Pump As Turbine Performance Research Articles

Effect of blade profile on the hydraulic performance of a double-suction centrifugal pump as turbine based on enstrophy dissipation theory

Pump as turbine (PAT) is widely used in micro hydropower stations and chemical industries as an economical energy recovery device. The special impeller with forward-curved blades can significantly improve the efficiency of PAT and expand its high-efficiency range due to the suitable blade profile, which is more appropriate for PAT's operation mode than the backward-curved blades. To study the influence of the forward blade on a double-suction centrifugal PAT performance, three forward-curved blade schemes with different blade angle conditions are compared with the original backward-curved blade scheme. The three forward-curved blade schemes have the highest efficiency when the inlet angles are 60°, 90°, and 120°, respectively, and the appropriate blade outlet angle. The results showed that the forward-curved blade is suitable for high-flow rate operating conditions in double-suction centrifugal PAT. The PAT with a forward-curved blade impeller has higher efficiency and a broader high-efficiency region than the backward-curved blade impeller. The double-suction centrifugal PAT's main energy loss comes from the impeller's turbulent loss. The forward-curved blade reduces the impeller's turbulence loss and improves the PAT's efficiency at large flow rates. The research in this paper provides a theoretical basis for the design and application of double-suction centrifugal PAT.

Enhancing Pump as Turbine (PAT) performances: A numerical investigation into the impact of impeller leading edge rounding

Pump As Turbine (PAT) utility represents a major advance in the field of hydraulic engineering. This work aims to improve the PAT performance characteristics. The sharp impeller leading edge (original impeller) was revealed by flow analysis as exhibiting negative effects on the PAT performances due to flow separation and flow misalignment. The performances of rounded and original impeller leading edge were studied by Computational Fluid Dynamic (CFD) method carried out on ANSYS CFX. Although impeller leading edge rounding has notably improved the performances in off design conditions, the difference of efficiency between the both impeller types was decreasing when increasing the discharge. The hydraulic head generated by the rounded impeller leading edge was also slightly higher at part load conditions, but when increasing the discharge, the difference between the both heads became negligible. It appeared from numerical simulations that the impeller leading edge rounding allows to decrease the hydraulic losses of the individual sub-domains except the outlet pipe. For the seek of a comprehensive analysis, the significant losses were computed for the two impeller geometries. It was observed that the shock losses and swirling losses of the rounded impeller leading edge were lower at part load conditions, but when increasing the discharge, the both losses were lower for the original impeller geometry. The rounded impeller leading edge exhibited as well lower wall frictional losses for the entire operating range of discharge.

Investigation and optimization into flow dynamics for an axial flow pump as turbine (PAT) with ultra-low water head

When the water downstream is abundant, axial flow pumps are often used as turbines (PAT) for reverse power generation to obtain additional benefits. However, due to the actual water level difference, PAT usually operates at off-design conditions. The CFD technology and entropy production theory were used to study the working performance, flow characteristics, and energy loss mechanism of the blade adjustable axial-flow PAT under a low water head. The results indicate that the strength of the tubular vortex at the pressure surface and the separation position of the tip-leakage-vortex at the suction surface depend on the inlet angle of the blade. The energy loss in the draft tube is influenced by the velocity circulation at the runner’s outlet, which depends on the blade’s outlet angle. Subsequently, the blade inlet and outlet angles in the runner were optimized. In turbine mode, the overall efficiency of the optimized runner increased by an average of 0.99 %. The blade pressure load distribution of the optimized runner has improved, and the pressure fluctuation in the draft tube is also under control. In pump mode, due to the increasing of the inlet angle within the same demand pump head range, the optimized runner’s pumping flow increases by an average of 2 m3/s.

