Pressure fluctuations in a Francis turbine draft tube calculated by cavitating flow simulation

Vortex cavitations in a Francis turbine draft tube were simulated to study the turbine safety by solving the 3-D unsteady Reynolds averaged Navier-Stokes equations with a transport-equation based cavitation model based on pseudo-homogeneous flow theory. The results show that the Thomas number, σp, has little influence on the main frequencies of the pressure fluctuations. However, for very small σp values, there are lower frequencies components in the pressure fluctuation spectrum since the troughs of the pressure waves are "cut-off" at the vaporization pressure. For large σp values, the pressure fluctuation amplitudes vary little with the σp value since the cavity rope diameter is small. As the σp value decreases the cavity vortex rope becomes thicker and the pressure amplitudes increase rapidly to the maximum. The calculated results agree well with experimental data to give reasonable explanations to the phenomena in model tests.