Cavitation Conditions Research Articles

Experimental investigation on the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff hydrofoils

We experimentally investigated the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff National Advisory Committee for Aeronautics 0015 hydrofoils in a low-pressure cavitation tunnel. We analyzed the cavitating dynamics by capturing the cavitation dynamics using two high-speed cameras at different cavitating regimes on the surface of the hydrofoils, made of polyvinyl chloride, brass, and aluminum. We then measured the associated structural deformations in specific cavitation regime such as cloud and partial cavitation dynamics, using a digital image correlation technique. The hydrofoil's angle of attack was set to 10°, and the flow's Reynolds number was adjusted to 0.6 × 106. Results showed that the cavity's shedding frequency on the flexible hydrofoil shifted faster to a higher frequency than on the stiff hydrofoils under similar cavitating conditions. The flexible hydrofoil underwent strong structural oscillations at the low cavitation number for the cloud cavitation regime. The associated amplitudes of the vibration were about 20 times higher than those of the hydrofoil made of brass. It was observed that the fluid–structure interaction can significantly affect the cavitation-induced vibration of the flexible hydrofoil.

Experimental investigation into passive control effect of micro vortex generator at various cavitation conditions

The objective of this study is to discuss a passive control method for different stages of cavitating flows. The passive flow control is implemented using micro delta-shaped vortex generator (mVG) inserted spanwise on the suction side of the NACA66 hydrofoil. A high-speed camera is applied to visualize the cavitating flow structures. The results show that for the inception cavitation, the mVG with different positions can cause different modes of inception cavity. The typical three-dimensional spot cavity, the fingerlike vortex cavity, and the patch cavity are respectively captured at the leading edge of baseline hydrofoil, mVG-1 hydrofoil, and mVG-2 hydrofoil. For the sheet attached cavity, a new cavitation mode is found on the mVG-1 hydrofoil which is the fingerlike vortex cavitation followed by the attached sheet cavity due to the vortex cavitation merging behavior. Furthermore, for the unsteady cloud cavitation, the two mechanisms of cavity quasi-periodic shedding caused by the re-entrant jet and shockwave are analyzed to reveal that the mVG-1 can enhance the instability while the mVG-2 can weaken it.

Numerical simulation of cryogenic cavitating flow in LRE oxygen turbopump inducer

Turbopump is a critical component of modern liquid rocket engine (LRE), which employs cryogen as propellant. Its working performance is closely relevant to the reliability of LRE. The performance improvement of turbopump is limited by cryogenic cavitation condition. While the cryogenic cavitating flow characteristic is not well understood till now. A novel numerical method for cryogenic cavitation simulation was developed based on a transport-based cavitation model in the present investigation. The real thermodynamic properties of fluids and an additional energy source item which takes the heat transfer during phase change into account were imbedded into the code, and the empirical constants of the cavitation model are unnecessary to alter. It is found that a moderate reduction of released heat by vapor condensation in the energy source item provides better prediction accuracy for temperature distributions, and an optimal reduction proportion has been suggested. The cavitating flow features inside a three-bladed inducer using liquid oxygen as working fluid have been investigated in detail based on the proposed method, thermal effect reduces both the cavitation regions and the vapor volume fraction inside cavities remarkably, producing less blocking effect on the blade flow channels, which delays the head breakdown. Higher temperature shows stronger thermal effect and better cavitation performance. A classical semiempirical theoretical model is introduced to quantitatively predict the influence of thermal effect on cavitation performance. The predicted results are in consistent with the calculated results. The developed numerical model shows favorable universality to act as a reliable tool for cryogenic turbopump design.

Research on flow resistance characteristics of reactor coolant pump under large break LOCA at inlet

In the event of a large break loss-of-coolant accident (LBLOCA) at the inlet of the nuclear Reactor Coolant Pump (RCP), the RCP is likely to enter an extreme working condition of the reverse pump mode. In this mode, the pump actually acts as a stopper to the coolant out of the circulation system. When the flow rate remains constant, the head reflects the flow resistance capacity. In previous studies, we have found that the head of the RCP increases and then decreases with increasing cavitation under the reverse pump mode. Objective to research into this situation, numerical simulation of the RCP under the reverse pump mode has been used to analyze the flow resistance characteristics inside the impeller. Results show that the high-velocity flow generated by the cavitation region is the crucial factor causing the variation in pump head in comparison with the vapor volume fractions of the impeller blade surface. To investigate the effect of high-velocity flow on the pump head under different cavitation conditions and to even better reveal the flow resistance characteristics, entropy production analysis and vorticity analysis are carried out for the internal flow field of the impeller.

