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Sagerman, Denton GregoryHypersonic Experimental Aero-thermal Capability Study Through Multilevel Fidelity Computational Fluid Dynamics
Master of Science (M.S.), University of Dayton, 2017, Aerospace Engineering
As true with all hypersonic flight, the ability to quickly and accurately predict the aero-thermodynamic response of an aircraft in the early design phase is important to not only lower cost, but also to lower the computational and experimental time required to test various parameters. The Mach 6 High Reynolds Number Facility at Wright-Patterson Air Force Base in Dayton, Ohio has been non-operational for the past twenty years, but a recent resurgence in the need for accurate hypersonic test facilities has led to the reactivation of the wind tunnel. With its restoration, new capabilities to assess hypersonic aero-thermodynamic effects on bodies in Mach 6 flow have emerged. Therefore, the objective of this research is to determine if obtaining aero-thermal data from the Mach 6 tunnel using temperature sensitive paint (TSP) is a viable option. Surface pressure and temperature readings, from pressure taps and thermocouples installed on the models, as well as TSP wall temperature distributions will be used for comparison with results from computational fluid dynamics (CFD) analysis codes of differing fidelity levels. The comparisons can then be utilized to gain confidence in the ability of the tunnel to capture the aero-thermal response of complex geometries. Three computational codes were used for numerical comparisons: the Configuration Based Aerodynamics (CBAero) tool set, an inviscid panel code with viscous approximation capabilities, Cart3d coupled with Unstructured Langley Approximate Three-Dimensional Convective Heating (UNLATCH) code to approximate viscous effects from the Euler solution, and finally Fun3d, a fully viscous RANS solver. The three tunnel model geometries that will be used for this research are the Reference Flight System model G (RFSG), a Generic Hypersonic Vehicle (GHV), and the Hypersonic International Flight Research Experimentation Program-Flight 1 (HIFiRE-1) payload geometry.

Committee:

Markus Rumpfkeil, Dr. (Committee Chair); Aaron Altman, Dr. (Committee Member); Jose Camberos, Dr. (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

hypersonics; hypersonic experimental testing; aero-thermal; temperature sensitive paint; CFD; hypersonic CFD; tsp

Dehner, Richard D.An Experimental and Computational Study of Surge in Turbocharger Compression Systems
Doctor of Philosophy, The Ohio State University, 2016, Mechanical Engineering
The objective of the present study is to predict compression system surge instabilities, including discrete sound peaks at low frequencies and their amplitudes at key locations. One-dimensional (1D) gas dynamics models were created for a turbocharger stand and a twin, parallel turbocharged V6 gasoline turbocharged direct injection (GTDI) engine, which have a common turbocharger design. In addition, a three-dimensional (3D) computational fluid dynamics (CFD) model was developed for the compression system of the turbocharger stand, and prediction results are utilized to study the details of compressor flow-field breakdown and the resulting instabilities at reduced flow rates. The turbocharger stand isolated surge from engine airborne pulsations, thereby providing a simplified bench-top environment for in-depth studies of pertinent physics and model development. Two different compression system configurations were studied on the turbocharger stand. The first configuration incorporated a plenum (large volume), which produced surge as the flow rate was reduced. To facilitate surge predictions, a compressor performance map from an extended flow range small volume system (second configuration) was incorporated into a 1D model of the large volume configuration. Mild and deep surge predictions were completed with the 1D model of the large volume turbocharger stand compression system and the dominant sound peaks of simulations agreed reasonably well with the corresponding measurements. Engine experiments and modeling were carried out in two phases, where the first phase was steady-state, full load operation at low engine speeds and the second phase perturbed one of the compressors into surge. The engine model incorporated loss coefficients from flow bench experiments with induction system components, and modified air cleaner box and charge air cooler models that accurately captured the frequency dependent interaction of pressure waves. Steady-state engine data confirmed the accuracy of predicted performance, pressure drops, and wave dynamics in the induction system. Once the models for compressor surge and stable engine operation were independently validated with experimental data from the turbocharger stand and engine dynamometer, respectively, the engine model was then utilized for predictions with the right-bank compressor in both mild and deep surge. The mild surge prediction provided reasonable agreement with experimental data, and the accuracy of predictions improved as the load was increased and the right-bank compressor entered deep surge. Simulations with a 3D CFD model were initially performed during stable operation with a domain that was confined to the compressor with short inlet and outlet duct extensions. Resulting predictions agreed well with measurements from the turbocharger stand, including compressor performance and the onset of temperature rise near the inducer blade tips. Next, the computational domain was expanded to include the (full) large volume turbocharger stand compression system. As the compressor flow rate was reduced below that at the peak pressure ratio, rotating stall cells formed near the shroud-side diffuser wall. A further reduction in flow rate resulted in the system entering mild surge, where two cycles were simulated. Mild surge predictions from this CFD model provided good agreement with compressor inlet and outlet pressure measurements, in terms of reproducing the amplitude and frequency.

Committee:

Ahmet Selamet (Advisor); Jen-Ping Chen (Committee Member); Sandip Mazumder (Committee Member); Junmin Wang (Committee Member); Philip Keller (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Automotive Engineering; Engineering; Experiments; Fluid Dynamics; Mechanical Engineering; Transportation

Keywords:

turbocharger; automotive; internal combustion engine; engine; centrifugal compressor; compressor; instabilities; surge; rotating stall; one-dimensional model; three-dimensional computational fluid dynamics; 3D CFD

Copeland, AlexaThe Impact of Patient Room Design on Airborne Hospital-Acquired Infections (HAI)
MS, Kent State University, 2016, College of Architecture and Environmental Design
Transmission of airborne diseases in healthcare facilities is an increasingly important concern. This, in part, is due to the reduction in funding from insurance companies for hospital-acquired infections (HAI) and the consequent economic impact of an influenza outbreak in a hospital. With recent cases of HAI in the USA, it became necessary to examine the current ventilation standards for healthcare facilities. Among the organizations that set standards for the prevention of disease transmission are Facilities Guidelines Institution and the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE). These standards have played an important role containing airborne transmitted diseases such as Influenza A in North American hospitals. While, design guidelines have focused on recommending appropriate ventilation rates, it ignored the delivery of conditioned air to occupied spaces and the impact of room layout relative to the placement of air supply and return. Airflow is often designed to contribute positively to air-mixing and distribution, improvement of thermal comfort and air quality conditions. Unfortunately, air distribution contributes to airborne pathogen transmission as well. Few studies focused on the relative location of air supply and as well as the impact of the movement of people, but none found focused on the patient room design and layout relative to recommended airflow patterns. This research focuses on a development of a guideline to help designers understand the effect of room design on the distribution of airborne diseases and the Influenza virus in particular. The paper will present the outcomes of several Computational Fluid Dynamic (CFD) simulations of typical room configurations and layouts under many room use scenarios. The model used for simulation is calibrated and verified by field measurements of air distribution patterns in patient rooms. In addition to airflow patterns, the CFD algorithm is used to determine the age of the air to measure the effectiveness of the recommended air changes in the standards. The paper will delineate through graphical and numerical means the potential location for concentrated contaminations and scenarios in which such concentrations may happen. The paper will also estimate the probability of infection based on air change effectiveness, and the relative spatial relationship between HVAC air delivery system and room design. The resulting outcome is suggestions intended to support health facility designers. It is also intended to encourage the development of design codes and standards that take into account airborne pathogen transmission in the room at which the patient is most vulnerable to infection from visitors or staff.

