Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 Experimental Study of Heat Transfer Through Window Assemblies Under External Heat Flux(2023) Schrader, Rebekah; Ni, Shuna; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Structure hardening is a key strategy to help mitigate building destruction during wildland-urban interface (WUI) fires. While hardening all exterior components of a structure is important, windows have specifically been identified as a vulnerable part of a building. The purpose of this study is to characterize the heat transfer through single- and double-pane windows constructed of plain and tempered glass. Double-pane windows with and without low-emissivity coatings and with either air or argon-filled gaps are included in this study. Small-scale experiments were performed on 23 cm x 23~cm windows exposed to a radiant panel producing centerpoint heat fluxes of 10, 20, 30, 40, and 50 kW/m2 to the exposed side of the glass. Each experimental condition was tested in triplicate. Total and radiative heat flux was measured 5.1 cm behind the unexposed side of the glass at the center of the window. Additionally, total heat flux was measured in the bottom corner of the window to characterize the difference in uniformity of heat transfer across the plane of the window. Surface temperatures on the exposed and unexposed side of the glass were measured in various locations using type K inconel-sheathed thermocouples. Tests lasted for either 20 minutes, until glass failure, or until frame failure. Times to glass crack and failure were recorded. Results showed that double-pane windows reduce heat transfer through a window compared to single-pane windows (13-43% and 39-60% of incident measured, respectively); additionally, the application of a low-emissivity coating is effective (heat fluxes measured were 5-17% of incident). Plain vs. tempered glass and air vs. argon-filled pane gaps do not yield statistically different results in heat flux measured behind the window. Temperatures were not uniform across the plane of the glass on both the exposed and unexposed sides. Finally, tempered glass had better survivability than plain glass (22/23 and 0/16 survived at incident heat fluxes up to 30 kW/m2, respectively), and double-pane argon-filled windows consistently survived longer than double-pane air-filled windows.Item Burning Emulations of Condensed Phase Fuels Aboard The International Space Station(2022) Dehghani, Parham; Sunderland, Peter B; Quintiere, James G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Little is known about the fire hazards of solids and liquids in microgravity. Ground-based tests are too short to overcome ignition transients and testing dozens of condensed fuels in orbit is prohibitively expensive. Burning rate emulation is one way to address this gap. It involves emulating condensed fuels with gases using a porous burner with embedded heat flux gages. This is a study of microgravity burning rate emulation aboard the International Space Station. The burner had porous round surfaces with a diameter of 25 mm. The fuel mixture was gaseous ethylene, and it was diluted with various amounts of nitrogen. The resulting heats of combustion were 15 – 47.2 kJ/g. The flow rate, oxygen concentration in the ambient, and pressure were varied. Heat flux to the burner was measured with two embedded heat flux gages and a slug calorimeter. The effective heat of gasification was determined from the heat flux divided by the fuel flow rate. Radiometers provided the radiative loss fractions. A dimensional analysis based on radiation theory yielded a relationship for radiative loss fraction. RADCAL, a narrow-band radiation model, yielded flame emissivities from the product concentrations, temperature, flame length, and pressure. Previously published analytical solutions to these flames allowed prediction of flame heights and radius, and when combined with the radiation empirical relationship led to corrections of total heat release rate from the flames due to radiative loss. Average convective and radiative heat flux were obtained from the analytical solution and a model based on the geometrical view factor of the burner surface with respect to the flame sheet, that was used to calculate the heat of gasification. All flames burning in 21% by volume oxygen self-extinguished within 40 s. However, steady flames were observed at 26.5, 34, and 40% oxygen. The analytical solution was used to quantify flame steadiness just before extinction. The steadiest flames reached more than 94% of their steady-state heat fluxes and heights. A flammability map as a plot of the heat of gasification versus heat of combustion was developed based on the measurement and theory for nominal ambient oxygen mole fractions of 0.265, 0.34, and 0.4.Item The Effects of Gravity on Flow Boiling Heat Transfer(2021) Hammer, Caleb Franklin; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow boiling is a method of phase change heat transfer used widely in electronics cooling, refrigeration, air conditioning, and other areas where stable temperatures are needed. An area of interest is spaceflight systems, where efficient heat transfer is desired to minimize mass, power requirements, and cost. When compared to terrestrial gravity conditions, the heat transfer of flow boiling in microgravity typically depreciates. This depreciation has been documented across multiple experimental studies performed by teams using different fluids, tube geometries, and flow regimes over the past three decades. Though select experimental microgravity flow boiling heat transfer data are available in the literature, holistic results are sparse due to the cost and limited availability of microgravity research. The two-phase heat transfer mechanisms responsible for the depreciation are therefore not well known, and so heat transfer models for variable gravity flow boiling do not exist. The goal of the proposed study is to develop models for flow boiling heat transfer through a tube as a function of gravity by identifying the effect of gravity