Air Holdup Research Articles

Numerical study on the mechanism of microplastic separation from water by cyclonic air flotation

Microplastics have attracted considerable attention as emerging contaminants that threaten water bodies. The removal of microplastics from a mini-hydrocyclone, enhanced by air flotation, was studied numerically. The three-phase flow was modeled using the Eulerian-Eulerian model coupled with interphase interactions. The characteristics of the flow field and distribution of microplastics and microbubbles were discussed, and the mechanism of cyclonic air flotation separation was analyzed. It was found that injecting microbubbles accelerated the axial migration of microplastics and moved the enriched area upward toward the overflow. The coalescence rate of the bubbles near the axis was higher than their breakage rate, which led to the formation of an air core. The length and diameter of the air core increased with the inlet gas holdup. When the air core size closely matched the overflow, the constrained flow channel prevented the discharge of microplastics. The optimal air holdup must be determined to ensure the efficiency of the cyclonic air flotation process. The sizes of the microbubbles used for cyclonic air flotation should be comparable to those of the separated microplastics. The upper cone angle significantly promoted the migration of microplastics to the axis. This study was conducted to purify microplastic-containing wastewater using an environmentally friendly and energy-efficient technique and to provide a theoretical basis and practical reference for applying microplastic separation technology in water.

Hold-up formation in bubble channel reactors: A bubble-scale investigation

Experiments were carried out to measure the air volume fraction and the hold-up in a bubble channel reactor fed by porous pipes.A new processing method was proposed for the signal acquired by means of a resistivity probe for the air volume fraction. This method avoids the loss of the bubble residence time on the probe tip, which affects the most used signal processing procedures. The probe measurements were compared with other studies regarding bubble curtains.The measured air hold-up was in agreement with correlations available in the literature and formulated for bubble columns fed by a porous sparger. A bubble-scale model was proposed to explain the functional form of the dependence of the hold-up on the air superficial velocity. Moreover, a link between the hold-up and the Koide equation for bubble dimensions was established.The experiments were reproduced by means of CFD simulations. The air volume fraction measurement method and the hold-up model found a confirmation in the numerical results. Moreover, the latter showed how the bubble plumes interact inside the reactor and the consequences on the air hold-up.

Design of a Robust MPC for Copper Recovery in an Industrial Flotation Column

This paper presents the design of a RMPC (Robust Model Predictive Controller) for effective control of the copper recovery in an industrial flotation column. Copper ore properties vary frequently causing uncertainties and disturbances in the dynamics of the recovery process in the flotation column. Using the system identification tools, a MIMO model of this plant was achieved, whose validation results exhibited adequacy degrees of 88.21% for froth depth, and 87.32% for air holdup. The design of the RMPC was carried out based on the nominal MIMO model and its obtained uncertainties. It was demonstrated that the designed RMPC makes it possible to control the process with high precision under various industrial operation scenarios. The simulation results of the control system designed with the RMPC and the GPC (Generalized Predictive Controller) revealed an improved performance when using the RMPC.

Microbubbles, oscillating flow, and mass transfer coefficients in air-water bubble columns

Mass transfer coefficient kLa determines the flux of dissolved oxygen DO in bubble columns. The kLa values can be determined experimentally or predicted using a correlation. Common correlations link kLa with superficial air velocity, air hold-up volume, bubble contact time, bubble rising velocity, or bubble Sauter diameter.We compared experimentally found and predicted kLa values to evidence that none of the above correlations is applicable for microbubbles flow. The only way to accurately predict the flow is by using mass transport models needed to calculate kL. The transition from kL to kLa requires a redefinition of the specific interfacial area a for microbubbles.Microbubbles distribute DO quicker than macrobubbles. Oscillating flow can raise the DO level to saturation. The generation of oscillating flow does not require extra energy turning the subsurface aeration more energy-efficient than the surface aeration.

