Cavitation Regime Research Articles

Changes in the Electrical Charge Accumulation Ability of Nanoporous Activated Carbon under Ultrasonic Radiation Exposure

AbstractThe influence of ultrasonic radiation in the cavitation regime on the structural and electrochemical properties of activated carbon was studied. It was established, as the result of the X‐ray diffraction and nitrogen adsorption/desorption analysis, that the ultrasound treatment causes substantial changes in the atomic and porous structure of the samples. The ultrasound treatment resulted in an increase of the graphitization degree of the carbon material and thus, in an increase of its density. The specific surface area and specific volume of the investigated samples decreased after the ultrasound treatment. The impedance dependencies for the supercapacitors made either from the initial and the modified carbon have been analyzed. The equivalent electrical circuits modelling the Nyquist diagrams have been constructed. De Levie impedance model, modified by series connection of parallel RC‐circuit, was used. It has been shown that ultrasonic radiation changes the Fermi level position by shifts to the energy region with high states density of delocalised electrons. This is the reason for unblocking of the Helmholtz layer capacitance due to the increase in the capacitance of the layer of the space charge region in the carbon material. It was shown that ultrasound treatment allows also controlling successfully the admixture and native defects distribution, existing on material surface and which are responsible for the surface electron states formation.

Investigation of condensation shocks and re-entrant jet dynamics in a cavitating nozzle flow by Large-Eddy Simulation

Cloud cavitation is related to an intrinsic instability where clouds are shed periodically. The shedding process is initiated either by the motion of a liquid re-entrant jet or a condensation shock. Cloud cavitation in nozzles interacts with the flow field in the nozzle, the mass flow and the spray break-up, and causes erosion damage. For nozzle geometries cloud shedding and the associated processes have not yet been studied in detail.In this paper, we investigate the process of cloud cavitation shedding, the re-entrant jet and condensation shocks in a scaled-up generic step nozzle with injection into gas using implicit Large-Eddy Simulations (LES). For modeling of the cavitating liquid we employ a barotropic equilibrium cavitation model, embedded in a homogeneous multi-component mixture model. Full compressibility of all components is taken into account to resolve the effects of collapsing vapor structures.We carry out simulations of two operating points exhibiting different cavitation regimes. The time-resolved, three-dimensional simulation results cover several shedding cycles and provide deeper insight into the flow field. Our results show that at lower cavitation numbers, shedding is initiated by condensation shocks, which has not yet been reported for nozzle flows with a constant cross-section. We analyze the cavitation dynamics and the shedding cycles of both operating points. Based on our observations we propose modifications to established schematics of the cloud shedding process. Additionally, we analyze the near-wall upstream flow in and underneath the vapor sheet and possible driving mechanism behind the formation of the re-entrant jet.

EFFECT OF GAS CONTENT ON THE CAVITATING AND NON-CAVITATING PERFORMANCE OF AN AXIAL THREE-BLADED INDUCER

In hydraulic systems, cavitation occurs in the liquid when its pressure locally drops below its vapor pressure. This can generate vibrations, noises, and instabilities in the flow that reduce the system performance. Cavitation has been the subject of many studies but the influence of dissolved gases in these operating conditions has remained unexplored. In the present work, an experimental study is conducted on a three-bladed axial inducer at partial flow rates in cavitating and non-cavitating regimes. A closed loop with a transparent test section and constant temperature water is used. In one of the tanks, a specific device is placed in order to inject CO2 and dissolve it in water. This device makes it possible to evaluate the dissolved CO2 rate effect on the performance of the inducer. A vacuum pump decreases the pressure in the loop. A high-speed camera filming the inducer suction in various conditions enables images to transmit at a sampling rate frequency of 1 kHz allowing comparison of the flow dynamics. Based on the pressure measurements, the flow rate, and the CO2 concentration, head versus net positive suction head (NPSH) curves are set for various Reynolds numbers. The results indicate that increasing the dissolved CO2 content decreases the inducer performance: the head breakdown occurs at higher NPSH, and it increases the cavitation intensity. In fact, as the degassing takes place small gas bubbles are noticed and the single-phase flow transforms into a two-phase homogeneous mixture.

