Gaseous Cavitation Research Articles

Stability analysis of electrorheological (ER) lubricant operated multilobe hydrodynamic journal bearing

Purpose An electrorheological (ER) fluid consists of dielectric particles blended in a nonconducting oil. ER lubricants are often considered smart lubricants. This paper aims to examine the steady state and dynamic response of multilobe journal bearings using an ER lubricant. Design/methodology/approach Reynold’s equation has been used to describe the lubricant flow in the journal-bearing clearance space. The Bingham model is used to characterize the nonlinear behavior of the lubricant. The solution of the Reynolds equation is obtained using the Newton–Raphson method, with gaseous cavitation in the fluid film numerically addressed by applying a mass-conserving algorithm. The effects of lobe geometry and the applied electric field are investigated on film pressure profile, fluid film thickness, direct stiffness and damping parameters. The equation of motion for journal center coordinates is solved using the fourth-order Runge–Kutta method, to predict journal center motion trajectories. Findings Using ER lubricant combined with two-lobe journal bearing significantly improved the minimum film thickness by 49.75%, the direct stiffness parameter by 132.18% and the damping parameter by 206.3%. However, the multilobe configuration was found to negatively impact the frictional powerloss of the bearing system. In the case of multilobe configurations of journal bearings using ER lubricant, linear motion journal trajectories are observed to be reduced and exhibit increased stability. Originality/value This study presents the effect of an ER lubricant and multilobe configuration on the rotor-dynamic performance and stability analysis of hydrodynamic journal bearings. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-06-2024-0201/

Removal of surface-attached micro- and nanobubbles by ultrasonic cavitation in microfluidics

Surface-attached micro- and nanobubbles are known for their resistance to external forces. This study experimentally and theoretically investigates their response to strong ultrasonic fields. Surface-attached micro- and nanobubbles with contact radii from 2 μm to 20 μm are generated in a microchannel and exposed to ultrasound through a vibrating glass substrate. At a driving frequency over 200 kHz up to 2 MHz tested, no significant response from the micro- and nanobubbles is observed. By contrast, at 100 kHz–200 kHz, ultrasonic cavitation bubbles appear in the microchannel and migrate toward the surface micro- and nanobubbles. Then the surface micro- and nanobubbles merge with the ultrasonic cavitation bubbles, detach from the substrate, and become free gaseous nuclei susceptible to further cavitation. Notably, the removal process leaves no observable residue. Theoretical analysis suggests that the directional migration of cavitation bubbles is driven by mutual acoustic radiation forces. This work demonstrates that ultrasonic fields can effectively remove surface micro- and nanobubbles, transforming them into free gaseous cavitation nuclei.

Investigation of unsteady friction effects in transient two-phase pipe flow

Water hammer and transient cavitating flow may induce large pressure fluctuations in pipelines. Transient events in pipeline may cause a drop in pressure large enough to break liquid homogeneity and continuity (liquid column separation, distributed vaporous cavitation). Trapped air pockets in liquid may be a major problem in piping systems. The paper deals with mathematical tools for modelling unsteady skin friction, transient vaporous cavitation (liquid column separation) and transient gaseous cavitation (trapped air pockets). The method of characteristics transformation of the unsteady liquid pipe flow equations gives the water hammer solution procedure. A convolution-based unsteady skin friction model is explicitly incorporated into the staggered grid of the method of characteristics. Incorporating discrete cavities into the water hammer model leads to the discrete cavity model. An advanced discrete gas cavity model (DGCM) with consideration of unsteady pipe flow friction effects is presented in the paper. The DGCM is capable to simulate vaporous and gaseous cavitation at the same time. Pipeline apparatus for measurement of transient liquid flow (water hammer) and transient two-phase flow (vaporous and gaseous cavitation) is briefly described. The paper concludes with two distinct case studies showing profound effects of unsteady skin friction on pressure histories.

Stability Analysis of the Rotor-Journal Bearing System Considering Shear and Gaseous Cavitation

Part of the gas phase within the bearing emanates from the gaseous lubricating medium generated by the phase transition of the liquid lubricant under low pressure, while the remaining portion originates from the expansion of gases, such as air, present in the lubricant. This study delves into the impact of vapor and gas cavitation on the stability of the rotor-journal bearing system. Utilizing computational fluid dynamics (CFD), a 3D transient lubrication model is developed for the rotor-journal bearing system. This model integrates a combined cavitation approach, encompassing both vaporous and gaseous cavitation phenomena. Based on a new structured dynamic mesh method, the journal orbits are obtained when the journal moves in the rotor-journal bearing system. In vaporous and gaseous cavitation, shear stress and non-condensable gases (NCG) are incorporated successively. Compared with the combined cavitation model, the basic cavitation model journal orbit amplitude is significantly larger than the combined cavitation model. The carrying capacity of journal bearings under the basic cavitation model is overestimated, leading to a more conservative prediction for system stability.

