Effect of multi-jet flow control on the vortex core in a simplified Francis turbine model

This work aims to advance methods for the active control of vortex phenomena in hydraulic turbines. The investigation was conducted on a Francis turbine model operating under partial load conditions, where large-scale vortex structures develop, inducing high levels of pressure pulsations detrimental to turbine structural integrity. Control of the vortex core characteristics is based on the injection of supplementary jets through the central streamline body of the runner. Experimental results are presented for velocity fields under various flow control strategies, differing in the spatial orientation of the jets and their total flow rate. An analysis is presented of the integral flow characteristics and the evolving vortex structures downstream of the Francis turbine model runner. The mechanisms governing vortex core control under different supplementary jet injection methods are described. It is found that, irrespective of jet direction, a total injected flow rate of 3% of the main flow significantly suppresses pressure pulsations. However, radial jet injection (parallel to the runner plane) proves more effective than axial injection or a combination of both directions. Alongside the injected flow fraction, the dimensionless momentum flux coefficient is identified as a key parameter governing the effectiveness of control on the precession vortex core.