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Insight into blade tip vibration and deformation characteristics of boiler circulation pumps induced by tip leakage vortex under different tip clearances
Abstract
For circulating pumps in large power plant boilers, tip leakage flow is the main cause of blade fatigue. To investigate the correlation between tip leakage vortex and blade fatigue, in this paper, the bidirectional fluid structure coupling method is used to simulate the full flow field of the boiler circulating pump under different tip clearance sizes. The accuracy of the delayed detached vortex simulation method is verified by combining the external characteristics and vibration characteristics of the pump. It is obtained that tip leakage vortex is the main cause of blade tip vibration and deformation. Under deep stall conditions, the increase in tip clearance size suppresses the vibration displacement of the blade leading edge, while the opposite is true under optimal conditions. After decomposing tip leakage vortex, it is found that the compression–expansion term played a major role in the deformation of the blade tip, while the viscous dissipation term and the stretching term mainly affected the vibration frequency. At optimal working conditions, the main frequency of blade vibration is basically consistent with the main frequency of vortex generation. In deep stall condition, as the tip clearance size increases, the amplitude of the vibration main frequency decreases and the number of harmonic frequencies decreases, while the optimal condition is the opposite.
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The complex flow inside the axial-flow pump device will cause the problem of hydraulic noise; in order to explore the influence of the law of rotation speed on the internal flow characteristics and hydraulic noise of the axial-flow pump conduit, a combination of Computational Fluid Dynamics (CFD) and Computational Acoustics (CA) was used to numerically solve the flow field and internal sound field in the pump device. The results showed that the flow in the elbow inlet conduit was smooth at different rotation speeds, and there was no obvious unstable flow. The higher the rotation speed, the more disordered the flow pattern in the left half of the elbow, which intensifies the unstable flow in the straight outlet conduit. The impeller is the main sound source of the internal hydrodynamic noise of the vertical axial-flow pump device. When the sound source propagates upstream and downstream along the conduit, the Total Sound Source Intensity (TSSI) gradually decays with the increase of distance; the greater the rotation speed is, the faster the Total Sound Source Intensity (TSSI) decays. When the rotation speed was increased from 1450 r/min to 2200 r/min, the TSSI in the straight outlet conduit was attenuated by 8.9 dB, 13.9 dB, and 16.0 dB respectively, and the TSSI in the elbow inlet conduit was attenuated by 11.0 dB, 13.5 dB, and 25.9 dB respectively. The vortex structure in the conduit induces flow noise and delays the attenuation of TSSI in the propagation process; with the increase of rotation speed, this delay will be more obvious.
Pump dynamic operational conditions result in extreme transient events that enhance the response of piping networks. Generally, the predominant transients during rapid startup and shutdown are mainly studied for centrifugal pumps and are scarce for reciprocating pumps. Our study extends the conventional steady-state analysis to include the effect of reciprocating pump dynamic loading on pulsatile flow-induced acoustics and vibrations in a complex piping network. The forced response resulting from acoustical-structural coupling is assessed by utilising the one-dimensional multiphysics piping acoustic model and beam structural model. The network responses to pulsatile flows during dynamic pump loading (rapid start-up-shutdown events) are compared to the pulsatile flows during steady-state pump loading. With pump startup-shutdowns accompanied by pulsatile flows, the network response is the result of the combination of the transient and steady-state characteristics of plane acoustic waves and structural vibrations. The dynamic pump loading generates a fundamental, low-frequency acoustic eigenmode that causes transient loading of pipelines similar to reservoir-pipe-valve (RPV) systems.
This study investigated the influence of the change in blade tip clearance on the internal flow characteristics of a vertical axial flow pump. Taking the actual running vertical axial flow pump of a pumping station as the research object, based on the SST k-ω turbulent flow model, the numerical simulation technology was used to study the effects of different tip clearances on the pressure, turbulent kinetic energy, Z–X section pressure and flow state of the impeller at the middle section. Furthermore, the impact of clearance layer tip leakage was also analyzed. Unsteady calculations of flow characteristics under the design conditions were performed. The research results showed that the variation trend of the pressure in the impeller was basically the same under different tip clearance values. With the increase in the clearance value, the pressure gradient along the water inlet direction of the blade decreased and the leakage vorticity increased. Observing the leakage vorticity distribution of the gap layer under the flow condition of 0.6Q0, it was found that when the tip clearance was smaller than 1 mm, the leakage flow was small and easily assimilated by the mainstream, and the leakage flow and mainstream had a certain ability to compete, which caused adverse effects on the performance of the pump device. The pressure pulsation characteristics showed that the leakage flow caused by the tip clearance caused a high-frequency distribution, and the clearance obviously influenced the pressure pulsation characteristics.
