Best Efficiency Point Research Articles

Efficacy of ancillary fluid injection technique for mitigation of vortex rope in hydraulic turbines: A review

Penetration of variable renewable energy into the electric grid requires parallel employment of power generation sources that can quickly balance the fluctuation in frequency of electric grid. Hydro power, being one of the most flexible power supplies, thus takes on an instrumental role in balancing the grid. However, in hydraulic turbines such flexibility is achieved at the expense of departure from turbine operation at the best efficiency point (BEP). Turbine operation at off-design conditions like part load (PL) or high load (HL) results in significant flow instability in the draft tube. Of particular prominence is the vortex breakdown phenomenon which manifests as a precessing vortex rope in the draft tube. The vortex rope causes dynamic pressure fluctuations in the system, additive to the compromise in efficiency. If resonant with the system natural frequency, these pressure fluctuations may affect the mechanical torque delivered by the turbine and cause a phenomenon called power swing. As such it is of paramount importance to circumvent the effects of the rope. Amidst many techniques developed by various investigators for mitigation of the rope, particularly recent the ancillary fluid injection technique exhibits promise. Ancillary fluid injection involves riddance of the stagnation zone that leads to rope formation, through transfer of momentum from a fluid injected at an appropriate location and velocity. This is usually accomplished using water. Admission of air for flow aeration is another method. An experimental as well as numerical perspective of these techniques as developed by various researchers is laid, to assess the benefit obtained alongside the entailing compromise.

Numerical Study of the Performance Loss of A Centrifugal Pump Carrying Emulsion

The performance and hydrodynamic behavior of centrifugal pumps when handling two-phase liquid-liquid flow and emulsion remain relatively unexplored, although they are of fundamental importance in optimizing the operating conditions of these pumps. Hence, this study aims at investigating the performance degradation of a centrifugal pump under emulsion flow by combined means of analytical and computational fluid dynamic (CFD) models. The analytical approach is based on internal energy loss equations while the CFD approach models the emulsion as a continuous and homogeneous single-phase fluid exhibiting shear thinning behavior. The results give a good insight into the performance degradation of such a system, especially at the best efficiency point (BEP).

Review of Methods Used for Selecting Pumps as Turbines (PATs) and Predicting Their Characteristic Curves

The use of Pumps As Turbines (PATs) can be the best solution for exploiting small hydraulic resources. Pump manufacturers do not provide the performances of their machines running in reverse mode, thus several authors developed appropriate prediction models. Some of them can only correlate the pump Best Efficiency Point (BEP) to the PAT corresponding one; other ones are able to obtain the characteristic curves. In this paper, a review of these methodologies is presented with the aim to find the best strategy that allows a designer of a small hydropower plant to select the PAT to be used and to predict its characteristic curves. The study also highlights the possibility of disassembling some models in order to merge the best parts in a more reliable strategy.

Effect of incidence angle on cavity blockage and flow stability in the net positive suction head drop of mixed-flow pumps

The cavitation is an inevitable factor in pumps used in the whole industry, which is a major cause of energy loss and mechanical breakdown. In this study, the cavitation phenomena at the design flow rate were numerically analyzed for two pumps with different incidence angles. The design flow rate for both models was located near the best efficiency point (BEP). The incidence angle was determined with the impeller inlet diameter and the blade angle. A pump with a smaller incidence angle consistently showed a stable flow pattern as the inlet pressure decreased, whereas a pump with a larger incidence angle contained non-uniform flow streamlines despite a very small amount of the generated cavities. The flow pattern at the impeller inlet was handled by the shape and thickness of the generated cavities which could act as an additional blockage in the pumps. The inception and growth of the cavity with a decrease of inlet pressure were also inferred, which was specifically quantified as the blockage ratio. A pump with a larger incidence angle performed poor cavitation characteristics and obtained the pressure fluctuation and cavity oscillation. The magnitude of pressure fluctuation was indicated using the fast Fourier transform (FFT) analysis. The experimental tests were performed on both pumps to validate the numerical results.

