Vaneless Region Research Articles

Using bionic tubercles to control swirling flow instabilities of a hydraulic turbine during the load rejection process

With the development of renewable energy sources, operations of hydroelectric units under off-designed conditions and frequently transition processes have gradually become normalized, which is accompanied by threats from swirling vortex ropes, high amplitude pressure fluctuations, and strong hydraulic vibrations. This study introduces bionic tubercles into the Francis turbine's draft tube and guide vane design to suppress pressure fluctuation and stabilize swirling vortex rope during the load rejection process. Results reveal that the bionic Francis turbine (BFT) reaches its maximum speed of 686.4 rpm at 3.804s, marking a 6.08 % reduction from the prototype Francis turbine's maximum speed during load rejection. Moreover, it significantly diminishes the principal frequency amplitude of low-frequency pressure fluctuations caused by swirling flow instability from the draft tube cone, achieving a reduction by one to two magnitude orders. It is interesting that the scale of the circumferential vortex in the vaneless region and blade vortices within the runner channels is verified to decrease. More importantly, it is attributed that the bionic tubercles effectively disrupt and disintegrate the walled circumferential vortex structures in the draft tube.

Research on the influence of labyrinth ring configuration on runaway characteristics of pump-turbines

The labyrinth ring can reduce the volume loss, mitigate the influence of force characteristics when the unit is operating, and ensure the efficient, safe, and stable operation of the unit. It is usually installed in the upper crown and lower ring of the pump-turbines runner. When the pump-turbines unit is running in runaway condition, the internal flow field structure is more complicated due to the different forms of labyrinth ring, so a new method needs to be adopted to analyze it from a new perspective. In this paper, a high head pump-turbines was selected as the research object. Combined with the experimental and numerical simulation results, the flow field structure corresponding to different upper labyrinth ring forms under runaway condition was analyzed and visualized in many aspects. The results show that the mixed type labyrinth ring can improve the undesirable flow regime in the runner and draft tube. The guide vanes under the serrated type labyrinth ring are more likely to be damaged due to greater force. The influence of the mixed type labyrinth ring on the radial force of the runner is only half that of the ladder type, which has a great advantage in maintaining the radial stability of the runner. The ladder and serrated type labyrinth ring can make the unit obtain less axial water thrust. In addition, based on the Finite-Time Lyapunov Exponent (FTLE) method, the flow field structure in the vaneless region was analyzed, and it was found that the labyrinth ring may affect the flow in the flow path of the guide vane and the vaneless region, thus causing the change of axial water thrust. This study reveals the internal flow and force characteristics of pump-turbines with different types of labyrinth rings under runaway condition and provides a technical reference for further understanding and studying the influence of labyrinth ring forms on pump-turbines performance and force characteristics.

Numerical investigation of energy dissipation and vortex characteristics on the S-shaped region of a reversible pump-turbine

In order to meet the regulation needs of power system, the pump-turbine is prone to operate in the S-shaped region during frequent start-up, leading to unit vibration and grid connection failure. In this paper, the S characteristic experiment was carried out, and the entropy production, unstable flow and vortex structure under turbine, runaway, turbine breaking, reverse pump conditions with optimum guide vane opening are detailed analyzed. The relationship between the energy dissipation and flow field characteristics is clarified. The results show that the runner entropy production is the largest in all components, and the wall entropy production is greater than the fluctuating entropy production under turbine condition. With the decrease in discharge, the fluctuating entropy production and wall entropy production first increase and then decrease, reaching the maximum values in the runaway condition. The circumferential movement trend of water enhances, and forms circumferential flow in the vaneless region. The separation vortex formed on the blade suction surface extends from the blade inlet to the middle of the flow channel, inducing a high entropy production. Under the turbine braking condition, the tangential velocity increases rapidly in the vaneless region. The circumferential flow develops into water ring, blocking the water flows into the runner. Under the reverse pump condition, the water flows in the opposite direction, with negative tangential and streamwise velocities. The absolute flow angle becomes obtuse, and a large-scale reverse vortex is formed. The runner inlet is blocked by the water ring, and the flow channel is almost completely filled with the low-speed region. The proportion of the guide vanes entropy production reaches maximum. Unstable flow leads to the increase in vorticity. Separation vortex, bypassed vortex, and reverse flow vortex leads to an increase in VST, forming a high VST region at corresponding positions. Due to the rotational motion accompanying the development of separated vortex, it leads to the increase in CFT. In addition, the circumferential flow and water ring have higher tangential velocities, which can lead to high CFT regions.

