Distribution Of Cavitation Bubbles Research Articles

Sonochemical reactor characterization in the presence of cylindrical and conical reflectors

Ultrasonic systems must be able to produce an acoustic field with the highest possible energy concentration in sonochemical reactors to accomplish maximum efficacy in the sonolytic degradation of water contaminants. In the present study, the impact of cylindrical and conical stainless-steel reflectors placed on the liquid surface on the sonochemical oxidation activity of ultrasonication reactors was investigated. The amount of effective acoustic power transferred to the ultrasonicated medium without and with reflectors was measured by calorimetric characterization of the sono-reactors at diverse ultrasonication frequencies in the interval of 300–800 kHz and different electrical powers in the range of 40–120 W. Iodide dosimetry without and with reflectors at diverse ultrasonication conditions (300–800 kHz and 40–120 W) and various aqueous solution volumes in the range of 300–500 mL was used to assess the sonochemical oxidation activity, i.e., the generation of oxidative species (mainly hydroxyl radicals). Sonochemiluminescence (SCL) imaging was used to study the active acoustic cavitation bubbles distribution in the sono-reactors without and with reflectors. Significant impacts of the position and shape of the reflectors on the active acoustic cavitation bubble distribution and the sonochemical oxidation activity were observed due to remarkable modifications of the ultrasonic field by directing and focusing of the ultrasonic waves. A significant augmentation in the triiodide formation rate was obtained in the presence of the conical reflector, especially at 630 kHz and 120 W (60.5% improvement), while iodide oxidation was quenched in the presence of the cylindrical reflector at all ultrasonication frequencies and powers. The SCL images show a noteworthy modification in the ultrasonic field and the acoustic cavitation bubble population when reflectors were used. The sonochemical oxidation activity was improved by the conical reflector when placed in the Fresnel zone (near field region).

Dynamic Characteristics of High-speed Water-Lubricated Spiral Groove Thrust Bearing Based on Turbulent Cavitating Flow Lubrication Model

The water-lubricated bearings are usually the state of turbulent cavitating flow under high-speed conditions. And the distribution of cavitation bubbles and the interface effect between the two phases have not been included in previous studies on high-speed water-lubricated bearings. In order to study the influence of interface effect and cavitation bubble distribution on the dynamic characteristics of high-speed water-lubricated spiral groove thrust bearings (SGTB).A turbulent cavitating flow lubrication model based on two-phase fluid and population balance equation of bubbles was established in this paper. Stiffness and the damping coefficients of the SGTB were calculated using the perturbation pressure equations. An experimental apparatus was developed to verify the theoretical model. Simulating and experimental results show that the small-sized bubbles tend to generate in the turbulent cavitating flow when at a high rotary speed, and the bubbles mainly locate at the edges of the spiral groove. The simulating results also show that the direct stiffness coefficients are increased due to cavitation effect, and cross stiffness coefficients and damping coefficients are hardly affected by the cavitation effect. Turbulent effect on the dynamic characteristics of SGTB is much stronger than the cavitating effect

Environment friendly dual-frequency ultrasonic exfoliation of few-layer graphene

Ultrasound-aided liquid phase exfoliation (ULPE) of graphene in pure water is environment-friendly. Two limiting factors of ULPE are the non-uniform thickness of few-layer graphene (FLG) and a relatively low graphene yield. Here we describe ULPE in water that enables us to produce FLG flakes with a thickness of 3 layers and the flake sizes exceeding 1 μm2 in just 2 h. This process is based on using a combination of two ultrasound sources of high and low frequencies: 1174 kHz and 20 kHz. Two different frequencies generate a wider population and size distribution of cavitation bubbles that act through a number of mechanisms towards the exfoliation of graphene. For the first time ULPE was characterized by acoustic measurements. Results show that a high graphene yield (10%) can be achieved. This study demonstrates that the use of a dual frequency ultrasonic source and control of acoustic pressure is critical in optimizing the quality and yield of the cavitation assisted LPE of graphene in pure water. It is suggested that the width of the acoustic pressure peak reflecting shock-wave emissions can be used as an indicator of ULPE completeness, opening for the first time a way of in-situ monitoring of the process.