Air filaments and cavities in plunging breaking waves, generically cylinders, produce bubbles through an interface instability. The effects of gravity, surface tension and surface curvature on cylinder breakup are explored. A generalized dispersion relation is obtained that spans the Rayleigh–Taylor and Plateau–Rayleigh instabilities as cylinder radius varies. The analysis provides insight into the role of surface tension in the formation of bubbles from filaments and cavities. Small filaments break up into bubbles through a Plateau–Rayleigh instability driven through the action of surface tension. Large air cavities produce bubbles through a Rayleigh–Taylor instability driven by gravity and moderated by surface tension, which has a stabilizing effect. Surface tension, interface curvature and gravity are all important for cases between these two extremes. Predicted unstable mode wavenumber and bubble size show good agreement with direct numerical simulations of plunging breaking waves and air cylinders.
To understand the formation mechanism and evolution process of the perpendicular cavitation vortex (PCV) of an axial flow pump for off-design conditions, turbulent cavitating flows were numerically investigated using the rotation curvature-corrected shear stress transport (SST-CC) turbulence model and the Zwart–Gerber–Belamri cavitation model. In this work, the origin and evolution of a PCV were analyzed through a high-speed photography experiment and numerical simulation. The results showed that the PCV came from a secondary tip leakage vortex (S-TLV) and was aggregated by the action of the re-entrant jet, combined with the cavitation bubbles driven by the radial flow to form the cavitation vortex (CV). With the joint action of leakage jet lifting and TLV entrainment, the PCV was reoriented and gradually became perpendicular to the chord direction. Then, the PCV and TLV collided, mixed, and entrained, which formed a strong pressure pulsation. The PCV was gradually divided into upper and lower parts. One part was combined with the residual part of the TLV and flowed to the next blade, and the other part flowed out of the impeller area along the axial direction. At the same time, the generation, evolution, and dissipation of the PCV formed high pulsation amplitudes and frequencies in the middle and rear above the blade suction.
Insufficient understanding of flow instability interaction between hump characteristic and the rotating stall is a major problem in the application of mixed flow pumps. In this paper, the unsteady flow characteristics of a mixed flow pump are studied numerically to understand the hump characteristic generating mechanism using a modified SST k-ω partially averaged Navier-Stokes (MSST PANS) model. The predicted pump characteristic curves show good agreement with experimental data. Fundamental results on time-averaged flow pattern and energy loss distributions depict that the substantial rise of energy loss in the pump impeller is the main reason for the hump characteristic. Detailed flow structures show the peak regions of circumferential vorticity (Ωθ) and boundary vortex flux (BVF) are located at the blade tip near the trailing edge (TE) and leading edge (LE). Further analysis on the flow angle and blade loading are carried out. The results indicate that the peak region located near the LE of spanwise = 0.8 and streamwise = 0.3 is the source of the rotating stall evolution. In this process, the substantial reduction in the blade loading occurs near the LE, eventually induces the head drop. Finally, pressure fluctuations analysis depicts that low-frequency signal is observed induced by the rotating stall evolution.
In this paper, the implicit assumptions in Leveque’s approximation are re-examined, and the dimensionless temperature distribution and the thermal boundary layer thickness were illustrated using the developed solution. By defining a similarity variable, the governing equations and boundary conditions are reduced to a typical dimensionless form in order to achieve an analytic solution in the entrance region. A relatively simple mathematical scheme was proposed by which the entrance-region temperature solution for laminar flow heat transfer with the similarity variable can be rigorously obtained The analytical solutions are then, checked against numerical solutions which that were programmed under FORTRAN code using fourth-order Runge–Kutta method (RK4). Finally, other important thermal results obtained from this analysis, such as; approximate Nusselt number for the thermal entrance region which was discussed in detail. Analytical results were compared with the published data available in the literature for limiting cases, and good agreement was noticed.
The AP1000 reactor coolant pump is a vertical shielded-mixed flow pump, is the most important coolant power supply and energy exchange equipment in nuclear reactor primary circuit system, whose steady-state and transient performance affect the safety of the whole nuclear island. Moreover, safety demonstration of reactor coolant pump is the most important step to judge whether it can be practiced, among which software simulation is the first step of theoretical verification. This paper mainly introduces the fluid-solid coupling simulation method applied to reactor coolant pump, studying the feasibility of simulation results based on workbench fluid-solid coupling technology. The study found that: for the unsteady calculations of the pure liquid media, the average head of the reactor coolant pump with bidirectional fluid-solid coupling decreases to a certain extent. And the coupling result is closer to the real experimental value. The large stress and deformation of rotor under different flow conditions are mainly distributed on impeller and idler, and the stress concentration mainly occurs at the junction of front cover plate and blade outlet. Among the factors that affect the dynamic stress change of rotor, the pressure load takes a dominant position.