Vibration and Flow Characteristics of a 200 MW Kaplan Turbine Unit under Off-Cam Conditions

Kaplan turbine units can adjust their blades to achieve wider outputs without a significant loss of efficiency. The combination of guide vane angle (GVA) and blade angle (BA) is selected based on efficiency curves obtained from cam tests. However, the vibration characteristics are not considered in the test. The vibration and flow characteristics are complex with different combinations of guide vane and blade angles. Different cam relation selection principles lead to varying machine vibration and flow characteristics. In this research, the flow and vibration characteristics were obtained by means of field test and numerical simulation. Vibration, pressure pulsation, and other stability indicators have been extracted and investigated under off-cam conditions. The flow and variation rules of different indicators have been thoroughly researched. The findings suggest that the magnitude of vibration in the X direction surpassed that in the Y direction for the head cover, upper frame, and lower frame under 22 experimental conditions. The disparity between the head cover and upper frame in both directions was not significant, whereas a substantial contrast existed between the lower frame in the X and Y directions. The calculation results indicate that when the guide vane angle was small, vortices appeared near the high-pressure edge of the runner in the vaneless region and caused disorganized flow lines in the runner, and this complex vortex behavior led to multiple frequency components in the pressure pulsation frequency domain. The conclusions provide references for the designers of Kaplan turbine units and improves the operating safety of Kaplan turbine power stations.

Numerical investigation of hydraulic instability of pump-turbines in fast pump-to-turbine transition

The fast pump-to-turbine transition of a pumped storage unit is highly responsive to electrical power system regulation demands and is a critical process for the unit itself. The correct prediction of this hydraulic transient process is crucial to avoid issues during machine operation. In this study, a three-dimensional numerical model of the transition process of a pumped storage hydraulic system was proposed and analyzed. Numerical simulations were performed to investigate the variations in external parameters, pressure pulsations, and the evolution of internal flow patterns and vortical structures in the pump turbine. The results showed that the stepwise variations in the flow rate, torque, and force were synchronized with the movement of the guide vanes. Notably, during the pump braking operation and runner speed reversal, a considerable time period with significant force oscillations and intense torque fluctuations occurred, particularly in the vaneless region, where pressure fluctuations were notable. Vortical structures that induce vibrations and intense pressure fluctuations were localized near the runner inlet and at the leading and trailing edges of the guide and stay vanes. This study offers theoretical insights that are crucial for enhancing the stability of the fast pump-to-turbine transitional process, thus optimizing its control strategies.

Pressure Fluctuation analysis of pump-turbine in turbine mode under the same head condition

Abstract As the key technical part of pumped-storage power station, the efficient and stable development of pump-turbine units is of great significance for renewable energy power generation. However, pump-turbine units have the characteristics of complex and variable operation conditions. In addition, under different guide vane openings, operation conditions of pump-turbine units also show different flow patterns and pressure fluctuation characteristics. For the sack of studying the pressure fluctuation characteristics of pump-turbine units in turbine mode, a dynamic model of a pump-turbine unit is established in this research. Based on SST k-ω turbulence model, combined numerical simulation and experimental test, pressure fluctuation laws in vaneless region and draft tube under three typical guide vane openings are carried out. The results indicate that, by increasing the guide vane opening, the pressure fluctuation intensity of each monitor point is decreased. Furthermore, the pressure fluctuation at the monitor points in vaneless region between guide vane and the runner presents a periodic change rule, which is mainly due to the two-stage rotor and stator interference between guide vane and runner, and between runner and draft tube. The research further clarifies the pressure fluctuation law in turbine mode, and provides theoretical reference for long-term sustainable development of pump-turbine units.

