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
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Item COMPARISON BETWEEN PARTICLE IMAGE VELOCIMETRY DATA AND COMPUTATIONAL FLUID DYNAMICS SIMULATIONS FOR AN IN-LINE SLOT AND TOOTH ROTOR-STATOR MIXER(2015) Kim, Jung W.; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rotor-stator mixers have a broad spectrum of applications in chemical, petrochemical and pharmaceutical processes since they produce the high shear fields for emulsification and dispersion processes. To assess device performance and quantify mixing and dispersion capabilities, analyzing the velocity field data due to the rotor-stator interactions is crucial. Experimental 2-D velocity data have previously been acquired using Particle Image Velocity (PIV) for an in-line IKA prototype mixer which contains single rows of 12 rotor teeth and 14 stator teeth. The working fluid was water in turbulent flow. In this thesis, the development and validation of a Computational Fluid Dynamics (CFD) model is reported along with the comparison between the CFD and PIV data. The CFD model geometry and mesh were developed within ANSYS Workbench with a fully transient sliding mesh 3-D RANS simulations performed with Fluent using the realizable k-ϵ turbulence model. To begin, the effect of mesh density and wall treatment were systematically tested to optimize the CFD simulation settings. With respect to post processing, the numerical data were sampled in a stator slot at 9 rotor tooth positions on a grid that closely mimicked that for PIV data acquisition. The comparisons were made for three different rotor speeds (10, 20, and 26 revolutions per second) but at the same volumetric throughput (1.3 liters per second). The study of near-wall modelling options considered Non-Equilibrium Wall Functions (NEWF) and Enhanced Wall Treatment (EWT). Both produced similar results but EWT showed advantage in computational efficiency. In the mesh independence study, 3 mesh levels were created with approximately 2, 6, and 16 million cells. The study revealed that the mesh level with 6 million cells was sufficient to insure grid independence at reasonable accuracy. The CFD and PIV data compared favorably in many aspects. On average, CFD predicted the location of mixing layer and rotor tip vortices within 6.0% of the stator slot width compared to the PIV data. CFD also successfully identified 23 out of 27 (85.1%) mixing layer and rotor tip vortices captured by PIV. Differences were observed as well. The CFD simulations consistently yielded higherItem Effect of Fin-Guided Fuel Injection on Supersonic Mixing and Combustion(2014) Aguilera Munoz, Camilo; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rapid mixing and combustion is a key challenge in supersonic combustors due to the extremely short flow residence time and the effect of compressibility. Mixing enhancement is therefore desirable to ensure timely mixing, reaction, and heat release. Fin-guided fuel injection is one approach that can be optimized for propulsion performance consideration. The present investigation examined the mixing and combustion characteristics of using this alternative fuel injection method to evaluate its performance in comparison to conventional transverse wall injection. This study was conducted in two parts: (1) fuel-air mixing experiments in a non-reacting Mach 2.2 flow with a test section Reynolds number of 1.15×106, and (2) combustion experiments using a high-enthalpy, vitiated air flow with a Mach 2.0 condition at the isolator inlet and Reynolds number of 1.14× 105. The non-reacting mixing study used either helium or ethanol, while the combustion study used either hydrogen or ethylene as fuel for each experiment. The mixing behavior of the gaseous and liquid jets was studied using schlieren and a laser sheet technique while quantitative assessments were made from pressure measurements. Similarly, the physical mechanisms in the reacting flow experiments were analyzed using schlieren visualizations while pressure measurements and chemiluminescence emission data were used for performance evaluation. The fuel-air mixing study highlighted possible tradeoffs between mixing enhancement and the stagnation pressure loss stemming from fuel jet-induced shocks. Since the fin was designed to weaken the oblique shock strength while shielding the fuel jet penetrating into the core airflow, it not only resulted in better mixing but also improved the pressure recovery. For gaseous fuel, fin-guided injection improved jet penetration by 100 to 200% for a momentum ratio between 0.15 and 0.03. It also resulted in 64 to 85% additional pressure recovery of the injection shock loss. Combustion experiments revealed that the fin could be used to extend the upper limit of supersonic combustion mode in the present configuration, from an equivalence ratio of 0.04 to 0.12, by preventing thermal choking caused by concentrated heat release near the baseline flame holder. This could be advantageous for certain systems by reducing the thermal protection requirements. However, the fin also made the wall cavity flame holder less effective by increasing fuel penetration away from the bottom wall. The net effects on propulsion system performance will ultimately depend on whether ramjet or scramjet mode is preferred for a given operation.Item A New Scale-Up Approach Through the Evaluation of Stress History Within a Twin-Screw Extruder(2014) Fukuda, Graeme Masuhiro; Bigio, David I.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of any new product manufactured by extrusion requires initial testing at the laboratory level. Once the behaviors of the product and process are understood, the operation is scaled-up to an industrial grade procedure. Scale-up/down is used in a wide variety of markets such as food processing, food packaging, tubing, and pharmaceutical industries. Product quality is critical to the success of all these applications, but is often made difficult when compounding with additives, fillers, pigments, plasticizers, and other supplemental ingredients. Product quality is achieved when the constituents are well mixed. Stress is a critical parameter in accomplishing good mixing. Through the use of polymeric stress beads, a methodology has been developed to measure residence stress distributions in real time. The methodology has enabled the analysis of both model and industrial grade extruders. Evaluation of both processes has led to the creation of a new scale-up approach for dispersive mixing. The new scale-up rule based on percent drag flow was shown to be a more accurate dispersive mixing scale-up approach for a range of operating conditions compared to the current industry standard.Item High Frequency Generation from Carbon Nanotube Field Effect Transistors Used as Passive Mixers(2012) Tunnell, Andrew Jacob; Williams, Ellen; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The high mobilities, low capacitances (due to small sizes), and high current densities of carbon nanotube field-effect transistors (CNT FETs) make them valid candidates for high frequency applications. The high cost of high frequency measurement equipment has been the largest hurdle to observing CNT transistor behavior at frequencies above 50 GHz. One economic solution to this barrier is to use an external harmonic mixer to convert high frequency signals to lower frequencies where they can be detected by a standard spectrum analyzer. By using this detection method, a new regime of high frequency CNT FET behavior is available for study. In this dissertation, we describe the design and fabrication of CNT FETs on quartz substrates using aligned arrays of CNTs as the device channel. The nonlinear input voltage to output drain current behavior of the devices is explained and approximated to the first order by using a Taylor expansion. For the high frequency mixing experiments, two input voltages of different frequencies are sourced on the gate of the devices without any device biasing. The input frequencies are limited to 100 kHz to 40 GHz by the signal generators used. The nonlinearities of the fabricated CNT FETs cause the input frequencies to be mixed together, even in the absence of a source-drain bias (passive mixing). The device output is the drain current, which contains sum and difference products of the input frequencies. By using an external harmonic mixer in combination with a spectrum analyzer to measure the drain current, output frequencies from 75 to 110 GHz can be observed. Up to 11th order mixing products are detected, due to the low noise floor of the spectrum analyzer. Control devices are also measured in the same experimental setup to ensure that the measured output signals are generated by the CNTs. The cutoff frequencies from previous passive mixing experiments predict that our devices should stop operating near 13 GHz, however our measurement setup extends and overcomes these cutoffs, and the generation of high frequency output signals is directly observed up to 110 GHz. This is the highest output frequency observed in CNT devices to date.