Tiny bubbles challenge giant turbines: Three Gorges puzzle

Abstract

Since the birth of the first prototype of the modern reaction turbine, cavitation as conjectured by Euler in 1754 always presents as a challenge. Following his theory, the evolution of modern reaction (Francis and Kaplan) turbines has been completed by adding the final piece of the element ‘draft-tube’ that enables turbines to explore water energy at efficiencies of almost 100%. However, during the last two and a half centuries, with increasing unit capacity and specific speed, the problem of cavitation has been manifested and complicated by the draft-tube surges rather than being solved. Particularly, during the last 20 years, the fierce competition in the international market for extremely large turbines with compact design has encouraged the development of giant Francis turbines of 700–1000 MW. The first group (24 units) of such giant turbines of 700 MW each was installed in the Three Gorges project. Immediately after commission, a strange erosion phenomenon appeared on the guide vane of the machines that has puzzled professionals. From a multi-disciplinary analysis, this Three Gorges puzzle could reflect an unknown type of cavitation inception presumably triggered by turbulence production from the boundary-layer streak transitional process. It thus presents a fresh challenge not only to this old turbine industry, but also to the fundamental sciences.

Lagrangian statistics of pressure fluctuation events in homogeneous isotropic turbulence

“…The specific motivation of the study reported herein is the phenomenon of incipient cavitation in turbulent liquid flows, which is a macroscopic consequence of pressure fluctuations on microscopic bubbles (or nuclei) present in the liquid 5,6 . Though the dynamics of cavitation bubbles has been well established following the pioneering work of Plesset 7 , accurate prediction of cavitation inception in technologically relevant flows remains a challenge 8,9 .…”

Instability of large-scale prototype Francis turbines of Three Gorges power station at part load

“…However, a serious damage was observed on the guide plate soon after the commissioning, accompanied by severe pressure fluctuations and machine vibrations. 22 In our previous work, 21,22 we found that the guide plate could possibly serve as a particular origin for the generations of the flow perturbations, which further interacts with the draft-tube surges. Our previous studies 23,24 also found that the guide plate increases the free-stream turbulence and adversely affects the flow field, particularly at the part load.…”

“…Introduction of the guide plate Figure 1 shows the comparison of the locations of spiral casing between cases with and without the guide plate (highlighted by the two red solid lines). 21 In Figure 1, R original is the radius of the spiral casing without the guide plate and R guide-plate is the radius of the spiral casing with the guide plate. It is notable that the unit size can be largely reduced by moving the spiral casing inwards while keeping the position of the butterfly edge of the stay ring as it used to be.…”

“…20 Consequently, instability problems such as severe pressure surges are further escalated, posing high risks to the system safety. 21,22 Pressure Fuctuations may also induce high-amplitude mechanical vibrations or serious failures of the turbine components, which reduce the operating period and shorten the predefined refurbishment period. 20,22 A device named 'guide plate' has been installed in the spiral casing of the Three Gorges turbines for reducing the unit size for the sake of economic benefits.…”

Introduction for amazing (cavitation) bubbles

“…The paper by Li [2] reveals a very strange erosion occurring on the giant turbines installed at the Three Gorges project that puzzles professionals. A hypothesis is proposed to illustrate this unknown cavitation-inception process.…”