Hydraulic Machinery Research Articles

The similarities and differences between the bubble collapse near a solid wall and in free water

In the actual working condition, the cavitation phenomenon is inevitably accompanied by the collapse of a large number of bubbles, and will affect the performance of hydraulic machinery. Therefore, it is of great significance to study the process of multi-bubble collapse. The double bubble system is the most simplified one in multi-bubble system, and it is also the basis of studying multi-bubble. However, the existing bubble dynamic studies mainly focused on single bubble and its interaction with the wall. This paper studies the interaction between two bubbles with different sizes and the influence of the wall on the process of bubble collapse by the VOF method. We have already investigated the interaction between two bubbles with different sizes without a wall (in free water) in our previous study. In this paper, we find that if there is a wall nearby, these two bubbles move close to each other when collapsing, and both move close to the wall at the same time. Through the study of bubble collapse time in the single bubble and the double bubble system, we find that the existence of the wall will delay the collapse time of bubble. However, the existence of the wall can also increase the intensity of collapse and rebound process of the bubble. Through the study of the center displacement of the bubble, we find that the double bubble system can increase the displacement of both bubbles. The method used in this paper can be easily extended to the study of multi-bubble, which provides a feasible method for the simulation of multi-bubble collapse near the wall.

Theoretical and numerical studies on the collapse of single‐ and double‐bubble system in water

AbstractThe cavitation bubbles that collapse in the hydraulic machinery can cause damage to the structure and the internal wall. In order to find feasible preventive measures, the mechanism of cavitation bubble collapse needs to be thoroughly studied. However, the influence of the initial radius or the interaction between multi‐bubbles has not been studied in depth. In this paper, the collapse process of single‐ and double‐bubble system is studied by the volume of fluid (VOF) method. Three conditions are considered in this paper, single‐bubble (R0 = 0.3 mm, R0 = 0.4 mm) and double‐bubble system (R0,1 = 0.3 mm and R0,2 = 0.4 mm). The time evolution of pressure, bubble radius, radial velocity, and acceleration during the bubble collapse can be obtained. The bubble collapse time, pressure distribution, and radial velocity of single bubble are verified by theoretical values. It is found that with the increase of bubble initial radius, the collapse, and rebound time is delayed, the maximum pressure of the bubble center increases, the minimum radius of the bubble decreases, the maximum radial velocity increases. In double‐bubble system, during the first rebound of the small bubble, the central pressure, radius and radial velocity of the small bubble fluctuate with time. And the collapse time of each bubble is delayed, the maximum central pressure decreases and the minimum radius of the bubble increases.

A methodology for detection of wear in hydraulic axial piston pumps

An effective asset management has a direct impact on maintenance costs, reliability, and equipment availability, especially in hydraulic machinery. Variable displacement axial piston pump is a major component used in the industry due to its load capacity ratio, pressure management, and high performance. Some of the main faults are wear and abrasion of the valve plates, increasing pressure losses as well as temperature, decreasing volumetric efficiency, and abnormal vibration. The off-line methodology implemented includes preprocessing of the vibration signals taken from the test bench available for this study, the feature extraction using wavelets, a stage of detection and classification through the use of artificial neural networks. Several networks were assessment, such as Adaline, nonlinear, and multilayer perceptron networks. Classification percentages greater than 90% are obtained taking into consideration 5 wear conditions related to the loss of volumetric efficiency.

A novel Omega-driven dynamic PANS model

A novel Omega (Ω) -driven dynamic partially-averaged Navier-Stokes (PANS) model is proposed in this paper. The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity ratio (the ratio of the rigid vorticity to the total vorticity), the key parameter of the Ω vortex identification method. Three classical flow cases with rotation and curvature are used to test the model. The results show that the turbulent viscosity is effectively adjusted by the new dynamic fk and the LES-like mode is activated, which can help the revelation of more turbulence information and improve the prediction accuracy. The new PANS model does not contain any explicit dependency on the grid size and enjoys good adaptability to the flow fields, and can be used for efficient engineering computations of the turbulent flows in the hydraulic machinery.

