One of the distinguishing characteristics of deregulated power markets is variable demand. The ability to operate efficiently at partial loads can determine whether a plant is profitable or not. This need creates special challenges for hydroelectric turbines, because at partial loads they often exhibit strong swirl in the draft tube at the outlet of the runner. As this swirling flow decelerates in the diffuser cone, it creates a hydrodynamic instability in the draft tube that looks like a rope twirling around a vortex.
This "vortex rope" causes two problems. The variable, high-pressure fluctuations in flow that it imposes on the walls of the draft tube reduce the efficiency of the turbine and foster fatigue damage that, over time, shortens turbine life. The phenomenon is most pronounced when the frequency of the vortex rope's twirling matches the turbine's mechanical resonance frequency.
In an operating turbine, it's impossible to measure in real time either the position of the vortex rope or the frequency and amplitude of the pulsations it produces. Being able to accurately simulate the twirling under various operating conditions would be the next-best capability. The simulations would allow design engineers to predict the effect of different hydroturbine geometries on the size and characteristics of the vortex rope. Using this information, they then could design draft tubes with smaller pressure fluctuations, to the benefit of turbine efficiency and longevity.
Atlanta-based GE Energy has confirmed its ability to accurately simulate the position and pressure fluctuations of vortex ropes. Enormous computing power is needed to achieve the spatial and temporal resolution necessary to maintain accuracy. GE engineers overcame this challenge by using computational fluid dynamics (CFD) software with powerful parallel and pre- and post-processing capabilities.
"Our simulation predictions matched experimental measurements within 5% for the position of the vortex rope and 18% for its amplitude," said Thi Vu, a senior hydraulic engineer at GE Energy's Hydropower Technology division in Montréal. "This is the first time that a simulation of the shape of a rotating vortex rope has been compared with empirical data. The excellent agreement between the two data sets will make it possible to improve the design of future hydropower turbines to minimize fatigue damage."
Crunching numbers, minimizing ropes
Manufacturers of a variety of plant systems through which steam, water, or gas flows increasingly use CFD tools to optimize the design of the equipment's internal structures (see related article on p. 58). CFD analysis can provide approximate solutions to the coupled fluid-flow equations that govern mass, momentum, and energy transport. The technique's flexibility makes it possible to solve these equations in very complex spaces—a big advantage over simpler modeling methods sometimes used for turbine design.
GE Energy engineers chose CFX software from ANSYS Inc. (Canonsburg, Pa.) as their modeling tool. Featuring an open architecture, the package also makes more-efficient use than competing products of the huge computing resources required for accurate simulations of the vortex rope. "Two characteristics of the rope create a need for huge processing power," explained Bernd Nennemann, a GE Energy research assistant. "The first is that the swirling movement of the rope often covers most of the draft tube. So a very fine CFD mesh is necessary to simulate the range and expanse of pressure fluctuations generated by rope movement. Second, the vortex rope is an unsteady phenomenon, so predicting its motion accurately requires solving flow equations repeatedly, in very small time steps."
Email
Comments
Print
Reuse
