Browsing by Author "William L. Roberts, Committee Member"

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Mathematical Modeling of Laminar and Turbulent Single-phase and Two-phase Flows in Straight and Helical Ducts
(2004-11-05) Cheng, Liping; Kevin M. Lyons, Committee Member; Zhilin Li, Committee Member; William L. Roberts, Committee Member; K. P. Sandeep, Committee Member; Andrey V. Kuznetsov, Committee Chair
The purpose of this research is to investigate numerically the dynamics and heat transfer of laminar or turbulent flows in different media and complicated geometries, including the flow in a composite domain whose central portion is occupied by a clear fluid (turbulent flow) and whose peripheral portion is occupied by a fluid saturated porous medium (laminar flow); a laminar flow of a non-Newtonian fluid in a helical pipe; a laminar flow in a helical pipe filled with a fluid saturated porous medium; a two-phase laminar flow (non-Newtonian carrying fluid and solid particles) in a helical pipe. To model forced convection in a composite porous/fluid domain, the Brinkma-Forchheimer-extended Darcy equation is utilized for the porous region and a two-layer algebraic turbulence model is utilized for the flow in the central region. The effects of turbulence on velocity and temperature distributions as well as on the Nusselt number are analyzed. To investigate a fully developed laminar flow of a non-Newtonian fluid in a helical pipe, an orthogonal helical coordinate system is utilized and the Navier-Stokes and energy equations for the non-Newtonian fluid in this coordinate system are derived. The effects of the curvature and torsion of a helical pipe, the Dean number and Germano number on the velocities, secondary flow and heat transfer are presented. A full momentum equation for the flow in porous media that accounts for the Brinkman and Forchheimer extensions of the Darcy law as well as for the flow inertia is adopted to study the fully developed laminar flow in a helical pipe filled with a fluid saturated porous medium. The effects of the geometry of the helical pipe and the physical properties of the porous medium are investigated. Accounting for the flow inertia is shown to be important for predicting the secondary flow in a helical pipe. For 3D modeling of two-phase laminar flow in a helical pipe, the Eulerian approach is utilized for fluid flow and the Lagrangian approach is utilized for tracking particles. The interaction between the solid particles and the fluid that carries them is accounted for by a source term in the momentum equation for the fluid. The influence of inter-particle and particle-wall collisions is also taken into account.
Mathematical Modeling of Single Phase Flow and Particulate Flow Subjected to Microwave Heating
(2006-12-19) Zhu, Jianxi; Andrey V. Kuznetsov, Committee Chair; K. P. Sandeep, Committee Member; William L. Roberts, Committee Member; Tarek Echekki, Committee Member
The purpose of this research is to numerically investigate heat transfer in liquids and liquids with carried solid particles as they flow continuously in a duct that is subjected to microwave irradiation. During this process, liquid flows in an applicator tube. When flow passes through the microwave cavity, the liquid absorbs microwave power and its temperature quickly increases. The spatial variation of the electromagnetic energy and temperature fields in the liquid was obtained by solving coupled momentum, energy and Maxwell's equations. A finite difference time domain method (FDTD) is used to solve Maxwell's equations simulating the electromagnetic field. The effects of dielectric properties of the liquid, the applicator diameter and its location, as well as the geometry of the microwave cavity on the heating process are analyzed. For modeling particulate flow subjected to microwave heating, the hydrodynamic interaction between the solid particle and the carrier fluid is simulated by the force-coupling method (FCM). The Lagrangian approach is utilized for tracking particles. The electromagnetic power absorption, temperature distribution inside both the liquid and the particles are taken into account. The effects of dielectric properties and the inlet position of the particle on electromagnetic energy and temperature distributions inside the particle are studied. The effect of the particle on power absorption in the carrier liquid is analyzed. The effect of the time interval between consecutive injections of two groups of particles on power absorption in particles is analyzed as well.
