Shear and vortex instabilities at deep part load of hydraulic turbines and their numerical prediction

Abstract

For all types of turbines, deep part load operation (DPL) poses a challenge. An example of a dynamic phenomenon due to vaneless space vortices occurring at DPL is presented for three turbine types: a diagonal, a propeller and a Francis turbine. The backflow region occurring at DPL is often considered to be a main factor in these phenomena. Our results confirm that these backflow regions play an important role, but other factors also seem to be significant in specific cases. In our example of a diagonal turbine, the intersection of the backflow region with the leading edge of the blades seems to generate particularly high pressure pulsations and vibrations. In the case of the propeller turbine, large vaneless space vortices are found in CFD, but vibrations on the prototype machine are well within acceptable levels. Inspecting the flow at an operating point before large vaneless space vortices occur, shows high shear levels near the inner head cover that generate the high-intensity vortices at an even lower load. In the Francis turbine, half the number of strong vaneless space vortices interact with equally strong inter-blade vortices with vorticity of opposite sign to result in high dynamic blade torque. CFD simulations are well capable of capturing these phenomena, allowing them to be considered in the mechanical design of the turbine components for safe operation.

Fatigue damage analysis of a Kaplan turbine model operating at off-design and transient conditions

“…20 Several researchers concluded that the flow at SNL is mainly stochastic, 10,[20][21][22][23][24][25][26][27][28] while others found that this condition also presents periodic phenomena, such as vaneless space vortex (VSV) structures which consist of a certain number of vortices rotating at a fixed frequency. [29][30][31][32][33][34][35][36][37] On the one hand, Melot et al 24 and Nennemann et al 23 performed numerical fatigue calculations on Francis runners working at SNL and found that most of the fatigue damage inflicted on the runner blades was induced by stochastic flow phenomena. On the other hand, Nennemann et al 37 studied, numerically and experimentally, the VSV structures occurring at deep part load (DPL) and SNL on a diagonal turbine, a propeller, and a Francis turbine.…”

“…[29][30][31][32][33][34][35][36][37] On the one hand, Melot et al 24 and Nennemann et al 23 performed numerical fatigue calculations on Francis runners working at SNL and found that most of the fatigue damage inflicted on the runner blades was induced by stochastic flow phenomena. On the other hand, Nennemann et al 37 studied, numerically and experimentally, the VSV structures occurring at deep part load (DPL) and SNL on a diagonal turbine, a propeller, and a Francis turbine. The backflow region was found to play a key role in their formation.…”

“…The potential energy is then dissipated through the formation of large-scale unsteady vortex structures breaking down into smaller vortices [20]. Several researchers concluded that the flow at SNL is mainly stochastic [10,[20][21][22][23][24][25][26][27][28], whilst others found that this condition also presents periodic phenomena, such as vaneless space vortex (VSV) structures which consist of a certain number of vortices rotating at a fixed frequency [29][30][31][32][33][34][35][36][37]. On the one hand, Melot et al [24] and Nennemann et al [23] performed numerical fatigue calculations on Francis runners working at SNL and found that most of the fatigue damage inflicted on the runner blades was induced by stochastic flow phenomena.…”

On the Vaneless Space Vortex Structures in a Kaplan Turbine Model Operating at Speed No Load

“…In axial turbines, SNL operation also presents stochastic flow phenomena and coherent vortex structures, which are usually originated in the vaneless space and cause low-frequency pressure fluctuations and significant vibrations [21][22][23][24][25][26][27][28][29][30][31][32][33]. Yang et al [21] investigated numerically the origin of strong pressure fluctuations and vibrations in an axial turbine operated at SNL.…”

Validation of deep part load dynamic stresses for axial runners

“…In addition to inter blade vortices, axial runners may also be afflicted by other types of vortices occurring in the vaneless space between guide vanes and runner blades. Those vaneless space vortices have been described hydraulically in previous publications [4]- [7] but attempts to predict dynamic stresses under their influence has not yet been done. This study presents strain gauge measurements on a fixed-blade axial prototype runner.…”