Cavitation Microjets Research Articles

Augmentation of pool boiling heat transfer using a microstructured aluminum surface fabricated by ultrasonic cavitation modification

Micro/nanostructure is a popular and effective structure for boiling heat transfer enhancement. In this study, ultrasonic cavitation modification is employed firstly to create microstructured aluminum surface for boiling heat transfer. With the impact of ultrasonic cavitation microjet, numerous micro pits and holes are formed on the surface in one step, constituting microstructures on aluminum plate. The saturated and subcooled pool boiling experiments are conducted at atmospheric pressure using ethanol as a working fluid. It is found that boiling hysteresis is apparent on the modified plate (MP) with the porous microstructure layer, which can be eliminated by periodically fluctuating heat flux. Under the stable saturated boiling condition, the maximum heat transfer coefficient is enhanced by 208% and wall superheat is decreased by 17.6 °C compared to the bare aluminum surface. Under subcooled boiling conditions, the boiling hysteresis on MP becomes weaker and the positive effect of microstructures on heat transfer performance is enhanced. The enhanced heat transfer coefficient of MP is mainly attributed to the increase of nucleation site density and further to the facilitation of bubble coalescence and departure, while the increased critical heat flux under subcooled boiling conditions is derived from the delaying occurrence of mushroom-like bubble.

SPH-FEM Analysis of Effect of Flow Impingement of Ultrasonic Honing Cavitation Microjet on Titanium-Tantalum Alloy Surface.

To investigate the machining effect of ultrasonic honing microjets on a titanium-tantalum alloy surface, a cavitation microjet flow impingement model was established using the smoothed particle hydrodynamics-finite element method (SPH-FEM) coupling method including the effects of wall elastic-plastic deformation, the ultrasonic field and the honing pressure field. Simulation analysis was conducted on a single impact with different initial speeds and a continuous impact at a constant initial speed. The results showed that the initial speed of the microjet needed to reach at least 580 to 610 m/s in order to obtain an obvious effect of the single impact. The single impact had almost no effect at low speeds. However, when the microjet continuously impacted the same position, obvious pits were produced via a cumulative effect. These pits were similar to that obtained by the single impact, and they had the maximum depth at the edge rather than the center. With the increase in the microjet's initial speed, the total number of shocks required to reach the same depth gradually decreases. When the number of impacts is large, with the increase in the number of impacts, the growth rate of the maximum pit depth gradually slows down, and even shows no growth or negative growth at some times. Using the continuous impacts of the microjet by prolonging the processing time can enhance titanium-tantalum alloy machining with ultrasonic honing for material removal.

Effect of ultrasonic cavitation micro-jet impact on corrosion of material in ultrasonic assisted electrochemical micromachining

Effect of ultrasonic cavitation micro-jet impact on corrosion of material in ultrasonic assisted electrochemical micromachining

Remediation of Lubricant Contaminated Soils by Cavitation Microjet Shock Wave Soil Washing System with Ozonation

ABSTRACT Adopting soil washing to clean up petroleum hydrocarbon contaminated soils with high sand content is considered as a fast and economical method. Cavitation microjet shock wave soil washing system (CMS) enhanced by the use of ozonation was developed for remediation of aged lubricant contaminated soil in this study. Contaminated soil with total petroleum hydrocarbon (TPH) ranging from 1890 mg/kg to 15735 mg/kg were applicable for CMS soil washing system. The impacts of factors including initial TPH concentration hydrocarbon (TPH) concentration, soil particle size, soil washing time, and ozone treatment time onto soil washing were investigated through a 21-month period of field operation. The contaminated soils with particle size between 0.074 mm and 0.25 mm, mainly very fine sand and find sand, demonstrated 31% to 98% of TPH removal by air-injected CMS and 19% to 92% of TPH removal by CMS with ozone addition. The optimal cavitation washing time was 20 minutes. The study indicated that optimal ozone concentration in the slurry was 1.08 mg/L which require air of 27 L/min for ozone produced. However, given the short contact time in the CMS system, ozone addition in the slurry did not improve TPH removal effectively. Considering the compatible TPH removal efficiency and energy cost of ozone addition in the soil washing system, use of air-injected CMS soil washing system may be considered as an alternative way.

