Date of Award

8-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Committee Member

Dr. John R. Saylor, Committee Chair

Committee Member

Dr. Hongseok Choi

Committee Member

Dr. Michael Porter

Abstract

Aerosols are solid particles or liquid drops typically less than 100 microns in diameter, suspended in a gas. Some types of aerosols can be very hazardous. Particulate pollution from the combustion of fossil fuels and other sources is of great concern due to health problems those exposed can experience. One method for removing these particles from industrial sources is wet scrubbing, where water sprays are used to capture particles before exhausting to the environment. Wet scrubbers work well for a large range of particle diameters, but are ineffective for particles on the order of 1 µm in diameter. Particles of this size are thought to be the most dangerous, as they can deposit deep in the human lungs. Liquid drops of this size can also be dangerous, but larger drops can as well, such as acid drops emitted from acid production plants. This work is concerned with collection of both solid particles and liquid drops. Previous researchers showed the ability to scavenge micron-scale particles from air streams using a combination of an ultrasonic standing wave field and water drops, where the standing wave field was generated with a disk shaped transducer. This prior setup was limited in both total particle removal efficiency and flow capacity. As such, improvements were needed to allow for both better overall particle collection, and ability to handle higher flow rates. These improvements could also translate into effective liquid aerosol removal in addition to particle removal. Cylindrical ultrasonic standing wave fields were studied as a method to remedy these problems and also provide an additional method for liquid aerosol capture. A customized cylindrical resonator was designed and constructed for use in removing both solid and liquid aerosols from air flows. The resonator consisted of a hollow metal cylinder driven by three Langevin transducers mounted on the midplane of the cylinder, evenly spaced around the circumference. Nodes in the cavity of the cylinder took the form of concentric cylinders which extended through the length of the cylinder. A frequency match was sought between the natural frequencies of the Langevins, the cylinder cavity, and the cylinder itself. With this setup, a strong cylindrical standing wave field was established. Experiments were performed to measure the aerosol collection capability of the cylin-drical resonator for two aerosol types: particle scrubbing of incense smoke with a water fog, and demisting of water drops. For particle scrubbing, the decrease in particle concentration measured by laser particle counters was used to find the particle collection efficiency for the setup. The particle collection efficiency was used as a metric to determine the effectiveness of the cylinder for different input powers and air flow rates. Demisting experiments were performed with a similar setup. Here, a mass based method was used to determine the amount of liquid collected by the system with the cylindrical resonator. The drop collec-tion efficiency, similar to the particle collection efficiency, was the primary metric used for determining the demisting effectiveness. The introduction of ultrasonics increased both the particle collection efficiency and the collection efficiency to greater than 0.8, or 80% re-moved. These values increased with the power applied to the resonator, but decreased with the air flow rate. The results were an improvement upon the results obtained previously for ultrasonic particle scrubbing, wherein maximum particle collection efficiencies of 0.2 were obtained. The proposed mechanism for the increase in performance with the addition of ultrasonics was coagulation of particles and/or drops due to diffusion. The closer drops and/or particles are to each other, the more effective this mechanism is, and for the setup used herein, the acoustic radiation force moved particles and/or drops close together in the nodal regions of the standing wave field. By moving these to the nodes of the field, and then allowing time for them to collide in a passive tube, significant increases in performance were achieved. The results were used to estimate the scalability of such a setup to an industrial setting for particle scrubbing. As is, the current limitation of the device is still the flow rate which can be accommodated by a single unit. It would require about 1,000 units per MW of generating capacity to filter the hazardous micron-scale particulate pollution from a coal fired power plant.

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