Acoustic wave propagation in bubbly flow with gas, vapor or their mixtures

Cavitation-induced shock wave behaviour in different liquids

This paper follows our earlier work where a strong high frequency pressure peak has been observed as a consequence of the formation of shock waves due to the collapse of cavitation bubbles in water, excited by an ultrasonic source at 24 kHz. We study here the effects of liquid physical properties on the shock wave characteristics by replacing water as the medium successively with ethanol, glycerol and finally a 1:1 ethanol-water solution. The pressure frequency spectra obtained in our experiments (from more than 1.5 million cavitation collapsing events) show that the expected prominent shockwave pressure peak was barely detected for ethanol and glycerol, particularly at low input powers, but was consistently observed for the 1:1 ethanol-water solution as well as in water, with a slight shift in peak frequency for the solution. We also report two distinct features of shock waves in raising the frequency peak at MHz (inherent) and contributing to the raising of sub-harmonics (periodic). Empirically constructed acoustic pressure maps revealed significantly higher overall pressure amplitudes for the ethanol-water solution than for other liquids. Furthermore, a qualitative analysis revealed that mist-like patterns are developed in ethanol-water solution leading to higher pressures.

A strict formulation of a nonlinear Helmholtz equation for the propagation of sound in bubbly liquids. Part II: Application to ultrasonic cavitation

This paper addresses one of the greatest challenges in sonochemistry that has impaired scaling up ultrasonic processes, which is the lack of models capable of predicting the pressure distribution in sonoreactors. This work studies the effect of acoustic pressure on the transmission of sound thought cavitating bubbly liquids by utilizing the nonlinear Helmholtz equation that was demonstrated on the paper part I. The model showed that the wave number and the attenuation can be estimated from the bubble dynamics of inertial bubbles and the local bubble density. The linear model of Commander and Prosperetti is encompassed by the nonlinear model. The model was employed to predict the pressure distribution below an ultrasonic horn tip achieving a relatively close prediction of the experimental data and certainly an accurate qualitative description of the distribution of the pressure field in spite of the simplifications of the model and the assumptions of unknown variables such as the bubble density, bubble distribution and the vessel boundary conditions.

Numerical simulations of stable cavitation bubble generation and primary Bjerknes forces in a three-dimensional nonlinear phased array focused ultrasound field

We present a model developed for studying the generation of stable cavitation bubbles and their motion in a three-dimensional volume of liquid with axial symmetry under the effect of finite-amplitude phased array focused ultrasound. The density of bubbles per unit volume is determined by a nonlinear law which is a threshold-dependent function of the negative acoustic pressure reached in the liquid, in which nuclei are initially distributed. The nonlinear mutual interaction of ultrasound and bubble oscillations is modeled by a nonlinear coupled differential system formed by the wave and a Rayleigh-Plesset equations, for which both the pressure and the bubble oscillation variables are unknown. The system, which accounts for nonlinearity, dispersion, and attenuation due to the bubbles, is solved by numerical approximations. The nonlinear acoustic pressure field is then used to evaluate the primary Bjerknes force field and to predict the subsequent motion of bubbles. In order to illustrate the procedure, a medium-high and a low ultrasonic frequency configurations are assumed. Simulation results show where bubbles are generated, the nonlinear effects they have on ultrasound, and where they are relocated. Despite many physical restrictions and thanks to its particularities (two nonlinear coupled fields, bubble generation, bubble motion), the numerical model used in this work gives results that show qualitative coherence with data observed experimentally in the framework of stable cavitation and suggest their usefulness in some application contexts.

Experimental investigations of the particle motions induced by a laser-generated cavitation bubble

The interactions between a laser-generated cavitation bubble and a spherical particle are investigated experimentally with the aid of high-speed camera. Both the cavitation bubble dynamics and its induced particle moving dynamics are clearly recorded and analyzed qualitatively and quantitatively. Influences of two essential parameters (including the bubble-particle distance and the particle/bubble radius ratio) on the phenomenon are given and discussed. Furthermore, the underlying physical mechanisms are discussed based on the calculations of the radiation pressure and the generations of the micro-jet. Our results reveal that the distance between the cavitation bubble and the particle shows significant influences on the phenomenon. For different radius ratios, the maximum particle displacement varies especially for the small stand-off distance.