Surface microfabrication using coaxial waterjet assisted laser-induced plasma micromachining

In this study, a novel coaxial water assisted laser-induced plasma micromachining (CW-LIPM) method was studied to process microstructures on the workpiece surface. In CW-LIPM, a pulsed laser beam with a wavelength 532 nm was focused within a coaxial water jet, leading to the optical breakdown and the subsequent formation of the localized plasma plume. Compared with conventional laser processing, a cleaner surface could be acquired by CW-LIPM, while maintaining the machining resolution and processing accuracy. The mechanism of CW-LIPM and the effects of microbubble explosions on the surface roughness of the workpiece in proximity to the entrance of the processing area have been studied. With the developed experimental setup, the influence of single laser pulse energy, pulse repetition frequency, and water-jet velocity on the plasma distribution was also researched. Moreover, the effects of waterjet speed on the distribution of cavitation bubbles and the relative position on the machining quality of the processed microchannels were also studied, considering the explosions of the micro cavitation bubbles near the workpiece surface due to the pressure difference. Compared with laser micromachining in the air, CW-LIPM could process microstructures with no protrusions and better precision. High-quality surface microstructures could be obtained by combining the higher-velocity waterjet, the lower single pulse energy, multiple scanning passes, and the processing positions 2–3 mm down from the plasma center and 4–5 mm above the bottom. The laser-induced plasma distribution was asymmetric, and the lower half plasma was more suitable for precision processing due to its focused shape, which is preferred to enhance the localization of the proposed CW-LIPM process.

Development of an integrated ultrasonic biofilm detachment model for biofilm thickness control in membrane aerated bioreactors

Controlling biofilm thickness in an aerated membrane biofilm reactor (MBfR) has been recognized as a key for MBfRs to achieve a long-term stable performance. In this context, acoustic cavitation is an effective strategy that can be used for controlling biofilm thickness. However, to maintain the biofilm thickness at an optimum value, it is necessary to understand the effect of acoustic parameters and cavitation bubble distribution on biofilm detachment and establish a link between biofilm detachment and regrowth. The purpose of this study is to provide an integrated mathematical model that describes biofilm development in an aerated membrane biofilm reactor using a nonlinear reaction-diffusion model and its response to mechanical stress generated from acoustic cavitation. The simulation results show that pressure, amplitude and frequency of transducer are two key factors that affect biofilm detachment. Moreover, uniform distribution of cavitation along the biofilm surface is critical to achieve an even biofilm thickness. Furthermore, periodic cavitation detachment with an appropriate resting time in between is important for maintaining the biofilm thickness at a desired value. Therefore, the purpose of this study is to present an integrated modeling approach to optimize acoustic cavitation parameters and methods to achieve effective biofilm thickness control in MBfRs.

Investigations on the effects of obstacles on the surfaces of blades of the centrifugal pump to suppress cavitation development

This paper proposes a new method that obstacles are attached to both the suction and pressure surfaces of the blades to suppress cavitation development. A centrifugal pump with a specific speed of 32 is selected as the physical model to perform the external characteristic and cavitation performance experiments. SST [Formula: see text] turbulence model and Zwart cavitation model were employed to simulate the unsteady cavitation flow in the pump. The results indicate that the numerical simulation results are in good agreement with the experimental counterparts. After the obstacles are arranged, the maximum head decrease is only 1.37%, and the relative maximum drop of efficiency is 1.12%. Obstacles have minimal impacts on the variations of head and efficiency under all flow rate conditions. The distribution of vapor volume in the centrifugal pump is significantly reduced after obstacles are arranged and the maximum fraction reduction is 53.6%. The amplitude of blade passing frequency decreases significantly. While obstacles decrease the intensity of turbulent kinetic energy near the wall in the impeller passages to effectively reduce the distribution of cavitation bubbles, and control the development of cavitation. After the obstacles are set, the strength of the vortex in the impeller passages is weakened significantly, the shedding of the vortex is suppressed, flow in the impeller becomes more stable.

