Hydrodynamics of Pumps

The subject of this monograph is the fluid dynamics of liquid turbomachines, particularly pumps. Rather than attempt a general treatise on turbomachines, we shall focus attention on those special problems and design issues associated with the flow of liquid through a rotating machine. There are two characteristics of a liquid that lead to these special problems, and cause a significantly different set of concerns than would occur in, say, a gas turbine. These are the potential for cavitation and the high density of liquids that enhances the possibility of damaging unsteady flows and forces

A Cavitation Susceptibility Meter with Optical Cavitation Monitoring -- Part Two: Experimental Apparatus and Results

This work in concerned with the development and operation of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. The pressure at the venturi throat is determined from the upstream pressure and the local flow velocity without corrections for viscous effects because the flow possesses a laminar potential core in all operational conditions. The detection of cavitation and the measurement of the flow velocity are carried out optically by means of a Laser Doppler Velocimeter. A custom-made electronic Signal Processor is used for real time data generation and temporary storage and a computerized system for final data acquisition and reduction. The implementation of the whole system is described and the results of the application of the Cavitation Susceptibility Meter to the measurement of the water quality of tap water samples are presented and critically discussed with reference to the current state of knowledge on cavitation inception

Torque converter turbine noise and cavitation noise over varying speed ratio

These investigations will discuss the operational noise caused by automotive torque converters during speed ratio operation. Two specific cases of torque converter noise will be studied; cavitation, and a monotonic turbine induced noise. Cavitation occurs at or near stall, or zero turbine speed. The bubbles produced due to the extreme torques at low speed ratio operation, upon collapse, may cause a broadband noise that is unwanted by those who are occupying the vehicle as other portions of the vehicle drive train improve acoustically. Turbine induced noise, which occurs at high engine torque at around 0.5 speed ratio, is a narrow-band phenomenon that is audible to vehicle occupants currently. The solution to the turbine induced noise is known, however this study is to gain a better understanding of the mechanics behind this occurrence. The automated torque converter dynamometer test cell was utilized in these experiments to determine the effect of torque converter design parameters on the offset of cavitation and to employ the use a microwave telemetry system to directly measure pressures and structural motion on the turbine. Nearfield acoustics were used as a detection method for all phenomena while using a standardized speed ratio sweep test. Changes in filtered sound pressure levels enabled the ability to detect cavitation desinence. This, in turn, was utilized to determine the effects of various torque converter design parameters, including diameter, torus dimensions, and pump and stator blade designs on cavitation. The on turbine pressures and motion measured with the microwave telemetry were used to understand better the effects of a notched trailing edge turbine blade on the turbine induced noise

Digital platform approach for hydraulic characterization and detection of hydrodynamic phenomena in reversible turbomachines

La energía hidroeléctrica es una de las fuentes de energía renovable más desarrolladas y utilizadas en términos de capacidad instalada y fiabilidad; por lo tanto, es uno de los métodos de producción de electricidad con mayor contribución a nivel mundial y con un nivel de madurez tecnológica esencial. En los últimos años, el sector hidroeléctrico ha pasado de las grandes a las pequeñas centrales, debido a las limitaciones de los recursos hídricos y a los efectos del cambio climático. Además, aunque menos utilizadas, las turbomáquinas hidráulicas reversibles se postulan como alternativas para compensar la demanda de tecnologías de energías renovables intermitentes y de difícil previsión. Este artículo presenta el desarrollo e implementación de una plataforma digital basada en mediciones en tiempo real para la monitorización, registro y análisis en los dominios de tiempo y frecuencia de magnitudes hidromecánicas, dinámicas y eléctricas. Los datos resultantes son adecuados para la caracterización de las máquinas hidráulicas, y su diagnóstico técnico, permitiendo la detección de fenómenos complejos que pueden ocurrir durante su funcionamiento. Para ilustrar algunas de las capacidades de la plataforma, se realizaron pruebas experimentales en una bomba centrífuga específica de baja velocidad, a través de su curva característica de cuatro cuadrantes. La atención se centró en la medición de la fluctuación de la presión y la aceleración (vibraciones) durante el funcionamiento en su punto de mejor rendimiento (BEP) y en un punto de funcionamiento fuera de diseño en el que se detectaron inestabilidades hidrodinámicas. Por último, se exponen los análisis y resultados, y se presentan las características.Hydropower is one of the most developed and utilized renewable energy sources in terms of installed capacity and reliability; therefore, it is one of the electricity production methods with the highest contribution worldwide and with an essential level of technological maturity. In recent years, the hydropower sector has shifted from large to small plants, due to water resource constraints and the effects of climate change. In addition, although less used, reversible hydro turbomachines are postulated as alternatives to offset the demand for intermittent and difficult to forecast renewable energy technologies. This paper presents the development and implementation of a digital platform based on real-time measurements for monitoring, recording, and analysis in the time and frequency domains of hydromechanical, dynamic, and electrical magnitudes. The resulting data are suitable for the characterization of hydraulic machines, and their technical diagnosis, allowing the detection of complex phenomena that may occur during their operation. To illustrate some of the capabilities of the platform, experimental tests were performed on a specific low-speed centrifugal pump, through its four-quadrant characteristic curve. The focus was on the measurement of pressure fluctuation and acceleration (vibrations) during operation at its best efficiency point (BEP) and at an off-design operating point where hydrodynamic instabilities were detected. Finally, the analysis and results are exposed, and the characteristics are presented

