A parametric study of axial flow jets for mitigation of vortex rope instabilities

Paper i proceeding, 2025

Hydraulic turbines are typically designed to operate continuously at their optimal efficiency. However, the growing reliance on intermittent energy sources has resulted in more frequent operation under transient and off-design conditions, which increases stress on the turbines and shortens their lifespan. Swirling flow deceleration in the draft tube of hydraulic turbines operating at part load often triggers a self-excited instability known as the vortex rope. This phenomenon generates significant pressure fluctuations that may compromise the structural durability of the turbine. Therefore, it is crucially important to study and mitigate such instabilities. The current work presents a detailed study of the impact of axial flow jets on the vortex rope instability in hydraulic turbines, particularly under Part Load (PL) conditions. Utilizing the Timisoara Swirl Generator (TSG) model, we conducted a comprehensive parametric study to analyze the effects of axial water injection through the runner hub on mitigating vortex rope instabilities. The numerical simulations are performed using the open-source CFD code OpenFOAM. The results indicate that increasing the jet flow rate moves the vortex rope and stagnant region downstream, significantly reducing pressure pulsations and hydrodynamic instabilities. The study identifies a critical jet flow rate at which the vortex rope diminishes, enhancing turbine performance and operational flexibility. The current parametric study can be considered a first step toward achieving the optimal strategy for active flow control of self-induced instabilities within hydraulic turbines.