Study on the design of short blade offset for the long and short blade runner of 1000 MW hydraulic turbine units

Abstract For such complex rotating machinery as hydraulic turbine, the hydraulic operation characteristics of the hydraulic turbine unit are significantly impacted by the short blade of the runner used as the splitter blade. To find the optimum position of the short blade, the study object is the 1000 MW hydraulic turbine, selects three circumferential offset positions of the short blade for numerical simulation calculation, obtains the runner flow pattern at different offsets, and explores the impact on the short blade offset on the vortex and pressure pulsation in the runner. The results show that the counterclockwise offset of the short blade will improve the flow pattern of the runner, reduce the scale of vortex in the runner and enhance the efficiency of the hydraulic turbine. The counterclockwise offset will increase the interference while decreasing the pressure pulsation in the vaneless region, at the runner’s inlet, and downstream of the short blade. When the offset δ=0.6, the peak-valley difference of pressure fluctuation in the vaneless zone is the smallest, accounting for only 1.38 % of the rated head. The main frequency of the vaneless zone gradually changes from 15fn to 30fn , and the main frequency of the runner inlet and the short blade downstream is 24fn . Due to the different weakening effects of short blades on the interference, low-frequency pulsation components of 4fn and 8fn appear.

Analysis of the Flow Behavior and Pressure Fluctuation of a Pump Turbine with Splitter Blades in Part-Load Pump Mode

The internal flow of a pump turbine is unstable in part-load pump mode for small guide-vane openings, and the strong vibration caused by pressure pulsation is related to the safe and stable operation of the unit. A pump turbine with a six-splitter-blade runner was chosen for unsteady simulation analyses. A standard k-epsilon turbulence model was adopted to study the unsteady flow and pressure pulsation in part-load pump mode. The predicted results show that the flow in the draft tube and the runner with splitter blades was relatively stable and the flow of the blade-to-blade channel was symmetrical. When the inlet and outlet velocity distribution of the vanes was not uniform, a vortex began to form in the stay-vane domain. The reason for this vortex formation is explained, and it is pointed out that the existence of the vortex and backflow leads to uneven velocity distribution. The unsteady calculation results showed that the pressure-pulsation peak-to-peak amplitudes in the vaneless area and guide vanes were much higher than those of other monitor points because of rotor–stator interference between the rotating runner and the vanes. In addition, the pulsation characteristics of the monitor points at different circumferential positions in the vaneless region were quite different. In the vaneless area, the velocity gradient along the circumferential direction was very large, and there was a phenomenon of backflow. Also, the pressure pulsation was 0.2 times that of the runner rotational frequency, and the blade-passing frequency was a third-order frequency. At the outlet of the guide vane, the pressure pulsation was mainly of a low frequency with a complex vortex flow. Finally, the pressure pulsation began to decrease rapidly in the stay-vane region.

Numerical investigation of no-load startup in a high-head Francis turbine: Insights into flow instabilities and energy dissipation

The presented paper numerically investigates the internal flow behaviors and energy dissipation during the no-load startup process toward a Francis turbine. Passive runner rotation is implemented through the angular momentum balance equation accompanied by dynamic mesh technology and user defined function. Three phases of rotational speed are identified: stationary, rapid increase, and slow increase. Head exhibits a monotonic decrease, rapid rise and fall, and eventual fluctuation. Flow rate shows quasi-linear increase. The pressure fluctuations in the vaneless region are primarily dominated by the frequencies induced by Rotor-Stator Interaction and a broad frequency range below 50 Hz, and below 30 Hz in the draft tube. Runner inlet experiences positive to negative incidence angles, causing intense flow separation and unstable structures. Draft tube exhibits large-scale recirculation and evolving vortex structures. Energy loss analysis based on the entropy production method highlights the runner and draft tube as primary contributors. The energy loss within the runner exhibits an initial increase, subsequent decrease, and then a rise again during the stationary and rapid speed increase phases. While the draft tube shows a rapid increase during the phase of rapid speed increase. Turbulent fluctuations significantly contribute to entropy production loss, with trends matching total entropy production. Maximum energy loss locations correspond to runner inlet and draft tube wall, emphasizing the importance of unstable flow and vortex generation. This study establishes foundational insights into unstable hydrodynamics and energy dissipation modes during hydraulic turbine no-load startup, paving the way for further research.

