Academic literature on the topic 'Unstable charge pump'

In this article, the effect of dust charge fluctuations on the parametric upconversion of a lower hybrid wave into an ion cyclotron wave and a side band wave in a two-ion species tokamak plasma is studied. When the oscillatory velocity of plasma electrons is a few percent of the sound velocity, the lower hybrid wave becomes unstable and decays into two modes: an ion cyclotron wave mode and a low frequency lower hybrid side band wave. Furthermore, a ponderomotive force by a lower hybrid pump and a side band wave is exerted on the existing electrons, which drives the ion cyclotron decay mode. The presence of negatively charged dust grains and their shape, size, radius, and density influence the instability. The growth rate of instability is calculated by considering typical existing D–T (Deuterium–Tritium) dusty plasma parameters, and it is observed that the growth rate increases with the relative density of dust grains, number density of dust grains, oscillatory velocity of electrons, and amplitude of pump waves. However, the normalized growth rate increases with the unstable wave frequency, and it also increases as we increase the ratio of deuterium to tritium density. Here, the growth rate decreases with the increase in the size of dust grains and electron cyclotron frequency. The theoretical results summarized in the present study are able to efficiently elaborate the complexity produced in plasma properties in a tokamak due to the dust–plasma interactions, which are briefly discussed here.

AbstractIntroduction:Unexpected disasters, such as earthquakes or fires, require preparation to address knowledge gaps that may negatively affect vulnerable patients. Training programs can promote natural disaster readiness to respond and evacuate patients safely, but also require evidence-based information to guide learning objectives.Problem:There is limited evidence on what skills and bedside equipment are most important to include in disaster training and evacuation programs for critically ill infants.Methods:An expert panel was used to create a 13-item mastery checklist of skills for bedside registered nurses (RNs) required to successfully evacuate a critically ill infant. Expert nurses were surveyed, and the Angoff method was used to determine which of the mastery checklist skills a newly graduated nurse (ie, the “minimally competent” nurse) should be able to do. Participants then rated the importance of 26 commonly available pieces of bedside equipment for use in evacuating a hemodynamically unstable, intubated infant during a disaster.Results:Twenty-three emergency department (ED) and neonatal intensive care unit (NICU) charge RNs responded to the survey with a mean of 19 (SD = 9) years of experience and 30% reporting personal experience with evacuating patients. The skills list scores showed an emphasis on the newly graduated nurse having more complete mastery of skills surrounding thermoregulation, documentation, infection control, respiratory support, and monitoring. Skills for communication, decision making, and anticipating future needs were assessed as less likely for a new nurse to have mastered. On a scale of one (not important) to seven (critically important), the perceived necessity of equipment ranged from a low of 1.6 (breast pump) to a high of 6.9 (face mask). The individual intraclass correlation coefficient (ICC) of 0.55 showed moderate reliability between raters and the average team ICC of 0.97 showed excellent agreement as a group.Conclusion:Experts rated the ability to manage physiological issues, such as thermoregulation and respiratory support, as skills that every nurse should master. Disaster preparedness activities for nurses in training may benefit from checklists of essential equipment and skills to ensure all nurses can independently manage patients’ physiologic needs when they enter the workforce. Advanced nursing training should include education on decision making, communication during emergencies, and anticipation of future issues to ensure that charge and resource nurses can support bedside nurses during evacuation events.

The unsteady flow process of waterjet pumps is related to the comprehensive performance and phenomenon of rotating stall and cavitation. To analyze the unsteady flow process on the unsteady condition, a computational domain containing nozzle, impeller, outlet guide vane (OGV), and shaft is established. The surface vortex of the blade is unstable at the valley point of the hydraulic unstable zone. The vortex core and morphological characteristics of the vortex will change in a small range with time. The flow of the best efficiency point and the start point of the hydraulic unstable zone on each turbo surface is relatively stable. At the valley point of the hydraulic unstable zone, the flow and pressure fields are unstable, which causes the flow on each turbo surface to change with time. The hydraulic performance parameters are measured by establishing the double cycle test loop of a waterjet propulsion device compared with numerical simulated data. The verification results show that the numerical simulation method is credible. In this paper, the outcome is helpful to comprehend the unsteady flow mechanism in the pump of waterjet propulsion devices, and improve and benefit their design and comprehensive performance.

