Best Efficiency Point Research Articles

Experimental and numerical study of oil drop motion within an ESP impeller

The Electrical Submersible Pump (ESP) is a multistage centrifugal pump used in the petroleum industry as an artificial lift method. The ESP usually works with the presence of two-phase liquid-liquid flows that constitute dispersions and emulsions, causing performance losses and operational problems. This research aims to investigate the behavior and evaluate the dynamics of individual oil drops in an oil-in-water dispersion within an ESP impeller. The study adopts experimental and numerical approaches. Initially, experiments were performed using an experimental facility with a high-speed camera and an ESP prototype working at 600 rpm and 900 rpm, for water flows around the Best Efficiency Point (BEP) and with the injection of oil drops at a low flow rate. The acquired images were processed, and a drop sample was tracked, enabling the analysis of the size, shape, path, velocity, and acceleration of the oil drops. Numerical simulations were executed in ANSYS® software to define relevant parameters related to water and oil drops, such as velocities, accelerations, forces, turbulent dissipation, and residence time. The images reveal a unique flow pattern of dispersed drops in a continuous water phase. The oil drops' diameters vary from tenths of a millimeter to around 3 mm. The drops' trajectories can be classified into three different regions within the impeller channels. The drops’ velocities stay in the order of 1 m/s, while accelerations can reach hundreds of m/s2. The velocity profiles show that the oil drops tend to decelerate during their trajectory, while the acceleration profiles suggest peaks at the channel inlet and outlet. High intense turbulence is present in the impeller entrance zone. The evaluation of the residence time and the particle Reynolds number suggest that smaller oil drops follow the water streamlines, while larger oil drops tend to be affected by external forces. The main forces that govern the oil drop motion are the drag, the pressure gradient, and the virtual mass forces. The force from the pressure gradient is tenfold greater than the force from the drag. The virtual mass effect is significant only in the impeller inlet. In general, in this research, numerical results show a satisfactory agreement with the experimental data.

Numerical Simulation of Centrifugal Pump and Effect of Impeller Geometry on Its Performance

In the presented paper, the effect of impeller geometric parameters on the performance of centrifugal pump has been investigated. This study was performed for different flow rates and rotational speeds, allowing to obtain the performance curve for the centrifugal pump. Three dimensional computational fluid dynamic simulation of the impeller and volute for a centrifugal pump has been performed using ANSYS CFX software (a high-performance computational fluid dynamics software tool that delivers reliable and accurate solutions). The pump has an outside impeller diameter of 205 mm, impeller outlet width of 16 mm, rotational speed 1450 rpm, seven impeller blade and a specific speed of 28. By increasing the impeller outer diameter and outlet width, both net head and power consumed are increased. In addition, it was noticed that the best efficiency point (BEP) was achieved at volume flow rate higher than design flow rate. The performed simulations indicated that; by changing the impeller outer diameter from 200 mm to 210 mm, the flow rate of BEP increases about by 14.7%. By changing the impeller outlet width from 14 mm to 18 mm, the flow rate of BEP increased by about 9%, and the efficiency of BEP reduced by approximately 0.5%. It was also noticed that, increasing the rotational speed will cause an increase in the net head and consumed power. An increase of 13.8% for the flow rate of BEP was observed when changing the rotational speed from 1400 rpm to 1500 rpm, with the same BEP.

CFD Investigation of a High Head Francis Turbine at Speed No-Load Using Advanced URANS Models

Due to the integration of new renewable energies, the electrical grid undergoes instabilities. Hydroelectric power plants are key players for grid control thanks to pumped storage power plants. However, this objective requires extending the operating range of the machines and increasing the number of start-up, stand-by, and shut-down procedures, which reduces the lifespan of the machines. CFD based on standard URANS turbulence modeling is currently able to predict accurately the performances of the hydraulic turbines for operating points close to the Best Efficiency Point (BEP). However, far from the BEP, the standard URANS approach is less efficient to capture the dynamics of 3D flows. The current study focuses on a hydraulic turbine, which has been investigated at the BEP and at the Speed-No-Load (SNL) operating conditions. Several “advanced” URANS models such as the Scale-Adaptive Simulation (SAS) SST k - ω and the BSL- EARSM have been considered and compared with the SST k - ω model. The main conclusion of this study is that, at the SNL operating condition, the prediction of the topology and the dynamics of the flow on the suction side of the runner blade channels close to the trailing edge are influenced by the turbulence model.