A high-efficiency mini-hydrocyclone for microplastic separation from water via air flotation

A mini-hydrocyclone was designed to remove microplastics from water through air flotation. The hydrocyclone geometry parameters were optimized through the combination of the orthogonal design method and numerical simulation. The performance of the optimized mini-hydrocyclone was experimentally evaluated. The experimental and numerical underflow pressure drop results showed a consistent trend in the considered flow range. The separation efficiency increased with increasing inlet flow rate, but increased, and then decreased, with the flow split ratio. The separation performance of microplastic in the entire size range could be improved through the injection of air bubbles into the water. The separation efficiency enhancement ranged from 5 % to 15 % within a split ratio range of 0.04 to 0.23. The separation efficiency increased rapidly with a low flow rate of air injection and remained high (> 92 %) with a further increase in the air flow rate. The concentration ratios of the hydrocyclone were all >7.8 owing to low flow split ratios. The maximum concentration ratio reached 9.7 at a feed flow rate of 0.45 m3/h, a split ratio of 0.1 and an air holdup of 5.4 %. The results demonstrated that the designed hydrocyclone with injected microbubbles can achieve high separation efficiency and concentration rate at the same time. The air flotation-induced increase in grade efficiency was mainly due to the density reduction effect of MP–bubble agglomerates, and the volume expansion effect was relatively small. The cyclonic air flotation technology can be applied to small-scale water treatment, such as separating microplastics in a washing machine.

Numerical Investigation on a Liquid–Gas Ejector for Carbon Dioxide Removal Using Amine Solution: Hydrodynamics and Mass Transfer Evaluation

The present study investigates the hydrodynamics and mass transfer of the liquid–gas ejector using three-dimensional (air–water) and two-dimensional (CO2/air-MEA (Monoethanolamine) solution) computational fluid dynamics (CFD) modeling. For 3D simulation, validation of the CFD results of this ejector with experimental data (error less than 5%) showed high simulation accuracy. The effects of motive liquid flow rate and outlet pressure parameters on the air entrainment rate and air hold-up are also investigated. It was found that by increasing the outlet pressure by about 70% (from 3587 to 6127 Pag), the rate of gas entrainment and gas hold up decreased by about 37% and 20%, respectively. On the contrary, these parameters showed increasing behavior of about 74% and 15%, respectively, when the mass flow rate of liquid increased by about 21%. In addition, three-dimensional phenomena such as mixing shock and the location of its occurrence are examined, which is the reason for recirculation and vortex in the ejector. Next, by simulating a two-dimensional simulation and changing the inlet fluids to CO2/air-methanol amine, the ejector was designed to simultaneously increase the gas pressure and absorb carbon dioxide. A user-defined function code was used to express the mass transfer from the gas to the liquid phase. The results, in this case, showed that with increasing the outlet pressure of the ejector (from 0 to 2000 Pag), and enhancing the concentration of MEA solution (from 10% to 30%), the CO2 removal boosted from 83% to 95%. A similar behavior was shown when the L/G ratio increased from 3.5 to 5.5. This study serves as a showcase on how to do an exact design and analysis for liquid–gas ejectors in flare gas recovery systems.

A new approach to measure gas holdup in industrial flotation Machines. Part III: Industrial prototype design and assessment

• An industrial gas holdup sensor prototype was validated in a 250 m 3 flotation cell at Los Pelambres. • A weakly dependence of sensoŕs discharge coefficient to Reynolds number was observed. • Gas holdup predictions were largely insensitive to errors in the anticipated pulp kinematic viscosity. • Submersible sensor provided robust accurate gas holdup measurement over long periods of time. An industrial gas holdup sensor was designed, constructed, and its performance assessed on a 250 m 3 self-aerated Wemco cell at Los Pelambres concentrator. The submersible sensor comprised a ceramic-liner IP68 magnetic flowmeter assembled to a stainless steel gas exclusion cell, which resembles an inverted truncated cone. The gas exclusion cell was designed to avoid low-velocity sections where particles could settle. When the sensor device is vertically immersed in an aerated pulp a continuous downward flow of pulp without bubbles is induced through it. A remotely installed processing unit converts the flow velocity signal, provided by the magnetic meter, into a gas holdup signal, by applying Bernoullís equation to the induced flow. The ideal velocity in the Bernoullís equation is obtained by multiplying the measured velocity and the discharge coefficient. The discharge coefficient was found to be fairly constant over a wide range of Reynolds numbers. Gas holdup measurements were compared against those obtained by sampling a fixed volume of aerated pulp using the JK Tech Air Holdup Probe. These tests were conducted on different days over nine months to allow for the pulp properties to change. Gas holdup changes in the flotation machine were achieved by restricting the air intake to the cell by placing a stainless steel cap on top of the air suction pipe. Short and long-term testing demonstrated the sensoŕs capability to provide reliable and accurate continuous-time gas holdup measurements for industrial flotation machines.

