Bulk Cavitation Research Articles

Bulk Cavitation in Model Gasoline Injectors and Their Correlation with the Instantaneous Liquid Flow Field

It is well established that spray characteristics from automotive injectors depend on, among other factors, whether cavitation arises in the injector nozzle. Bulk cavitation, which refers to the cavitation development distant from walls and thus far from the streamline curvature associated with salient points on a wall, has not been thoroughly investigated experimentally in injector nozzles. Consequently, it is not clear what is causing this phenomenon. The research objective of this study was to visualize cavitation in three different injector models (designated as Type A, Type B, and Type C) and quantify the liquid flow field in relation to the bulk cavitation phenomenon. In all models, bulk cavitation was present. We expected this bulk cavitation to be associated with a swirling flow with its axis parallel to that of the nozzle. However, liquid velocity measurements obtained through particle image velocimetry (PIV) demonstrated the absence of a swirling flow structure in the mean flow field just upstream of the nozzle exit, at a plane normal to the hypothetical axis of the injector. Consequently, we applied proper orthogonal decomposition (POD) to analyze the instantaneous liquid velocity data records in order to capture the dominant coherent structures potentially related to cavitation. It was found that the most energetic mode of the liquid flow field corresponded to the expected instantaneous swirling flow structure when bulk cavitation was present in the flow.

Ultrasound treated asphaltene laden W/O Interface: Insights into emulsion destabilization mechanism using a Microrheology approach

In this study, the effect of different levels of ultrasonic fields on the microrheology of an asphaltene laden flat interface and on the behavior of asphaltene-stabilized emulsions was evaluated. Experiments were performed at frequencies of 20, 40, 50, 80, 120, and 150 kHz, and voltages of 20, 30, 40, and 50 V. In the flat interface experiments, four distinct microrheology regimes were observed: I. no movement, II. slow streaming, III. strong streaming and interfacial cavitation, IV. bulk cavitation and mixing. The regime encountered depended on the inertial effect of ultrasound on the subphase (Reynolds number) as well as the thermodynamic impact of ultrasonic fields. Treating the emulsions with the same ultrasonic fields resulted in three categories of behavior: I. no coalescence, II. coalescence, III. emulsification. There was a strong correlation between emulsion behavior with the microrheology regime. Below the threshold of interfacial cavitation (Microrheology Regimes I and II), no change in the emulsions was detected. When interfacial cavitation occurred (Microrheology Regime II), the emulsions coalesced significantly. This behavior was attributed to the ability of the cavitation process in breaking the asphaltene barrier between the droplets. However, increasing the frequency and the voltage of the field increased the mechanical effects of the ultrasonic field (Microrheology Regime IV) and caused secondary emulsification in the system.

Study on the mechanism and load characteristics of secondary cavitation near free surface in underwater explosion

Although the cavitation phenomenon in underwater explosion has been researched for more than 100 years, the phase transition models based on mass and heat exchange between liquid and its vapor phases have only been established in the past decade. In this study, the secondary cavitation phenomenon was first captured by phase transition based on a four-equation system. The bulk cavitation near the free surface induced by underwater explosion was numerically investigated, and three typical bulk cavitation cases were investigated to explore their motion mechanisms and load characteristics on hydrodynamics and phase transition generation. It was found that secondary bulk cavitation will occur only under the condition that both the initial shock wave intensity and the distance between the water surface and the explosion bubble are satisfied in a specific relationship. Producing bulk cavitation was difficult at a relatively deep detonation depth because of the weak rarefaction wave reflected from the water surface by smaller charges. The statistical data under the condition of small charge indicated that the duration of cavitation increases with the increase in charge weight but the growth trend slows down gradually. However, the maximum volume of cavitation increased linearly with an increase in the charge weight. The present results can expand the currently limited database of underwater explosion multiphase fluids and provide insight into the interactions between the shock wave, bulk cavitation, explosion bubble, and water surface.

Interfacial cavitation.

