Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item COMPACT ABSORBER FOR ADVANCED ABSORPTION HEAT PUMPS(2018) Bangerth, Stefan; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Almost half of all energy contained in primary energy carriers is discarded as low temperature waste heat. One of few application areas for low temperature waste heat recovery is to drive absorption cooling systems for conversion of waste heat to cooling energy. However, absorption chillers are often not economical due to their bulky, and hence expensive, heat and mass exchangers; the absorber heat/mass exchanger being the largest among them. This dissertation introduces original contributions to advance next generation, more economical absorption chillers by utilizing a novel, highly compact absorber. The novel absorber designed in this work enhances absorption performance by combining rotation of the heat transfer surface for solution-side heat and mass transfer enhancement with innovative high-performance heat transfer technology on the water-side. A numerical model was developed to describe the absorption process and promote design optimization. The replacement of gravitation force by the stronger centrifugal acceleration thins and mixes the solution film and thereby decreases solution-side thermal and mass transfer resistance. The development of an original adaptation of manifold-microchannel technology leads to significant water-side heat transfer enhancement. This dissertation includes the first publication of an experimental characterization of exothermic absorption on a spinning disk. The overall and film-side heat transfer coefficients were 4.7 and 5.5 times higher, respectively, than conventional horizontal tube banks. The absorption rate increased by a factor of 4 to 10 folds over those of the conventional tube absorbers. The power required for spinning the disk was modest and ranged between 1.1% and 2.3% of the cooling capacity. The results suggest that a spinning disk absorber could substantially reduce the size of absorber in the absorption machines. The technology developed in this dissertation can lead to more compact and hence more economical absorption chillers, thereby easing higher market penetration of absorption chillers which in turn can reduce the amount of primary energy spent on cooling applications. Spinning disk absorbers may be especially useful if combined with a new generation of absorbents that promise improved system efficiency and/or expanded application range but exhibit challenging thermophysical properties.Item ENHANCED DIFFUSIOOSMOSIS AND THERMOOSMOSIS IN POLYELECTROLYTE-BRUSH-FUNCTIONALIZED NANOCHANNELS(2018) Maheedhara, Raja; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the holy grails of nanofluidic systems is to ensure significant flow rates without applying a large pressure gradient. This has motivated researchers to study different mechanisms of liquid transport in nanochannels involving physical effects that exploit the large surface-to-volume ratio of such nanochannels. This thesis will focus on two highly efficient non-pressure-driven flow mechanisms in nanochannels functionalized by grafting the inner walls of nanochannels with end-charged polyelectrolyte (PE) brushes. We study two mechanisms to achieve flow augmentation: (i) ionic diffusioosmosis (IDO), triggered by the application of an external concentration gradient, and (ii) ionic thermoosmosis (ITO), triggered by a temperature gradient. We find a non-intuitive scenario where the flow in nanochannels can be significantly augmented by grafting the nanochannels with PE brushes. Given the difficulty in attaining a desirable flow strength in nanochannels, we anticipate that this thesis will serve as an important milestone in the area of nanofluidics.Item ENHANCEMENT OF THERMAL INACTIVATION OF CRONOBACTER SAKAZAKII WITH INCLUSION OF PARABENS(2016) Ruan, Luxi; Buchanan, Robert L; Food Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Parabens are a family of p-hydroxybenzoic acid esters, which have antimicrobial activity over a broad pH range (4-8). This study was designed to evaluate the enhanced thermal inactivation of Cronobacter sakazakii by the inclusion of “parabens” and to ultimately develop mathematical models to describe this effect. A heat-resistant strain, Cronobacter sakazakii 607, was heated at three mild heating temperatures in combination with treatments with five parabens in various concentrations. Results showed the presence of parabens significantly enhanced thermal inactivation in a concentration-dependent manner, and the effect increased with increasing alkyl chain length. The concentration of parabens, alkyl side chain length, and heating temperature acted synergistically, causing bacterial inactivation even at low temperatures that were not effective in killing C. sakazakii. The survival data were used to develop primary and secondary mathematical models that accurately describe how this synergistic activity can be applied in the food industry.