Analytical Evaluation of Head and Flow Rate Off-Design Characteristics for Pump as Turbine Application

Head and flow rate are the important parameters for proper selection of centrifugal pump. However, the reversed operation of centrifugal pump leads to the off-design characteristic of head and flow rate. This paper presents an analytical model developed by using the system curves and velocity relations derived from pump application. Also, the differential technique is applied to the analytical model to develop the off-design characteristics of head ratio and flow rate ratio relations. The off-design characteristic relations were compared with literature and available conversion methods. Then, the analytical model coefficient (AMC) with the range between −4 and +4 was developed from the off-design characteristics of head ratio and flow rate ratio relations. The AMC value was equal to 1 when the pump operates in turbine mode and pump mode at the pump best efficiency point (BEP) and extended to either side up to ±4 when tested with literature data. Therefore, the analytical model consists of the off-design head and flow rate characteristics, when simplified leading to the AMC that could be applied to select the possible boundary limits of head and flow rate for different pumps.

Solar photovoltaics pumps operating head selection for the optimum efficiency

Solar Photovoltaic Water Pumping (SPVWP) systems have established their potential as the most dependable and economically viable systems compared to the diesel based or grid-based electrical pumps. This paper presents an in-depth investigation of the energy efficiency of SPVWP system based on solar radiation, temperature, and operational heads. The study also identifies the shortcomings in the conventional design method based on Best Efficiency Point (BEP) concept that is applicable only in case of fixed frequency and voltage type pumps. However, in the case of SPVWP systems, due to variations in the solar intensity, ambient temperature, and water head, BEP concept does not offer the best efficiency design. The study experimentally proves that the model based on weighted system efficiency and Solar Operational Duty Head (SODH) increases the performance of SPVWP system (∼9% gain) and is consistently provide higher efficiencies in any season or under any climatic conditions.

On the Interactions of a Rotor Blade Tip Flow With Axial Casing Grooves in an Axial Compressor Near the Best Efficiency Point

Experiments in a refractive index-matched axial turbomachine facility show that semicircular skewed axial casing grooves (ACGs) reduce the stall flowrate by 40% but cause a 2.4% decrease in the maximum efficiency. Aiming to elucidate mechanism that might cause the reduced efficiency, stereo-PIV measurements examine the impact of the ACGs on the flow structure and turbulence in the tip region near the best efficiency point (BEP), and compare them to those occurring without grooves and at low flowrates. Results show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the suction side (SS). Entrainment of the PS boundary layer and its vorticity generates a vortical loop at the entrance to the groove, and a “discontinuity” in the tip leakage vortex (TLV) trajectory. During exposure to the SS, the backward tip leakage flow separates at the entrance to the groove, generating a counter-rotating circumferential “corner vortex,” which the TLV entrains into the passage at high flowrates. Interactions among these structures enlarge the TLV and create a broad area with secondary flows and elevated turbulence near the groove's downstream corner. A growing shear layer with weaker turbulence also originates from the upstream corner. The groove also increases the flow angle upstream of the blade tip and varies it periodically. Accordingly, the circulation shed from the blade tip and strength of leakage flow increase near the blade leading edge (LE).

Parametric study of unsteady-flow-induced volute casing vibro-acoustics in a centrifugal fan

A numerical parametric analysis of a vibro-acoustic coupling method that considered the influence of vibro-acoustic coupling was carried out to investigate the casing vibrations and feathers of vibrational noise induced by unsteady flow of the centrifugal fan at the best-efficiency point (BEP). There are three important aspects of this method. First, an unsteady flow-field with a whole impeller-volute configuration was solved based on three-dimensional incompressible Navier-Stokes equations and a standard k-ε turbulence mode to obtain the source of the vibro-acoustics. Second, a one-way-flow structural acoustic coupling method was implemented to study the volute vibrations and behaviors of vibrational noise by adoption. The generation mechanism of vibrational noise of the volute casing was revealed. Third, the parametric analysis method was used to explore the parametric relationship between the panel thicknesses (such as front-panel thickness [FT], side-panel thickness [ST], and back-panel thickness [BT]) and the outlet acoustical power of the volute casing surface. The parametric analysis provides a reasonable range of values of three panel thicknesses that result in minimal vibrational sound radiation.