Cavitation has long been recognized as a crucial predictor, or precursor, to the ultimate failure of various materials, ranging from ductile metals to soft and biological materials. Traditionally, cavitation in solids is defined as an unstable expansion of a void or a defect within a material. The critical applied load needed to trigger this instability -- the critical pressure -- is a lengthscale independent material property and has been predicted by numerous theoretical studies for a breadth of constitutive models. While these studies usually assume that cavitation initiates from defects in the bulk of an otherwise homogeneous medium, an alternative and potentially more ubiquitous scenario can occur if the defects are found at interfaces between two distinct media within the body. Such interfaces are becoming increasingly common in modern materials with the use of multimaterial composites and layer-by-layer additive manufacturing methods. However, a criterion to determine the threshold for interfacial failure, in analogy to the bulk cavitation limit, has yet to be reported. In this work, we fill this gap. Our theoretical model captures a lengthscale independent limit for interfacial cavitation, and is shown to agree with our observations at two distinct lengthscales, via two different experimental systems. To further understand the competition between the two cavitation modes (bulk versus interface), we expand our investigation beyond the elastic response to understand the ensuing unstable propagation of delamination at the interface. A phase diagram summarizes these results, showing regimes in which interfacial failure becomes the dominant mechanism.

Numerical simulation of underwater explosion cavitation characteristics based on phase transition model in compressible multicomponent fluids

Underwater explosion cavitation has an important influence on the shock wave, explosion bubble, and structural deformation. The one-fluid model is usually used in underwater explosion cavitation. It is assumed that the cavitation is induced suddenly when the pressure is lower than the saturated pressure. The biggest deficiency of the model is that it is difficult to consider the temperature change in the cavitation domain. It is known that cavitation is sensitive to the temperature. In this paper, we study UNDEX cavitation characteristics based on the two-fluid model proposed by Chiapolino et al. (2017), which holds that the cavitation phenomenon is the result of the phase transition between the gas and liquid phases. This model is composed of the 4-equation model with phase transition relaxation, which is solved by a simple fractional step. In this article, we briefly introduce the compressible multiphase fluids model and successfully extent it to the engineering research field of underwater explosion cavitation. Through the quantitative analysis of the vapor phase content, it is suggested that the initial mass fraction of vapor phase in water should be set to less than 10−7. Meanwhile, it can be observed that the bulk cavitation near the free surface can evolve into a vortex ring until it collapses. Numerical results show that the collapsed pressure cannot be ignored relative to the shock wave in the bulk and local cavitation. Numerical results of this paper display that the phase transition model shows a great prospect of engineering application in underwater explosion cavitation.

Measurement and quantification of acoustic bulk cavitation extent

Measurement and quantification of acoustic bulk cavitation extent

Effect of Fuel Mass Flow at the End of Injection on Cavitation and Gas Ingestion in the Nozzle

The fuel flow in the diesel engine nozzle has a vital impact on the fuel atomization and spray, and the fuel mass flux affects the internal flow of the nozzle. The visual experimental platform for a transparent nozzle was built to obtain the image of fuel flow in a nozzle with a small sac combining the back-light imaging technology and a high-speed framing camera. A two-phase three-component numerical model, based on the OpenFOAM solver, was calculated to quantitatively analyze gas ingestion and cavitation in the nozzle. The results indicate that at the end of injection (EOI), fuel cavitation and external air backflow (gas ingestion) occur successively in the nozzle, and both phenomena first appear in the orifice and then transition to the sac. Cavitation collapse is the major factor of gas ingestion, and the total amount of gas ingestion and cavitation mainly depends on the sac. The outflow of fuel largely depends on the total amount of cavitation and the inertial outflow of fuel at the EOI. The type of cavitation in the nozzle mainly presents annular and bulk cavitation, the former primarily exists in the sac, while the latter is established within the orifice. Therefore, larger mass flows will contribute to stronger cavitation and gas ingestion.

Visualization of shock wave propagation due to underwater explosion

Researchers have a great interest in determining the characteristics of shock wave propagation by an underwater explosion. But it involves complex physical processes near boundaries during the underwater explosion. The major issues are explosions, fluid interactions, fluid–structure interactions, and shock wave propagation. In general, the underwater explosion leads to shock wave propagation and it moves toward two types of boundaries. The first one is the free surface of the water, and the second is fluid–structure interaction. The underwater explosion also deals with bulk cavitation behind shock waves. This paper deals with the shock wave propagation and bulk cavitation because of the underwater explosion near the free surface. The computation and analytical results of the underwater explosion were discussed and compared. Also, polynomial and Mie–Gruneisen (shock) equation of state (EOS) of water was implemented to determine the incompressible nature of water and to get realistic results. The results of both the EOS of water were compared against each other and validated against analytical results. These results show that the equation of state of water and water–air interface has a significant influence on the characteristics of the shock wave propagation and unsteady cavitation near the free surface.