Numerical investigation of flow field and performance of the Francis turbine of Bhilangana-III hydropower plant

Computational fluid dynamics is an alternate tool to predict the performance as well as to investigate the flow field of a hydraulic turbine at different operating conditions. The objective of this paper is to analyze the flow field numerically and to predict the performance of Francis turbine of Bhilangana–III power plant. A numerical model of the prototype Francis turbine is undertaken to perform the simulation under the actual working condition of hydropower. The steady-state simulation has been performed at different operating conditions such as high load, part load, and best efficiency point using two turbulence model k-ω shear stress transport (SST) and standard k-ε with zero clearance gap and with a gap due to erosion. Based on computational results, an attempt has been made to compare the performance of the turbine under erosive wear conditions. The computational results are very close to the expected performance characteristics of the prototype turbine under no-gap condition. A difference of 0.24% and 1.71% in efficiency is observed between the numerical and prototype hill chart curve at the best efficiency point (BEP) and full load conditions, respectively. The simulation has also been performed with a clearance gap of 1.85 mm to study the effect of the gap, due to erosion, on the efficiency. The leakage flow-through 1.85 mm clearance gap has resulted in a decrease in the efficiency of the turbine. The efficiency drops around 5.73 % at BEP condition and 5.83% at full load condition. These computational results may be useful for further study of the turbine of Bhilangana–III power plant.

Analysis and improvement of a mathematical turbine model

A mathematical turbine model is necessary for dynamic and transient analysis of hydro power plants, also in the early stages of a project. Such models can be based on a first principles approach or on empirical data, many models are a combination. A first principles approach is practical when specific laboratory data are unavailable, for example in the design phase of a new plant. These models can easily be simplified and linearized without losing physical correctness or generality, even though accuracy may vary when applied to a specific system.The model studied in this work, was developed several years ago from the Euler turbine equation and the opening degree definition, and has later been modified and linearized for simple implementation into simulation software. The model captures losses for varying rotational speed quite well, but struggles to capture losses for varying flow rate. The hydraulic efficiency is over-predicted for flow far off the design point, because irreversible hydraulic loss phenomena such as 3D turbulence and dissipation are not inherently included in this 1D model. Empirical relations or Hill charts for exact fit to a measured turbine can be included, however generality is then lost.This paper analyzes and discusses the mathematical model compared to laboratory measurements, highlighting its performance in predicting the efficiency off best efficiency point (BEP). A mathematical improvement based on turbine data is presented and implemented for three specific Francis turbines. The possibility to generalize the procedure is also discussed.

Investigating the Performance of a Super High-head Francis Turbine under Variable Discharge Conditions Using Numerical and Experimental Approach

A super high-head Francis turbine with a gross head of nearly 700 m was designed with computational fluid dynamics (CFD) simulation and laboratory tests. Reduced-scale (1:3.7) physical and numerical models of the real-scale prototype were created to investigate the hydraulic performance. According to the CFD analysis, a strong rotor–stator interaction (RSI) between guide vanes and runner blades is observed as a result of the high-speed tangential flow towards runner created by the super high water head as well as the small gaps between the radial blades. At the designed best efficiency point (BEP), there is no significant flow recirculation inside the flow passage and minor loss occurs at the trailing edge of the stay vanes and guide vanes. Maximum velocity is observed at runner inlets due to flow acceleration through the narrow passages between the guide vanes. The elbow-shaped draft tube gradually decreases the flow velocity to keep the kinetic energy loss at a minimum. The laboratory test was conducted on a reduced-scale physical model to investigate the pressure pulsations and guide vane torque (GVT) under variable-discharge configurations, which are key concerns in the design of a high head turbine. Pressure sensor networks were installed at the inlet pipe, vaneless space and draft tube, respectively. The most intense pressure variation occurs at the inlet pipe and elbow at 0.04–0.2 GVOBEP and 1.5–1.8 GVOBEP with a low frequency about 0.3 times of the runner frequency, while the vibration in vaneless zone performs stable with the blade passing frequency caused by RSI. The GVT shows a declining trend and then keeps stable as GVOs increases at synchronized condition. For the misaligned conditions, the torque of adjacent guide vanes differs a lot except at the synchronous angle and maximum absolute value at least doubles than the synchronized condition.

A methodology to forecast the main non-dimensional performance parameters of pumps-as-turbines (PaTs) operating at Best Efficiency Point (BEP)

Abstract This work presents a model based on analytical equations to identify the Best Efficiency Point (BEP) of Pumps-as-Turbines (PaTs). The equations are developed exploiting an experimental data-set of 59 PaTs, obtained in both pump and turbine modes, in form of non-dimensional parameters. Data analysis shows a linear correlation between specific speeds in pump and turbine modes, as well as between specific diameters in both operating modes. In addition, the prediction of the PaT efficiency in turbine mode, whose evaluation is often disregarded in literature works, is presented: a second order polynomial equation to forecast the mechanical efficiency of PaTs in turbine mode is developed using the values of specific speed and mechanical efficiency in pump mode as independent variables. Performance experimental data of four PaTs, which were not used in the development of the model, are employed to validate and assess the accuracy of the proposed analytical equations. The prediction capability of the model is also compared to other four models available in literature. Results demonstrate a good forecast capability and a general better agreement with experimental data. A further improvement of the model can be achieved by extending the experimental data-set with additional PaTs typologies.