Metodología de apoyo para la programación a corto plazo de la operación de unidades de generación tipo Francis en rangos extendidos

La generación hidroeléctrica goza de una gran aceptación a nivel mundial debido a sus ventajas: es limpia, renovable, flexible, controlable y madura -- En Colombia, representa aproximadamente el 75% de las fuentes de energía -- A partir de los años noventa, el mecanismo para comercializar al corto plazo este producto es la Bolsa de Energía -- Para participar en ella, cada generador debe presentar diariamente una oferta en la cual se declara una disponibilidad en kilovatio-hora a un precio determinado -- Esta modalidad de mercado representa un desafío permanente para los encargados de la planeación a mediano y largo plazo, y para los responsables de la programación diaria de las unidades de generación -- En los últimos años se reportan en la literatura numerosos trabajos que pretenden modelar la producción, encontrándose diferentes enfoques -- En este proyecto se tiene en cuenta para la programación de unidades de generación tipo Francis la posibilidad de operar por fuera de los márgenes establecidos por el fabricante -- Las turbinas tipo Francis son una de las máquinas más utilizadas para la generación hidroeléctrica, son versátiles en un amplio rango de operación e incluso expertos comienzan a indagar acerca de sus comportamientos en rangos extendidos -- En estos rangos se presentan fenómenos como la cavitación y las vibraciones por la interacción fluido-estructura que pueden acelerar el desgaste de los componentes de la turbina -- En este trabajo se propone una metodología teórica en la cual las partes de la máquina sometidas al desgaste acelerado, se convertirían en repuestos o material de sacrificio para sacar provecho de escenarios que de otro modo serían desventajosos, por ejemplo en temporadas con precios especialmente altos del kilovatio-hora por disminución en las afluencias -- A partir de modelos de desgaste se formulan restricciones a la maximización del beneficio -- Esta propuesta puede interesar al generador, al fabricante y proveedor de componentes quienes podrían encontrar aquí nuevos lineamientos de diseño y desarrollo de nuevos materiale

