Tiny bubbles challenge giant turbines: Three Gorges puzzle.

Area of Science:

• Fluid mechanics
• Turbomachinery
• Hydroelectric power

Background:

• Cavitation has been a persistent challenge in reaction turbines since their inception.
• The addition of draft tubes improved efficiency but did not solve cavitation issues, which are exacerbated by draft-tube surges.
• Modern giant Francis turbines (700-1000 MW) face new complexities due to increased capacity and compact designs.

Purpose of the Study:

• To investigate a novel erosion phenomenon observed on guide vanes of giant Francis turbines at the Three Gorges project.
• To explore potential new mechanisms of cavitation inception in large-scale turbomachinery.
• To address the challenges posed by these findings to the turbine industry and fundamental science.

Main Methods:

• Multi-disciplinary analysis of the erosion phenomenon.
• Investigation of turbulence production from boundary-layer streak transitional processes.
• Analysis of operating conditions in giant Francis turbines.

Main Results:

• A strange erosion phenomenon was observed on the guide vanes of 700 MW Francis turbines at the Three Gorges project shortly after commissioning.
• This phenomenon is hypothesized to stem from an unknown type of cavitation inception.
• The inception is potentially triggered by turbulence generated during boundary-layer streak transitional processes.

Conclusions:

• The observed erosion suggests a previously unrecognized cavitation mechanism in large Francis turbines.
• This finding presents a significant challenge to the established understanding of cavitation in turbomachinery.
• Further research is needed to understand and mitigate this new cavitation phenomenon for the advancement of turbine technology and fluid dynamics.