Pressure pulsation of pump turbine at runaway condition based on Hilbert Huang transform

Pumped storage is an important component of electrified wire netting. The safe and stable operation of pump turbines is extremely important. Among them, pressure pulsation is one of the main causes of pump turbine vibration. The characteristics of pressure pulsation are relatively complex, and it is difficult to directly observe their temporal changes using commonly used FFT methods. The division of frequency characteristics is often vague. Meanwhile, it is difficult to explain some phenomena such as frequency doubling. This article focuses on a certain model of pump turbine and uses SST model to numerically simulate the runaway condition of the pump turbine. And the Hilbert Huang transform method is used to analyze the pressure pulsation in the vaneless region and draft tube. The results show that the main characteristic frequencies of the vaneless region are blade passing frequency 112.5 Hz and rotational frequency 12.5 Hz. The main characteristic frequencies of the draft tube are vortex rope frequency near 3 Hz which energy ratio is up to 50%, rotational frequency, and blade passing frequency. The pressure pulsation characteristics in the vaneless region have changed from a complex composition of double blade passing frequency and rotational frequency to a distribution dominated by blade passing frequency. In the passage of the guide vane, the pressure pulsation is almost only characterized by blade passing frequency. The frequency characteristics of the vaneless region between the runner and the guide vane become complex again. Meanwhile, the results show that the characteristic frequencies of the vaneless region and the draft tube propagate upstream and downstream.

Energy loss analysis of transition simulation for a prototype reversible pump turbine during load rejection process

The dynamic behavior of a reversible pump turbine (RPT) during the load rejection process was investigated through numerical simulation and experimental testing. The study focused on various parameters, such as the operation of guide vanes, torque, flow, head, and runner forces. The pressure fluctuations at different monitoring points were analyzed using the time-frequency method, specifically the continuous wavelet transforms (CWT). The results revealed significant pressure pulsation amplitudes in the vaneless region. Additionally, the entropy production rate was employed to quantify energy loss in different domains, including direct dissipation, turbulent dissipation, and wall shear stress. Moreover, the study utilized the vorticity transport equation to examine vortex characteristics and transport in the stay vanes, guide vanes, and runner flow channels. The vortex characteristics were classified into relative vortex stretching (RVS), Coriolis force (CORF), and viscous diffusion (VISD). The findings highlight significant energy loss in flow patterns associated with high RVS and CORF regions. This study provides valuable insights into the transition process and energy loss of RPTs during load rejection, serving as a basis for future research aiming to enhance the safety and reliability of RPTs under load rejection conditions.

Comprehensive hydraulic performance improvement in a pump-turbine: An experimental investigation

Pumped storage power plants, which is known as only large-scale energy management equipment, plays a vital important role in energy field. Hump characteristic and S characteristic are two classical unsteady hydraulic characteristics when pump-turbine operating at turbine mode and pump mode, respectively. It has been found that these two characteristics are both strongly related to the complex vortex evolution process in vaneless region while only few papers focus on the elimination mechanism of them. In present paper, a scaled runner with optimized high-pressure side (HPS) is designed and manufactured based on multi-objective optimization process arming at eliminating unsteady characteristic, i.e. hump characteristic and S characteristic, while maintaining efficiency characteristic unchanged. Thereafter, hydraulic experiments are conducted to investigate the impact of HPS geometry on hydraulic performance (efficiency, hump margin, S margin and pressure fluctuation) of object pump turbine. The experimental results show that compared with the original runner, the weighted average efficiency for pump mode increases by 0.1% while the weighted average efficiency for turbine mode decreases by 0.1%. Moreover, the S margin increases from 76.7 m to 87.2 m and the S2 unsteady region is largely increased. The hump margin decreases a little while it can recover to the original level through increasing 0.6% of the original ratio scale. Moreover, the runner with optimized HPS can effectively reduce the pressure fluctuation amplitude in vaneless region up to 33.3% and 21.4% for turbine mode and pump mode, respectively.

