Mitigation of vibrations caused by inter-blade vortices using pumping cap for natural aeration

In 2014, Andritz received a contract to refurbish six Francis turbines with a total capacity of 840 MW. During the commissioning of the first unit, unexpected vibrations occurred within a power range of 25 to 65 MW, and Andritz was requested to find a solution. Unsteady computational fluid dynamics CFD simulations, revealed that the vibrations originated from intermittently cavitating inter-blade vortices. Conventional solutions involving compressed air injection were rejected, leaving natural aeration as the only viable option. Andritz explored various options for natural aeration by CFD and later, by means of homologous model tests. The solution was able to accommodate suction heads of approximately 10 meters and consisted of a specialized runner cap, referred to as pumping cap, which allowed for natural aeration through the hollow turbine shaft. The cap was optimized using two-phase CFD, and once installed on the prototype, it demonstrated a reduction of aproximately 50% in the peak level of vibration and noise. This achievement fully satisfied the customer, leading to the acceptance of the machine.