The effect of runner cone design on pressure oscillation characteristics in a Francis hydraulic turbine

Three-dimensional unsteady turbulent flow through the entire flow passage of a model Francis hydraulic turbine is simulated using the transition shear-stress transport turbulence model and verified with experimental data. Pressure oscillations with the original runner cone, an extended runner cone, an extended runner cone with grooves, and a round-top runner cone are analysed under five operating conditions. The computational results show that (1) runner cone design can change the distribution pattern of the vortex rope in the draft tube and narrow the zone of special pressure oscillation; (2) the dominant frequency of pressure oscillation in the draft tube at guide vane opening angle, alpha = 17 degrees, is significantly decreased by runner cone design while the peak-to-peak amplitude increased; (3) varying guide vane opening alters the influence of runner cone design on pressure oscillation; (4) the modified runner cones are effective for low frequency pressure oscillations induced by the vortex rope in the draft tube but have little effect on blade frequency pressure oscillation induced by rotor-stator interaction; (5) the cavitation performance at alpha = 17 degrees is decreased while that at alpha = 18 degrees, 19 degrees, and 20 degrees is increased by runner cone design; and (6) the round-top runner cone is most effective in decreasing pressure oscillations and increasing turbine cavitation performance.