A Study on the Development of Sub-micron Bubble Generator and Characterization of Sub-micron Bubble

Abstract

This study examined the possibility of sub-micron bubble generation through the experiment of microbubble generation and the modeling of bubble breaking up. And using the developed sub-micron bubble generator and the method to measure the size of bubble by DO concentration, the effect of factors in sub-micron bubble generator on the bubble generation efficiency, was investigated. And the characteristics of sub-micron bubbles was measured such as rise velocity, the mass transfer rate, and the zeta potential.

1. Modeling of the Sub-micron Bubble Generation 1.1 The principle of microbubble generation in conventional microbubble generator Microbubble generation principle in a conventional microbubble generator has been established through comparison of experiments and three theories about bubble generation

nucleation, direct injection, and breaking up. The experiment was operated with different flowrate and nozzle sizes. And experiment results were compared the tendency of bubble generation and the size of generated bubbles with bubble generation theories to determine the suitable theory. The results displayed that the nucleation showed a considerable gap in the condition of microbubble generation, and physical injection in the size of bubbles. In contrast, the bubble generation theory by breaking up is suitable for explaining generation of bubbles in conventional microbubble generator. Lastly, as a result of the comparison of bubble size, it was found that the bubble size is irrelevant to the size of nozzle and flow.

1.2 Modeling of the hydraulic force exerted on bubbles in the nozzle The hydraulic force exerted on a bubble in nozzle flow was formulated by fluid momentum equation and Bernoulli's equation. And based on this, the impact of radius of the nozzle, pressure, relative velocity, length of the nozzle, initial bubble size and friction coefficient to the force exerted on bubble, was investigated. The results show that the hydraulic force exerted on a bubble gets larger as the radius of the nozzle gets smaller and the pressure, relative velocity, the length of the nozzle, initial bubble size and friction coefficient get larger.

2. Development of Sub-micron Bubble Generator 2.1 Outline of sub-micron bubble generator Based on the result of the experiment and modeling above, a sub-micron bubble generator was developed by using a pump and a hose. Under a certain condition of diameter and length, sub-micron bubble is generated.

2.2 Method of measuring the sub-micron bubble size Existing methods to measure sub-micron bubble size have difficulty in measuring and adaptation to field in common. One of the methods to complement this weakness is DO concentration method. On this study, the interaction formula between DO concentration and bubble size is derived to improve the accuracy of the method. To do this, the size of sub-micron bubble and DO concentration at that time are measured first. The size of bubble was measured by counting the number of pixels after photographed by an optical microscope and revising. And the interaction formula between DO concentration and bubble size was presumed as a cumulative curve based on the idea that bubble size distribution follows normal distribution. Estimating coefficients from the result of this experiment, the interaction formular is derived.

2.3 The effect of design and operating conditions on sub-micron bubble generator A high capacity sub-micron bubble generator was built for investigating the effect of design and operating conditions on sub-micron bubble generator. DO concentration was measured and the bubble size at the experiment was indirectly measured by DO concentration. As a result, it was found that the size of sub-micron bubble gets smaller as the diameter gets smaller, and the pressure, length, air flowrate and friction coefficient get larger. This shows a similar tendency with the result of the previous modeling, so that it represents that a hydraulic force in hose affects the generation of sub-micron bubble. And the size of bubbles have been estimated as 2-4 ㎛ approximately.

3. Characteristic of sub-micron bubble Understanding characteristics of bubble is crucial for operating treatment process and its optimization. Therefore, the basic characteristics of sub-mircon bubble such as rise velocity, mass transfer rate, and zeta potential were measured and compared to those of microbubble. As for rise velocity, while microbubble showed a similar behaviour in accordance with the existing theory, sub-micron bubble was found incongruent for the theory. It is assumed that this is attributable to the small size of sub-micron bubble which gets easily affected by the surrounding environment. As for mass transfer rate, it was modeled based on the existing theory. While a single bubble has a higher mass transfer rate as the size gets larger, but the mass transfer rate in same volume has a much higher transfer rate as the size gets smaller. As for zeta potential, the values were measured by a zeta potential measuring device and compared to the conventional studies. As a result, the zeta potential of sub-micron bubble showed nearer to zero value than that of microbubble under every pH and NaCl conditions. This is assumed to be attributable to the weaker force for ions to adsorb to its surface than that of microbubble.