A sustainable approach for removal of microplastics from water matrix using Colloidal Gas Aphrons: New insights on flotation potential and interfacial mechanism

Microplastics (<5 mm) are becoming one of the most emerging contaminants found in the environment. This study proposes an eco-friendly generation of novel Colloidal Gas Aphrons (CGA) covered flotation technique to remove the microplastics (MPs) from different water matrix. Sapindus mukorossi soap nuts were used to prepare the surfactant solution. Polyethylene terephthalate (PET), polystyrene (PS), polypropylene (PP) and polyethylene (PE) MPs were taken as target pollutants. The efficiency of prepared CGA was tested in deionized water (DW), seawater (SW), river water (SW), wastewater (WW) and leachate (LE). The CGA generation, air holdup, half-life, critical-micelle-concentration, stability, kinetic study, and size of CGA microbubbles were studied effectively. The impact of MPs dosages, zeta potential and dissolved organic carbon (DOC) was also examined in a real water matrix. The electrostatic attraction played a major role in removing the target MPs from DW, RW, WW and LE samples whereas ion concentration could be attributed for the removal of MPs in SW. From the experimental results, it can be concluded that the CGA could function as an active agent in removing MPs from the water matrix successfully.

Impact of salinity on colloidal ozone aphrons in removing phenanthrene from sediments

Polycyclic aromatic hydrocarbons (PAHs) tend to adsorb and accumulate on sediments owing to their hydrophobicity and persistence. Salinity is the predominant factor determining the PAH partition between aqueous and solid phases in freshwater, estuaries and seawater. This study focuses on the impact of salinity on the phenanthrene (PHE) removal from sediments using an in situ and targeted remediation technology – colloidal ozone aphrons (COAs). The ozone-encapsulated colloidal aphrons exhibited increasing air holdup but decreasing stability with the salinity increasing from 0.5‰ to 35‰. The hydrophobic attraction between Tween-20-coated bubbles and the hydrophobic solid surface weakened at high salinities. The presence of inorganic ions in the aqueous phase could lead to the salting-out of nonionic compounds (PHE, Tween-20 and even ozone), hindering detaching and degrading PHE from the solid phase. Anyhow, COAs achieved high efficiencies of washing (88.0–90.2%) and oxidative degradation (74.0–76.5%) particularly for the hydrophobic sediments with highly concentrated PHE (200.4 μg/kg) over the investigated salinities. The flushing effect imposed by the bubble flow played an important role, which was not greatly influenced by salinity. Although the dissolved natural organic matter competed with PHE for COAs and led to low PHE removal, the efficiency was improved by successive COA addition.

A comprehensive hydrodynamic analysis of a full-scale oxidation ditch using Population Balance Modelling in CFD simulation

This work exhibits the importance of the experimental validation when full-scale computational fluid dynamics (CFD) models are developed to provide a detailed analysis of the spatial variations in 3D of the fluid flow inside aerated tanks. Single-phase and two-phase CFD models were performed to study the fluid behaviour carefully by means of the velocity profiles and the aeration pattern in a full-scale oxidation ditch. Air hold-up, bubble size distribution and interfacial area density were calculated by polydisperse models where Population Balance Model (PBM) was governed by break-up and coalescence; the free-surface approach allowed the CFD model to describe the three-dimensional effect of bubbly plumes in large scales in detail. Tracer tests were carried out to obtain the flow pattern and the hydraulic distribution of the flow into two wastewater treatment lanes in order to define the boundary conditions for the model correctly. Despite the difficulty of performing velocity measurements of the fluid in 3D, with and without air bubbles, these provided essential information to validate the CFD model. From this analysis, several simulations were performed to improve the hydrodynamics and the operation of the process by relocating the propellers.