Research on Cavitation Flow Dynamics and Entropy Generation Analysis in an Axial Flow Pump

The entropy generation theory is introduced to investigate the effects of different NPSH and tip clearance size on the cavitation flow dynamics and mechanical energy dissipation intuitively and quantitatively within an axial flow pump through numerical simulations. The results indicate that main mechanical energy dissipation of the pump gathers in part impeller and diffuser, and most are turbulent dissipation. Meanwhile, the impeller is the largest place of mechanical energy dissipation of the pump under cavitation conditions, accounting for more than 50%. NPSH has significant effects on the cavitation pattern, which reflects on the field that the areas of attached sheet cavitation and tip leakage vortex cavitation around blades increase obviously with NPSH reducing under the tip clearance of 0.1% span. With NPSH decreasing, high regions of turbulent dissipation in the impeller mainly expands along blades and move downstream, with span S0.98 near the shroud having larger turbulent dissipation. Besides, high regions of turbulent dissipation are mainly distributed at the rear part of the cavity for every corresponding span of the impeller, which indicates that the turbulent dissipation has a strong relation with the cavitation pattern. In the impeller, the unstable flows cause cavity shedding at the rear of the cavity and wake flows near the blade trail induce higher turbulent kinetic energy, finally resulting in higher turbulent dissipation there. Under the same NPSH, areas of tip leakage vortex cavitation and areas of tip clearance cavitation around the tip both expand with the tip clearance increasing from 0.1% span to 0.8% span. And high areas of turbulent dissipation also are distributed at the rear of the cavity and moving downstream along the blade suction side, especially at span S0.98. Therefore, the tip clearance width mainly affects the cavitation development and turbulent dissipation distribution near the impeller’s shroud under same NPSH.

Numerical investigation of Cavitation in Francis Turbine

Cavitation is one of the major challenges in the Himalayan rivers of Nepal that cause damage and degradation of the performance of hydraulic turbine components. Numerical analysis has been performed to investigate the cavitation effect in the Francis turbine. For detection of cavitation, the SST turbulence model using Rayleigh plesset equation has been used in ANSYS-CFX. The turbine is investigated for three different operating conditions i.e. part load, BEP and full load under cavitation and without cavitation. The results are shown based on efficiency, pressure fluctuation, vortex rope and vapor volume fraction. It has been observed that changes in efficiency and pressure distribution are found to be minimal between cavitation and without cavitation conditions at rated discharge and head. Under part load condition, turbine efficiency loss and vapor bubbles formation at the leading edge of the runner blade is found to be maximum.

Numerical study on the effect of guide vane opening and draft tube outlet pressure on cavitation in Kaplan turbine

variations in electricity demand and site conditions like mass flow rate and net head, force the turbines to operate under off-design conditions. This increases cyclic loading in stationary and rotating components and severe cavitation in low head turbines like Kaplan turbine. The present analysis aims to examine the influence of various operating conditions on hydraulic efficiency and cavitation characteristics using ANSYS CFX. The stages of cavitation development in the runner are extensively studied for different operating conditions. The key point of present work is the critical analysis of various flow parameters for cavitating and non-cavitating conditions. The value of critical cavitation factor is found to increase from 0.34 to 0.61 with an increase in guide vane opening from 35º to 60º. The blade loading and flow parameters are found to be affected due to changes in guide vane opening and cavitation. The computed results are validated with the experimental model testing results and found to bear close comparison.

