•Interactions between a cavitation bubble and a spherical particle are experimentally investigated with a high-speed camera.•Three cases are identified together with characteristics.•Transitions ...between cases are shown with special cases.•Scales of particles and bubbles do not show significant influences on the phenonmenon.
In the present paper, the interactions between a cavitation bubble and a spherical particle are experimentally investigated with the aid of high-speed camera. The cavitation bubble is generated by a focused laser beam with the bubble size adjusted through energy dissipater. A series of experimental work is conducted with variations of the bubble sizes, the particle sizes and the distance between the bubble and the particle. Based on the collapsing process of cavitation bubble near the particle, three cases (mushroom-shaped, pear-shaped and spherical-shaped collapses) are quantitatively identified and the criterions for separating three cases are also given in terms of non-dimensional parameters. Furthermore, transitions between the aforementioned cases during parameter variations are also shown together with several examples. Finally, some special cases (e.g. collapse of two bubbles near the particle) are also discussed with typical collapsing process shown.
Fluidization of cohesive pharmaceutical powders is difficult to achieve and typically requires the introduction of external forces. This study investigates the fluidization of the fine inhalation ...grade of lactose powders (size range from 0.1‐20 μm) that are specifically developed for dry powder inhalation (DPI) applications. The fluidization behaviour of fine lactose powders was evaluated under six conditions: without fluidization aids, with only vertical vibration (VFA), with only a downward‐pointing micro‐jet (MFA), with both vibration and pre‐mixing with coarse particles (VCFA), with both vibration and micro‐jet (VMFA), and with the combined assistance of vibration, micro‐jet, and addition of coarse particles (VMCFA). The enhancement of fluidization due to the use of different assistance methods is reflected by the increase of bed expansion and the decrease in both the minimum fluidization velocity and agglomerate formation. However, applying micro‐jet results in considerable powder losses due to the high fraction of fine particles stuck to the wall. Combining any two assisting methods leads to better fluidization than using a single approach. In particular, the combination of vibration and micro‐jet shows the best performance in improving fluidization. Further addition of coarse particles does not play a significant influence on promoting fluidization. Finally, the analysis of the forces acting on the lactose agglomerates shows the enhancement of separation forces by introducing the fluidization assistance, which leads to a decrease in agglomerate size.
•Effect of surface tension on acoustic cavitation is studied by adding surfactant.•Micro bubble in certain size is generated in liquid with different surface tension σ.•Acoustic-driven bubble ...evolution near a rigid wall in liquid with various σ are recorded.•Effect mechanisms of σ on bubble dynamics in ultrasonic field are discussed.•Mechanisms of lower σ on efficiency increase of ultrasonic applications are analyzed.
Acoustic cavitation is a very important hydrodynamic phenomenon, and is often implicated in a myriad of industrial, medical, and daily living applications. In these applications, the effect mechanism of liquid surface tension on improving the efficiency of acoustic cavitation is a crucial concern for researchers. In this study, the effects of liquid surface tension on the dynamics of an ultrasonic driven bubble near a rigid wall, which could be the main mechanism of efficiency improvement in the applications of acoustic cavitation, were investigated at the microscale level. A synchronous high-speed microscopic imaging method was used to clearly record the temporary evolution of single acoustic cavitation bubble in the liquids with different surface tension. Meanwhile, the bubble dynamic characteristics, such as the position and time of bubble collapse, the size and stability of the bubbles, the speed of bubble boundaries and the micro-jets, were analyzed and compared. In the case of the single bubbles near a rigid wall, it was found that low surface tension reduces the stability of the bubbles in the liquid medium. Meanwhile, the bubbles collapse earlier and farther from the rigid wall in the liquids with lower surface tension. In addition, the surface tension has no significant influence on the speed of the first micro-jet, but it can substantially increase the speed of second and the third micro-jets after the first collapse of the bubble. These effects of liquid surface tension on the bubble dynamics can explain the mechanism of surfactants in numerous fields of acoustic cavitation for facilitating its optimization and application.
•A resonant piezoelectric micro-jet using a longitudinal transducer was proposed for high-viscosity liquid.•The driving principle of piezoelectric micro-jet was analyzed to reveal the pressure change ...and jetting status.•The structure parameters were optimized by simulation analyses and the performances were also simulated.•A prototype was fabricated and tested to verify the driving principles, designs and simulations.•The silicone oil with the large viscosity of 150 cps could be easily jetted at 300 Vp-p for 19.4 kHz.
A resonant piezoelectric micro-jet (RPMJ) using a longitudinal transducer (LT) is proposed for achieving high-speed jetting of high-viscosity liquid. The continuous droplets are realized by the pressure change of micro-jet element (MJE) which can be generated by the high frequency vibration of the LT. The RPMJ is designed and its driving principle is simulated to reveal the pressure change and the jetting status. The modal and harmonic response analyses are performed to determine the resonant frequency of the LT. The transient and two-phase coupling analyses are launched to optimize the parameters of the MJE; and the jetting performances under the various viscosities of silicone oils are analyzed. A prototype of the RPMJ is fabricated and its tested system is established. The jetting of the silicone oil with the viscosity of 150 cps is achieved under the voltage of 300 Vp-p and frequency of 19.4 kHz, and the average flowrate is about 0.07 ml/s. The maximum average flowrate can reach to 0.51 ml/s when the viscosity is 2 cps. In a word, the viscosity range of operable liquid has been broadened using the proposed RPMJ.