Ozone-encapsulated colloidal gas aphrons for in situ and targeting remediation of phenanthrene-contaminated sediment-aquifer

The hydrophobic polycyclic aromatic hydrocarbons (PAHs) are apt to adhere tightly to the sediments in aquifer and thus pose great threats to the aquatic environment of groundwater and surface water as well as human health. The present study constructed functionalized microbubbles, named colloidal ozone aphrons (COAs), by dissolving ozone-contained air into the nonionic surfactant (Tween-20) solution at the pressure of 300 kPa for the in situ remediation of phenanthrene (PHE)-contaminated sediments. The COA system aimed at improving the PHE elimination in terms of (i) enhancing the migration and transportation ability of the bubble system in the contaminated aquifer matrix, (ii) accurately desorbing the target hydrophobic contaminants from sediments, and (iii) reinforcing the in situ oxidation degradation immediately after or simultaneously when the PAHs are desorbed into the aqueous phase. Experimental results demonstrated that the COAs exhibited similar characteristics as the classical colloidal gas aphrons (CGAs), including the high stability (half-life time > 200 s), typical morphology and average bubble size (114–162 μm); higher air hold-up of COAs was achieved (i. e. > 20%) compared with the air-microbubbles (1–2%) obtained under the same generation conditions. Although the encapsulated ozone could oxidize the surfactant-layers, the properties and behaviors of COAs were not greatly affected. The surfactant multi-layers endowed the COAs with strong hydrophobic attraction with PHE, great migration capacity and enlarged oxidation area in the sediment matrix. Approximately 96.9% of PHE was removed from the sediments and 84.9% of the overall PHE was oxidized at the high ozone concentration of 0.6 mg/L when the initial PHE concentration was 240.0 μg/kg. The COA-involved remediation technology provided the insight of combining the processes of washing and oxidizing through adopting the particularly conceived microbubbles. The in situ and selective removal of hydrophobic organic contaminants from sediments in aquifer was well achieved in this study.

Experience coating bubbles with solvent in a downcomer

A downcomer, as employed in the Jameson Cell, was considered for producing solvent-coated bubbles for possible application in a technique referred to as “air-assisted solvent extraction” (AASX). It was found that the bubble bed in the downcomer started to collapse upon introducing solvent, evident from decreasing air holdup, decreasing aspirated air rate, and formation of air slugs. Neither changing solvent introduction method, solvent composition nor addition of frother or salt alleviated the collapse, which could become complete (air rate became zero) at about 900 ppm solvent. To explain, the bubble bed, which in the downcomer can approach 60% air holdup (Atkinson et al., 1993; Jameson and Manlapig, 1991), was likened to a foam that is known to collapse in the presence of hydrophobic liquid (oil) droplets due to a bridging/spreading mechanism that expels water between contacting bubbles. Tests with hydrophobic solids, graphite and talc, gave increased bubble bed stability showing it is the liquid nature of the solvent that is the key factor causing the collapse. One combination, talc and MIBC, did provide stability against solvent addition. Froth collapse/stability mechanisms are discussed. It is noted that the Jameson Cell is successfully employed to remove solvent droplets in SX plants. The solvent levels in those cases appear to be less than 200 ppm where collapse is only partial and the phenomenon may not be noticed.

Nanobubbles generation in a high-rate hydrodynamic cavitation tube

Gas dispersion parameters (air holdup-ɛg, superficial air velocity-Jg and bubble surface area flux-Sb) and especially the concentration of formed nanobubbles in a hydrodynamic cavitation tube were measured. Best results were obtained at 30% air/liquid volume ratio; 49 mN m−1 air/liquid interfacial tension resulting in an air holdup of 16%, a Jg of 0.87 cm s−1, a Sb of 85 s−1 and a nanobubbles (230–280 nm) concentration of 6.4 × 108 NBs mL−1. The lower the air/liquid interfacial tension, the higher the air holdup; this proceed by assisting cavitation and formation of micro and nano-sized bubbles. Data obtained are discussed in terms of solution, hydrodynamics and interfacial phenomena. Main mechanisms involving the role of the nanobubbles and their interactions with solids and bigger bubbles were envisaged. It is believed that these findings are important because of the need for techniques and devices producing nanobubbles at low cost and high rate. This work shows that the cavitation tube studied has a high potential, due to the wide size of bubbles formed, high bubble surface flux obtained and especially the generation of a high concentration of nanobubbles. These bubbles are very important in modern flotation of mineral fines and in pollutant separation in wastewater treatment and some examples are shown and discussed.

