Abstract To investigate the effect of different bionic grooves on the tip leakage vortex, four bionic grooves with different shapes were designed, which are semicircular, rectangular, triangular and trapezoid. The streamline, pressure, vortex strength and turbulence kinetic energy were analyzed to clarify the suppression mechanism. When the groove is arranged, the velocity of vortex center and the area of high-intensity vortex area decrease. The vortex strength in the triangular groove is higher than other groove shapes. The pressure in the center of the tip leakage vortex core increases. The low pressure area reduces obviously, which is the smallest in the trapezoidal groove condition. The turbulent kinetic energy in the main flow field increases, but decreases in the tip clearance. The high turbulent kinetic energy region is divided into small pieces by the bionic groove, which is the smallest in the rectangular groove condition. The bionic groove leads to the decrease in lift-to-drag ratio, which is the largest in the semicircular groove and smallest in the rectangular groove condition.

Abstract In order to solve the problems of difficult and inaccurate cavitation simulation in centrifugal pumps, this paper adopts the commercial software CFX and realizes the same-platform comparison of Zwart-Gerber-Belamri model, Kunz model and Schner-Saer model in the simulation of centrifugal pumps’ cavitation by using its own language. The research results show that the Schner-Saer model can predict the external characteristics of the pump more accurately without considering the coefficient correction; By adjusting the empirical coefficients, different cavitation models can achieve the same effect; The Kunz model can predict the pressure distribution in the impeller more accurately, and the error is basically kept within 10% except for the local complex flow area; The change of the pump head is more obviously affected by the volume of the vapour in impeller, but weakly affected by the morphology of the vapour.

In order to study the influence of inlet gas volume fraction on performance of a gas liquid centrifugal pump, the CFD commercial software ANSYS CFX 16.0 was used to solve the three-dimensional turbulent flow field in a gas-liquid two-phase flow centrifugal pump. The non-homogeneous two-fluid model was chosen for the purpose of obtaining phase distribution and its influence on the pressure and velocity fields, and the SST k-omega model was chosen as turbulence model. The result showed that the performance of the pump is stable under single-phase flow operating points. Under gas-liquid two phase flow, with the increase of inlet gas volume fraction, the pressure increment would be reduced, and the internal flow would be unstable. As the inlet gas volume fraction reaches a certain value, surge would occur under small flow rate conditions. At this time, a large amount of gas was concentrated in the impeller flow channel, and the channel of the impeller was blocked evenly, which lead to the pump's head dropped suddenly. Under design flow and large flow rate operating points, with the inlet gas volume fraction increase, the performance of the pump was relatively stable, and the head of the pump decreases slowly.

Unsteady cavitating flow is difficult to be predicted accurately with traditional RANS turbulence model, especially cavities shedding and evolution from sheet cavity to cloud cavity. In order to determine an appropriate numerical setup for accurate and reliable simulations of cavitating flow, the unsteady-state cavitating flow simulations are performed with the Zwart cavitation model accompanying with the DCM(Density correction model) model based on the RNG k-E turbulence model after grid convergence test. The details about pressure distribution and cavities evolution around the hydrofoil show that the used models predict the cavities shedding frequency well, the pressure distribution along chord is quasi-periodic. Due to the dynamic of the shed cavities, periodic lift and drag coefficient are pretty clear with both positive and negative values. The cavities formation, breakup, shedding and collapse in a cycle are also presented.

In industrial processing, a centrifugal pump is mostly used reversely as the micro hydraulic turbine for its lower cost. In some application situation, the working fluid will contain a part of gas which will reduce the efficiency, stability, and the power output of the machine. Due to the lower efficiency and stability of the pump as turbine (PAT) working with liquid flow, the working characteristic will deteriorate furtherly when the working fluid contains gas. Therefore, a micro-hydraulic turbine based on Francis turbine model is designed and proposed to except to obtain good performance under two-phase flow working condition. In this study, the three-dimensional transient two-phase flow model which adapts the k-omega model as turbulence model and Eulerian-Eulerian model as two-phase flow model with the commercial code ANSYS-CFX was established and solved to study the two-phase flow characteristics of the micro-hydraulic turbine with fixed guide vane openings under different gas volume fraction (GVF). The numerical simulation results show that the power, efficiency and torque of the turbine decrease with GVF. Due to the expansion of the gas in working fluid, the velocity of fluid increases with the gas, which leads to the bigger lower pressure area near the exit of the impeller. The vortex at the back of the blade is also intensified when the gas fraction increase. When the GVF is larger than 10%, the gas distribution is inhomogeneous, and most of them gather at the back side of the blade near the exit of the impeller where the GVF can reach 90%. The efficiency of the micro- hydraulic turbine is about 60% when the GVF is 20%, which needs to be improved in the future.

