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 CFD INVESTIGATION OF A PULSE JET MIXED VESSEL WITH RANS, LES, AND LBM SIMULATION MODELS(2023) Kim, Jung; Calabrese, Richard V.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pulse Jet Mixed (PJM) vessels are used to process nuclear waste due to their maintenance free operation. In this study we model the turbulent velocity field in water during normal PJM operation to gain insight into vessel operations and to evolve a modeling strategy for process design and operator training. Three transient simulation models, developed using Large Eddy Simulation (LES), unsteady Reynolds-Averaged Navier-Stokes (RANS), and Lattice Boltzmann Method (LBM) techniques, are compared to velocity measurements acquired for 3 test scenarios at 3 locations in a pilot scale vessel at the US DOE National Energy Technology Laboratory (NETL). The LES and RANS simulations are performed in ANSYS Fluent, and the LBM simulations in M-STAR.The LES model well predicts the experimental data provided that the operational pressure profile within the individual pulse tubes is considered. While the RANS model failed to predict the data and exhibited significant differences from LES with respect to turbulence quantities, it is a useful comparison tool that can quickly predict averaged flow parameters. The LBM model’s rigid grid system is deemed unsuitable, as currently configured, for the NETL PJM vessel’s wide range of length scales and curved boundaries, resulting in the longest simulation time and least accurate velocity predictions. Predicted velocity and turbulence metrics are explored to better understand the strengths and failures of the three models. Because the LES model produced the most accurate predictions, it is exploited to generate animations and still images on various 2D planes that depict extremely complex flow patterns throughout the vessel with numerous local jets and mixing layer vortices The study concludes with recommendations for future research to improve the model development and validation strategy.Item COMPUTATIONAL FLUID DYNAMICS SIMULATIONS OF A PIPELINE ROTOR-STATOR MIXER(2017) Minnick, Benjamin Austin; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rotor-stator mixers provide high deformation rates to a limited volume, resulting in intensive mixing, milling, and/or dispersion/emulsification. CFD simulations of mixers provide flow field information that benefit designers and end users. This thesis focuses on transient three-dimensional simulations of the Greerco pipeline mixer, using ANSYS FLUENT. The modeled unit consists of two conical rotor-stator stages aligned for axial discharge flow. Flow and turbulence quantities are studied on a per stator slot and per rotor stage basis. Comparisons are made between the LES and RANS realizable k-ε model predictions at various mesh resolutions. Both simulations predict similar mean velocity, flow rate, and torque profiles. However, prediction of deformation rates and turbulence quantities, such as turbulent kinetic energy and its production and dissipation rates, show strong dependencies on mesh resolution and simulation method. The effect of operating conditions on power draw, throughput, and other quantities of practical utility are also discussed.Item EXPERIMENTAL AND NUMERICAL INVESTIGATION OF TANGENTIALLY-INJECTED SLOT FILM COOLING(2013) Voegele, Andrew; Trouve, Arnaud; Marshall, Andre; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Film cooling is a technique used in gas turbine engines, and blades and rocket nozzles to protect critical surfaces from the hot combustion gases. In film cooling applications, a relatively cool thin fluid is injected along surfaces and subsequently mix with the hot mainstream, thus leading to a reduction of protection at the wall. The breakdown of this film involves complex physics including intense turbulent mixing, heat transfer, conduction, radiation and variable density effects to name a few. In this dissertation, film cooling is both experimentally measured and numerically simulated. The experiments feature non-intrusive Particle Image Velocimetry to provide two-dimensional planes of mean and fluctuating velocity, which are critical in order to characterize and understand the turbulent flow phenomena involved in film cooling. Additionally, through the use of micro-thermocouples, the thermal flow fields and wall temperatures are non-intrusively measured, with very small radiative errors. The film cooling flows are experimentally varied to cover a variety of breakdown regimes for both adiabatic (or idealized walls with no heat loss) and on-adiabatic walls (or walls with a carefully controlled heat loss through them). The subsequent experimental dataset is a unique and comprehensive set of turbulent measurements characterizing and demonstrating the film breakdown and the turbulent flow physics. The experiments are then numerically simulated using an in-house variable density, Large Eddy Simulation (LES) Computational Fluid Dynamics (CFD) code developed as part of this dissertation. In addition to accurately predicting important turbulent kinematic and thermal flow phenomena, the key wall parameters were predicted to within 3% for the adiabatic cases and to within 6% for the non-adiabatic cases, with a few exceptions. Turbulent inflow techniques, crucial for the success of LES of film cooling, are examined. In addition to the turbulent flow physics, radiation and conduction physics at the wall were also simulated with good fidelity. The combined experimental and numerical approach was used to uniquely form a comprehensive study, examining many aspects of film cooling phenomena relevant for engineering applications.Item Numerical Characterization and Modeling of Adiabatic Slot Film Cooling(2011) Voegele, Andrew; Marshall, André W; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Film cooling is a technique used to protect critical surfaces in combustors, thrust chambers, turbines and nozzles from hot, chemically reacting gases. Accurately predicting the film's performance is especially challenging in the vicinity of the wall and the film injection plane due to the complex interactions of two highly turbulent, shearing, boundary layer flows. Properly characterizing the streams at the inlet of a numerical simulation and the choice of turbulence model are crucial to accurately predicting the decay of the film. To address these issues, this study employs a RANS solver that is used to model a film cooled wall. Menter's baseline model, and shear-stress transport model and the Spalart-Allmaras model are employed to determine the effect on film cooling predictions. Several methods for prescribing the inlet planes are explored. These numerical studies are compared with experimental data obtained in a UMD film cooling wind tunnel.Item Experimental and Numerical Characterization of Turbulent Slot FIlm Cooling(2008-05-08) CRUZ, Carlos Alberto; MARSHALL, André W.