The tip leakage vortex (TLV), which develops in the clearance between the rotor and the stator of axial hydro turbines, has been studied for decades. Yet, many associated phenomena are still not understood. For instance, it remains unclear how the clearance size is related to the occurrence of cavitation in the vortex, which can lead to severe erosion. Experiments are here carried out on the influence of the clearance size on the tip vortex structure in a simplified case study. A NACA0009 hydrofoil is used as a generic blade in a water tunnel while the clearance between the blade tip and the wall is varied. The 3D velocity fields are measured using Stereo Particle Image Velocimetry (SPIV) in three planes located downstream of the hydrofoil for different values of the upstream velocity, the incidence angle and a large number of tip clearances. The influence of the flow conditions on the structure of the TLV is described through changes in the vortex intensity, core axial flow, vortex center position and wandering motion amplitude. Moreover, high-speed visualizations are used to highlight the vortex core trajectory and clearance flow alteration, turning into a wall jet as the tip clearance is reduced. The measurements clearly reveal the existence of a specific tip clearance for which the vortex strength is maximum and most prone to generating cavitation.

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Mind the gap: a new insight into the tip leakage vortex using stereo-PIV

Experimental and numerical studies on a high head model Francis turbine were carried out over the entire range of turbine operation. A complete Hill diagram was constructed and pressure-time measurements were performed at several operating conditions over the entire range of power generation by installing pressure sensors in the rotating and stationary domains of the turbine. Unsteady numerical simulations were performed at five operating conditions using two turbulent models, shear stress transport (SST) k-ω and standard k-e and two advection schemes, high resolution and second order upwind. There was a very small difference (0.85%) between the experimental and numerical hydraulic efficiencies at the best efficiency point (BEP); the maximum difference (14%) between the experimental and numerical efficiencies was found at lower discharge turbine operation. Investigation of both the numerical and experimental pressure-time signals showed that the complex interaction between the rotor and stator caused an output torque oscillation over a particular power generation range. The pressure oscillations that developed due to guide vanes and runner blades interaction propagate up to the trailing edge of the blades. Fourier analysis of the signals revealed the presence of a vortex rope in the draft tube during turbine operation away from the BEP.

Experimental and Numerical Studies for a High Head Francis Turbine at Several Operating Points

Well knowing a good-running hydraulic turbine has important operational and financial benefits to those who operate a plant. Fatigue damage is the most fundamental failure type of hydraulic turbines. Existing flaws due to fatigue and their risk limit the operating time of a unit. Some plants have been temporarily shut down for up to a month and sometimes longer to repair these fatigue-induced damage, which has resulted in enormous economic losses. Fatigue problems must be solved or effectively prevented to ensure that turbine units run safely and steadily within their design life. This paper reviews loading features and some key issues (e.g. different load operations, start-up, emergency shut-down, load rejections, and runaway) on the fatigue damage, and provides the latest information about different prediction approaches. At last, it also attempts to present an overview of the complete failure modes, therefore other types of failure including cavitation, erosion and ingested bodies are introduced briefly.

A review on fatigue damage mechanism in hydro turbines

In high-head pump-turbines, observations in the engineering field show that the most detrimental hydraulic instabilities are pressure fluctuations in the vaneless space. For example, these pressure fluctuations can cause vibrations and fatigue failures in hydraulic components. It has been recognized that pressure fluctuations in vaneless space are induced primarily by the interactions between runner blades and guide vanes, i.e., rotor–stator interactions (RSI). The present paper presents and discusses studies in this field, which are carried out by various investigators. Studies include mechanism research on RSI, experimental and numerical investigations on RSI characteristics, discussions on the relationship between vibrations and pressure fluctuations, studies on the geometric and operating parameters of the unit that influence the pressure fluctuations in vaneless space, and precautions and countermeasures to reduce these pressure fluctuations.