The impact of blade inlet angle (β2) influence on pump‐as‐turbine performance by using entropy production theory

AbstractThe effect of the blade inlet angle on the pump‐as‐turbine (PAT) has not been adequately investigated. The hydraulic performance of PAT was analyzed using an available test rig. The accuracy of the numerical data was established by the comparative analysis of the experimental data. The study demonstrated that as the blade inlet angle increases, so does hydraulic performance, but only to a certain degree. The required pressure head and the shaft power increased with the increase in blade inlet angle. As compared to the volute and outlet pipe, the hydraulic power loss within the impeller accounts for a large portion of the total power loss. The impeller power loss decreases and increases at low and high flow rates, respectively, as the blade inlet angle increases. The fluid angle of attack decreases as the blade inlet angle increases at high flow, allowing the impeller power loss to drop and performance to increase. To visualize PAT internal energy loss, the distribution of the variable of pressure, velocity, and entropy dissipation theory was applied. The impeller had the highest energy loss of the PAT geometrical parameters. The shock loss, flow separation, and asymmetric volute structure are the primary causes of impeller loss. The unsteady simulation results showed that rotor–stator interaction greatly influences the gap between the volute tongue and the impeller tip. The research results provide a reference for the design of the PAT.

Analyzing the impact of blade geometrical parameters on energy recovery and efficiency of centrifugal pump as turbine installed in the pressure-reducing station

Improving renewable energy efficiency has received great attention globally by increasing demand for energy supplies and air pollution issues. The Soft Pressure Regulation System (SPRS) as a potential technology provides the possibility of regulating pressure and energy recovery in pressure-reducing stations. Pump as Turbine (PAT) is a good choice for installation in Water Distribution Network (WDN) pipelines due to its availability and economic benefits, and it can play a key role in decreasing startup time and return on investment. In this paper, the performance of centrifugal PAT under WDN conditions has been numerically and experimentally evaluated, and simulation validation was performed, too. Analyzing the work conditions of the pressure reduction station shows that the hourly flow rate trend changes considerably based on end-user demand. A major portion of the SPRS's operation time occurs in off-design conditions of the PAT, and efficiency is drastically decreased considering the lack of flow control tools. The effect of blade inlet angle, blade warp angle, blade curvature, and change of these parameters simultaneously was investigated to increase the high-efficiency working range. Losses resulting from flow separation and formation of vortices are significantly decreased by simultaneous change of blade geometrical parameters in the range of design point and higher flow rates (Q ≥ 1.0QBEP). Since the highest frequency of flow rate based on statistical analysis occurs in this range, the total power generation of SPRS improves significantly. The PAT efficiency with the improved impeller at part-load condition (0.8QBEP), design condition (QBEP), and over-load condition (1.2QBEP) is improved by 0.83 %, 2.69 %, and 6.71 % in comparison with the original impeller, respectively.

Performance and unsteady flow characteristic of forward-curved impeller with different blade inlet swept angles in a pump as turbine

As an economical energy recovery device, a pump as turbine (PAT) is widely used in micro hydropower and energy recovery fields. Compared with an original impeller, a special impeller with forward-curved blades can dramatically enhance a PAT's efficiency and broaden its high-efficiency range because the blade angles created by the latter more effectively match the PAT's operating mode. The shape of blade curve and sweep has obvious influence on the performance of a PAT. To investigate how the inlet swept angle of forward-curved blade influences the performance of PAT, five different forward-curved impellers with different inlet swept angle blades were designed and numerically investigated based on a verified computational fluid dynamics (CFD) method. The effects of blade inlet swept angles on the external characteristics, energy loss and unsteady characteristics of PAT are analyzed. Compared with the blade whose inlet swept angle is 0°, the maximum efficiency of back-swept PAT with swept angle of −8° increases by 0.58% while the head decreases by 0.49 m. However, the efficiency of forward-swept PAT of 8° reduces by 0.9%, and the head increases by 0.68 m. The maximum reduction rates of the main frequency amplitude of pressure pulsation in the volute, impeller and draft tube of back-swept PAT are 35.2%, 45.47% and 67.24%, respectively, indicating that the back-swept blade can contribute to improve the operation stability of PAT. Moreover, the vortex in PAT with back-swept blade is greatly improved under partial and design flow rate, which can improve the unsteady flow characteristics of PAT.