A model for the extrapolation of the characteristic curves of Pumps as Turbines from a datum Best Efficiency Point

Pumps used as Turbines (PATs) are a viable and low-cost technology of energy converters suitable for small-scale and in-pipe energy generation from water resources. However, amongst the main barriers to PAT technology diffusion is the unavailability of the characteristic curves of most hydraulic pumps on the market when used as turbines. Several methods exist in the literature to derive the shape of head and power characteristic curves of a PAT knowing its Best Efficiency Point (BEP). Such methods based on fixed-coefficient polynomials have been assessed here in a cross-comparison and a new method was developed based on a database of 113 experimentally tested PAT curves, which proved to reproduce the behaviour of the sampled machines more accurately by improving the overall goodness of fit up to 60%, 37% and 5% according to the different selected indicators. Finally, the mechanical efficiency and system efficiency of a PAT under variable flow rates has been compared and contrasted with that of conventional hydro turbines.

Study of a Pump-as-Turbine (PaT) speed control for a Water Distribution Network (WDN) in South-Tyrol subjected to high variable water flow rates

The development of renewable energy technologies for producing clean energy has more and more become a priority worldwide. Research activities have not just to target the technological improvement of such systems, but they have also to consider their market deployment. In such a scenario, hydraulic machines, in particular Pumps-as-Turbines (PaTs), can play a key role in energy recovery applications. One of the main open issues of PaTs is the performance forecast in turbine mode, due to the lack of data from manufacturers, and their use in some applications with high flow rate and pressure variability, especially at part-load operating conditions like in energy recovery applications within Water Distribution Networks (WDNs). In this work, a MATLAB® Simulink model is developed for simulating a branch of the WDN located in Laives (South-Tyrol), where specific PaTs have been selected and used to substitute Pressure Reducing Valves (PRVs). A speed control by means of an inverter is performed due to the high variability of the flow rate inside the grid branch, allowing the machines to operate at their Best Efficiency Point (BEP). A preliminary analysis showed that it is possible to increase the energy production of about 23% with respect to a constant-speed machine, leading to a significant decrease of the PayBack Period (PBP).

Numerical calculation and analysis of gas-liquid two-phase flow in a centrifugal multiphase pump for deep-sea mining

With the development of offshore oilfield, the gas-liquid centrifugal multiphase pump (CMP) has become the focus of research for its more effective way of working and economic benefits. In order to transport two-phase media, the internal flow of gas-liquid multiphase pump is complicated. In this study, the gas content effect on CMP performance was investigated by computational fluid dynamics (CFD). Gas and water are flowed through a DN127 CMP at varying gas content. This paper based on the Euler Model, the numerical simulation and analysis of three stages gas-liquid two-phase pump internal flow field are conducted, with the standard k-ε, SST and SSG turbulence model. The influence of different turbulence models on the numerical results and the effects of different gas-liquid fraction on the characteristics of pump and internal flow law are analyzed. The results shows that the standard SST turbulence model is more suitable for the numerical calculation of gas-liquid two-phase flow; and at pump best efficiency point (BEP), the head of the multiphase pump decreases by 4.5%∼26% and the efficiency of CMP decreases by 4.6%∼20% when the gas content increases from 0% to 20%, the CMP becomes ineffective when the gas content is higher than 40%.

Experimental and numerical investigation of the transient characteristics and volute casing wall pressure fluctuations of a single-blade pump

In contrast to conventional multiblade centrifugal pumps, single-blade pumps are characterized by a significant fluctuation of head and highly transient and circumferentially nonuniform flow field even in the best-efficiency point. For a contribution to a better understanding of the flow field and an improvement of numerical methods, a combined experimental and numerical study is performed with special emphasis on the analysis of the transient pressure field. In an open test rig, piezoresistive pressure sensors are utilized for the measurement of transient in- and outflow conditions and the volute casing wall pressure fluctuations. The quality of the numerical simulations is ensured by a careful adoption of the real geometry details in the simulation model, a grid study and a time step study. While the power curve is well reproduced by the numerical simulations, the time-averaged head is systematically overpredicted, probably due to underestimation of losses. Transient pressure boundary conditions for the numerical simulation show a better prediction of the measured pressure amplitude than constant boundary conditions, whereas the time-averaged head prediction is not improved. For a more accurate prediction of the transient flow field and the time-averaged characteristics, the utilization of scale-resolving turbulence models is assumed to be indispensable.