RETRACTED ARTICLE: Investigation of the key factors influencing cavity collapse using molecular dynamics simulation

Cavitation greatly affects the safe and stable operation of hydraulic machineries and thus has been key challenge in this field. Cavity collapse caused by cavitation can form transient characteristics such as high temperature, high pressure, strong shock wave, and high-speed micro-jet, which may cause serious damage to nearby solids. Cavitation can be affected by many intercoupled factors during the collapse transition. However, existing research rarely focuses on the analysis of its influencing factors. This research aims to reveal the collapse mechanism of cavitation, the parameters of each factor, and the influencing rules of the cavity collapse process. Molecular dynamics method was employed to analyze the evolution process of cavity collapse in the computational domain of liquid argon under different conditions. The influences of relaxation time, system temperature, and cavity size on cavity collapse were investigated. These simulation results showed that when the relaxation time was within 1 ns, the longer the relaxation time, the more likely the system molecules formed a stable state, and the more easily the cavities collapsed. When the relaxation time was greater than 0.2 ns, the difference in the process of collapse was smaller. When the temperature of the system was within 80 K, as the temperature increased, the thermal motion of the molecules intensified to increase the kinetic energy and weaken the intermolecular gravitation, shortening the time for cavities to reach the collapse point. When the temperature reached 80 K, as the temperature continued to increase, the time of collapse point and the process basically unchanged. When the initial cavity size was within 15 A, the larger the initial cavity size, the less likely the cavities tended to collapse. When the cavity size was larger than 15 A, the cavities were further less likely to collapse as the cavity size increased. The research results can provide theoretical support for the study of cavitation characteristics.

Computational investigation of the cavitation vortex dynamics in flow over a three-dimensional hydrofoil by a new transport-based model

Cavitation is of significant practical interest due to its unsteady features which could induce destructive effects such as drastic drop in efficiency, noise, vibration, and corrosion for propulsion systems, rudders and other hydraulic machinery. A thorough understanding of the hydrodynamics in the cavitating flow past a three-dimensional hydrofoil makes indicators for an improved control performance of these hydraulic systems. Hence, a computational investigation of the cavitating flow over the Delft Twist-11 hydrofoil was performed with special emphasis on the cavitation vortex dynamics. A new transport-based cavitation model was proposed and compared with the conventional Schnerr–Sauer model through the predictions of cavitation characteristics, for which available experimental data were also provided. The results show that, as compared with the Schnerr–Sauer model, the proposed model can predict closer engineering parameters, including time-mean lift coefficient and vapor shedding frequency with the experiments. In addition, more reasonable structure and dynamics of the three-dimensional unsteady sheet/cloud cavitation patterns, including the cavity growth, break-off, and collapse downstream are captured by the proposed model. With the help of the vorticity transport equation in a variable density flow, further analysis of the flow field predicted by the proposed model reveals that cavitation promotes the production of vortex as well as the flow instabilities. Vorticity production in the cavitating flow is mainly induced by the terms of vortex stretching and vortex dilatation, while the baroclinic torque only contributes in the region of shedding and collapse of the cloud cavity and the contribution of the viscous diffusion term is negligible as compared with the other three terms. The main significance of this study is that it demonstrates the potential of a robust transport-based cavitation model to investigate the unsteady dynamics in the cavitating flow past a three-dimensional twisted hydrofoil and expected to make sense for other hydraulic machinery.

