Unsteady regimes and pressure pulsations in draft tube of a model hydro turbine in a range of off-design conditions

Varying the generator load of a hydro turbine results in short-term changes in the rotation frequency of the runner, leading inevitably to flow instability and strong flow swirling behind the turbine. This may lead to the formation of unsteady flow regimes featured by vortex instability of the swirling flow behind the runner, known as the precessing vortex core (PVC) Dorfler et al. (2012). This effect causes dangerous periodic pressure pulsations that propagate throughout the water column in the draft tube. The present study reports on stereo PIV measurements of the air flow field inside a transparent draft tube of a model hydro turbine for a wide range of operation conditions. The research is focused on the time-averaged flow properties (mean velocity field and the second-order moments of velocity fluctuations), pressure pulsations and coherent flow structures in the velocity field.

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On the generation mechanisms of low-frequency synchronous pressure pulsations in a simplified draft-tube cone

International Journal of Heat and Fluid Flow ◽

10.1016/j.ijheatfluidflow.2021.108912 ◽

2022 ◽

Vol 93 ◽

pp. 108912

Author(s):

Mohammad Hossein Khozaei ◽

Arthur Favrel ◽

Kazuyoshi Miyagawa

Keyword(s):

Draft Tube ◽

Low Frequency ◽

Pressure Pulsations ◽

Generation Mechanisms

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Comparison of Axial Water and Air Injections in the Draft Tube of a Francis Turbine for RVR Mitigation

10.1115/fedsm2021-65503 ◽

2021 ◽

Author(s):

Subodh Khullar ◽

Krishna M. Singh ◽

Michel J. Cervantes ◽

Bhupendra K. Gandhi

Keyword(s):

Pressure Pulsation ◽

Draft Tube ◽

Water Injection ◽

Air Injection ◽

Flow Instabilities ◽

Pressure Recovery ◽

Francis Turbine ◽

Tube Flow ◽

Pressure Pulsations ◽

Jet Injection

Abstract The presence of excessive swirl at the runner outlet in Francis turbines operating at part load leads to the development of flow instabilities such as the rotating vortex rope (RVR). The presence of RVR causes severe pressure pulsations, power swings, and fatigue damage in the turbine unit. Air and water injection in the draft tube have been reported to reduce the detrimental effects of RVR formation in the Francis turbines. Air injection is one of the oldest and most widely used methods. In contrast, water jet injection is a relatively new methodology. The present work reports the numerical simulations performed to compare the respective effectiveness of these methods to mitigate the RVR and the related flow instabilities. The efficacy of the two methods has been compared based on the pressure pulsations and pressure recovery in the draft tube cone. The results show that the air and water injection influence the draft tube flow field in different ways. Both air and water injection led to a reduction in pressure pulsation magnitudes in the draft tube cone. However, the air injection led to a negative pressure recovery while the water injection improved the draft tube action.

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Investigations Of Pressure Pulsations In A Model Of Draft Tube Of Hydraulic Turbine Caused By Vortex Rings

Siberian Journal of Physics ◽

10.54362/1818-7919-2016-11-4-25-32 ◽

2016 ◽

Vol 11 (4) ◽

pp. 25-32

Author(s):

Sergey Skripkin ◽

Mikhail Tsoy ◽

Sergey Shtork ◽

Pavel Kuibin

Keyword(s):

Vortex Ring ◽

High Speed ◽

Draft Tube ◽

Hydraulic Turbine ◽

Vortex Rings ◽

Experimental Investigations ◽

Visualization Technique ◽

Vortex Rope ◽

Hydro Turbine ◽

Hydraulic Turbines

Current work is devoted to experimental investigations of behavior of precessing vortex rope in a draft tube model of hydraulic turbine. We used combination of stationary and freely rotating swirlers as a hydro turbine model. Such construction provides velocity distribution on the draft tube inlet close to distribution in natural hydraulic turbines operated at non-optimal conditions. The phenomenon of precessing vortex rope reconnection with further formation of vortex ring was founded in this experimental research using high-speed visualization technique. Synchronization of highspeed visualization and pressure measurements allowed us to relate pressure shock on the draft tube wall with vortex ring moving along wall.

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