Numerical Simulation of the Flow in a Kaplan Turbine Model during Transient Operation from the Best Efficiency Point to Part Load

The aim of this study is to develop a reliable numerical model that provides additional information to experimental measurements and contributes to a better exploitation of hydraulic turbines during transient operation. The paper presents a numerical analysis of the flow inside a Kaplan turbine model operated at a fixed runner blade angle during load variation from the best efficiency point (BEP) to part load (PL) operation. A mesh displacement is defined in order to model the closure of the guide vanes. Two different types of inlet boundary conditions are tested for the transient numerical simulations: linear flow rate variation (InletFlow) and constant total pressure (InletTotalPressure). A time step analysis is performed and the influence of the time discretization over the fluctuating quantities is discussed. Velocity measurements at the corresponding operating points are available to validate the simulation. Spectrogram plots of the pressure signals show the times of appearance of the plunging and rotating modes of the rotating vortex rope (RVR) and the stagnation region developed around the centerline of the draft tube is captured.

Numerical assessment of the vulnerability to impact erosion of a pump as turbine in a water supply system

Abstract Nowadays, the installation of pumps as turbines (PATs) in water supply systems (WSSs) is considered attractive because it is able to effectively combine the pressure regulation with the small-scale hydropower generation. One critical aspect concerns the behaviour of the PAT in the presence of solid particles in the flow which impinge against the inner surface of the device, producing a loss of material termed impact erosion. In this paper, the numerical assessment of PAT erosion is performed, by referring, as a case study, to an existing pressure control station in Southern Italy. The variable operative strategy (VOS) was applied to derive the frequency distribution of flow rates over the PAT operation, under the hypothesis of performing the hydraulic regulation of the hydropower plant. The commercial computational fluid dynamics (CFD) code Ansys Fluent was employed for simulating the liquid–solid flow inside the PAT and then coupled with an in-house code to estimate the erosion. The vulnerability of the PAT to wear was analysed by varying its flow rate, aiming at characterizing the decay of PAT components at a constant rotational speed. Finally, a detailed characterization of the PAT response to the particle impingement at its best efficiency point (BEP) was developed and discussed.

Energy recovery in oil refineries by means of a Hydraulic Power Recovery Turbine (HPRT) handling viscous liquids

Chemical plants like oil refineries involve energy intensive processes. Several energy efficiency interventions are being performed in oil refineries to meet the latest emission targets, but few of them are related to the energy recovery from process liquids. In this paper, an energy recovery study of an Italian oil refinery is presented: specifically, a Hydraulic Power Recovery Turbine (HPRT), coupled with the shaft of the feed pump used in the same process, is used to replace a Pressure Reducing Valve (PRV). The machine is installed in the H2S removal process and it exploits the Selexol® solvent. A new model that predicts the Best Efficiency Point (BEP) of the HPRT in turbine mode when handling liquids different from water is discussed and validated through operational BEP data. The HPRT supplies 349.3 kW to the feed pump, leading to a yearly electric energy recovery of 2966 MWh and a maximum PayBack Period (PBP) close to 9 years considering the installation, the Carbon Dioxide Equivalent (CDE) allowances and both operational and maintenance costs. The obtained PBP is quite high, but the installation costs would be fairly lower if the HPRT is already considered in the design phase of the chemical process.

Study of the internal flow field in a pump-as-turbine (PaT): Numerical investigation, overall performance prediction model and velocity vector analysis

This work presents a detailed analysis of the three-dimensional flows in a Pump-as-Turbine (PaT). The flow field is studied through URANS simulations and a velocity vector analysis. Moreover, a prediction model is used to forecast the performance of the PaT. The results of these methodologies were validated using experimental results. Flow rate and head at Best Efficiency Point (BEP) in turbine mode were 27% and 41% higher than the ones in pump mode, respectively; the mechanical efficiency was 4% lower. To give a fluid flow interpretation of this behaviour, velocity triangles in turbine mode were analytically calculated, as well as numerically evaluated. The analytically identified PaT flow rate at BEP is in agreement with CFD and measurements. Only small differences appear, which might be explained by three reasons: i) the real flow angle is influenced by the finite number of blades and their thickness, ii) the PaT leading edge has a sharp ending since the impeller is designed to operate only in pump mode and iii) tapering of the meridional cross-section from volute to impeller. While close to the BEP the mechanical efficiency is flat, a sensible drop caused by flow detachments and swirling flows was detected at part-load.