Numerical analysis of unstable turbulent flows in a centrifugal pump impeller considering the curvature and rotation effect

In this study, a modified partially averaged Navier-Stokes model (MSST PANS) is proposed by treating a modified shear stress transport (SST) k-ω model as the parent turbulence model. The unstable turbulent flow in a centrifugal pump that considers the curvature and rotation effect is investigated as the test case to evaluate the performance of the MSST PANS model and analyze the flow instability in a centrifugal pump. The SST k-ω and the standard k-e PANS models are also evaluated for comparison. Results show that the MSST PANS model exhibits excellent performance and delivers the most satisfactory prediction results of the positive slope of the characteristic curve, time-averaged internal flows, and velocity profiles. The energy loss based on the energy balance equations is adopted to provide an explanation of the internal flow evolutions in pumps. The findings also indicate energy loss distribution is associated with the positive slope phenomenon. The high-velocity gradient flows at the entrance of the blade-to-blade passage and the reverse flows at the impeller exit are the main reasons for the high turbulent kinetic energy in the impeller. The MSST PANS model demonstrates promising applications in the field of hydraulic machinery, where unstable turbulent flows are prevalent.

The Effect of Flow Variations to Vibration Tendency in A Hydraulic Manifold

A hydraulic manifold is an important component in hydraulic machinery to convey hydraulic oil under high pressure into hydraulic tube and hose for cleaning purpose. Due to high pressure during operation, the process induces vibration and leakage can occur at the exit ports of hydraulic manifold. The aim of this study is to determine the effects of pressure and velocity variations in hydraulic manifold with respect to vibration tendency. In this study, computational fluid dynamics (CFD) is used to simulate the hydraulic manifold fluid behaviours, under various operating conditions. The result shows that fluctuations in pressure and velocity occurs at each branch in a mainstream due to changes of the area and geometrical shape. We also observed surge of pressure but reduction in velocity at each branch. Overall, the results show the most affected areas are those near the inlet. Areas further downstream are not significantly changed by the increments.

Application of the MST turbulence model to predict the tip leakage vortex flows

PurposeTip leakage vortex flow (TLV) is a common flow phenomenon in the axial-flow hydraulic machinery. High-efficiency simulation of TLV is still not an easy task because of the complex turbulent vortex-cavitation interactions. As an important basis of CFD, turbulence model directly affects the efficient computation of TLV. The purpose of this paper is to evaluate the newly developed MST turbulence model in predicting the TLV flows.Design/methodology/approachBy using the MST turbulence model and the ZGB cavitation model, numerical simulations of the TLV generated by a NACA0009 hydrofoil were performed under the cavitation-free and cavitation conditions, and the results were compared with the available experimental data.FindingsThe important features of TLV are well captured by the MST-based simulation scheme, and the problem of under-predicting the cavitating TLV tube is well solved. Turbulent viscosity is reasonably adjusted in the TLV core regions, and the LES-like mode is activated, which is beneficial to obtain more turbulent information on the same URANS grids. The requirements of grid size and time step of the MST model are much lower than that of the LES method, thereby weighing a good balance between the simulation accuracy and computation cost.Originality/valueThe MST turbulence model is suitable for the high-efficiency simulation of the TLV flows, which can lay a good foundation for efficient engineering computations of the cavitating TLV in the axial-flow hydraulic machinery.

Suppressing tip-leakage vortex cavitation by overhanging grooves

In the present paper, a new control method for tip-leakage vortex (TLV) cavitation is proposed. This method, which combines the effects of grooves and winglets, is made of overhanging grooves (OHGs) fitted at the tip of a hydrofoil. Experimental and numerical investigations are conducted to evaluate the performances of OHGs in terms of TLV cavitation suppression. The results are systematically compared with the baseline, conventional grooves (CGs) and anti-cavitation lip (ACL) and a significant improvement of TLV cavitation suppression is obtained with the OHGs. We also carried out a primary optimization design of the OHGs and a more effective suppression on TLV cavitation is obtained for small gaps and TLV cavitation is almost suppressed for middle and large gaps. The underlying reasons for TLV cavitation mitigation are discussed in detail with the help of numerical simulations. It indicates that for small gap sizes, OHGs can effectively weaken the strength of both TLV and tip-separation vortex (TSV) and mitigate TLV cavitation. For middle and large gap sizes, it is found that OHGs will induce an increase in the TLV core size, which further increases local minimum pressure. The influence of OHGs on the performance of hydrofoil is also examined, which indicates that the fluctuation of TLV cavitation can be effectively suppressed by OHGs and no significant alteration of time-averaged drag and lift is induced by OHGs. Our work shows that the OHGs can effectively suppress TLV cavitation with limited influence on the performances of hydrofoil in a large range of the gap sizes, which is a promising method for the control of TLV cavitation in hydraulic machinery.