Theoretical modeling and experimental investigation of a V-shaped traveling wave piezoelectric transducer for ultrasonic cavitation Peening: Part A

Ultrasonic cavitation peening is a surface enhancement technology that can increase surface hardness without obviously introducing a high surface roughness of the treatment surfaces. However, the most commonly used traditional transducers for ultrasonic cavitation peening are operated in the longitudinal mode and generate a standing wave in the working gap between a sonotrode end and a treated surface, causing a small cavitation treated area and low efficiency for treating the axis components. To achieve a high efficiency on the surfaces of axis components, a new V-shaped traveling wave transducer is proposed in this paper. Two Langevin transducers are employed to couple on the ring sonotrode, generating two third order in-plane bending vibrations with a spatial phase difference of 90° in the ring. Therefore, same elliptical vibrations are generated in all points on inner surface of the ring, resulting in a uniform cavitation effect around the axis component which is located in the center of the ring sonotrode. It is the first time to propose that cavitation bubbles are generated utilizing traveling waves. An analytical model is created utilizing the transfer matrix method for the vibrating system to describe the dynamic behavior and predict the acoustic field of the working area in the ring sonotrode. To validate the developed transfer matrix model, experimental investigations were conducted to measure the vibration characteristics of the proposed transducer and compared to the transfer matrix model calculation results. For further proving the feasibility of the cavitation peening, sonochemiluminescence was carried out to evaluate the distribution of cavitation bubbles in the working area. The results show that calculation results have a good agreement with the experimental results and a higher voltage and smaller working gap were beneficial for the ultrasonic cavitation treatment utilizing this novel transducer.

Study on Unstable Characteristics of Centrifugal Pump under Different Cavitation Stages

In order to reveal the regularity of unsteady flow of centrifugal pump under different cavitation stages, a visual closed test-bed is built to collect signals such as the distribution of cavitation bubbles at the impeller inlet and external characteristics, etc. in the process of cavitation of centrifugal pumps. Combined with the shape and distribution of bubbles captured by high-speed photography, the cavitation stage of the centrifugal pump is divided. In addition, the variation of vorticity distribution, pressure pulsation and radial force of centrifugal pump under different cavitation stages are studied using the standard k-e turbulence model and the Kunz cavitation model. Main contributions are as follows: The cavitation bubbles can absorb the energy of vortex core to a certain extent and increase the volume of vortex core. Cavitation bubbles can also block the flow-path and induce the distortion of the internal flow field, resulting in unstable pressure waves that cause a significant increase in pressure pulsation rate. Besides, with the development of cavitation, the radial force on the impeller tends to remain invariable first and then decrease, and trajectory of the radial force changes from closed to open.

Research of cavitation characteristics of RCP based on section-constrained blades

To make sure that the bubbles will not block the impeller passage in the process of the transient cavitation, a section-constrained design is adopted in the hydraulic design of the CAP1400 Reactor Coolant Pump’s(RCP’s) impeller blade, and the cavitation bubble distribution on the suction side of the blade in the CAP1400 RCP under the transient cavitation condition and the internal pressure pulsation are studied. To study the performance of the section-constrained impeller blade, simulations are carried out by using the CFX software, the cavitation bubble distribution on the suction side and the pulsation dynamics at each monitoring point in different extents of cavitation are obtained based on the Reynolds-averaged N-S equation and the SST k - ω equation. The results show that the section-constrained blade has a good constraint capacity on the cavitation, and could enhance the passing ability of the blade in some extent. The main causes of the pressure pulsation inside a pump are the rotor-stator interaction, the separated flow at the round-like pump chamber, and the reflux inside the flow passage. The pressure pulsation inside the impeller and the guide vane is characterized by the significant periodicity, which are both affected by the rotor-stator interaction. And the impeller outlet and the guide vane inlet are more susceptible to the rotor-stator interaction. Studies of the vapor-liquid two-phase flow and the pressure pulsation inside the RCP’s impeller with the transient cavitation are of great significance for the prediction of the RCP’s dynamic characteristics after the cavitation, and for the safety and the stable running of the RCP.