Committee:

Adil Sharag-Eldin (Advisor); Margaret Calkins (Committee Member); Christopher Woolverton (Committee Member)

Subjects:

Architecture

Keywords:

Airborne pathogen transmission; Hospital Acquired Infection; Patient room design; CFD; Design Guide; Infection Control

Rajagopalan, Arun GopalGeometric Modifications and their Impact on the performance of the Vortex Tube
MS, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering
A vortex tube is a simple device that can separate a single stream of compressed air into two streams, one at a temperature higher than the inlet gas stream and the other at a temperature lower than the inlet stream. Vortex tubes are widely used in cooling applications where compactness, safety, and low equipment cost are basic factors. To better understand gas separation performance, a 3D model of vortex tube was designed using Ansys Workbench, a commercial CFD package. The turbulent flow within the device was modeled using the k-e model. Since the aim of this study was to investigate geometric modifications and their impact on the performance of the vortex tube, several changes in the vortex tube geometry was examined. Model validation was achieved using a base vortex tube model and compared to published results. Then the effect of the number of inlet nozzles, Length to tube Diameter ratio (L/D), Cold Diameter to tube Diameter ratio ((Dc/D) and Cone valve inner diameter (Dco/D) variation on the performance of the vortex tube were studied. This was followed by examining the introduction of vortex chamber and the effect of the non-dimensional chamber diameter ((Dch/D) on the performance of the vortex tube. Experimental validation tests were also conducted on vortex tube currently on the market and the CFD results were compared. The results obtained from the parametric (geometrical and operating) and experimental validation study was used to design a computational model of vortex tube with better thermal performance.

Committee:

Peter Disimile, Ph.D. (Committee Chair); Shaaban Abdallah, Ph.D. (Committee Member); Kelly Cohen, Ph.D. (Committee Member)

Subjects:

Aerospace Engineering; Mechanics

Keywords:

CFD;Parametric Study;Temperature Separation;Vortex Tube;Chamber Diameter

List, Michael GNumerical Quantification of Interaction Effects in a Closely-Coupled Diffuser-Fan System
PhD, University of Cincinnati, 2014, Engineering and Applied Science: Aerospace Engineering
Distorted inflow conditions have been a plague for compression system operability and aeromechanics since the dawn of gas turbine engines. This will likely continue to be an issue for all systems, but modern inlet designs, particularly serpentine diffusers, accentuate the existing issues through the production of both total pressure deficits and flow angularity distortion. For the first time, unsteady, full annulus CFD analysis of a coupled diffuser-fan system was pursued to determine coupling effects between the serpentine diffuser and transonic fan stage. Additionally, the harmonic balance method was used for the first time to demonstrate development and transfer of the distortion. These efforts provided a novel demonstration of 3-D analysis using a generated distortion relevant to this class of inlet and fan. Prior distortion analyses have been conducted using prescribed inlet distortion fields typically matching experimental total pressure or bulk swirl profiles. In the present study, an offset serpentine diffuser was used, coupled directly with the fan stage, to generate the distortion pattern. The Star-CCM+ solver was used to complete mixing-plane, harmonic balance, and transient, full annulus simulations of the coupled system. Mixing plane analysis provided a computationally inexpensive way to assess a nominal mass flow rate at which the components were matched, but provided little additional information beyond a flow field initialization for the higher-fidelity methods. The ability of any distortion analysis method to compute the generation of total temperature and swirl distortion through the fan stage is critical. The harmonic balance and full annulus results show appropriate capture of the distortion transfer, generation of intra-stage total temperature distortion, as well as the impact of the turbomachinery upon the incoming flow field. Redistribution of the distorted flow entering the fan was captured reasonably well by the harmonic balance simulation. Entering the rotor region, however, the vortices generated by the diffuser were not fully captured due to an inadequate combination of modes being solved to resolve the vortex structures. This led to an inability of the harmonic balance simulation to match the peak total pressure and total temperature rise generated by the rotor in the distorted regions, though the mass-averaged circumferential distribution and the amplification of distortion per-rev components showed remarkable similarities. A reduction in the total computational requirements was seen using harmonic balance making it of value for detailed design and analysis, though current implementations have limitations prohibiting more rapid turnaround for use in earlier design iterations. Its use in distortion applications, particularly for stage analysis, has been shown to be meaningful and suitable to bridge the gap between reduced order modeling and full annulus simulations. Better understanding of the unsteady aerodynamics will provide a more accurate view of performance in distorted flow and the sources of aerodynamic excitation and damping that dramatically affect the physics in modern fans.

Committee:

Mark Turner, Sc.D. (Committee Chair); Steven Puterbaugh, Ph.D. (Committee Member); Shaaban Abdallah, Ph.D. (Committee Member); Paul Orkwis, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

distortion transfer;inlet-fan interaction;harmonic balance;full annulus;CFD

Laukiavich, CraigThe Fluid-Solid Interactions and Thermoelastic Behavior (with Rotordynamic Considerations) of the "OIL Transfer Sleeve" in a Turboprop Engine: A Numerical and Experimental Investigation
Doctor of Philosophy, University of Akron, 2015, Mechanical Engineering
A constant speed propeller turboprop engine relies on its lubricating oil to also act as a lever for varying the pitch of the propeller blades. Two inlets, a high pressure and a low pressure, supply the lubricating oil to the shaft. The propeller shaft has radial holes placed both axially and circumferentially allowing oil to enter the shaft’s interior. On the low pressure side, the oil exits the shaft to lubricate other portions of the engine, while the high pressure side is used to vary the pitch on the blades. This pressurized oil interacts with a spring that affects the pitch of the propeller blades; with the pitch of the blades transitioning from a zero to full thrust position depending on the amount this spring is compressed. The internal lubrication is numerical modeled using a commercially available computational fluid dynamic code, ANSYS CFX. The numerical simulations account for the full 3D Navier Stokes equations coupled with: the energy equation, gaseous cavitation using mass transfer between liquid and gaseous phases, and variable properties of the fluids. A finite element solver, ANSYS Mechanical, was used in conjunction with ANSYS CFX to simulate fluid-solid interactions between the lubricating fluid and the solid shaft and sleeve. Four numerical models were implemented and compared between one another in order to isolate the effects of temperature, deformations, and rotordynamics on the system. The results present: pressures and temperatures of the fluid film, the cavitation bubble formed due to film rupture, deformation of the shaft and sleeve boundaries, and rotordynamic trajectories and orbits. A test loop was designed from a shaft and sleeve used in generic constant speed propeller engine. The sleeve was instrumented with pressure transducers and thermocouples placed both axially and circumferentially to map the pressures and temperatures of the lubricating film. The experimental test section will examine the effects of inlet temperature, rotational speed, and applied load on the pressures and temperatures of the oil film.

