Pressure measurements and high speed visualizations of the cavitation phenomena at deep part load condition in a Francis turbine

The unsteady phenomena of a low specific speed pump–turbine operating in pump mode were characterized by dynamic pressure measurements and high-speed flow visualization of injected air bubbles. Analyses were carried out on the pressure signals both in frequency and time–frequency domains and by bispectral protocol. The results obtained by high-speed camera were used to reveal the flow pattern in the diffuser and return vanes channels The unsteady structure identified in the return vane channel appeared both at full and part load condition. Furthermore, a rotating stall structure was found and characterized in the diffuser when the pump operated at part load. The characteristics of these two unsteady structures are described in the paper.

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Generation of Twin Vortex Rope in the Draft-Tube Elbow of a Francis Turbine During Deep Part-Load Operation

Journal of Fluids Engineering ◽

10.1115/1.4051150 ◽

2021 ◽

Author(s):

Mohammad Hossein Khozaei ◽

Arthur Favrel ◽

Toshitake Masuko ◽

Naoki Yamaguchi ◽

Kazuyoshi Miyagawa

Keyword(s):

Linear Correlation ◽

Draft Tube ◽

Pressure Fluctuations ◽

Low Frequency ◽

Model Tests ◽

Francis Turbine ◽

Deep Part ◽

Swirl Number ◽

Vortex Rope ◽

Part Load

Abstract This paper focuses on the generation of twin vortex rope in the draft-tube elbow of a Francis turbine at deep part-load operation through analyzing the results of model tests along with numerical simulations. Model tests, including pressure fluctuations measurements, are conducted over 10 speed factors. By considering the frequency of the pressure fluctuations with respect to the swirl intensity at the runner outlet, the part-load operating range is divided into three regimes, with two clear transitions between each occurring at swirl numbers 0.4 and 1.7. For operating conditions with a swirl number S>0.4, a linear correlation between the frequency of the precessing vortex core and the swirl number is established. During deep part-load regime (S>1.7), low-frequency pressure fluctuations appear. Their frequency feature another linear correlation with the swirl number. Unsteady CFD simulation of the full domain is performed to elucidate the generation mechanisms of the low-frequency fluctuations. By tracking the center of the vortical structures along the draft-tube, generation of three vortices in the elbow responsible for the pressure fluctuations at the lowest frequency is highlighted: the main PVC hits the draft-tube wall in the elbow resulting in its break down into three vortices rotating with half the rotational speed of the PVC. Two of the vortices rotate with opposite angular position, constituting a structure of twin vortices. The periodic rotation of these three vortices in the elbow induces the low-frequency pressure fluctuations.

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Characteristics and Control of the Draft-Tube Flow in Part-Load Francis Turbine

Journal of Fluids Engineering ◽

10.1115/1.3002318 ◽

2009 ◽

Vol 131 (2) ◽

Cited By ~ 50

Author(s):

Ri-kui Zhang ◽

Feng Mao ◽

Jie-Zhi Wu ◽

Shi-Yi Chen ◽

Yu-Lin Wu ◽

...

Keyword(s):

Swirling Flow ◽

Draft Tube ◽

Flow Behavior ◽

Load Condition ◽

Francis Turbine ◽

Tube Flow ◽

Reversed Flow ◽

Part Load ◽

Axial Pressure Gradient ◽

Flow Phenomena

Under part-load conditions, a Francis turbine often suffers from very severe low-frequency and large-amplitude pressure fluctuation, which is caused by the unsteady motion of vortices (known as “vortex ropes”) in the draft tube. This paper first reports our numerical investigation of relevant complex flow phenomena in the entire draft tube, based on the Reynolds-averaged Navier–Stokes (RANS) equations. We then focus on the physical mechanisms underlying these complex and somewhat chaotic flow phenomena of the draft-tube flow under a part-load condition. The flow stability and robustness are our special concern, since they determine what kind of control methodology will be effective for eliminating or alleviating those adverse phenomena. Our main findings about the flow behavior in the three segments of the draft tube, i.e., the cone inlet, the elbow segment, and the outlet segment with three exits, are as follows. (1) In the cone segment, we reconfirmed a previous finding of our research group based on the turbine’s whole-flow RANS computation that the harmful vortex rope is an inevitable consequence of the global instability of the swirling flow. We further identified that this instability is caused crucially by the reversed axial flow at the inlet of the draft tube. (2) In the elbow segment, we found a reversed flow continued from the inlet cone, which evolves to slow and chaotic motion. There is also a fast forward stream driven by a localized favorable axial pressure gradient, which carries the whole mass flux downstream. The forward stream and reversed flow coexist side-by-side in the elbow, with a complex and unstable shear layer in between. (3) In the outlet segment with three exits, the forward stream always goes through a fixed exit, leaving the other two exits with a chaotic and low-speed fluid motion. Based on these findings, we propose a few control principles to suppress the reversed flow and to eliminate the harmful helical vortex ropes. Of the methods we tested numerically, a simple jet injection in the inlet is proven successful.

