Investigation of geometric effect on the ultrasonic processing of liquids

Ultrasonic processing of liquids is used in many engineering fields, from sonochemistry to material processing. Acoustic cavitation and acoustic streaming are the two key phenomena responsible for the ultrasonic processing applications. Both of them are non-linear effects of ultrasonic wave propagation in liquids and are extremely hard to characterize either analytically or experimentally. This meant that there is limited knowledge about the interactions between the various parameters that affect the extent of the acoustic cavitation and streaming generated during the ultrasonic processing of liquids. In the current study, it was hypothesized that the geometric configuration of the ultrasonic processing equipment has an effect on the resultant acoustic pressure field, which in turn affects the acoustic cavitation and the acoustic streaming flow.;Numerical modeling serves as a powerful tool to overcome the practical difficulties involved in experiments. Over the years, various finite element models have been developed to resolve the acoustic pressure field inside the ultrasonic processing cell. The majority of them have used a linear modeling of the Helmholtz equation with infinitely hard ultrasonic processing cell boundaries. In the current study, a non-linear numerical model was developed to resolve the acoustic pressure inside the ultrasonic processing cell. The viscous dissipation loss during the ultrasonic wave propagation is taken into account by replacing the general liquid material properties with complex material properties in the Helmholtz equation. The model developed was then validated with experimental results. An error analysis revealed that the simulation results show a mean error of about 33 %, with a maximum error of 78 % and a minimum error of 5 % in comparison with the experimental results. Following this, a method was introduced for the quantification of the acoustic cavitation zone size from the numerical modeling results of acoustic pressure field in the ultrasonic processing cell.;The acoustic cavitation zone size was used as the response variable and analysis of variance (ANOVA) was used to study the effect of different geometric parameters and identify their significance. A full factorial design with 27 orthogonal arrays was formed with three geometric parameters, diameter ratio to the probe diameter, immersion depth ratio to the probe diameter, and the liquid volume as three factors. Three levels of each factor, namely high, low and mid-point values were chosen based on a literature review. The values for the diameter ratio to the probe diameter are 1.25, 3.0, and 2.25 respectively. Similarly, 0.25, 3.0, and 1.625 for immersion depth ratio to the probe diameter and 20 ml, 200 ml, and 125 ml for volume. Based on the ANOVA results, it was determined that the ultrasonic processing cell diameter is the most significant of all the geometric parameters considered in this study. Parametric analyses involving these parameters was then conducted. The results show the variation of cavitation zone size with the change in the significant geometric parameters. In addition, the geometric configuration offering the largest acoustic cavitation zone size for a volume of 57 ml in the considered parameter range was determined.;Acoustic streaming is a second order non-linear effect of ultrasonic propagation and can be obtained using the acoustic pressure field values a in every time interval. The model with Navier-Stokes equations coupled to the non-linear transient acoustic wave equation will anticipate the acoustic streaming flow inside the ultrasonic processing cell. The acoustic streaming field model is then applied to the geometric configuration with parameters that give the largest cavitation zone size. It was found that a probe immersion depth of 2.54 cm would produce the maximum acoustic cavitation zone and a relatively uniform acoustic streaming flow in the ultrasonic processing cell with a diameter of 3.59 cm for processing 57 ml of liquid.;The experimental verification of the current study is performed for manufacturing metal matrix nanocomposites (MMNCs) using ultrasonic processing of the master nanocomposite with carbon nanofibers with appropriately selected processing parameters, the area of pores in the nanocomposite was significantly decreased by 46.5 % and a deviation of the hardness was also decreased by 46.0 % due to further dispersion and distribution of the carbon nanofibers. The significance of this study lies in the formation of an understanding of non-linear acoustic phenomena in liquids and the development of a proper process design methodology for ultrasonic processing of liquids.