Detailed images of hydropower turbine reveals a region vital to a destructive phenomenon

Francis turbines are the hydropower industry’s dominant technology for supplying electricity to grids, but suffer from damaging phenomena related to off-design operation, where flow conditions are far from their so-called best efficiency point (BEP).

Reported in Physics of Fluids, researchers from India, Sweden and Norway studied the destructive breakdown of the spiral vortex on a scale model of a turbine at Norway’s Tokke Power Plant. They produced detailed accounts of the evolution and mechanisms of the efficiency-lowering breakdown that develops in the draft tube, a diffuser situated after the blade, when the turbine is operating at partial load.

The turbulent condition has become a major concern as hydropower plants are called upon to maintain grid stability. As energy demands on Francis turbines fluctuate, they must operate at loads far from their BEP as guide vanes open and close to control flow, impacting turbine operating lifetimes.

The research team used particle image velocimetry and strategically placed pressure and flow sensors to capture events as the guide vanes increased in angle more than 3 degrees, from part load to the BEP. At initial partial load, they detected a high swirling flow exiting the runner and the emergence of a fast-growing vortex rope (the vortex breakdown) around the draft tube’s center axis. The resulting flow velocity and pressure fluctuations led to significant structural vibrations.

Velocity contours revealed that this imbalance occurred near a stagnant region in the draft tube, the disappearance of which mitigates the breakdown. This result, confirmed with over 20 test runs, shows that stagnation is the primary instability for these vortices. The group observed a new wake-to-jet transition develop, which eliminated the vortex breakdown and restored optimal flow in the draft tube.

Source: “Experimental study of mitigation of a spiral vortex breakdown at high Reynolds number under an adverse pressure gradient,” by Rahul Goyal, Bhupendra K. Gandhi, and Michel J. Cervantes, Physics of Fluids (2017). The article can be accessed at https://doi.org/10.1063/1.4999123 .