Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Advisor

Saylor, John R

Committee Member

Miller , Richard S

Committee Member

Xuan , Xiangchun

Committee Member

Thompson , Lonny L

Abstract

Currently the backbone of the world's energy supply is composed of fossil fuels. However, the combustion of fossil fuels results in the production of enormous quantities of particulate pollutants. The smog resulting from these particulate pollutants causes significant health problem for city dwellers. Wet scrubbers, which use a water spray to scavenge airborne particles, is one of the most widely used devices to control particulate pollutants. Typical wet scrubbers can scavenge particles with diameters bigger than 10 um, but it is inefficient in scavenging particles with diameters on the order of 1 um. Unfortunately these fine particles are more dangerous than the coarse particles since fine particles can penetrate deep into human lungs. This dissertation is an investigation into the use of ultrasonics to enhance the ability of wet scrubbers to scavenge fine particles. The first part of the investigation involves testing a combination of water spray and ultrasonics on the scavenging of fine particles in a small scale scrubber. A stream of air laden with particles was flowed into the scrubber with a water spray. Experiments were conducted with and without the presence of an ultrasonic standing wave field inside the scrubber over a range of parameters: water flow rate, air flow rate, particle size and spray drop size. Compared to the water spray alone, significant increases in the scavenging of particles were observed when the water spray was combined with the standing wave field in these experiments. The second part of the investigation involves a determination of the mechanism that causes the increase in particle scavenging of a water spray in the presence of an ultrasonic standing wave field. A review of existing theories showed that the acoustic radiation force generated by an ultrasonic standing wave field can influence the motion of the aerosols in the standing wave field. These theories predict that the spray drops used in these experiments would migrate toward the pressure nodes of the standing wave field. However, for the micron-scaled particles investigated here, some theories predict that the particles would migrate toward the pressure nodes, while other theories predict that they would migrate toward the pressure anti-nodes. Experiments were conducted where particles having a range of diameters were flowed into the region of a standing wave field and their locations in the standing wave field were recorded. Results obtained from these experiments show that the particles with diameters larger than 0.3±0.1 um would migrate toward the pressure nodes while the particles with diameters smaller than 0.3±0.1 um would migrate toward the pressure anti-nodes. A theory of the acoustic radiation force that agrees with these results was selected to build a model. This model was used to simulate the trajectories of the spray drops and the particles in the scrubber. Results obtained from the simulations show that the increased scavenging is caused by an increase in particles combining with spray drops in the pressure nodes of the standing wave field.

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