Air return in brook intake shafts.
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In Norwegian practice, it is common to construct additional intakes (brook intakes) to collect the water from smaller streams and transport the water to the reservoir to increase the production of a hydropower plant. Undesired air entrainment can lead to head loss, supersaturation in downstream water or harmful blowouts in these intakes. Therefore, it is essential to try to understand the air transport phenomena in inclined conduits in order to provide guidance in brook intakes design and operation that would prevent hazardous blowouts from these intakes.Experimental studies were conducted in order to understand the phenomenon of air movement and air accumulation in the inclined conduits that can lead to blowouts in brook intakes. A physical model was built in the laboratory for this purpose. The model included a circular shaft with diameter 210 mm connected to a horseshoe shaped tunnel with dimensions 400 mm x 400 mm. Tests were performed for three model configurations with the 450, 600 and 900 inclined shafts.Detailed investigations of the flow patterns in the shaft and flow conditions in the tunnel were performed. It was concluded that a prerequisite for a blowout to occur is a change of flow regime and formation of air pockets in the intake shaft. This happens for conditions at which air can accumulate in the tunnel. Results of experiments allowed for determination of threshold velocities for the continuous air return and for the beginning of air accumulation in the tunnel. A threshold velocity for the continuous air return is considered a limiting velocity for an occurrence of hazardous blowouts.