- Title
- Ultrasonic-assisted freezing of micro-sized water droplets
- Creator
- Gai, Shaolei
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This research is concerned with a combined theoretical, experimental and numerical study of the ultrasonic-assisted freezing of micro-sized water droplets. The main objectives of the study are to gain an improved understanding of ice nucleation and the freezing of micro-sized water droplets induced by the high pressures from acoustic cavitation, and the subsequent effects on the freezing temperatures of the droplets. This work is motivated by the need to develop effective approaches to generating engineered ice particles with uniform sizes and spherical shapes for a range of industrial applications, such as in air-conditioning systems, medical cooling, food storage, ice pigging, and the broader field of cold energy storage. In this study, a theoretical framework was developed to describe the relationship between ice nucleation and pressure. Specifically, fundamental improvements were incorporated into the existing models to capture the transition of the ice-water interface using an approach based on the distribution of the molecular kinetic energy which, in turn, eliminated the need to specify the ice-water interface energy and activation energy typically required in conventional methods. A good agreement was obtained between the predictions of the model and the experimental data from the literature for temperatures down to 190 K, indicating that the model was able to capture all of the essential elements of the ice nucleation phenomenon using a simplified approach. With the measurable parameters as the inputs, including the enthalpy of fusion, the hydrogen-bond energy, the pressure-dependent melting temperatures and the pressure-dependent densities of the solids/liquids, this theoretical framework can be readily extended and applied to analyse the nucleation of other liquids with hydrogen-bonds. The experimental work is focused on the adaptation of the classic ice nucleation triggering techniques, which are based on acoustic cavitation, to systems with confined volumes of water (i.e., micro-sized droplets). In particular, two mechanical triggering methods were developed. In the first method, the fine solid particles submerged in a droplet were used to provide free sites for the inception of cavitation bubbles. In the second method, cavitation bubbles were formed within a continuous medium carrying the suspended droplets. Both methods used acoustic vibrations to trigger the ice nucleation. In the first method, it was found that the fraction of the frozen droplets increased with an increase in the concentration of the particle numbers, the intensity of the vibrations and the vibration induction time. It was evident that the nucleation sites in this approach were limited to the regions between the solid particles and the vibrating substrate which, in turn, indicated that the contact pressures due to the collisions of the particles with the substrate greatly influenced the onset of the ice nucleation. In the second method, the fraction of the frozen droplets was also found to increase with an increase in the intensity of the vibrations and the vibration induction time. The experimental observations showed that the sites of the onset of the ice nucleation were on the droplet’s surface, where strong interactions were encountered between the cavitation bubbles (formed in the continuous phase) and the droplets (in the dispersed phase). It was evident that the cavitation bubbles triggered the onset of the ice nucleation. Numerical studies based on the lattice Boltzmann method (LBM) were carried out to gain a better understanding of the underlying mesoscale physics of the ice nucleation and freezing processes for the above approaches. Specifically, the study developed a model which coupled the conventional pseudo-potential multi-relaxation-time LBM (MRT-LBM) with a thermal LBM to investigate the dynamics of the cavitation bubbles, including their growth and collapse and the subsequent ice nucleation and freezing processes. The thermal LBM was extended by: (i) the inclusion of the recalescence stage (rapid growth of dendritic ice); and (ii) the inclusion of a criterion for the pressure-dependent onset of the ice nucleation. In this model, the Stefan number was used to determine the initial ice fraction for the entire spectrum of the degrees of supercooling in the recalescence stage. The Simon-Glatzel equation was applied to correlate the ice melting curve with the local pressure field which, in turn, governed the onset of the ice nucleation. Both the data in the literature data and the experimental data collected as part of this study were used to validate the model. It was found that the deviations fell within the limits of experimental error. The model was then used to gain insights into a number of phenomena, including: (i) the effects of the recalescence stage on the freezing process; (ii) the ice nucleation induced by the cavitation bubbles; (iii) the evolution of the pressures of the cavitation bubbles inside the crevices; and (iv) the evolution of the pressures on the surfaces of the droplets in the vicinity of the collapsing cavitation bubbles. The simulation results showed that the inclusion of the recalescence stage had a significant effect on the accuracy of predicting ice-water interface evolution for supercooling degrees greater than 20 K. Given that the freezing of a small droplet often bears a supercooling degree of more than 30 K, and the local supercooling degree could be significantly increased by the high pressures created by the collapse of the cavitation bubbles, therefore an accurate description of the freezing can be achieved only when the recalescence stage has been taken into account. Simulation results also captured the sudden rise in local temperature following the rapid (isentropic) process of cavitation bubble collapse. The local temperature rose significantly exceeded the ice melting temperature preventing the freezing process to proceed. These results suggest that maintaining a sufficiently large initial supercooling degree or use of additional mechanisms to force the ice crystals to migrate to the low-temperature regions are essential to achieve complete freezing of water droplets. The collapse dynamics of the air bubbles trapped in the crevices on the surfaces of the solid particles was found to be sensitive to the morphological characteristics of the crevices, with the collapse of the bubbles in a hemispherical crevice generating the highest pressures. More importantly, the results provided a potential theoretical explanation on how the presence of solid particles assists with the initialisation of the ice nucleation in a droplet. Lastly, the simulation results showed that the onset of the ice nucleation on the surfaces of the droplets was strongly dependent on the distance between the bubble collapse point and the surface of the droplet. The simulation results were used to develop a correlation for predicting the minimum distance required to initiate ice nucleation in the droplet as a function of the key operating parameters, including the sizes of the cavitation bubbles and the amplitudes of the external pressures. The effectiveness of the method of triggering the ice nucleation with the cavitation bubbles for continuous production of micro-sized ice particles dispersed in an immiscible liquid was examined experimentally. Specifically, the yield and quality of ice particles, in terms of the fractions of the frozen droplets, the distributions of the particle sizes and the roundness ratios of the ice particles, were examined as a function of the characteristics of the ultrasonic vibrations, namely the power output of the sonicator, the duty cycle of the vibrations and the offset distance of the sonicator probe. It was found that the production yield could be increased with increases in the power output of the sonicator, the duty cycle of the vibrations and the offset distance of the sonicator, but that it was decreased with a reduction in the temperature of the cooling module. However, the increase in the yield led to a loss of quality as the mean diameter of the water droplets used for producing the ice particles was in a range from 535 μm to 567 μm. After the ice nucleation using the cavitation bubbles, the mean diameters of the ice particles produced were measured in a range from 450 μm to 590 μm. The freezing temperature achieved was as high as 269 K with a fraction of frozen droplets of 2% and a roundness ratio for the ice particles of 1. The highest fraction of frozen droplets was about 92% which was obtained at 262 K, whereas the roundness ratio of the ice particles decreased drastically to around 45%. The results of the present study should prove useful in the application of the ultrasonic vibration-assisted nucleation of supercooled liquids in other fields, such as freezing of saline water droplets, solidification of molten metals, and freezing of biomaterials, in which the supercooling phenomenon is often encountered.
- Subject
- ice nucleation; water droplets; cavitation bubble; supercooling; ice particle; LBM; ultrasonic
- Identifier
- http://hdl.handle.net/1959.13/1426417
- Identifier
- uon:38414
- Rights
- Copyright 2021 Shaolei Gai
- Language
- eng
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