on different heat transfer mechanisms. The scope of this proposal involves modeling three microgravity flow regimes (bubbly, slug, and annular flow) to serve as baseline predictions for flow boiling heat transfer without the influence of gravity. Additional gravity effects can be identified using partial and hyper-gravity data. Experiments have been performed aboard parabolic flights and on the ground at various flow rates, heating rates, and inlet subcoolings in microgravity, hyper-gravity, Lunar gravity, Martian gravity, and terrestrial gravity. Results from the experiments showed that negligible slip velocity plays an important role in modeling flow boiling heat transfer. Simulations using modified single-phase models of an accelerating flow were performed which predicted microgravity flow boiling heat transfer well in the nucleate boiling regime.Additional experiments concerning terrestrial gravity quenching heat transfer have been performed to address research gaps in microgravity cryogen chilldown studies. Quenching heat transfer coefficients were recorded in the nucleate boiling regime and compared with correlations. The correlations were able to predict heat transfer for room temperature fluids much more accurately than for cryogenic fluids. Scaling parameters must be tuned to match cryogen data to examine the large disparity between cryogenic quenching heat transfer data and correlations observed in the literature.Item DESIGN AND ANALYSIS OF A NOVEL, ULTRA-LIGHT, CRYOGENIC DEWAR FOR BALLOON-BORNE OBSERVATORIES(2020) Denker, Samuel; diMarzo, Marino; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The deployment of cryogenic Dewars aboard high-altitude balloons is critical to astronomical observation at submillimeter wavelengths. Balloon-borne, cryogenically cooled telescopes are limited in size by weight restrictions of the balloons, which is dominated by the Dewars. This thesis presents a portion of the multi-phase BOBCAT project which reduces Dewar weight with the use of thin-walled designs and a novel multi-layer insulation (MLI) system. The BOBCAT-1 mission used conventional Dewar technology to demonstrate cryogen transfer at float altitude and establish baseline thermal performance of balloon-borne Dewars. Design and assembly of the BOBCAT-2 ultra-light Dewar showed successful fabrication of the thin-walled vessel and novel MLI system. Thermal modelling predicts that the BOBCAT-2 Dewar will experience an order of magnitude increase in heat transfer through the MLI, equivalent to a 60% increase through the Dewar in total, due to its larger volume and decreased number of radiation shields relative to the BOBCAT-1 Dewar.Item Experimental Investigation into the Heat Transfer Mechanism of Oscillating Heat Pipes using Temperature Sensitive Paints(2020) Francom, Matthew Brent; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Oscillating heat pipes (OHPs) represent a promising passive mechanism for the removal or spreading of heat. While simple to construct, the fluid and thermodynamics of these devices are still poorly understood. There is debate over whether the primary heat transfer mechanism is due to sensible heating of the liquid phase or due to latent heat transfer through phase change. To provide experimental data answering this question, an experimental apparatus was constructed to provide local temperature and heat transfer data across the face of an OHP during operation. This experiment utilized temperature sensitive paint alongside visual recording of the fluid motion in order to determine the relative latent and sensible contribution to the overall heat transfer. The OHP was tested with input powers ranging from 2.6 W to 10.1 W. It found that latent heat transfer was dominant, representing between 65% and 85% of the total heat transferred in all cases.Item 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.Item Heat Transfer Measurements in a Supersonic Film Flow(2016) Adamson, Colin Sawyer; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents measurements of wall heat flux and flow structure in a canonical film cooling configuration with Mach 2.3 core flow in which the coolant is injected parallel to the wall through a two-dimensional louver. Four operating conditions are investigated: no film (i.e. flow over a rearward-facing step), subsonic film, pressure-matched film, and supersonic film. The overall objective is to provide a set of experimental data with well characterized boundary conditions that can be used for code validation. The results are compared to RANS and LES simulations which overpredict heat transfer in the subsonic film cases and underpredict heat transfer in supersonic cases after film breakdown. The thesis also describes a number of improvements that were made to the experimental facility including new Schlieren optics, a better film heater, more data at more locations, and a verification of the heat flux measurement hardware and data reduction methods.Item Slot Film Cooling: A Comprehensive Experimental Characterization(2016) Raffan Montoya, Fernando; Marshall, Andre W; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When components of a propulsion system are exposed to elevated flow temperatures there is a risk for catastrophic failure if the components are not properly protected from the thermal loads. Among several strategies, slot film cooling is one of the most commonly used, yet poorly understood active cooling techniques. Tangential injection of a relatively cool fluid layer protects the surface(s) in question, but the turbulent mixing between the hot mainstream and cooler film along with the presence of the wall presents an inherently complex problem where kinematics, thermal transport and multimodal heat transfer are coupled. Furthermore, new propulsion designs rely heavily on CFD analysis to verify their viability. These CFD models require validation of their results, and the current literature does not provide a comprehensive data set for film cooling that meets all the demands for proper validation, namely a comprehensive (kinematic, thermal and boundary condition data) data set obtained over a wide range of conditions. This body of work aims at solving the fundamental issue of validation by providing high quality comprehensive film cooling data (kinematics, thermal mixing, heat transfer). 