Mechanism of runner high-pressure side on stall characteristics at typical unsteady operating points in both modes of a pump turbine

Stall phenomenon, a classical physical phenomenon which is located in the vaneless region of a pump–turbine and accompanied by a complex vortex evolution process, is strongly related to the formation of hump unsteady region at the pump mode and S unsteady region at the turbine mode. In the present paper, a detached eddy simulation model is employed to numerically investigate the impact of runner high-pressure side (HPS) on stall characteristics at typical unsteady operating points, namely, a valley point in the hump region at the pump mode and a runaway point in the S region at the turbine mode. It is found that the stall characteristics at both investigated points are obviously changed: For the valley point, only three fixed stall cells exist in the original plan, while four additional rotating stall cells appear and rotate at the speed of 0.02nr (nr, runner rotation speed) in the optimized plan (OPT). The distinctive coexistence phenomenon of both fixed stall and rotating stall is reported for the first time and is attributed to the complex vortex evolution controlled by optimized HPS; for the runaway point, both the intensity and frequency of the stall characteristic are slightly increased in OPT. Moreover, for both operating points, the optimized HPS can effectively decrease the backflow at shroud, resulting in a significant decrease in the relative backflow rate within a complete flow period, of which 17.3% is for the valley point and 4.8% is for the runaway point. Finally, a local hydraulic loss rate (LHLR) method is adopted to investigate the hydraulic loss evolution process, and it is found that the high LHLR region in OPT is more concentrated in both circumferential direction and radial direction in the vanless region at both operating points. Based on the runner with optimized HPS proposed in the present paper, many unsteady hydraulic characteristics that is related to the stall phenomenon might be eliminated to some extent.

Numerical Investigation of the Flow Regime in the Vanes and the Torsional Self-Excited Vibration of Guide Vane in the Pump Mode of a Reversible Pump-Turbine

Guide vanes (GVs) are installed between the runner and the stay vanes for flow guidance and discharge regulation in reversible pump-turbines. The unstable torsional self-excited vibration of the guide vane (GV) may occur when running at small guide vane opening angles during the transient operations involving pump flow. In addition, the double-stage radial vanes may induce complex flow in the vanes and influence the stability of torsional self-excited vibration of the guide vane. In this study, numerical simulations were conducted at small guide vane opening angles in pump mode for two different guide vanes based on the three-dimensional computational fluid dynamics (CFD) method. The flow regime with a deflection was formed on the trailing edge with a circle in the vaneless region that rotated reversely against the runner rotation when operating at smaller guide vane opening angles for both of the two guide vanes. Based on this, the coupling simulations based on the CFD method with a single-degree-of-freedom (1DOF) oscillator were carried out under these operating conditions. Two flow types were formed at small opening angles when adopting different inlet boundary conditions. The results showed the flow regime with a deflection on the trailing edge may aggravate the instability of torsional vibration when applied as an initial flow field. Moreover, the vibration instability of the torsional self-excitation for two guide vanes was analyzed, showing that modifying the profile of guide vane airfoil is an efficient and reliable approach for weakening the torsional vibration instability.