In this article, the influence of rotor–stator distance on the pump performance and rotor–stator interaction strength within mixed-flow centrifugal pump was investigated based on numerical calculation and test verification. Firstly, the performances of mixed-flow centrifugal pumps with two different rotor–stator distances were obtained and compared with the numerical results, which confirms the high accuracy of the numerical simulation. Next, the performances of mixed-flow centrifugal pumps with five different rotor–stator distances were compared and analyzed. It was found that the hydraulic performance of the mixed-flow centrifugal pump varies slightly as the rotor–stator distance increases. The mean values of the standard deviation of the head and efficiency of the mixed-flow centrifugal pump at each rotor–stator distance under full flow conditions are only 0.16 m and 0.11%, respectively. Then, the strengths of the rotor–stator interaction with different rotor–stator distances were analyzed. It was found that the strengths of the shock interaction, the wake interaction, and the potential interaction were all reduced with increasing rotor–stator distance. Moreover, when the rotor–stator distance is 1.5 mm, the pressure distribution in the circumferential direction of the rotor–stator interference zone shows obvious unstable characteristics: the pressure change amplitude is significantly greater than the other rotor–stator distance of the pressure change amplitude, the maximum and minimum pressure amplitude difference being 56.9 kPa, and with the increase in the rotor–stator distance, the maximum and minimum pressure amplitude difference gradually decreases, with an average value of 32.3 kPa. These findings could provide useful insight into prospects for the improvement of the operational stability of mixed-flow centrifugal pumps, and the results of this study can be extended to all centrifugal pumps using diffusers in the form of vanes as the pressure chamber, which has strong practical application and theoretical value.

Introduction. The preliminary study of new developed pumps for circulatory support on the hydrodynamic circulation model is an important step in the process of their designing. Hydrodynamic circulation models that can closely imitate cardio – vascular system are important to defi ne the range of effective functioning of the pumps under normal and heart disease conditions which is of great importance for defi ning the mode of these pumps in real clinical conditions.The aim of study is to create a new hydrodynamic circulation model of the systemic circulation to study the processes of interaction of heart left ventricle and continuous – fl ow pumps.Materials and methods. The main components of the mock circulation model (arterial and venous blocks) are designed as closed reservoirs with an air bag providing the necessary elasticity value of these reservoirs. The heart left ventricle was simulated with an artifi cial heart ventricle with a pneumatic drive Sinus-IS which allows to change its options in a wide range. As a test pump we used the fi rst native implantable axial pump VISH – 1. In the course of research we made the registration and recording of the basic hemodynamic parameters (pressure, fl ow) with a multichannel module Pumpax for the measurement of pressure parameters.Results. The designed circulation model allows to adequately reproduce the main hemodynamic parameters of the circulatory system in normal (arterial pressure – 110/77 mmHg, left atrium pressure – 7 mmHg and cardiac output – 4.2 l/min) and heart failure conditions (arterial pressure – 79/53 mmHg, left atrium pressure – 15 mmHg and cardiac output – 3.1 l/min). On the circulation model the interaction of heart left ventricle and continuous-fl ow pump in heart failure simulation was studied. The dynamics of the main circulation fi gures is shown under conditions of changing of the pump rotor speed. Meanwhile, the conditions of the closing of aortic valve are identifi ed which is important for theclinical use of this pump. Using a special separately fl exible camera on the pump inlet we modeled phenomena of negative pressure leading to unstable pump operation and reduction of pump fl ow.Conclusion. Characteristics of the developed hydrodynamic circulation model allow us to reproduce parameters of systemic circulation in a wide range and to use it in designing of new pumps for circulatory support.