Computational Investigation of Flow Over Nose Cap of Closed Impeller of Mixed Flow Centrifugal Pump

Abstract Flow turbulence near the suction region and over the eye of a closed impeller may be troublesome during the operation of a centrifugal pump. This article aims to bring out a methodology for minimization of flow turbulence and for providing guidance to suction flow by modifying the surface design of the lock nut into a nose cap of optimized profile. The effects of this modification on the performance parameters of closed impeller of a low head, mixed flow centrifugal pump are presented here. Based on geometric constraints and impeller eye diameter, nose caps of 65 mm diameter and various nose tip angles (15–90 deg) and lengths (50–80 mm) are evaluated. Parametric studies are carried out via computational fluid dynamics (CFD) analyses using ansysfluent 17.0 as well as experimental investigations to investigate the effects on head, energy consumption, and overall efficiency for flowrates in the range of 0.25–1.5 times the flowrate at best efficiency point (BEP). The estimates of head developed obtained using CFD and experimental studies match within 4.23%. Results confirm improvement in performance parameters like head and overall efficiency due to the reduction of losses and better flow guidance in the suction region. The performance improvement is greatest for the nose cap of 900 tip angle and 50 mm length at all flowrates.

Numerical Study of the Winter–Kennedy Flow Measurement Method in Transient Flows

This paper explores the possibility of using the Winter–Kennedy (WK) method for transient flow rate measurement in hydraulic turbines. Computational fluid dynamic (CFD) analysis of a numerical model of an axial turbine was carried out for accelerating and decelerating flows. Those were obtained by linearly opening and closing of the guide vanes, respectively, while retaining the inlet pressure constant during the simulations. The behavior of several WK configurations on a cross-sectional plane and along the azimuthal direction of the spiral casing was studied during the transients. The study showed that there are certain WK configurations that are more stable than others. The physical mechanism behind the stability (or instability) of the WK method during transients is presented. Using the steady WK coefficient obtained at the best efficiency point (BEP), the WK method could estimate the transient flow rate with a deviation of about 7.5% and 3.5%, for accelerating and decelerating flow, respectively.

Experimental Study of Phase Inversion Phenomena in Electrical Submersible Pumps Under Oil/Water Flow

Abstract Despite the common presence of water in oil production, just recently, the scientific community has devoted efforts to studying the influence of emulsion phenomena effects related to oil production using pumps. In the context of this study of phase inversion phenomena, the influence of viscosities and rotational speeds in electrical submersible pumps (ESPs) are evaluated as part of this effort. This study is aimed at investigating the influence of viscosity in phase inversion phenomena. An eight-stage ESP was tested with three different rotational speeds and two different oil viscosities for the best efficiency point (BEP) flow rates. Initially, the total flow rate was obtained in relation to BEP using ESP performance curves for pure oil at 52 cP and 298 cP and rotational speeds of 800 rpm, 1200 rpm, and 2400 rpm. The total flow rate was kept constant and the water cut was increased from 0 to 100%. The inversion phase phenomenon was detected by performance improvement when the water cut increased. The factors analyzed were the head and efficiency of the ESP as a function of the water cut. The phase inversion experimental data obtained in this study were compared with literature models for horizontal pipes. The results of this comparison presented satisfactory agreement. The phase inversion phenomena occur in all eight stages at the same time. Hysteresis was observed in ESPs for oil viscosity of 52 cP and rotating speed of 800 rpm and 1200 rpm.

Improved Planning of Energy Recovery in Water Systems Using a New Analytic Approach to PAT Performance Curves

The use of pumps working as turbines (PATs) to improve the energy efficiency of water networks has been studied in the last years. This recovery system is justified due to a low investment contrasting with the capacity to take advantage in certain points with low and medium recoverable heads. Analyses of water systems using simulation software and/or optimization algorithms need the characteristic curves (head and efficiency) of the machines, which should be known with minor error by the water managers. The knowledge of the best efficiency point (BEP) as a turbine is one of the major limitations when the user wants to choose PATs. In this sense, the present research defines new approach equations to estimate the BEP of the PAT, as well as to predict the characteristic curves, comparing the results with the rest of the published methods. The comparison demonstrated that the new proposal reduced the error indexes, improved the R2 and increased the accuracy of the error ellipse using an experimental database of 181 different PATs.