Performance prediction of cavitating marine current turbine by BEMT based on CFD

In the present study, the hydrodynamic performance of cavitating and non-cavitating marine current turbines (MCTs) has been investigated by blade element momentum theory (BEMT) coupled with Reynolds-averaged Navier–Stokes (RANS) technique. Before employing the BEMT, two-dimensional (2-D) lift and drag forces of cavitating and non-cavitating sections of MCT have been computed by RANS. The lift and drag forces, including cavity shapes, depend on Reynolds number (Re), cavitation number, and angle of attack. The hydrodynamic characteristics of cavitating 2-D hydrofoils (sections) were calculated via a multiphase solver “interPhaseChangeFoam” from the open-source code OpenFOAM. Cavitating flow simulations around a hydrofoil were carried out with Shear Stress Transport (SST) k−ω turbulence and Schnerr–Sauer cavitation model. The grid convergence index (GCI) was applied to confirm the numerical accuracy of the simulations. Validation of the proposed BEMT model has been done with two different model MCTs under non-cavitating conditions. Then, cavitating flow over a hemispherical head-form axisymmetric body, the NACA 66, and the Clark-Y were selected to evaluate the accuracy of the numerical methods and turbulence models available in the solver. Then, cavitating National Renewable Energy Laboratory (NREL) S814 2-D hydrofoil was later simulated for various Reynolds numbers, cavitation numbers, and angles of attacks. By using cavitating NREL S814 2-D hydrofoil data, the cavitation phenomenon was included in the method of BEMT. The distribution of cavitation on the blade has been modeled with BEMT coupled with RANS. Power coefficients at a wide range of tip speed ratios, including cavitation distribution on turbine blades obtained from BEMT, have been compared with each other and experiments. A reliable agreement has been found for all cases studied here. It is shown that cavitation has a negative effect on the power output of MCT for all tip speed ratios and cases discussed in this study. In addition, under cavitating conditions, the tip speed ratio of the operation point that corresponds to the highest power coefficient shifts to the right side of the power curve.

Flow in fuel nozzles under cavitation and flash-boiling conditions

Cavitation plays a crucial role in various fuel systems and spray applications. Due to the limitation in experimentally measuring the cavitation flow in fuel nozzles, numerical simulations can be used as an alternative in exploring the underlying physics. Most of the previous simulations of cavitation flow in nozzles were carried out under isothermal conditions, which become invalid when the thermal effect is strong. In this study, we use OpenFOAM and a modified cavitation model to investigate the influence of thermal effect on fuel nozzle flow characteristics under cavitation and flash boiling conditions. The comparison with experimental data shows that the modified cavitation model can predict well the cavitation flow in nozzles. Vaporization caused by cavitation at the nozzle throat and vaporization caused by the flash boiling near the outlet are simulated under different conditions. Phenomena such as mass flow reduction, outlet velocity blockage, and the transition from cavitation flow to flash boiling flow are well predicted. Further results show that with the increase in the inlet fuel temperature, the cavitation at the throat and flash boiling at the outlet occur gradually and then mix under high-temperature conditions. With the decrease in the ambient pressure, the flow in the nozzle gradually transits from single-phase flow to cavitation flow and then to flash-boiling flow. Increasing the injection pressure can inhibit the generation and the growth of superheated vapor near the nozzle outlet.

Experimental cavitation investigation of the electromagnetic PbBi pump with rotating permanent magnets

Electromagnetic induction pumps with rotating permanent magnets (EMP) are used mostly in experimental setups, where operation safety and reliability are crucial. Such pumps can face flow parameter decrease when operating at a low inlet pressure compared with those gained at higher inlet pressure values. The pump's developed flow rate increase does not correspond to the magnetic rotor rotation rate increase at the lower inlet pressure caused by the cavitation process at the pump’s inlet tract. The study focuses on critical cavitation numbers at several inlet tract sections under severe cavitation conditions which prevent the flow rate increase. The current study is an experimental approach to characterize the pump operation parameters using cavitation numbers and comparing them with the literature data. Figs 7, Refs 13.