With the rapid development of industrialization, the excessive use of fossil fuels has caused problems such as increased greenhouse gas emissions and energy shortages. The development and use of renewable energy has attracted increased attention. In recent years, solar heat pump heating technology that uses clean solar energy combined with high-efficiency heat pump units is the development direction of clean heating in winter in northern regions. However, the use of solar energy is intermittent and unstable. The low-valley electricity policy is a night-time electricity price policy. Heat pump heating has problems such as frosting and low efficiencies in cold northern regions. To solve these problems, an exergy analysis model of each component of a phase-change heat-storage coupled solar heat pump heating system was established. Exergy analysis was performed on each component of the system to determine the direction of optimization and improvement of the phase-change heat-storage coupled solar heat pump heating system. The results showed that optimizing the heating-end heat exchanger of the system can reduce the exergy loss of the system. When the phase-change heat-storage tank meets the heating demand, its volume should be reduced to lower the exergy loss of the tank heat dissipation. Air-type solar collectors can increase the income exergies of solar collectors.

Abstract High stability in axial direction is required for rocket pumps operated under extremely low-temperature and high-pressure conditions, turbopumps therefore uses balance piston (BP) system for balancing their axial thrust. The BP system is stable under quasi-static conditions. However, BP system might become dynamically unstable under some condition. Thus it is fundamental for stability evaluation of turbopumps to predict static/dynamic characteristics in axial direction of BP system. Furthermore, we focus on characteristic change by cavitation which often occurs in the pump inlet. In this paper, an experimental study of a model turbopump which had an unshrouded impeller equipped with BP system was carried out. We experimented it with an active magnetic bearing (AMB) test facility in order forcibly to oscillate it with an optional amplitude and frequency. In addition, we examined characteristics of BP system by three-dimensional computational fluid dynamics (3D-CFD) simulations. The results of 3D-CFD simulations were in good agreement with these tendency of BP system, and were effective in predicting its static/dynamic characteristics. Some cases showed that dynamic characteristic of BP system became unstable by growth of cavitation, therefore we suggest that the influence of cavitation must also be considered in the design of turbopump.

The mixed-flow pump is wildly used in industrial and agricultural fields, and the requirements for its stability become more and more restricted with the improvement of the capacities. When the pump is operated under part-loading conditions, unstable characteristic like rotating stall happens and brings intensive vibration and noise. To reveal the internal unstable flow characteristics in mixed-flow pump, unsteady numerical simulation is conducted under different flow rates in the current research. The secondary flow and the vortex induced by the unsteady flow in the pump, which vary with the change of the flow rate, are analyzed according to the simulation results. Pressure fluctuation in different parts of the impeller is numerically predicted and correlated to the rotating stall propagation process. With the reduction of the flow discharge, a critical flow rate (0.6 Qdes in the current model pump) is found related to the rotating stall inception. Unstable vortex structure is found in part of the flow passages of the impeller under the stall inception stage; 0.56 Qdes is the flow rate related to the deep rotating stall under which stable stall vortexes are found in each flow passage of the impeller. Three types of pressure fluctuation characteristics representing different flow field pattern in the impeller are found. When the pump is working at designed flow rate, the pressure fluctuation is regular with the dominant frequency being the rotation frequency fs of the rotor. At the stall inception condition, when the stall core appears in the flow passages of the impeller in turn, the amplitude of the pressure fluctuation is increased and the dominant frequency is changed to a low-frequency signal. When the flow rate is reduced below deep stall condition, owning to the reflux from the guide vane and the intensive rotor–stator interaction, the blade passing frequency of the guide vane fb2 becomes the dominant frequency. Finally, unsteady characteristics of the flow field under stall inception condition are analyzed to demonstrate the propagation process of the rotating stall. The stall vortex is found to propagate from the stalled flow passage to the next, through the tip area of the blade leading edge. During the propagation process, the first vortex shrinks and disappears gradually, while the second vortex grows continuously, and the third vortex shows up at the vanishment of the first stall vortex. This research provides detailed information for the unstable flow especially related to rotating stall evolution with the variation of the operating flow rate of the mixed-flow pump with guide vane.