Theoretical and experimental investigations of ultrasonic sound fields in thin bubbly liquid layers for ultrasonic cavitation peening

Ultrasonic cavitation peening is a potential surface enhancement process. During this process a high input power is necessary to obtain an effective process result. A small gap, usually less than 1 mm, between the sonotrode tip and the treated surface is also required to avoid substantial energy loss. Due to the high vibration of the sonotrode, many cavitation bubbles are generated, forming a thin bubbly liquid layer in the small gap. The cavitation bubbles in the layer seriously disturb the sound wave propagation and interact with each other. The disturbances and interactions change the intensity and the spatial distribution of cavitation bubbles, resulting in the different interactions between cavitation bubbles and workpiece surfaces. The variations of the interactions cause different surface properties of the workpieces after ultrasonic cavitation peening. Therefore, quantifying the ultrasound field in different conditions is of great important to improve the ultrasonic cavitation peening process. A current model of the sound propagation in the bubbly liquid was already developed but did not include the bubble interactions. In this work, the bubble interactions are taken into account to improve the current model. The calculated results of the sound field with the improved model are validated by sonochemiluminescence experiments in various standoff distances and vibration amplitudes. Both of the experimental and the calculated results show that the highest sound pressure is generated when the vibration amplitude is around 25 µm. The strongest cavitation intensity occurs at the gap width of 0.5–0.7 mm.

Optimization of protein yields by ultrasound assisted extraction from Eurycoma longifolia roots and effect of agitation speed

The traditional ways of protein extraction is disadvantageous from both economic and environment perspective. In this study, ultrasonic assisted extraction, UAE technique was used for the first time for the optimization process of extracting proteins from Eurycoma longifolia roots. The experiments demonstrated the successful effect of applying eccentric agitation with UAE to provide a combination of agitation and sonication to extract the proteins in short extraction times. Central composite design (CCD) was used to optimize the Ultrasonic-assisted extraction of proteins from roots of Eurycoma longifolia by water. Response surface methodology (RSM) was employed to investigate the effects of five independent variables; particle sizes (A), extraction temperatures (B), agitation speeds (C), amplitude (D) and duty cycle (E). The second enhanced model (Y2) prevailed to be more efficient in protein recovery with optimum conditions of A: 0.022 ± 0.022 mm, B: 49 °C, C: 1314 rpm, D: 9 W and E: 63% UI: 2.94 W·cm−2 was 9.543 ± 0. 946%. High agitation speed didn’t only affect the solvent properties, the diffusion of the solutes from the particles, cooling the extraction temperature, but influenced the distribution of cavitation bubbles and their impact on the solid particles. UAE was found to minimize the extraction of the same amount by conventional extraction 7-fold times. The results obtained in this study have exposed the capability of ultrasonic assisted extraction, UAE technology in extraction of protein from E. longifolia roots. Further works are nevertheless required to provide deeper understanding of the mechanisms involved to facilitate the development of optimum system applicable to the industry.

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu2+ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.

Prediction of Cavitation Depth in an Al-Cu Alloy Melt with Bubble Characteristics Based on Synchrotron X-ray Radiography

The size of cavitation region is a key parameter to estimate the metallurgical effect of ultrasonic melt treatment (UST) on preferential structure refinement. We present a simple numerical model to predict the characteristic length of the cavitation region, termed cavitation depth, in a metal melt. The model is based on wave propagation with acoustic attenuation caused by cavitation bubbles which are dependent on bubble characteristics and ultrasonic intensity. In situ synchrotron X-ray imaging of cavitation bubbles has been made to quantitatively measure the size of cavitation region and volume fraction and size distribution of cavitation bubbles in an Al-Cu melt. The results show that cavitation bubbles maintain a log-normal size distribution, and the volume fraction of cavitation bubbles obeys a tanh function with the applied ultrasonic intensity. Using the experimental values of bubble characteristics as input, the predicted cavitation depth agrees well with observations except for a slight deviation at higher acoustic intensities. Further analysis shows that the increase of bubble volume and bubble size both leads to higher attenuation by cavitation bubbles, and hence, smaller cavitation depth. The current model offers a guideline to implement UST, especially for structural refinement.