Simplified hydrodynamic analysis on the general shape of the hill charts of Francis turbines using shroud-streamline modeling

The paper presents a simplified one-dimensional calculation of the efficiency hill-chart for Francis turbines, based on the velocity triangles at the inlet and outlet of the runner’s blade. Calculation is done for one streamline, namely the shroud streamline in the meridional section, where an efficiency model is established and iteratively approximated in order to satisfy the Euler equation for turbomachines at a wide operating range around the best efficiency point (BEP). Using the presented method, hill charts are calculated for one splitter-bladed Francis turbine runner and one Reversible Pump-Turbine (RPT) runner operated in the turbine mode. Both turbines have similar and relatively low specific speeds of nsQ = 23.3 and nsQ = 27, equal inlet and outlet diameters and are designed to fit in the same turbine rig for laboratory measurements (i.e. spiral casing and draft tube are the same). Calculated hill charts are compared against performance data obtained experimentally from model tests according to IEC standards for both turbines. Good agreement between theoretical and experimental results is observed when comparing the shapes of the efficiency contours in the hill-charts. The simplified analysis identifies the design parameters that defines the general shape and inclination of the turbine’s hill charts and, with some additional improvements in the loss models used, it can be used for quick assessment of the performance at off-design conditions during the design process of hydraulic turbines.

Experimental study of viscosity effects on a pump as turbine performance

Abstract An experimental study is carried out to find the effects of viscosity on the performance of a pump as turbine (PAT) system. A pump is tested in an established test rig for five kinds of fluids with different viscosities by the solution of Water-Glycerin with various volume fractions. By analyzing the results of performance tests, two opposite effects are observed. In the part-load region, increasing the viscosity leads to an increase in hydraulic friction losses in the impeller, which decreases the PAT efficiency. On the other hand, at the best efficiency point (BEP) and overload region, by increasing the viscosity, the lubrication effects cause a reduction in the mechanical losses. This effect improves the machine's efficiency. However, at the higher rotational speeds, the hydraulic losses are dominated. Dimensionless correlations for the pure water are compared with other available data in the literature. Three viscosity correction factors for the head, efficiency, and power are obtained, which can be beneficial for selection of a PAT system with the tested viscosity values. Finally, based on the obtained results, a correlation is proposed for calculation of the efficiency as a function of flow coefficient and Reynolds number.

Power Swing Generated in Francis Turbines by Part Load and Overload Instabilities

Hydropower plays a key role in the actual energy market due to its fast response and regulation capacity. In that way, hydraulic turbines are increasingly demanded to work at off-design conditions, where complex flow patterns and cavitation appear, especially in Francis turbines. The draft tube cavitation surge is a hydraulic phenomenon that appears in Francis turbines below and above its Best Efficiency Point (BEP). It is a low frequency phenomenon consisting of a vortex rope in the runner outlet and draft tube, which can become unstable when its frequency coincides with a natural frequency of the hydraulic circuit. At this situation, the output power can significantly swing, endangering the electrical grid stability. This study is focused on the detection of these instabilities in Francis turbines and their relationship with the output power swings. To do so, extensive experimental tests for different operating conditions have been carried out in a large prototype Francis turbine (444 MW of rated power) within the frame of the European Project Hyperbole (FP7-ENERGY-2013-1). Several sensors have been installed in the hydraulic circuit (pressure sensors in the draft tube, spiral casing, and penstock), in the rotating and static structures (vibration sensors, proximity probes, and strain gauges in the runner and in the shaft), as well as in the electrical side (output power, intensity, and voltage). Moreover, a numerical Finite Element Method (FEM) has been also used to relate the hydraulic excitation with the output power swing.

Numerical and Economic Study of Performance of Centrifugal Pump as Turbine

In this paper, performance of centrifugal pump as turbine (PAT) is investigated numerically. Three different specific speeds are considered and three pumps are designed using diagrams from catalogues and CFturbo V.9 software. Next, models are analyzed by Ansys CFX 16 software and results are compared with those of CFturbo software. Also, a mesh study analysis for one case is performed in order to show the effect of grid size on the solution. In addition, three different flow rates of 75%, 100%, and 125% of best efficiency point (BEP) are considered for extracting head-flow rate diagrams and comparing results of CFX and CFturbo software. In next step, using relations between pump and turbine modes (PAT formulations) and by changing boundary conditions in CFX, turbine mode is investigated and efficiency is compared with pump mode. Finally, by an economic analysis a comparison between PATs and turbines with same nominal output powers are performed to distinguish which case is more profitable. Results showed that PATs have lower payback time in comparison with turbines with equal output power (in low capacities), although they have lower efficiencies.