The role of blade rotational angle in the energy performance and pressure fluctuation of a mixed-flow pump is investigated through an experimental measurement and numerical simulation. The mixed-flow pump head increases at a blade rotational angle of 4° and decreases at a blade rotational angle of −4° compared with a blade rotational angle of 0°. Meanwhile, the highest efficiency decreases by 0.3% at a blade rotational angle of 4° and increases by 0.8% at a blade rotational angle of −4°. The pressure fluctuation characteristics in the mixed-flow pump at different blade rotational angles are also revealed. The dominant frequencies of pressure fluctuations in the impeller are the axis rotation frequency or six times this frequency corresponding to six guide vanes. The dominant frequencies of pressure fluctuations at the middle plane of impeller and guide vane are the blade-passing frequencies or twice this frequency. The maximum amplitude of pressure fluctuation in the impeller at a blade rotational angle of −4° is greater than that of blade rotational angle 0° and 4° because of strong vortex intensity. The maximum amplitude of pressure fluctuation at the middle span of the impeller and vane occurs at a blade rotational angle of 4° because of the largest pressure gradient.
This numerical study is aimed at investigating the convective heat transfer and flow fluid inside a horizontal circular tube in the fully-developed laminar flow regime under the constant wall temperature boundary condition,, is commonly called the Graetz Problem that our goal is to get the steady temperature distribution in the fluid. The complexity of the partial differential equation that describes the temperature field with the associated linear or nonlinear boundary conditions is simplified by means of numerical methods using current computational tools. The simplified energy equation is solved numerically by the orthogonal collocation method followed by the finite difference method (Crank– Nicholson method).The calculations were effected through a FORTRAN computer program and the results show that orthogonal collocation method giving better results than Crank– Nicholson method.
Abstract ................................ ................................ ................................ ......................... iii Acknowledgements,................................ ................................ ................................ ........ iii Nomenclature................................ ................................ ................................ ................. iv 1 Introduction ................................ ................................ ................................ ................. 1 2 Verification and Validation Procedures ................................ ................................ ......... 2 3 Verification and Validation Methodology................................ ................................ ...... 3 3.1 Concepts and Definitions................................ ................................ ................ 3 3.2 Verification ................................ ................................ ................................ .... 4 3.2.1 Convergence Studies ................................ ................................ ....... 5 3.2.2 Iterative Convergence ................................ ................................ ...... 8 3.2.3 Monotonic Convergence: Generalized Richardson Extrapolation ..... 9 3.2.4 Oscillatory Convergence ................................ ................................ 12
An axial flow circulating pump (AFCP), as a key facility of the large vessel power system, is designed for conveying cooling water of the power system. The pump will be rotated actively under the drive of motor when the vessel is navigated at a low speed, which is called the pump operating condition (POC). Besides, it will be rotated passively under the drive of Inlet flow fluid energy in the case of the vessel at a high speed, which is called the unpowered driven condition (UDC). It is worth noting that the characteristics of internal flow, energy generation and dissipation laws of the AFCP are varied under UDC and POC. This paper analyzes the energy transfer laws of the AFCP under UDC and POC using the modified PANS model and energy conveying theory. Pressure propulsion power (accounting for 82.6%) is a primary factor affecting energy conveying, while Lamb vector divergence and enstrophy (accounting for 17.4%) have limited effects on energy conveying, according to the research findings. Moreover, more intense energy generation and dissipation can be found under UDC with a rising proportion between Lamb vector divergence and enstrophy.
To explore the flow characteristics around the tip clearance and the evolution of tip leakage vortex, this paper adopted the delayed detached eddy simulation model to calculate the blade tip region and employed the vorticity transport equation to analyze the forms and strength of tip leakage vortex. It is found that under the effect of tip leakage vortex, the unsteady flow at blade edges are serious under unpowered driven condition. Tip leakage vortex is generated from the leading edge of the blade. From the leading edge to the trailing edge of the blade, the intensity of tip leakage vortex gradually decreases and the generation frequency gradually increases. The core intensity of tip leakage vortex at L1 to L5 points decreases gradually, with a decreased range of about 24.8%, 34.4%, 55.03% and 60.48%.
Axial flow circulating pumps (AFCP) are one of large marine steam turbine unit for large-sized ships. One peculiar operation condition for AFCP is when a ship cruises beyond a certain speed, the energy of pump inflow is able to completely overcome the frictional resisting moment of the pump itself, thereby driving the impeller to rotate. Such a condition is also known as the unpowered driven condition (UDC). At this time, the fluid is in the artesian flow state.
In this paper, pressure fluctuation and inner flow of the AFCP under UDCs and different inflow conditions are analyzed using DDES turbulence model. It is found that the intensity of TLV decreases from the leading edge to the trailing edge of the blade, and the amplitude of pressure pulsation caused by TLV also decreases. Due to the jet wake structure at the blade trailing edge, the amplitude of pressure fluctuation at the trailing edge of the blade increases by 7.8% under the optimal UDC. In addition, the compression-expansion term determines the strength of TLV's core, thus affecting the amplitude of pressure fluctuation. The viscous dissipation effect of TLV can cause high frequency components of pressure fluctuation.
