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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2005 - 200912010 - 20111

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    Improvements in Microboiling Device Design
    (2011) Carrier, Michael James; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Small ribbon heaters (10 μm - 20 μm wide) have been used for many years to study the formation of microbubbles in liquids when short voltage pulses are applied. This thesis describes improvements in the device design with an emphasis on smaller and more sensitive heaters. I used a novel method of creating 250 nanometer wide heaters to keep both the fabrication time and costs as low as possible by using a focused ion beam to create the heaters from a set of larger devices. Ribbon heaters are usually fabricated on a thin SiO2 layer on a silicon wafer which acts as a large heat sink whose effect becomes more pronounced the smaller the heater width. Suspending the heaters on a thin membrane dramatically increased their sensitivity in microboiling experiments. The suspended devices required the development of a very low stress platinum deposition process.
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    Nucleate Pool Boiling Characteristics From a Horizontal Microheater Array
    (2005-12-14) Henry, Christopher Douglas; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Pool boiling heat transfer measurements from different heater sizes and shapes were obtained in low-g (0.01 g) and high-g (1.7 g) aboard the NASA operated KC-135 aircraft. Boiling on 4 square heater arrays of different size (0.65 mm2, 2.62 mm2, 7.29 mm2, 49 mm2) was investigated. The heater arrays consist of 96 independent square heaters that were maintained at an isothermal boundary condition using control circuitry. A fractional factorial experimental method was designed to investigate the effects of bulk liquid subcooling, wall superheat, gravitational level, heater size and aspect ratio, and dissolved gas concentration on pool boiling behavior. In high-g, pool boiling behavior was found to be consistent with classical models for nucleate pool boiling in 1-g. For heater sizes larger than the isolated bubble departure diameter predicted from the Fritz correlation, the transport process was dominated by the ebullition cycle and the primary mechanisms for heat transfer were transient conduction and microconvection to the rewetting liquid in addition to latent heat transfer. For heater sizes smaller than this value, the boiling process is dominated by surface tension effects which can cause the formation of a single primary bubble that does not depart the heater surface and a strong reduction in heat transfer. In low-g, pool boiling performance is always dominated by surface tension effects and two mechanisms were identified to dominate heat and mass transport: 1) satellite bubble coalescence with the primary bubble which tends to occur at lower wall superheats and 2) thermocapillary convection at higher wall superheats and higher bulk subcoolings. Satellite bubble coalescence was identified to be the CHF mechanism under certain conditions. Thermocapillary convection caused a dramatic enhancement in heat transfer at higher subcoolings and is modeled analytically. Lastly, lower dissolved gas concentrations were found to enhance the heat transfer in low-g. At higher dissolved gas concentrations, bubbles grow larger and dryout a larger portion of the heater surface.