Studying the impact of impeller geometrical parameters on the high-efficiency working range of pump as turbine (PAT) installed in the water distribution network

The development plan for micro hydropower is expanding with an increasing focus on renewable energy sources. There is increasing attention to using PATs to recover extra hydraulic energy in the water distribution network (WDN) pipelines. In this paper, the performance curves of PAT installed instead of pressure-reducing valves in WDN were investigated. The quality and reliability of numerical simulations were validated compared to the experimental results. Investigating hourly flow rate variations in WDN shows that the PAT working conditions change frequently and operate for a long time under off-design conditions. Since there are no tools to control and guide the fluid to the impeller, there is no proper conformance between the inlet flow and the blade profile, and in this condition loss increases and efficiency decreases. In order to improve the PAT performance in the working range, the effect of modifying parameters of blade thickness, blade number, and blade inlet width is numerically investigated. Optimal geometric parameters for designing a special impeller were determined by considering the flow rate changes of the PRS over 4 working months. Analyzing the PAT performance with a special impeller indicates that losses caused by the flow separation at the leading and trailing edges of the blade as well as the formation of vortices are significantly reduced. Moreover, the fluid's tendency to follow the blade profile is improved due to the flow field form in impeller passages, and the PAT's high-efficiency working range is extended. The greatest increase in efficiency occurs in the overload flow rate (1.2QBEP) and is improved by 6.48%. In this case study, the power generation capacity of the soft pressure regulating system (SPRS) will increase by 5.1% using the special impeller.

Development and validation of a comprehensive methodology for predicting PAT performance curves

This paper presents a physics-based approach aimed to predict the performance curves of PATs (pumps as turbines). The methodology includes a tuning procedure, which allows the calibration of the physics-based model on a given dataset of pumps/PATs, and a prediction procedure, which allows the estimation of PAT performance curves for previously unknown PATs. The methodology is applied to six pumps/PATs, of which the performance curves were experimentally determined at different rotational speeds. A cross-validation process is applied in such manner that one of the six pumps/PATs is not employed for the tuning procedure and is used for validating the reliability of the prediction procedure.The results show that, in the range ±20% with respect to the experimental BEP, the deviation of the predicted PAT performance curves from experimental curves can be acceptable from an engineering point of view, with an average value of 13.4%, 14.0% and 12.4%, for head, power and efficiency, respectively.

Performance improvement of forward-curved impeller with an adequate outlet swirl using in centrifugal pump as turbine

Pump as turbine (PAT) is a type of economical energy recovery device that is widely used in micro-hydropower plants and energy recovery. A special impeller with forward-curved blades can dramatically improve the PAT's performance relative to an original impeller with backward-curved blades because the inlet angle created by the former more effectively matches volute outlet flow. The blade outlet angle is another important factor in energy conversion. To investigate the effects of blade outlet angle on the performance of a special impeller, four outlet velocity moments were chosen according to the same inlet velocity moment value vu1r1 (0.02vu1r1, 0, −0.02vu1r1, -0.04vu1r1, were used). Four special impellers with different blade outlet angles were modeled and simulated via verified computational fluid dynamics. The results show that, the highest efficiency among PATs at −0.02vu1r1 was higher than that at 0.02vu1r1 by 1.46%. A suitable negative outlet velocity moment also helps eliminate swirl at the impeller outlet caused by the asymmetric inlet velocity moment and helps reduce the formation of vortex in the draft tube. This further decreases hydraulic loss and improves the operating stability of PAT. This paper presents a method for determining the blade outlet angle of the special impeller of PAT.

Effect of blade layout on performance of low specific speed pump operating in turbine mode

The very low specific speed pump (nq = 8.9) operated in turbine mode was analyzed. The experimental and numerical studies were carried out in order to show effect of different blade layout on pump-as-turbine (PaT) performance. In total three different impellers were analyzed. One impeller consisting of four main blades and two impellers consisting of four main blades and different arrangement of splitter blades. Either single splitter blade or two splitter blades are placed between each of main blades. While measurements pointed out the main PaT performance, the simulations enable to analyze internal flow fields and point out the mechanisms of performance variation using different impellers. The main aim of this study is to clarify usability of very low specific speed pump for energy recovery in terms of pump as turbine operation or for storage capability in terms of pump-turbine.