Cavitation recognition of axial piston pumps in noisy environment based on Grad‐CAM visualization technique

AbstractThe cavitation in axial piston pumps threatens the reliability and safety of the overall hydraulic system. Vibration signal can reflect the cavitation conditions in axial piston pumps and it has been combined with machine learning to detect the pump cavitation. However, the vibration signal usually contains noise in real working conditions, which raises concerns about accurate recognition of cavitation in noisy environment. This paper presents an intelligent method to recognise the cavitation in axial piston pumps in noisy environment. First, we train a convolutional neural network (CNN) using the spectrogram images transformed from raw vibration data under different cavitation conditions. Second, we employ the technique of gradient‐weighted class activation mapping (Grad‐CAM) to visualise class‐discriminative regions in the spectrogram image. Finally, we propose a novel image processing method based on Grad‐CAM heatmap to automatically remove entrained noise and enhance class features in the spectrogram image. The experimental results show that the proposed method greatly improves the diagnostic performance of the CNN model in noisy environments. The classification accuracy of cavitation conditions increases from 0.50 to 0.89 and from 0.80 to 0.92 at signal‐to‐noise ratios of 4 and 6 dB, respectively.

Investigations of Cavitation Erosion and Corrosion Behavior of Flame-Sprayed NiCrBSi/WC-12Co Composite Coatings.

Flame-sprayed NiCrBSi/WC-12Co composite coatings were deposited in different ratios on the surface of stainless steel. Oxyacetylene flame remelting treatment was applied to surfaces for refinement of the morphology of the layers and improvement of the coating/substrate adhesion. The performance of the coated specimens to cavitation erosion and electrochemical corrosion was evaluated by an ultrasonic vibratory method and, respectively, by polarization measurements. The microstructure was investigated by means of scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX). The obtained results demonstrated that the addition of 15 wt.% WC-12Co to the self-fluxing alloy improves the resistance to cavitation erosion (the terminal erosion rate (Vs) decreased with 15% related to that of the NiCrBSi coating) without influencing the good corrosion resistance in NaCl solution. However, a further increase in WC-Co content led to a deterioration of these coating properties (the Vs has doubled related to that of the NiCrBSi coating). Moreover, the corrosion behavior of the latter composite coating was negatively influenced, a fact confirmed by increased values for the corrosion current density (icorr). Based on the achieved experimental results, one may summarize that NiCrBSi/WC-Co composite coatings are able to increase the life cycle of expensive, high-performance components exposed to severe cavitation conditions.

ANN modelling of Hydrodynamic Cavitation for the degradation of Rhodamine B dye

Hydrodynamic cavitation is one of the promising advanced oxidation processes for wastewater treatment. The stochastic nature of cavity implosions, cavity dynamics in the reactor and limited understanding of thermodynamic properties of reactants at extreme temperature and pressure makes its modelling difficult. Artificial neural network being a purely data-driven approach can be a possible solution to model hydrodynamic cavitation. In this study, a 2D-Model is first developed, followed by a 4D-Model for the degradation of a representative pollutant - Rhodamine B (Rh B) dye. The first model builds a correlation of pH and number of passes with degradation of Rh B (2D-Model) and is used to optimize hyperparameters for training the models. The second model correlates four parameters: pH, initial dye concentration, input pressure and the number of passes with degradation of dye (4D-Model).It was observed that both these models (2D-Model and 4D-Model) have shown a good match with experimental results/test data with low mean square error (0.0008 and 0.0002) and high regression coefficient values (0.9935 and 0.998). 2D-Model simulated correct trends in both interpolation and extrapolation range, while 4D-Model simulated accurate interpolation trends but only limited extrapolation because of the availability of limited data during model development. Augmentation methods have proven effective for training these models, especially 4D-Model, with a dataset having sparse data for multiple input parameters. These models are expected to help in reducing the number of experiments to decide the optimum operating conditions for hydrodynamic cavitation and other applications which require exhaustive and expensive experimentation.