Committee:

Minel Braun, Dr. (Advisor); Abhilash Chandy, Dr. (Advisor); Alex Povitsky, Dr. (Committee Member); Xiaosheng Gao, Dr. (Committee Member); Gerald Young, Dr. (Committee Member); Atef Saleeb, Dr. (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Thermal effects; fluid-solid interactions; cavitation; CFD; FEA; rotordynamics; tribology

Rattermann, Dale NA Fast Poisson Solver with Periodic Boundary Conditions for GPU Clusters in Various Configurations
MS, University of Cincinnati, 2014, Engineering and Applied Science: Aerospace Engineering
Fast Poisson solvers using the Fast Fourier Transform on uniform grids are especially suited for parallel implementation, making them appropriate for portability on graphical processing unit (GPU) devices. The goal of the following work was to implement, test, and evaluate a fast Poisson solver for periodic boundary conditions for use on a variety of GPU configurations. The solver used in this research was FLASH, an immersed-boundary-based method, which is well suited for complex, time-dependent geometries, has robust adaptive mesh refinement/de-refinement capabilities to capture evolving flow structures, and has been successfully implemented on conventional, parallel supercomputers. However, these solvers are still computationally costly to employ, and the total solver time is dominated by the solution of the pressure Poisson equation using state-of-the-art multigrid methods. FLASH improves the performance of its multigrid solvers by integrating a parallel FFT solver on a uniform grid during a coarse level. This hybrid solver could then be theoretically improved by replacing the highly-parallelizable FFT solver with one that utilizes GPUs, and, thus, was the motivation for my research. In the present work, the CPU-utilizing parallel FFT solver (PFFT) used in the base version of FLASH for solving the Poisson equation on uniform grids has been modified to enable parallel execution on CUDA-enabled GPU devices. New algorithms have been implemented to replace the Poisson solver that decompose the computational domain and send each new block to a GPU for parallel computation. One-dimensional (1-D) decomposition of the computational domain minimizes the amount of network traffic involved in this bandwidth-intensive computation by limiting the amount of all-to-all communication required between processes. Advanced techniques have been incorporated and implemented in a GPU-centric code design, while allowing end users the flexibility of parameter control at runtime in order to maximize throughput with all data sizes. The new code also allows the use of multiple GPU devices in a variety of configurations. The elapsed solution time for the newly implemented GPU-based solvers for a Poisson equation with known source terms demonstrate speed-ups of up to 3.5 times faster than the CPU-based solver.

Committee:

Kirti Ghia, Ph.D. (Committee Chair); Bracy Elton, Ph.D (Committee Member); Shaaban Abdallah, Ph.D. (Committee Member); Urmila Ghia, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

GPU;Poisson;Incompressible;CFD;FFT;CUDA

Resor, Michael IrvinCOMPUTATIONAL INVESTIGATION OF ROTARY ENGINE HOMOGENEOUS CHARGE COMPRESSION IGNITION FEASIBILITY
Master of Science in Engineering (MSEgr), Wright State University, 2014, Mechanical Engineering
The Air Force Research Laboratory (AFRL) has been investigating the heavy fuel conversion of small scale Unmanned Aerial Vehicles (UAV). One particular platform is the Army Shadow 200, powered by a UEL Wankel rotary engine. The rotary engine historically is a proven multi-fuel capable engine when operating on spark ignition however, little research into advanced more efficient compression concepts have been investigated. A computational fluid dynamics model has been created to investigate the feasibility of a Homogeneous Charge Compression Ignition (HCCI) rotary engine. This research evaluates the effects, rotor radius to crankshaft eccentricity ratio, known as K factor, equivalence ratio, and engine speed and how they affect the response of horsepower, maximum temperature, and peak pressure to determine the feasibility of HCCI operation. The results show that the advanced HCCI strategy is promising to significantly improve efficiency of the rotary engine.

Committee:

George Huang, Ph.D. (Advisor); Haibo Dong, Ph.D. (Advisor); Greg Minkiewicz, Ph.D. (Committee Member); Scott Thomas, Ph.D. (Committee Member); Zifeng Yang, Ph.D. (Committee Member)

Subjects:

Aerospace Engineering; Automotive Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Rotary Engine Wankel HCCI Homogeneous Charge Compression Ignition UAV Fluid Dynamics CFD

Ingraham, DanielVerification of a Computational Aeroacoustics Code Using External Verification Analysis (EVA)
Master of Science in Mechanical Engineering, University of Toledo, 2010, College of Engineering
As Computational Aeroacoustics (CAA) codes become more complex andwidely used, robust Verification of such codes becomes more and more important. Recently, Hixon et al. proposed a variation of the Method of Manufactured Solutions of Roache especially suited for Verifying unsteady CFD and CAA codes that does not require the generation of source terms or any modification of the code being Verified. This work will present the development of the External Verification Analysis (EVA) method and the results of its application to some popular model equations of CFD/CAA and a high-order nonlinear CAA code.

Committee:

Ray Hixon, PhD (Committee Chair); Douglas Oliver, PhD (Committee Member); Chunhua Sheng, PhD (Committee Member)

Subjects:

Acoustics; Mechanical Engineering

Keywords:

Computational Aeroacoustics; Computational Fluid Dynamics; Code Verification; External Verification Analysis; Method of Manufactured Solutions; CAA; CFD; EVA; MMS

Chinchore, Asmita CComputational Study of Savonius Wind Turbine
Master of Science in Mechanical Engineering, Cleveland State University, 2013, Fenn College of Engineering
This project involves study of 2-Blade and 3-Blade Savonius vertical wind turbines positioned at different orientations. For a 2-Blade turbine the orientations considered were 0 degree, 45 degree, 90 degree and 135 degree in reference to the direction of the prevailing wind and for the 3-Blade turbine the orientations taken into account were 0 degree, 30 degree, 60 degree and 90 degree in reference to the direction of the prevailing wind. The basic aim of this thesis was to study how the two designs are different from each other and which design produces more power when applied with constant wind velocity of 10mps. Computational Fluid Dynamics (CFD) analyses were conducted for every case to find out the torque and power generated by the turbines for each orientation. To ensure the accuracy of the results, CFD techniques were applied using Gambit 2.2.30 and Fluent 6.2.16. All cases were run using “transition-SST” flow model and the faces were meshed using `Quadrilateral Pave’ meshing scheme. The turbine was also tested for varying wind velocities of 5mps, 20mps, and 30mps for a constant orientation of turbine. The results were later compared and graphs were created for easy comparison of power and torque generated by turbines at different velocities. Maximum change in pressure occurs when 2-Blade turbine in perpendicular to direction of wind flow direction i.e. at 90 degree and when 3-Blade turbine is at 60 degree orientation. The 2-Blade Turbine generates higher value of torque (215.28 N) as compared to 3-Blade turbine, generating torque of value 110.92 N for any given constant wind velocity; 30mps in this case. This information can help the designer of the system to select the proper wind turbine considering the efficiency and stability along with other factors.

