Flow Regime Transition and Air Entrapment in Combined Sewer Storage Tunnels
The rapid filling of a stormwater storage tunnel is accompanied by a flow regime transition from free surface to pressurized flow with the presence of a hydraulic bore. Nearly all previous experimental investigations of flow regime transition have involved the study of pipe-filling bores; this seems to be a consequence of the experimental setup implemented as opposed to a necessarily common occurrence in prototype systems. In analyzing transient filling conditions in a large combined sewer overflow storage tunnel proposed for Washington DC, it became clear that the flow regime transition more commonly occurs as a gradual flow regime transition which involves a regular free surface bore followed by a gradually sloping free surface up to the pipe crown. This unsteady process can result in air pockets being trapped and pressurized in the system. As these air pockets reach a ventilation shaft, they can be quickly released upward in an air-water mixture. This phenomenon known as geysering potentially results in the untreated poor quality water returning to the surface. Laboratory experiments were used to observe the formation of pressurized air pockets and geysering events associated with gradual flow regime conditions.
Geysers that occur due to the release of these air pockets do not necessarily correspond to high pressures in the system. The consequences of these observations for the requirements of numerical models are discussed. It is also shown that the geysering phenomenon can be significantly reduced by altering the geometry of the ventilation shafts.
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