Theses and Dissertations from UMD
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Item NEAR-LIMIT SPHERICAL DIFFUSION FLAMES AND COOL DIFFUSION FLAMES(2023) Waddell, Kendyl; Sunderland, Peter B; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To combat the rising threats of climate change, current combustion technologies must evolve to become cleaner and more efficient. This requires a better understanding of the fundamental properties of combustion. One way to gain this is through microgravity experiments, where the lack of buoyancy reduces flames to their most basic components, simplifying modeling efforts. The low-temperature combustion of warm and cool flames, which has applications in advanced engine technologies and implications in terrestrial and spacecraft fire safety, is favored in microgravity. In this work, microgravity spherical diffusion flames are generated aboard the International Space Station using a spherical porous burner. A transient numerical model with detailed chemistry, transport, and radiation is used to simulate the flames. This incorporates the UCSD mechanism with 57 species and 270 reactions. Hot, warm, and cool diffusion flames are all studied. Experimental flame temperature was measured using thin-filament pyrometry, which was calibrated using a blackbody furnace. The measured temperatures agreed reasonably well with numerical simulations for a wide range of conditions, and were in the range of 950-1600 K, with an estimated uncertainty of ± 100 K. The temperatures of the porous spherical burner were measured by a thermocouple embedded in its surface. These measured temperatures, combined with numerical simulations of the gas phase, yield insight into the complex heat transfer processes that occur in and near the porous sphere. Previous work has found that ethylene microgravity spherical diffusion flames extinguish near 1130 K at atmospheric pressure, regardless of the level of reactant dilution. The chemical kinetics associated with this consistent extinction temperature are explored using the transient numerical model. Species concentrations, reaction rates, and heat release rates are examined. Upon ignition, the peak temperature is above 2000 K, but this decreases until extinction due to radiative losses. This allows the kinetics to be studied over a wide range of temperatures for the same fuel and oxidizer. At high temperature, the dominant kinetics are similar to those reported for typical normal-gravity hydrocarbon diffusion flames. There are well defined zones of pyrolysis and oxidation, and negligible reactant leakage through the reaction zone. As the flame cools, there is increased reactant leakage leading to higher O, OH, and HO2 concentrations in the fuel-rich regions. The pyrolysis and oxidation zones overlap, and most reactions occur in a narrow region near the peak temperature. Reactions involving HO2 become more significant and warm flame chemistry appears. At atmospheric pressure, this low-temperature chemistry delays extinction, but does not produce enough heat to prevent it. As ambient pressure is increased, low-temperature chemistry is enhanced, allowing the flame to extend into the warm flame and cool flame regimes. Experimental results show that increasing the pressure from 1 atm to 3 atm decreased the ethylene extinction temperature by almost 60 K. Numerical simulations showed similar behavior, as well as the emergence of cool flame behavior when the pressure was increased to 50 atm. This allows the kinetics of spherical warm and cool diffusion flames and the role of increased HO2 participation to be examined. There are few options for studying cool diffusion flames experimentally that do not require expensive facilities that are unavailable to the average researcher. A method is presented for observing cool diffusion flames inexpensively using a pool of liquid n-heptane and parallel plates heated to produce a stably stratified stagnation flow. The flames were imaged with a color camera and an intensified camera. Measurements included gas phase temperatures, fuel evaporation rates, and formaldehyde yields. These are the first observations of cool flames burning near the surfaces of fuel pools. The measured peak temperatures were between 705 – 760 K and were 70 K above the temperature of the surrounding air. Autoignition first occurred at 550 K.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 LAMINAR SMOKE POINTS OF COFLOWING DIFFUSION FLAMES IN MICROGRAVITY(2012) DeBold, Thomas; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nonbuoyant laminar jet diffusion flames in coflowing air were observed aboard the International Space Station with an emphasis on laminar smoke points. The tests extended the 2009 Smoke Points In Coflow Experiment (SPICE) experiment to new fuels and burner diameters. Smoke points were found for methane, ethane, ethylene, and propane burning in air. Conditions included burner diameters of 0.76, 1.6, 2.1, and 3.2 mm and coflow velocities of 3.0 - 47 cm/s. This study yielded 57 new smoke points to increase the total number of smoke points observed to 112. Smoke point lengths were found to scale with burner diameter raised to the -0.67 power times coflow velocity raised to the 0.27 power. Sooting propensity was observed to rank according to methane < ethane < ethylene < propane < 50% propylene < 75% propylene < propylene. This agrees with past normal gravity measurements except for the exchanged positions of ethylene and propane. This is the first time a laminar smoke point has been observed for methane at atmospheric pressure.Item 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.