The cavitating flow around the asymmetric leading edge (ALE) 15 hydrofoil is investigated through large eddy simulation with the modified Schnerr–Sauer cavitation model, which considers the effect of noncondensable gas. The statistical average velocity profiles obtained by simulation and experimentation show good agreement. The time evolution of cavity shape shows that cavity growth and separation start from the short side and spread toward the long side due to a side-entrant jet. The variation frequency of the cavity length of ALE15 hydrofoil at the long side is 163.93 Hz, and the cavitation shedding frequency at the short side is 306.67 Hz, which is about twice the value of the former. The filtered vorticity transport equation is employed to investigate the cavitation–vortex–turbulence interaction. Results indicate that vortex stretching is the major promoter of cavitation development, and vortex dilatation links vapor cavity and vortices. Baroclinic torque is noticeable at the liquid–vapor interface, and turbulent stress is related to cavitation inception. Moreover, a one-dimensional model for predicting pressure fluctuation is proposed, and results show that the model can effectively predict cavitation-induced pressure fluctuation on a hydrofoil, even on a three-dimensional ALE15 hydrofoil.

Cavitation–Vortex–Turbulence Interaction and One-Dimensional Model Prediction of Pressure for Hydrofoil ALE15 by Large Eddy Simulation

The physical mechanism of flow unsteadiness is one of the key problems in cavitating flow. Significant efforts have been exerted to explain the cavitation-vortex interaction mechanism. As well, the process of kinetic energy transport during the evolution of unsteady cavitating flow must be elucidated. In this work, 2D calculations of cavitating flow around a NACA66 hydrofoil were performed based on the open source software OpenFOAM. The modified shear stress transport k-ω turbulence model, which considers curvature and turbulent eddy viscosity corrections, was employed to close the governing equations. The Schnerr-Sauer cavitation model was adopted to capture the cavitation phase change process. Numerical results showed reasonable consistency with the results of the experiments conducted by Leroux et al. (2004). The results showed that cavitation promotes turbulence intensity and flow unsteadiness around the hydrofoil. During the attached sheet cavity growth stage, high-value regions of turbulent kinetic energy are located substantially at the interface of the cavity, particularly at the rear portion of the cavity region. During the cloud cavity shed-off stage, the cavity begins to break off and the maximum value of turbulent kinetic energy is observed inside the shed cavity. Finally, the influence of cavitation on the turbulence intensity is illustrated using the turbulent kinetic energy transport equation, which shows that the pressure diffusion and turbulent transport terms dominate as cavitation occurs. In addition, cavitation promotes turbulence production and increases dissipation with fluid viscosity and flow unsteadiness. The viscous transport term only acts in the cavitation shedding stage under large-scale vortex shedding. Overall, these findings are of considerable interest in engineering applications.

Calculation of cavitation evolution and associated turbulent kinetic energy transport around a NACA66 hydrofoil

Blade trailing edge position influencing pump as turbine (PAT) pressure field under part-load conditions

Cavitation is a challenging flow abnormality that leads to undesirable effects on the energy performance of the centrifugal pump and the reliable operation of the pump system. The onset and mechanism of a phenomenon that results in unsteady cavitation must be realised to ensure a reliable operation of pumps under the cavitation state. This study focuses on cavitation instability at normal flow rate, at which point the unsteady cavitation occurs as the available net positive suction head (NPSHa) falls below 5.61 m for the researched pump. An ameliorative algorithm–united algorithm for cavitation vibration analysis is proposed on the basis of short time Fourier transform (STFT) and Wigner–Ville distribution (WVD). The STFT–WVD method is then tested using vibration data measured from the centrifugal pump. The relationship between vibration and suction performance indicates that the inception and development of cavitation can be effectively detected by the distribution and intensity of the united algorithm at the testing points. Intermediate frequency components at approximately 6 kHz fluctuate initially with the development of cavitation. A time–frequency characteristic is found to be conducive to monitoring the cavitation performance of centrifugal pumps.

An experimental study on the cavitation vibration characteristics of a centrifugal pump at normal flow rate

This review mainly summarizes the latest developments in the internal flow field and external characteristics of centrifugal pumps. In particular, the latest findings of centrifugal pumps focused on turbulence and cavitation models, flow visualization methods, and fault detection based on noise and vibration. The external characteristics, cavitation, and vibration of the centrifugal pump were extensively discussed. In addition, advanced multi-objective optimization methods for improving impeller’s efficiency and reducing net positive suction head (NPSH) were briefed. Although some progress was made in this field, there remain many unsolved problems, such as monitoring and modeling of cavitation, rotational stall phenomenon, and discrepancies between simulation and measurement. In the future, researchers are encouraged to employ multi-dimensional flow visualization technologies and high-performance computing facilities to advance existing understandings on these issues and create new research directions.