Transient simulation of the rapid guide vane adjustment under constant head of pump turbine in pump mode

In order to understand the flow mechanism of pump turbine during the rapid guide vane adjustment (RGVA), the shear stress transition (SST) k-ω turbulence model was used for transient simulation. The dynamic mesh technique was used to simulate the closing process of the guide vane. Combined with the verification of grid independence and the validation of experimental results, the variations of performance parameters (head, flow, torque), entropy production rate and pressure pulsation were calculated. And this process can be basically divided into three stages, including the preparation stage before the guide vane closing (Stage I), the guide vane closing stage (Stage II) and the completion stage of the guide vane closing (Stage III). The numerical simulation results showed that the performance parameters were varying in the form of exponential function during RGVA. The variation trend of the torque of each guide vane in the Stage II is basically the same, but the pulsation intensity in Stage III is different. In the Stage II, the guide vane is the component with the largest change in hydraulic loss, rising by about 362.5 %. The high energy dispersion sites are filling the flow passages of the guide vanes and the static vanes. The position with high pressure pulsation amplitude in the vaneless region also shifted, and the offset angle was 2–3°. The composition and evolution of the vortex structure in the vaneless region and the flow passage are determined by the Q-criterion. These structures, including horseshoe vortex and spanwise vortex, are the main reasons for the increase of pressure pulsation amplitude.

Entropy Production Evaluation within a Prototype Pump-Turbine Operated in Pump Mode for a Wide Range of Flow Conditions

Inside the pump-turbine, energy is irreversibly lost due to turbulent pulsations in the high Reynolds number zone and actions of viscous forces close to the wall. The conventional differential pressure method cannot obtain specific details of the hydraulic loss within the machine’s flow passages; on the other hand, the entropy production method can provide accurate information on the location of irreversible losses and the spatial distribution of energy dissipation. Therefore, based on the entropy production theory, this study investigates the composition and distribution of hydraulic losses under different flow conditions for a prototype pump-turbine in pump mode. Study results indicated that total hydraulic losses significantly decreased, then slowly increased with an increase in flow rate. The entropy production rate caused by turbulence dissipation (EPTD), direct dissipation (EPDD), and wall shear stress (EPWS) displayed the same variation patterns as that of total hydraulic losses, with EPTD and EPDD being the most dominating. The location of hydraulic loss within the pump-turbine’s flow domain strongly depended on flow conditions. High hydraulic losses primarily occurred in the guide vanes (GV) and draft tube under low flow rates. Under high flow conditions, however, high hydraulic losses were mostly concentrated in the stay vanes (SV), spiral casing, and GV. Hydraulic losses at low flow rates were primarily caused by flow separation within the GV flow channels, vortices in the vaneless region, and inlet flow impacts on the runner blade’s leading edge. On the other hand, large vortices within the GV and SV flow channels, GV wake flow, and unsteady flow at the spiral casing were the main contributors to hydraulic loss under high flow conditions. EPDD was mainly caused by strain rate, so it was closer to the main vortex regions, whereas EPTD was affected by turbulence intensity and had a wider distribution range in the unsteady flow.

Influence of eccentric impeller on pressure pulsation of large-scale vaned-voluted centrifugal pump

Large-scale vaned-voluted centrifugal pump is a key component in water conveyance engineering. Hydraulic performance and pressure pulsation are important assessment indicators for operation efficiency and stability. During the common operation, the impeller eccentricity is easy to occur due to installation error, radial force and other reasons, which results the changes of hydraulic performance and pressure pulsation. In this study, the hydraulic performance and pressure pulsation in the vaneless region are analyzed by computational fluid dynamics method. The influence of eccentric impeller on the hydraulic performance in each components and the pressure pulsation is studied. Results show that the eccentric impeller causes the increasing of hydraulic dissipation especially in the volute. However, decreasing of the hydraulic dissipation happens in the draft tube. Impeller eccentricity significantly enhances the amplitude of rotation frequency, but has no obvious effect on the blade passing frequency and its harmonics. The peak-peak value of pressure pulsation of eccentric impeller increases rapidly with the eccentricity ratio. The results of this study provide scientific support for solving the stability problem caused by eccentric impeller in engineering cases.