Abstract Hydropower is a major renewable clean energy and is widely used worldwide. The reversible pump-turbine unit of the pumped-storage power station is able to work in two main operating modes as required by the power grid: turbine-mode for power generation and pump-mode for power storage. In order to absorb unstable energies such as wind and solar energy and improve the quality of the electricity, reversible pump-turbines need to frequently change operating conditions, and experience more start-stops under different operating modes in a short period. The unstable flow during these transient processes will lead to high-level stresses on the structural components of the pump-turbine units. Therefore, it is of great engineering and academic significance to study the flow characteristics and structural characteristics of the unit during the transient processes. This paper has established a numerical calculation model for a prototype reversible pump-turbine unit, has carried out the CFD calculations of the pump-turbine fluid domains during the pump shutdown transient process, and has analysed the corresponding structural dynamic characteristics of the stationary components of the unit with the fluid-structure coupling method. The pressure variation trend of the spiral case outlet during pump shutdown has the same trend as that of the spiral case domain, and the guide vane flow domain. The maximum flow-induced deformation and stress of the stationary structures have a strong correlation with the axial thrust values of the head cover. The maximum deformation occurs at the inner edge of the head cover, and the maximum stress appears in the fillet of the stay vane leading edge. An increase in the number of shutdowns will result in a higher real risk of fatigue damage to the stay vanes. The conclusions obtained are of great value for safe operation, field condition monitoring, fault diagnosis, and predictive maintenance of the pump-turbine units.

Submersible drainage sump pumps work in a highly corrosive environment, forming films with corrosive reaction products on the surface. Pump rotors are high-demand parts, so they are made of quality materials with good wear and corrosion resistance properties such as nodular graphite cast iron. This paper analyses the corrosion behaviour of cast iron used in the manufacture of rotors in three types of wastewaters, with variable pH. Nodular graphite cast iron samples were immersed in wastewater for 30, 60, and 90 days and tested by linear polarisation and electrochemical impedance spectroscopy (EIS). Also, the layers of reaction products formed on the surface of the material were analysed by SEM and EDS. The results showed that nodular cast-iron immersed in wastewater with acidic pH showed intense corrosion, the oxide layer formed on its surface is unstable. Also, the final structure of the product layer is that of a tri-layer with cations and anions absorbed from the corrosion media: the double-electric layer directly connected to the metal surface, an internal layer consisting of ferrous compounds and ferric compounds that control the diffusion of oxygen, an outer layer, and a compact crust of ferric compounds. The change in the pH of the wastewater has a major influence on the corrosion rate of the cast iron, which increases from 356.4 µm/year in DWW-1 (6.5 pH) to 1440 µm/year in DWW-2 (3 pH) and 1743 µm/year DWWW-3 (11 pH) respectively. As can be seen, the experimental study covers the problem of the corrosion behaviour of the pump rotor in various types of wastewaters this aspect is particularly important for the good use of wastewater pumps and to predict possible deviations for the operation of the equipment within the treatment plants.