Internal flow structure, fault detection, and performance optimization of centrifugal pumps

Concerning the development of cavitation in a centrifugal pump, numerical simulations and experimental investigations have been carried out in a closed hydraulic test rig. The internal flow characteristics and pressure pulsation at pump inlet and outlet have been analyzed during the process of cavitation development. The results of the research reveal the occurrence and development of cavitation in the centrifugal pump which has been confirmed through experiments and numerical simulation. The degree of pump cavitation could be monitored through pump inlet and outlet pressure pulsation. Compared with pump outlet pressure pulsation, pump inlet pressure pulsation is more sensitive to the change of cavitation. According to the results of the investigation, the typical frequency of pump inlet pressure pulsation could be regarded as around 30 Hz in the severe cavitation conditions. Meanwhile, the pump head dropped by 0.77% from noncavitation conditions which could be regarded as a symbol of incipient cavitation.

Numerical and experimental investigation on the development of cavitation in a centrifugal pump

Numerical simulation and experimental method are combined to investigate the pump inlet and outlet pressure fluctuations, the vibration characteristics and the internal flow instabilities under the unsteady cavitation condition in a centrifugal pump. It is found that the unsteady cavitation starts to generate as the NPSHa is lower than 5.93 m. Apparent asymmetric and uneven cavity volume distribution on each blade and in the impeller can be observed as the NPSHa decreases from 4.39 m to 1.44 m, which includes the cavitation develops from cavitation surge, rotating cavitation to asymmetric cavitation. The flow vortexes in each blade channel are produced in the cavity trailing edges by the shedding and collapse of cavitation, which interfere with each other and aggravate the flow instabilities. The dominant frequencies of the pump inlet and outlet pressure fluctuations are the shaft frequency and blade passing frequency under the unsteady cavitation conditions, respectively. Broadband pulses are obtained from both the pump inlet and outlet pressure pulsations, which results from the random shedding and collapse of unsteady cavitation bubbles. Obvious corresponding relationship between the root mean squares of the vibration measured in different positions and the suction performance curve is found under both the non-cavitation and unsteady cavitation conditions.

The characteristics investigation under the unsteady cavitation condition in a centrifugal pump

In the present work, a cavitation model based on a new truncated form of the full Rayleigh-Plesset (R-P) equation for the source terms controlling vapor generation and destruction has been developed and implemented in the ANSYS FLUENT platform. Coupled with a Filter-based density corrected model (FBDCM), the cavitating flow over a 2-D Clark-Y hydrofoil is investigated numerically with particular emphasis on understanding the effect of cavitation structures and the shedding dynamics on the hydrodynamics coefficients and surrounding flow velocity structures. The hydrofoil has a fixed angle of attack of α = 8° with a moderate Reynolds number of Re = 7.0×105. Simulations have been carried out for various cavitation numbers ranging from non-cavitating flows to the cloud cavitation regime (σ = 0.80). In particular, we compared the lift and drag coefficients, the cavitation dynamics and the time-averaged velocity with available experimental data for two cavitation models, i.e. the proposed model and Schnerr-Sauer model. The comparisons between the numerical and experimental results show that the proposed model has a better capability than Schnerr-Sauer model to capture the characteristics of lift and drag coefficients under cavitation conditions. Meanwhile, the proposed model is sufficiently robust to predict the initiation of the sheet/cloud cavity, growth towards the trailing edge, and subsequent shedding downstream, which is in accordance with the experimental quantitative features in literature.

Application of a new cavitation model for computations of unsteady turbulent cavitating flows around a hydrofoil

Axial-flow fan with advantages such as large air volume, high head pressure, and low noise is commonly used in the work of air-conditioner outdoor unit. In order to investigate the internal flow me...

https://journals.sagepub.com/doi/pdf/10.1177/1687814018811745

The influence of axial-flow fan trailing edge structure on internal flow:

Axial thrust in centrifugal pumps attracts extensive attention in order to improve the operating reliability of pumps. High axial thrust can cause rapid thrust bearing wear and subsequent pump failure or frequent overhauls. A centrifugal pump (XA65/20) was selected in this study, based on L16 (43) orthogonal array and CFD methods. The time-averaged Navier-Stokes equation was calculated for a 3D steady flow in the model pump in ANSYS CFX with the standard k-ω turbulence model and standard wall function applied. The structured meshes with different numbers were used for comparison in order to confirm that the computational results were not influenced by the mesh. Meanwhile, the effects of impeller back pump-out vane geometrical parameters, including its thickness Sk, its outlet diameter De and axial clearance δ, on the axial thrust and performances of the model centrifugal pump were analyzed. The different orthogonal schemes were obtained on the different values of Sk, De, and δ.Finally, when the parameters of the impeller Sk, De, and δ are 5mm, 100mm, 1.5mm, respectively. The Best Efficiency Point (BEF) of 69.9% was achieved with 60.12m for the designed head and −952.133N for the minimum total axial force. The corresponding impeller with minimum total axial force was considered as the optimal scheme and manufactured for experimental test. The external characteristics by CFD have a good agreement with their experimental data, which also better verified the accuracy of the numerical method of axial thrust applied in this research.© 2013 ASME

Banglun Zhou

Papers

Numerical and experimental investigation on the development of cavitation in a centrifugal pump

The characteristics investigation under the unsteady cavitation condition in a centrifugal pump

Application of a new cavitation model for computations of unsteady turbulent cavitating flows around a hydrofoil

The influence of axial-flow fan trailing edge structure on internal flow:

Numerical Optimal Design of Impeller Back Pump-Out Vanes on Axial Thrust in Centrifugal Pumps