Numerical and Theoretical Analysis of Beam Vibration Induced Acoustic Streaming and the Associated Heat Transfer
(2004-02-23) Wan, Qun; Paul I. Ro, Committee Member; Andrey V. Kuznetsov, Committee Chair; Paul D. Franzon, Committee Member; William L. Roberts, Committee Member
The purpose of this research is to numerically and analytically investigate the acoustic streaming and the associated heat transfer, which are induced by a beam vibrating in either standing or traveling waveforms. Analytical results show that the beam vibrating in standing waveforms scatters the acoustic waves into the free space, which have a larger attenuation coefficient and longer propagating traveling wavelength than those of the plane wave. In contrast to a constant Reynolds stress in the plane wave, the Reynolds stress generated by such acoustic wave is expected to drive the free space streaming away from the anti-nodes and towards nodes of the standing wave vibration. The sonic and ultrasonic streamings within the channel between the vibrating beam and a parallel stationary beam are also investigated. The acoustic streaming is utilized to cool the stationary beam, which has either a heat source attached to it or subjected to a uniform heat flux. The sonic streaming is found to be mainly the boundary layer streaming dominating the whole channel while the ultrasonic streaming is clearly composed of two boundary layer streamings near both beams and a core region streaming, which is driven by the streaming velocity at the edge of the boundary layer near the vibrating beam. The standing wave vibration of the beam induces acoustic streaming in a series of counterclockwise eddies, which is directed away from the anti-nodes and towards the nodes. The magnitude of the sonic streaming is proportional to ω²A while that of the ultrasonic streaming is proportional to Ω[superscript 3/2]A². Numerical results show that the acoustic streaming induced by the beam vibrating in either standing or traveling waveforms has almost the same cooling efficiency for the heat source and the heat flux cases although the flow and temperature fields within the channel are different. The hysteresis of the ultrasonic streaming flow patterns associated with the change of the aspect ratio of the channel is numerically investigated. Present research is also extended to a cavity which is driven by a vibrating lid. The ultrasonic streaming induced in the cavity reveals some interesting interactions among the primary eddies, which have never been observed in the classical driven cavity problem.
Numerical Simulation of Scramjet Combustion in a Shock Tunnel
(2005-12-09) Star, Jason Blue; Jack R. Edwards Jr., Committee Chair; Hassan A. Hassan, Committee Member; D. S. McRae, Committee Member; William L. Roberts, Committee Member
Three-dimensional computational simulations of reactive flowfields within a hydrogen-fueled scramjet-like geometry experimentally tested in a free piston shock tunnel are presented. The experimental configuration (Odam and Paull, AIAA Paper 2003-5244) involves injection of hydrogen fuel into the scramjet inlet, followed by mixing, shock-induced ignition, and combustion. The predictions for both fuel-off and fuel-on conditions were observed to be sensitive to the choice of the wall temperature boundary conditions. The best comparison with experimental data were achieved through the implementation of an approach that involves a simplified conjugate heat transfer model that couples the heat conduction through the wall with the heat conduction of the fluid within the boundary layer. This approach is able to predict thermal loads on the walls of the scramjet model due to shock wave interactions and due to heat release. As such, it is able to more accurately represent the physical temperature response of the engine model. Also shown to produce very good agreement with the statistically-steady experimental data was the isothermal ghost-cell boundary condition, which is based on a simplification of the time-dependent conjugate heat transfer boundary condition. This simplified boundary condition assumes a linear temperature distribution within the wall based on the effective depth that an applied heat load would penetrate, thus, it also allows the actual wall temperature to vary in response to the applied heat load. Results for fuel-off simulations showed that the solution generated by a steady-state simulation implementing the isothermal ghost-cell wall boundary condition was very comparable with the statistically-steady solution obtained from a fully transient simulation with coupled heat conduction within the walls. When integrated in a fully time-accurate manner, the fuel-on simulations showed a striking sensitivity to the modeled rate of air ingestion into the engine. For experimental data that showed steady combustion, the transient simulations resulted in either a steady combusting solution or a progression toward engine unstart, depending on the modeled rate of air ingestion. Also, for experimental data that showed an unsteady thermal choking event leading to eventual unstart, the transient simulations were able to predict both unstart and steady combustion, once again depending on the air ingestion rate. In all cases, the modeled air ingestion process is an approximation of the actual experimental process, in that uniform conditions are imposed as linear functions of time over the inlet plane. The computational results also provide some support for a 'radical-farming' hypothesis, proposed to explain the ability of the hydrogen-air mixture to auto-ignite at relatively low inlet contraction ratios.
