Numerical Estimation of Torsional Dynamic Coefficients of a Hydraulic Turbine

The rotordynamic behavior of a hydraulic turbine is influenced by fluid-rotor interactions at theturbine runner. In this paper computational fluid dynamics (CFDs) are used to numerically predictthe torsional dynamic coefficients due to added polar inertia, damping, and stiffness of a Kaplanturbine runner. The simulations are carried out for three operating conditions, one at about 35%load, one at about 60% load (near best efficiency), and one at about 70% load.The runner rotational speed is perturbed with a sinusoidal function with different frequencies inorder to estimate the coefficients of added polar inertia and damping. It is shown that the addedcoefficients are dependent of the load and the oscillation frequency of the runner. This affect thesystem's eigenfrequencies and damping. The eigenfrequency is reduced with up to 65% comparedto the eigenfrequency of the mechanical system without the fluid interaction. The contribution tothe damping ratio varies between 30–80% depending on the load. Hence, it is important to considerthese added coefficients while carrying out dynamic analysis of the mechanical system