La récupération de l'énergie est le processus qui consiste à convertir l'énergie inutilisée présente dans notre environnement en énergie électrique utilisable pour alimenter un système électronique. Les récupérateurs d'énergie cinétique à transduction électrostatique (eKEH) utilisent l'énergie cinétique présente dans l’environnement, qui provient d'un objet en mouvement ou de vibrations, afin de la convertir en énergie électrique. Le principe employé est basé sur un condensateur variable polarisé. En ajoutant une couche triboélectrique entre ses armatures et en créant un contact entre elles, un eKEH devient ce qui l’on appelle communément un nanogénérateur triboélectrique (TENG). Ce type de transducteur accumule des charges par contact dans la couche triboélectrique qui devient ainsi un électret dont le champs électrique semi-permanent généré permet une variation de la distribution des charges électriques dans les électrodes par induction électrostatique.La modification de l'architecture d'un TENG par l'ajout d'une troisième électrode permet de transformer un transducteur à capacité simple en un TENG à capacité double. Le fait de doubler l'élément de conversion dans un transducteur doit permettre d’augmenter la quantité d'énergie convertie. Le circuit électronique choisi pour redresser le signal obtenu en sortie du générateur est le doubleur de Bennet. L’absence de tension de saturation en sortie de ce circuit est une caractéristique particulière de cette pompe de charge dite instable. Elle se traduit par une augmentation exponentielle de la tension de sortie et des charges accumulées dans le condensateur de stockage qui augmentent (en théorie) de manière infinie. Cela signifie que la surface du cycle charge-tension aux bornes du TENG, et donc l’énergie convertie du domaine mécanique, augmente à chaque itération du cycle mécanique du transducteur.Cette thèse comprend l'analyse, la simulation et la démonstration expérimentale d’un doubleur de Bennet comportant deux capacités variables asymétriques incluant chacune une couche triboélectrique. Il est montré que les performances du système « double TENG » - « double Bennet » sont supérieures au doubleur de Bennet classique. L'étude analytique s'aligne sur les simulations. Le système a été testé expérimentalement. Il en est conclu que les résultats expérimentaux sont pertinents lorsqu'ils sont comparés aux performances du système classique d’un « TENG mono-capacitif » - « Bennet simple». Par rapport au doubleur de Bennet classique, le double Bennet atteint les mêmes niveaux de tension en moins de temps. Cela est dû à l'avantage du double condensateur TENG qui augmente le nombre de charges accumulées par cycle mécanique. L'analyse a montré que les deux condensateurs TENG sont co-dépendants et qu’ils s’influencent mutuellement lorsqu’ils sont mis en fonction.Le signal de sortie du double Bennet est caractérisé par des tensions élevées allant de plusieurs centaines à plusieurs milliers de volts. Pour abaisser la tension redressée en sortie à un niveau compatible avec une application commerciale, un convertisseur DC-DC de type Buck est implémenté. Celui-ci nécessite un interrupteur. Cette thèse propose et étudie l'utilisation d'un interrupteur haute-tension MEMS dit à micro-plasma dont la tension d’actionnement est définie par la loi de Paschen. Cette thèse se conclue par une étude théorique et expérimentale de cette loi à l'échelle micrométrique dans le but de proposer des tensions d’actionnement optimales pour une meilleure gestion de l'énergie captée
Energy harvesting is the process that involves converting otherwise unused energy present in our environment into usable electrical energy that can be used to power an electronic system. Electrostatic kinetic energy harvesters (eKEHs) utilize vastly present kinetic energy that originates either from an object in motion or vibrations and converts it into electrical energy. The employed principle is based on a polarized variable electrostatic capacitor. With an addition of a triboelectric layer between its plates and utilizing the triboelectrification effect, an eKEH is transformed into a triboelectric nanogenerator (TENG). This type of transducer accumulates charges by contact in the triboelectric layer which thus becomes an electret whose generated semi-permanent electric field allows a variation of the distribution of the electric charges in the electrodes by electrostatic induction.Altering the architecture of a TENG by adding the third electrode, a single-capacitive transducer is converted into a double-capacitive TENG. Doubling the conversion element in a transducer is expected to increase the amount of converted energy. Chosen electronics circuit to condition obtained signal from the generator is Bennet’s charge doubler. An increase without saturation point at the output of this circuit is the unique characteristic of this unstable charge pump. It reflects through an exponential increase of output voltage and a number of charges accumulated in the storage capacitor which increase (in theory) in an infinite way. This means that the surface of the charge-voltage cycle at the terminals of the TENG, and thus the converted energy of the mechanical domain, increases at each iteration of the mechanical cycle of the transducer.The scope of this thesis encompasses the simulation, analytical and experimental research of Bennet’s charge doubler with two asymmetric variable capacitors each containing a triboelectric layer. It is postulated that the performance of the "double TENG" - "double Bennet" system is superior to the classic Bennet’s double. The results of analytical and simulation analysis have shown that the expected behavior of this circuit aligns with hypothesized performance results. The system has been tested experimentally. It is concluded that the results of the constructed system are relevant when compared with the reported performance of the classic "single-capacitive TENG" - "Bennet’s doubler" system.When compared with classic Bennet’s doubler, double Bennet reaches the same voltage levels in less time. That is due to the advantage of double capacitive TENG which increases the number of accumulated charges per mechanical cycle. In analytical analysis, it was found that the two TENG capacitors are codependent and that in operation they affect one another.The output signal of double Bennet is characterized by high voltages ranging from a few hundreds of volts to a few kilovolts (kV). To reduce the rectified output voltage to a level compatible with a commercial application, a Buck DC-DC converter is implemented. This requires a switch. This thesis proposes and studies the use of a high-voltage MEMS micro-plasma switch whose actuation voltages is defined by Paschen’s law. Within the scope of this thesis, the theoretical and experimental studies of this law at the micrometer scale propose optimal actuation voltages for better management of the converted energy

