Mechanical Engineering

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    Evaluation of solder-joint reliability for a 10mm Quad Flat Leadless package with top-side paddle using classical models for a leadless device and accelerated life testing
    (2009) Levin, Mark Alan; Barker, Dr. Donald; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The standard QFN package consists of a leadless perimeter array and a bottom solderable thermal paddle. The thermal performance of the package can be improved by moving the paddle to the topside. The soldered surface area of the package reduces by about 80% with a top-side paddle. The soldered-joint life will also reduce due to the significant thermal coefficient of expansion mismatch between the QFN package and the circuit board. The solder-joint reliability of a large QFN package with top-side paddle is not well understood. This thesis evaluates the solder-joint reliability of a 10mm square leadless QFN package with top-side paddle. The analysis includes several classical models for a leadless package and compares modeling results to accelerated reliability testing. The accelerated tests include the influence mold compound and lead finish play on solder-joint life and ways to improve solder-joint reliability.
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    SEDIMENT SUSPENSION EVENTS FROM RIPPLE BEDS IN OSCILLATORY FLOW: EXPERIMENTS
    (2009) Knowles, Philip Leland; Kiger, Kenneth T; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    An experimental sediment flume is used to investigate sediment transport mechanics within an oscillatory turbulent boundary layer over a mobile sediment bed in the ripple bed regime. Two-phase PIV is utilized to simultaneously capture data from each phase, allowing examination of suspension mechanisms, carrier phase stresses, and to obtain statistics to describe the momentum exchange between the phases. The technique employs median filtering, as well as size and brightness criteria to separate and accurately identify each phase. Independent well-conditioned tests have been conducted to improve the algorithm to account for the imaging conditions encountered in the vicinity of a mobile bed in order to minimize cross-talk between the phases and allow quantification of the dispersed phase concentration. Results show that large-scale vortical structures are responsible for the ejection of sediment from the bed into the outer flow. These structures are a significant source of turbulent transport, but their overall contribution to the bed stress is small compared to the mean flow. Triple decomposition of the equations of motion show that long time averaged sediment flux is of similar magnitude to cyclic fluctuations and the time averaged flow consists of two counter rotating cells. Turbulent kinetic energy created at flow reversal advects over the sediment bed and keeps particles suspended in the flow. Calculation of the vertical particle drag, body force, and convection terms revels that at flow reversal the body force terms are larger than the drag causing the particles on average to settle. The particle convection terms are small compared to particle drag and body force terms. Some of the terms most significant in the particle drag are the fluctuating components indicating that the turbulence is keeping the particles suspended in the outer flow.
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    Performance and Oil Retention Characteristics of a CO2 Heat Pump Water Heater
    (2008) Fernandez, Nicholas Edward Peter; Radermacher, Reinhard K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A CO2 heat pump water heater (HPWH) was investigated experimentally and analytically. In the first stage of the study, a baseline performance was measured, investigating the effect of operating parameters on the system performance under typical tank heating scenarios. In the second, the CO2 HPWH was modeled to investigate the effect of optimizing key components. In the third, the oil retention mass, the increase in pressure drop, and the COP degradation were measured as a function of oil mass fraction. In the fourth, two alternative system configurations were investigated for potential performance enhancement; a two-stage compression cycle with internal heat exchanger and a system with a suction line heat exchanger. Overall, the CO2 cycle seems uniquely suited for water heating. CO2 HPWHs have enormous energy savings potential if the cooling from the evaporator can be harnessed during the summer months, and rejected to the environment during the colder months.