Numerical study of the synergistic effect of cavitation and micro-abrasive particles

In ultrasonic-assisted machining, the synergistic effect of the cavitation effect and micro-abrasive particles plays a crucial role. Studies have focused on the investigation of the micro-abrasive particles, cavitation micro-jets, and cavitation shock waves either individually or in pairs. To investigate the synergy of shock waves and micro-jets generated by cavitation with micro-abrasive particles in ultrasonic-assisted machining, the continuous control equations of a cavitation bubble, shock wave, micro-jet, and micro-abrasive particle influenced by the dimensionless amount (R/R0), a particle size-velocity–pressure model of the micro-abrasive particle was established. The effects of ultrasonic frequency, sound pressure amplitude, and changes in particle size on micro-abrasive particle velocity and pressure were numerically simulated. At an ultrasonic frequency of 20 kHz and ultrasonic sound pressure of 0.1125 MPa, a smooth spherical SiO2 micro-abrasive particle (size = 5 µm) was obtained, with a maximum velocity of 190.3–209.4 m/s and pressure of 79.69–89.41 MPa. The results show that in the range of 5–50 μm, smaller particle sizes of the micro-abrasive particles led to greater velocity and pressure. The shock waves, micro-jets, and micro-abrasive particles were all positively affected by the dimensionless amount (R/R0) of cavitation bubble collapse, the larger the dimensionless quantity, the faster their velocity and the higher their pressure.

A new ultrasonic cleaning model for predicting the flux recovery of the UF membrane fouled with humic acid

Ultrasonic cleaning is an alternative promising method to control the membrane fouling. At present, the urgent task of further promoting its application is to establish a model to predict the instantaneous membrane flux. In this study, a new model was proposed by considering the power intensity and temperature, and the ultrasonic cleaning mechanism was revealed with ultrasonic cavitation theory and extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. Results showed that the predictions of the model were in good agreement with the experimental data (σ < 1.23%) for the polyethersulfone (PES) ultrafiltration (UF) membrane fouled with humic acid (HA) and its accuracy was higher for the polyvinylidene fluoride (PVDF) UF membrane (σ < 0.72%) or actual sewage (σ < 1.18%). Meanwhile, the HA cake resistance was removed 79.10~88.91% and the corresponding available membrane area increased 0.42–0.57 times within 1 min, and the membrane flux recovery (FR) could be reached to 91.07% within 5 min (100 W and 30 °C). In addition, the main mechanism of ultrasonic cleaning was that the kinetic energy of the ultrasonic cavitation microjet (6.28 ×10−10 J) was much bigger than the interaction energy between the HA foulants and the membrane surface (7.92 ×10−13 J).

Numerical study on the desorption processes of oil droplets inside oil-contaminated sand under cavitation micro-jets

The removal of the adsorbed oil droplet is critical to deoiling treatment of oil-bearing solid waste. Ultrasonic cavitation is regarded as an extremely useful method to assist the oil droplets desorption in the deoiling treatment. In this paper, the effects of cavitation micro-jets on the oil droplets desorption were studied. The adsorbed states of oil droplets in the oil-contaminated sand were investigated using a microscope. Three representative absorbed states of the oil droplets can be summarized as: (1) the individual oil droplet adsorbed on the particle surface (2) the clustered oil droplets adsorbed on the particle surface; (3) the oil droplet adsorbed in a gap between particles. The micro-jet generation during the bubble collapse near a rigid wall under different acoustic pressure amplitudes at an ultrasonic frequency of 20 kHz was investigated numerically. The desorption processes of the oil droplets at the three representative absorbed states under micro-jets were also simulated subsequently. The results showed that the acoustic pressure has a great influence on the velocity of micro-jet, and the initial diameter of cavitation bubbles is significant for the cross-sectional area of micro-jets. The wall jet caused by a micro-jet impacting on the solid wall is the most important factor for the removal of the absorbed oil droplets. The oil droplet is broken by the jet impinging, and then it breaks away from the solid wall due to the shear force generated by the wall jet. In addition to a higher sound pressure, the cavitation bubble at a larger initial diameter is more important for the desorption of the clustered oil droplets. Conversely, the micro-jet generated by the cavitation bubble at a smaller initial diameter (0.1 mm) is more appropriate for the desorption of the oil droplet in a narrow or sharp-angled gap.

