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

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

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

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

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

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

Detección de vórtices en dársenas de bombeo mediante modelación matemática

El trabajo tiene como finalidad verificar la capacidad de un modelo matemático comercial de fluidodinámica computacional (CFD) para predecir la formación de vórtices en una dársena de bombeo. Se propone identificar vórtices de distinto origen e intensidad en una dársena de bombeo de geometría sencilla, de la cual se cuenta con resultados experimentales de las mismas condiciones de operación. Las velocidades calculadas muestran tendencias y magnitudes similares a las medidas, mientras que los valores máximos de vorticidad calculados resultan varios órdenes mayores que los medidos, lo cual se explica por las características de la medición en modelo físico. La modelación predice la presencia de vórtices superficiales cuya ocurrencia fue detectada en el modelo físico de referencia, pero además detecta vórtices de pared y de fondo que no fueron registrados en el trabajo de referencia. La representación de la vorticidad total, expresada por su valor absoluto, y seleccionada como superficie equipotencial, resulta ser una herramienta de visualización muy útil para realizar un seguimiento de la ubicación, trayectoria y variación temporal de los vórtices concentrados.The present work is aimed at verifying the ability of a commercial computational fluid dynamic (CFD) mathematical model to predict the formation of vortices in a pump basin. It was intended to identify vortices of diverse origin and intensity in a geometrically simple pump basin of which experimental results under the same operating conditions are known. Calculated velocities correlate well to trends and magnitudes of measured ones, whereas the maximum values of vorticity calculated are several orders of magnitude higher than those measured, which is explained by the characteristics of measurement in the physical model. Model results predict the presence of surface vortices which were seen in the reference physical model, but also wall vortices and bottom vortices which were not reported in the reference work under the same working conditions. For lower submergences, the presence of the bottom vortex in the physical model is inferred from the cavitation of its core. The representation of total vorticity, in terms of its absolute value, and selected as an equipotential surface, may turn into a very useful tool to visualize and follow the actual location, trajectory and time variation of concentrated vortices.Facultad de Ingenierí

Implementation of pressurized air injection system in a Kaplan prototype for the reduction of vibration caused by tip vortex cavitation

Blade tip cavitation is a well-known phenomenon that affects the performance of large-diameter Kaplan turbines and induces structural vibration. Injection of pressurized air has been found to yield promising results in reducing those damaging effects. In this work, the results of an experimental test of air injection on a 9.5-m-diameter Kaplan turbine are reported. Experiments were performed for several load conditions and for two different net heads. Accelerations, pressure pulsation and noise emission were monitored for every tested condition. Results show that, at the expense of a maximum efficiency drop of 0.2%, air injection induces a decrease on the level of vibration from 57% up to 84%, depending on the load condition. Such decrease is seen to be proportional to the air flow rate, in the range from 0.06 to 0.8‰ (respect to the discharge at the best efficiency point).Facultad de Ingenierí

Corrección de vórtices en dársenas de grandes estaciones de bombeo

La formación de vórtices de superficie libre y sumergidos es un problema que puede afectar severamente el funcionamiento de las bombas, por ser causales de: vibraciones que se traducen en desgaste de los cojinetes, caída de rendimiento por el ingreso de aire, caída de rendimiento por existir componentes tangenciales del flujo en el ingreso a las bombas, eventualmente, cavitación inducida por los vórtices de fondo y posibles sobrepresiones en la impulsión por la compresión rápida del aire incorporado. Esta problemática se intensifica con el aumento de los caudales de bombeo y la búsqueda de minimización del volumen de las obras y simplicidad de los diseños (para favorecer los procesos constructivos). En el caso que se presenta, además se pretende operar en condiciones de operación muy diferentes en cuanto a rango de caudales y niveles, lo cual hace que la etapa de optimización del diseño demande una serie de correcciones que deben verificarse en las distintas exigencias operativas.Facultad de Ingenierí