Nanobubbles: Generation using a multiphase pump, properties and features in flotation

The importance of nanobubbles is now widely known, particularly their applications and potential in mineral processing. The actual challenge is to generate a high concentration of bubbles (number per water volume) in a sustainable manner at high flow rates. The main objective of this work was to develop a new method for generating highly-loaded nanobubbles aqueous solutions by hydrodynamic cavitation using a centrifugal multiphase pump (CMP) and a needle valve. Nanobubbles (150–200nm) were formed, at 22°C, with the pump and a recycle column, at various operating pressures and air/liquid surface tension. Nanobubbles were resistant to shearing caused by pump impellers and to high operating pressures (up to 5bar) throughout several bubble generation cycles. The size of the nanobubbles remained constant, and their numeric concentration increased as a function of these cycles, reaching equilibrium after 29 cycles; this was dependent on pump pressure and the surface tension of the solution. The highest concentration (4×109nanobubblesmL−1) was obtained at 5bar and 49mNm−1 surface tension (air holdup=6.8% and D32 of microbubbles in the range between 62 and 70µm). These phenomena can be explained by Henry's Law and the lower energy required for bubble formation when the interfacial tension (air/water) decreases and when the differential pressure in the cavitation zone increases. The mean diameter and concentration of these nanobubbles did not vary significantly over a period of two months, demonstrating the high stability of these concentrated nanobubbles. It is concluded that the procedure has great potential in future applications in ore flotation and wastewater treatment and reuse.

Effect of Surfactant on Bubble Size and Air Holdup on Column Flotation

Flotation is a complex process in which is present physicochemical and hydrodynamic phenomena. The flotation performance is related to the probability of bubble-particle collisions and stability of aggregate formed. The collision efficiency is a function of particle and bubble diameters. Currently one of the biggest challenges of the mining industry is the flotation of fine and ultrafine particles and a possible solution to increase the recovery of these fine particles is the use of bubbles with intermediate size (100-1000μm). Therefore, determining and controlling the bubble size is very important for further recoveries in the flotation of fine particles. It is known that the bubble size and air holdup are influenced by variations on the superficial gas velocity and by addition of surfactants. Thus this work aimed to study the effect of adding surfactants on bubbles formation. The results showed that the addition of surfactant was a good alternative to decrease bubble size and increase the air holdup in a bubble column. Three different surfactants had similar behavior on bubble size and holdup. It was possible to define the range of concentration values which are sufficient for forming bubbles with intermediate size and holdup within the range recommended for mineral flotation.

Raw water clarification by flotation with microbubbles and nanobubbles generated with a multiphase pump.

Raw water clarification by flotation was studied by injecting air into a centrifugal multiphase pump to generate microbubbles (MBs) and nanobubbles (NBs). Measurements of gas dispersion parameters were performed and optimal conditions were obtained using a pump pressure of 4 bar. Values showed a bubble Sauter diameter of 75 μm, an air holdup of 1.2%, a bubble surface area flux of 34 s-1 and an NB concentration of 1 × 108 NBs mL-1 (measuring 220 nm). Then, a study compared flotation with bubbles formed with the multiphase pump (F-MP) to lamellar settling at the clarification stage of a water treatment plant (WTP), in Brazil. The F-MP showed a higher separation efficiency at high hydraulic loads (9-15 m h-1), even without the use of a polymer, reaching 2 NTU (10-25 NTU raw water feed), which was much lower than the technical goal of the WTP (5 NTU). The results and the technical aspects are discussed, and it is concluded that the employment of MBs and NBs with pumps widens new research lines and applications in modern flotation.

A study on bubble formation and its relation with the performance of apatite flotation

Abstract The flotation performance is associated with the probability of bubble–particle collision and the aggregate stability. Currently, one of the major challenges of the mining industry is the flotation of fine particles and the generation of medium-sized bubbles (100–1000 μm) can be a good alternative to increase the recovery of these particles. Therefore, new strategies to determine and control bubble sizes, to achieve the desired diameters are very important. This study aimed to characterize and control the bubble size distribution and to consider its relation with the flotation performance. Two different methods (offline and online) were used to measure the bubble size distribution. The results showed that the addition of surfactant was a good alternative to generate bubbles with high percentages of medium-sized bubbles (80–90%) and high air hold-up values (10–12%), simultaneously. Kinetic test showed a direct relation between the bubble size distribution and the P2O5 content.