An experimental study on size distribution and zeta potential of bulk cavitation nanobubbles

Nanobubble flotation technology is an important research topic in the field of fine mineral particle separation. The basic characteristics of nanobubbles, including their size, concentration, surface zeta potential, and stability have a significant impact on the nanobubble flotation performance. In this paper, bulk nanobubbles generated based on the principle of hydrodynamic cavitation were investigated to determine the effects of different parameters (e.g., surfactant (frother) dosage, air flow, air pressure, liquid flow rate, and solution pH value) on their size distribution and zeta potential, as measured using a nanoparticle analyzer. The results demonstrated that the nanobubble size decreased with increasing pH value, surfactant concentration, and cavitation-tube liquid flow rate but increased with increasing air pressure and increasing air flow rate. The magnitude of the negative surface charge of the nanobubbles was positively correlated with the pH value, and a certain relationship was observed between the zeta potential of the nanobubbles and their size. The structural parameters of the cavitation tube also strongly affected the characteristics of the nanobubbles. The results of this study offer certain guidance for optimizing the nanobubble flotation technology.

Influence of sac structure on cavitation and gas ingestion at the end of injection

A comparative study using different sac structures was conducted focusing on the cavitation morphology inside a diesel nozzle. Cavitation phenomena play important roles in the flow field during fuel injection at the end of injection, which causes high-temperature gas ingestion. This can lead to coke formation inside the orifice (a hole of the nozzle where cavitation mainly occurs), further influencing the injection atomization in the cylinder. Using visual experiments and a finite volume method two-phase, three-component multiphase flow solver, the flow process in the nozzle at the end of injection was tested and simulated, and the influence of sacs with three different wall curvatures on the cavitation and gas ingestion was analyzed. The results demonstrated that cavitation occurred successively in the orifice and sac at the end of injection. The cavitation amount in the sac was larger, and two cavitation forms were produced: vortex and bulk cavitation. For narrow and straight sacs, bulk cavitation played an important role, and the amount of cavitation increased. The prolongation of the lower pressure duration inside of the nozzle decreased the fuel outflow, eventually leading to a decrease in gas ingestion. When vortex cavitation occurred in the sac, the second matrix invariant Q of the velocity gradient tensor ∇V was positively correlated with the volume of the vortex cavitation. The smaller the vortex core zone was, the lower the friction dissipation in the flow field was; under these conditions, cavitation occurred more easily in the sac.

Impact of liquid layer thickness on the dynamics of nano- to sub-microsecond phenomena of nanosecond pulsed laser ablation in liquid

The dynamics of nanosecond pulsed laser ablation in liquid (ns-PLAL) are significantly altered in the immediate vicinity of a free boundary. To improve the control and efficiency of ns-PLAL, more knowledge is needed on the evolution of the plasma, shock waves, and cavitation bubbles that form when this technique is performed within a thin liquid layer. Here, we present time-resolved photoelastic observations of ns-PLAL using an epoxy resin target covered with a sub-mm liquid paraffin layer. The investigation was conducted on a nanosecond time scale. When the liquid layer thickness approximates the plasma size that is induced in bulk-liquid ablation, part of the plasma plume is formed outside the liquid. This plasma is more extensive than that produced in bulk-liquid ablation, but more compact than in air ablation. At greater liquid layer thicknesses, the plasma is entirely confined within the liquid. The layer surface is elevated due to the expansion of the cavitation bubble on the target. The laser-induced shock wave is reflected back-and-forth between the liquid-air boundary and target surface, causing the formation of a bulk cavitation zone in the liquid layer. This study also discusses the mechanism by which liquid layer thickness affects the strength of laser-induced stress.

