Investigation of on-set of trailing edge cavitation in a Francis runner - Undersøkelse av dannelse av kavitasjon på avløpskanten til et Francis løpehjul
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Numerical simulations has been run on the Francis99 model runner at full load and deep part load conditions. Simulations has also been performed on plant Thoma number for for varying discharge. The simulations were performed using a blade channel modelling approach instead of the entire runner. This was done to reduce computational effort. The full load simulations were based on experimental data from cavitation tests in 2007. The full load simulations shows a gradual onset of cavitation with an increase in area where cavitation occurs as the Thoma number is decreased. There is no break of efficiency in any of the simulations, which is the same as in the experiments. The cavitation happening in the simulations occurs on the trailing edge. Regarding the simulations at plant Thoma number cavitation can be seen in the simulations with 12 degree opening, but not in the simulation with 9 degree opening. The deep part load simulations shows several vortices in the blade channel, among them inter-blade vortices. The inter-blade vortices starts at the leading edge of the runner. The inter-blade is found to be a transient phenomenon since it is not always present in the channel. The deep part load simulations does not include cavitating inter-blade vortices since the local pressure is too high for cavitation to occur inside the core.