Doctor of Philosophy, University of Akron, 2013, Mechanical Engineering

Due to their physical properties, gallium nitride crystals are in high demand in applications including light emitting diodes, high power and high frequency devices. One way of growing the crystals is the ammonothermal growth process. The process consists of chemical reactions occurring in reactors under high temperature and high pressure conditions. A basket of gallium nitride nutrient is inserted inside the reactor filled with ammonia and a mineralizer, whereupon gallium nitride is dissolved and transported by natural convection and then deposited onto the seeds. Because the etching and deposition reactions require temperatures in the range of 600 to 1,000 K (620 to 1,340 °F) and pressures in the range of 1,000 to 6,000 bar (14,504 to 87,022 psi), it is impossible to visualize the flow or measure its parameters. This dissertation presents a way to look inside an ammonothermal crystal growth reactor by simulating the process using CFD software. An equivalent reactor and crystal growth environment are created to provide experimental validation. The equivalent reactor respects geometric and dynamic similitude with respect to the actual ammonothermal crystal growth reactor. Experimental temperatures and velocities are recorded and compared with numerical results. Three turbulence models and the laminar model were tested. The laminar and the standard k-omega models performed better compared with experimental results. By simulating an equivalent reactor that allows visualization and measurements, the CFD model was validated. With the validated model, simulations of the actual growth process including mass transfer were performed. The wall temperature profile, the geometry of the nutrient basket, and the baffle were used as parameters to investigate their influence on the deposition rates. The temperatures on the outer walls of the reactor have a great influence on the deposition rate and can lead to etching of the seeds instead of crystal growing. The presence of a ba (open full item for complete abstract)

Committee: Minel Braun Dr. (Advisor); Abhilash Chandy Dr. (Advisor); Alex Povitsky Dr. (Committee Member); Gaurav Mittal Dr. (Committee Member); S. I. Hariharan Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2023, Mechanical Engineering

A 32.5% water-urea mixture, commercially known as AdBlue®, is stored onboard diesel vehicles as a liquid within storage tanks and is used for exhaust aftertreatment. In cold weather conditions, the mixture may freeze and expand over the span of several hours or days, resulting in the damage of the enclosing tank. However, computational modelling of the solidification/melting process in tanks of such “large” size and over such “long” durations is a challenging task, partly due to the simultaneous presence of all three phases (solid, liquid and gas). Furthermore, as natural convection plays an important role during the freezing process, it cannot be ignored. Capturing the dynamics of natural convection requires the use of extremely small time-step sizes, in relation to the overall freezing time scales, which significantly affects the computational speed of these simulations. This fact is demonstrated in the preliminary assessment phase of this study, where the in-built models of the commercial CFD solver ANSYS FluentTM are utilized to study the freezing process in a simple, small, partially filled 2D tank. Results show that though the models are able to provide great physical details of the solidification process, they result in impractically long simulation run times (~year). This led to the main objective of this work: the development, validation, and demonstration of an efficient 3D computational model that can be used to model the solidification process in large, partially-filled tanks containing either water or Adblue®. The first part of this work developed a new “reduced” model that accounts for the heat transfer due to natural convection during solidification/melting but, ignores the movement of the gas-(solid/liquid) interface due to expansion of ice. This new reduced natural convection model bypasses solving for flow and reduces the energy equation to a pure conduction equation by modeling convective heat fluxes using an equivalent conductive heat flux via (open full item for complete abstract)

Committee: Sandip Mazumder (Advisor); Seung Hyun Kim (Committee Member); Datta Gaitonde (Committee Member); Marcello Canova (Committee Member) Subjects: Fluid Dynamics; Mechanical Engineering

