CFD-simuleringer med plasserte hindringer i utløpet til Brokke kraftstasjon for reduksjon av gassovermetning i Otra

Abstract

If air is drawn into the intake system of a power plant, super saturated gas may develop.Super saturated gas can occur if air is dissolved in water under a pressure higher than theatmospheric pressure. Downstream Brokke power plant measurements of super saturatedgas values up to 176% have been reported. It has been shown that super saturated gasover 110% can lead to fish death, and measures should be considered to reduce the totaldissolved gas in Otra.In this study, various measures for reducing super saturated gas in the outlet of Brokkepower station are investigated by using CFD-simulations in Ansys and modeling in Mat-lab. The method of the study assumes that increased turbulence will lead to a reductionof super saturated gas. Super saturated gas data from measuring stations downstreamBrokke power plant were used to determine the gas constant, and the effect of an installeddeflector is evaluated.Ansys Fluent was used for 2D- and 3D-simulations to determine the most effective ob-stacle design for reduction of super saturated gas in the outlet. By investigating variousobstacle designs, it was found that an obstacle with an angle of 120◦against the flowdirection led to the highest average turbulence kinetic energy (TKE) within a givenarea. Various distances between obstacles were further investigated, and modeling ofthe average TKE showed that obstacles placed close to each other led to the highestturbulence.The simulation results which gave the highest average TKE was three obstacles placedon a line over the outlet width. The super saturated gas reduction for this experimentwas modeled with variousβ2-values. Forβ2=15, the super saturated gas was reducedfrom 170% to 154% over the last 20 meters of the outlet, and forβ2=1.5, the supersaturated gas was reduced from 170% to 168% over the same area. The parameterβ2should be calibrated to fit the outlet at Brokke before a super saturated gas reductionfor the outlet can be modeled. Further measures for insertion of additional obstacles, andinvestigations of damming in the outlet when placing obstacles close together, should beconsidered.iii