Vortex is one of typical structures of the unsteady flow inside the hydroturbines, leading to significant pressure fluctuation, prominent vibration of the units, and fatigue of turbine components. In order to reveal the complex vortex structures in the hydroturbines, a large amount of advanced methods for vortex identification and visualization have been developed and also are currently being intensively investigated by researchers. In this review, the vortex identification methods are reviewed in great detail with many illustrating examples and quantitative comparisons between different methods. The vortex identification methods are classified based on five different taxonomies. The identification of several typical vortices (e.g. vortex rope in draft tube, Karman vortex, and inter-blade vortex) in hydroturbines (including reversible pump turbines, Francis turbine, Kaplan turbine etc.) have been shown and discussed. Furthermore, experimental techniques for vortex observation have been also summarized and discussed. This review provides a practical guidance to the researchers for performing vortex identification.

A review of methods for vortex identification in hydroturbines

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

Currently, operations of reversible pump turbines in pumped hydro energy storage plant suffer great instability problems in the well-known S-shaped characteristic regions, leading to the failure of...

https://journals.sagepub.com/doi/pdf/10.1177/0954406216640579

A review of rotating stall in reversible pump turbine

Applications of computational fluid dynamic (CFD) techniques to hydropower have increased rapidly in the last three decades. The majority of the experimental investigations of hydraulic turbines were supported by numerical studies and this has become a standard practice. In the paper, applied numerical techniques and flow modeling approaches to simulate the hydraulic turbines are discussed. Both steady-state and transient operating conditions of the turbines are considered for the review. The steady-state conditions include the best efficiency point (BEP), high load (HL), and part load (PL). The transient conditions include load variation, startup, shutdown, and total load rejection. The performance of the applied numerical models and turbulence modeling with respect to the operating conditions are discussed. The recently developed numerical technique (transient blade row modeling) using the Fourier transformation (FT) method is discussed. This technique allows guide vane and blade passages to be modeled with the pitch ratio other than unity. Numerical modeling and simulation of hydraulic turbines during the transient operating conditions is one of the most challenging tasks because guide vanes' angular movement is time-dependent and mesh should be dynamic/moving. Different approaches applied to simulate the transient conditions and their limitations are discussed. Overall, this review summarizes the role of numerical techniques, advantages, limitations, and upcoming challenges within hydropower.

Numerical Techniques Applied to Hydraulic Turbines: A Perspective Review

Hydraulic turbines are widely used to meet real-time electricity demands. Computational fluid dynamic (CFD) techniques have played an important role in the design and development of such turbines. ...

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Experimental and Numerical Studies of a High-Head Francis Turbine: A Review of the Francis-99 Test Case

Within the framework of an international research consortium on low-head hydraulic turbine flow dynamics, the predictive behavior of Reynolds Averaged Navier-Stokes (RANS) simulations of the efficiency (η) hill chart of a bulb turbine is investigated. The paper presents the impacts of the blade tip gap and the hub gaps on performance predictions.

https://iopscience.iop.org/article/10.1088/1755-1315/15/3/032007/pdf

Numerical prediction of a bulb turbine performance hill chart through RANS simulations

The paper presents the averaged velocity field inside the inter-blade channel of a propeller turbine runner measured using a stereoscopic particle image velocimetry (SPIV) technique. In this manner measurements have been performed without any modification of the flow patterns, with the averaged three-dimensional velocity fields reconstructed from phase-averaged acquisition data based on the blade azimuth. Main and secondary flows were analyzed for nine operating conditions, ranging from part to full load. The radial velocities and gradients play major roles in the inter-blade flow development. Using the λ 2 -definition for vortex detection, leading edge vortices were detected and identified under part load conditions.

Inter-blade flow analysis of a propeller turbine runner using stereoscopic PIV

Transient conditions such as load rejection will often lead to high amplitude pressure fluctuations that will affect a turbine residual-life. If Computational Fluid Dynamic offers a promising tool to study the flow dynamic under transient regime, focused validation data on the runner are still lacking to assess the accuracy of different simulation strategies. Hence within the framework of the AxialT project of the International Consortium on Hydraulic Machines, exploratory measurements of the pressure field on the runner blades of a propeller turbine model were performed in transient conditions. The model was setup on the test stand of the LAMH of Laval University. The test stand control procedures were adapted to mimic transient condition such as load rejection or the transition from a normal operating condition to a speed-no-load condition. The pressure on the runner blades were measured using miniature piezo-resistive transducer linked to a high frequency telemetric system. Using specifically adapted data processing routines, it was possible to characterize the variations of the energy content during the transient runs. Specifically, the main fluctuations appear to occur in the sub-synchronous range in both cases.

http://iopscience.iop.org/article/10.1088/1755-1315/15/6/062061/pdf

Experimental study of the pressure fluctuations on propeller turbine runner blades: part 2, transient conditions

The draft tube of reaction hydraulic turbines is subject to numerous investigations since it accounts for a significant portion of the energy recovery. But even with up-to-date computational fluid dynamics methodologies, simulating the draft tube flow remains highly challenging since it is a diverging swirling flow that may undergo flow separations and become dominated by unsteady secondary flows. Within the framework of a collaborative research project on the flow dynamics of a propeller turbine model, the flow at the inlet region of the draft tube was studied using 2D-laser Doppler velocimetry (2D-LDV). Measurements were used to detect and characterize the flow structures at three operating conditions: partial discharge, near best efficiency, and full-load conditions. The paper presents analysis based on phased-averaged velocity fields to yield information on fluctuations and dominant frequencies according to runner positions. The main features detected are the flow nonuniformity at the runner exit and the secondary flow structures associated with the runner hub wake. Those results are part of a larger database aimed at providing test cases for the validation of numerical simulation strategies.

Experimental Investigation of Draft Tube Inlet Velocity Field of a Propeller Turbine

Hydraulic turbines are more frequently used for power regulation and thus spend more time providing spinning reserve for electrical grids. Spinning reserve requires the turbine to operate at its synchronous rotation speed, ready to be linked to the grid in what is termed the speed-no-load (SNL) condition. The turbine's runner flow in SNL is characterized by low discharge and high swirl leading to low-frequency high amplitude pressure fluctuations potentially leading to blade damage and more maintenance downtime. For low-head hydraulic turbines operating at SNL, the large pressure fluctuations in the runner are sometimes attributed to rotating stall. Using embedded pressure transducer measurements, mounted on runner blades of a model propeller turbine, and numerical flow simulations, this paper provides an insight into the inception mechanism associated with rotating stall in SNL conditions. The results offer evidence that the rotating stall is in fact associated with an unstable vorticity distribution not associated with the runner blades themselves.

Sébastien Houde

Papers

Numerical prediction of a bulb turbine performance hill chart through RANS simulations

Inter-blade flow analysis of a propeller turbine runner using stereoscopic PIV

Experimental study of the pressure fluctuations on propeller turbine runner blades: part 2, transient conditions

Experimental Investigation of Draft Tube Inlet Velocity Field of a Propeller Turbine

Experimental and Numerical Investigations on the Origins of Rotating Stall in a Propeller Turbine Runner Operating in No-Load Conditions