The aim of this work is to investigate the unsteady characteristics of tip leakage vortex (TLV) structure in an axial flow pump experimentally and numerically. Experiments were carried out to study the performance of an axial flow pump under cavitating cavitation. The numerical simulation was conducted by employing the Large Eddy Simulation turbulence and Zwart cavitation models, and the results show a good agreement with the experimental ones. The evolution mechanisms and influencing factors of the transient TLV morphology was revealed, and the correlation between pressure difference and leakage velocity were investigated in a systematic way. The TLV evolution appears periodic development, coinciding with the variation of leakage velocity driven by the pressure difference. The cylindrical coordinate system is more suitable for analyzing the TLV dynamics, due to the impeller rotation. The vorticity in the tip region is dominated by tangential vorticity ωθ and axial vorticity ωz. The transport terms of ωθ and ωz present a great relationship with TLV and TLV-induced cavitation. Relative vortex stretching dominants the vorticity generation in the tip region, and relative vortex dilation and Coriolis force are also important sources. In contrast, baroclinic torque has the least influence on the vorticity in the flow passage.
The stable operation of axial flow turbines (AFTs) in the energy conversion process directly affects their safety and performance. Because of its unstable pressure features, a dynamic analysis of its internal flow characteristics is required. To analyze, evaluate, and compare the pressure variation within the turbine under the base gap (Ƣ = 0.05D) and different axial gaps (Ƣ) of 0.09D, 0.14D, and 0.18D, a reliable measurement approach is required. Pressure sensors were installed on the flow domain of the AFT to measure dynamic pressure pulsation under various Ƣ and running flow conditions. Based on the experimental result, a 28% and 2.8% increase in turbine efficiency was recorded when the Ƣ was increased from 0.05D to 0.09D at 0.8Qd (design flow rate) and 1.0Qd, respectively.
Further analysis of the frequency spectra reveals different unsteadiness in the flow structures due to changes in Ƣ which resulted in various excitation signals. A decrease in the Ƣ leads to an increase in pressure pulsation intensity. Consequently, AFT with Ƣ of 0.09D was recommended since it provides maximum efficiency with fewer pulses compared with Ƣ = 0.05D, where the vibrations are at peak value. The findings above will aid in the optimum use of energy resources since energy generated from AFT is clean and free of pollution and can lead to a sustainable energy generation process.
An axial-flow pump (AFP)is a key hydraulic component in the circulating water system of large ships. When the speed of a large ship meets certain requirements, the AFP operates in the unpowered driven condition and has the characteristics of low speed, positive rotation and driven rotation, thereby achieving energy recovery efficiency without investing in the driving device. The unsteady internal flow characteristics of the AFP under the unpowered driven condition differ from those of conventional conditions. The blade tip clearance (TC) is an essential basis for the comprehensive technical indicators of the lift pump and the safe and stable operation of the circulating water system. In this study, the performance of the AFP with different TCs under the unpowered driven condition is investigated based on experimental tests and numerical simulations .Based on the entropy production theory, the energy loss characteristics of the AFP with different TCs are studied, and it is concluded that increasing the TC increases the strength of tip leakage flow .Particularly at large TCs, the energy loss of the pump increases significantly, causing flow separation on the blade surface and the formation of large-area vortex structures on the blade .In addition, under the large spacing TC, the instantaneous entropy production of the pump is unstable, and the entropy production frequency and amplitude are much larger than those of the conventional TC.
The effects of wavy leading edge on noise of an axial flow waterjet pump are investigated by using numerical simulation method. Based on bionics, five wavy leading edges with different combinations of wavenumber and amplitude are proposed. The physical mechanism is also revealed. The results show that the rotor with wavy leading edge does not change the dominant frequency of the noise, but reduces the amplitude. The optimal combination of wavenumber and amplitude in the study is 4/s (s is spanwise length) and 0.12C (C is the averaged chord length). The reason is that the wavy leading edge changes the pressure distribution of the blade, resulting in a radial pressure gradient. Meanwhile, vortex pairs around the leading edge of the blade occur. The vortex pairs cause the momentum exchange of the fluid in the boundary layer and outside the boundary layer, which makes the velocity distribution more uniform. In view of turbulent kinetic energy, the wavy leading edge can greatly reduce the turbulent kinetic energy in rotor under large mass flow rates. Therefore, the wavy leading edge may be applied to design the rotor of the axial flow waterjet pump to reduce its pressure pulsation and noise.