Superior Slurry Erosion Behavior of a Casting NiCoCrFeNb0.45 Eutectic High Entropy Alloy

The flow passage components in hydraulic machinery will be eroded rapidly when running in sand water. The wear resistance of materials commonly used in hydraulic machinery needs to be improved to prevent the failure of hydraulic machinery. New wear-resistant materials are urgently needed to improve the service life of hydraulic machinery. Eutectic high entropy alloy (EHEA) is a new kind of alloys with in situ lamellar structure. NiCoCrFeNb0.45 EHEA has combined excellent strength and toughness. In this study, slurry erosion tests of NiCoCrFeNb0.45 EHEA were carried out with different impact angles and impact velocities using a rotary jet erosion test apparatus. The NiCoCrFeNb0.45 EHEA exhibited significant slurry erosion resistance compared with medium carbon steel 1045 and stainless steel 04Cr13Ni5Mo. The erosion rate of EHEA is much slower. Compared with medium carbon steel and stainless steel, the erosion of EHEA is completed via the accumulation and removal of platelets after plowing and work hardening, accompanied by partial micro-cutting.

Replacement and Maintenance Decision Analysis for Hydraulic Machinery Facilities at Reservoirs under Imperfect Maintenance

After the long-term operation of reservoir facilities, they will become nonoperational due to the material deterioration and the performance degradation. One of crucial decisions is to determine the maintenance or replacement of the facilities in a cost-effective manner. Conventional replacement models seldom consider the maintenance effect. The facilities after maintenance are generally not as good as new, but are relatively restored. The target of this study is to establish a replacement decision model of the reservoir facilities under imperfect maintenance. By combining the theories of reliability analysis, imperfect maintenance, and engineering economics, the best timing of replacement that achieves cost-effectiveness is analyzed and proposed. Lastly, based on the design of experiments (DOE) and simulation, the regression curve chart for the economical replacement decision is established. Once the failure rate, the age of recovery after maintenance, and the ratio of maintenance cost to replacement cost are estimated based on historical data, the cost-effective replacement time of hydraulic machinery facilities will be efficiently determined.

Control of unsteady partial cavitation and cloud cavitation in marine engineering and hydraulic systems

Cavitation is a process of liquid evaporation, bubble or vapor sheet formation, and further collapse of vapor structures, which plays a destructive role in many industrial applications. In marine transport and hydraulic machinery, cavitation usually occurs nearby the surface of a ship propeller and rudder, impeller blades in a pump, and distributor vanes and runner blades in a hydroturbine and causes various undesirable effects such as vibrations of frameworks and/or moving parts, material erosion, and noise enhancement. Based on an extensive literature review, this research is aimed at an experimental investigation of a passive approach to control cavitation on a benchmark hydrofoil using a wedge-type vortex generator in different flow regimes with a high Reynolds number. In this study, we employed a high-speed imaging method to explore the spatial patterns and time evolutions of cavitation structures and utilized a hydroacoustic pressure transducer to record and analyze local pressure pulsations due to the collapse of the cavities in the hydrofoil wake region. The results show that the examined control technique is quite effective and capable of hindering the formation of cloud cavities and reducing the amplitude of pressure pulsations associated with unsteady cavitation dynamics. This study provides important experimental information, which can be useful for improving industrial technologies and for promoting new developments in this particular research field.