Abstract Centrifugal pump performance curves instability, characterized by a local dent, can be the consequence of flow instabilities in rotating or stationary parts. Such flow instabilities often result in abnormal operating conditions, causing severe problems such as increased pressure pulsation, noise and vibration which can damage both pump and system. For the pump to have reliable operation, it is necessary to understand the onset and the mechanism of the phenomenon resulting in performance curves instability. Present paper focuses on performance curves instability of a centrifugal pump of low specific speed (ωs = 0.65, Ns = 1776) and aims at a better understanding of the mechanism leading to the head drop observed during tests at part load. For that purpose, Computation Fluid Dynamic (CFD) was performed using a Large-Eddy Simulation (LES) approach. The geometry used for present research is in fact the first stage of a multi-stage centrifugal pump and is composed of a suction chamber, a closed-type impeller, a vaned diffuser and return guide vanes to next stage (not included). Leakages at wear ring and stage bush were also included in the computed geometry in order to consider their potential influence on pump stability. The occurrence of the instability in CFD is found at a higher flow rate than in the experiments. It is observed that the pre-swirl angle is under-predicted by several degrees which leads to change the impeller operating conditions. Nevertheless, the analysis of the CFD results is still useful to have a better understanding of the onset of the head drop. When the head drops, a switching of low radial and axial velocities at the impeller outlet from the hub side to the shroud side is observed. This change of flow pattern goes along with a strong increase of the diffuser inlet throat recirculation and the development of stall, that impairs pressure recovery between the impeller outlet and the diffuser inlet. As the pump flow rate is further decreased below the head drop flow rate, recirculation at the diffuser throat extend toward the impeller outlet and impact Euler head. Conversely, the pressure recovery from the impeller outlet to the diffuser inlet throat increases again as the flow velocity slowdown can be effective again. Consequently, the pump head increases again.

Static and dynamic characteristics of a mixed flow pump were measured at low flow rate, where the steady characteristic curves show a positive slope. Unsteady flow was measured at upstream and downstream of impeller, as well as on the casing wall of diffuser passage by using a fast response five-hole pitot tube and semi-conductor type pressure transducers. Dynamic response of the pump to the sinusoidal change in flow rate was measured under constant rotational speed. As a result of the unsteady flow measurements, the positive slope of steady characteristic was found to be caused by the impeller tip separation, inlet backflow, and pre-rotation. Moreover, the dynamic characteristics of the pump were found to become unstable with the increased frequency of flow rate.

