Laboratorio de Hidromecánica. Facultad de Ingeniería UNLP : Investigaciones y servicios de obras y turbomáquinas hidráulicas

En particular hablando de este laboratorio de hidráulica, de hidromecánica, que tiene un perfil de ensayos de turbinas a escala reducida, de estaciones de bombeo, de obras hídricas, donde también tiene un perfil que la tecnológica nos permitió llevar esas obras a lo que se denomina modelaciones numéricas, es decir, o hacemos una maqueta pero con las fuerzas escala del agua o eso lo simulamos numéricamente con las grandes computadoras que hay hoy en día a disposición, que la tecnología puso a disposición de la investigación científica tecnología y a disposición de que el hombre pueda hacer su trabajo y desarrollarse en la sociedad.Academia de la Ingeniería de la provincia de Buenos Aire

Modelación matemática de los fenómenos transitorios en acueductos

El diseño de una conducción a presión se concibe con la intención de albergar, sin riesgos de colapso estructural, el escurrimiento del agua en régimen estacionario. Sin embargo, aun el régimen permanente puede manifestarse en, al menos, tres formas disímiles: estático, en el que la velocidad del agua es nula; dinámico a caudal mínimo, ligado a la menor velocidad admisible del agua; o dinámico a caudal máximo, asociado, análogamente, a la mayor velocidad admisible del agua. Este último es, por cierto, determinante en el dimensionado del diámetro de la conducción. Un análisis pormenorizado de estos tres casos extremos parecería bastar, entonces, para garantizar la seguridad del acueducto; en efecto, ello sería estrictamente cierto en caso de que las transiciones entre unos y otros fueran lo suficientemente suaves. Desafortunadamente, ello es incompatible con los tiempos de operación y de maniobra de los elementos hidráulicos que regulan el escurrimiento: válvulas o compuertas, en tanto se abren o cierran en cuestión de segundos; o turbomáquinas, en tanto se encienden, apagan o cambian su régimen de operación casi instantáneamente, por ejemplo. Los fenómenos transitorios o impermanentes son entonces aquellos inducidos por un cambio (por lo general, brusco) en las condiciones de escurrimiento, y su importancia para un buen diseño es tal que pueden ocasionar la destrucción o la salida de servicio de un acueducto. Estadísticamente, las fallas o roturas más frecuentes de las conducciones a presión involucran efectos transitorios subestimados o pobremente evaluados: sobrepresiones o depresiones extremas, al punto de inducir cavitación o ingreso de aire por encima de lo tolerable, etcétera. Las características de los estados impermanentes resultantes están vinculadas a la tipología del acueducto, es decir: según funcionen a gravedad o por bombeo. En este último caso, además, importa si involucra una sola unidad o un complejo de varias unidades y su arreglo: bien en serie o en paralelo, bien en la toma de una cisterna, bien en línea, en alguna sección intermedia del acueducto. Todas estas configuraciones comportan situaciones previsibles, como el encendido de las bombas o su salida de servicio; la regulación del caudal conducido, ya sea mediante válvulas o la variación de la frecuencia de los motores de las turbomáquinas; en fin, cualquier maniobra que propenda a satisfacer la demanda cambiante de la provisión de agua o de energía. Se trata de maniobras programadas, que hacen al funcionamiento mismo del acueducto. Existen, asimismo, situaciones accidentales e indeseables, que pueden surgir en cualquier momento de manera inesperada, tales como cortes de energía o fallas hidromecánicas. El estudio de los fenómenos transitorios resulta de transcendental importancia para unos y otros casos, pues sirve igualmente como guía para implementar óptimas secuencias de operación cuanto como para diseñar los elementos de protección que eviten daños a las conducciones y a las propias máquinas. A fin de asimilar las distintas capas de complejidad involucradas, el estado impermanente se aborda habitualmente por medio de la integración de un sistema de ecuaciones diferenciales, habida cuenta de que las condiciones de contorno condigan con los elementos hidráulicos de una configuración dada del acueducto. En la práctica, este procedimiento se realiza por medios numéricos antes que analíticos y es el objeto de las secciones siguientes.Facultad de Ingenierí

