Liquid Slug Motion in Vertical Risers with Limited Ventilation

Dangerous intermittent slug flows occur when large air pockets entrapped in deep tunnels escape from vertical risers, a phenomenon known as stormwater-induced geysers. Although the dynamics of liquid slug flow in fully ventilated pipelines has been well studied, investigations on the mass shedding of liquid slugs moving in vertical risers with a top orifice are lacking. By means of a three-dimensional computational fluid dynamics model, the dynamic behavior of liquid slugs with different initial slug lengths under different driving pressures and orifice openings was analyzed. Experiments were conducted to provide physical insights and data for model validation. It was found that the length of the rising liquid slug decreased due to a constant mass shedding rate. The latter was independent of the driving pressure and initial slug length, but dependent on the size of the orifice at the top end. The velocities of the slug’s tail and front were linearly related via the mass shedding rate. In the fully open riser, the net force driving the slug decreased but remained positive in the geyser event, while in the riser with limited ventilation it depended on both the driving air pressure and the orifice size and could become negative. A new relationship between the air bubble nose velocity, the water slug front velocity, and the film flow velocity has been derived. The upward film flow velocity can be laminar or turbulent. Moreover, a relationship between the film thickness, mass shedding rate, and pipe diameter has been derived.