The behavior to cavitation erosion-corrosion of the CuZn39Pb3 brass structures, obtained by in-depth heat treatments

The paper highlights the behavior at the vibratory cavitation erosion of CuZn39Pb3 brass microstructures, as obtained by in-depth heat treatments. Two heat treatment cycles are analysed (hardening at 800ºC, followed by cooling in water, and hardening at 800ºC, with cooling in water, followed by tempering at 400ºC with cooling in air). The micro-hardness measurements show that the change in the microstructure, relative to the initial status of the material (semi-finished), determines higher values of the micro-hardness. The studies of the cavitation erosion, using the vibrating standard piezoelectric crystal assembly, available in the Cavitation Research Laboratory at the Polytechnic University from Timisoara, presented in the paper, shows significant increases in the surface resistance to the stress determined by the cavitation micro-jets, as compared to the initial state of the material (semi-finished).

Mechanism of ultrasonic-assisted transient liquid phase bonding of 6061 Al alloy with cladded Zn-Al alloy in air

Abstract Full α-Al solid solution of the 6061 Al alloy joint was successfully achieved using ultrasonic-assisted transient liquid phase (TLP) bonding at 450 °C. The process contained two steps, both assisted by ultrasonic vibration: 1) cladding the bonding material; 2) TLP bonding in air. The molten Zn-Al alloy, with optimized composition, was cladded on the surface of 6061 Al alloy. Ultrasonic assisted TLP bonding using the cladded layer can produce a sound joint with shear strength reaching 95 % of the base metal. The optimized joint consisted of Zn diffusion layer and full α-Al solid solution of interlayer, and the oxide film was completely removed owe to the ultrasonic vibration. The existence of Zn-Al eutectic phase and voids deteriorated the joint strength when the Al content of the interlayer was 20 wt.%. The isothermal solidification completed at 450–480 °C using interlayer of Zn-15Al. Longer ultrasonic time promoted the microstructure homogenization, while residual liquid phase in the interlayer would degrade the joint strength if ultrasonic time was deficient. The ultrasonic vibration accelerated the dissolution of Al into the liquid and Zn into the substrate because of the acoustic cavitation micro-jet and acoustic streaming effects. The isothermal solidification can be finished within a bonding time of 60 s with residual liquid squeezing out during bonding process.

Soft material perforation via double-bubble laser-induced cavitation microjets

The resulting jet of two interacting laser-induced cavitation bubbles is optimized and studied as a technique for micro-scale targeting of soft materials. High controllability of double-bubble microjets can make such configurations favorable over single bubbles for applications where risk of ablation or thermal damage should be minimized such as in soft biological structures. In this study, double-bubble jets are directed toward an agar gel-based skin phantom to explore the application of micro-scale injection and toward a soft paraffin to quantify the targeting effectiveness of double-bubble over single-bubble jetting. The sharp elongation during the double-bubble process leads to fast, focused jets reaching average magnitudes of Ujet = 87.6 ± 9.9 m/s. When directed to agar, the penetration length and injected volume increase at ∼250 μm and 5 nl per subsequent jets. Such values are achieved without the use of fabricated micro-nozzles seen in existing needle-free laser injection systems. In soft paraffin, double-bubble jetting produces the same penetration length as single-bubble jetting, but with ∼45% reduction in damage area at a 3× greater target distance. Thus, double-bubble jetting can achieve smaller impact areas and greater target distances, potentially reducing collateral thermal damage and effects of strong shockwave pressures.

Study of Cavitation Erosion Pits on 1045 Carbon Steel Surface in Corrosive Waters

Cavitation erosion resistance of steels is important in many applications. The investigation of such resistance, under different conditions, should be very useful. Cavitation erosion tests were carried out on carbon steel AISI-1045 using an ultrasonic induced cavitation facility. Cavitation erosion pits and their effect on the localized corrosion were investigated in detail in three different corrosive media: distilled water, tap water, and 3% NaCl water.The results of the investigation using SEM indicated the formation of three types of pits on cavitating specimen surfaces: corrosion pits, erosion pits, and erosion-corrosion pits. The corrosion pits have different shapes, however, the lamellar structure is the dominant structure, and has a large size of about 100 μm. The erosion pits that were formed by the cavitation microjet impacts have sizes of a few micrometers. The erosion-corrosion pits were similar to the corrosion pits, except the erosion pits formed on the corrosion pit surface due to dissolution. The eroded surface removal was the largest in the case of saline water.