Air injection test on a Kaplan turbine: Prototype - Model comparison

Air injection is a very well-known resource to reduce pressure pulsation magnitude in turbines, especially on Francis type. In the case of large Kaplan designs, even when not so usual, it could be a solution to mitigate vibrations arising when tip vortex cavitation phenomenon becomes erosive and induces structural vibrations. In order to study this alternative, aeration tests were performed on a Kaplan turbine at model and prototype scales. The research was focused on efficiency of different air flow rates injected in reducing vibrations, especially at the draft tube and the discharge ring and also in the efficiency drop magnitude. It was found that results on both scales presents the same trend in particular for vibration levels at the discharge ring. The efficiency drop was overestimated on model tests while on prototype were less than 0.2 % for all power output. On prototype, air has a beneficial effect in reducing pressure fluctuations up to 0.2 ‰ of air flow rate. On model high speed image computing helped to quantify the volume of tip vortex cavitation that is strongly correlated with the vibration level. The hydrophone measurements did not capture the cavitation intensity when air is injected, however on prototype, it was detected by a sonometer installed at the draft tube access gallery.Facultad de Ingenierí

Air injection test on a Kaplan turbine: Prototype - Model comparison

Air injection is a very well-known resource to reduce pressure pulsation magnitude in turbines, especially on Francis type. In the case of large Kaplan designs, even when not so usual, it could be a solution to mitigate vibrations arising when tip vortex cavitation phenomenon becomes erosive and induces structural vibrations. In order to study this alternative, aeration tests were performed on a Kaplan turbine at model and prototype scales. The research was focused on efficiency of different air flow rates injected in reducing vibrations, especially at the draft tube and the discharge ring and also in the efficiency drop magnitude. It was found that results on both scales presents the same trend in particular for vibration levels at the discharge ring. The efficiency drop was overestimated on model tests while on prototype were less than 0.2 % for all power output. On prototype, air has a beneficial effect in reducing pressure fluctuations up to 0.2 ‰ of air flow rate. On model high speed image computing helped to quantify the volume of tip vortex cavitation that is strongly correlated with the vibration level. The hydrophone measurements did not capture the cavitation intensity when air is injected, however on prototype, it was detected by a sonometer installed at the draft tube access gallery.Facultad de Ingenierí

Mitigation of tip vortex cavitation by means of air injection on a Kaplan turbine scale model

Kaplan turbines operating at full-load conditions may undergo excessive vibration, noise and cavitation. In such cases, damage by erosion associated to tip vortex cavitation can be observed at the discharge ring. This phenomenon involves design features such as (1) overhang of guide vanes; (2) blade profile; (3) gap increasing size with blade opening; (4) suction head; (5) operation point; and (6) discharge ring stiffness, among others. Tip vortex cavitation may cause erosion at the discharge ring and draft tube inlet following a wavy pattern, in which the number of vanes can be clearly identified. Injection of pressurized air above the runner blade centerline was tested as a mean to mitigate discharge ring cavitation damage on a scale model. Air entrance was observed by means of a high-speed camera in order to track the air trajectory toward its mergence with the tip vortex cavitation core. Post-processing of acceleration signals shows that the level of vibration and the RSI frequency amplitude decrease proportionally with air flow rate injected. These findings reveal the potential mitigating effect of air injection in preventing cavitation damage and will be useful in further tests to be performed on prototype, aiming at determining the optimum air flow rate, size and distribution of the injectors.Facultad de Ingenierí

Pressurized air injection in an axial hydro-turbine model for the mitigation of tip leakage cavitation

Tip leakage vortex cavitation in axial hydro-turbines may cause erosion, noise and vibration. Damage due to cavitation can be found at the tip of the runner blades on the low pressure side and the discharge ring. In some cases, the erosion follows an oscillatory pattern that is related to the number of guide vanes. That might suggest that a relationship exists between the flow through the guide vanes and the tip vortex cavitating core that induces this kind of erosion. On the other hand, it is known that air injection has a beneficial effect on reducing the damage by cavitation. In this paper, a methodology to identify the interaction between guide vanes and tip vortex cavitation is presented and the effect of air injection in reducing this particular kind of erosion was studied over a range of operating conditions on a Kaplan scale model. It was found that air injection, at the expense of slightly reducing the efficiency of the turbine, mitigates the erosive potential of tip leakage cavitation, attenuates the interaction between the flow through the guide vanes and the tip vortex and decreases the level of vibration of the structural components.Facultad de Ingenierí

