Highest Cavitation Intensity Research Articles

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2−) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2−. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2− comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2− productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2−, and ·O2− comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2−. This paper shed light to the generation mechanism of 1O2 and ·O2− in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

Enhanced microbubble-mediated cavitation by using acoustic droplet vaporization

Acoustic cavitation serves various therapeutic purposes, with both microbubbles and phase-transit droplets serving as potential cavitation nuclei. This project focused on investigating the intricate interaction between microbubbles and droplets and the resultant impact on acoustic cavitation.To facilitate this investigation, microbubbles with a negative potential of −51.58 mV were prepared, while droplets were endowed with a positive potential of +4.22 mV, facilitating the formation of microbubble-droplet clusters. Microscopic analysis was employed to observe the morphology of these clusters, revealing an initial mean size of 4.85 µm before ultrasound activation and subsequent transformation into larger bubbles with a mean size of 12.54 µm after ultrasound activation. The experimental approach involved the use of varying driven voltages to generate ultrasound waves with different intensities. A passive cavitation detector (PCD) was then employed to capture cavitation signals. Results demonstrated that, when maintaining a microbubble ratio of 50% and varying droplet ratios from 0% to 50%, the driven voltage at which the cavitation peak appeared decreased from 180 mV to 140 mV. Additionally, the mean cavitation intensity exhibited an initial increase from 0.58 to 0.78, followed by a subsequent decrease from 0.78 to 0.63 after normalization.This study provided valuable insights into the dynamic interactions between microbubbles and droplets. Specifically, it showed that microbubble cavitation transferred energy to droplets, leading to the generation of larger bubbles and higher cavitation intensity. Nevertheless, the observed cavitation intensity and threshold were contingent upon the ratios of microbubbles and droplets. Notably, to achieve an ideal cavitation intensity, the energy generated by microbubble cavitation must be substantial enough to activate all droplets, inducing acoustic droplet vaporization (ADV) and cavitation.

Thermal-Elastohydrodynamic Contacts Lubricated With Oil/Refrigerant Solutions: A New Cavitation Modeling Approach Based on Refrigerant Solubility

Abstract A first cavitation modeling with thermal effects for oil/refrigerant solutions lubricated elastohydrodynamic point contacts is reported in this work. The solubility of the oil/refrigerant system is introduced into the generalized Reynolds equation coupled with the elasticity equation and the energy conservation equation. The numerical results show a very good agreement with the published experimental results concerning film thickness prediction. Moreover, the present model describes the cavitation region on a physical basis. A discussion with other cavitation models from the literature is proposed. It puts into light the necessity of taking into account the solubility of the refrigerant into oil for such problems. Compared to pure oil, oil/refrigerant solutions can potentially reduce the amount of liquid oil for the next contact due to its higher cavitation intensity.

Effect of Cavitation Intensity on the Cavitation Erosion Behavior of 316L Stainless Steel in 3.5 wt.% NaCl Solution

In this study, the cavitation erosion behavior of 316L stainless steel under different cavitation intensities in 3.5 wt.% NaCl solution was investigated with scanning electron microscopy and various electrochemical tests. Results indicated that cavitation intensity corresponding to CE amplitude of 5 μm was lower than that of the mechanical bearing capacity of passive films. When subjected to cavitation erosion (CE), Open circuit potential (OCP) shifted to the noble direction instantaneously, which was mainly attributed to enhanced oxygen transfer of the cathode due to stirring effects of CE. By contrast, high cavitation intensities corresponding to CE amplitudes of 25 μm and 55 μm exceeded the mechanical bearing capacity of passive films, causing significantly reduced OCP associated with metal dissolution of the anode from mechanical damage. Potentiostatic polarization and Mott–Schottky tests showed that 316L SS subjected to low cavitation intensities displayed good repassivation properties. However, repassivation performance was markedly weakened when high cavitation intensity was applied, resulting in weaker protection of the passive films with high carrier density.

The Influence of Inflow Swirl on Cavitating and Mixing Processes in a Venturi Tube

A study of the mixing flows (Schmidt number = 103) in a cavitating Venturi tube that feature linear and swirling flows is presented in this paper. The Large Eddy Simulation (LES) turbulence model, the Schnerr–Sauer cavitation model, and the mixture multiphase model, as implemented in the commercial CFD ANSYS FLUENT 16.2, were employed. The main emphasis is spending on the influence of different inlet swirling ratios on the generation of cavitation and mixing behaviors in a Venturi tube. Four different inflow regimes were investigated for the Reynolds number Re = 19,044, 19,250, 19,622, 21,276: zero swirl, 15% swirl, 25% swirl and 50% swirl velocity relative to the transverse inflow velocity, respectively. The computed velocity and pressure profiles were shown in good agreement with the experiment data from the literature. The predicted results indicate that the imposed swirl flow moves the cavitation bubbles away from throat surfaces toward the throat axis. The rapid mixing between two volumetric components is promoted in the divergent section when the intense swirl is introduced. Additionally, the increase in the swirl ratio from 0.15 to 0.5 leads to a linear increase in the static pressure drop and a nonlinear increase in the vapor production. The reduction in the fluid viscosity ratio from μ2μ1=10 to μ2μ1=1 generates a high cavitation intensity in the throat of the Venturi tube. However, the changes in the pressure drop and vapor volume fraction are significantly small of pure water flow.

