of a specific tip clearance for which the vortex strength is maximum and most prone to generating cavitation. List of symbols c Hydrofoil chord h Maximum foil thickness W ∞ Inlet velocity p ∞ Inlet pressure x, y, z Cartesian coordinates u, v, w Spanwise, transverse and axial velocity x c , y c Vortex center coordinates r c Vortex core radius Re c Reynolds number (W ∞ c/ν) α Incidence angle τ Normalized tip clearance (gap/h) ω Vorticity Γ Circulation Γ * Normalized circulation (

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The development of energy recovery systems on existing water utility networks is a new challenge for increasing the sustainability of electricity generation mix. The concept of the new micro-turbine with counter rotating runners braces this goal by allowing the recovery of the energy which is spoiled in pressure reducing valves in many water supply networks. The hydraulic performances

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Variable speed hydroelectric units equipped with full size frequency converter (FSFC) offer high operational flexibility enabling fast operating point transitions which increase grid regulation capacities. The XFLEX HYDRO H2020 European research project aims to demonstrate flexibility of such technology at prototype scale. The Z’Mutt pumping station, part of the Grande Dixence hydroelectric scheme located in Switzerland, is one of the demonstrators focused on the FSFC technology with a new 5 MW reversible Francis pump-turbine which will be commissioned in 2021. This paper, divided in two parts, aims to simulate the turbine mode fast start-up sequence made possible with the use of a FSFC and to assess the unit damage by means of 1D and 3D CFD simulations. The part I of this paper presents the 1D hydraulic transient simulation results of start-up sequences of unit U5 considering both conventional fixed speed technology and variable speed technology. The time evolution of the unit’s operating point is used as input data for 3D CFD simulations of part II, aiming to assess the impeller damage. Different control strategies to use the FSFC for turbine mode start-up sequence are analysed. Advantages and limits of each strategy are discussed, and recommendation is made for the Z’Mutt prototype demonstrator.

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