Doctor of Philosophy, University of Akron, 2022, Mechanical Engineering

Natural convection in laterally heated vertical cylindrical enclosures (LHVCE) has been studied in the past; however, few studies have thoroughly investigated the characteristic length scales. The studied parameters of this enclosure included: the heated and cooled length, diameter, aspect ratio, hot cold wall temperature difference, and the fluid properties. The characteristic length and form of two correction functions were derived from a logical set of assumptions based upon the enclosure configuration. Utilizing the derived dimensionless functions and length scale in conjunction with numerical simulation results, a best fit correlation was formed. The correlation, a first of its kind for this geometry, successfully predicted heat transfer and the flow regime changes from laminar to turbulence. The correlation and the underlying foundation from which it was formed showed to be in agreement with other similar studies. This study then proceeded from enclosures to internal reactor configurations containing baffles. The baffles were grouped into three geometries: rings, single hole openings, and a uniquely shaped (flow enhancing) baffle. The dimensions of each baffle were parametrically varied, typically by scaling the openings; and the velocity and temperature contour results of the simulations are presented. It was found that the central hole and shaped baffle exhibited more desirable flow patterns and thermal environments than the ring baffles. The final portion of the study investigated the effect of the porous media within the enclosure. The porous media cannot be studied independently, thus, two baffles from the previous investigation were utilized for the porous media. Three porous media configurations were studied: homogenous block, extruded concentric rings, and disks. Each of the basic configurations were laid out with a set of scaling parameters and were simulated parametrically in conjunction with the two baffles. The resistance of the porous media was (open full item for complete abstract)

Committee: Nicholas Garafolo (Advisor); Minel Braun (Advisor); Sergio Felicelli (Committee Chair); Alex Povitsky (Committee Member); Scott Sawyer (Committee Member); Kevin Kreider (Committee Member); Edward Evans (Committee Member) Subjects: Engineering

Master of Science, The Ohio State University, 2019, Mechanical Engineering

A mixture of 32.5% urea and water, commercially known as AdBlue®, is conventionally used for Selective Catalytic Reduction (SCR) of NOx in diesel vehicles. AdBlue® freezes under cold winter conditions (below −11°C or 12°F). Complete freezing is detrimental to the tank in which it is stored, and exact knowledge of the freezing front (solid-liquid interface) propagation is key to mitigating complete freezing. However, modeling the solidification/melting process in the tank is made complicated by the fact that three phases have to be accounted for: gas (air), liquid (water) and solid (ice). Moreover, the effects of natural convection are important and cannot be neglected. Numerical constraints in modeling natural convection necessitate the use of very small time-step sizes, making full-blown Computational Fluid Dynamics (CFD) calculations of the freezing process computationally expensive, especially when the physical time scales of the problem are of several days. This thesis has two objectives: (1) to perform a CFD study of the freezing process in a partially-filled tank using the in-built modeling capabilities of the commercial CFD code ANSYS Fluent™ in order to establish the limits of such models and, (2) to develop and validate a computationally efficient reduced model for freeze/thaw.The thesis begins with a systematic study to investigate the capabilities of the in-built physical models of Fluent pertaining to freezing/thawing, particularly with respect to computational efficiency. A canonical three-phase system was modeled using the Volume Of Fluid (VOF) method coupled with the solidification sub-model of Fluent. Results indicated that though Fluent was successfully able to capture the physical effects of solidification, extremely small time-step sizes (as low as 5×10-4 seconds) were needed to model the solidification process. This study successfully established the limits of Fluent's in-built models to model freeze/thaw in actual tanks for SCR applications.Foll (open full item for complete abstract)

Committee: Sandip Mazumder (Advisor) Subjects: Fluid Dynamics; Mechanical Engineering