Committee:

Majid Rashidi, PhD (Committee Chair); Rama Gorla, PhD (Committee Member); Asuquo Ebiana, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Savonius Wind Turbine;Wind Turbines;2-Blade Turbine;3-Blade Turbine;CFD;Betz Limit;Fluid Flow Analysis;Ansys;Torque;Power

Shi, LimingComputational Fluid Dynamics Simulation of Steam Reforming and Autothermal Reforming for Fuel Cell Applications
Master of Science (MS), Ohio University, 2009, Chemical Engineering (Engineering and Technology)
With the increasing demand for fuel cell applications in transportation, the performance of reformers using gasoline or diesel as the fuel needs to be optimized. Numerical models based on computational fluid dynamics (CFD) were used to simulate the performance of these reformers. A CFD model of steam reforming and a CFD model of autothermal reforming were developed and validated for two reformers. Each model included submodels for the reactor and reaction chemistry. A single channel was used in the model of steam reforming and a whole reactor was modeled in the model of autothermal reforming. A reaction rate expression was developed for the steam reforming reaction to form hydrogen and carbon dioxide. The CFD results provided an adequate match to the experimental data from the literature. The percentage of difference between each experimental measurement of the mole fraction of hydrogen and the corresponding CFD prediction was less than 17.7% for the model of steam reforming and 16.8% for the model of autothermal reforming. The CFD models were used to predict reformer performance. For steam reforming, the inlet steam-to-carbon molar ratio had a negligible effect on reforming efficiency when it was varied from 2 to 4. The reforming efficiency decreased slightly as the inlet velocity was increased from 2.9 to 8.7 m/s, which was mainly caused by the steam reforming reaction. For autothermal reforming, the thermal conductivity of the catalyst support affected the temperature profile in the reactor, but its effect on the mole fraction of hydrogen in the products was negligible. The reforming efficiency decreased by 11.5% as power input was increased from 1.7 to 8.4 kW.

Committee:

Michael E. Prudich (Advisor)

Subjects:

Chemical Engineering; Energy

Keywords:

Steam Reforming; Autothermal Reforming; CFD; Modeling; Fuel Processing; Fuel Cells

Uddandam, Vinay R.Computer Simulation of an Electrostatic Cyclonic Emissions Separator
Master of Science (MS), Ohio University, 2008, Mechanical Engineering (Engineering and Technology)
In 1997, the United States Environmental Protection Agency strengthened its health protection standard for particulate matter by introducing the PM 2.5 standard. This standard has since lead to control of fine particulates with even more importance and better technology. Although, fabric filters and electrostatic precipitators on coal-burning facilities are successful in attaining standards, these technologies are relatively expensive, need a huge amount of space, and require longer downtimes for installation. The primary focus of this research is to use Fluent and Gambit software to simulate an electro cyclone technology with a novel slipstream idea aimed at reducing fine particulate matter efficiently, combined with a conventional PM control technology. The computer model, once validated appropriately using a bench-scale cyclone, would serve as a design tool to further improve the novel electro cyclone concept.

Committee:

David J. Bayless (Committee Chair); Gregory Kremer (Committee Member); Ben Stuart (Committee Member); Michele Morrone (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Computer; Electrocyclone; CFD; Fluent; Gambit; Validation; 5-Hole; 3D; Pitot Probe

XUE, JIANQINGCOMPUTATIONAL SIMULATION OF FLOW INSIDE PRESSURE-SWIRL ATOMIZERS
PhD, University of Cincinnati, 2004, Engineering : Mechanical Engineering
Simplex atomizers (pressure-swirl atomizers) are widely used in air-breathing gas turbine engines as they have good atomization characteristics and are relatively simple and inexpensive to manufacture. To reduce emissions, it is critical to design fuel atomizers that can produce spray with a predetermined droplet size distribution at the desired combustor location (small mean droplet diameters and uniform local air/fuel ratios). Manufacturing methods are now available where complex atomizer geometries can be easily obtained. However to use such methods, the influence of atomizer geometry on its performance must be well understood. In this dissertation, a two-dimensional axisymmetric computational fluid dynamics (CFD) model based on the Arbitrary-Lagrangian-Eulerian (ALE) method to predict the flow in pressure-swirl atomizers was developed. The Arbitrary-Lagrangian-Eulerian method was applied so that the free interface between gas and liquid could be tracked sharply and accurately. The developed code was validated by comparison of predictions with experimental data for large scale prototype and with semi-empirical correlations at small scale. The computational predictions agreed well with experimental data for the film thickness at the exit, spray cone angle, and the pressure drop across the atomizer as well as velocity field in the swirl chamber. Using the validated code, a comprehensive parametric study on simplex atomizer performance was conducted. The geometric parameters of atomizer covered in this study include: atomizer constant, the ratio of length to diameter in swirl chamber, the ratio of length to diameter in orifice, the swirl chamber to orifice diameter ratio, inlet slot angle, trumpet angle, trumpet length, and swirl chamber convergent angle. The effects of these geometric parameters on the atomizer performance were studied for a fixed mass flow rate through the atomizer as well as for a fixed pressure drop across the atomizer. The atomizer performance was described in term of dimensionless film thickness at the exit, discharge coefficient and spray cone half angle. To address applications in pharmaceutical and food processing industry, flow of non-Newtonian power-law fluids through pressure-swirl atomizers was considered. Detailed flow patterns inside the atomizer for shear-thinning, Newtonian and shear-thickening fluids were investigated. A range of power-law index from 0.7 to 1.3 was considered. With a fixed flow rate through the atomizer, the shear-thickening fluids exhibited higher film thickness at exit, lower spray angle, and higher discharge coefficient compared to Newtonian fluids. For the range of power-law index considered in this study, the atomizer performance parameters for shear-thinning fluids showed small change from Newtonian fluids. The variation of atomizer performance with the atomizer constant was delineated for different power-law index.