Transposition of the mechanical behavior from model to prototype of Francis turbines

Hydropower is nowadays essential for balancing the electrical grid providing flexibility and fast response. The role of hydraulic turbines has changed from working at their Best Efficiency Point (BEP) to work in the whole operating range when demanded. Francis turbines working at off-design conditions suffer from dynamic problems that affect the useful life of their components, especially of the runner.Reduced scale physical models of Francis turbines are widely used to determine their hydraulic behavior. Most of the hydraulic parameters obtained in model tests are transposable to prototype by using the similarity theories as long as they meet similitude requirements. However, from the mechanical behavior point of view, both model and prototype structures are not always similar and the mechanical behavior of the model is often not considered for transposition.In this paper, a transposition method for the mechanical behavior of Francis turbines models is presented and the results are compared with the real size prototype. The modal behavior of the runner, its stress and fatigue life estimation for different operating conditions are experimentally compared for both model and prototype. Results present the possibilities and limitations of transposing the mechanical parameters from model to prototype.

PERFORMANCE EVALUATION OF END SUCTION, SINGLE STAGE, RADIAL DISCHARGE CENTRIFUGAL PUMP IN TURBINE MODE OPERATION

Pump as Turbine (PAT) provides a cost-effective alternative to conventional hydro turbines for high terrain rural areas where grid supply is not feasible. PAT technology is simple, low in cost and easy to harness. End suction centrifugal pumps are readily available as compared to conventional custom made hydro turbines. This paper presents theoretical, numerical and experimental study of radial discharge centrifugal pump in hydro turbine mode. Experimental setup is used to investigate performance of radial discharge end suction mono block centrifugal pump in pump and turbine mode and the results are compared with theoretical and numerical results. Performance characteristic of mono block pump of specific speed 20.28 (m, m3 /s) in turbine mode operation are determined through experiment. The experiment results reveal suitability of using a mono block centrifugal pump in hydro turbine mode. The results confirm that the pump in hydro turbine mode operate at higher heads and discharge as compared to the pump mode operation. BEP (best efficiency point) for the pump in hydro turbine mode is lower than in pump mode operation. Correlations proposed by earlier researchers for performance prediction of pump in hydro turbine mode are also tested.

CFD Analysis of Impeller Trimming

Impeller trimming is a key impeller modification. It allows the best efficiency point (BEP) to be moved towards the region of lower flowrate and height, thus optimizing pump performance for specific piping systems. The paper deals with prediction of trimming results using CFD methods. Experience shows that modification of the BEP position depends on the trimming method as well as on the pump type and its specific speed. The analysed pump is of diagonal type with a specific speed of nb = 0.168. Its impeller is of a 4-blade design and a spiral casing is used as a volute. Seven cases of trimming are presented (including a non-trimmed original version). The paper compares CFD obtained data and data measured on an experimental stand. Additionally, the approach to CFD analysis, as well as the use of a turbulence model and characteristics of internal pump volume meshing are described.

Numerical study on the effect of viscosity on a multistage pump running in reverse mode

Utilization of pump as turbine (PAT) systems is known as an economically beneficial method to harvest hydropower energy, especially in a remote area. Despite the extensive investigations on the operation and parametric study of the PAT components, the turbine mode of multistage PATs in case of a viscous working fluid is found as a literature gap. In this paper, a numerical simulation method is employed as a predicting tool to investigate the consequences of changing the fluid viscosity affecting the flow patterns within the multistage PATs. The commercial solver ANSYS CFX 16 was used for simulation. Validation of numerical solution was done for the single-stage pump considering water and 48 cSt viscous fluid. The numerical results in the single-stage PAT showed that increasing the viscosity causes an efficiency drop mainly at the part-load condition and translates the maximum efficiency point to higher flow rates, where the vortices at impeller passage and draft tube are weaker. Increasing the viscosity up to 48 cSt, results in 12.5% reduction of efficiency at the best efficiency point (BEP) in two stage PAT. Based on the two-stage PAT results, the total hydraulic efficiency at the BEP has been increased by 1% and 1.5% for water and the 48 cSt viscous fluid, respectively, in comparison with the single-stage PAT. According to the post-processed streamlines at different stages of the two-stage PAT, the undesired vortical structures were caused by the diffuser and the return channel, which were mainly designed for the pump mode. The hydraulic analysis of two-stage PAT shows that using multistage pumps in reverse mode is reasonable for power generation in high head and flow rate sites.