Doctor of Philosophy, University of Akron, 2019, Mechanical Engineering

Light industry has been revolutionized by using light emitting diodes, high power, and high-frequency devices. The base of this revolution relies on the usage of crystals such as gallium nitride (GaN). There are three main crystal growth techniques; Hydride Vapor Phase Epitaxy (HVPE), Amonothermal Crystal Growth and Solution Growth. Ammmonothermal crystal growth is the most effective method for growing high-quality GaN crystals. Gallium nitride crystals are dissolved and transported by natural convection and then deposited on the seeds in the reactor in a process that requires high temperatures (315C to 760C), as well as high-pressure environments (1000 to 3000 atm). Fluid flow study in such a domain is expensive and has its own experimental difficulties. Yet, experimental results are essentials to validate the numerical simulations. The first part of the present work, aims at establishing the threshold parameters for dynamic and geometric similitude (based on the Rayleigh number (Ra)), for natural convection in a cylindrical enclosure heated laterally. Ra considered for this purpose spanned a range between 750 and 8.8e8, where the characteristic length, L, used in its definition is represented by the ratio of cylindrical enclosure's volume to its lateral surface area (R/2). The commercial CFD code, ANSYS FLUENT is used to model a 2-D axisymmetric cylindrical geometry using the laminar and k-w SST turbulent solvers. When comparatively analyzed, the results allowed conclusions associating values of the Ra to flow development from laminar to transitional and to turbulent flow. The next part of the present work aims to investigate the effects of using different RANS models (k-e and k-w), in ANSYS FLUENT, on the fluid and thermal maps for a fixed Ra of 8.8e6, with similar thermal and geometrical boundary conditions using the 2-D axisymmetric model. The results will provide an overall insight into the differences and how a new turbulent model can potentially change (open full item for complete abstract)

Committee: Minel. J. Braun PhD (Advisor); Alex Povitsky PhD (Committee Member); Scott Sawyer PhD (Committee Member); George Chase PhD (Committee Member); Kevin. L. Kreider PhD (Committee Member) Subjects: Mechanical Engineering

MS, University of Cincinnati, 2019, Engineering and Applied Science: Electrical Engineering

This work aims at demonstrating the concept of utilizing waste heat of Photovoltaic cell by thermoelectric devices. Experiments are performed to study the effect of variation of sun concentration from 0.25 to 1.8 suns (1 sun = 1000 W/m2) on the efficiency of the Photovoltaic cell having 5 cm x 5 cm dimensions. It is found that with the increase of sun intensity from 400 (x0.25) to 1600 W/m2 (x1.6) the photovoltaic cell shows variation of the power output from 0.10 W to 0.25 W. Also, this sun intensity variation resulted in the heating of photovoltaic cell. We measured the top and bottom surface temperatures of the photovoltaic cell. The top surface temperature varied in range from 304.7 K to 380.3 K with increasing solar intensity, while the bottom surface has a lower temperature compared to the top surface varying from 303.85 K to 370.55 K. Thermoelectric devices are attached to the bottom surface of the photovoltaic cell to recover the waste heat from the photovoltaic cell. In order to demonstrate the thermoelectric principle, we have first measured the power output we could fetch through the K type thermocouple junction; the same TC junction we have used to measure the temperature on the bottom surface of photovoltaic cell. At input sun intensity of 1800 W/m2 we have measured the maximum power output of around 0.28 µW from this single thermocouple junction. Finally, a V- type longitudinal thermoelectric device made of multiple TE legs is fabricated and attached at the back surface of the photovoltaic cell to demonstrate the increase of the power output. We have also discussed how increasing in the thermal contact area of the thermocouple device could help in increasing the power output from the PV cell. Also, in this work we have performed full three-dimensional numerical simulation on the hybrid photovoltaic and transverse thermoelectric device with natural air convection using ANSYS to gauge the power generation performance for future research.