Committee:

Dr. Milind Jog (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

CFD; Spray; Atomization; Pressure-Swirl Atomizer; Two Phase Flow; ALE

Shuster, James LouisNumerically Modeling the Flow and Friction Within a Helically-Finned Tube
Master of Science in Engineering, Youngstown State University, 2010, Department of Mechanical and Industrial Engineering
As the populations and the economies of the world grow, the demand for electricity rises and necessitates an increase in the supply of electricity as the primary fuels that are used to generate electricity are finite and exhausting. Moreover, mounting concerns about carbon emissions and the current direction of environmental legislation are pushing for lower emissions and higher efficiencies of energy producing facilities. One approach to abate such dilemmas is to increase the efficiency of the modern steam cycle, which is used to generate most of the world's electricity. Improving the components of the steam cycle, or the boiler component in particular, can affect the overall efficiency of the steam cycle significantly. An integral constituent of the boiler is the boiler tube. There are several types of boiler tubes, and the helically-finned tube is one type that has proven to increase the efficiency of the boiler. However, insight to the internal flow within the helically-finned tube is still developing and incomplete. The objective of this study was to computationally model the internal flow and measure the friction factor of a helically-finned tube for which experimental data was already published. Using three different modeling techniques, the flow was solved numerically with Fluent, a computational fluid dynamics software package. With respect to the experimental data, the Fluent solutions reflected percent errors ranging between 14% and 27%. Although the results are acceptable, suggestions for future work are included.

Committee:

Hazel Marie, PhD (Advisor); Daniel Suchora, PhD (Committee Member); Yogendra Panta, PhD (Committee Member)

Subjects:

Fluid Dynamics

Keywords:

Fluent; CFD; Finned tube; Helically-finned tube

Gan, SubhadeepActive Separation Control of High-Re Turbulent Separated Flow over a Wall-Mounted Hump using RANS, DES, and LES Turbulence Modeling Approaches
PhD, University of Cincinnati, 2010, Engineering : Mechanical Engineering
Most practical flows in engineering applications are turbulent, and exhibit separation which is generally undesirable because of its adverse effects on performance and efficiency. Therefore, control of turbulent separated flows has been a topic of significant interest as it can reduce separation losses. Often, flow control work employs passive techniques to manipulate the flow. Passive-flow control does not require any additional energy source to achieve the control, but is accompanied by additional viscous losses. It is more desirable to employ active techniques as these can be turned on and off, depending on the flow control requirement. The primary goal of the present work is to numerically investigate a high Reynolds number turbulent separated flow. It is Case 3 of the 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop, http://cfdval2004.larc.nasa.gov/case3.html, conducted by NASA for the flow over a wall-mounted hump. Followed by the baseline flow simulation, i.e, without flow control, active flow control will be investigated using both steady suction jet as well as a "synthetic" jet. The present work also implements the use of two jets (steady suction and synthetic jets) as have not been previously implemented for this flow model. For the synthetic-jets case, the work also studies the effect of two jets in opposite phase. The secondary goal of this work is to bring together a variety of turbulence models and simulation approaches for one flow problem. The flow is simulated using steady and unsteady-state three-dimensional RANS equations-based turbulence models and three-dimensional time-dependent DES and LES methods. Multiple turbulence modeling approaches help to ascertain what models are most appropriate for capturing the physics of this complex separated flow. The results will help us better decide what models to choose for flows with adverse pressure gradients, flow separation and control of separated flows. For the flow over the wall-mounted hump, the simulation results agree well with experiment. Significant computational-resources savings was realized by using an analytical exit velocity profile for the active flow control jets, instead of simulating the entire flow-control manifold without sacrificing the quality of the work. Results compared with experimental values were surface pressure coefficient, skin friction coefficient, mean velocity profiles, Reynolds stresses and flow reattachment locations. Simulation results show some degree of variation with experimental results in the separated flow region. The steady-suction active control was able to reduce the reattachment length the most. The region of negative streamwise velocity was the smallest in the active flow control with steady suction. The multiple jets cases, with steady suction and synthetic jets, were able to reduce the length of separation bubble in comparison to the corresponding single jet cases. The synthetic jets case, using two jets in opposite phase, was able to achieve the most uniform velocity field in the separation bubble region. The work shows great promise in implementing active flow control, using single and multiple jets, for separated flow at high Reynolds number.

Committee:

Urmila Ghia, PhD (Committee Chair); Milind Jog, PhD (Committee Member); Kirti Ghia, PhD (Committee Member); Donald French, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

CFD;Separated flow;Active flow control;Steady Suction;Synthetic Jet

Votaw, Zachary StevenComputational Study on Micro-Pilot Flame Ignition Strategy for a Direct Injection Stratified Charge Rotary Engine
Master of Science in Engineering (MSEgr), Wright State University, 2012, Mechanical Engineering
The Office of Security of Defense's Assured Fuels Initiative has recently been pressing for a single fuel battle space. This endeavor requires modifying many of the vehicle power plants currently in operation throughout the Armed Forces. The RQ-7 Shadow, an unmanned aerial vehicle (UAV) utilized by the Marine Corp and Army for reconnaissance purposes, is powered by UEL's AR741 rotary engine and functions on aviation fuel. One effort underway has been focused on developing this rotary engine system to operate on heavy fuels using direct injection technology and charge stratification. Although the rotary engine has many advantages over standard reciprocating engines, providing a reliable ignition source for the stratified charge within the sweeping combustion chamber presents challenges. This work made effort to compensate for those challenges by utilizing a pilot flame ignition system. The system incorporated a micro-diesel injector and spark plug recessed within an ignition cavity along the housing of the rotary engine. The pilot flame ignition approach was thoroughly evaluated by conducting a parametric study using computational methods to simulate the combustion process. Gambit meshing software was used to build the 3D rotary engine mesh. ANSYS Fluent was used to formulate and apply the various numerical models describing the combustion phenomena. And lastly, JMP software was used to perform a response surface analysis in effort to determine the optimal parameter values for the ignition system. The goal of the parametric study was to maximize power output and likewise minimize specific fuel consumption. A total of thirty one cases were performed to complete the study. For the rotary engine operating at 6000rpm an optimal solution was successfully realized within the design space. The rotary engine model generated 5.313 horsepower (HP) for the complete cycle of one chamber. The overall equivalency ratio allocated in the combustion chamber for the simulations was 0.55. This resulted in a specific fuel consumption of 0.395 lb/hp-h. The study not only provided evidence to confirm the profitable use of a pilot flame ignition system applied to the direct injection stratified charge rotary engine (DISCRE), but also provided multiple insights on the design and operation of such a system.

