Every Francis turbine has a thin gap between rotating and non-rotating parts, which prevents contact between the two units. Although necessary, hydraulic seals create energetic losses: some fluid does not flow through the runner (leakage loss) and exerts a torque on the rotor (friction loss). Only analytical and empirical prediction methods of a seal efficiency had been developed before 1980. Numerical methods are now used to predict seals performance. However, most of the studies known to the authors deal with gas labyrinth seals and use the k–ε turbulence model. In hydraulic seals, since the viscous losses in the boundary layer influence the leakage loss, low Reynolds turbulence models appear more appropriate. Our study aims to implement an accurate model to predict losses in labyrinth seals using a low Reynolds model, and validate it using experimental results. The issues of the mesh and boundary conditions are addressed. The commercial code ANSYS CFX 12 is used.

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