inner diameter of 0.15 m and a slope of 0.01, equipped with a vertical vent shaft located near the upstream end. Eighteen vent orifice diameters were tested, covering orifice diameter ratios from 0 to 0.5. High-frequency pressure measurements and high-speed visualizations were combined to capture air compression, discharge, and flow regime transitions. The results indicate that thresholds at diameter ratios of 0.075 and 0.375 delineate three distinct pressure types, each associated with a different flow pattern. For d/D<0.075, the flow exhibits a long continuous air–water front; for intermediate orifices, a highly aerated bubbly interface; and for d/D>0.375, large bullet-shaped bubbles. At a diameter ratio of 0.10, restricted venting leads to intensified air compression and pronounced pressure oscillations, with the maximum peaks observed. In addition, the quantified flow velocities upstream and downstream of the vent shaft reveal, from a physical perspective, that the discharge of trapped air induces local deceleration of the water column. These findings provide robust experimental evidence and mechanistic insights for vent design, contributing to improved safety, stability, and operational efficiency of hydraulic pipeline systems.