Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 155-170).

Yes / Positron emission particle tracking (PEPT) was used to visualise the flow patterns established by mixing in two laboratory-scale anaerobic digesters fitted with mechanical mixing or gas mixing apparatus. PEPT allows the visualisation of flow patterns within a digester without necessitating the use of a transparent synthetic sludge. In the case of the mechanically-mixed digester, the mixing characteristics of opaque sewage sludge was compared to a transparent synthetic sludge at different mixing speeds. In the gas-mixed apparatus, two synthetic sludges were compared. In all scenarios, quasi-toroidal flow paths were established. However, mixing was less successful in more viscous liquids unless mixing power was increased to compensate for the increase in viscosity. The robustness of the PEPT derived velocities was found to be significantly affected by the frequency with which the particle enters a given volume of the vessel, with the accuracy of the calculated velocity decreasing in regions with low data capture. Nevertheless, PEPT was found to offer a means of accurate validation of computational fluid dynamics models which in turn can help to optimise flow patterns for biogas production. / The first author was funded via an EPSRC CASE award in conjunction with Severn Trent Water. The second author was funded via a University of Birmingham Postgraduate Teaching Assistantship award.

Master of Science / Department of Chemistry / Christopher T. Culbertson / Mixing is an essential process to many areas of science for example it is important in studying chemical reaction kinetics, chemical synthesis, DNA hybridization and PCR amplification. Mixing on the macroscale level is readily achieved through convection. Rapid mixing on microchips however, is problematic as the low Reynolds numbers and high Peclet numbers indicate that fluid flow is in the laminar regime and limits mixing on microchips to diffusion. Because of these limitations mixing on microchips is often relegated to diffusional mixing which requires long channels and long time periods. Several methods have been developed to increase the speed and efficiency of mixing on microfluidic devices. A variety of techniques have been employed to overcome these obstacles including for example 1) 3 dimensional channel designs to split up and recombine flows 2) employing sophisticated lithographic techniques to make grooves within a channel to generate transverse flows and 3) using lateral flow created by using spiral channels. Other groups have used outside energy sources to achieve mixing by changing of the zeta potential within the channel, using induced charge electroosmosis, and also by modifying the electrokinetic flow. We propose using dielectric elastomers (DEs) to modulate flow as a means to achieve rapid and active mixing on the microchip format. Electroactive polymers such as poly(dimethylsiloxane) function as DEs and are capable of converting electrical energy into mechanical energy. The application of an electrical potential across the PDMS results in a change in the dimensions of the PDMS dielectric layer between the two actuating electrodes creating an actuator. When employed in microfluidic devices this actuator can be used to change the volumes of the microfluidic channels on the PDMS. If the actuators are placed near a T-intersection where two components are entering the intersection the actuators can serve to improve mixing on microfluidic devices. Studies were conducted on how on the magnitude of the actuation, the frequency of actuation, the field strength, the electrode design and position relative to the T intersection, the channel dimensions and the overall channel design impacted mixing efficiency. Mixing results showed promise but further development of technology is necessary to achieve adequate mixing in microfluidic channels using DEs.

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Due to strong motivation to reduce costs and increase performances of stationary diesel after-treatment systems, computational modeling has become a necessary step in system design and improvement. A unique mixing duct typified by significant changes in scale and strong flow curvature was evaluated for its potential to improve flow distribution across the SCR catalyst inlet face. The flow dynamics were investigated with a steady three-dimensional turbulence model and detailed chemistry was studied separately using a one-dimensional channel reactive flow model. Aqueous urea injection was modeled using Discrete Phase Modeling. The mixing duct performance relative to reactor dimensions and engine loads is discussed. The Impact of injector positions was studied using massless particle tracking. A total of three geometries were evaluated using a Uniformity Index of the ammonia-to-NOx feed ratio. It was found that a higher mixing duct height to inlet diameter ratio yielded better mixing.

Due to strong motivation to reduce costs and increase performances of stationary diesel after-treatment systems, computational modeling has become a necessary step in system design and improvement. A unique mixing duct typified by significant changes in scale and strong flow curvature was evaluated for its potential to improve flow distribution across the SCR catalyst inlet face. The flow dynamics were investigated with a steady three-dimensional turbulence model and detailed chemistry was studied separately using a one-dimensional channel reactive flow model. Aqueous urea injection was modeled using Discrete Phase Modeling. The mixing duct performance relative to reactor dimensions and engine loads is discussed. The Impact of injector positions was studied using massless particle tracking. A total of three geometries were evaluated using a Uniformity Index of the ammonia-to-NOx feed ratio. It was found that a higher mixing duct height to inlet diameter ratio yielded better mixing.

No description available.