3 distinct velocity ratios (VR=uc/u∞) are examined corresponding to wall-wake (VR~0.5), min-shear (VR ~ 1.0), and wall-jet (VR~2.0) type flows at injection, while the temperature ratio TR= T∞/Tc is approximately 1.5 for all cases. Turbulence intensities at injection are 2-4% for the mainstream (urms/u∞, vrms/u∞,), and on the order of 8-10% for the coolant (urms/uc, vrms/uc,). A special emphasis is placed on inlet characterization, since inlet data in the literature is often incomplete or is of relatively low quality for CFD development. The data reveals that min-shear injection provides the best performance, followed by the wall-jet. The wall-wake case is comparably poor in performance. The comprehensive data suggests that this relative performance is due to the mixing strength of each case, as well as the location of regions of strong mixing with respect to the wall. Kinematic and thermal data show that strong mixing occurs in the wall-jet away from the wall (y/s>1), while strong mixing in the wall-wake occurs much closer to the wall (y/s<1). Min-shear cases exhibit noticeably weaker mixing confined to about y/s=1. Additionally to these general observations, the experimental data obtained in this work is analyzed to reveal scaling laws for the inlets, near-wall scaling, detecting and characterizing coherent structures in the flow as well as to provide data reduction strategies for comparison to CFD models (RANS and LES).Item A STUDY OF HEAT TRANSFER AND FLOW CHARACTERISTICS OF RISING TAYLOR BUBBLES(2016) Scammell, Alexander; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Practical application of flow boiling to ground- and space-based thermal management systems hinges on the ability to predict the system’s heat removal capabilities under expected operating conditions. Research in this field has shown that the heat transfer coefficient within two-phase heat exchangers can be largely dependent on the experienced flow regime. This finding has inspired an effort to develop mechanistic heat transfer models for each flow pattern which are likely to outperform traditional empirical correlations. As a contribution to the effort, this work aimed to identify the heat transfer mechanisms for the slug flow regime through analysis of individual Taylor bubbles. An experimental apparatus was developed to inject single vapor Taylor bubbles into co-currently flowing liquid HFE 7100. The heat transfer was measured as the bubble rose through a 6 mm inner diameter heated tube using an infrared thermography technique. High-speed flow visualization was obtained and the bubble film thickness measured in an adiabatic section. Experiments were conducted at various liquid mass fluxes (43-200 kg/m2s) and gravity levels (0.01g-1.8g) to characterize the effect of bubble drift velocity on the heat transfer mechanisms. Variable gravity testing was conducted during a NASA parabolic flight campaign. Results from the experiments showed that the drift velocity strongly affects the hydrodynamics and heat transfer of single elongated bubbles. At low gravity levels, bubbles exhibited shapes characteristic of capillary flows and the heat transfer enhancement due to the bubble was dominated by conduction through the thin film. At moderate to high gravity, traditional Taylor bubbles provided small values of enhancement within the film, but large peaks in the wake heat transfer occurred due to turbulent vortices induced by the film plunging into the trailing liquid slug. Characteristics of the wake heat transfer profiles were analyzed and related to the predicted velocity field. Results were compared and shown to agree with numerical simulations of colleagues from EPFL, Switzerland. In addition, a preliminary study was completed on the effect of a Taylor bubble passing through nucleate flow boiling, showing that the thinning thermal boundary layer within the film suppressed nucleation, thereby decreasing the heat transfer coefficient.Item CHARACTERIZATION OF HEAT TRANSFER AND PRESSURE DROP OF NORMAL FLOW HEAT EXCHANGERS IN COUNTER FLOW CONFIGURATION(2014) Andhare, Rohit Subhash; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In today's times, successful technology advancement lies in making systems that are highly compact, offer superior energy efficiency, while sustainable and cost effective . There is interest in developing small heat exchangers having better flow distribution control rather than bulky heat exchangers which are energy intensive. Microchannels and microreactors controlled by microprocessors are slowly taking over energy conversion, transportation and process industry. The nature inspired - Fractal arrangement of manifold-microchannels has the potential to provide enormous heat transfer capabilities at an attractive coefficient of performance. However majority of such fractal flow manifolds are very short and operate with short counterpart microchannel. They have not been completely adopted for counter flow configuration required by majority of the industrial processes. The work covered under this thesis is focused on adopting of high performance fractal microchannel arrangement to counter flow configuration heat exchangers that are required by industrial processes. Two single phase solution heat exchangers were developed using this approach. The solution heat exchanger is an essential component in absorption refrigeration cycle to convert waste heat into cooling. The study also utilized the novel additive manufacturing process of 3D printing to develop a tubular manifold in order to promote the fractal normal flow on tubular surfaces. The heat exchangers developed as a part of this thesis show enhancement in the overall performance and demonstrate high potential of the proposed technology.
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