Committee: Je-Hyeong Bahk Ph.D. (Committee Chair); Marc Cahay Ph.D. (Committee Member); Peter Kosel Ph.D. (Committee Member) Subjects: Engineering

Master of Science, University of Akron, 2018, Mechanical Engineering

This study numerically investigates the laterally heated vertical cylinder and the length scale associated with this reactor. For natural convection the important dimensionless characteristic is the Rayleigh number, which predicts the flow regime as laminar, transitional, or turbulent. The Rayleigh number is useful as a design tool for scaling a reactor. Up to this point the associated length scale has been assumed as various definitions of length and diameter and has not yet been thoroughly investigated. The current assumed definitions for the length scale: height, diameter, and volume to lateral surface area, are directly studied in a multi-dimensional (2D and 3D) numerical parametric study involving these length scales and aspect ratio (height/diameter). Other important characteristics such as the ratio of heating to cooling and thickness of the divider (insulator) between heating and cooling are also studied. The study begins with turbulent transient 2D axisymmetric simulations and proceeds to turbulent transient 3D simulations then compares the 3D and 2D simulations. Finally, 2D and 3D laminar simulations are investigated. Presented are the results of the fluid flow speeds, thermal environments, flow patterns, boundary layer thickness, boundary layer velocity, and normal probability density functions which provide a unique way of studying how the Rayleigh number is influenced by variables. The numerical simulations are examined for spatial step, time step, and relative convergence by a mesh study, time step study, and thermal analysis, respectively. The turbulence model used (k-ω SST) is based on recent published studies. All simulations were conducted with the commercially available software ANSYS FLUENT. Findings are discussed when they prove significant, and aid in developing a fundamental understanding of the physics occurring inside this reactor setup. The results indicate that the current the length scales assumed for this reactor are incorrec (open full item for complete abstract)

Committee: Braun Minel J (Advisor); Nicholas Garafolo G (Committee Member); Scott Sawyer (Committee Member); Abhilash Chandy J (Other) Subjects: Mechanical Engineering

Doctor of Engineering, Cleveland State University, 2017, Washkewicz College of Engineering

This dissertation explores the role of different types of convection on macrosegregation and on dendritic array morphology of two aluminum alloys directionally solidified through cylindrical graphite molds having both cross-section decrease and increase. Al- 19 wt. % Cu and Al-7 wt. % Si alloys were directionally solidified at two growth speed of 10 and 29.1 µm s-1 and examined for longitudinal and radial macrosegregation, and for primary dendrite spacing and dendrite trunk diameter. Directional solidification of these alloys through constant cross-section showed clustering of primary dendrites and parabolic-shaped radial macrosegregation profile, indicative of “steepling convection” in the mushy-zone. The degree of radial macrosegregation increased with decreased growth speed. The Al- 19 wt. % Cu samples, grown under similar conditions as Al-7 wt. % Si, showed more radial macrosegregation because of more intense “stepling convection” caused by their one order of magnitude larger coefficient of solutal expansion. Positive macrosegregation right before, followed by negative macrosegregation right after an abrupt cross-section decrease (from 9.5 mm diameter to 3.2 mm diameter), were observed in both alloys; this is because of the combined effect of thermosolutal convection and area-change-driven shrinkage flow in the contraction region. The degree of macrosegregation was found to be higher in the Al- 19 wt. % Cu samples. Strong area-change-driven shrinkage flow changes the parabolic-shape radial macrosegregation in the larger diameter section before contraction to “S-shaped” profile. But in the smaller diameter section after the contraction very low degree of radial macrosegregation was found. The samples solidified through an abrupt cross-section increase (from 3.2 mm diameter to 9.5 mm diameter) showed negative macrosegregation right after the cross-section increase on the expansion platform. During the transition to steady-state after the expansion, radia (open full item for complete abstract)

Committee: Surendra Tewari Ph.D. (Advisor); Jorge Gatica Ph.D. (Committee Member); Orhan Talu Ph.D. (Committee Member); Rolf Lustig Ph.D. (Committee Member); Kiril Streletzky Ph.D. (Committee Member) Subjects: Aerospace Materials; Automotive Materials; Chemical Engineering; Condensed Matter Physics; Engineering; Fluid Dynamics; High Temperature Physics; Materials Science; Metallurgy