Numerical study of cavitation regimes in flow over a circular cylinder

Cavitating flow over a circular cylinder is investigated over a range of cavitation numbers for both laminar and turbulent regimes. We observe non-cavitating, cyclic and transitional cavitation regimes with reduction in freestream $\sigma$. The cavitation inside the Karman vortices in the cyclic regime, is significantly altered by the onset of "condensation front" propagation in the transitional regime. At the transition, an order of magnitude jump in shedding Strouhal number is observed as the dominant frequency shifts from periodic vortex shedding in the cyclic regime, to irregular-regular vortex shedding in the transitional regime. In addition, a peak in pressure fluctuations, and a maximum in St versus $\sigma$ based on cavity length are observed at the transition. Shedding characteristics in each regime are discussed using dynamic mode decomposition. It is demonstrated that the condensation fronts observed in the transitional regime are supersonic (referred as "condensation shocks"). In the presence of NCG, multiple condensation shocks in a given cycle are required for complete cavity condensation and detachment, as compared to a single condensation shock when only vapor is present. This is explained by the reduction in pressure ratio across the shock in the presence of NCG effectively reducing its strength. In addition, at $\sigma=0.85$ (near transition from the cyclic to the transitional regime), presence of NCG suppresses the low frequency irregular-regular vortex shedding. Vorticity transport at $Re=3900$, in the transitional regime, indicates that the region of attached cavity is nearly two-dimensional with very low vorticity, affecting Karman shedding in the near wake. In addition, the boundary-layer separation point is found to be strongly dependent on the amounts of vapor and gas in the freestream for both laminar and turbulent regimes.

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

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

Numerical investigation of partial cavitation regimes over a wedge using large eddy simulation

Partial cavitation over incipient, transitory and periodic regimes is investigated using large eddy simulation (LES) in the (experimental) sharp wedge configuration of Ganesh et al. (2016). The numerical approach is based on a compressible homogeneous mixture model with finite rate mass transfer between the phases. Physical mechanisms of cavity transition observed in the experiments; i.e. re-entrant jet and bubbly shock wave, are both captured in the LES over their respective regimes. Vapor volume fraction data obtained from the LES is quantitatively compared to X-ray densitometry. In the transitory and periodic regimes, void fractions resulting from complex interactions of large regions of vapor in the sheet/cloud show very good comparison with the experiments. In addition, very good agreement with the experiments is obtained for the shedding frequency and the bubbly shock wave propagation speed. In the incipient regime, the qualitative characteristics of the flow (e.g. cavitation inside spanwise vortices in the shear layer) are captured in the simulations. Conditions favoring either the formation of the re-entrant jet or the bubbly shock wave are analyzed by contrasting the LES results between the regimes. In the transitory regime, large pressure recovery from within the cavity to outside, and the resulting high adverse pressure gradient at the cavity closure support the formation of re-entrant jet. In the periodic regime, overall low pressures lead to reduced speed of sound and increased medium compressibility, favoring the propagation of shock waves. In a re-entrant jet cycle, vapor production occurs predominantly in the shear layer, and intermittently within the cavity. In a bubbly shock cycle, vapor production is observed spanning the entire thickness of the cavity. Bubbly shock wave propagation is observed to be initiated by the impingement of the collapse-induced pressure waves from the previously shed cloud. Supersonic Mach numbers are observed in the cavity closure regions, while the regions within the grown cavity are subsonic due to the negligible flow velocities.

LES investigation of sheet-cloud cavitation around a 3-D twisted wing with a NACA 16012 hydrofoil

In this paper, we sought to investigate unsteady cavitating turbulent flows around a twisted three-dimensional NACA 16012 hydrofoil using the large eddy simulation (LES) and volume of fluid (VOF) methods. The one-equation eddy viscosity model (OEEVM) was used to calculate the sub-grid scale (SGS) stress tensor. The compressive velocity VOF technique and the Kunz cavitation model were also employed. Numerical simulation was performed using the interPhaseChangeFoam solver within the OpenFOAM framework. Cavitation simulation was performed at three cavitation numbers within the cloud cavitation regime; that is to say, cavitation numbers: 0.95, 1.15, and 1.35. Afterwards, the details of flow predictions including the pressure, velocity, streamlines, volume fraction of water phase, and turbulent kinetic energy were reported. Cavity behaviours, including growth, shedding, and collapse of the cavity, were considered in detail. Three-dimensional cavity structures such as primary and secondary shedding and the shedding of the U-shaped horseshoe vortex were reported as well. The present work illustrated the side-entrant jets and the radially diverging re-entrant jet corresponding to the three-dimensional effect of the twisted wing. The cavity pattern and the shedding cycle frequency agreed well with the available experimental observations.

