Hydraulics | EPFL Graph Search

Hydraulics () is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counterpart of pneumatics, which concerns gases. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on applied engineering using the properties of fluids. In its fluid power applications, hydraulics is used for the generation, control, and transmission of power by the use of pressurized liquids. Hydraulic topics range through some parts of science and most of engineering modules, and cover concepts such as pipe flow, dam design, fluidics and fluid control circuitry. The principles of hydraulics are in use naturally in the human body within the vascular system and erectile tissue. Free surface hydraulics is the branch of hydraulics dealing with free surface flow, such as occurring in rivers, canals, lakes, estuaries and seas. Its sub-field open-channel

Gondo Throttled Surge Tank Numerical Modeling and Design Review

Mona Seyfeddine

In order to increase the installed capacity, the refurbishment of Gondo high-head power plant required a modification of the existing surge tank by installing a throttle at its entrance. In a previous study, the geometry of this throttle was optimized by physical modeling to achieve the target loss coefficients as identified by a transient 1D numerical analysis. This study complements previous analyses through 3D numerical modeling using the commercial software ANSYS-CFX 19 R1. Results show that: (i) a 3D CFD model predicts sufficiently accurate local head loss coefficients that agree closely with the findings of the physical model; (ii) in contrast to a standard surge tank, the presence of a connecting gallery proved to be of insignificant effect on a surge tank equipped with a throttle, as the variations in the section of the tank cause negligible local losses compared to the ones induced by the throttle; (iii) CFD investigations of transient flow regimes revealed that the head loss coefficient of the throttle only varies for flow ratios below 20% of the total flow in the system, without significantly affecting the conclusions of the 1D transient analysis concerning minimum and maximum water level in the surge tank as well as pressure peaks below the surge tank. This study highlights the importance of examining the characteristics of a hydraulic system from a holistic approach involving hybrid modeling (1D, 3D numerical and physical) backed by calibration as well as validation with in-situ measurements. This results in a more comprehensive design of throttled surge tanks that makes full use of the advantages associated with each modeling strategy.

2020

Hydroacoustic Modeling of a Cavitation Vortex Rope for a Francis Turbine

Christian Landry

Hydraulic machines subject to off-design operation involve the presence of cavitating flow regimes in the draft tube. The cavitation vortex rope at part load conditions is described as an excitation source for the hydraulic system, and interactions between this excitation source and system eigenfrequency may result in resonance phenomena and induce a draft tube surge and electrical power swings. To accurately predict and simulate a part load resonance, proper modeling of the draft tube is critical. The presence of this cavitation vortex rope requires a numerical pipe element taking into account the complexity of the two-phase flow. Among the parameters describing the numerical model of the cavitating draft tube flow, three hydroacoustic parameters require a special attention. The first hydroacoustic parameter is called cavitation compliance. This dynamic parameter represents the variation of the cavitation volume with respect to a variation of pressure and implicitly defines the local wave speed in the draft tube. The second parameter corresponds to the bulk viscosity and is related to internal processes breaking a thermodynamic equilibrium between the cavitation volume and the surrounding liquid. The third parameter is the excitation source induced by the precessing vortex rope. The methodology to identify these hydroacoustic parameters is based on the direct link that exists between the natural frequency of the hydraulic system and the wave speed in the draft tube. First, the natural frequency is identified with the help of an external excitation system. Then, the wave speed is determined thanks to an accurate numerical model of the experimental hydraulic system. By applying this identification procedure for different values of Thoma number, it is possible to quantify the cavitation compliance and the void fraction of the cavitation vortex rope. In order to determine the energy dissipation induced by the cavitation volume, the experimental hydraulic system is excited at the natural frequency. With a Pressure-Time method, the amount of excitation energy is quantified and is injected into the numerical model. A spectral analysis of the forced harmonic response is used to identify the bulk viscosity and the pressure source induced by vortex rope precession. Thus, the identification of the hydroacoustic parameters requires the development of a new numerical draft tube model taking into account the divergent geometry and the convective terms of the momentum equation. Different numerical draft tube models are compared to determine the impact of convective and divergent geometry terms on identification of the hydroacoustic parameters. Furthermore, to predict the hydroacoustic parameters for non-studied operating conditions and to break free from the dependence upon the level setting of the Francis turbine, dimensionless numbers are proposed. They have the advantage of being independent from the selected numerical model and they define a behavior law of hydroacoustic parameters when the cavitation volume oscillates at resonance operating conditions. Finally, to investigate the stability operation of the prototype, the hydroacoustic parameters need to be transposed to the prototype conditions according to transposition laws. By assuming both Thoma similitude and Froude similitude conditions, transposition laws are developed and the hydroacoustic parameters are predicted for the prototype.

EPFL2015

Small Flex : Modèle d’exploitation flexible d’aménagements hydroélectriques de haute chute au fil de l’eau en Valais

Baku Mbenza

The Kraftwerk Rhone Oberwald hydropower was selected as a pilot site to analyze the possibility of running hydropower plants along a river in alpine regions during winter months. The KRO Gletsch-Oberwald hydropower facility has been operating since the beginning of 2018 is a run-of-the-river installation that has been designed without restraint and only works when the Rhone's water supply is abundant, between the May and September. The flow of the Rhone outside this range cannot be turbined and are therefore lost. The project studies the possibility of storing those debris and turbining them in order to provide peak energy despite the lack of upstream basin. These waters will be stored in the settling basin, which is used as a settling basin when flows are plentiful and will pass through holes installed between the Settling basin and the forebay. Secondly, the project studies the sediment dynamics upstream of the intake in order to establish a threshold for the use of the Settling basin as a storage unit and to define the periods during which the settling basin will be used as a sand drap. The project also lists the improvements that should be made at the instrumentation level to reduce uncertainties about the estimates made. This project is part of the activities carried out at the Laboratory of Hydraulic Constructions at the Swiss Federal Institute of Technology in Lausanne as part of the energy transition in Switzerland (Swiss Competence Center for Energy Research - Supply of Electricity) including, in parallel, environmental monitoring activities and studies on the hydraulic machines of the plant.

2020

Hydroacoustic Modeling of a Cavitation Vortex Rope for a Francis Turbine

Christian Landry

EPFL2015