Processing and Characterization of Functionally Graded Hydroxyapatite Coatings for Biomedical Implants
(2010-08-07) Bai, Xiao; William L. Roberts, Committee Member; Michael Rigsbee, Committee Member; Afsaneh Rabiei, Committee Chair; Gregory Buckner, Committee Member
Hydroxyapatite [Ca10(PO4)6(OH)2, HA] has been widely applied as a coating on various biomedical bone/dental implants to improve biocompatibility and bioactivity. It has been observed that primary reasons leading to implantation failure of commercial HA coated implants processed by plasma spraying are the poor mechanical properties of coatings and infections accompanied by implantation. It has been also reported an ideal coating should be able to stimulate new bone growth at the initial stage of implantation and stay stable both mechanically and chemically thereafter. This research has investigated a functionally graded hydroxyapatite (FGHA) coating that is capable of improving the stability of implants, facilitating recovery, and preventing infections after implantation. A series of FGHA coatings with incorporated Ag 0 ~ 13.53 wt. % has been deposited onto Ti substrate using ion beam assisted deposition (IBAD) with in-situ heat treatment. The compositional, microstructural, mechanical, and biological properties of coatings have been analyzed via various tests. The relationship among processing parameters, coating properties and biological behaviors has been established and the processing parameters for processing FGHA coatings with/without incorporated Ag have been optimized. Microstructure observations of coating cross section via transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) for set temperature coatings deposited at 450 oC ~ 750 oC reveals that in-situ substrate temperature is the primary factor controlling the crystallinity of the coatings. The microstructure observation of cross section via TEM / STEM for both FGHA coatings with / without incorporated Ag has shown that coatings are dense and have a gradually decreased crystallinity from substrate/coating interface to top surface. In particular, the interface has an atomically intermixed structure; the region near the interface has a columnar grain structure whereas the region near coating top surface is mostly amorphous. TEM / STEM observation of FGHA coating with incorporated Ag has also demonstrated that the metallic silver particles in size of 10 ~ 50 nm distribute at the coating cross section throughout the coating thickness. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis have shown that coatings consist of HA and various calcium phosphate compounds. The pull off tests have shown that the average adhesion strength of FGHA coatings (both with and without Ag) to substrate are in the range of 83.44 Â± 5.71 ~ 89.36 Â± 5.13 MPa. Further optical observation of pull off area of coating shows that no coating delamination is observed and epoxy failure is dominant, indicating a well-boned interface and a strong coating itself. It has been concluded that the high adhesion strength of coating to substrate is attributed to the atomic intermixed interface and dense structure of coating, which is resulted from the increased mobility of coating atoms at high substrate temperature under bombardment of assisted ion beam. Culture tests have shown a distinct increase in osteoblast cell attachment to FGHA surface after 24 hours culture test when compared to blank Ti controls. Both calcium and silver release tests of Ag-doped FGHA coatings have shown the release rate is high at the initial stage and it steadily decreases, which is the expected performance of FGHA coatings. Antibacterial test using S. aureus has revealed that Ag doped FGHA coatings show an inhibitory effect when compared to coating without Ag and blank Ti. In particular, with higher amounts of Ag in coatings, the inhibition of S. aureus is stronger. Cytotoxicity test indicates that the FGHA coating with the highest amounts of Ag shows a negative effect on the osteoblast response.
Synthesis and Monte Carlo Simulation of Metallic Nanoparticles and Thermophysical Property Studies of Nanofluids
(2007-01-11) Wu, Chunwei; Kevin M. Lyons, Committee Co-Chair; William L. Roberts, Committee Member; Carl C. Koch, Committee Member; Taofang Zeng, Committee Chair
Nanostructured materials, including versatile nano-objects such as nanoparticles, nanotubes, nanowires, quantum dots and other nano-units as the building blocks for new bottom-up approaches to device and system assembly, are at the leading edge of the rapid developing field of nanoscience and nanotechnology. Metallic nanoparticles have captivated scientists' enduring attention and passion for their novel physical and chemical properties and promising application in numerous areas. In this work, we present for the first time, a whole new metallic nanoparticles synthetic strategy based on a heterogeneous metal displacement reduction mechanism. In association with this underlying principle, we developed hydrodynamically and mechanically-assisted, and ultrasonication-assisted displacement reduction methods to successfully prepare a series of silver, copper, iron oxide, gold and platinum nanoparticles. By controlling reactant concentration and particle average residence time, we achieve size selectivity and size distribution control, which provides the possibility for exploitable scalability in commercial production. Based on our experimental system, we established a kinetic model using a Monte Carlo stochastic algorithm and FORTRAN programming to explain the formation of dispersions of various sizes and size distributions. The model was tested with parameters of our real system of silver nanoparticles formation with a variety of mean size and size distribution. The simulated average size, size distribution and the time scale of the process agree reasonably well with the experimental values. Thus the established theoretical model was proven to simulate and predict the practical system adequately and effectively. Thermophysical property of copper nanofluids we produced was studied, and the effective thermal conductivity of nanofluids at room temperature enhances with increased nanoparticles volume fraction; a 7.4 % of enhancement was obtained with 1% volume fraction.