Two-phase lubrication characteristics of journal bearing in refrigerant-oil system under high-pressure environment considering gaseous cavitation

The journal-bearing system is crucial for the reliability of refrigerant compressors that operate under high-ambient-pressure environments. The pressure changes rapidly, which hereby results in the variation in solubility of refrigerant in oil, and causes gaseous cavitation of the refrigerant-oil mixture in the bearing. However, the cavitation mechanism under high-pressure environments is not well understood, and its impact on lubrication characteristics has been rarely discussed. In this paper, a numerical model is developed to simultaneously solve the gaseous cavitation and two-phase lubrication problems. The first systematic comparison of lubrication characteristics for constant-eccentricity journal bearings, in such a special environment, between the classical Reynolds boundary and gaseous cavitation boundary is presented. Results indicate the load-carrying capacity is reduced by about 3% at a lower eccentricity of 0.3, but is greatly enhanced by about 21% at a higher eccentricity of 0.9 due to the gaseous cavitation. Meanwhile, the ‘reverse’ flow rate is found to reach -0.057 at the end side of the bearing owing to sub-ambient pressure up to -1 MPa at the eccentricity of 0.8. The cavitation-induced sub-ambient pressure should be addressed for the design of the bearing operating in such pressurized refrigerant environments as ambient gas might be sucked into the bearing.

Saffman–Taylor instability in eccentric cylinders at gaseous cavitation

A flow of silicon fluid in the gap between eccentric cylinders was studied experimentally. The condition of gaseous cavitation inception during the rotation of internal cylinder was considered. It was shown that at reduction of the gap between cylinders Saffman–Taylor instability appeared on surface of the internal cylinder and then gaseous cavitation was observed. Possibility of one uniform gas formation appearance under this type of instability was demonstrated.

Spatial-temporal evolution mechanism of mass transfer under synergetic gaseous and vapour cavitating effects in a micropump

As the vital power and the energy management systems for satellite life extension, the on-orbit refuelling system pressurizes the gas-containing propellants via the micropump, where the mixed transportation involves gas-liquid transitions and hydrodynamic oscillation. To achieve accurate predictions for the complex gas-liquid dynamic processes, the coupling methods of vapour and gaseous cavitations were proposed for the first time. The multifactor instabilities induced by the mass-transfer behaviors and rigid disturbance of the impeller were investigated, and the synergetic cavitating mechanism on its frequency spectrum, amplitude distribution and propagation law was revealed. The results indicated that the large absorbed concentration suppressed the fluctuating frequency, but enhanced the fluctuating amplitude. The absorbed-evolved characteristic frequency occupied the largest proportion of the mixing frequency about 5.5 % in the suction chamber. For the impeller, rigid disturbances by the impeller were the principal cause of hydraulic instability, and the amplitude of the absorbed–evolved condition accounted for 11–23 % of the amplitude of rotational frequency, demonstrating the non–negligible role of gaseous cavitation. In the volute, the degree of the gas consumption attenuation decreased as the total gas content increased, and the surges of turbulence kinetic energy and mass–transfer rate demonstrated the coupling mechanism between the flow and mass transfer.

Multi‐modality imaging characteristics of a surgically confirmed emphysematous torsion of papillary process of the caudate liver lobe and pneumoperitoneum in a dog

AbstractA 7‐year‐old, female, spayed dog presented with a 1‐week history of vomiting, diarrhoea and lethargy. Abdominal radiographs and ultrasound revealed bicavitary effusion, a large, gas‐cavitated, soft tissue mass in the region of the papillary process of the caudate lobe of the liver, with associated peritonitis and mild pneumoperitoneum. The location of the mass and gas cavitation were confirmed on computed tomography. The absence of vascular and parenchymal contrast enhancement and the distortion of the caudal vena cava was consistent with hilar torsion of the papillary process of the liver. The torsion was surgically confirmed and excised, and ischemic necrosis was histologically confirmed. The patient fully recovered following surgery.