A physical mechanism to explain the delivery of chemical penetration enhancers into skin during transdermal sonophoresis — Insight into the observed synergism

The synergism between low-frequency sonophoresis (LFS) and chemical penetration enhancers (CPEs), especially surfactants, in transdermal enhancement has been investigated extensively since this phenomenon was first observed over a decade ago. In spite of the identifying that the origin of this synergism is the increased penetration and subsequent dispersion of CPEs in the skin in response to LFS treatment, to date, no mechanism has been directly proposed to explain how LFS induces the observed increased transport of CPEs. In this study, we propose a plausible physical mechanism by which the transport of all CPEs is expected to have significantly increased flux into the localized-transport regions (LTRs) of LFS-treated skin. Specifically, the collapse of acoustic cavitation microjets within LTRs induces a convective flux. In addition, because amphiphilic molecules preferentially adsorb onto the gas/water interface of cavitation bubbles, amphiphiles have an additional adsorptive flux. In this sense, the cavitation bubbles effectively act as carriers for amphiphilic molecules, delivering surfactants directly into the skin when they collapse at the skin surface as cavitation microjets. The flux equations derived for CPE delivery into the LTRs and non-LTRs during LFS treatment, compared to that for untreated skin, explain why the transport of all CPEs, and to an even greater extent amphiphilic CPEs, is increased during LFS treatment. The flux model is tested with a non-amphiphilic CPE (propylene glycol) and both nonionic and ionic amphiphilic CPEs (octyl glucoside and sodium lauryl sulfate, respectively), by measuring the flux of each CPE into untreated skin and the LTRs and non-LTRs of LFS-treated skin. The resulting data shows very good agreement with the proposed flux model.

Effect of Escalating Versus Fixed Voltage Treatment on Stone Comminution and Renal Injury During Extracorporeal Shock Wave Lithotripsy: A Prospective Randomized Trial

ESWL is a minimally invasive, efficacious therapy for most renal stones. However, an optimal voltage treatment protocol ensuring effective stone comminution while minimizing tissue injury is not well established. We performed a prospective, randomized trial of the stone-free rate and renoprotective effect of an escalating vs a fixed voltage treatment strategy during ESWL. Between February 2006 and June 2008 we enrolled 45 patients undergoing ESWL for a renal stone in this institutional review board approved trial. A Dornier DoLi 50 lithotriptor was used. Patients were randomized to receive the escalating strategy of 500 shocks at 14k V, 1,000 at 16 kV and 1,000 at 18 kV or the fixed strategy of 2,500 shocks at 18 kV. Abdominal x-ray was done to determine the stone-free rate at 1 month. Voided urine was analyzed for beta2-microglobulin and microalbumin before, immediately after and 1 week after ESWL to evaluate renal damage. Median patient age was 48 years. Median stone size was 8 mm. Of patients in the escalating group 81% were stone-free vs 48% in the fixed group (p <0.03). There was a significant difference between microalbumin and beta2-microglobulin 1 week after the procedure (p = 0.046 vs 0.045). There was trend toward a difference in microalbumin and beta2-microglobulin immediately after the procedure (p = 0.17 and 0.25, respectively). This prospective, randomized study shows that an escalating voltage treatment strategy produces better stone comminution than a fixed strategy. The study suggests that there may be a protective effect against damage caused by ESWL with an escalating treatment strategy.