Investigations of Compressible Turbulent Flow in a High-Head Francis Turbine

Dynamic stability of the high-head Francis turbines is one of the challenging problems. Unsteady rotor–stator interaction (RSI) develops dynamic stresses and leads to crack in the blades. In a high-head turbine, vaneless space is small and the amplitudes of RSI frequencies are very high. Credible estimation of the amplitudes is vital for the runner design. The current study is aimed to investigate the amplitudes of RSI frequencies considering a compressible flow. The hydro-acoustic phenomenon is dominating the turbines, and the compressibility effect should be accounted for accurate estimation of the pressure amplitudes. Unsteady pressure measurements were performed in the turbine during the best efficiency point (BEP) and part load (PL) operations. The pressure data were used to validate the numerical model. The compressible flow simulations showed 0.5–3% improvement in the time-averaged pressure and the amplitudes over incompressible flow. The maximum numerical errors in the vaneless space and runner were 6% and 10%, respectively. Numerical errors in the instantaneous pressure amplitudes at the vaneless space, runner, and draft tube were ±1.6%, ±0.9%, and ±1.8%, respectively. In the draft tube, the incompressible flow study showed the pressure amplitudes up to eight times smaller than those of the compressible. Unexpectedly, the strong effect of RSI was seen in the upper and lower labyrinth seals, which was absent for the incompressible flow.

Numerical Investigation of Back Vane Design and Its Impact on Pump Performance

Adding back vanes to the rear shroud of centrifugal pumps is sometimes practiced in order to alleviate large axial forces. Effective design and flow characteristics of back vanes remain obscure due to lack of knowledge associated with experimental complexities in study of this area. In this study, various design parameters of the conventional noncurved rectangular back vanes are evaluated using computational fluid dynamics (CFD). Furthermore, the complex flow structure at the rear chamber of these pumps is illustrated and discussed with the advantage of CFD which is a highly costly and taxing job if one chooses to capture it using experimental methods. Effect of back vanes outer radius, width, clearance, thickness, vane angle, and number of vanes on pump characteristics and axial thrust has been investigated. New findings of this study show that back vanes are capable of canceling the axial thrust in a large range of flow rates without a penalty to the machine efficiency, provided that suitable design parameters are selected. In addition, the best efficiency point (BEP) will not be affected by usage of back vanes. The rear chamber’s flow pattern suggest that back vanes have a repumping effect causing increased pump head at longer back vane configurations.

Analytical Prediction Models for Evaluating Pumps-As-Turbines (PaTs) Performance

Abstract: The hydropower sector is moving to the small-scale generation due to the exploitation of the majority water reservoirs with the aim of providing electrical energy in rural zones. Pump-as-Turbines (PaTs) is one of the most interesting technology due to their use for recovering energy in different industrial applications. Several studies aimed to study the performance of the tested PaTs and to describe their performance curves. The efficiency of these machines at their Best Efficiency Point (BEP) is comparable as much as the pump mode. In this work, a general analytical method for forecasting PaTs performance in turbine mode was investigated.

Numerical Study on the Internal Flow Field of a Reversible Turbine during Continuous Guide Vane Closing

The unsteady flow field in a reversible pump-turbine is investigated during the continuous load rejection using a 3D computational fluid dynamic analysis. Numerical calculations are carried out using the detached eddy simulation (DES) turbulence model and a new approach involving automatic mesh motion. In this way, the instability of the flow field is analyzed by continuously changing the guide vane openings from the best efficiency point (BEP). Unsteady flow characteristics are described by post-processing signals for several monitoring points including mass flow, torque, head and pressure in the frequency and time-frequency domains. The formation of vortices of different scales is observed from the origin to further enlargement and stabilization; the effect of the rotating structures on the flow passage is analyzed, and the influence of unsteady flow development on the performance of the turbine is investigated. Finally, the evolution during the period of load rejection is characterized in order to determine the hydrodynamic conditions causing the vibrations in the machine.

Study of Pressure Shock Caused by a Vortex Ring Separated From a Vortex Rope in a Draft Tube Model

Operating hydraulic turbines under part- or over-load conditions leads to the development of the precessing vortex rope downstream of the turbine runner. In a regime close to the best efficiency point (BEP), the vortex rope is very unstable because of the low residual swirl of the flow. However, strong pressure pulsations have been detected in the regime. These oscillations can be caused by self-merging and reconnection of a vortex helix with the formation of a vortex ring. The vortex ring moves along the wall of the draft tube and generates a sharp pressure pulse that is registered by pressure transducer. This phenomenon was investigated on a simplified draft tube model using a swirl generator consisting of a stationary swirler and a freely rotating runner. The experiments were performed at Reynolds number (Re) = 105. The measurements involved a high-speed visualization technique synchronized with pressure measurements on the draft tube wall, which enables an analysis of the key stages of vortex ring formation by comparing it with the pressure on the draft tube wall. Quantitative information regarding the average velocity distribution was obtained via the laser Doppler anemometer (LDA) technique.