Committee:

Haibo Dong, PhD (Advisor); Rory Roberts, PhD (Committee Member); Greg Minkiewicz, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

CFD; Rotary Engine; Wankel Engine; Stratified Charge; Direct Injection; Pilot Flame Ignition; Multi-fuel; Parameter Optimization; Combustion Simulation; Computational Fluid Dynamics

MAGAR, YOGESH NARESHCONVECTIVE COOLING AND THERMAL MANAGEMENT OPTIMIZATION OF PLANAR ANODE-SUPPORTED SOLID OXIDE FUEL CELLS
MS, University of Cincinnati, 2006, Engineering : Mechanical Engineering
Convective heat and mass transfer in a planar, tri-layer, solid oxide fuel cell (SOFC) module is considered for a uniform supply of volatile species (80% hydrogen + 20% water vapor) and oxidant (20% oxygen + 80% nitrogen) to the electrolyte surface. With an Arrhenius electrochemical reaction rate, the coupled heat and mass transfer is modeled by steady incompressible fully-developed laminar flow in the interconnect ducts of rectangular cross section for both the anode-side fuel and cathode-side oxidant flows. The governing three-dimensional equations for mass, momentum, energy, and species transfer along with those for electrochemical kinetics are solved computationally using commercial CFD software. The homogeneous porous-layer flows, which are in thermal equilibrium with the solid matrix, are coupled with the electrochemical reaction rate to properly account for the flow-duct and anode/cathode interface heat/mass transfer. Parametric effects of rectangular flow duct aspect ratio and anode porous-layer thickness on the variations in species mass concentration and temperature distributions, flow friction factor, and convective heat transfer coefficient are presented. The combined effects of porous layer and electrochemical reaction are seen to alter the flow and heat transfer behavior of SOFC. The hydrodynamic and thermal behavior is characterized for effective performance and cooling, and interconnect channels of rectangular cross-section aspect ratio of ~ 2 are seen to provide optimal thermal management benefits. Parametric effects of flow duct cross-sectional shapes (triangle, trapezoid and, rectangle) and geometry along with heat transfer enhancing flow arrangements (offset-strip fin and wavy fin flows) are characterized by the variations in mass and temperature distributions, flow friction factor, and convective heat transfer coefficient. Though triangular cross section al interconnects showed lowest heat transfer coefficient it might be preferred de to its highest structural stability. Offset-strip flow arrangement has shown the best convective cooling and is probably the best interconnect design for the efficient thermal management in planar anode-supported SOFC.

Committee:

Dr. Raj Manglik (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Solid oxide fuel cells; Thermal analysis; Fluid flow; CFD; Compact Heat Exchangers; Electrochemistry

Bhari, AnilA Rapid Compression Machine with the Novel Concept of Crevice Containment
Master of Science in Engineering, University of Akron, 2010, Mechanical Engineering
Rapid Compression Machines (RCMs) typically incorporate creviced pistons to suppress the formation of the roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effects inside the RCM due to the crevice zone being at a lower temperature than the main reaction chamber. In this work, such undesirable effects of the creviced piston are first highlighted through computational fluid dynamics simulations of n-heptane ignition in an RCM. Specifically, the results show that in an RCM with a creviced piston, additional mass flow takes place from the main combustion chamber to the crevice zone during the first-stage ignition. This phenomenon is not captured by the conventional zero-dimensional modeling approaches. Consequently, a novel approach of ‘crevice containment’ is introduced and evaluated. According to this approach, in order to avoid the undesirable effects of the creviced piston, the crevice zone is separated from the main reaction chamber at the end of compression. The computational results with this novel approach show significant improvement in the fidelity of the zero-dimensional modeling in terms of predicting the overall ignition delay and pressure rise in the first-stage ignition. In addition, this approach also offers other advantages, namely a reduction in the rate of post-compression pressure drop and improved data during species sampling experiments. An RCM is subsequently designed and successfully fabricated with the feature of ‘crevice containment’ for the purpose of chemical kinetics studies at elevated pressures and temperatures. Characterization experiments for the newly built RCM show that the operation of the RCM is free from any vibrations, allows fast compression (22 ms), compressed pressures up to 100 bar and the experimental data obtained is highly reproducible. Using this facility, autoignition investigations are conducted for Hydrogen at a pressure of 50 bar. The experiments are modeled using the kinetic mechanism of O’Conaire et al. (2004). Results showed that the mechanism of O’Conaire et al. agree very well with the experimental data.

Committee:

Gaurav Mittal, Dr. (Advisor); Alex Povitsky, Dr. (Committee Member); Abhilash J. Chandy, Dr. (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

RCMs; ignition; pistons; CREVICE CONTAINMENT; COMPRESSION; CFD

Pundi Ramu, ArunCFD Studies on the Flow and Shear Stress Distribution of Aneurysms
Master of Science, University of Akron, 2009, Mechanical Engineering

Rupture of cerebral aneurysms followed by subarachnoid hemorrhage is a very fatal disease condition associated with high mortality and morbidity rates. Studies show that over 10 to 15 million people in the United States harbor intracranial aneurysms. Most aneurysms do not rupture, and the management of these lesions is difficult, due to the lack of complete understanding of their natural history and the risks associated with surgery. In this work, 3-dimensional Computational Fluid Dynamic (CFD) approach has been undertaken to study and quantify the flow mechanics of aneurysms.

The hemodynamic factors leading to the formation, growth and rupture of aneurysms have not been fully understood. We provide a possible hypothesis based on low WSS distribution to assess the hemodynamic environment along the dome of spherical aneuryms, prior to rupture. Our hypothesis is based on immunohistochemical findings, of rigorous remodeling attempts along the wall of the aneurysm dome, prior to rupture. We hypothesize WSS around ±0.4 Pa, to favorably initiate such remodeling and study the variation of the area prone to remodeling for varying arterial curvature and angle of tilt of dorsal aneurysms. The study is aimed towards demonstrating a better understanding of the factors that lead to the degeneration of aneurysms, by coupling findings of clinical studies with their corresponding hemodynamic causes. Impact of size, curvature of the parent artery, and angle of tilt and pulsatility on the various dependent variables such as, the distribution of WSS along the aneurysm, the primary and secondary flow patterns, the impact zone and the area prone to vascular remodeling are quantified.

Committee:

Guo-Xiang Wang, PhD (Advisor)

Subjects:

Mechanical Engineering

Keywords:

Aneurysms; hemodynamics; Bio Fluids; CFD; Intracranial Aneurysms

Guarendi, Andrew NNumerical Investigations of Magnetohydrodynamic Hypersonic Flows
Master of Science, University of Akron, 2013, Mechanical Engineering
Numerical simulations of magnetohydrodynamic (MHD) hypersonic flow are presented for both laminar and turbulent flow over a cylinder and flow entering a scramjet inlet. ANSYS CFX is used to carry out calculations for steady flow at hypersonic speeds (Mach number > 5). The low magnetic Reynolds number (<<1) calculated based on the velocity and length scales in this problem justifies the quasistatic approximation, which assumes negligible effect of velocity on magnetic fields. Therefore the governing equations employed in the simulations are the compressible Navier-Stokes and the energy equations with MHD-related source terms such as Lorentz force and Joule dissipation. Turbulence effects are accounted for when applicable and multiple turbulence models are compared. The results demonstrate the ability of the magnetic field to affect the flowfield, and variables such as location and magnitude of the applied magnetic field are examined. An examination of future work is provided through the implementation of a semi-discrete central scheme in-house code toward the solution of the Orszag-Tang vortex system.