Analysis of the Influence of Camber on Hydrodynamic Characteristics of Airfoil Based on FLUENT

Based on hydraulic machinery blades airfoil design, numerical simulation was carried out for the hydrodynamics force of airfoil with different camber at Re = 107. Firstly, the numerical simulation of airfoil was performed with structured grid and the Reynolds-averaged Navier-stokes-method, which coupled with the k – ε turbulence model. Secondly, the drag and lift characteristics of airfoil with different camber were analysed. Finally, the flow field and pressure distribution of the airfoil was discussed. The result shows that as camber increases, drag, lift and lift-to-drag ratio increase, while the attack angle corresponding to maximum lift-to-drag ratio decreases. The results are useful for understanding the hydrodynamic performance of the airfoil, and provide an important theoretical basis for the design, selection and development of the airfoil in hydraulic machine.

Development and application of the entropy production diagnostic model to the cavitation flow of a pump-turbine in pump mode

A novel mathematical model, entropy production diagnostic model (EPDM) with phase transition (EPDMS), was developed, which is practical to analyze the irreversible loss for cavitation flow in hydraulic machinery by including mass transfer and slip velocity. To model the corresponding flow, two-fluid model with main interphase forces was employed. Compared with the EPDM in homogeneous model, two extra contributions, diffusion and interface entropy production (DifE, IntE), are added in EPDMS. Then, EPDMS was validated by experiments of a pump-turbine in pump mode at three off-design conditions. The improved numerical model is more accurate in capturing the typical cavitation points, while the original one is obviously lower due to poor consideration of phase interaction. Therefore, the loss predicted by EPDMS is more rational. A detailed comparison among four overlapping items of EPDM and EPDMS was performed. It shows that these items merely interpret the accumulative effect of cavitation on the flow, while as a key part of EPDMS, IntE is closely related to the evolution of cavitation. Moreover, low-frequency excitation that caused by cavitation-induced vortex is captured by the probe at the edge of the cavity, which is consistent with the frequency-domain characteristics of IntE instead of other items.

Dynamic measurements on unsteady pressure pulsations and flow distributions in a nuclear reactor coolant pump

The operational stability of the nuclear reactor coolant pump (RCP) will directly affect the safety of the nuclear power plants (NPPs). A comprehensive analysis of the dynamic characteristics of the internal flow structure in the RCP is extremely important to assess its operational stability. Because the unsteady pressure characteristics of the spherical casing wall of the RCP and the dynamic flow field inside the RCP are complicated, it is highly demanding to be accurately measured by a reliable approach. Dynamic pressure pulsation and Laser Doppler Velocimetry (LDV) measurements have been carried out on the spherical casing wall and internal flow of the RCP under different operating conditions. Based on the pressure pulsation measurement, it is convinced that different unsteadiness flow structures directly affect the pressure pulsation characteristics, which will generate various excitation signals. The intercoupling between rotor-stator interaction (RSI) and the collision of the fluid discharged from the diffuser with the circulating flow to the casing bottom is the main reason for leading to strong pressure pulsations. Dynamic LDV measurement result shows that there is a significant counterclockwise periodic vortex shedding from the diffuser blade trailing edge, especially under high flow rate (1.2QN), which is also the main reason of strong velocity pulsations. The core of this paper is to comprehensively elaborate the dynamic flow characteristics in such a complex hydraulic machinery through combining multiple dynamic measurement methods. Moreover, an exhaustive understanding of dynamic flow characteristics is the basis for optimizing the RCP. The experimental results can provide ideas and basis for the optimization of the RCP with stable operation characteristics.

Cavitating Flow Suppression in the Draft Tube of a Cryogenic Turbine Expander through Runner Optimization

The application of a cryogenic liquefied natural gas expander can reduce the production of flash steam and improve the efficiency of natural gas liquefaction. Like traditional hydraulic machinery, cavitation will occur during the operation of a liquefied natural gas expander, in particular, there is a strong vortex flow in the draft tube, and the cavitation phenomenon is serious. In this paper, the energy loss coefficient of the draft tube is used to describe the cavitation flow in the draft tube, and the goal of reducing the cavitation in the draft tube is achieved through the optimization design of the runner. Different runner models within the range of design parameters were obtained using the Latin hypercube test, and the relationship between design parameters and objective functions is constructed by a second-order response surface model. Finally, the optimized runners were obtained using a genetic algorithm. The effects of blade loading distribution and blade lean angles on the cavitation in the draft tube were studied. According to the optimization results, the blade loading distribution and blade lean angles are recommended in the end.