Performance analysis of geometrically optimized PaT at turbine mode: A perspective of entropy production evaluation

In the field of hydroelectric power, Pump as Turbine (PaT) is an important tool to achieve recycling the redundant kinetic energy in small stream/rivers. However, due to the contradiction between the reverse fluid flow working at turbine mode and the designed flow passage of pump geometry, the internal hydraulic loss in PaT is excessive. As a result, it renders the PaT performance extremely low affecting its wide applications with the demand of high efficiency operation. Therefore, this paper aims to optimize the geometric parameters of the impeller in PaT turbine mode under three different typical flow conditions (i.e. rated, best efficiency, and overload working conditions). After the geometric optimization, the internal energy losses among the flow passages are analyzed by using the entropy production method. The results show that the impeller passage exhibits obviously higher entropy loss in comparison to the volute and the outlet pipe. Inside the impeller, the blade leading and trailing edges on both suction and pressure sides are the locations where the majority of the impeller energy losses occur, and the high entropy area is concentrated at the impeller inlet area. Moreover, The energy loss gradually decreases from blade leading edge (BLE) to blade trailing edge (BTE), and the entropy production decreases from the blade pressure side (PS) to the blade suction side (SS) due to the large mixing flow loss from PS to SS. It is concluded that the entropy production analysis is effective in explaining the PaT power losses caused by the geometric mismatch of different components including inlet, impeller, and outlet, providing the reference value for the geometric optimization of other turbine machinery.

Numerical and experimental investigations on inflow loss in the energy recovery turbines with back-curved and front-curved impeller based on the entropy generation theory

Pump as turbine (PAT) is one of the economical and effective energy recovery devices in small hydropower stations. A back-curved PAT and a front-curved PAT were designed, and performance characteristics were studied, the accuracy of the numerical calculation was verified by comparing with the experimental results. The entropy generation theory was used to compare performance and energy loss of PATs. The results show that the high efficiency range of front-curved PAT is significantly wider than that of back-curved PAT. Under part-load condition (0.8Qd), design flow condition (1.0Qd) and over-load condition (1.2Qd), the efficiency of the front-curved PAT is 0.6%, 5.9% and 7.9% higher than that of the back-curved PAT, respectively. The energy loss in the PAT impeller mainly comes from the turbulent entropy generation rate which is mainly concentrated on the blade leading edge and trailing edge. Flow separation and flow impact caused by the mismatch between the relative flow angle and the blade setting angle are the main mechanisms of energy loss in impeller. In addition, the loss caused by the wall friction in the front-curved impeller is less than that in the back-curved impeller. Therefore, the entropy generation theory can provide guidance for the performance optimization of PAT.

A Numerical Analysis of the Effect of Impeller Rounding on Centrifugal Pump as Turbine

Pump as turbine (PAT) is one of the micro-hydro system components. It can be used alone as a generator in remote areas without power supply, and in hydraulic networks instead of pressure-reducing valves. However, its hydraulic optimization still remains an open research problem. One of the optimization techniques is the rounding of the sharp edges at the blade periphery. Existing studies are mostly based on prototype experiments to obtain the optimization effect. In order to more intuitively analyze the influence of this structural optimization on the internal flow of PAT, this paper uses the CFD method to study the influence of the leading edge of the centrifugal pump blade and the fillet of the impeller on the turbine performance. By simulating the PAT performance at different flow rates and speeds, the internal hydraulic performance changes caused by the inverted circular blade are analyzed. Simulation results show that, under various operating conditions, the impeller inverted circle improves the efficiency of PAT to different degrees. At the speed of 1500 rpm, the efficiency is most obviously improved, which can reach 8.09%. Internal flow results show that the efficiency increases along with the decrease in impeller inlet resistance and the flow separation region in the impeller. This paper provides an effective method for studying the PAT hydraulic optimization problem.