Void fraction measurements in partial cavitation regimes by X-ray computed tomography

Cavitation is a complicated multiphase phenomenon, where the production of vapor cavities leads to an opaque flow. Exploring the internal structures of the cavitating flows is one of the most significant challenges in this field of study. While it is not possible to visualize the interior of the cavity with visible light, we use X-ray computed tomography to obtain the time-averaged void fraction distribution in an axisymmetric converging-diverging nozzle (’venturi’). This technique is based on the amount of energy absorbed by the material, which in turn depends on its density and thickness. Using this technique, two different partial cavitation mechanisms are examined: the re-entrant jet mechanism and the bubbly shock mechanism. 3D reconstruction of the X-ray images is used (i) to differentiate between vapor and liquid phase, (ii) to obtain radial geometric features of the flow, and (iii) to quantify the local void fraction. The void fraction downstream of the venturi in the bubbly shock mechanism is found to be more than twice compared to the re-entrant jet mechanism. The results show the presence of intense cavitation at the walls of the venturi. Moreover, the vapor phase mixes with the liquid phase downstream of the venturi, resulting in cloud-like cavitation.

A review of transient flow structure and unsteady mechanism of cavitating flow

The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street’s forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented.

КАВИТАЦИЯ НА РИФЛЕНОМ ДВУМЕРНОМ ГИДРОКРЫЛЕ ПРИ МАЛОМ УГЛЕ АТАКИ

Кавитация представляет собой один из основных источников неустойчивостей течения, возникающих при эксплуатации гидравлического оборудования, а также является причиной эрозионного износа его рабочих элементов. В этой связи разработка и развитие различных методов управления кавитационными течениями является актуальной задачей для задержки развития кавитации и снижения ее негативного влияния. Одним из таких методов является модификация поверхности гидрокрыла. Цель работы: экспериментальное исследование кавитационного обтекания рифленого гидрокрыла с продольными бороздками полукруглого профиля на поверхности (НЛ2), представляющего собой уменьшенную модель направляющей лопатки высоконапорной гидротурбины, в сравнении с моделью оригинальной направляющей лопатки (НЛ1). Методы. Для анализа пространственной структуры и временной эволюции паровых каверн и оценки их интегральных характеристик была применена высокоскоростная визуализация. Скорость течения над гидрокрыльями и в их следе измерялась с помощью метода PIV, на основе измеренных ансамблей полей мгновенной скорости были получены распределения средних и турбулентных характеристик течения. Результаты. На лопатке с модифицированной поверхностью (НЛ2) кавитация зарождается в виде одиночных изолированных пузырей в канавках, которые при уменьшении числа кавитации переходят в кавитирующие стрики. Пока стрики локализованы в канавках и не взаимодействуют друг с другом, режим обтекания остается стационарным. Однако когда их размер становится больше диаметра желобков, они выходят за пределы этих углублений, взаимодействуют и образуют единую каверну, которая теряет устойчивость и начинает пульсировать. В целом бороздки на поверхности гидрокрыла позволяют в некоторой степени задержать развитие кавитации и воспрепятствовать переходу к нестационарным режимам обтекания. На нестационарном режиме динамика каверн на оригинальном гидрокрыле (НЛ1) и НЛ2 сильно отличается. Так, на НЛ1 в отличие от НЛ2 каверна никогда не исчезает полностью, каверна на НЛ1 в среднем оказывается длиннее и пульсирует с большей частотой (St=0,09) по сравнению с НЛ2 (St=0,06). Кроме того, поведение каверны в течение одного периода пульсаций оказалось довольно необычным для обеих моделей: сначала она увеличивается до максимального размера, затем несколько уменьшается и снова вырастает до максимума, после чего возвращается в исходное состояние. Причина такой динамики пока остается невыясненной. На переходном режиме обтекания, когда внутри бороздок формируются кавитирующие стрики, интенсивность турбулентных флуктуаций скорости над поверхностью НЛ2 снижается по сравнению с режимом пузырьковой кавитации. Это происходит потому, что изолированные каверны в желобках как бы восстанавливают форму модифицированного гидрокрыла, делая геометрию его поверхности более приближенной к оригинальной (НЛ1). Таким образом, профиль НЛ2 становится более заполненным благодаря локальной кавитации в бороздках. Вместе с тем наличие канавок на поверхности НЛ2 приводит к дополнительной турбулизации течения вблизи поверхности лопатки для всех рассматриваемых режимов течения, что, вероятно, и является причиной задержки развития кавитации на гидрокрыле с бороздками.