With the increasing adoption of renewable energy sources globally, hydropower contributes significantly to energy generation through various schemes ranging from big to small-scale plants. In small-scale hydropower plants, the preference for reverse-operated pumps (known as pump as turbines or PATs) over small-scale hydroturbines has increased. However, apart from the associated economic advantages, PATs, like any other hydraulic machinery, are not free from common problems such as cavitation. Cavitation is a phenomenon in which air bubbles are formed within the fluid medium due to substantial local pressure drop and their eventual collapse causes material erosion and degrades the overall machine efficiency. Several studies have focused on PAT conventional operating mode, while its reverse mode just begun to gain research interest. Nevertheless, cavitation remains a common problem in PATs at various hydro-sites. Therefore, to analyze PAT cavitation performance and highlight the differences between its two operating modes in terms of their development mechanisms, this article presents a thorough review of PAT cavitation dynamics and influencing parameters, as well as the future research directions. It is found that PAT reverse mode is more prone to cavitation, but more damages would occur in the conventional mode. Nevertheless, modifying the PAT geometric design parameters can considerably improve its cavitation performance. However, this approach has not been sufficiently investigated for PAT reverse operating mode and hence requires further research. Note that the terms “PAT conventional mode,” “PAT pumping mode,” and “pump” are equally used throughout this paper.
Owing to its ability to handle large flows, an axial flow pump as turbine (PAT) can generate considerable amounts of electricity in small-scale hydropower plants. However, a PAT's efficiency can be hindered by tip leakage flow (TLF), namely, flow through the clearance between the impeller blade tip and shroud. Accordingly, this study investigates the influences of TLF on the PAT's energy performance through numerical simulations in which the entropy production method has been adopted. TLF and the associated tip leakage vortex (TLV) are found to both decrease the hydraulic efficiency and increase the flow rate; the shaft power output is also affected, especially near the machine's best efficiency point. The effect of TLF on the pressure distribution along the blade depends on the flow conditions, and the form of the TLV directly generated by TLF is affected by the flow incidence angle. The vorticity transport equation reveals that the vortex stretching term plays a dominant role in the spatial evolution of the TLV and has the greatest impact on the pressure distribution. Finally, different operating conditions lead to different energy loss mechanisms: turbulent dissipation is the main cause of energy loss, and high flow conditions are marked by an increase in TLF-dependent wall shear stress dissipation.
Axial flow pump system is widely used in marine jet propulsion equipment and pump stations along the river coastal areas. In order to explore the safety and reliability of axial flow pump system, especially the axial-flow pump system in pump stations in coastal areas, in response to various extreme conditions. In this paper, a high-precision full-feature test bench for axial flow pump system is built, and the axial flow pump system model in a coastal area is manufactured. The axial flow pump system is experimentally tested under various extreme conditions. Then a numerical model for predicting the full characteristics of axial flow pump system is proposed, and the numerical results are compared with the experimental results. The full working condition hydrodynamic characteristics of axial flow pump system are revealed, including pump condition, turbine condition, braking condition and various extreme operating conditions. The research results provide reference for the safe and stable operation of axial flow pump system, especially coastal axial flow pump stations under various extreme conditions.
The requirements of Liquid Propellant Rocket Engine (LPRE) are high for thrust, specific impulse, and flow rate; thus, its components also have strict requirements. For the turbopumps (TPs), this means high flow rate, high rotational speed, and high pressure ratio, which makes their operations susceptible to the cavitation phenomenon, as observed in two previous works. In the first, cavitation regions were observed in the first stage of the Space Shuttle Main Engine (SSME) Liquid Oxygen (LOX) booster turbine, for 3.0%, 5.5%, and 8.0% tip clearances (relative to rotor blade height), using monophase flow [1]. In the second, the simulations were performed with multiphase flow, producing results more physically coherent for the 3.0% gap configuration [2]. The characteristics of both types of simulations in space propulsion applications still require better understanding. Therefore, to compare monophase and multiphase results at various operating points and turbine configurations, steady-state turbulent 3-D Computational Fluid Dynamics (CFD) simulations were performed, based on Reynolds-Averaged Navier-Stokes (RANS) formulation. The same three tip configurations for the turbine first stage were simulated, and the calculations were validated using experimental results from the National Aeronautics and Space Administration (NASA) [3]. This made it possible to verify the effect of the tip clearance on the machine performance and internal flowfield. When the gap increased, the pressure loading decreased in a large region of the blade tip, the interaction was greater between the Tip Clearance Vortex (TCV) and a vortex generated around the shroud cavitation region (Cavitation Vortex - CV), and this interaction moved towards the middle of the blade-to-blade passage. Thus, the losses increased and the efficiency decreased. Various comparative aspects between the simulations using both mono and multiphase numerical schemes are also discussed.
