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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    THERMAL AND HYDRAULIC PERFORMANCE OF SPINE FIN TUBE HEAT EXCHANGERS AT LOW REYNOLDS NUMBER CONDITIONS
    (2017) Herrera, Carlos; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The goal of this work is to present the air-side thermal and hydraulic performance of heat exchangers with spine-fin surface augmentation. Although not as common as plain / plate fin, spine-fin heat exchangers have been used for decades in household refrigeration evaporators and in the outdoor coils of household air-conditioning systems. Of particular interest in this study, was the performance at low air-side Reynolds numbers (500 – 900). Heat transfer coefficients for this geometry were evaluated for samples of varying fin pitch, fin height and tube diameter in both parallel and angled bank arrangements. Water was selected as the hot fluid operating in the turbulent regime with mass flow rates varying at each airflow rate test point. Static cold and hot stream temperatures were maintained for all tests. Air-side heat transfer coefficient (AHTC) is highest for the lower diameter tube heat exchangers and increases in fin pitch lowered the AHTC. This behavior is not seen in plain fin, microchannel and other heat exchangers.
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    PREDICTION OF HEAT TRANSFER AND PRESSURE DROP OF CONDENSING REFRIGERANT FLOW IN A HIGH ASPECT RATIO MICRO-CHANNELS
    (2009) Al-Hajri, Ebrahim Saeed Abdulla; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis presents a detailed study of parametric characterization of two-phase condensing flow of two selected refrigerants R134a and R-245fa in a single water-cooled micro-channel of 0.4 mm X 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and 190 mm in length. To avoid flow mal-distribution associated with typical micro-channel tube banks, a single micro-channel was fabricated utilizing an innovative approach and used for the present study experiments. The study investigated the effects of variations in saturation temperature ranging from 30 oC to 70 oC, mass flux from 50 to 500 kg/m2s, and inlet super heat from 0 oC to 15 oC on the average heat transfer and overall pressure drop coefficient of the micro-channel condenser. In all cases the inlet vapor quality was kept at 100% quality (saturated vapor) and the outlet condition was always kept at 0% quality (saturated liquid). Accuracy of the fabricated channel geometry with careful design and choice of instrumentation of the test setup resulted in energy balance and average heat transfer coefficient uncertainties within +/-11% and +/-12%, respectively. It is observed that saturation temperature and mass flux have a significant effect on both heat transfer coefficient and overall pressure drop coefficient, where as the inlet super heat has little effect. This study provides further understanding of the potential micro-scale effects on the condensation phenomenon for the tube geometry and the dimensions investigated in the present study combined with flow visualization study. No previous study has addressed the unique single micro-channel geometry being investigated in the present work combined with the two-phase flow visualization of the flow regimes in the present micro-channel geometry. The letter was a major undertaking of the present work and represents one of the main contributions of the present work. The results of the present work shall prove useful in contributing to better understanding of any micro-scale effects on the condensation flow of the two selected refrigerants (one commonly used high pressure refrigerant, R134a) and the other a new low pressure refrigerant (R245fa). It is also expected that the results of this study will lead to future work in this area, realizing the fast penetration of the micro-channel technology in various compact/ultra compact heat exchangers, including refrigeration, petrochemical, electronics, transportation, and process industries.