Experimental validation of a low-head turbine intake designed by CFD following Fisher and Franke guidelines

Model acceptance tests evaluate the response of the turbines at different operating conditions. Model tests are mounted so that the velocity profile at the inlet section is uniform, a condition which is not often met in practice. In fact, divergences might render inaccurate model results, obtaining at prototype scale an efficiency drop, structural vibrations and even component failures, in extreme cases. This concern becomes all the more relevant for low-head turbines, as the intake is closer to the turbine runner. With the aim of best estimating the actual flow conditions at the turbine inlet section as a function of the intake design, Voith designers, Fisher and Franke recommended performing scale model tests of the intake structure and listed a series of requirements that a good intake design should meet. These guidelines have not yet been applied on numerical modeling design but rather on more expensive and time-consuming scale model tests. This work presents the results of a computational fluid dynamics (CFD) design of a low-head turbine intake taking into account an upgraded version of Fisher and Franke recommendations. The optimization process was aimed at obtaining the design that best matches the ideal flow conditions at the inlet section. The physical model was built in a scale of 1:40 and involves the complete turbine intake geometry. Different designs were tested on the basis of the evaluation of their corresponding velocity field distributions at a reference section and the best design was measured with an acoustic Doppler velocimeter (Vectrino). The results show that intake design guidelines are very useful tools that allow hydraulic designers to test their proposals with CFD more quickly, objectively and with enough degree of sensitivity to optimize the intake geometry.Publicado en IOP Conference Series: Earth and Environmental Science, vol. 22.Facultad de Ingenierí

Pressurized air injection in an axial hydro-turbine model for the mitigation of tip leakage cavitation

Tip leakage vortex cavitation in axial hydro-turbines may cause erosion, noise and vibration. Damage due to cavitation can be found at the tip of the runner blades on the low pressure side and the discharge ring. In some cases, the erosion follows an oscillatory pattern that is related to the number of guide vanes. That might suggest that a relationship exists between the flow through the guide vanes and the tip vortex cavitating core that induces this kind of erosion. On the other hand, it is known that air injection has a beneficial effect on reducing the damage by cavitation. In this paper, a methodology to identify the interaction between guide vanes and tip vortex cavitation is presented and the effect of air injection in reducing this particular kind of erosion was studied over a range of operating conditions on a Kaplan scale model. It was found that air injection, at the expense of slightly reducing the efficiency of the turbine, mitigates the erosive potential of tip leakage cavitation, attenuates the interaction between the flow through the guide vanes and the tip vortex and decreases the level of vibration of the structural components.Facultad de Ingenierí

Modelación matemática aplicada al estudio de estaciones de bombeo

El trabajo se basó en la correspondencia de la ubicación y estructura de distintos tipos de vórtices visualizados sobre un modelo físico con las vorticidades que pudieran ser identificadas en la modelación de CFD. La construcción y operación del modelo físico estuvo a cargo del grupo de trabajo de Jun Matsui en Japón. Así mediante el software comercial FLOW-3D se simularon las mismas condiciones de ensayo utilizadas en el modelo físico, para luego ver si los vórtices que se identificaron se representaban satisfactoriamente mediante este modelo matemático.Facultad de Ingenierí

Pressure pulsation in Kaplan turbines: Prototype-CFD comparison

Pressure pulsation phenomena in a large Kaplan turbine are investigated by means of numerical simulations (CFD) and prototype measurements in order to study the dynamic behavior of flow due to the blade passage and its interaction with other components of the turbine. Numerical simulations are performed with the commercial software Ansys CFX code, solving the incompressible Unsteady Reynolds-Averaged-Navier Stokes equations under a finite volume scheme. The computational domain involves the entire machine at prototype scale. Special care is taken in the discretization of the wicket gate overhang and runner blade gap. Prototype measurements are performed using pressure transducers at different locations among the wicket gate outlet and the draft tube inlet. Then, CFD results are compared with temporary signals of prototype measurements at identical locations to validate the numerical model. A detailed analysis was focused on the tip gap flow and the pressure field at the discharge ring. From a rotating reference frame perspective, it is found that the mean pressure fluctuates accordingly the wicket gate passage. Moreover, in prototype measurements the pressure frequency that reveals the presence of modulated cavitation at the discharge ring is distinguished, as also verified from the shape of erosion patches in concordance with the number of wicket gates.Facultad de Ingenierí