This project is an investigation of how ultrasonics and sprays can be used together to improve the scavenging of micron-scale particles. Sprays are widely used to reduce the level of particles in air in applications such as pollution control and dust control in mines. However micron scale particles are very difficult to remove from the air using sprays, and so significant levels of these micron scale particles exist even when sprays are vigorously implemented. An acoustic radiation force exists whenever a sound wave interacts with an object. In this work the acoustic radiation force from ultrasonic waves is used to force particles and drops to come into contact with each other thereby scavenging, or removing, these particles from the air. The goal is to cause greater scavenging of micron-scale particles than would have existed without the use of ultrasonics.
Several theories for the acoustic radiation force exist, however there are significant disagreements between these theories. This is a significant issue, since without a theory, any attempts to improve the scavenging of particles using acoustics will be forced to rely on a time-consuming trial and error approach. Accordingly, the first step in this project is to experimentally validate and correct a theory. This will be a significant result in and of itself. This result will also be used to guide further experimentation in this project to show how to best improve scavenging of micron-scale particles by sprays using ultrasonics. An especially important goal of this project will be to determine if the acoustic radiation force should be configured to act on both particles and drops, forcing them into a small region, thereby increasing the scavenging of particles, or alternatively (or in addition), to use the acoustic radiation force to make the particles combine with each other, causing them to more easily be removed.
Particles that are on the order of one micron in size (about one-fiftieth of the diameter of a human hair) are very dangerous to the pulmonary health of human beings. This is of concern for two reasons. Firstly, particles in this range are very difficult for the human body to reject. Particles larger than a micron tend to be blocked in the pharynx and upper respiratory system, while particles much smaller than a micron may be exhaled after entering the lung. However, particles that are about one micron in in size tend to make it all the way into the alveolar region of our lung, and then stay there. Water sprays are often used to help eliminate particles from the air. For example, devices called wet scrubbers are used in smokestacks to reduce the level of particulate pollutants. However, due to technical reasons, water sprays are not very good at eliminating particles that have a size on the order of a micron, leading to the second point of concern which is that sprays are good at removing particles that are much larger than or much smaller than a micron, but not those on the order of a micron. Hence, micron size particles can cause lung damage, and sprays are not very good at keeping them out of the air that we breathe. In this project, ultrasonic sound waves (acoustic energy just outside of the range of human hearing) are being used to improve the ability of sprays do remove particles from the air. By using high intensity ultrasound, particles and spray drops can be pushed into close proximity to each other, enabling the drops to remove the dangerous particles in a way that would not otherwise occur. The potential importance of this work is that it may be implemented in smoke stacks, automobile and truck exhaust systems, the cabs of mining vehicles, and in underground mines. In this way, the technology developed in this project may be used to reduce the level of particles in the air, thereby reducing the incidence of lung ailments such as asthma, lung cancer and COPD.