Committee:

Abhilash Chandy, Dr. (Advisor); Scott Sawyer, Dr. (Committee Member); Alex Povitsky, Dr. (Committee Member)

Subjects:

Aerospace Engineering; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Hypersonic; Hypersonic Flow; Flow over a cylinder; Magnetohydrodynamic; MHD; Lorentz; Hypersonic MHD; Numerical Methods; CFD; Computational fluid dynamics; fluid dynamics; Aerospace;

Wegman, Kevin RNumerical Modeling of a Printed Circuit Heat Exchanger Based on Experimental Results from the High-Temperature Helium Test Facility
Master of Science, The Ohio State University, 2016, Nuclear Engineering
The U.S. Department of Energy’s Generation IV Program has identified six advanced reactor technologies to be investigated for possible deployment in both energy and process heat generation. Most of these reactor concepts, such as the Very-High-Temperature Reactor (VHTR) and the Molten Salt Reactor (MSR), operate at high temperatures and/or pressures, requiring intermediate heat exchangers (IHXs) to transfer heat from the primary loop to secondary and tertiary loops. The VHTR has a design core outlet temperature of up to 1000 °C, and thus a robust high temperature IHX is required for full VHTR technology maturity. One such candidate for the IHX in these advanced reactors is the printed circuit heat exchanger (PCHE). The PCHE has an extremely high effectiveness and compactness, and the fabrication methods lead to great robustness as well. In this study, numerical simulations using a commercial code, COMSOL Multiphysics, were investigated and compared to the experimental results obtained from straight channel PCHE testing at the High-Temperature Helium test Facility (HTHF) at The Ohio State University (OSU). A post-machining analysis was completed for the frontal face geometry of the PCHE flow channels, and the results were compared to the nominal geometric values. The actual channel diameter was found to be 2.04±0.12 mm, compared to the nominal value of 2.0 mm, and the actual channel height was found to be 0.9±0.11 mm, compared to the nominal value of 1.0 mm. These new values were tested in the numerical model geometry as well as the nominal values. Three model were created for numerical investigation of the experimental results; a two-channel model, a two-plate model and a full-geometry model. A grid sensitivity study was completed for the two-channel model using a laminar flow model. Results were obtained for the two-channel model and was compared to the results obtained in the experiment. The heat transfer characteristics were over predicted in the numerical results, while the numerical pressure drops predicted the experimental values well. Preliminary results using a coarsened mesh were obtained for the two-plate and full-geometry model. A methodology for calculations of local friction factor and Nusselt number effects from numerical data is presented, and the resulting analyses are discussed. The globally calculated values are compared to the locally calculated values. The global and locally calculated results do not always match, explained by numerical errors related to the use of differentials for first ordered mesh cell elements.

Committee:

Xiaodong Sun, Dr. (Advisor); Tunc Aldemir, Dr. (Committee Member); James O'Brien, Dr. (Committee Member)

Subjects:

Nuclear Engineering

Keywords:

Printed Circuit Heat Exchanger; PCHE; COMSOL Multiphysics; CFD

Gogar, Ravikumar LeelamchandFlow Investigation in Spacers of Membrane Modules.
Master of Science, University of Toledo, 2015, Chemical Engineering
Spacers are used in membrane modules to keep membrane leaves separated. Introduction of a spacer between membrane layers create a flow channel of definite height. Typically, conventional spacers are woven or layered filaments. The spacer also disrupts fluid flow and can thereby improve mixing. The improvement in mixing comes at the cost of an increase in pressure drop. Particle Image Velocimetry (PIV) and Computational Fluid dynamics (CFD) was used to investigate the flow of water in symmetric and asymmetric spacer channels. In PIV, motion of the tracer particles in the fluid is captured to obtain the spatial distribution of velocity. 2D-PIV provides details of two components of velocity in a planar layer while 2D-3C-PIV provides details of all three components of velocity. Velocity profiles obtained from the PIV and simulations of fluid flow in spacer-filled channels are compared in terms of flow direction and velocity magnitude for different positions within the channel. Analysis of the velocity profile from PIV validates the simulations conducted using COMSOL Multiphysics and the assumptions made therein.

Committee:

Glenn Lipscomb (Committee Chair); Dong-Shik Kim (Committee Member); Sridhar Viamajala (Committee Member)

Subjects:

Chemical Engineering

Keywords:

Spacers, CFD, Particle Image Velocimetry

Seidu, IddrisuAnalytical and Numerical Validation of Nozzle Spray Measurement Data Obtained from a Newly Developed Production System
Master of Science in Mechanical Engineering, Cleveland State University, 2015, Washkewicz College of Engineering
A newly developed production test stand for measuring the spray angle of a pressure swirl atomizer was constructed and used to measure a product line of these pressure swirl atomizers – the macrospray atomizer. This new test stand, utilizing constant temperature hot wire anemometers, captures the spray angle data based on the voltage drop the hot wire probes see as they traverse the spray cone of the atomizer and as fluid droplets impinge upon the wire. Datasets acquired from the experiments are compared and correlated with computational fluid dynamics (CFD) simulation data. In addition, angles obtained from another type of spray characterization technique, the spray angle device, are also compared to see how closely CFD can predict the angle as captured by this new stand and how reliable and independent of human error it is. Another nozzle with a pressure swirl atomizer, the conventional atomizer, is also simulated to compare its agreement with experimental values obtained from the spray angle device. Finally, the datasets are compared to understand if the CFD results, when compared to the two spray characterization techniques used in this thesis for both the nozzle and atomizer can be utilized to assist in future atomizer designs. For the macrospray atomizer, it was found through the experiments that the hot wire stand predicts the spray angle more accurately within 10% error. The spray angle device measured the spray angles within an error of 29% while the CFD introduced more error into the spray angle measurement obtained, within 7% to 93%. The conventional atomizer was found to have an error up to 18% with CFD results and up to 28% with the manual spray angle device.