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study presents an experimental and numerical characterization of the turbulent mixing in two-dimensional slot film cooling flows. Three different flows are considered by varying the coolant to mainstream velocity ratio (VR): a wall jet case (VR ≈ 2.0), a boundary layer case (VR ≈ 1.0) and a wall-wake case (VR ≈ 0.5). For each flow, detailed measurements of the film cooling effectiveness, the heat flux, and the heat transfer coefficient are obtained for adiabatic and backside cooled wall conditions. Additionally, detailed flow velocity and temperature are measured under hot conditions using Particle Image Velocimetry (PIV) and a micro-thermocouple probe, respectively. These comprehensive measurements provide a unique data set for characterizing the momentum and thermal mixing of the turbulent flows, and for validating turbulence models in Reynolds averaged Navier-Stokes (RANS) simulations and large-eddy simulations (LES). The three flow families display different performances. The mixing of the film is strongly influenced by the mean shear between the coolant and the hot mainstream, thus explaining that the boundary layer case provides the best effectiveness. Initially governed by the film kinematics at the injection point, the convective heat transfer is influence by the mainstream when the film mixes. Additionally, measurements indicate that semi-empirical correlations largely overpredict the mixing of the film. The results obtained with the Spalart-Allmaras RANS model compare favorably with the measurements, thereby proving that this model is a viable alternative to using correlations for the film cooling effectiveness. A Large-Eddy Simulation (LES) with the dynamic models is performed for the wall jet case under adiabatic wall conditions with inflow conditions prescribed from precursor simulations. The LES results show good agreement with measured adiabatic wall temperatures and provide unique insight into the turbulent transport mechanism and interaction between the near wall and outer shear regions responsible for the mixing of the film.Item On The Fluid Dynamics of Virtual Impaction and The Design of a Slit Aerosol Sampler(2006-09-18) Charrouf, Marwan; Calabrese, Richard V.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It has been long established that Reynolds number effects can lead to flow instabilities and/or transition from laminar to turbulent flow regimes. The nature of free shear jets is well understood and heavily covered in the fluid mechanics literature. On the other hand, the study of confined nozzles presents some challenges and is still a developing area of research. In this work, we focus on quasi-impinging jets, such as the ones feeding into a virtual impactor. Virtual impactors are popular, inexpensive aerosol collection devices capable of separating airborne solid particles. Recently they found increased application in areas that require concentration of dilute aerosols, such as biological-laden flows. In essence, this research is motivated by the need to fundamentally understand the fluid-particle interaction mechanisms entailed during virtual impaction. To this end, we rely on theoretical insight gained by numerical analysis of the classical equations within a one-way coupled Lagrangian framework. In the first part of this investigation we perform a direct transient simulation of the two-dimensional incompressible Navier-Stokes equations for air as the carrier phase. The momentum and continuity equations are solved by FLUENT. The solutions of three separate computations with jet Reynolds numbers equal to 350, 2100, and 3500 are analyzed. The 2-D time-mean results established the nature of the jet potential core and clarifications about the role of the Reynolds number were proposed. Transient analysis deciphered the characteristics of the mirrored Kelvin-Helmholtz instability, along with particle-eddy interaction mechanisms. In the second part we perform a large eddy simulation (LES) on a domain of a real-life sampler. The Lagrangian dynamic residual stress model is implemented and validated for two canonical turbulent flows. The newly contrived code is then applied to the study of a prototype device. A three-dimensional growth mechanism is proposed for the jet mixing layers. The Lagrangian dynamic model LES exhibited significant regions of high subgrid turbulent viscosity, compared to the dynamic Lilly-model simulation, and we were able to identify the origin, and learn the dynamics of five key coherent structures dominant during transition. Comparison with preliminary experimental data for the aerosol separation efficiency showed fairly good agreement.Item PASSIVE SCALAR DISPERSION IN A TURBULENT MIXING LAYER(2004-08-23) Li, Ning; Wallace, James M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experimental and numerical studies of spatially developed turbulent mixing layers with passive scalar concentrations was performed. In the experiment, a mixing layer was created by an S-shaped splitter plate in a wind tunnel, with a velocity ratio of 2:1. A concentration field was realized by injecting incense smoke into the high-speed side of the mixing layer. Simultaneous measurements of the velocity, vorticity and concentration fields were performed. A 12-sensor hot-wire probe was used to measure the velocity field and its gradients, while the concentration field was recorded by taking digital pictures of the laser-illuminated smoke. The statistics of the velocity and vorticity fields agree well with previous research. By synchronizing the velocity and concentration measurements, concentration fluxes were determined. Octant analysis was performed on the flux data to explore the scalar transport processes. Conditional planar average of flow properties was also performed to determine their spatial distribution with respect to the large-scale vortices. A large-eddy simulation, designed to match the experimental conditions, was performed to provide three-dimensional pictures of the mixing layer. A new approach to effectively specify the inflow boundary condition was proposed. Passive particles were released and tracked to simulate the scalar concentration field. Numerical interpolation schemes were examined for performing the particle tracking tasks. The simulation statistically supported the experimental result while providing insight about the flow topology, from which scalar transport models by the rib vortices and roller vortices were proposed and examined.