Factors affecting the floto-elutriation process efficiency of a copper sulfide mineral

Coarse mineral particles exhibit poor conventional flotation efficiency because of many factors, including the low carrying capacity of bubbles, bubble/particle adhesion problems due to cell turbulence, and low degrees of liberation (low hydrophobicity). Many attempts to improve the recovery of coarse fractions have been explored, such as floto-elutriation operating at a high solid content while dispersed in a fluidized (or expanded) bed formed with a continuous injection of compressed air and an uprising water flow. This work analyzed the comparative performances of floto-elutriation (FE) and conventional flotation (CF) on a classified copper sulfide mineral feed as an example of a difficult-to-liberate low-grade ore. Contrary to expectations, CF and FE (Hydrofloat) displayed similar particle recovery rates with feed size distributions for P80s of 130, 240 and 280μm. However, metallurgical recoveries from classified fractions of −297+210μm were 25% higher in FE than in CF and as expected, coarse (+297μm) particles were not recovered in the CF, but in the FE. The recovery of fine fractions in the FE process was due to high hydraulic entrainment and surprisingly the recovery of intermediate and liberated fractions (+74−149μm) was very low, due to its low air hold-up. However, the enhancement of the holdup in FE increased the recovery of these mid-sized fractions. Because of the hydraulic carryover caused by the bubbles and water elutriation, the metallurgical grades obtained in all cases were very low compared to conventional bench flotation. It is believed that this FE equipment works better with coarse, narrowly classified particles and high-grade feeds and that performance decreases for low-grade ores requiring high liberation. Certain features of these findings are visualized.

Numerical Simulations of Two-Phase Flow in a Self-Aerated Flotation Machine and Kinetics Modeling

A new boundary condition treatment has been devised for two-phase flow numerical simulations in a self-aerated minerals flotation machine and applied to a Wemco 0.8 m3 pilot cell. Airflow rate is not specified a priori but is predicted by the simulations as well as power consumption. Time-dependent simulations of two-phase flow in flotation machines are essential to understanding flow behavior and physics in self-aerated machines such as the Wemco machines. In this paper, simulations have been conducted for three different uniform bubble sizes (db = 0.5, 0.7 and 1.0 mm) to study the effects of bubble size on air holdup and hydrodynamics in Wemco pilot cells. Moreover, a computational fluid dynamics (CFD)-based flotation model has been developed to predict the pulp recovery rate of minerals from a flotation cell for different bubble sizes, different particle sizes and particle size distribution. The model uses a first-order rate equation, where models for probabilities of collision, adhesion and stabilization and collisions frequency estimated by Zaitchik-2010 model are used for the calculation of rate constant. Spatial distributions of dissipation rate and air volume fraction (also called void fraction) determined by the two-phase simulations are the input for the flotation kinetics model. The average pulp recovery rate has been calculated locally for different uniform bubble and particle diameters. The CFD-based flotation kinetics model is also used to predict pulp recovery rate in the presence of particle size distribution. Particle number density pdf and the data generated for single particle size are used to compute the recovery rate for a specific mean particle diameter. Our computational model gives a figure of merit for the recovery rate of a flotation machine, and as such can be used to assess incremental design improvements as well as design of new machines.

Analytical and experimental investigations of the pulsed air–water jet

This paper describes a new way of generating pulsed air–water jet by entraining and mixing air into the cavity of a pulsed water jet nozzle. Based on the theory of hydro-acoustics and fluid dynamics, a theoretical model which describes the frequency characteristic of the pulsed air–water jet is outlined aimed at gaining a better understanding of this nozzle for generating pulses. The calculated result indicates that as the air hold-up increases, the jet oscillation frequency has an abrupt decrease firstly, and then reaches a minimum gradually at α (air hold-up)=0.5, finally it gets increased slightly. Furthermore, a vibration test was conducted to validate the present theoretical result. By this way, the jet oscillation frequency can be obtained by analyzing the vibration acceleration of the equal strength beam affected by the jet impinging. Thereby, it is found that the experimental result shows similar trend with the prediction of the present model. Also, the relationship between vibration acceleration and cavity length for the pulsed water jet follows a similar tendency in accord with the pulsed air–water jet, i.e. there exists a maximum for each curve and the maximum occurs at the ratio of L/d1 (the ratio of cavity length and upstream nozzle diameter) =2.5 and 2.2, respectively. In addition, experimental results on specimens impinged by the pulsed water jet and pulsed air–water jet show that the erosion depth increases slightly with air addition within a certain range of cavity length. Further, this behavior is very close to the vibration test results. As for erosion volume, the air entrained into the cavity significantly affects the material removal rate.