Plasmon-Enhanced Pan-Microbial Pathogen Inactivation in the Cavitation Regime: Selectivity Without Targeting

Pan-microbial inactivation technologies that do not require high temperatures, reactive chemical compounds, or UV radiation could address gaps in current infection control strategies and provide efficient sterilization of biologics in the biotechnological industry. Here, we demonstrate that femtosecond (fs) laser irradiation of resonant gold nanoparticles (NPs) under conditions that allow for E-field mediated cavitation and shockwave generation achieve an efficient plasmon-enhanced photonic microbial pathogen inactivation. We demonstrate that this NP-enhanced, physical inactivation approach is effective against a diverse group of pathogens, including both enveloped and non-enveloped viruses, and a variety of bacteria and mycoplasma. Photonic inactivation is wavelength-dependent and in the absence of plasmonic enhancement from NPs, negligible levels of microbial inactivation are observed in the near-infrared (NIR) at 800 nm. This changes upon addition of resonant plasmonic NPs, which provide a strong enhancement of inactivation of viral and bacterial contaminants. Importantly, the plasmon-enhanced 800 nm femtosecond (fs)-pulse induced inactivation was selective to pathogens. No measurable damage was observed for antibodies included as representative biologics under identical conditions.

Characterization of unsteady cavitating flow regimes around a hydrofoil, based on an extended Schnerr–Sauer model coupled with a nucleation model

The present work is aimed at the numerical spatio-temporal characterization of the unsteady cavitating flow structures on a hydrofoil by means of Proper Orthogonal Decomposition (POD) and Fast Fourier Transform (FFT) techniques. Three different cavitation regimes have been investigated: bubble cavitation, cloud cavitation and supercavitation. The homogeneous mixture approach has been used, in combination with an extended Schnerr–Sauer cavitation model. The accuracy of the numerical predictions has been improved by means of the implementation of a Density Correction Model of the turbulent viscosity, and a simplified Population Balance Modeling (PBM) which solved the spatial distribution and the temporal evolution of the nuclei. In particular, the PBM has led to a reduction of the intensity of the evaporation inside the vapor cavities and a consequent condensation enhancement at the cavity closure and in the wake downstream. This phenomenon mainly impacted on the vapor cavity dynamics during supercavition by facilitating the formation of the re-entrant jet and the vapor cavity detachment. Also, during supercavitation the nuclei density nb exhibited maximum variations of about 35.6% with respect to the inlet nuclei density. As the cavitation number increased, both the intensity and the extension of oscillations significantly reduced, and in bubble cavitation regime nb fluctuated at amplitudes of about 10% of the inlet nuclei density.The characterization of the cavitation regimes revealed that the bubble cavitation regime had a more stable and periodic dynamics highlighted by a higher hydrodynamic efficiency and a reduced root mean square of the lift force. The cloud cavitation and the supercavitation exhibited a more violent bubble detachment which caused stronger oscillations of the vapor cavity as well as the pressure upstream. This was retrieved in an increase of the average drag coefficient of about the 38% due to the presence of vapor cloud transported downstream, which promoted the surge of the flow. The vorticity analysis underlined that the formation of the re-entrant jet and the bubble detachment were promoted by the baroclinic vorticity, while the dilatation vorticity drove the dynamics of the detached clouds, governed by the phase change phenomena. The FFT analysis of the dynamics of the vapor cavity and the pressure upstream led to the detection of the most representative frequencies and Strouhal numbers of each cavitation regimes, in particular (fs=16.6Hz,St=0.355) for bubble cavitation, (fs=10.74Hz,St=0.358) for cloud cavitation, and (fs=8.79Hz,St=0.300) during supercavitation.The POD analysis allowed for the detection of the most relevant cavitating structures, in relation to the vapor cavity fluctuations and their frequency content. Furthermore, the FFT analysis of the temporal eigenfunctions demonstrated that the first POD mode of the liquid volume fraction described the overall unsteady behavior previously detected. Instead, high order POD modes revealed frequency values well above the overall ones of the main flow previously detected.