Numerical analysis of interaction between turbulent structures and transient sheet/cloud cavitation

This paper through the in-house code numerically examines the cavitation–vortex–turbulence interaction mechanism. The high grid resolution can obtain a more detailed flow field structure, which is helpful to reveal the relationship between cavitation occurrence and development and local turbulent flow field. Results are presented for a three-dimensional NACA66 hydrofoil fixed at an 8° angle of attack under a moderate Reynolds number of 1 × 106 and sheet/cloud cavitating conditions. Numerical simulations are performed via the boundary data immersion method coupled with the artificial compressibility method through a Fortran-based code. The results show that the numerical predictions are capable of capturing the unsteady cavitation characteristics, in accordance with the quantitative features observed in high-speed cavitation tunnel experiments. The evolution of the transient cavitating flow can be divided into three stages: growth of the attached sheet cavity, development of a re-entrant jet, and cloud shedding downstream. The Liutex method is applied to capture the vortex structure. Further analysis of the process of enstrophy transport reveals that cavitation promotes vortex production and increases the enstrophy as the cavity becomes more unstable. Moreover, the structure of the vortex gradually evolves from a vortex tube to a U-type vortex, Ω-type vortex, and streamwise vortex. Finally, the interaction between cavitation and turbulence is expounded using the turbulent energy transport equation, which demonstrates that cavitation promotes the production, diffusion, and dissipation of turbulent kinetic energy, while the viscous transport term only acts during the process of cloud cavity shedding.

Effects of dissolved gas on surface pressure of a hydrofoil under cavitation condition

Abstract Dissolved gas in water is known as one of the factors which affect cavitation. The present study focuses on the effect of dissolved gas on cavitation around a hydrofoil, more specifically on the hysteresis of cavitation and surface pressure of the hydrofoil. The pressure measurement on a hydrofoil of Clark Y-11.7% was carried out under various dissolved gas conditions. Pressure in the test section was reduced stepwise from an atmospheric pressure to that in a super-cavitation condition, then was increased stepwise to the atmospheric pressure condition. Amount of dissolved oxygen (DO) was used as a parameter representing the degree of air content, and three DO conditions were set at low DO: under 30%, middle DO: around 50%, and high DO: over 70% of saturation under atmospheric pressure. Also, two angles of attack, 8 and 20 degrees were selected. Although there is no significant difference of the surface pressure and its hysteresis at the angle of attack of 8 degrees, dissolved gas seems to affect the behavior of cavitation. At the angle of attack of 20 degrees, it was found that the appearance of cavity was changed by DO conditions, and it would affect surface pressure of the foil. Under high DO condition, when the cavity oscillated with large amplitude, dissolved gas seemed to enhance the growth of the cavity, and the cavity tended to grow as covering the suction surface of the foil; it would make the time-averaged surface pressure of high DO condition lower than that of the other DO condition.

Research on Noise-Induced Characteristics of Unsteady Cavitation of a Jet Pump.

The dynamic cavitation characteristics of normal-temperature water flowing through a transparent jet pump under different cavitation conditions were experimentally studied by adjusting the pressure ratio. The common results are presented at different pressure ratios, including the temporal and spatial changes of the pressure and noise, together with the visual observation of the cavitation unsteady behaviors using a high-speed camera. The analyses on the measured data and images reveal that the cavitation cloud is generated by periodic oscillations of the jet traveling pressure wave and the bubble traveling pressure wave. The oscillation of the two kinds of interface waves is caused by the collapse of the bubbles, which is the main mechanism of the bubble cloud shedding. As the pressure ratio increases, the maximum length of the jet cloud and bubble cloud linearly decreases, while their oscillation frequency increases gradually. Combined with the cavitation-cloud visualization data and noise frequency analysis, it is proposed that the strong impact between the jet traveling pressure wave and the bubble traveling pressure wave is the main cause of noise. Specially, the acoustic pressure reaches the maximum when the oscillation frequency of the jet traveling pressure wave is the same as that of the bubble traveling pressure wave. Also, the jet traveling pressure wave has a great influence on the migration of bubbles in the cavity. The results can provide guidance for the optimal operating condition in cavitation applications such as jet aerator and quantitative addition.