•The size effect of materials should be considered in the formation of cavitation pits.•The prediction models of cavitation impact load, impact pressure and velocity of micro-jet with size effect ...were established.•A size effect coefficient 1+54hpα2μ2bdp2σJC2 was defined.•The predicted values of parameters increased by about 11%–88% for the ten typical cavitation pits when considering the size effect.
High-speed micro-jet produced by cavitation collapse near the wall is the main mechanism of material damage, and cavitation pit is the most typical damage feature. The reason why high-pressure and high-speed micro-jet can only cause nano- and microscale cavitation pit is that the micro-jet is a short-term impact load of nano- and microscale, and the material shows size effect during the formation of pits. To further explore the cavitation damage characteristics and deformation mechanism of materials, the theoretical framework of indentation test and J-C constitutive model were adopted, and the size effect of materials during the process of cavitation pit formation was mainly considered, and the prediction models of cavitation impact load, impact pressure and velocity of micro-jet were established. The results showed that the equivalent stress and strain of cavitation pit and the impact pressure and velocity of micro-jet are only related to the diameter-to-depth ratio of pit without size effect, and also to the diameter of pit with size effect. Larger diameter and deeper depth of the pit infers greater cavitation impact load, and the influence of the pit diameter is more obvious. When considering the size effect, there is an additional size effect coefficient: 1+54hpα2μ2bdp2σJC2. In the selected size range of pit, the cavitation impact load, impact pressure and velocity of micro-jet predicted with size effect increase by 0.9408%–322.5% compared with those without size effect. The maximum increase ratio appears at the minimum of diameter-to-depth ratio of pit (dp = 2 μm and dh = 2 μm), that is, the smaller the pit diameter is and the greater the depth is, the greater the increase ratio is. Ten typical cavitation pits were selected for inversion analysis. The impact pressure and velocity of micro-jet with and without size effect are 473–1131 MPa and 355–848 m/s, and 427–604 MPa and 320–453 m/s, respectively. The predicted values increase by about 11%-88% when considering the size effect, and the micro-jet velocity predicted is closer to that observed by high-speed cameras, which confirms the necessity and rationality of size effect in the inversion analysis of cavitation pits.
•Ultrasonic capillary effect in liquid melt was observed and analysed for first time.•The filling of a pre-existing channel was monitored using X-ray radiography.•Analytical results showed that ...bubble collapse is responsible for the channel filling.•Liquid micro-jet from bubble collapse increase the Al2O3 concentration in the channel.•Discovery of that filling is of high technological importance e.g. melt-filtration.
An in situ synchrotron radiographic study of a molten Al–10wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30kHz, with a peak to peak amplitude at 140microns, was used, producing a pressure output of 16.9MPa at the radiation surface of the 1-mm diameter sonotrode.
This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.
In this study, the flame structures of a fuel-rich premixed H2–air mixture issued from a divergent micronozzle were numerically studied and compared with their counterparts. The flame dynamics of the ...divergent micronozzle at relatively low jet velocities were also analyzed. A flame pattern of “dual flames with a smaller one surrounded by a larger one” was discovered for both the straight and divergent micro-nozzles at high jet velocities. This flame structure began to appear at a lower jet velocity (Vin) as the divergence angle (θ) decreased. Specifically, it first appeared at Vin = 7.5 and 20 m/s for θ =0° and 5°, respectively. In addition, a flame mode consisting of dual flames with a smaller one inside the nozzle occurred only for divergent micronozzles under low and medium jet velocities. This flame structure could emerge at a higher jet velocity as the divergence angle increased. For example, it could occur until Vin = 5 m/s and 17.5 m/s for θ =1° and 5°, respectively. A global map of all flame patterns was drawn based on the results obtained by varying the jet velocity and divergence angle. Moreover, periodic flame dynamics appeared when the jet velocity was less than the critical value (<1.7 m/s). Specifically, a small flame (i.e., secondary flame) split from the jet flame (i.e., main flame) and propagated towards the nozzle inlet; however, its propagation direction was inverted and it finally extinguished owing to the heat loss to the nozzle wall. The next cycle was initiated after a short duration. The analysis revealed that the periodic flame dynamics are a result of the thermal interactions between the flame and nozzle wall.
Using H2 or natural gas/H2 blends as the fuel of domestic gas stoves is a promising technology to reduce CO2 emission. In this study, the flame characteristics of pure H2 jet flame issued from a micro divergent nozzle was numerically investigated. As shown in the figure above, new flame dynamics have been observed. A small flame splits from the bottom of H2 jet flame and propagates towards the inlet of divergent nozzle, then alters its direction and extinguishes owing to the heat loss to the wall. Such flame dynamics occur periodically. Display omitted
•A smaller flame surrounded by a larger one appears at high jet velocities.•A smaller flame occurs in the divergent nozzle at low and medium jet velocities.•A global flame pattern map is drawn by varying jet velocity and divergence angle.•Periodic flame dynamics appear under low jet velocities for divergent nozzles.•The flame dynamics are a result of thermal coupling of the flame and nozzle wall.
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