Numerical methodology to predict and analyze cavitating flows in a Kaplan turbine

A computational methodology to predict the appearance and the evolution of the cavitation phenomena in a scale model of a 5–blades Kaplan turbine was developed. The cavitating flow was modeled by a homogeneous approach with a barotropic state law and the k − ω SST turbulence model. RANS simulations were performed, near the Best Efficiency Point, on two computational domains (D1 and D2) comprising one periodic interblades channel consisting of the guide vanes, the runner blade and the cone (domain D1) or taking into account the entire draft tube (domain D2). At first, non–cavitating simulations were carried out in domain D1 in order to match numerical conditions to the experimental ones (similar mass flow Q, head H and mainly the torque T) via an iterative procedure. From this reference operating point obtained in free–cavitation regime, cavitating simulations were performed on both computational domains by reducing the Thoma number, σ. Two different inlet boundary conditions were tested: the classical one, imposing the mass flow rate Q; the new one, fixing the total pressure and, consequently, keeping the machine head H constant during the cavitation drop, as in experiments. In all the performed calculations, the outlet boundary condition is given by a constant static pressure. Computed torque and efficiency drop curves were compared to available experimental data. The best results were obtained with the computational domain D2 applying a constant total pressure on the inlet. The torque and efficiency evolutions were well–predicted with the proposed calculation methodology, and the numerical cavitation structures agreed with experimental observations. Analyses of the blade loading during cavitation breakdown are also proposed in the article. Unsteady simulations are under investigation to improve the prediction and the analyses of more developed cavitating regimes.

Assessment of cavitation regime in divergent vortex flows

The analysis of a cavitating swirling flow in a configuration similar to the flow at the entrance of a hydraulic turbine draft tube, except the rotational component, is proposed. This flow behaviour is essential for the hydraulic turbine operation in extended range and its understanding is essential for the turbine design improvement. Advanced 2D measurements of the velocity field using PIV techniques and unsteady pressure measurements are used to determine the σ number and velocity field for various stages of cavitation development.

Study of a Cavitating Venturi Tube by Lumped Parameters

The hydrodynamic cavitation in a Venturi tube is studied both theoretically and experimentally. A lumped parameter model was developed to describe the accumulation and dissipation of energy in the biphasic flow as a function of the bubble population characteristics (mean volume, standard deviation, and void fraction). Resistance, capacitance, inductance, and frequency lumped-parameters were identified applying the fluid conservation equations (mass and momentum) along with electrical/hydraulic analogies. Experiments with 1,2-propanediol were carried out in a hydraulic circuit composed of valves, a pump, and a Venturi nozzle. The acoustic noise generated (at different cavitation regimes) by the passage of the fluid through the tube was acquired with a piezoelectric sensor. After processing the experimental signals, the system frequency at each operation condition was determined. Plausible estimations of the void fraction were obtained at different experimental frequencies by evaluating a theoretical expression of the frequency lumped-parameter. This semi-empirical technique might be a low-cost alternative when the void fraction of a flow needs to be determined and tomography devices are not available.

Study of Venturi tube geometry on the hydrodynamic cavitation for the generation of microbubbles

Venturi tubes have been extensively used to generate microbubbles through hydrodynamic cavitation. Due to their simple and flexible design, Venturi tubes have been used in a wide range of applications including mineral flotation. Although the impacts of the geometries of Venturi tubes on hydrodynamic cavitation have been studied in different specific contexts, no clear relation between cavitation and geometry parameters of Venturi tubes has been reported. In this work, we investigated numerically and experimentally the influence of several geometrical parameters on the cavitation inception detected by a hydrophone and the microbubble generation measured by a high-speed camera. Using a dimensionless number analysis, we found that the cavitation inception was determined by flow resistance, which significantly depends on the geometrical design of Venturi tube. In the cavitation regime, the flow resistance induced by cavitation increases linearly with reducing downstream cavitation number while the upstream cavitation number becomes constant regardless of the geometry of Venturi tube. A small outlet angle results in a low cavitation inception and a high microbubble production. Moreover, the degree of microbubble generation was found to increase with extra flow resistance and the dissolved gas concentration. The insights from this study provide a guideline for the design of efficient Venturi tubes for hydrodynamic cavitation system.

