Vortex rope interaction with radially protruded solid bodies in an axial turbine: a numerical study

Abstract

Radially protruded solid rods and their interaction with the rotating vortex rope at part load condition are investigated numerically on an axial model turbine.The commercially available software ANSYS CFX was used to perform the simulation, and the test case was the Porjus U9 Kaplan turbine model operating at a fixed runner blade angle at part load condition. Four rods, with a rod diameter equal to 15% of the runner diameter were evenly distributed in a horizontal plane in the draft tube cone and protruded to a length set to intercept the RVR in its unperturbed trajectory. It is shown that the RVR plunging (synchronous) mode is completely mitigated upstream and downstream of the protruded rods. The RVR rotating (asynchronous) mode is reduced by 47% and 63% at the two monitor positions located upstream of the protruding rods, while only a minor reduction occurs to the first RVR harmonic at the monitor positions located downstream of the protruded rods. The perturbed RVR experiences an increased angular velocity due to the flow area decrease caused by the protruding rods, thus increasing the RVR frequency by approximately 53% compared to the unperturbed value. Investigation of the swirling flow indicates a locally increased swirl in the center of the draft tube downstream of the protruded rods which could explain the reduction of the RVR pressure amplitude. The overall turbine efficiency with solid rods protruded causes a marginally efficiency reduction of 0.85%. However, as the RVR pressure pulsations are reduced significantly, a more comprehensive investigation of the rods impact on the turbine performance and life time should be performed to elucidate the suitability of using solid rod protrusion for RVR mitigation.