Tubular turbine running environment will be affected by ocean tidal wave, under the influence of the tidal wave, the hydrodynamic performance of the tubular turbine and its water-energy conversion efficiency have become the focus of our research and discussion. This paper explores the mechanism of wave-current coupling under surface fluctuations, considering the influence of tidal wave flow and gravity influence in flow field and flow characteristics of tubular turbine. The study on the performance of the influence of hydraulic characteristics of tubular turbine under the condition of wave flow has been carried out. Results show that the wave flow conditions with considering gravity leads to mechanical runner blade be damaged by tidal wave of pressure fluctuation and cyclic extrusion, causing more serious than fatigue stress damage than no wave flow condition does which make this kind of fluid machinery practical fatigue life is reduced greatly. The condition of the wave flow condition is universal, which is one of the reasons that the actual service life is always smaller than the theoretical calculation life. Considering the case of wave flow, the calculated efficiency is larger than that without considering this condition, and the difference between the two is the maximum at the optimal operating point. In the hydraulic turbine flow passage, significant pressure and velocity gradient are generated in the vertical direction due to wave action, resulting in no longer symmetrical distribution of its position. The hydraulic characteristics in the runner are obviously different from those without flow.

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

In the case of small load condition, the water flowing impact angle is very large that the secondary flow and the backflow are generated between the flow passage of runner, resulting in flow separation in runner and producing inter-blade vortex when cavitation is serious. In order to improve the stability of the Francis turbine in the small load conditions, this paper presents a controlling method of double-row guide vane cascade, and its numerical simulation is carried out. In this paper, it is found that there are obvious inter-blade vortices in the runner and the vibrations in the tube are violent under the small load conditions on the original model turbine. Secondly, the relationship between the opening method of double guide vane cascade and the flow separation condition in the runner is studied, the suitable opening combination is obtained and the difference in the performance of the original turbine under the small flow condition is analysed. The results show that: Through adjusting the opening combination of outer guide vanes and inner guide vanes, it can effectively control the outflow angle of the guide vane and the runner, thus the influences of the vortex in the draft tube and the inter-blade vortex can be obviously reduced. At the same time, double-row guide vanes can effectively reduce the pressure fluctuation amplitude of the draft tube. It has provided a certain theoretical basis for improving the stability of Francis hydraulic turbine.

Rotating stall is a common unstable flow phenomenon in centrifugal pumps, which usuallyoccurs in part-load conditions. Rotating stall will cause performance instability when thepump is running, and even excite the resonance of the whole pump system in someextreme cases. In this paper, the stall phenomena at part-load conditions have beeninvestigated by CFDsimulation for a centrifugal pump. The CFD results have beencompared with the experiments, including characteristic curves, internal flow structures andcorresponding frequency spectrum analyses. Furthermore, a parameter named theblockage coefficient for the pump impeller passage is put forward to quantitativelyevaluate the magnitude of stall.The large-scale vortex in the stalled channel is defined as astall cell.At the operation condition Q=0.35Q0, five rotating stall cells can be observedin the impeller, which propagate among different impeller channels with a rotatingfrequency lower than that of the impeller. From the frequency spectrum analysis, the mainfrequency of the stall cells is about 23.4% of the impeller rotational frequency, resulting in4.68% for each stall cells. For the operation condition of Q=0.38Q0, a “stationary stall”phenomenon has been observed in the impeller passages.

The paper deals with the influence of the tongue shape on the flow characteristics at the near-tongue region of the volute in the double suction centrifugal pump with long-tongue, middle-tongue and short-tongue respectively. For this study, the computational fluid domains including suction chamber, impeller and volute was simulated by means of the commercial CFD software that solved the Navier-Stokes equations for three-dimensional steady flow. The results show that the tongue shape has a significant impact on the pump head and efficiency in a range of 0.4Qd-1.4Qd flow rate. Under small flow conditions, the head and efficiency of the short-tongue model are higher than that of the middle-tongue model and long-tongue model. Under large flow conditions, the head and efficiency curves of the three models are relatively close. In addition, the tongue shape has a great influence on the pressure and the streamline distribution in the near-tongue region. Therefore, effectively changing the tongue shape can improve the internal flow pattern of the double suction centrifugal pump and reduce the pressure pulsations. The tongue shape also has a great influence on the turbulent kinetic energy distribution. By changing the tongue shape the numerical value of the turbulent kinetic energy can be effectively reduced, thus the generation of the turbulent vortex and the energy loss of the pump will be reduced. It is shown that the tongue shape has a great influence on the performance of the double suction centrifugal pump. This study has certain reference value to improve the pressure pulsation of the double suction centrifugal pump.

