Página 2 dos resultados de 452 itens digitais encontrados em 0.022 segundos

Investigation of pressure-tube and calandria-tube deformation following a single channel blockage event in ACR-700

Gerardi, Craig Douglas
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 115 leaves
Português
Relevância na Pesquisa
1386.5912%
The ACR-700 is an advanced pressure-tube (PT) reactor being developed by Atomic Energy of Canada Limited (AECL). As in conventional CANDU reactors, the PTs are horizontal. Each PT is surrounded by a calandria tube (CT), and the gap in between is filled with carbon dioxide gas. The space between the CTs is filled with the heavy-water moderator. One postulated accident scenario for ACR-700 is the complete coolant flow blockage of a single PT. The flow is not monitored within each individual PT, thus during the early stages of this accident the reactor remains at full power and full pressure, resulting in rapid coolant boil-off and fuel overheating. Melting of the Zircaloy (Zry) components of the fuel bundle (cladding, end plates and end caps) can occur, with relocation of some molten material to the bottom of the PT. The hot spot caused by the molten Zry/PT interaction may cause PT/CT failure due to localized plastic strains. Failure of the PT/CT results in depressurization of the primary system, which triggers a reactor scram, after which the decay heat is removed via reflooding, thus PT/CT rupture effectively terminates the accident. Clearly, prediction of the time scale and conditions under which PT/CT failure occurs is of great importance for this accident. We analyzed the following key phenomena occurring after the blockage: (a) Coolant boil-off (b) Cladding heat-up and melting (c) Dripping of molten Zircaloy (Zry) from the fuel pin (d) Thermal interaction between the molten Zry and the PT (e) Localized bulging of the PT (f) Interaction of the bulged PT with the CT Simple one-dimensional models were adequate to describe (a)...

Advanced design concepts for PWR and BWR high-performance annular fuel assemblies; Advanced design concepts for boiling water reactor and pressurized water reactor high-performance annular fuel assemblies

Ellis, Tyler Shawn
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 183 p.
Português
Relevância na Pesquisa
1386.5567%
Sobering electricity supply and demand projections, coupled with the current volatility of energy prices, have underscored the seriousness of the challenges which lay ahead for the utility industry. This research addresses the impending global need for electricity through the development of advanced annular fuel designs with both internal and external cooling which can achieve higher power densities and hence, higher electricity output from the same basic reactor vessel and containment. Therefore the objectives of this project are to determine the optimal geometrical design parameters of an annular fuel assembly for both PWRs and BWRs for the purpose of achieving maximum power density. It is theorized that utility companies can utilize this design through either retrofitting of their existing reactor facilities or incorporation of the fuel design into new plant concepts. For the case of annular fuel for PWRs, a high performance uranium nitride fuel assembly concept capable of achieving a 50% higher power density was successfully developed. It is shown that a 5% enriched UN annular-fuel assembly can operate at 150% power density for about 50 effective-full-power-days more than that of the nominal 17xl7 solid-fuel-pin assembly operating at 100% power density. Furthermore...

Effects of thermal aging on Stress Corrosion Cracking and mechanical properties of stainless steel weld metals

Hixon, Jeff
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 589 p.
Português
Relevância na Pesquisa
1386.5366%
Stress Corrosion Cracking (SCC) in and around primary loop piping welds in Boiling Water Reactors has been observed worldwide as plants continue to operate at temperatures and pressures near 2880C (5500F) and 6.9 MPa (1000 PSI). An experimental program was designed to explore the effect of thermal aging on the SCC crack growth rate in weld materials for type 316 and 304 stainless steels. An autoclave facility was designed and constructed for the measurement of SCC crack growth rates under BWR conditions and testing was underway at the time of this writing. The effects of composition and thermal aging on mechanical properties (i.e. tensile, micro-hardness, nano-hardness, Jic, and Charpy-impact toughness) was in process and initial results show an increase in yield strength and a decrease in fracture toughness after aging for 1000 hours at 430 and 400 *C. Thermal aging results in no discernable changes to the 6-ferrite morphology when viewed optically at 500 X agnifications and in the scanning electron microscope.; by Jeff Hixon.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006.; Includes bibliographical references (p. 589).