On PEO-Based MWCNT and Graphene Composite Electrolyte Structure

Novel and highly effective polyethylene oxide (PEO) based composite electrolytes were prepared by combining the graphene nanoplatelets (GR) and multiwall carbon nanotubes (MWCNT) for the application as solid polymer electrolyte. MWCNT and GR were used as reinforcing filler and as electrical conductivity enhancement agent. Dispersions in N,N-dimethylformamide (DMF) of MWCNT and GR within the PEO matrix were prepared. DMF are featured by high electron-pair donor number and low hydrogen-bonding parameters, therefore DMF is considered a standard for liquid-phase exfoliation of MWCNT and GR. In our study, the MWCNT and GR solutions were tip sonicated using an ultrasonic processor, operated at 80% amplitude. A pulse-mode (cycle of 0.5 s) sonication was used because of the system relaxation role for the off phase, allowing a higher cavitation intensity and lower heat generation to be reached. Subsequent heat pressing was applied to obtain thin solid PEO composite electrolytes. Analyses of the experimental and theoretical density of prepared solid PEO composite electrolytes are calculated and discussed. GR and MWCNT functionalization effect on void content of polymer composites is evaluated. FTIR analysis was carried out to further investigate the effect of fillers content. The SEM results showed that surface of electrolyte film became rougher after the addition of MWCNT and GR. It is concluded, that the higher is filler fraction, the lower is void content and greater is composite density.

Use of Hydrodynamic Cavitation for Algae Removal: Effect on the Inactivation of Microalgae Belonging to Genus Scenedesmus

Excessive algae growth has generated conflicts on the use of water supplies; therefore, the focus on new technologies to remove algae from water bodies is demanding. The aim of the present study was to assess the effect of hydrodynamic cavitation on the inactivation of microalgae belonging to genus Scenedesmus. A laboratory-scale experimental apparatus was built in order to accomplish this goal; it consisted of a Venturi device designed to generate the cavitation phenomenon. Suspended microalgae samples were treated for 60 minutes under different cavitation intensities (cavitation number—Cv—ranging from 0.17 to 0.27). Results evidenced that microalgae decay over time can be modeled through first-order kinetics. The maximum removal efficiency (85%) was recorded at the highest cavitation intensity (Cv = 0.17). The removal efficiency decreased as the cavitation number increased. Hydrodynamic cavitation was effective in inactivating Scenedesmus; it produced irreversible damages to cell morphology such as flotation spines removal, cell wall lesions, cytoplasm extravasation, and cavity formation. Assumingly, hydrodynamic cavitation has great potential to treat eutrophic water bodies. Furthermore, it represents a sustainable removal technique, since it does not produce secondary pollution.

The influence of cavitation on the flow characteristics of liquid nitrogen through spray nozzles: A CFD study

Spray cooling with cryogen could achieve lower temperature level than refrigerant spray. The internal flow conditions within spray nozzles have crucial impacts on the mass flow rate, particle size, spray angle and spray penetration, thereby influencing the cooling performance. In this paper, CFD simulations based on mixture model are performed to study the cavitating flow of liquid nitrogen in spray nozzles. The cavitation model is verified using the experimental results of liquid nitrogen flow over hydrofoil. The numerical models of spray nozzle are validated against the experimental data of the mass flow rate of liquid nitrogen flow through different types of nozzles including the pressure swirl nozzle and the simple convergent nozzle. The numerical studies are performed under a wide range of pressure difference and inflow temperature, and the vapor volume fraction distribution, outlet vapor quality, mass flow rate and discharge coefficient are obtained. The results show that the outlet diameter, the pressure difference, and the inflow temperature significantly influence the mass flow rate of spray nozzles. The increase of the inflow temperature leads to higher saturation pressure, higher cavitation intensity, and more vapor at nozzle outlet, which can significantly reduce mass flow rate. While the discharge coefficient is mainly determined by the inflow temperature and has little dependence on the pressure difference and outlet diameter. Based on the numerical results, correlations of discharge coefficient are proposed for pressure swirl nozzle and simple convergent nozzles, respectively, and the deviation is less than 20% for 93% of data.