For the purpose of comparing the capability for improved Schnerr-Sauer model, unsteady and turbulent cavitating flows over the three-dimensional NACA66 hydrofoil with fixed angle of attack of α=6° were numerically investigated based on Schnerr-Sauer (S-S) model and improved Schnerr-Sauer model, respectively. The SST k-ω model with local density correction was applied for the turbulence modeling and the effect of turbulence fluctuation was considered as well. Hydrodynamic coefficients, time-evolution of cloud cavity and the surface average pressure based on different cavitation models were obtained from simulations. in the stage of unsteady cavitation, the drag coefficient obtained from the improved S-S model close better with the value measured in experiment. The time-evolution of cloud cavity predicted by improved S-S model is much stronger and has better agreement with the experimental results as compared with the solution of S-S model.

Hydraulic and structural performances are two main subjects in the optimization design of hydraulic machines. This article gives consideration to both hydraulic and structural performances during the optimization process of hydraulic turbine with main and splitter blades on the basis of Multidisciplinary Design Optimization (MDO). The aim is to improve the optimization methods of hydraulic turbine, shorten the development cycle time and ensure stable and efficient operation of the unit. Computational fluid dynamics(CFD) and finite element method(FEM) analysis are combined to improve the overall performance of the runner in the multidisciplinary optimization process. The optimization considers the whole runner including the crown and band to get more accurate results of stress distribution. Open Grip program was used to parameterize runner blade and Bezier curve was used to match the bone line and it changed the blades through transforming the bone lines of different profile. NSGA-II algorithm was adopted during the optimization calculation. What's more, super transfer approximation method was used to obtain the weighted coefficients considering the overall performance under three operating conditions, based on this, the final optimization result was obtained. The whole performance of optimized runner gets improved and has a better outflow quality. Also, it provides a feasible method for engineering application in the multidisciplinary optimization design of runner blades.

When operating at part load conditions, flow separations may occur in the impeller of an axial flow pump due to the increase of the incidence angle at the impeller leading edge, probably resulting in instability in head curve. In this paper, the hump characteristic in head curve of an axial flow pump has been examined by CFD method with the help of ANSYS CFX 16.0. The results show that the head of the axial flow pump in the hump area decreased by 40% at 61% of the design flow rate, caused by stalls in the pump impeller. Furthermore, the improvement of head curve has been conducted by applying axial grooves at the wall of the pump inlet section. It is found that under the condition of small flow rates, the axial groove can effectively reduce the inlet circulation and the attack angle at the leading edge of the impeller. As a result, the back flow on the suction side of the impeller has been reduced. Consequently, the hump phenomenon in the head curve of the axial flow pump has been effectively eliminated.

Pressure reducing valve is an extremely significant equipment of energy dissipation for the water supply by gravity with pressure reducing technology in hydropower stations, and which has a pronounced effect on the normal technical water supply even safety operation for the hydropower units. A three-dimensional numerical calculation of flow field and cavitation characteristics towards a combined type pressure reducing valves was carried out based on the system of technical water supply in this paper. The numerical results show that the investigated valve could meet the requirements of technological supply water pressure and great pressure loss was caused when the water flow was accelerated by narrow overflowing section between throttling cone and valve seat. At working operation, obvious cavitation phenomenon was observed on the surface of throttling cone, and the maximum volume fraction of vapor reached 0.537%. Based on above researches, this paper introduces an optimization model for profile line design of throttling cone. The optimal results show that the cavitation performance is effectively improved with identical pressure drop compared with original results.

The cavitation flow phenomena may occur in the bulb tubular turbine at some certain operation conditions, which even decrease the performance of units and causes insatiably noise and vibration when it goes worse. A steady cavitating flow numerical simulations study is carried out on the bulb tubular unit with the same blade pitch angle and different guide vane openings by using the commercial code ANSYS CFX in this paper. The phenomena of cavitation induction areas and development process are obtained and draws cavitation performance curves. The numerical results show that the travelling bubble cavity is the main types of cavitation development over a wide operating range of discharge and this type of cavitation begins to sensitive to the value of cavitation number when the discharge exceeding a certain valve, in this condition, it can lead to a severe free bubble formation with the gradually decrement of cavitation number. The reported cavitation performance curves results indicate that the flow blockage incident would happen because of a mount of free bubble formation in the flow passage when the cavity developed to certain extend, which caused head drop behavior and power broken dramatically and influenced the output power.