Water hydraulic axial piston pump (WHAPP) is widely used in underwater vehicles. The noise and vibration of WHAPP are important for the concealment of underwater vehicles. In some hydraulic system, the oil and water inevitably permeate each other, resulting in changes of working medium characteristics and the vibration and noise of the WHAPP. In this study, the effect of working medium on the noise and vibration of WHAPP was researched through experimental and theory simulation methods. The noise and vibration are measured and compared when the WHAPP use different fluid (water or oil) as working medium. The results show that noise and vibration of WHAPP using oil as working medium are larger than that of using water as working medium, when the pressure is greater than 4 MPa. The simulation results show that the flow and pressure pulsation of oil is larger than that of water, which contributes to larger noise and vibration of oil. The differences of flow and pressure pulsation are mainly caused by the bulk modulus of working medium. This work also expresses that the effect of working medium characteristics must be considered for reducing the noise and vibration of axial piston pump.
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.
In this paper, the cavitating flow and pressure fluctuation in the tip region were simulated based on Delayed Detached Eddy Simulation (DDES). The high-speed photography and transient pressure measurements were employed to capture the cavitation structures and pressure fluctuation. The numerical results show a reasonable agreement with the available experiments. The maximum errors of head and efficiency are 2.9% and 2.2%. The impeller rotation dominants the tip pressure field, with some obvious high-frequency components induced by the cavitating flow. Increasing the flow rate from 0.8Qopt to 1.2Qopt, the amplitudes of the frequency domain in P2 decrease from 0.13 to 0.08. There are more harmonic-frequency components at 0.8Qopt, including 5BPF, 6BPF, 7BPF. Suction-side-perpendicular cavitating vortices (SSPCVs) appeared in severe cavitation conditions, which leads to the collapse of the triangular cavitation cloud. The spatial-temporal evolution of SSPCV was divided into three stages: Generating stage, Shedding stage, and Dissipating stage. The pressure fluctuates significantly in the flow passage caused by SSPCV. The amplitudes of dominant frequency in P3 vary from 0.031 to 0.089, as the cavitation number decreased from 0.582 to 0.231. In particular, there is an obvious low-frequency N∗ = 0.5 shown in the frequency domains of P2 and P3.
In order to further enrich the theory of rotational stall, this paper explored the mechanism of internal energy loss in the mixed-flow pump under stall condition based on the shear stress transport (SST) k-ω turbulence model, identified the vortices in the impeller by the Q-criterion method, and characterized the turbulence intensity by the turbulent kinetic energy (TKE). The results show that the SST turbulence model can well predict the positive slope characteristics of the energy performance curve of a mixed-flow pump, and there are obvious characteristics of rotating stall in impeller. The numerical results of the energy performance curve and internal flow field are in good agreement with the experimental results. Under stall conditions, there exist swirls and vortices at impeller inlet, which makes the flow inception angle increase by 38°. Meanwhile, the tip leakage flow causes certain energy loss in the rim region, but the high loss area caused by the leakage vortex does not coincide with the core region of the leakage vortex. The interaction between stall vortex and the main flow in the flow passage results in the large-scale blockage area, which is the most fatal effect on the head drop of the mixed-flow pump. The backflow appears on the suction surface of the blade outlet, which has a great impact on the main flow and leads to a large hydraulic loss. The typical characteristics of critical stall and deep stall are found. Under the critical stall condition, part of the leakage flow overflows the leading edge of the lower blade and affects the flow of the next passage. While under deep stall condition, the streamlines of the leading edge of stall vortex reaches the inlet of the next stage blade and overflows, causing a lot of energy loss. And the interference effect between tip leakage flow and stall vortex is strengthened. In general, under the deep stall condition, the average turbulent kinetic energy value in the passages is the highest, and the uniformity of flow rate distribution in the four passages is the worst, resulting in the largest energy loss, which is at the lowest head in the saddle area.
When an axial flow pump operates at small flow rates, rotating stall may occur, which seriously restricts the pump performance and poses a great threat to the safe and stable operation of the whole system. The present work proposed a new groove flow control technology for the axial flow pump and studied its improvement effect and mechanism under stall conditions by numerical simulation. Results show that the groove flow control technology can significantly improve the hydraulic performance of the axial flow pump under stall conditions, with the pump head increment as high as 85.55% and the unstable region on the hydraulic performance curve was completely eliminated. In the inlet pipe region, it can effectively improve the bad flow state near the inlet of the impeller by increasing the axial velocity component and decreasing the velocity circulation dramatically. In the impeller region, the flow separation on the blade surface was improved and the cross-passage vortexes were suppressed, and the reasons were explained by analyzing the flow fields and pressure distributions in the representative grooves. In addition, compared with the diagonal groove (DG) scheme, the axial groove (AG) scheme showed better performance.
The influence of the floor-attached vortex on the pressure pulsation at the pump sump was investigated using the experimental analysis, and the theoretical analysis according to the theory of eddy dynamics. When the floor-attached vortex appeared under the large flow rate conditions, the time-domain characteristics curves for the pressure pulsation were significantly disordered and the pressure pulsation amplitudes at different points with floor-attached vortex varied significantly. The position of the vortex was consistent with the position of the low-pressure area at the floor of the pump sump and a large pressure gradient existed at the vortex appearing area. Through the theoretical and experimental analysis, it was found that the pressure pulsation induced by the floor-attached vortex fluctuates periodically with time, and the shape of the pressure pulsation curve is approximately the cosine functional curve.