Characterization of cavitation-radiated acoustic power using single-element detectors

Standard approaches to quantifying cavitation activity using emission measurements made by single-element passive cavitation detectors (PCD) would be facilitated by improved quantitative and system-independent characterization techniques. Although the strength of an individual emission source can be determined from absolute pressure measurements by a calibrated PCD, this approach requires spatially resolved detection of single bubbles at known locations. Here, a method is shown for characterizing an ensemble of emission sources, quantified by their radiated acoustic power per unit area or volume of a defined region of interest (ROI). An analytic diffraction-correction factor relating frequency-dependent PCD-measured pressures to cavitation-radiated acoustic power is derived using a spatial integral of the PCD sensitivity. This approach can be applied to measurements made by any PCD without a priori knowledge of the number or spatiotemporal distribution of cavitation bubbles. Simulations show that cavitation-radiated acoustic power per unit ROI volume or area is accurately recovered by compensation of emissions received by focused or unfocused PCDs. Measurements from previous sonophoresis experiments are analyzed, showing that skin permeability changes from 0.41 or 2.0 MHz sonication are comparably correlated to the radiated acoustic power of subharmonic emissions per unit area.

Synchrotron x-ray imaging of acoustic cavitation bubbles induced by acoustic excitation

The cavitation induced by acoustic excitation has been widely applied in various biomedical applications because cavitation bubbles can enhance the exchanges of mass and energy. In order to minimize the hazardous effects of the induced cavitation, it is essential to understand the spatial distribution of cavitation bubbles. The spatial distribution of cavitation bubbles visualized by the synchrotron x-ray imaging technique is compared to that obtained with a conventional x-ray tube. Cavitation bubbles with high density in the region close to the tip of the probe are visualized using the synchrotron x-ray imaging technique, however, the spatial distribution of cavitation bubbles in the whole ultrasound field is not detected. In this study, the effects of the ultrasound power of acoustic excitation and working medium on the shape and density of the induced cavitation bubbles are examined. As a result, the synchrotron x-ray imaging technique is useful for visualizing spatial distributions of cavitation bubbles, and it could be used for optimizing the operation conditions of acoustic cavitation.

Visualization and optimization of cavitation activity at a solid surface in high frequency ultrasound fields

Despite the increasing use of high frequency ultrasound in heterogeneous reactions, knowledge about the spatial distribution of cavitation bubbles at the irradiated solid surface is still lacking. This gap hinders controllable surface sonoreactions. Here we present an optimization study of the cavitation bubble distribution at a solid sample using sonoluminescence and sonochemiluminescence imaging. The experiments were performed at three ultrasound frequencies, namely 580, 860 and 1142kHz. We found that position and orientation of the sample to the transducer, as well as its material properties influence the distribution of active cavitation bubbles at the sample surface in the reactor. The reason is a significant modification of the acoustic field due to reflections and absorption of the ultrasonic wave by the solid. This is retraced by numerical simulations employing the Finite Element Method, yielding reasonable agreement of luminescent zones and high acoustic pressure amplitudes in 2D simulations. A homogeneous coverage of the test sample surface with cavitation is finally reached at nearly vertical inclination with respect to the incident wave.

Quasi-emulsion between water and cavitation bubble cloud in an ultrasonic field

The cavitation bubble distribution (cavitation structure) is spatially inhomogeneous in ultrasonic field. A quasi-emulsion phenomenon of cavitation structure is reported in this paper. The inception process of cavitation bubble cloud in a thin liquid layer (the thin liquid layer is trapped between a radiating surface and a hard reflector) was investigated experimentally with high-speed photography. It is revealed that cavitation bubble cloud can be considered as a uniform fluid (cloud), and water without cavitation can be considered as another uniform fluid (water). The conversion from cloud-in-water emulsion to water-in-cloud emulsion occurs with the increase in the number of bubbles. The formation and stability of cloud-water emulsions is analyzed theoretically. It is found that surface tension of cavitation bubble cloud played a leading role. Findings of this research proved that cavitation bubble clusters can be considered and investigated as a whole. [This work was supported by the National Natural Science Foundation of China (Grant No. 11174315).]

