Topic of the Lecture: A New Type of Cavitation Triggered By Boundary-Layer Turblent Production
Reporter： Prof Shengcai Li,
Date of the lecture： 3：00pm on September 26th
Place of the report: the conference room in the 4th floor of the old hydraulic building of the institute of the Mechatronics Contror Engineering,
Brief introduction of the lecture:
The Francis turbines (710 MW) of the Three Gorge Project are the world largest. They are developed by world leading manufactures (Alstom, GE and Voith-Siemens), reflecting cutting-edge technologies. However, all these turbines have developed an unusual pattern of damage neither identified nor reported before. This puzzled professionals around world. Li identified this new type of cavitation and now works to determine its mechanism which we know little currently.
The fact that this cavitation occurs on all prototype turbines but no cavitation was detected during model tests has raised a fundamental question to the turbine similarity laws. Fluid dynamic and metallurgical analysis suggests that this cavitation is triggered by the boundary-layer turbulent production; and the damaged (roughened) spot in turn trigger subsequent cavitation (damage) immediately down stream. This forms a sustainable dynamic process, resulting in long and equal-width stream-wise damage-strips with span-wise regularity reflecting the span-wise stochastic characteristics of turbulent production. Owing to the heat effect of cavitation, inter-granular corrosion takes place through sensitization process, leaving damage surface a corrosion appearance. Also, bluing presents at the damage tails owing to the nature of low-intensity damage. Extremely large turbines are much more susceptible to this type of cavitation (damage) owing to the scale-effect.
This phenomenon of highly multi-disciplinary nature presents a grand challenge to fundamental sciences as well as to development of modern turbines and other devices.