When an axial-flow pump works in low flow rate conditions, rotating stall phenomena will probably occur, and the pump will enter hydraulic unsteady conditions. The rotating stall can lead to violent vibration, noise, turbulent flow, and a sharp drop in efficiency. This affects the safety and stability of the pump unit. To study the rotating stall flow characteristics of an axial-flow pump, the steady and unsteady internal flow field in a large vertical axial-flow pump was investigated using 3D computational fluid dynamic (CFD) technology. Numerical calculations were carried out using the Reynolds-averaged Navier–Stokes (RANS) solver and Menter's shear stress transport (SST) k-ω turbulence model. Steady flow characteristics including streamline, velocity vector, pressure and turbulent kinetic energy are presented and analyzed. Unsteady flow characteristics are described using post-processing signals for pressure monitoring points in the time and frequency domains. Using Q-criterion, the locations and evolution rules of the core region of the vortex structure in guide vanes under deep stall conditions were investigated. The reliability of the numerical simulation results was verified using the experimental prototype pressure fluctuation test. In this way, typical flow structure and pressure fluctuation characteristics in an axial-flow pump were analyzed, with contrastive analysis in design condition and stall conditions. Finally, the mechanism of low-frequency pressure fluctuation in a pump unit under the stall condition was revealed.
The flow characteristics in a volute-type centrifugal pump operating at design (Qd = 35 m³/h) and off-design (Qoff = 20 m³/h) conditions are investigated using large eddy simulation. Numerical results indicate that separation bubbles are generated on both the pressure and suction sides of impeller blades. At the off-design condition, the blade pressure side contains a larger recirculation zone with highly unsteady characteristics due to impeller-volute interactions. The vortices shed from a blade trailing edge due to its rotation strongly interact with those from the following blade and leakage through radial gaps at the off-design condition, generating stronger vortices in a wider region inside the volute, whereas this mutual interaction is weak at the design condition. Flow separation also occurs around the volute tongue at both operating conditions. At the off-design condition, a part of high-pressure fluid discharged from the volute does not follow the main stream to the outlet duct but re-enters into the volute area near the volute tongue. This pressurized fluid forms a high adverse pressure gradient on the blade pressure side, resulting in strong unsteady separation there. Also, a high pressure gradient in the axial direction at the radial gaps is formed especially near the volute tongue, creating the leakage into the cavities. Inside the volute, azimuthal vortices exist and grow along the volute passage. A secondary motion induced by these vortices also significantly affects the leakage to the cavities. All of these flow losses contain unsteady features that are strongly influenced by impeller-volute interactions, especially at the off-design condition.
Purpose
This paper aims to study the transient flow characteristics in a mixed-flow pump during the start-up period.
Design/methodology/approach
In this study, numerical calculation of the internal flow field in a mixed-flow pump using the sliding mesh method was carried out. The regulation of the pressure, streamline and the relative speed during the start-up period was analyzed.
Findings
The trend of the simulated head is consistent with the experimental results, and the calculated head is around 0.3 m higher than the experimental head when the rotation speed reached the stable stage, indicating that the numerical method for the start-up process simulation of the mixed-flow pump has a high accuracy. At the beginning, the velocity inside the impeller changes little along the radius direction and the flow rate increases slowly during the start-up process. As the rotation speed reached the stable stage, the flow inside the impeller became steady, the vortex reduced and transient effects disappeared gradually.
Originality/value
The study results have significant value for revealing the internal unsteady flow characteristics of the mixed-flow pump and providing the reference for the design optimization of the mixed-flow pump.
Purpose
To study the transient hydraulic impact and overall performance during startup accelerating process of mixed-flow pump experimentally and numerically.
Design/methodology/approach
In this study, the impeller rotor vibration characteristics during starting period under the action of fluid-structure interaction was investigated, which is based on the bidirectional synchronization cooperative solving method for the flow field and impeller structural response of the mixed flow pump. Experimental transient external characteristic and the transient dimensionless head results were compared with the numerical calculation results, to validate the accuracy of numerical calculation method. Besides, the deformation and dynamic stress distribution of the blade under the stable rotating speed and accelerating condition were studied based on the bidirectional fluid-structure interaction.
Findings
The results show that the combined action of complex hydrodynamic environment and impeller centrifugal force in startup accelerating process makes the deformation and dynamic stress of blade have the rising trend of reciprocating oscillation. At the end of acceleration, the stress and strain appears transient peak value and the transient effect is nonignorable. The starting acceleration has a great impact on the deformation and dynamic stress of blade, and the maximum deformation near the rim of impeller outlet edge increases 5% above the stable condition. The maximum stress value increases by about 68.7% more than the steady state condition at the impeller outlet edge near the hub. The quick change of rotating speed makes the vibration problem around the blade tip area more serious, and then it takes the excessive stress concentration and destruction at the blade root.