CRUD removal via hydrodynamic cavitation in micro-orifices

Chalk river unidentified deposit (CRUD) is an acronym used to describe the corrosion deposits that form in nuclear power plants. Such deposits are often associated to reduction of heat transfer in fuel rods and to blockage of channels in regions of fast hydrodynamic flow. Given the significance of the operational consequences associated to CRUD deposition, the development of methodologies to remove it is of scientific and technical interest. In this study, a microscale flow loop setup, replicating conditions relevant to plant operations, was used to explore the potential of exploiting hydrodynamic cavitation for CRUD removal. Tests were performed on discs with micro-orifices that were previously exposed to high temperature water flow to induce CRUD deposition. CRUD removal from within the micro-orifices was performed by inducing cavitation, and monitored via periodic microstructural examinations. The results indicate that a cavitating regime can reduce the volume of CRUD deposit by 80–90% within minutes, with no detectable damage to the metal surface.

The role of compressibility in computing noise generated at a cavitating orifice

The computational fluid dynamics calculation can be accomplished by solving either compressible or incompressible Navier–Stokes equations to determine the flow-field variables of the noise source. The proper assumption depends on both the physical situation and the Mach number. Although in cavitating devices usually we are dealing with low Mach number flow, cavitation is an acoustic phenomenon that can be affected by compressibility. Cavitation behaves acoustically as a monopole and it is mentioned by some researchers that incompressible solution is sufficient to study the dipole sources. However, in order to study the monopole (and quadrupole) sources a compressible solution may be required. In this study, the role of compressibility in computing noise generated at a cavitating single-hole orifice was investigated using large eddy simulation and Ffowcs Williams–Hawkings formulation. The fluid zone downstream of the orifice where the cavitation occurs was evaluated as the acoustic source which generates sound. Time-accurate solutions of the flow-field variables on source surfaces were obtained from both compressible and incompressible flow simulations. Three cases of cavitation were studied and the sound pressure signals far downstream of the orifice were computed by the Ffowcs Williams–Hawkings formulation. For a developed cavitation regime at low frequencies, there is a big discrepancy between the computed values of sound pressure level from compressible and incompressible simulations, and at higher frequencies greater than 6 kHz, both simulation methods provide almost the same values for sound pressure levels. For a super cavitation regime, both compressible and incompressible simulations provide similar values for sound pressure levels at frequencies greater than 2 kHz. The results of this work demonstrate that the compressibility has a significant role in terms of computing noise generated at a cavitating orifice and cannot be ignored, especially when the noise generated by developed cavitation regimes at low frequencies is investigated.

Cavitation in Diesel Fuel Injector Nozzles and its Influence on Atomization and Spray

AbstractIn combustors, high‐pressure fuel injector nozzles are employed for liquid atomization and spray generation. The internal flow within nozzles, especially cavitation, plays an important role in promoting the breakup of liquid jets. The formation mechanism and evolvement of cavitation are reviewed and described. Different cavitation regimes were characterized experimentally and the positive effect of cavitation on liquid atomization was confirmed. Different empirical formulas correlating the discharge coefficient with fluid‐dynamical parameters are summarized. The applicability, advantages, and disadvantages of two popular computational models for flow modeling, i.e., the interface tracking model and the homogeneous equilibrium model, are reviewed. The effects of cavitation on the generated spray are discussed.

Cavitation-Dispersion Method for Copper Cementation from Wastewater by Iron Powder

The circular economy for sustainable economic deployment is strongly based on the re-use of secondary products and waste utilization. In the present study, a new effective cementation method for recovering valuable metallic copper from industrial wastewater using Fe0 powders is reported. A high-speed mixer-disperser (HSMD) capable of providing a cavitation effect was used for the rapid intake, dispersion, and mixing of Fe0 powder in an acidic wastewater solution (pH ≈ 2.9) containing copper ions mainly in the form of CuSO4. Three iron powders/particles were tested as the cementation agent: particles collected from industrial dust filters (CMS), water-atomized iron-based powder AHC100.29, and sponge-iron powder NC100.24. The effects of mixing regimes and related mixing conditions on the effectiveness of the Cu cementation process were evaluated by comparison between the HSMD and a laboratory paddle mixer. It was observed that the use of cavitation provided more efficient copper removal during the copper cementation process in comparison to the standard experiments with the propeller mixer. Under the cavitation regime, about 90% of copper was cemented in the first five minutes and the final copper removal of 95% was achieved using all three Fe0 powders after seven minutes of cementation. In comparison, only around 55% of copper was cemented in the first seven minutes of cementation using the traditional mixing method.