The role of cavitation microjets in the therapeutic applications of ultrasound

The dynamics of a gas bubble situated in a sound-irradiated liquid and near a rigid boundary was studied theoretically to get a better understanding of the role of cavitation microjets in the therapeutic applications of ultrasound (US). The boundary integral method was adopted to simulate the temporal development of the bubble shape, jet formation during bubble collapse and bubble migration. It was found that the dynamic behaviour of the jet and the migratory characteristics of the bubble depend not only on the distance between bubble and boundary but, also, on the properties of the acoustic field. For frequencies of sound fields smaller than or equal to the resonance frequency of the bubble, jet formation and bubble migration toward the boundary are the main features of the interaction. No jet formation was observed for frequencies of sound fields larger than the resonance frequency of the bubble, and the bubble kept its initial position from the boundary throughout its motion. The pressure generated by the impact of the jet developed during bubble collapse close to the boundary may result in the fragmentation of brittle objects, such as renal calculi, dental tartar or intraocular lens. (E-mail: eabrujan@yahoo.com)

Diagnosis of Oxide Films by Cavitation Micro-Jet Impact

Oxide films form readily when aluminum alloy castings are melted and/or poured. They could be primary or secondary types of oxide films. The former type is inherited from the ingot and has existed in aluminum alloy casting for a long period of time. During the filling of the mold cavity, the free unstable surface of the molten metal causes a secondary oxide film to exist on the aluminum alloy castings. The oxide films are usually rich in oxygen and are difficult to observe by optical micrographs. This paper presents a simple but powerful method to observe the shape and size of oxide films on the aluminum matrix. During ultrasonic-vibration treatment, cavitation bubbles nucleated, grew and collapsed, generating micro-jets on the surface of sample. The water micro-jets then had an impact on the oxide film initiating micro-cracks. The cracks grew or became linked together which caused fractures in the oxide film. Small or tiny oxide particles detached from the oxide film to erode the surface of the treated sample. This eroded surface would show as a foggy mark via visual or optical observation. A series of photographs were made and are shown to illustrate the cavitation erosion process of oxide film on the surface of an aluminum sample. In addition, the presented method was shown to be useful in the diagnosis of oxide films that formed on aluminum and magnesium alloys, including the ingots, casting or wrought products.

Ultrasonic Heat Transfer Enhancement Using a Horn-Type Transducer

The purpose of this study is to clarify experimentally the influence of streaming induced by ultrasonic vibration on heat transfer using a horn-type ultrasonic vibrator. A horn tip of 6 mm diameter and 60.7 kHz resonant frequency was used as the ultrasonic transducer. Heat transfer experiments for a downward-facing horizontal heating surface with ultrasonic vibration from below were carried out in a natural convection region. The acoustic jet in the water from the horn tip of the transducer regarded as a nozzle exit was induced by this transducer, and as a result, up to a ten-fold increase in heat transfer coefficient was obtained by application of 20 W in both tap water and degassed water. It was found that the mechanism of heat transfer enhancement by ultrasonic vibration in tap water can be classified into four categories. In degassed water, heat transfer enhancement is influenced not by the acoustic jet, but by small-scale perturbations by cavitation microjets.

RELAXATION OF MICRO INDENTATIONS IN CALCIUM OXALATE URINARY STONES

We define energy requirements for stone micro indentation as a quantifiable event equivalent to in vivo energy delivery and investigate the change in indentation characteristics with time. The 7 stones extracted from 7 patients were cut, embedded in resin and polished. Multiple micro indentations were performed on each stone section using a diamond Vickers micro indentor with a 500x light microscope and video system. The resulting indentations were observed by optical and scanning electron microscopy as a function of time. Organic matrix content was determined by dissolving stones in ethylenediaminetetraacetic acid solution. The energy requirement for stone indentation varies among stones (median range 43.6 to 109.9 kg/mm2) and at different locations in the same stone. Indentations relaxed by 10 to 70% during the first 2 weeks after indentation. Stones with a high organic matrix content were ductile and the phenomenon of indentation relaxation was pronounced. Brittle, low matrix stones relaxed to a lesser extent. The relaxation phenomena may have a practical implementation when considering repeat shock wave lithotripsy. A significant fraction of the energy invested in a stone which did not cause fracture or critical cracks is lost within 1 to 2 weeks after the procedure, particularly in elastic stones with a high organic matrix content. We suggest that the preferred interval for repeat shock wave lithotripsy be less than 2 weeks.