Parametric design and multi-objective optimization on blade high-pressure side of a pump-turbine

High pressure side, as the runner outlet at pump mode and runner inlet at turbine mode, playing an important role in controlling hydraulic loss and flow characteristics in vaneless region, affecting both steady characteristics and unsteady characteristics. While when it comes to three-dimensional inverse design of a pump-turbine blade, scholars are forces on the design method in runner flow channel, ignoring the geometry profile of high-pressure side. Hence, in present paper, concepts “swept”, “bowed (lean)” and “twisted” are innovatively introduced, eight new parameters are proposed to control the profile of blade high pressure side. And then a multi-objective optimization design system consisting of geometry generation, computational fluid dynamics, design of experiment, approximation model, multi-objective genetic algorithm, and self-organization map is built. Efficiency at both pump mode and turbine mode and S margin at turbine mode are selected as optimization objectives in the first optimization step. Then, the hump margin is regard as the objective in the second optimization step. Based on the two-step optimization, a runner with optimized high-pressure side is obtained and CFD results show that compared with the original runner, the efficiency at rated points and the margin of unsteady characteristics can be increased. The parametric design method on high-pressure side presented in our paper can be regard as an essential supplement to the traditional design method in hydraulic machinery.

Investigation on hydraulic loss component and distribution in hydraulic machinery: A case study of pump-turbine in pump mode

The sudden increase of hydraulic loss in hydraulic machinery and the attendant unsteady flow characteristics like rotating stall and secondary flow seriously threaten the security and stability of operation. The entropy production method is widely used to estimate the hydraulic loss distribution in hydraulic machinery even if it is a thermodynamic concept expressing the irreversible energy transformation from kinetic energy into internal energy. In this study, the conception and the form of hydraulic loss is derived from the kinetic energy equation under incompressibility assumption according to the conception of hydraulic energy. Then, the conception of local hydraulic loss is proposed and defined as the combined action of dissipation effect and transportation effect. By contrast, the entropy production method can only reflect the dissipation effect while ignoring the transportation effect in depicting hydraulic loss. Finally, the hydraulic loss analysis in pump mode of a pump-turbine is conducted under SST k-ω model based on local hydraulic loss rate method. With the help of entropy wall function, the local hydraulic loss rate method shows good agreement with the total pressure difference and the error is within 5% apart from the hump region. Transportation effect plays an equal important role as dissipation effect in hydraulic loss and the hydraulic loss proportion in wall region is within 3.1%. Moreover, the local hydraulic loss rate method can better depict hydraulic loss distribution and hydraulic loss evolution process at draft tube outlet, runner inlet and the vaneless region between guide vanes and runner outlet in part load operating points compared to the entropy production method. The local hydraulic loss rate method proposed in this paper might become the most promising method in depicting the hydraulic loss in hydraulic machinery.

Optimization design and unsteady flow analysis of stall characteristics in a vertical centrifugal pump

Abstract In this paper, an optimization design of the vertical centrifugal pump with vaned diffuser was accomplished, and analyzed the internal flow pattern of the model before and after optimization under stall conditions to explore the relationship between design parameters and the stall characteristics. The reasons for the improvement of efficiency and hump characteristics curve after optimization were revealed combined with streamline distributions, entropy production theory and pressure fluctuation. The results show that the error between the external characteristic result calculated by Computational Fluid Dynamics (CFD) and the experimental value is less than 5%, which proves the reliability of the numerical simulation method adopted in this paper. The hydraulic efficiency under multiple operating conditions and stall characteristics of the optimized model have been significantly improved. After optimization, the entropy production rate in the vaned diffuser is greatly reduced, and the improvement of the internal flow pattern in the vaned diffuser is the main reason for slowing down the hump characteristic curve. The hydraulic performance and the stall characteristics of the vertical centrifugal pump can be significantly improved by properly increasing the radial distance of the vaneless region. The matching relationship between the vaned diffuser and the impeller is one of the important factors that affect the stall characteristics. The large-scale vortices in the vaned diffuser is the main reason for the low-frequency pressure fluctuation signal when operating in the hump region. The low-frequency signal in the vaned diffuser of the optimized model is obviously weakened.