Numerical analysis of the cavitating flow in an axial flow waterjet pump with special emphasis on the tip leakage flow and tip leakage vortex

Abstract Due to the structural design requirements, tip leakage flow is a common phenomenon in the field of axial flow rotating machinery. The tip leakage flow interacts with the mainstream and induce complex tip leakage vortex. When the local pressure drops to the saturation pressure, cavitation may occur in flow passage and the vortex core. The tip leakage cavitating flow has a huge effect on the flow stability, which may reduce the efficiency of the waterjet pump. In the present study, Zwart cavitation model and SST k-ω turbulence model are employed to simulate the cavitating flow. The new proposed Liutex criterion has been used to capture the vortex structures in the flow passage and the tip gap. The vorticity transport equation in cylindrical coordinates is also used to discuss the variation tendency of the vorticity in the vicinity of the gap region under cavitation condition. The evolution of the cavitation is proved to be vital important for the formation and the breakdown of the tip leakage vortex. By analyzing the vorticity transport equation, the stretching term is shown to be the decisive factor.

Cavitating flow investigation in low specific speed axial flow waterjet pumps

Abstract In this paper, the cavitating flow in low specific speed axial flow waterjet pumps are treated at different cavitation conditions. In order to analyze the cavitating turbulent flow comprehensively, three axial flow waterjet pumps with the specific speed less than 650 min-1·m3s-1·m are chosen to compare the internal flow phenomenon. The investigations are conducted by numerical simulation, where RANS method is applied with k-ω SST turbulence model and homogeneous cavitation model. The results show that there are several types of cavitation such as tip leakage vortex cavitation, sheet cavitation attached on blade suction surface, and shedding cavity in the pumps. The vortex structure in the low specific speed waterjet pump contains tip leakage vortex, shedding vortex downstream of the sheet cavitation and perpendicular cavitation vortex. It is noted that the cavity firstly occurs at the leading edge near blade tip, and the sheet cavity covering the blade suction surface extends from leading edge to the middle part, while the tip leakage vortex cavitation develops with the decreasing cavitation number. Further analysis using relative vortex transport equation depicts that the vortex stretching term is the main contributor for the vorticity production in the pump, and the vortex dilation and baroclinic torque occur at the interface between cavity and water.

Comparison of cavitation erosion behavior of 45 steel and 0Cr18Ni9Ti stainless steel

Abstract A cavitation erosion test of 0Cr18Ni9Ti stainless steel and 45 steel sample was performed in an ultrasonic vibration apparatus. The cavitation erosion behavior was studied by means of metallographic microscope, Vickers hardness test, scanning electron microscopy and mass loss analysis. The results showed that the surface hardness had an obvious influence on cavitation erosion, the higher the hardness, the better the cavitation erosion resistance was. Plastic deformation and fatigue fracture were the main mechanics of cavitation erosion of 0Cr18Ni9Ti stainless steel, whereas cavitation erosion of 45 steel was accompanied by oxidation, which will intensify cavitation erosion. Reasonable surface modification technology should be adopted for 0Cr18Ni9Ti stainless steel to improve surface hardness, and suitable surface protection technology must be used to resist cavitation erosion and oxidation for 45 steel under cavitation conditions.

Investigation on the Influence of Entrained Air to Internal Flow of Centrifugal Pumps under Cavitation Condition

The introduction of air is often used in engineering to delay the occurrence of cavitation. A steady simulation on a single-stage and single-suction centrifugal pump was used to study the influence of the air on the internal flow under cavitation condition at 1% inlet void fraction. Steady simulation based on SST k-ω turbulence model and Rayleigh-Plesset cavitation model included in Ansys CFX 17.0 were processed to obtain vapor volume fraction, gas volume fraction and the turbulent energy of the impeller under the condition of natural cavitation and cavitation with entrained air. A homogeneous two-phase flow model was adopted to describe the air-water mixed flow. The experimental cavitation performance curve provides boundary condition support for the numerical simulation. In conclusion, it shows that the head of the pump keeps steady after 1% air entrained like natural cavitation. Then it goes worse when the cavitation number decreases but with lower NPSHr which means cavitation performance is improved by 1% air entrained. It indicated that the entrained air changes the distribution of the vapor in the passage of the impeller. However, the descent of the pressure over a certain value will also do harm to the internal flow of the model pump whether it is ventilating or not. The internal flow of the pump will be seriously damaged and even be blocked completely due to more and more vapor generated causing a dramatic decline of the head.