Propagation of shock waves in elastic solids caused by cavitation microjet impact. II: Application in extracorporeal shock wave lithotripsy

To better understand the mechanism of stone fragmentation during extracorporeal shock wave lithotripsy (ESWL), the model developed in Part I [P. Zhong and C.J. Chuong, J. Acoust. Soc. Am. 94, 19-28 (1993)] is applied to study cavitation microjet impingement and its resultant shock wave propagation in renal calculi. Impact pressure at the stone boundary and stress, strain at the propagating shock fronts in the stone were calculated for typical ESWL loading conditions. At the anterior surface of the stone, the jet induced compressive stress can vary from 0.82 approximately 4 times that of the water hammer pressure depending on the contact angles; whereas the jet-induced shear stress can achieve its maximum, with a magnitude of 30% approximately 54% of the water hammer pressure, near the detachment of the longitudinal (or P) wave in the solid. Comparison of model predictions with material failure strengths of renal calculi suggests that jet impact can lead to stone surface erosion by combined compressive and shear loadings at the jet impacting surface, and spalling failure by tensile forces at the distal surface of the stone. Comparing responses from four different stone types suggests that cystine is the most difficult stone to fragment in ESWL, as observed from clinical experience.

Propagation of shock waves in elastic solids caused by cavitation microjet impact. I: Theoretical formulation

To understand the physical process of the impingement of cavitation microjet and the resultant shock wave propagation in an elastic solid, a theoretical model using geometrical acoustics was developed. Shock waves induced in both the jet head (water) and the solid were analyzed during a tri-supersonic impact configuration when the contact edge between the jet head and the elastic boundary expands faster than the longitudinal wave speed in the solid. Impact pressure at the boundary was solved using continuity conditions along the boundary normal. Reflection and refraction of shock waves from a solid-water interface were also included in the model. With this model, the impact pressure at the solid boundary and the stress, strain as well as velocity discontinuities at the propagating shock fronts were calculated. A comparison with results from previous studies shows that this model provides a more complete and general solution for the jet impact problem.

Characterization of fracture toughness of renal calculi using a microindentation technique

Since the early 1980s extracorporeal shock-wave lithotripsy (ESWL) has become the primary treatment modality for urinary stone disease [1]. Renal stones are fragmented by high-energy shock waves [2] or by the impingement of cavitation microjets [3, 4] into small pieces and are passed spontaneously. Because of the differences in chemical compositions and structural features of renal calculi [5], shock wave-stone interaction during ESWL [6], and thus the efficacy of stone fragmentation, may vary significantly. Clinically, among common stone types, phosphate (struvite and calcium apatite) stones are found to be easy to fragment, whereas uric acid and brushite stones are in the middle range, and calcium oxalate monohydrate and cystine stones are the most resistant types to ESWL therapy [7-9]. To improve the efficacy of ESWL treatment, it is desirable to identify the physical properties of renal calculi that can offer direct correlation with their fragilities during ESWL and thus can be used to guide treatment procedures for more effective stone fragmentation. The fracture toughness of renal calculi offers a direct measurement of stone resistance to fracture failure during ESWL. In this letter we report our results for the fracture toughness of renal calculi of various compositions. A microindentation technique was used to determine stone hardness. An ultrasound transmission technique was used to measure both longitudinal and transverse wave speeds, from which the Young's and shear moduli of stone materials were derived. Based on these measurements and the lengths of crack lines produced by the hardness indentor, the fracture toughness of renal calculi were calculated and compared with stone fragility obtained from previous studies [7-9]. Implications of the measurement results on an improved understanding of the mechanism of stone fragility and on enhancing the effectiveness of stone fragmentation during ESWL are discussed. Six stone specimens with crystal compositions determined by crystallographic analysis (Urolithiasis Laboratory, Baylor College of Medicine, Houston, Texas, USA) were used. The densities of the specimens were determined using a pycnometer (2 ml in capacity) based on ~ Archimedes' principle [6]. Stone specimens were mounted in a self-curing cold-mounting material (Koldmount, VernonBenshoff Co., New York, USA), polished and prepared for microhardness measurement as described in [10]. Ten indentations were made in homogeneous crystalline regions of each specimen using a Vickers indentor at a load of 50 g with a dwell time of 15 s. Since renal calculi are essentially brittle materials, radial cracks (Fig. 1) were formed along the diagonals of the indentation after unloading [10]. The lengths of these crack lines reflect the degree of elastic recovery of the material and can be used to determine the fracture toughness of the stone [11]. We measured the crack length (c) and the size of indentor impression (l), and calculated the fracture toughness (Kic) for the stone material using [11]