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A Turbulence Model Assessment for Deep Part Load Conditions of a Francis Turbine

IOP Conference Series Earth and Environmental Science ◽

10.1088/1755-1315/774/1/012072 ◽

2021 ◽

Vol 774 (1) ◽

pp. 012072

Author(s):

J Wack ◽

J Beck ◽

P Conrad ◽

F von Locquenghien ◽

R Jester-Zürker ◽

...

Keyword(s):

Turbulence Model ◽

Francis Turbine ◽

Model Assessment ◽

Deep Part ◽

Part Load ◽

Load Conditions

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Prediction of the Pressure Pulsation in a Draft Tube for a Part Load Condition Using the LES Approach

Volume 1A: Symposia, Part 2 ◽

10.1115/ajkfluids2015-09281 ◽

2015 ◽

Author(s):

Olivier Pacot ◽

Chisachi Kato ◽

Yang Guo ◽

Yoshinobu Yamade

Keyword(s):

Load Condition ◽

Francis Turbine ◽

Sharp Change ◽

Back Side ◽

Vortex Rope ◽

Part Load ◽

Eddy Simulation ◽

Computational Mesh ◽

Large Eddy ◽

Lift And Drag

The present paper focuses on the vortex rope that arises when operating a model Francis turbine at a part load condition: 65% of the Best Efficiency Point (BEP). The investigation is performed numerically using the Large Eddy Simulation (LES) approach with the Dynamic Smagorinsky Model (DSM). Such approach and turbulence model are implemented in the overset finite element open source code, FrontFlow/blue (FFB). Furthermore, a cavitation model is implemented allowing computations for non-cavitating and cavitating conditions. Thanks to the use of the K supercomputer, located at Kobe in Japan, and to the use of large computational mesh (123 million elements), it is shown that the frequency of the precession of the vortex rope as well as the head can be accurately computed. However, the predicted amplitude of the fluctuation did not fully agree with the experiment. Differences in a particular region near the back side of the elbow are about 35%. A comparison between the variation of the size of the vortex rope and the swirl number has been investigated and showed a clear relation. The location of the vortex rope and the minimum of the pressure were also investigated and showed that they do not fully share the same location. Furthermore, in a preliminary study to the computation of the cavitating vortex rope, computations of the flow around a Clark-11.7% hydrofoil under cavitation condition and for angles of attack of 2° and 8° are carried out. The results showed the common issue for this computation, i.e. the sharp change of the lift and drag coefficients could not be accurately predicted. Currently underway are the computation of the cavitating vortex rope. The effect of the cavitation on the vortex rope will be studied and reported at a later stage.

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Physical Mechanism of Interblade Vortex Development at Deep Part Load Operation of a Francis Turbine

Journal of Fluids Engineering ◽

10.1115/1.4043989 ◽

2019 ◽

Vol 141 (11) ◽

Cited By ~ 5

Author(s):

Keita Yamamoto ◽

Andres Müller ◽

Arthur Favrel ◽

François Avellan

Keyword(s):

Energy Loss ◽

Physical Mechanism ◽

Vortex Structures ◽

Operating Conditions ◽

Francis Turbine ◽

Deep Part ◽

Seamless Integration ◽

Part Load ◽

Recirculating Flow ◽

Runner Blade

For seamless integration of growing electricity production from intermittent renewable energy sources, Francis turbines are under increasing demand to extend their operating range. This requires Francis turbines to operate under off-design conditions, where various types of cavitation are induced. At deep part load condition, an interblade cavitation vortex observed in a runner blade channel is a typical cavitation phenomenon causing pressure fluctuations and erosion, which prevent a reliable operation of Francis turbines at deep part load. The underlying mechanisms of its development are, however, yet to be understood. In an objective of revealing its developing mechanisms, the present study is aimed at investigating flow structures inside runner blade channels by comparison of three different operating conditions at deep part load using numerical simulation results. After demonstrating interblade vortex structures are successfully simulated by performed computations, it is shown that flow inside the runner at deep part load operation is characterized by a remarkable development of recirculating flow on the hub near the runner outlet. This recirculating flow is concluded to be closely associated with interblade vortex development. The skin-friction analyses applied to the hub identify the flow separation caused by a nonuniform distribution of flow, which describes the underlying physical mechanism of interblade vortex development. Investigations are further extended to include a quantitative evaluation of the specific energy loss induced by interblade vortex development. The integration of energy flux defined by rothalpy evidences the energy loss due to the presence of strong interblade vortex structures.

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