Abstract: The installation of an internal air circulationflow system in the plunging-type dropshaft can effectively slow down the air pressure and air accumulation in downstream drainage pipelines. In order to determine the critical position where the air in the internal air circulation flow system can form a circulation loop, the theoretical calculation method was used to explore the pressure distribution in the wet shaft and the air shaft in the plunging-type dropshaft with an internal air circulation system. The results showed that the system played an effective role only when the air-circulated route was above the critical position (corresponding to the critical pressure). The calculated air pressure distributions in both shafts through the theoretical calculation method agree with the measurements well. For the plunging-type dropshaft with a drop height of H = 7.72 m, the critical position was about 0.65H ~ 0.84H. Moreover, the critical position was closer to the bottom of the dropshaft with the increase of the water flow rate.

Key words: plunging-type dropshaft, theoretical model, internal circulation air flow, critical pressure, critical position