Flow inside a tall plunging dropshaft.

Yiyi Ma, David Zhu, N. Rajaratnam

Monday 29 june 2015

17:36 - 17:39h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation, Poster pitches

Parallel session: Poster pitch: 3F. Engineering - Instrument

Dropshafts are widely used in urban drainage systems to transfer stormwater or wastewater from a high elevation to a low one. A model of a tall plunging dropshaft, with a 7.72 m drop height and a 0.38 m-in-diameter vertical shaft, was built for the research. The visual observation of the flow within the dropshaft at a flowrate of 3.8, 10.8 and 18.0 L/s was made using a high speed camera, at three heights: 4.5, 2.5 and 1.4 m. The size distributions and velocities of water drops were measured from video images. The air demand and pressure distribution along the dropshaft were measured under eight flowrates from 3.8 to 48.0 L/s. The air demand was obtained with an anemometer, and air pressure was measured by six pressure transducers installed at various heights of the dropshaft. The water flow inside the dropshaft was found to disintegrate gradually and majority of the water broke up into drops near the bottom. A total of 60 drops were randomly chosen for calculating the drop size distribution in each case. All the tests showed that most drops had a diameter around 2 mm. The drop velocities were also found similar in all tests at a velocity between 6-8 m/s, closed to the terminal velocity 7 m/s for 2 mm water drops moving in the air. The air demand was found fitted well with an exponential form of the water flowrate. The maximum negative air pressure was found at the highest measuring point, about 7 m above the dropshaft bottom. It increased as approaching the bottom and got close to the ambient pressure in the horizontal outlet. The energy dissipation of the dropshaft model could reach above 80%. Overall, the results give a good understanding of the flow inside a dropshaft.