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
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Item Microturbopump Utilizing Microball Bearings(2008-08-05) Waits, Christopher Michael; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the development of a microfabricated turbopump capable of delivering fuel with the flow rates and pressures required for portable power generation. The device is composed of a spiral-groove viscous pump that is driven by a radial in-flow air turbine and supported using a novel encapsulated microball bearing. First, the encapsulated microball bearing and methods to investigate the wear and friction behaviors were developed. Two primary raceway designs, point-contact and planar-contact designs, were developed with the key design factor being wearing of the raceway. A modification to the planar-contact design was made for the final turbopump that reduced both wear and debris generation. Second, two air turbine platforms were developed using the encapsulated microball bearings to characterize both the bearing and the turbine drive mechanism. A tangential air turbine platform was first developed and characterized using the point-contact bearing mechanism. Rotational speeds >37,000 rpm were demonstrated and long-term operation (>24 hours) using this platform, but with large driving pressures (tens of psi) and large raceway wear (tens of microns). Furthermore, the circumferential asymmetry of the turbine design led to difficulty in measuring pressure distribution and sealing for pump applications. Results from the tangential air turbine platform led to an axisymmetric radial in-flow air turbine platform using a planar-contact bearing design. Rotational speeds greater than 85,000 rpm with turbine pressure differentials in the range of 1 psi were demonstrated using this platform. The wear of the raceway was observed to be on the order of single microns (a 10x improvement). The radial in-flow air turbine platform allowed an empirical model to be developed relating the friction torque to the rotational speed and load for the planar-contact bearing. This enabled calculation of the power balance for pumping and a method to characterize future bearing designs and materials. Lastly, a microfabricated turbopump was demonstrated based on a spiral-groove viscous pump and the radial in-flow turbine platform using the planar-contact bearing. Pumping operation was demonstrated with a differential pressure up to +0.3 psi and flow rates ranging from 35 mL/hour to 70 mL/hour, within the range relevant to portable power generation.Item Design and Characterization of an Electrohydrodynamic (EHD) Micropump for Cryogenic Spot Cooling Applications(2008-04-21) Foroughi, Parisa; Ohadi, Michael M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-temperature superconducting (HTSC) components are being incorporated into communication and monitoring electronic devices to increase their signal-to-noise ratio or their channel capacity. Those devices must be maintained at cryogenic temperatures to prevent the loss of their superconducting properties and retain their performance superiority. They are conventionally cooled via direct heat conduction, which leads to undesirable temperature differences among the various components being cooled. Compact micropumps capable of pumping liquid nitrogen at 77 K into liquid-cooling circuits would enable a much more compact and lightweight method of maintaining a uniform temperature across the cooling circuit. These pumps can also address the demand for delivering small doses of LN2 to particular spots in bioengineering applications.
One of the main objectives of the present study was to develop an electrohydrodynamic (EHD) ion-drag micropump with LN2 as the working liquid. EHD ion-drag pumping phenomenon refers to liquid motion caused by an interaction between electric and hydrodynamic fields in a dielectric liquid.
To investigate the effect of each design parameter on the performance of the micropump, several prototypes with four distinct designs were fabricated and packaged. The designs included a variety of emitter shapes, inter-electrode spacings, electrode-pair spacings, and channel heights. The micropumps were tested at different DC voltages ranging from 0 to 2.5 kV. Two test rigs with novel measurement techniques were also designed, built, and calibrated to measure the generated static pressure head, electric current, and flow rate with an acceptable level of accuracy.
The relationships between pressure/current (P-I) and pressure/voltage (P-V) for various designs were investigated experimentally. The results showed good agreement with the general analytical trends reported for EHD pumping in the literature. The experimental results also demonstrated that electrode geometry and gaps are effective in determining the pressure onset voltage. The results also show that a maximum static pressure head of 160 Pa at 1400 V is achievable for a design with a combination of a 50-μm emitter-collector gap, a 200-μm electrode-pair gap, and a saw-tooth shaped emitter/flat collector.Item EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF PLANAR ION DRAG MICROPUMP GEOMETRICAL DESIGN PARAMETERS(2005-06-07) Benetis, Vytenis; Ohadi, Michael; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To deal with increasing heat fluxes in electronic devices and sensors, innovative new thermal management systems are needed. Proper cooling is essential to increasing reliability, operating speeds, and signal-to-noise ratio. This can be achieved only with precise spatial and temporal temperature control. In addition, miniaturization of electric circuits in sensors and detectors limits the size of the associated cooling systems, thereby posing an added challenge. An innovative answer to the problem is to employ an electrohydrodynamic (EHD) pumping mechanism to remove heat from precise locations in a strictly controlled fashion. This can potentially be achieved by micro-cooling loops with micro-EHD pumps. Such pumps are easily manufactured using conventional microfabrication batch technologies. The present work investigates ion drag pumping for applications in reliable and cost effective EHD micropumps for spot cooling. The study examines the development, fabrication, and operation of micropumps under static and dynamic conditions. An optimization study is performed using the experimental data from the micropump prototype tests, and a numerical model is built using finite element methods. Many factors were involved in the optimization of the micropump design. A thorough analysis was performed of the major performance-controlling variables: electrode and inter-electrode pair spacing, electrode thickness and shape, and flow channel height. Electrode spacing was varied from 10 µm to 200 µm and channel heights from 50 µm to 500 µm. Also, degradation of the electrodes under the influence of an intense electric field was addressed. This design factor, though important in the reliability of EHD micropumps, has received little attention in the scientific and industrial applications literature. Experimental tests were conducted with prototype micropumps using the electronic liquid HFE7100 (3M®). Flow rates of up to 15 ml/min under 15 mW power consumption and static pumping heads up to 750 Pa were achieved. Such performance values are acceptable for some electronic cooling applications, where small but precise temperature gradients are required.