Analysis and evaluation of the performance and utilization of regenerative flow pump as turbine (PAT) in Pico-hydropower plants

In this paper, a novel PAT system was analyzed and evaluated for utilization at very low specific speeds. Thus, a regenerative pump was employed due to the simplicity, compact size, stable features, low manufacturing costs, and very low specific speed. A theoretical analysis was developed based on the momentum exchange theory to predict the performance characteristics of the regenerative pump in reverse operation. Also, a 3D numerical simulation was implemented to study the performance of the pump in reverse mode. The numerical simulations were validated with experimental data of the pump. Results revealed that except for very low flow rates the presented model provides an acceptable prediction of the PAT performance. Results showed that at BEP, efficiency has decreased by about 20%. The head ratio and discharge ratio was obtained a bout 3.5 and 3.6, respectively. According to the results of entropy generation method, the impeller blades showed the most potential for energy losses. It was concluded that regenerative pump could be easily run as a low specific speed hydraulic turbine or a pressure reducing valve due to the cheapness, reliability and availability.

Experimental assessment of the impact of number of stages on vertical axis multi-stage centrifugal PATs

The pursuit of Best Management Practices (BMPs) for pressurized water systems has intensified interest in Pumps As Turbines (PATs), given that PAT use enables coupling of pressure regulation with small-scale hydropower generation. Installing PATs instead of micro-turbines provides for several advantages, such as cheaper procurement and maintenance costs, and importantly, the ability to select the most reliable model from among a wide variety of devices available in the market. Nevertheless, the current state of knowledge regarding PAT performance leaves out critical aspects that remain to be adequately addressed, especially for some models, such as centrifugal vertical axis multi-stage pumps running in reverse mode, despite such devices having attractive features that offer particularly advantageous benefits when high head in the system is exploitable.In contributing key experimental findings on four vertical multi-stage (single-, two-, three-, and four-stage) PATs, this paper thus extends the performance related knowledge about vertical multi-stage PATs: prior models from the literature are demonstrated to reliably predict performance, with the contribution additionally consisting in assessing performance dependence on number of pump stages. The results showed both head and power to be highly correlated with the variable number of stages, especially for predicting PAT characteristics at the Best Efficiency Point (BEP) for multi-stage models. However, the single-stage model exhibited slightly lower performance values.

Numerical investigation on the volute cutwater for pumps running in turbine mode

The hydropower sector has recently raised the interest in pump as turbine (PaT) that can be a valid trade-off between capital cost and performance in micro-scale installations. Nevertheless, the modest efficiency of PaTs often restricts their exploitation. In this paper, available experimental data are used to tune a numerical model which aims to investigate the effect of the pump cutwater design on the PaT performance to improve the hydraulic efficiency. Because of its finite thickness, the cutwater interferes with the flow at the runner inlet, and generates local flaws in the velocity field such as swirl and deviations of the streamlines which limit the machine performance. To identify the geometrical characteristics of the cutwater impacting on the PaT performances, different values of stretching and thickness of the cutwater are studied at variable inclination by computational fluid dynamics (CFD) simulations. A multivariate regression method is applied on the CFD results to build a surrogate model of the PaT hydraulic characteristics as a function of the geometrical parameters of the machine cutwater. Based on this model, an optimization problem is solved to identify the most advantageous geometrical asset of the PaT cutwater to maximize the efficiency. The results highlight that the length and the cutwater angle are the most affecting variables in favouring a tangential component at the entrance of the runner. The hydraulic efficiency peak of the optimized geometry results to be 86.3%, while the baseline configuration records an efficiency of 82.4%, and the Ψ−ϕ characteristic moves the best efficiency point towards higher head (+7.5%) and lower discharge (−13.0%). The proposed methodology allows identifying the best geometrical characteristics of the PaT cutwater to maximize the performances while significantly reducing the computational time.

Investigation of Flow Separation Characteristics in a Pump as Turbines Impeller Under the Best Efficiency Point Condition

Abstract The impeller, which is the main energy conversion component of a pump as turbine (PAT), is designed for pumping mode, and its internal flow characteristics are quite complicated even at the best efficiency point (BEP) of the turbine mode. This study aims to investigate the flow separation characteristics in a PAT impeller under the BEP condition by numerical method. The hydraulic performance and transient pressure characteristics of PAT predicted numerically were verified through experimentation. The surface friction lines and flow topological structure were applied to diagnose the flow separation at the surface of the blade. The relationship between flow topological structure and vortex in the impeller and static pressure at the blade were analyzed. Analysis results show that the backflow and open flow separation are observed significantly in the leading region and near the shroud of the trailing region of suction side. The passage vortex always appears near the spiral node. The saddle point and spiral node correspond to the peak position of adverse pressure and the lowest position between two peak values of the static pressure of the blade, respectively. The inflow conditions of blade and shape of the trailing edge significantly influence the flow separations in the impeller.

