Intrusion of air into hydropower tunnels is known to be the cause of several problems, e.g.head losses, explosions due to escaping compressed air, and nitrogen pollution. This paper concentrates on the head loss problem. THe triggering example is a tunnel where the total head losses exceed expectation by about 10 m. Hydraulic analysis indicate that a 5 km long, nearly continuous air pocket may form in the tunnel during steady operation of the power plant. Field investigations have shown that air pockets indeed exist, but so far only some of the excess head loss seems to be caused by the air pocket. The rather simple steady case in question has been supplemented by an analysis of unsteady operation of the power plant, coupled with more marginal intrusion of air, and taking the ability of the water to absorb or transport air away from the pocket.
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