Energy is unarguably the key factor for today's economic and social development within nations Electricity as one of many energy forms is a critical input to developing countries in the struggle to the national self-satisfaction in all domains Rural electricity supply involved institutions have recently recommended the pump as turbine (PAT)-based micro hydropower plant (MHP) schemes for remote off-grid electrification, mostly from their economic advantages However, from different published research findings, PAT-based MHP is not only simple and economically feasible, but has presented bottlenecks in the move to its full understanding Moreover, compared to other clean energy technologies, PAT technology has not found much literature in academic published researches, thus contributing to its limited understanding within the community Therefore, the PAT literature availability is one way to level up its understanding, which can be helpful to academic and professional communities In the present study, a state-of-the-art review on the two most challenging PAT aspects, namely PAT performance prediction and PAT flow stability aspects are presented In the presented literature, the selected energy sources history leading to the actual MHP global adoption was first briefly explained, followed by an intensive literature on PAT operations, where details about PAT selection and performance prediction were provided Finally, the PAT flow stability aspects where pump-turbine S-shape and Saddle-type characteristics constitute the main focus, were discussed It is worth an attention to mention that the words “pump-turbine”, “Pump as turbine”, and “reversible pump turbine”; are equally used throughout the whole literature It is within the authors wish that this paper can scale up the reader's PAT technology understanding, thus serving awareness in the same

Investigation on pump as turbine (PAT) technical aspects for micro hydropower schemes: A state-of-the-art review

S-shaped characteristics on the performance curves of pump-turbines in turbine mode promote instability problems in associated transient processes, e.g., difficulties in synchronizing with power grid in turbine start-ups, unstable performance in turbine load rejections, etc. The present paper presents and discusses relative studies on the S-shaped characteristics, which have been carried out by various investigators. Studies include system instability analysis, experimental and numerical investigations on the flow mechanism of pump-turbines, studies on the influencing geometric and operating parameters of the unit, and precautions and countermeasures to improve the S-shaped characteristics and instability problems in associated transient processes.

S-shaped characteristics on the performance curves of pump-turbines in turbine mode – A review

Over the past decade, the use of conventional one-dimensional numerical-simulation methods has been demonstrated to be inadequate in terms of their usefulness in investigations concerning transient processes in hydraulic machines systems—their theoretical analyses and engineering applications. Consequently, numerous three-dimensional numerical methods capable of accurately simulating transient processes in hydraulic-machine systems have been proposed and improved upon in recent years. Through use of these novel methods and strategies, many researchers have investigated transient characteristics of processes occurring within hydraulic-machine systems along with corresponding formation mechanisms. This study presents a comprehensive review of related experimental studies, novel numerical methods and strategies along with transient characteristics and formation mechanisms in hydraulic-machine systems. Based on this study, suggestions have been made concerning the selection of simulation methods to be used and directions for future research have been proposed.

Investigation methods for analysis of transient phenomena concerning design and operation of hydraulic-machine systems—A review

Evolutions of Pressure Fluctuations and Runner Loads During Runaway Processes of a Pump-Turbine

Transient flow analysis in reactor coolant pump systems during flow coastdown period

Numerical investigation on the unsteady characteristics of reactor coolant pumps with non-uniform inflow

The diffuser of a reactor coolant pump was optimized using an orthogonal approach with numerical simulation to improve the pump hydraulic performance. Steady simulation was conducted by solving Reynolds-averaged Naiver-Stokes equations with the SST k-ω turbulence model using CFX code. The influence of the diffuser geometric parameters, namely, S, φ, α
 4, b
 4, δ
 2, R
 t and R
 4, on the pump performance were determined. L18 (37) orthogonal table was chosen for the optimization process. Best indicators were determined, and range analysis of energy losses, head, and efficiency at the rated condition was performed. Optimal parameters of the diffuser were S = 490 mm, φ = 36°, α
 4 = 30°, b
 4 = 200 mm, δ
 2 = 20 mm, R
 t = 5 mm and R
 4 = 565 mm. The final design was experimentally tested. Simulation results showed more remarkable performance than the experimental result. However, the numerical predictions and experimental results were consistent, validating the design procedure. Loading of the impeller and diffuser blades was analyzed to investigate the direct impact on the hydrodynamic flow field. The head was 14.74 m, efficiency was 79.6 %, and efficiency of the prototype pump was 83.3 % when the model pump functioned at the rated conditions. Optimization results showed that efficiency and head were improved at the design condition.