Comparison of dispersion behavior of agglomerated particles in liquid between ultrasonic irradiation and mechanical stirring

The particle dispersion behavior was compared for ultrasonic irradiation and mechanical stirring. The experiment and calculation were carried out with polymethylmethacrylate (PMMA) particles. The dispersion rate of the agglomerated particles increased with the decreasing ultrasonic frequency and the increasing electric power, whereas it increased with the increasing rotation speed for the mechanical stirring. The temporal change in the particle dispersion proceeded stably after passage of a long time. The dispersion of the ultrasonic irradiation was suggested to occur by the erosion from the surface of the cluster one by one due to the bulk cavitation as well as the division into smaller particles because of the inner cavitation, and that of the mechanical stirring mainly by the division into smaller clusters due to the shear stress flow. Based on the experimental results, mathematical models for the ultrasonic irradiation and mechanical stirring were developed with the dispersion and agglomeration terms and the calculation of the temporal change in the total cluster number at the different operational factors agreed with the experiments. The dispersion efficiency of the ultrasonic irradiation was larger than that of the mechanical stirring at the lower input power, but it was reversed at the higher input power.

Examining a change in the properties of coarse wool fiber under the influence of electrical discharge treatment

Modification of coarse wool fibre is a basic condition for improving quality of the manufactured woolen textile products while reducing their cost. Traditional methods of modification of wool are expensive, that is why the implementation of these methods into existing technological modes is not always economically justified. The obtained effect of modification is often associated with undesirable damage of protein structure of fiber. A solution to this problem is the application of electric discharge treatment, accompanied by the emergence of electric discharge non-linear bulk cavitation. It is shown that the influence of electrohydrolic shock and products of water cleavage on wool does not cause profound destruction of keratin, in contrast to enzymatic and plasmochemical methods of treatment. Based on the performed experimental studies, it was determined that during electric discharge treatment of coarse wool there occurs the compaction of fiber cuticles and a change in supramolecular structure of keratin. This fact predetermines a decrease in felting ability of wool fibre, an increase in sorption properties, as well as in resistance to hydrolysis and influence of oxidants and reductants. It was established that the application of electric discharge treatment as a method of coarse wool modification would contribute to a decrease in shrinking and improvement of optimized dyeing of woolen textile material. Thus, the proposed technology of wool improvement will make it possible to use less expensive raw materials for the production of textile materials for different purposes.

Upon Bulk Cavitation Appearing in Underwater Explosions

Abstract This paper presents some results of the author’s researching in connection with SPH (smoothed particle hydrodynamics) method and underwater explosion numerical modelling. All about cavitation fundamentals are considered known and about cavitation effects upon the structures. The authors, deeply preoccupied in using of SPH method, as well in modelling of the underwater explosion effects upon structures, had to take into consideration the bulk cavitation. A main issue in this study was the knowing of the bulk cavitation domain and its characteristic parameters. Such researching was possible to be successfully carried out, only by using of the SPH method. Finally, the paper presents the relations and the working way for knowing of the bulk cavitation domain and also a numerical model using SPH method is presented. The numerical example regarding shape and dimensions of the bulk cavitation is presented together putting in evidence of some parameters which can make damages upon a structure that is in the bulk cavitation area.

Bulk cavitation extent modeling: An energy-based approach

Bulk cavitation is a phenomenon that occurs when a negative-pressure or tension wave causes a liquid to rupture, or cavitate, over space. It is a process which causes resident microbubbles to grow to many times their original size, forming a bubble cloud. Such bubble clouds are observed in shallow underwater explosions, where negative-pressure waves are formed after shock waves reflect off the water surface; they are also observed in shock wave lithotripsy, shock wave histotripsy, ultrasonic cleaning, and other applications. Models had been developed for predicting the size and shape of such bulk cavitation regions. This work introduces a model that accounts for energy “lost” to bulk cavitation which in turn influences the extent that is dependent on the rate at which the passing negative-pressure wave dissipates. In-laboratory underwater experiments utilizing a spark source for high-amplitude pressure pulse generation, hydrophones and high-speed videography validate the energy transfer from tension wave to bubble cloud formation. These experiments are supplemented by computational fluid dynamics simulations. A cavitation absorption coefficient is introduced and parameterized for accurate prediction of cloud extent.