Hydrolink 2020/2. Hydraulic Transients

Topic: Hydraulic Transient

Turbomachinery Developments and Cavitation

After a brief review of flow-induced instabilities in turbopumps for liquid propellant feed systems of modern rocket engines, the lecture illustrates some recent results of the work carried out at Alta on the hydrodynamics and unsteady cavitation phenomena of these machines. A reduced order model for preliminary design and noncavitating performance prediction of tapered axial inducers is illustrated. In the incompressible, inviscid, irrotational flow approximation the model expresses the 3D flow field in the blade channels by superposing a 2D cross-sectional vorticity correction to a fully-guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow allows for the control of the blade loading and the estimate of the boundary layer blockage at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carter’s rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation and Euler’s equation are used to derive a simple 2nd order boundary value problem, whose numerical solution describes the far field axisymmetric flow at the inducer discharge. A closed form approximate solution is also provided, which proved to yield equivalently accurate results in the prediction of the inducer performance. Finally, the noncavitating pumping characteristic is obtained by introducing suitably adapted correlations of pressure losses and flow deviation effects. The model has been verified to closely approximate the geometry and noncavitating performance of a number of tapered-hub high-head inducers for space application. The results of a series of tests conducted in water under similarity conditions on the four-bladed DAPAMITO4 inducer, designed and manufactured by means of the above reduced-order model, are illustrated. Several non-synchronous instabilities have been observed on the inducer, including an axial surge, a backflow oscillation and, at higher temperatures, incipient rotating cavitation and backflow vortex instability. In addition, synchronous rotating cavitation (leading to the characteristic “one step” shape of the cavitating performance curve near head breakdown conditions) has been detected at all the flow conditions investigated. It has been found that the amplitude of the flow oscillations associated to this instability generally tends to decrease at higher water temperatures. The characterization of the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch high-head inducer, under different load and cavitation conditions is presented. The results have obtained in the Cavitating Pump Rotordynamic Test Facility at Alta by means of a novel experimental technique, allowing for the continuous measurement of the rotordynamic force spectra as functions of the whirl ratio. Comparison with simultaneous high-speed movies of the inducer inlet flow highlighted the relationship between the cavitation dynamics in the inducer backflow and the spectral behavior of the rotordynamic force as functions of the whirl ratio. Finally, the future perspectives of the work carried out at Alta on the hydrodynamics and unsteady cavitation phenomena of high performance turbopumps for liquid propellant feed systems of modern rocket engines are briefly illustrated

Analysis of the flow through a Francis\u27 turbine runner using computational fluid dynamics

This study investigates the fluid flow through the runner of an exceptionally low specific speed Francis’ turbine using computational fluid dynamics (CFD) to predict some performance characteristic of the runner, and to determine the effect of cavitation on the flow features within the turbine. It employs FLUENT CFD codes for the analysis. The study was carried out under steady conditions in two- dimensional analysis, primarily using the RNG k-ε turbulence model. It is mainly on single phase analysis; however a single case of two-phase cavitation modeling was performed in order to evaluate cavitation effects. The hydraulic losses were much dependent on the inlet flow velocity and less dependent on the rotational speed. Cavitation occurred throughout the flow domain and this led to non-physical negative pressure predictions for the single phase analysis. For a fixed discharge, cavitation had significant effect on the prediction of the torque and hence the runner efficiency obtained from the CFD analysis. It causes a significant reduction of the runner efficiency by increasing the hydraulic losses and should be considered in the analysis of the flow features within a runner which is likely to experiences cavitation in order to overcome some of the shortcomings of the single phase predictions

A control system for preventing cavitation of centrifugal pumps

Cavitation is a well-known phenomenon that may occur, among other turbo-machines, in centrifugal pumps and can result in severe damage of both the pump and the whole hydraulic system. There are situations in which, in principle, the cavitation could be avoided by detecting the condition of incipient cavitation, and changing slightly the working point of the whole system in order to move away from that condition. In the present paper two simple closed-loop control strategies are implemented, acting on the pump's rotational speed and fed by the measurements of a set of inertial sensors. In particular, the research is focused on a centrifugal pump normally employed in hydraulic systems. The pump operates in a dedicated test rig, where cavitation can be induced by acting on a reservoir's pressure. Three accelerometers are installed in the pump body along three orthogonal axes. An extensive set of experiments has been carried out at different flow rates and a number of signals' features both in the time domain and in the frequency domain have been considered as indicators of incipient cavitation. The amount of energy of the signal captured by the accelerometer in the component orthogonal to the flow direction, in the band from 10 to 12.8 kHz, demonstrated to be effective in detecting the incipient cavitation, by selecting a proper (condition-dependent) threshold. Therefore, two simple controllers have been designed: the first regulates the speed of the pump, to recover from cavitation, bringing the indicator back to the nominal value, while the second allows to reduce the pump's rotational speed when the cavitation detector indicates the incipient cavitation and restoring the nominal speed when possible. The latter approach is rather general, because the threshold-based detector can be substituted by any detector providing binary output. Experimental results are reported that demonstrate the effectiveness of the approach