Numerical and experimental investigation on the diffuser optimization of a reactor coolant pump with orthogonal test approach

Instability of pump turbine with S-shaped curve is characterized by large fluctuations of rotational speed during the transient processes. For investigating this phenomenon, a numerical model based on the dynamic sliding mesh method (DSSM) is presented and used to numerically solve the 3D transient flow which is characterized by the variable rotation speed of runner. The method is validated by comparison with measured data for a load rejection process in a prototype pump turbine. The results show that the calculated rotation speed agrees well with the experimental data. Based on the validated model, simulations were performed for the runaway process using an artificially assumed operating condition under which the unstable rotation speed is expected to appear. The results confirm that the instability of runner rotational speed can be effectively captured with the proposed method. Presented results include the time history profiles of unit flow rate and unit rotating speed. The internal flow characteristics in a typical unstable period are discussed in detail and the mechanism of the unstable hydraulic phenomenon is explained. Overall, the results suggest that the method presented here can be a viable alternative to predict the dynamic characteristics of pump turbines during transient processes. pump turbine, transient process, dynamic sliding mesh, flow stability

Investigation of the unstable flow phenomenon in a pump turbine

The problem of non-uniform inflow exists in many practical engineering applications, such as the elbow suction pipe of waterjet pump and, the channel head of steam generator which is directly connect with reactor coolant pump. Generally, pumps are identical designs and are selected based on performance under uniform inflow with the straight pipe, but actually non-uniform suction flow is induced by upstream equipment. In this paper, CFD approach was employed to analyze unsteady hydrodynamic characteristics of reactor coolant pumps with different inflows. The Reynolds-averaged Naiver-Stokes equations with the k-ε turbulence model were solved by the computational fluid dynamics software CFX to conduct the steady and unsteady numerical simulation. The numerical results of the straight pipe and channel head were validated with experimental data for the heads at different flow coefficients. In the nominal flow rate, the head of the pump with the channel head decreases by 1.19% when compared to the straight pipe. The complicated structure of channel head induces the inlet flow non-uniform. The non-uniformity of the inflow induces the difference of vorticity distribution at the outlet of the pump. The variation law of blade to blade velocity at different flow rate and the difference of blade to blade velocity with different inflow are researched. The effects of non-uniform inflow on radial forces are absolutely different from the uniform inflow. For the radial forces at the frequency fR, the corresponding amplitude of channel head are higher than the straight pipe at 1.0Фd and 1.2Фd flow rates, and the corresponding amplitude of channel head are lower than the straight pipe at 0.8Фd flow rates.

The Effect of Different Inflows on the Unsteady Hydrodynamic Characteristics of a Mixed Flow Pump

The theoretical and experimental studies of reactor coolant pump accidents encountered nation-wide and world-wide were described. To investigate the transient hydrodynamic performance of reactor coolant pump (RCP) during the period of rotational inertia in the station blackout accident, some theoretical and experimental studies were carried out, and the analysis of the test results was presented. The experiment parameters, conditions and test methods were introduced. The flow-rate, rotate speed and vibrations were analyzed emphatically. The quadruplicate polynomial curve equation was used to simulate the flow-rate,rotate speed along with time. The test results indicate that the flow-rate and rotator speed decrease rapidly at the very beginning of cut power and the test results accord with the regulation of safety standard. The vibrant displacement of bearing seat is intensified at the moment of lose power, but after a certain period rotor shaft libration changes. The test and analysis results help to understand the hydrodynamic performance of nuclear primary pump under lost of power accident, and provide the basic reference for safety evaluation. (authors)

Wang Dezhong

Papers

Numerical investigation on the unsteady characteristics of reactor coolant pumps with non-uniform inflow

Numerical and experimental investigation on the diffuser optimization of a reactor coolant pump with orthogonal test approach

Investigation of the unstable flow phenomenon in a pump turbine

The Effect of Different Inflows on the Unsteady Hydrodynamic Characteristics of a Mixed Flow Pump

Test Study on Safety Features of Station Blackout Accident for Nuclear Main Pump