Enhancement of ultrasound-assisted degradation of Eosin B in the presence of nanoparticles of ZnO as sonocatalyst

In this research, ZnO nanoparticles, as a sonocatalyst for degradation of Eosin B dye under ultrasonic irradiation, were synthesized. Various experimental conditions (ultrasound irradiation power: 50-250 W, ultrasound irradiation time: 10-70 min, catalyst dosage: 1-3 g/L and initial dye concentration: 5-25 mg/L), using ZnO nanoparticles were investigated in order to find the optimal condition for the degradation of Eosin B. The crystalline and grain size of samples were obtained using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), 15 and 30 nm, respectively. The nanoparticles structure was observed in the form of hexagonal. The band-gap of the prepared nanoparticles was measured as 2.9 eV which is appropriate for sonodegradation process under ultrasonic irradiation. Results demonstrated that Eosin B degradation efficiency was enhanced considerably in sonicated samples compared to non-sonicated ones. The experiments were analyzed via response surface methodology (RSM) based on central composite design (CCD). Analysis of variance (ANOVA) confirmed a good reliability of quadratic response surface model for predicting the sonocatalytic efficiency at various operational parameters (R² = 0.9918 and Adjusted R² = 0.9841). Results indicated that increasing the ultrasound power and time led to enhancement of Eosin B removal efficiency, while increasing the dye concentration caused to its decreasing. The degradation of dye, increased by enhancement of the catalyst dosage, where in the specified value (2.17 g/L) it began to decrease. The optimization of the process showed the maximum sonocatalyst degradation of 93.46% at irradiation power, irradiation time, catalyst dosage and dye concentration of 250 W, 70 min, 2.17 g/L and 5.08 mg/L, respectively. Kinetic studies showed that the sonodegradation of Eosin B corresponds well to first-order reaction.

A strict formulation of a nonlinear Helmholtz equation for the propagation of sound in bubbly liquids. Part I: Theory and validation at low acoustic pressure amplitudes

This paper strictly demonstrated a nonlinear Helmholtz equation, with its corresponding new expressions for the wave number of the mixture, for the propagation of sound trough a bubbly liquid. The demonstration was conducted under the assumption of periodicity of volume fluctuations, the acoustic approximation and considering only mono-harmonic pressure oscillations. The model revealed a beautiful symmetry between the average acoustic energy density and the average energy dissipation, as well as between the time average of the first and second derivatives of such fluctuations. The nonlinear model was validated with available experimental data at very low pressure amplitudes yielding the same results as the linear model. However, unlike the linear model, the advantage of the nonlinear model is that the wave number of the mixture is function of the pressure amplitude, which has great implications to model the sound propagation on cavitating bubbly liquids where the linear theory greatly under-predicts.

Cavitation field analysis for an increased efficiency of ultrasonic sludge pre-treatment using a novel hydrophone system

The generation of cavitation fields for the pre-treatment of anaerobic sludge was studied by means of a novel acoustic measuring system. The influence of different reactor dimensions (i.e., choosing reaction chamber widths of 40, 60 and 80 mm) on the cavitation intensity was determined at various solid contents, flow rates and static pressures. Results suggest that the cavitation intensity is significantly reduced by the sonication of liquids with a high solid content. By increasing the pressure to 1 bar, the intensity of bubble implosions can be enhanced and the sound attenuation in the solid fraction is partly compensated compared to ambient pressure. However, a further increase in pressure to 2 bars has a detrimental effect due to the suppression of powerful bubbles. A reduction of the reactor gap permits an intensification of the treatment of waste activated sludge (WAS) by concentrating the ultrasound power from 6 to 18 dB. This effect is less relevant in digested sludge (DS) with its markedly lower total solids content (2.2% vs. 6.9% of solids in WAS). Increasing the flow rate, resulting in a flow velocity of up to 7 m/min, has no influence on the cavitation intensity. By adapting the reactor design and the static pressure to the substrate characteristics, the intensity of the sonication can be notably improved. This allows the design of sonication devices that are suitable for the intensive treatment of wastewater sludge.