Master of Science in Mechanical Engineering, Cleveland State University, 2010, Fenn College of Engineering

A boundary layer analysis is presented for the natural convection heat and mass transfer flow past along two different geometries i.e., a vertical cone and an isothermal sphere in a Non-Darcy porous medium saturated with a nanofluid. A co-ordinate transformation is used to transform the governing equations into non-dimensional non-similar boundary layer equations. These equations are then solved numerically using implicit finite difference method (Keller-box method). Numerical solutions for heat transfer rate, mass transfer rate and friction factor have been presented for parametric variations of the buoyancy ratio parameter N¬r, Brownian motion parameter Nb, thermophoresis parameter Nt and Lewis number Le. The dependency of the local friction factor, surface heat transfer rate (Nusselt number), and mass transfer rate (Sherwood number) on these parameters has been discussed.

Committee: Rama Subba Reddy Gorla Ph.D. (Committee Chair); Majid Rashidi Ph.D. (Committee Member); Hanz Richter Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 1991, Mechanical Engineering

An experimental study is made of natural convection oscillations in gallium melts enclosed by right circular cylinders with differentially heated end walls. Heated-from-below cases are examined for angles of inclination ranging from 0° (vertical) to 75°, with aspect ratios cal A (height/radius) of 2, 4, and 6 and Rayleigh numbers (based on radius) ranging from 500 to 10,000. Temperature measurements are made at three levels along the circumference of the cylinder enclosing the gallium to detect the oscillations. Critical Rayleigh numbers for the onset of oscillations are found to be invariant to changes in aspect ratio for aspect ratios 4 and 6 at all angles of inclination. The experiments confirm predictions from scaling analysis that frequencies for these oscillations scale with the square root of the Rayleigh number. A parameter is found to relate the cal A = 4 and cal A = 6 frequencies at different values of tilt angle φ. For cal A = 6, 5° ≤ φ ≤ 20° a range of supercritical Rayleigh numbers is found where oscillations disappear or are greatly reduced in amplitude. Measurements are also made of temperature magnitudes for aspect ratios 2 and 6 in vertical and 1 0°-inclined cases. Results for the inclined cases were consistent with steady, unicellular convection. Although no conclusions could be drawn for the cal A = 6 vertical case, the cal A = 2 vertical case indicates some effects of convection below the critical Rayleigh number for onset of oscillation.

Committee: Ostrach Simon (Advisor) Subjects: Engineering, Mechanical

Master of Science in Engineering, University of Akron, 2012, Mechanical Engineering

The accurate simulation of magnetohydrodynamic (MHD) ows and heat transfer is critical to the understanding of the complex physics in liquid metal flows for coolant and breeder blankets used in fusion power plants such as tokamaks, metallurgy,aerospace applications and crystal growth. These issues will be addressed through an effort investigating low-magnetic Reynolds number (Rem), high-Hartmann number(Ha), MHD flows in a square cavity, using a 2D parallel vorticity-streamfunction-based incompressible Navier-Stokes code. Numerical simulations of MHD flow and heat transfer are carried out in a square cavity with emphasis on specific classical benchmark problems like the lid-driven cavity with and without heat transfer (forced convection), natural convection and mixed convection (forced + natural). Effects of Ha and Pr are investigated to assess MHD in heat transfer under various conditions. Quantities analyzed include streamfunction, vorticity, velocities, temperature and Nusselt number. Computations are carried out on in-house supercomputing desktops and clusters. This study provides a fundamental understanding of MHD effects on flow behavior and heat transfer in the context of a series of simple benchmark problems. Also created through this eort is an accurate and efficient incompressible code capable of calculating a variety of added complex physics in fluid mechanics.

Committee: Abhilash J. Chandy Dr. (Advisor); Alex Povitsky Dr. (Committee Member); Gaurav Mittal Dr. (Committee Member) Subjects: Mechanical Engineering