Gas Release and Solution as Possible Mechanism of Oscillation Damping in Water Hammer Flow

Water hammer flow is examined, putting into evidence that unsteady friction cannot be fully responsible for observed oscillation damping. The measured piezometric head oscillations of water hammer flow experimental tests carried out for very long time (about 70 periods) are presented and compared with the numerical results of a quasi-two-dimensional (2D) flow model. The hypothesis is made that the energy dissipation could be partially due to the process of gas release and solution. An equation for the balance of gas mass is taken into account, already successfully used to improve the comparison between numerical and experimental head oscillations for transient gaseous cavitation. The models are based on a particular implementation of the method of characteristics (MOC-Z). The calibration of the empirical parameters of the models is carried out with a micro-genetic algorithm (micro-GA). The better performance of the proposed model is quantified with comparison of the mean absolute errors for three experimental tests at different Reynolds numbers, ranging from 5300 to 15,400. The corresponding ratios between the mean absolute errors of the models with and without gas release range between 47.3% and 17.7%. It is also shown that different turbulence models give very similar results. The results have some relevance in water hammer research, because sometimes dissipation that is not due to unsteady friction is attributed to it. However, the hypothesized mechanism has to be deepened and validated with further studies.

Hump and mass-transfer characteristics of dissolved oxygen induced by gaseous cavitation in a space micropump

Fuel transportation in on-orbit refueling and circulation systems used in space involves dynamic transitions in the gas–liquid behaviors. Based on our verified innovative computational model of dynamic dissolution and evolution, the interesting “hump characteristics” of the dissolved oxygen concentration (DOC) were discussed, and the space-spatial evolution of gas–liquid mass transfer were compared. The hump phenomenon is related to the evolution of gas in the suction chamber. The processes undergone by the released gas, including the local evolution, evolution expansion, and global evolution, which were observed for the first time in rotating machinery owing to the pressure fluctuations caused by the impeller. Limited by the mass-transfer rate, the pressure increase in the impeller was significantly greater than the increase in the DOC, and thus there was no gas evolution inside the impeller. With an increase in the initial DOC, the near-zero mass-transfer rate band split into two and moved downstream of the blade, indicating a dynamic balance between the dissolution and evolution. Moreover, the flow separation and vortices near the clearance widened the band and decreased the dissolution rate. The current research provides an in-depth understanding of the dynamic changes in the gas–liquid behavior of on-orbit refueling systems.

Synergistic effects of vapor and gaseous cavitation and mass-transfer mechanism in a mechanical pump

Dynamic gas–liquid mass-transfer processes are extensively encountered in gas–liquid mixture transport systems, where mechanical pumps pressurize the mixture and are accompanied by flow and mass-transfer instabilities. Herein, our proposed gaseous cavitation model was innovatively developed to revolutionize the independent unidirectional absorbed or evolved mass transfers. Complex gas–liquid behaviors under the synergetic effects of gaseous and vapor cavitations were achieved for the first time in an on-orbit refueling mechanical pump. Four coupled mass-transfer processes, namely, evolution, evaporation, absorption, condensation, and gas–liquid distribution, were investigated through numerical calculations. The results indicated that when the solution was close to critical saturation and conversion of the mass-transfer direction, a surge in the mass-transfer rate, and more intense hydrodynamic instability occurred. The vapor drove the accumulation of the evolved gas along the edge of the vapor in the impeller, where the evolved-dominated mass-transfer bands existed on the suction surfaces of the long blade, exhibiting the degassing characteristics of the vapor cavity, and other regions belonged to absorption-dominated region. Continuous dissolution induced by significant positive pressure gradient led to the maximum absorbed oxygen concentration at the impeller outlet. The maximal increments of absorbed oxygen in the suction chamber, impeller, and volute were 98%, 447%, and 694%, respectively, and the volume fractions were attenuated by 18.3%, 12.5%, and 5.0%, respectively. Notably, an increase in the gas volume fraction was the dominant reason for exacerbating the instability of the impeller forces, and the range of the radial force tended to be narrow and concentrated as the concentration increased.