Pressure fluctuations in the vaneless space of High-head pump-turbines—A review

Energy is unarguably the key factor for today's economic and social development within nations Electricity as one of many energy forms is a critical input to developing countries in the struggle to the national self-satisfaction in all domains Rural electricity supply involved institutions have recently recommended the pump as turbine (PAT)-based micro hydropower plant (MHP) schemes for remote off-grid electrification, mostly from their economic advantages However, from different published research findings, PAT-based MHP is not only simple and economically feasible, but has presented bottlenecks in the move to its full understanding Moreover, compared to other clean energy technologies, PAT technology has not found much literature in academic published researches, thus contributing to its limited understanding within the community Therefore, the PAT literature availability is one way to level up its understanding, which can be helpful to academic and professional communities In the present study, a state-of-the-art review on the two most challenging PAT aspects, namely PAT performance prediction and PAT flow stability aspects are presented In the presented literature, the selected energy sources history leading to the actual MHP global adoption was first briefly explained, followed by an intensive literature on PAT operations, where details about PAT selection and performance prediction were provided Finally, the PAT flow stability aspects where pump-turbine S-shape and Saddle-type characteristics constitute the main focus, were discussed It is worth an attention to mention that the words “pump-turbine”, “Pump as turbine”, and “reversible pump turbine”; are equally used throughout the whole literature It is within the authors wish that this paper can scale up the reader's PAT technology understanding, thus serving awareness in the same

Investigation on pump as turbine (PAT) technical aspects for micro hydropower schemes: A state-of-the-art review

Investigation of the rotor-stator interactions of a reduced-scale model 0.19 ν= pump-turbine in generating mode is presented for the maximum discharge operating condition. This operating point is chosen in order to have the most important rotor-stator interactions. The numerical simulation of the unsteady flow is performed with CFX 5.7™ for a computing domain which is extended to the full pump-turbine from the spiral casing to the draft tube. A computing domain embracing the full geometry enables to minimize the errors, streaming focus the boundary conditions, the periodic interface or the pitch ratio of rotor-stator interface. It also allows considering the fully non uniformity of the in coming flow field from the spiral casing. The pressure measurements are performed with piezoresitive miniature pressure sensors located in the distributor channels. The pressure fluctuations for one distributor channel obtained from the numerical simulation present a very good agreement with experimental data. The numerical result analysis shows, how the pressure fluctuations at blade passage frequency (BPF) and its harmonics vary along a distributor channel of the pumpturbine. The maximum pressure amplitude of BPF occurs in the rotor-stator zone, but it decreases very fast backward to the stay vane. However, the pressure amplitude of the first harmonic corresponding to 2 times the blade passage frequency spreads to the spiral casing highlighting the -2 precessing diametrical mode resulting from the modulation of the interacting stationary and rotating flow field.

Pump-Turbine Rotor-Stator Interactions in Generating Mode: Pressure Fluctuation in Distributor Channel

How oblique trailing edge of a hydrofoil reduces the vortex-induced vibration

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 02 to 04 times the runner rotational frequency resulting from the so-called vortex breakdown For low cavitation number, the flow features a cavitation vortex rope animated with precession motion Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations For the upper part load range, between 07 and 085 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines The selected operating point corresponds to a discharge of 083 times the best efficiency discharge Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed

Experimental Investigations on Upper Part Load Vortex Rope Pressure Fluctuations in Francis Turbine Draft Tube

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

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On the upper part load vortex rope in Francis turbine: Experimental investigation

Experiments on vortex shedding from a blunt trailing edge symmetric hydrofoil operating at zero angle of attack in a uniform high speed flow, Re-h = 16.1 . 10(3) - 96.6 . 10(3), where the reference length h is the trailing edge thickness, are reported. The effects of a tripped turbulent boundary layer on the wake characteristics are analyzed and compared with the condition of a natural turbulent transition. The foil surface is hydraulically smooth and a fully effective boundary layer tripping at the leading edge is achieved with the help of a distributed roughness. The vortex shedding process is found to be strongly influenced by the boundary layer development: the tripped turbulent transition promotes the re-establishment of organized vortex shedding. In the context of the tripped transition and in comparison with the natural one, significant increases in the vortex span-wise organization, the vortex-induced hydrofoil vibration, the wake velocity fluctuations, and the vortex strength are revealed. Although the vortex shedding frequency is decreased, a modified Strouhal number based on the wake width at the end of the vortex formation region is constant and evidences the similarity of the wakes in terms of spatial distribution for the two considered boundary layer transition processes. [DOI: 10.1115/1.4006700]

Amirreza Zobeiri

Papers

Pump-Turbine Rotor-Stator Interactions in Generating Mode: Pressure Fluctuation in Distributor Channel

How oblique trailing edge of a hydrofoil reduces the vortex-induced vibration

Experimental Investigations on Upper Part Load Vortex Rope Pressure Fluctuations in Francis Turbine Draft Tube

On the upper part load vortex rope in Francis turbine: Experimental investigation

The Effects of a Tripped Turbulent Boundary Layer on Vortex Shedding from a Blunt Trailing Edge Hydrofoil