Numerical and Experimental Investigations on Inflow Loss in the Energy Recovery Turbines with Back-Curved and Front-Curved Impeller Based on the Entropy Generation Theory

Pump as turbine (PAT) is one of the economical and effective energy recovery devices in small hydropower stations. A back-curved PAT and a front-curved PAT were designed, and performance characteristics were studied, the accuracy of the numerical calculation was verified by comparing with the experimental results. The entropy generation theory was used to compare performance and energy loss of PATs. The results show that the high efficiency range of front-curved PAT is significantly wider than that of back-curved PAT. Under part-load condition (0.8Qd), design flow condition (1.0Qd) and over-load condition (1.2Qd), the efficiency of the front-curved PAT is 0.6%, 5.9% and 7.9% higher than that of the back-curved PAT, respectively. The energy loss in the PAT impeller mainly comes from the turbulent entropy generation rate which is mainly concentrated on the blade leading edge and trailing edge. Flow separation and flow shock caused by the mismatch between the relative flow angle and the blade setting angle are the main mechanisms of energy loss in impeller. In addition, the loss caused by the wall friction in the front-curved impeller is less than that in the back-curved impeller. Therefore, the entropy generation theory can provide guidance for the performance optimization of PAT.

Numerical simulation and experimental investigation on the influence of the clocking effect on the hydraulic performance of the centrifugal pump as turbine

With the increasing attention to small hydropower, research on improving the hydraulic performance of the pump as turbine (PAT) has become more important. Therefore, to study the influence of clocking effects on the hydraulic performance of PAT, numerical simulation and experimental verification of different guide vane clocking positions are carried out in this paper. The angle between the trailing edge of the guide vane and the volute tongue, θ, is set in eight schemes from 0 to 70 deg. The maximum difference in the efficiency of the eight schemes can reach 2.45% at 80 m3/h. When the flow rate is more than 48 m3/h, the scheme with θ of 50° shows the highest efficiency. The regression equations of entropy generation and total pressure loss are established to obtain visual energy loss. The results indicate that energy loss of the stationary components changes significantly under different schemes, which is the main reason for the change in the efficiency of PAT. Besides, the stator-rotor interaction makes the energy loss near the wake region of the impeller change periodically. This research not only reveals the cause of the clocking effect but also provides a new perspective for the optimization of PAT structures.

Improvement of Internal Flow Performance of a Centrifugal Pump-As-Turbine (PAT) by Impeller Geometric Optimization

Rotor-stator interaction (RSI) in the centrifugal pump-as-turbine (PAT) is a significant source of high amplitude of the pressure pulsation and the flow-induced vibration, which is detrimental to the stable operation of PAT. It is therefore imperative to analyze the rotor-stator interaction, which can subsequently be used as a guideline for reducing the output of PAT noise, vibration and cavitation. In addition, it is important for a PAT to have a wide operating range preferably at maximum efficiency. In order to broaden the operating range, this work proposes a multi-condition optimization scheme based on numerical simulations to improve the performance of a centrifugal PAT. In this paper, the optimization of PAT impeller design variables (b2, β1, β2 and z) was investigated to shed light upon its influence on the output efficiency and its internal flow characteristics. Thus, the aim of the study is to examine the unsteady pressure pulsation distributions within the PAT flow zones as a result of the impeller geometric optimization. The numerical results of the baseline model are validated by the experimental test for numerical accuracy of the PAT. The optimized efficiencies based on three operating conditions (1.0Qd, 1.2Qd, and 1.4Qd) were maximally increased by 13.1%, 8.67% and 10.62%, respectively. The numerical results show that for the distribution of PAT pressure pulsations, the RSI is the main controlling factor where the dominant frequencies were the blade passing frequency (BPF) and its harmonics. In addition, among the three selected optimum cases, the optimized case C model exhibited the highest level of pressure pulsation amplitudes, while optimized case B reported the lowest level of pressure pulsation.