Cavitation Modeling Using Lumped Parameter Approach Accounting for Bubble Dynamics and Mass Transport Through the Bubble Interface

AbstractVaporous and gaseous cavitation cause several physical phenomena which are typically undesirable, such as reduction in compressibility and material damage. Therefore, the ability to capture these effects in simulation is highly valued. In the fluid power field, lumped parameter modeling technique has proven effective for analyzing components and systems, allowing for fast simulations. Past efforts in modeling cavitation using lumped parameter approach have assumed dependence of fluid properties such as bulk modulus, density, and viscosity directly to pressure and temperature. This cannot be considered as the fluid mixture is composed of different phases of matter. Some other formulations account for gaseous cavitation based on the equations that are derived from vaporous cavitation. This paper illustrates a better approach that combines the two cavitation effects by considering that both vapor and undissolved gas co-occupy a spherical bubble. The size of the spherical bubble is solved using the Rayleigh–Plesset equation, and the transfer of gas through the bubble interface is solved using Henry's law and diffusion of the dissolved gas in the liquid. These equations are coupled with a novel pressure derivative equation. To show the validity of the proposed approach, the instantaneous pressure of a closed fluid volume undergoing expansion/compression is compared with multiple experimental sources, showing an improvement in accuracy when compared to existing models. Integrating this modeling technique with current displacement chamber simulation can further improve the understanding of cavitation in hydraulic systems.

Computational Fluid Dynamics Modeling of Gaseous Cavitation in Lubricating Vane Pumps: On Metering Grooves for Pressure Ripple Optimization

Abstract This paper addresses the topic of balanced vane pump simulations for pressure ripple optimization by means of metering grooves at the leading edge of delivery ports. It presents a computational fluid dynamics case study based on a transient three-dimensional model developed using an up-to-date commercial software tool. Special attention is dedicated to gaseous cavitation and the impact of the related modeling choices on predictions. The effect of groove dimensions and interaction with the precompression features of the cam ring of the pump are addressed. Experimental data obtained using a dedicated test rig are presented for comparison with numerical results in terms of delivery pressure ripple and volumetric flowrate. A validation strategy based on tuning of the dynamic cavitation model adopted for simulations is proposed. The results demonstrate the critical importance of gas release modeling for the simulation of porting details in the case of significant aeration effects.

Study on the Cavitation Suppression Mechanism of Axial Piston Pump

Gaseous and vaporous cavitation is extremely harmful to axial piston pumps, such as reducing flow rate, increasing flow pulsation, increasing vibration, increasing noise, and shortening life. To suppress the cavitation and improve the performance of axial piston pumps, a mathematical model for suppressing cavitation in the plunger chamber with a constant theoretical flow rate is innovatively established by combining the flow equation of the plunger pump and the pressure drop equation of the plunger chamber. Based on the model, two methods to suppress cavitation in the plunger chamber under the condition of a constant theoretical flow rate are proposed. The first method is to increase the distribution circle radius and correspondingly reduce the rotation speed, and the second method is to increase the plunger chamber radius and correspondingly reduce the rotation speed. To verify the effectiveness of these two methods, the CFD model of the axial piston pump is established, and the correctness of the model is verified by experiments. The results show that the two methods can effectively suppress cavitation in the plunger chamber, improve the actual flow rate, and reduce the flow pulsation under the condition of a constant theoretical flow rate. The research results can provide an important reference for the design and optimization of the plunger pump.

Experimental Study of Cavitation Development and Secondary Circulation Flow between Two Eccentric Cylinders

The flow of a hydrophobic fluid in the gap between eccentric cylinders has been experimentally studied. The experimental setup was designed and built for this study. Experimental setup consists of two eccentric cylinders with the ability to rotate and a camera, a microscope, and a pressure sensor. The conditions for gaseous cavitation occurrence during the rotation of the outer cylinder was considered in this study. The discreteness of gaseous cavitation occurrence in the form of individual bubbles is shown. When cavitation bubbles merge, the charge is redistributed at the gas–liquid interface, and bubble luminescence is observed. It has been shown that near the surface of the inner cylinder, in the area of flow expansion and compression, reverse flows occur.

Experiments and Computational Fluid Dynamics on Vapor and Gas Cavitation for Oil Hydraulics

AbstractA compressible Euler‐Euler computational fluid dynamics (CFD) model for vapor, gas, and pseudo‐cavitation in oil‐hydraulic flows is presented. For vapor, the Zwart‐Gerber‐Belamri (ZGB) model is used and for gas cavitation, the Lifante model. The aim is to determine the empirical parameters within the cavitation models for hydraulic oil by comparing CFD results to experiments in a realistic valve. The cavitating flow is visualized and measured for numerous operating points. By degassing, states of pure vapor cavitation are generated. The major findings are: (1) large eddy simulation turbulence modeling is essential, (2) vapor cavitation in mineral oil can be simulated very well with the ZGB model using the determined parameter, and (3) gas cavitation model provides useful results although not all details can be reflected and its scope is limited.