Effect of sediment concentration and particle size on erosion behavior of forged stainless steel

Erosion due to slurry is a main impediment in a number of industrial applications like hydraulic machinery components, slurry pipe lines, valves, thermal power plants, etc. Forged stainless steel alloy 13Cr-4Ni-(Mo) is extensively used in many hydraulic machinery components owing to its good corrosion and wear resistance. In the light of above current study is an investigation of the effect of sediment concentration and particle size on the slurry erosion of forged 13Cr-4Ni-(Mo). Experiments were performed in an erosive wear tester using sand and water slurry to evaluate the erosive wear. Wear rate was calculated using mass loss measurement. The tests were carried out at four levels of sediment concentration (10–25 wt%) and at varied particle size (125–425 µm). It is found that the increase in the sediment concentration of the slurry results in increase of wear loss of material due to increased probability of sediment particles attacking the surface. Increased particle size increases the area of contact between sand particles and target material and resulted in higher material loss. SEM and optical confocal studies were carried out on worn surfaces and it also shows that increased sediment concentration and higher sized particles caused more degradation on target surfaces.

Efficiency Analysis of Pump-controlled Motor System of Hydraulic Machinery Continuously Variable Transmission for Agricultural Machinery

In order to improve the efficiency of Hydraulic Machinery Continuously Variable Transmission (HMCVT), studying the efficiency of its pump-controlled motor system is of great significance to promote the practical application of HMCVT in agricultural machinery. By analyzing the HMCVT transmission principle and the driving parameters of medium and large vehicles, determine the HMCVT transmission scheme that meets the actual requirements. The simulation model of the whole HMCVT system was established by SimulationX software, and the relationship curves between different engine speeds and HMCVT output speeds were given. And determine the effect of the pump-controlled motor system on the response parameters of HMCVT under different volumetric efficiency.

Cavitation control using Cylindrical Cavitating-bubble Generators (CCGs): Experiments on a benchmark CAV2003 hydrofoil

Cavitation often causes a destructive impact on the performance of hydraulic machinery, such as erosive wear, noise and vibrations of the framework and moving parts of marine propellers, pumps, hydraulic turbines and other equipments, which eventually leads to a degradation of overall system effectiveness. The paper reports on an experimental investigation of a passive method of flow control for different cavitation conditions: starting from the cavitation inception, including quasi-steady partial cavitation with shedding of small-scale vortical structures and finishing by unsteady cloud cavitation. The passive flow control was implemented using miniature vortex generators of a cylindrical type referred to as Cylindrical Cavitating-bubble Generators (CCGs) that were placed on the surface of a benchmark CAV2003 hydrofoil. First, we performed high-speed visualization of cavitation on the suction side of the original hydrofoil (without the control element) to find the cavitation inception point near the leading edge and to analyze the spatial structure and time evolution of partial cavities. In order to improve our understanding of the mechanism of cavitating flow unsteadiness and the effect of CCGs on the cavitation dynamics, we also applied a PIV technique to measure the mean flow velocity profiles and a hydroacoustic pressure transducer to record local pressure pulsations in the hydrofoil wake. As a result, this allowed us to determine the influence of CCGs on turbulent structure of the flow at different cavitation regimes and amplitude-frequency spectra of the pressure pulsations associated with attached cavity length oscillations for unsteady flow conditions. It was revealed that, in the case of unsteady cloud cavitation, CCGs were capable to mitigate large-scale cloud cavities. In addition, a substantial decrease in the amplitude of pressure pulsations was registered for the modified hydrofoil (with the control element). In general, CCGs appeared to be quite effective to hinder the cavitation development and to reduce the strength of side- and middle-entrant jets as the primary mechanisms of unsteady cloud cavitation.