Numerical analysis of flow characteristics in diesel injector nozzles with convergent-divergent orifices

The geometry of diesel injector nozzles is known to significantly affect the characteristic spray behavior and emissions formation. In this paper, a novel nozzle concept, consisting of orifices with a convergent–divergent shape, is investigated through Computational Fluid Dynamics techniques. Three of these nozzles, characterized by different degrees of conicity, are compared to a nozzle with cylindrical orifices, which acts as a baseline. A homogeneous equilibrium model, validated against experimental data in previous works by the authors, is used to calculate the eventual cavitation formation inside these orifices. Additionally, the characteristics of the flow at the orifice outlet are analyzed for the four aforementioned nozzles in terms of their steady-state mass flow, effective outlet velocity and area coefficient. The results show that convergent-divergent nozzles exhibit a high cavitation intensity, located in the transition between the convergent and the divergent sections. This high cavitation intensity tends to compensate for the expected velocity decrease induced by the divergent shape, producing effective velocity values similar to those achieved by the cylindrical nozzle in many of the simulated conditions. The characteristics of the flow, together with the higher spray opening angles expected due to the divergent section of the nozzle, may improve atomization and fuel-air mixing processes.

Fundamental studies on cavitation melt processing

The application of ultrasound to industrial casting processes has attracted research interest during the last 50 years. However, the transfer and scale-up of this advanced and promising technology to industry has been hindered by difficulties in treating large volumes of liquid metal due to the lack of understanding of certain fundamentals. In the current study experimental results on ultrasonic processing in deionised water and in liquid aluminium (Al) are reported. Cavitation activity was determined in both liquid environments and acoustic pressures were successfully measured using an advanced high-temperature cavitometer sensor. Results showed that highest cavitation intensity in the liquid bulk is achieved at lower amplitudes of the sonotrode tip than the maximum available, suggesting nonlinearity in energy transfer to the liquid, while the location of the sonotrode is seen to substantially affect cavitation activity within the liquid. Estimation of real-time acoustic pressures distributed inside a crucible with liquid Al was performed for the first time.

Investigation of the factors influencing cavitation intensity during the ultrasonic treatment of molten aluminium

The application of ultrasound to casting processes is a subject of great interest: the resulting degassing, sonocrystallization, wetting, fragmentation, de-agglomeration and dispersion yield an improved cast material with fine grain structure. However, due to the lack of understanding of certain fundamentals involved in the process, the transfer and scale-up of this promising technology to industry has been hindered by difficulties in treating large volumes of liquid metal. Experimental results of ultrasonic processing of liquid aluminium with a 5-kW magnetostrictive transducer and a 20-mm niobium sonotrode producing 17-kHz ultrasonic waves are reported in this study. A high-temperature cavitometer sensor that is placed at different locations in the liquid melt, measured cavitation activity at various acoustic power levels and in different temperature ranges. The highest cavitation intensity in the liquid bulk is achieved below the surface of the sonotrode, at the lowest temperature, and when the applied power was 3.5kW. Understanding these ultrasonication mechanisms in liquid metals will result in a major breakthrough for the optimization of ultrasound applications in metal industries.

Effect of Input Power and Temperature on the Cavitation Intensity During the Ultrasonic Treatment of Molten Aluminium

Experimental results of ultrasonic processing of liquid aluminium with a 5 kW magnetostrictive transducer and a 20 mm titanium sonotrode excited at 17 kHz are reported in this study. A unique high-temperature cavitometer sensor, placed at various locations in the liquid melt, measured cavitation activity at various acoustic power levels and different temperature ranges. The highest cavitation intensity in the liquid bulk is achieved below the surface of the sonotrode, at the lowest temperature and with an applied power of 3.5 kW. This two-fold mechanism is related to (a) acoustic shielding and (b) the tendency of liquid aluminium to release hydrogen when the temperature drops, thus promoting multiple cavitation events. Understanding these mechanisms in liquid metals can result in a major breakthrough for the optimization of ultrasound applications to liquid metal processing.

Chemical support of cavitation intensity in ultrasonic cleaning

In processes of ultrasonic cleaning in aqueous alkaline solutions, due to the intensity of the chemical reactions occurring during cleaning, it is advisable to use high temperatures. But the intensity of cavitation, being the main causative agent of cleaning in these liquids, reduces with increasing temperature. Moreover, a great reduction in cavitation intensity is observed when a liquid is heated compared with its intensity when the same liquid is cooling. This phenomenon disturbs the process of cleaning when the temperature is kept constant. It is also known that a high cavitation intensity in water may be sustained by adding a substance that fixes the oxygen dissolved in the water. In the present work, a method of stabilising cavitation intensity is proposed by adding such a substance, i.e. sodium sulphite, to aqueous alkaline solutions used in ultrasonic cleaning.