Cavitation Morphology Study between Hemispherical Textured Rotating Friction Pairs

A non-direct contact rotary interface uses a viscous fluid as the lubricant working medium. Because the oil film friction coefficient formed is extremely small, so it has great application potential in sealing, fluid transmission, thermosolutal convection, and bionics. Research on mechanical seals, wet clutches, and dynamic load bearing have proven that micro-textures can effectively improve friction and lubrication performance. However, when the fluid flows through the texture boundary, pressure disturbances can induce hydrodynamic cavitation. A pair of rotating disks are selected as our research objects. From the simulation and experiment research, we found that cavitation volume does not always increase with an increase in the texture rate, and cavitation always occurs preferentially at the outer diameter, so it is necessary to avoid machining the texture structure at the outer diameter of the mechanical seal end. Once the conditions for cavitation are met, a complete cavity is formed in approximately 0.015 s. The study also verifies that the cavitation gas originates from the phase change of the oil.

Numerical Simulation of a Spool Valve in Gaseous Cavitation Conditions

Spool valves are subject to a deteriorated performance and noise due to the occurrence of cavitation phenomena. It is well known that cavitation effects the performance of the component and causes an unwanted noise. The noise sound levels are influenced by many parameters like geometries and opening areas. In this paper a simple valve body made in plexiglass has been tested analyzing the cavitating area in U-grooves. A dedicated test rig has been equipped with a high-speed camera to acquire images of the phenomenon. A numerical three-dimensional CFD model has been built up using the commercial code. Experimental imagines have been compared with the numerical results showing a high accuracy on the prediction of the gaseous cavitation. The numerical results allow separate examination of several distinctive flow characteristics, which show favorable consistency with experimental observation and a periodic evolution of cavitation structure.

Identification approach of acoustic cavitation via frequency spectrum of sound pressure wave signals in numerical simulation

In a sono-reactor, complex ultrasound pressure wave signal can be detected, containing multiple information related to acoustic cavitation. In this present study, acoustic cavitation in a cylinder is investigated numerically. Via Fast Fourier Transfer (FFT), the sound pressure signals from sonotrode emitting surface are separated into harmonics, sub/ultra-harmonics and cavitation white noise: (1) the appearance of harmonics proved the non-linear propagation of ultrasound, (2) at the vibratory amplitude from 5∼20μm, only harmonics exists in the frequency spectra, corresponding to expansion and compression of non-condensable gas (NCG), (3) at the vibratory amplitude range of 30∼50μm, the occurrence of sub/ultra-harmonics demonstrated gaseous cavitation occurred, and (4) at the vibratory amplitude higher than 55μm, cavitation white noise arose, pointing out the initiation of vaporous cavitation. Based on the combination of frequency spectra and cavitation zones distribution, the acoustic cavitation state in water liquid is determined.

Gaseous cavitation characteristics of high-speed hydraulic electric motor-pump based on centrifugal pressurization: A numerical study

Hydraulic electric motor-pump is an integrated hydraulic power unit, raising rotational speed will further increase its power density. However, gaseous cavitation at high speed is the primary problem to be solved. In the present study, a new but simple centrifugal pressurization structure that integrated with the hollow shaft of hydraulic electric motor-pump was proposed to increase the inlet pressure of conjugated straight-line internal gear pump, and then to suppress gaseous cavitation at high speed. The numerical simulation is the main means of this work to investigate the gaseous cavitation characteristics of high-speed hydraulic electric motor-pump. In addition, a common conjugated straight-line internal gear pump model was developed as a reference to evaluate the pressurization effect of centrifugal tubes in terms of gas volume fraction. The simulation results present that compared with the significant increase of the suction chamber pressure of conjugated straight-line internal gear pump due to increasing the inclined angle of centrifugal tubes, the blocking effect of its inside wall to the fluid flow can be ignored. Under the same gas volume fraction in rotating volume, the rotational speed of hydraulic electric motor-pump is much higher than conjugated straight-line internal gear pump due to the pressurization effect of centrifugal tubes. On the premise of no obvious gaseous cavitation of centrifugal tubes, the recommended speed of hydraulic electric motor-pump is up to 13,000 r/min.