Numerical investigation of tip leakage vortex
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The Kaplan turbine has a small clearance gap between the blade tip and casing to allow the blades to rotate freely. This clearance gap is the cause of an undesirable Tip Leakage Vortex (TLV). A TLV might reduce the turbine efficiency, erode the turbine blades or cause instabilities for the power output. A literature study indicated that the tip clearance gap was a critical parameter affecting the behavior of the TLV. A research gap was observed for an operating Kaplan turbine where the turbine shafts tend to wobble slightly, and thus fluctuate the tip clearance gap.The aim of this thesis is to gain a better understanding of the Tip Leakage Vortex and investigate the research gap observing the effects generated by a fluctuating tip clearance gap. To examine the effects, a numerical study was completed on a mesh of 7 million elements. The selected turbulence models were Shear stress transport (SST) and Scale-adaptive simulation (SAS). The numerical results produced values close to the experimental results. Simulated results showed that the negative effects associated with TLV worsened when including the wobbling from the turbine shaft. For the smallest clearance gap, the maximum vortex strength showed an increase of 4 % compared to the maximum vortex strength of a stationary test. Tested on for stationary case, the radial forces showed a peak with a clearance gap size of 5 mm. The effects of a fluctuating clearance gap showed a significant increase of the maximum radial forces. Obtained results show that further improvements are possible in the hydropower industry.