Committee:

Mounir Ibrahim, Ph.D. (Committee Chair); Vikram Shyam, Ph.D. (Committee Member); Ralph Volino, Ph.D. (Committee Member)

Subjects:

Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

atomization; spray; angles; hot wire anemometry; CFD; simulation; computational fluid dynamics; atomizers; atomizer; pressure swirl atomizer; spray cone; spray; nozzle; nozzles; sprays; measurement; spray angle; cone; air core; hot wire anemometers;

Hylton, Sonya LynnNumerical Investigation of Boiling in a Sealed Tank in Microgravity
Master of Science in Mechanical Engineering, Cleveland State University, 2014, Washkewicz College of Engineering
NASA’s missions in space depend on the storage of cryogenic fluids for fuel and for life support. During long-term storage, heat can leak into the cryogenic fluid tanks. Heat leaks can cause evaporation of the liquid, which pressurizes the tank. However, when the tanks are in a microgravity environment, with reduced natural convection, heat leaks can also create superheated regions in the liquid. This may lead to boiling, resulting in much greater pressure rises than evaporation at the interface between the liquid and vapor phases. Models for predicting the pressure rise are needed to aid in developing methods to control the pressure rise, so that the safety of the storage tank is ensured for microgravity operations. In this work, a CFD model for predicting the pressure rise in a tank due to boiling has been developed and validated against experimental data. The tank was modeled as 2D axisymmetric. The Volume of Fluid (VOF) model in ANSYS Fluent version 15 was modified using a User Defined Function (UDF) to calculate mass transfer between the liquid and vapor phases. A kinetic based Schrage equation was used to calculate the mass flux for evaporation and condensation at the interface. The Schrage equation and the Lee model were compared for calculating the evaporation due to boiling that occurred in the bulk liquid. The results of this model were validated against microgravity data provided by the Tank Pressure Control Experiment, a tank pressurization and pressure control experiment performed aboard the Space Shuttle Mission STS-52 that experienced boiling. During this experiment, the tank pressure rose from about 43400 Pa to about 47200Pa, a difference of about 3800 Pa. The heater temperature rose from about 296K to about 303K, a difference of about 7K. The tank pressure predicted by the CFD model compared well with the experimental pressure data for self-pressurization and boiling in the tank. The validated CFD model uses the Schrage equation to calculate the mass transfer. Three different accommodation coefficients were used, one for evaporation at the interface, one for condensation at the interface, and one for boiling in the bulk liquid. The implicit VOF model with bounded second order time discretization was used.

Committee:

Mounir Ibrahim, PhD (Advisor); Mohammad Kassemi, PhD (Committee Member); Olga Kartuzova, PhD (Committee Member)

Subjects:

Fluid Dynamics; Mechanical Engineering

Keywords:

CFD; Computational Fluid Dynamics; model; boiling; pressure rise; microgravity; tank; engineering; modeling;

Ham, Tae KyuCFD Analyses of Air-Ingress Accident for VHTRs
Doctor of Philosophy, The Ohio State University, 2014, Nuclear Engineering
The Very High Temperature Reactor (VHTR) is one of six proposed Generation-IV concepts for the next generation of nuclear powered plants. The VHTR is advantageous because it is able to operate at very high temperatures, thus producing highly efficient electrical generation and hydrogen production. A critical safety event of the VHTR is a loss-of-coolant accident. This accident is initiated, in its worst-case scenario, by a double-ended guillotine break of the cross vessel that connects the reactor vessel and the power conversion unit. Following the depressurization process, the air (i.e., the air and helium mixture) in the reactor cavity could enter the reactor core causing an air-ingress event. In the event of air-ingress into the reactor core, the high-temperature in-core graphite structures will chemically react with the air and could lose their structural integrity. Complex multiple phenomena (i.e., diffusion, gravity-driven density-driven flow, natural circulation, and chemical reactions) are involved in an air-ingress accident; however, there is limited experimental data available to understand the air-ingress phenomena. Therefore, Ohio State University (OSU) designed a 1/8th scaled-down test facility to develop an experimental database for studying the mechanisms involved in the air-ingress phenomenon. The current research focuses on the analysis of the air-ingress phenomenon using the computational fluid dynamics (CFD) tool ANSYS FLUENT for better understanding of the air-ingress phenomenon. CFD is the process of obtaining solutions to a set of coupled non-linear partial differential equations by numerical approximation. Due to physical modeling and discretization limitations, it is inevitable that the CFD solutions will have some errors. Therefore, the uncertainty of the CFD physical models were investigated by benchmark studies, and mesh refinement studies were performed to minimize the error from the mesh structure and transient time step. The grid conversion index (GCI) and Richardson extrapolation method were used to quantify the uncertainty of the simulation. The anticipated key steps in the air-ingress scenario for guillotine break of VHTR cross vessel are: 1) depressurization; 2) density-driven stratified flow; 3) local hot plenum natural circulation; 4) diffusion into the reactor core; and 5) global natural circulation. However, the OSU air-ingress test facility covers the time from depressurization to local hot plenum natural circulation. Prior to beginning the CFD simulations for the OSU air-ingress test facility, benchmark studies for the mechanisms which are related to the air-ingress accident, were performed to decide the appropriate physical models for the accident analysis. In addition, preliminary experiments were performed with a simplified 1/30th scaled down acrylic set-up to understand the air-ingress mechanism and to utilize the CFD simulation in the analysis of the phenomenon. Previous air-ingress studies simulated the depressurization process using simple assumptions or 1-D system code results. However, recent studies found flow oscillations near the end of the depressurization which could influence the next stage of the air-ingress accident. Therefore, CFD simulations were performed to examine the air-ingress mechanisms from the depressurization through the establishment of local natural circulation initiate. In addition to the double-guillotine break scenario, there are other scenarios that can lead to an air-ingress event such as a partial break were in the cross vessel with various break locations, orientations, and shapes. These additional situations were also investigated. The simulation results for the OSU test facility showed that the discharged helium coolant from a reactor vessel during the depressurization process will be mixed with the air in the containment. This process makes the density of the gas mixture in the containment lower and the density-driven air-ingress flow slower because the density-driven flow is established by the density difference of the gas species between the reactor vessel and the containment. In addition, for the simulations with various initial and boundary conditions, the simulation results showed that the total accumulated air in the containment collapsed within 10% standard deviation by: 1. multiplying the density ratio and viscosity ratio of the gas species between the containment and the reactor vessel and 2. multiplying the ratio of the air mole fraction and gas temperature to the reference value. By replacing the gas mixture in the reactor cavity with a gas heavier than the air, the air-ingress speed slowed down. Based on the understanding of the air-ingress phenomena for the GT-MHR air-ingress scenario, several mitigation measures of air-ingress accident are proposed. The CFD results are utilized to plan experimental strategy and apparatus installation to obtain the best results when conducting an experiment. The validation of the generated CFD solutions will be performed with the OSU air-ingress experimental results.

Committee:

Xiaodong Sun (Advisor); Tunc Aldemir (Committee Member); Richard Christensen (Committee Member)

Subjects:

Nuclear Engineering

Keywords:

Air-ingress, CFD, Uncertainty quantification

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