Pumped storage system contributes to adjust the electric power unbalance between day and night, in general. The pumping operation, however, may be unstable in the rising portion of the head characteristics, and/or bring the cavitation at the low suction head. To simultaneously overcome both weak points, the authors have proposed a superior pumping unit that is composed of counter-rotating type impellers and a peculiar motor with double rotational armatures. This paper discusses the operation at the turbine mode of the above unit. It is concluded with the numerical simulations that this type unit can be also operated acceptably at the turbine mode, because the unit works so as to coincide the angular momentum change through the front runners/impellers with that thorough the rear runners/impellers, namely to take the axial flow at not only the inlet but also the outlet without the guide vanes.

Abstract The Detached Eddy Simulation (DES) has been used to simulate the pressure fluctuation of the impeller in an axial flow pump. The results were combined with experiments including high-speed photography and transient pressure measurements to investigate the unstable flow induced by tip leakage vortex (TLV). Numerical results show that maximum predictive error values of head is 2.9%, compared with experimental results. The pressure fluctuation at different monitoring points present a certain regularity, with 3 peaks and 3 troughs in a period, corresponding to the number of blades. The amplitude of pressure fluctuation at P1 (impeller inlet) is the highest among those monitoring points, where the amplitude decreases with the flow rates. The dominant frequency of pressure fluctuation at impeller under cavitation condition is the blade passing frequency (BPF). Besides, there are also N* = 6, 9, 12 and other more harmonic frequencies. The cavitation flow was analyzed with the pressure fluctuation of the blade tip. For the existence of the pressure difference between pressure side and suction side, the pressure at monitoring points change alternately. The amplitude of the fluctuation near tip is affected seriously by the cavitation bubbles, as the cavitation could is a low pressure region with unstable fluctuation.

The objective of the study was the experimental and computational investigation of the unsteady flow in the centrifugal pumps. This paper analysed the effect of the vaneless stator interference on the exit flow field of a radial pump operated in the DIM facility. High-response pressure transducers were used to determine unsteady pressure field at three planes at the pump and at diffuser inflows. The experimental data showed that unsteady pressure disturbances modes change when the flow was reduced. Detailed analysis showed that disturbances occur at distinct frequencies and that these rotated in the circumferential direction. Comparison of the pressure signals measured at two circumferential locations on the casing confirmed the characteristic frequency pattern to be a so called “rotating instability”. This unsteady phenomenon was highlighted both at design flow rate and at low flow rates. The azimuthal distributions exhibited significant nonuniformities. The amplitude of this non-uniformity was sensitive to the flow rate. A simple model showed that, contrary to the common belief, the transport of the vane wake and secondary flows across the rotor was not enough to explain the magnitude of the variations. In this paper numerical investigations of the unsteady three-dimensional flow through the pump stage were also presented. Turbulence was modelled both by the k-ω transport equations model, and Reynolds Stress Model based on the ω-equation. The effects of the tip leakage flow were considered by meshing the tip clearance between rotor blade and casing. Results showed the jet-wake flow pattern induced an unstable vortex, which influenced flow discharging from the adjacent passage and destabilised jet-wake flow in the passage. Both calculations and measurements detected the periodic fluctuations at impeller discharge which were found to be coherent from blade to blade and possessed a rich harmonic content.

Velocity measurements have been obtained in a centrifugal pump with a volute. The measurements have been obtained between the blade passages of a shrouded impeller and in the volute region using laser Doppler velocimetry (LDV). Complimentary measurements have been obtained with pressure transducers circumferentially mounted on the volute. The flow through the impeller is measured at both design load and at several off-design conditions including severe off-design conditions. Results show that flow behaviour at design conditions Qd is well behaved. At reduced flow off-design conditions (0.25Qd) the measured flow pattern was altered significantly showing alternating stable stationary stalled and unstalled passages. Spectral analysis of the data from the LDV probe and the pressure probes showed a marked change in the spectrum as the stall phenomena occurs. This result is in contrast with previous researchers who have reported partially stalled flow passages or rotating stall conditions where the stall pattern rotated relative to the impeller with a regular frequency.