Relationship between thrombolysis efficiency induced by pulsed focused ultrasound and cavitation bubble size

In this study, the relationship between the efficiency of pulsed focused ultrasound (FUS)-induced thrombolysis and the size distribution of cavitation bubbles has been studied. Firstly, the thrombolysis efficiency, evaluated by degree of mechanical fragmentation was investigated with varying duty cycle. Secondly, the size distribution of cavitation bubbles after the 1st, 103th and 105th pulse during experiments for various duty cycles was studied. It was revealed that the thrombolysis efficiency was highest when the cavitation bubble size distribution was centred around linear resonance radius of the emission frequency of the FUS transducer. Therefore, in cavitation enhanced therapeutic applications, the essential of using a pulsed FUS may be controlling the size distribution of cavitation nuclei within an active size range so as to increase the treatment efficiency.

Application of analyzer based X-ray imaging technique for detection of ultrasound induced cavitation bubbles from a physical therapy unit.

BackgroundThe observation of ultrasound generated cavitation bubbles deep in tissue is very difficult. The development of an imaging method capable of investigating cavitation bubbles in tissue would improve the efficiency and application of ultrasound in the clinic. Among the previous imaging modalities capable of detecting cavitation bubbles in vivo, the acoustic detection technique has the positive aspect of in vivo application. However the size of the initial cavitation bubble and the amplitude of the ultrasound that produced the cavitation bubbles, affect the timing and amplitude of the cavitation bubbles’ emissions.MethodsThe spatial distribution of cavitation bubbles, driven by 0.8835 MHz therapeutic ultrasound system at output power of 14 Watt, was studied in water using a synchrotron X-ray imaging technique, Analyzer Based Imaging (ABI). The cavitation bubble distribution was investigated by repeated application of the ultrasound and imaging the water tank. The spatial frequency of the cavitation bubble pattern was evaluated by Fourier analysis. Acoustic cavitation was imaged at four different locations through the acoustic beam in water at a fixed power level. The pattern of cavitation bubbles in water was detected by synchrotron X-ray ABI.ResultsThe spatial distribution of cavitation bubbles driven by the therapeutic ultrasound system was observed using ABI X-ray imaging technique. It was observed that the cavitation bubbles appeared in a periodic pattern. The calculated distance between intervals revealed that the distance of frequent cavitation lines (intervals) is one-half of the acoustic wave length consistent with standing waves.ConclusionThis set of experiments demonstrates the utility of synchrotron ABI for visualizing cavitation bubbles formed in water by clinical ultrasound systems working at high frequency and output powers as low as a therapeutic system.

Ultrafast active cavitation imaging with enhanced cavitation to tissue ratio based on wavelet transform and pulse inversion.

The quality of ultrafast active cavitation imaging (UACI) using plane wave transmission is hindered by low transmission pressure, which is necessary to prevent bubble destruction. In this study, a UACI method that combined wavelet transform with pulse inversion (PI) was proposed to enhance the contrast between the cavitation bubbles and surrounding tissues. The main challenge in using wavelet transform is the selection of the optimum mother wavelet. A mother wavelet named "cavitation bubble wavelet" and constructed according to Rayleigh-Plesset-Noltingk-Neppiras-Poritsky model was expected to obtain a high correlation between the bubbles and beamformed echoes. The method was validated by in vitro experiments. Results showed that the image quality was associated with the initial radius of bubble and the scale. The signal-to-noise ratio (SNR) of the best optimum cavitation bubble wavelet transform (CBWT) mode image was improved by 3.2 dB compared with that of the B-mode image in free-field experiments. The cavitation-to-tissue ratio of the best optimum PI-based CBWT mode image was improved by 2.3 dB compared with that of the PI-based B-mode image in tissue experiments. Furthermore, the SNR versus initial radius curve had the potential to estimate the size distribution of cavitation bubbles.