Phase diagrams for dual frequency sonic processors using organic liquid medium

This paper presents phase diagrams for the cavitation bubble dynamics in a dual frequency ultrasonic processor employing organic medium. These phase diagrams depict spatial dispersal of the cavitation intensity (as indicated by peak temperature and pressure reached in the bubble at transient collapse) as a function of two design parameters, viz. ratio of frequencies of two ultrasound waves and phase difference between the waves. n-hexane has been chosen as model organic solvent with cavitation bubbles made of argon. The choice of the design parameters for simulations has been made on the basis of two criteria, viz. maximum total energy dissipation and most uniform energy dissipation in the processor. Simulations of cavitation bubble dynamics have been made for harmonic and ultra-harmonic coupling of two ultrasound waves with base frequency of 20kHz and coupling frequency ratio in the range of 1.5–5. The range of phase difference between waves is 0°–270°. The phase diagrams clearly indicate the regions of constructive and destructive interference between the waves. For similar maxima and minima of acoustic pressure amplitudes, higher cavitation intensity was seen for lower frequency ratios. Moreover, in the phase diagrams, the span of parameter space of constructive interference was higher for ultra-harmonic coupling. However, due to higher vapor pressure and lower density of n-hexane, the peak temperatures attained in the bubble, even for the conditions of maximum constructive interference between two ultrasound waves, were relatively lower than those observed for water. Thus, merits of dual frequency ultrasound field for augmentation of cavitation intensity are not so marked for organic medium. Despite this limitation, the phase diagrams presented in this paper could form a useful tool for optimization of the dual frequency ultrasound processor.

Prediction of Cavitation Intensity and Erosion Area in Centrifugal Pump by Using Cavitating Flow Simulation with Bubble Flow Model

We developed a numerical simulation code and a method of estimation for predicting the cavitation erosion in pumps. Cavitation erosion is closely related to cavitation intensity based on bubble dynamics. A “bubble flow model” simulates detailed bubble behavior in a cavitating flow. Cavitation intensity was estimated by analyzing the bubble pressure and bubble nuclei distribution in a centrifugal pump. We simulated the impulsive bubble pressure that varied in microseconds. The impulsive pressure was considered to be related to actual bubble collapse, which caused cavitation erosion. The erosion area was experimentally detected using a paint method. The predicted high cavitation intensity area agreed well with the experimental erosion area, since the predicted and experimental areas were both located between the shroud and mid-point of the blade near the leading edge. Our code was thus effective for estimating the cavitation intensity and predicting the erosion area around the impeller of a centrifugal pump.

Prediction of Cavitation Intensity and Erosion Area in Centrifugal Pump by Using Cavitating Flow Simulation with Bubble Flow Model

We developed a numerical simulation code and an estimation method for predicting the cavitation erosion in pumps. The cavitation erosion is closely related to the cavitation intensity based on the bubble dynamics. A 'bubble flow model' simulates the detailed bubble behavior in a cavitating flow. The cavitation intensity was estimated by analyzing the bubble pressure and the bubble nuclei distribution in a centrifugal pump. We simulated impulsive bubble pressure that varied in microseconds. The impulsive pressure is considered to be related to actual bubble collapse, which causes the cavitation erosion. The erosion area was experimentally detected using a paint method. The predicted high cavitation intensity area agreed well with the experimental erosion area, since the predicted and experimental areas were both located between the shroud and mid-point of the blade near the leading edge. Our code is thus effective for estimating the cavitation intensity and predicting the erosion area around the impeller of a centrifugal pump.

Behaviors of oxide film at the ultrasonic aided interaction interface of Zn–Al alloy and Al 2O 3p/6061Al composites in air

Ultrasonic aided interactions between Zn–Al alloy and Al 2O 3p/6061Al composites were investigated. Ultrasonic vibration imposed on the composite plate can cause high cavitation intensity in the liquid Zn–Al alloy which disrupts and flakes off surface oxides, thereby allowing the Zn–Al alloy to wet the bare composite surfaces and form a metallurgical bond. Whether the undermining phenomena occurred or not, the spreading of Zn–Al liquid over the composite did not occur until the ultrasonic vibration amplitude reached 10 μm. The Zn–Al liquid spread quickly over the composite despite that the surface oxide had not been fully disrupted when the ultrasonic vibration higher than 10 μm was applied. When undermining phenomena occurred during interaction, the oxide layer at the interaction interface was firstly lifted up by the undermining alloy, suspending in the Zn–Al liquid, and subsequently broke up by the ultrasonic excitation. Gas escaping from the base metal was observed to be helpful in decohesion of the oxide layer from the composite. When undermining phenomena did not occur during interaction, the removal of the oxide layer was based upon the melting of the nether base metal. The disruption of oxide initiated from the composite surface where the drop center located and the surface oxide layer near the drop edge was lastly removed.