Centrifugal pumps of low specific speed display an inherent tendency to generate an unstable pump performance curve [1]. These curves are characterized by a head dropping at low flow rates that limits the operational range. Hence, for example centrifugal pumps with such performance curves are not suitable for a usage in firefighting applications or parallel operation. However, there are a few actions that influence positively the stability of the performance curve [1, 2]. One is adding slots at the rear shroud, e.g. on the pressure side or the suction side of the blade. Slots at the pressure side of the blade stabilize the characteristic curve by increasing the head, while suction-side slots stabilize the characteristic curve by dropping it down [2]. The part load flow pattern of a centrifugal pump includes two recirculation zones. The first is located at the inlet of the impeller and caused by the blade suction geometry. The second recirculation zone forms at the outlet of the impeller. It is known that the recirculation zone at the pressure side of a radial impeller has various positions, sizes and structures depending on initial conditions [3]. This paper deals with the assumption that influencing the pressure side recirculation zone leads to a stable pump performance curve. Therefore the structure of the recirculation zone at the impeller outlet is being investigated and analyzed whereas geometrical changes on a centrifugal pump impeller are performed. The tests contain an experimental setup and compare the results to numerical simulations. Subject of the experimental investigations is a centrifugal pump with a specific speed of 33 min−1, a flow rate of 650 m3/h and head of 47 m for the Nominal Point. Measurements are performed for analyzing the time resolved pressure fluctuations and visualizing the flow structures in the volute casing by using pressure transducers and particle image velocimetry (PIV). These data show the changing pressure and velocity field and enable an analyzing of the part load recirculation. Furthermore, the measured operational points and the time resolved pressure data are compared to numerical simulations that are carried out by Computational Fluid Dynamics (hereafter: CFD). The flow pattern gained by CFD allows analyzing the phenomena of the pressure side recirculation in detail, also in areas where the access with measuring instruments is limited. Within the present study different geometrical parameters are subsequently changed on the original impeller design. This concerns, for example, the earlier named slots in the rear shroud both on suction and pressure side of the blade. Results show an influence of these subsequent design methods on the performance curve as well as on the efficiency of the centrifugal pump. Additionally, the time resolved pressure data are used for a validation of the CFD simulations and both results show a significant influence of the flow structure at the impeller outlet on the performance curve. Therefore, it can be shown that the recirculation zone of the impeller is affected by these actions.

When the rotating stall occurs in a mixed-flow pump, the periodic generation and disappearance of the stall core often cause unstable pressure pulsation. Based on the k-ε turbulence model, the time domain and frequency domain responses of the pressure fluctuation of the mixed-flow pump with 3 kinds of different flange clearances are studied. The results show that the time-domain curves of pressure fluctuation at each monitoring point show periodic changes near stall condition, but the peak and trough characteristics of impeller rotation are not obvious. There is a large phase difference between adjacent monitoring points, accompanied by a strong pressure drop. When the flange clearance is 0.5 mm and 0.8 mm, the time domain curve of pressure fluctuation has 2 wave troughs in one cycle. In the near stall condition, the main frequency of pressure fluctuation at 3 monitoring points in the same channel is 0.2 times the rotational frequency, i.e. stall frequency. In addition, the main frequency amplitude in the middle of the outlet is the largest, and the larger the gap is, the larger the main frequency amplitude is.

Detailed measurements have been made of the stalled flow in a centrifugal pump-turbine model operating in air. Instantaneous velocity, total pressure and flow angle have been measured with hot wire and pressure probes at the impeller inlet and outlet and in the vaned diffuser for several pump operating points, ranging from nominal to very low flow rates. Three distinct stall phenomena have been found to affect the pump operation at reduced flow rates. The unsteady characteristics of these unstable flows have been analysed both in time and in frequency domain by means of Fourier transforms of the velocity signals. The changes in the flow structure, correlated to the occurrence of the observed stall conditions, have been investigated by means of phase-locked ensemble averages of the instantaneous velocity components at the impeller outlet and surveys of the mean total pressure and absolute flow angle distributions within the diffuser passages.