Originality/value
This study provides basis and reference for the safety operation of pumps during starting period
In order to study the rotor-stator interaction mechanism in impeller and guide vanes of mixed-flow pump under part loading condition, the flow field between the impeller outlet and the guide vane inlet under part loading condition was measured based on Particle Image Velocimetry (PIV). Besides, the relative velocity distribution along the monitoring lines in the impeller inlet and outlet sections, the relative velocity distribution and the vorticity distribution of interference field were analyzed under 0.2 times of the design flow condition at different phases. The results showed that at 0.2 times of the design flow rate, the internal flow between impeller and guide blades is affected by the rotor-stator interaction and as a result, the internal flow is disordered, conspicuous backflow and vortex flow occurred at different phases. The positive vortex structure and the reverse vortex structure both exist in the interaction zones, but the former exists near the wall while the latter exists near the hub. With the change of phase angle from impeller blade leading edge to impeller blade trailing edge, the positive vortex structure strength increases while the reverse vortex structure strength is reduced first and then increased. The relative velocity distributions at different phases in the monitor line near the guide vane inlet, due to the rectifying action of guide vane, are similar and steady. The closer the monitoring line to the impeller inlet, the more disordered the relative velocity distribution at different phase is, and the more conspicuous the rotor-stator interaction is. The maximum relative velocity difference can reach about 5m/s on the same place at different phases. All these phenomena indicate that the rotor-stator interaction is the major source of the unsteady flow field at the part loading condition. The research results provide significant reference value for revealing the internal flow characteristics under part loading condition as well as for optimization of mixed-flow pump.
The objective of this work is to simulate and analyze the formations of three-dimensional tip leakage vortex (TLV) cavitation cloud and the periodic collapse of TLV-induced suction-side-perpendicular cavitating vortice (SSPCV). Firstly, the improved SST k-. ω turbulence model and the homogeneous cavitation model were validated by comparing the simulation result with the experiment of unsteady cavitation shedding flow around the NACA66-mod hydrofoil, and then the unsteady TLV cloud cavitation and unstable SSPCV in an axial flow pump were predicted using the improved numerical method. The predicted three-dimensional cavitation structures of TLV and SSPCV as well as the collapsing features show a good qualitative agreement with the high speed photography results. Numerical results show that the TLV cavitation cloud in the axial flow pump mainly includes tip clearance cavitation, shear layer cavitation, and TLV cavitation. The unsteady TLV cavitation cloud occurs near the blade trailing edge (TE) where the shapes of sheet cavitation and TLV cavitation fluctuate. The inception of SSPCV is attributed to the tail of the shedding cavitation cloud originally attached on the suction side (SS) surface of blade, and the entrainment affect of the TLV and the influence of the tip leakage flow at the tailing edge contribute to the orientation and development of the SSPCV. The existence of SSPCV was evidently approved to be a universal phenomenon in axial flow pumps. At the part-load flow rate condition, the SSPCV may trigger cavitation instability and suppress the tip cavitation in the neighboring blade. The cavitation cloud on the SS surface of the neighboring blade grows massively, accompanying with a new SSPCV in the neighboring flow passage, and this SSPCV collapses in a relatively short time.
Purpose – The purpose of this paper is to present a new eddy-viscosity formulation designed to exhibit a correct response to streamline curvature and flow rotation. The formulation is implemented into a linear k- e turbulence model with a two-layer near-wall treatment in a commercial computational fluid dynamics (CFD) solver. Design/methodology/approach – A simple, robust formula is developed for the eddy-viscosity that is curvature/rotation sensitive and also satisfies realizability and invariance principles. The new model is tested on several two- and three-dimensional problems, including rotating channel flow, U-bend flow and internally cooled turbine airfoil conjugate heat transfer. Predictions are compared to those with popular eddy-viscosity models. Findings – Converged solutions to a variety of turbulent flow problems are obtained with no additional computational expense over existing two-equation models. In all cases, results with the new model are superior to two other popular k- e model variants, especially for regions in which rapid rotation or strong streamline curvature exists. Research limitations/implications – The approach adopted here for linear eddy-viscosity models may be extended in a straightforward manner to non-linear eddy-viscosity or explicit algebraic stress models. Practical implications – The new model is a simple “plug-in” formula that contains important physics not included in most linear eddy-viscosity models and is easy to implement in most flow solvers. Originality/value – The present model for curved and rotating flows is developed without the need for second derivatives of velocity in the formulation, which are known to present difficulties with unstructured meshes.
Research on DES and DDES numerical simulation based on unstructured mesh solver