The pressure reducing valves are widely used in the technological water supplied ways of gravity flow. A credible pressure reducing valve can provide stable cooling water for units with extremely low maintenance cost and labor intensity in a fairly long period of time. In this paper, a three-dimensional numerical simulation of flow field and acoustic characteristics towards a combined type pressure reducing valve was carried out based on ANSYS Fluent and the FW-H equation. The numerical results achieve the regulation of noise generation, transmission and attenuation. It shows that the sound pressure level of monitoring points seem to be higher and large gradient at low frequencies under the same flow velocity, while it presents reverse results with the increment of frequency and maintains a constant valve finally. At the same time, the monitoring points in the vicinity of throttling cone shows higher sound pressure level and upstream noise is lower than downstream's. Aiming at the problem of valve noise, a modified measure to reduce the flow-induced noise was proposed.

Depending on the long-term hydraulic development of Francis turbine, the blade channel vortex phenomenon was investigated systematically from hydraulic design, experimental and numerical computation in this paper. The blade channel vortex difference between the high water head and low water head turbine was also analyzed. Meanwhile, the relationship between the blade channel vortex and the operating stability of hydraulic turbine was also investigated. The results show that the phenomenon of blade channel vortex is an intrinsic property for Francis turbine under small flow rate condition, the turning-point of the blade channel vortex inception curve appears at low unit speed region, and the variation trend of the blade channel vortex inception curve is closely related to the blade inlet edge profile. In addition to, the vortex of the high water head turbine can generally be excluded from the stable operation region, while which is more different for the one of the low water head turbine.

This paper presents numerical analysis of unsteady flow in a scroll hydraulic pump to discover its flow mechanism. The dynamic mesh model has to be used to simulate the flow field unsteadily. The unsteady flow patterns and pressure distributions in the suction, squeezing and discharge chamber are analysed. The suction process continues until the crank angle reaches the 320 degree. Then the pressure in the chamber rises instantaneously, and the fluid begins to flow out from the chamber. Because of the high pressure difference at the clearance, the jet flow and the vortex appear, and the large flow losses generates with them. In addition, the velocity and static pressure distribution in the two symmetry crescent suction chamber is different remarkably. One reason is that the location of suction port cannot be set symmetrically for the simplification of the pump structure. Another reason for that is the fluid is impelled by different part of the orbiting scroll. The asymmetric pressure distribution will result in the extra force on the scroll. The cavitation generates at the negative pressure region. Therefore, the unsteady simulation shows some important phenomena. The structure of the scroll pump need to be optimized to reduce the maximum pressure, weaken the jet flow, vortex and the uneven pressure distribution to ensure the pump working safely and efficiently.

The fluid lubricant force in the journal bearing is an important factor for the stability and dynamic characteristics of rotating machine. In order to obtain the dynamic coefficients of journal bearing, the equilibrium position must be known for further calculation. In this paper, the Reynolds equation is solved by finite difference method and the dichotomy is applied to acquire the equilibrium position of journal bearing by means of double loop. The effects of length, radius and clearance of journal bearing on the equilibrium position are also researched. The calculated results show that the dichotomy is an effective method for the equilibrium position of journal bearing and the geometry parameters play an important effect on the equilibrium position.

For a prototype turbine operating under part load conditions, the turbine output is fluctuating strongly, leading to the power station incapable of connecting to the grid. The field test of the prototype turbine shows that the main reason is the resonance between the draft tube vortex frequency and the generator natural vibration frequency. In order to reduce the fluctuation of power output, different measures including the air admission, water admission and adding flow deflectors in the draft tube are put forward. CFD method is adopted to simulate the three-dimensional unsteady flow in the Francis turbine, to calculate pressure fluctuations in draft tube under three schemes and to compare with the field test result of the prototype turbine. Calculation results show that all the three measures can reduce the pressure pulsation amplitude in the draft tube. The method of water supply and adding flow deflector both can effectively change the frequency and avoid resonance, thus solving the output fluctuation problem. However, the method of air admission could not change the pressure fluctuation frequency.