Numerical simulation of underwater explosion near air–water free surface using a five-equation reduced model

Underwater explosion phenomena is a complicated problem, however, it has different and important applications. This type of explosion includes strong shock waves with generation of low pressure cavitation׳s zone and deformable interfaces. The main objective of this article is simulation of interaction of shock wave with interface of two-phase gas–liquid flow and capturing the complicated interface generated from explosion. For this reason, a five equations reduced model is considered with using a new cavitation model including gravity force effect. From the numerical point of view, a Godunov method was applied using HLLC solver. By using MUSCL-Hancock strategy, second order accuracy is achieved. To verify the developed computer code, a one dimensional shock tube test case and two test cases including one dimensional cavitation in open tube and a two dimensional underwater explosion where its experimental results can be found in the related literature are used for comparison to justify the obtained results. The comparison of the results confirms an excellent accuracy of the numerical results. The proposed new cavitation model, on the other hand, is capable of calculating low pressures and simulating the dynamic creation and evolution of the bulk cavitation below the free surface.

The Influence of Electrical Discharge Nonlinear Bulk Cavitation on the Structural and Chemical Changes in Water during the Wool Fiber Bleaching

The influence of the electric-discharge treatment on changes in the physical and chemical properties of water has been investigated. It is shown that the influence of the electrical discharge nonlinear bulk cavitation changes the structure of water as well as the formation of free radicals and hydrogen peroxide. The electric-discharge treatment increases the whiteness of wool due to the changes in water properties.

Nanoscale particles in technological processes of beneficiation.

Background: Cavitation is a rather common and important effect in the processes of destruction of nano- and microscale particles in natural and technological processes. A possible cavitation disintegration of polymineral nano- and microparticles, which are placed into a liquid, as a result of the interaction of the particles with collapsed cavitation bubbles is considered. The emphasis is put on the cavitation processes on the interface between liquid and fine solid particles, which is suitable for the description of the real situations.Results: The results are illustrated for the minerals that are most abundant in gold ore. The bubbles are generated by shock loading of the liquid heated to the boiling temperature. Possibilities of cavitation separation of nano- and microscale monomineral fractions from polymineral nano- and microparticles and of the use of cavitation for beneficiation are demonstrated.Conclusion: The cavitation disintegration mechanism is important because the availability of high-grade deposits in the process of mining and production of noble metals is decreasing. This demands for an enhancement of the efficiency in developing low-grade deposits and in reprocessing ore dumps and tailings, which contain a certain amount of noble metals in the form of finely disseminated fractions. The cavitation processes occuring on the interface between liquid and fine solid particles are occasionally more effective than the bulk cavitation processes that were considered earlier.

Investigation of the shock wave propagation characteristics and cavitation effects of underwater explosion near boundaries

The shock wave propagation characteristics of underwater explosion have been of great interest to researchers. While the physical processes during an underwater explosion near boundaries are extremely complex, which involve lots of complex issues such as the explosion, shock wave propagation, water–air or water–structure interaction, etc. After the underwater detonation of an explosive, the shock wave may approach two main types of boundaries: the free surface of the water and the fluid–structure interface. The presence of these boundaries will significantly affect the wave propagation phenomena, and lead to bulk cavitation near the free surface or the structure surface. This paper deals with the behavior of shock wave propagation and cavitation from underwater explosion near different boundaries. A coupled numerical approach with combined Lagrangian and Eulerian methods is used to simulate the water–air interface and shock wave–structure interaction. A numerical model of free-field explosion in water is established, and the results have been compared with the published empirical formulas to verify the results of numerical solutions. The shock wave propagation characteristics from explosions in water near different boundaries are simulated and compared. In addition, the unsteady cavitations just near the free surface and the structure surface are described and captured. The water–air and water–structure interaction effects are examined. The results show that the free surface and structure surface boundaries have significant influence on the shock wave propagation characteristics.

Rheology, adhesion, and debonding mechanisms in fluorosilicone polymer gels

AbstractPolymer gels are complex materials used in myriad applications and industries including foods, consumer products, and adhesives. We examine the rheology and adhesion characteristics of three fluorosilicone gels of varying equilibrium modulus. Adhesion is studied in terms of confinement and separation velocity or initial strain rate. Further, the role of debonding mechanism on the adhesion properties is also elucidated. At low initial strain rates or low degrees of confinement, interfacial failure dominates while at high initial strain rates or high degrees of confinement bulk cavitation is the dominant debonding mechanism. We also report for the first time a transition region where both interfacial failure and bulk cavitation are observed. The adhesion results are explained in light of the rheological properties of the gels examined. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 40034.