Lower hybrid modeling and experiments on Alcator C-Mod

Liptac, John E
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 256 p.
Português
Relevância na Pesquisa
1386.5138%
A highly flexible phase control system was developed in order to dynamically adjust the parallel wave number spectrum of driven LH waves on Alcator C-Mod. Wave coupling between the antenna and the plasma was studied using this system. A coupling code was modified to allow arbitrary phase and amplitude in each waveguide, which was required for agreement between the theory and experiment. Remarkable agreement was achieved when a small vacuum gap of 0.6 to 1.4 mm was included in the model, similar to results obtained on ASDEX. The location of the fast electron population is critical in diagnosing the performance of the lower hybrid system and can be inferred by measuring nonthermal bremsstrahlung emission. A state-of-the-art diagnostic for hard x-ray (HXR) emission was designed and used to make measurements of fast electrons bremsstrahlung during LHCD experiments on C-Mod. The HXR diagnostic consists of an array of 32 CdZnTe detectors to image energies in the 20-200 keV range. Detectors and pulse processing electronics were integrated into a compact and modular package. The system also makes use of fast digitization and software signal processing techniques allowing a maximum counting rate of 1 MHz. HXR experimental results indicate a spatially broad and centered fast electron population. HXR profiles predicted by a synthetic diagnostic in the Fokker-Planck code CQL3D are narrower than the observed profiles...

Effects of chromium and silicon on corrosion of iron alloys in lead-bismuth eutectic

Lim, Jeongyoun
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 134 leaves
Português
Relevância na Pesquisa
1388.7188%
The high power densities and temperatures expected for next generation nuclear applications, including power generation and transmutation systems, will require new types of heat transport systems to be economic. Present interest in heavy liquid metal coolants, especially in lead and lead-bismuth eutectic, originates from such requirements as increased heat removal capacity and enhanced safety features. However, corrosion of structural metals represents a major limiting factor in developing advanced liquid Pb-alloy coolant technology. In fact, the development of advanced structural and cladding alloys that are resistant to corrosion over a wide range of oxygen potentials in this environment would represent the enabling technology for these systems. The goal of this research was to develop a class of Fe-Cr Si alloys that are resistant to corrosion in Pb and Pb alloys at temperatures of 6000C or higher. As a necessary part of this development effort, an additional goal was to further develop the fundamental understandings of the mechanisms by which corrosion protection is achieved. A series of alloys based on the Fe-Cr-Si system were proposed as potential candidates for this application. These alloys were then produced and evaluated. The results of this evaluation verified the hypothesis that an Fe alloys with suitable levels of Cr (>12 wt%) and Si (> 2.5 wt%) will be protected by either a tenacious oxide film (over a wide range of oxygen potentials above the formation potential for Cr and Si oxides) or by a low solubility surface region (at low oxygen potentials) Experimental results obtained from model alloys after lead-bismuth eutectic exposure at 6000C demonstrated the film formation process.; (cont.) The hypothesis that Si addition would promote the formation of a diffusion barrier was confirmed by the actual reduction of oxide thickness over time. The Si effect was magnified by the addition of Cr to the system. Based on a kinetic data assessment on the experimental results of Fe-Si and Fe-Cr-Si alloys...

Transport modeling of simple fluids and nano-colloids : thermal conduction mechanisms and coarse projection

Eapen, Jacob, 1968-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 181 p.
Português
Relevância na Pesquisa
1386.5567%
In the first part of this thesis, the modes of microscopic energy fluctuations governing heat flow in nano-colloids are quantitatively assessed by combining linear response theory with molecular dynamics (MD) simulations. The intrinsic thermal conductivity is decomposed into self and cross correlations of the three modes that make up the microscopic heat flux vector, namely, the kinetic, the potential and the virial. By this decomposition analysis, the interplay between the molecular mechanisms that govern the variation of the thermal conductivity with volume fraction and solid-fluid interaction is examined. For a specific system of nanosized platinum clusters which interact strongly with host liquid xenon, a significant thermal conductivity enhancement is obtained as a result of self correlation in the potential energy flux. The effect saturates at higher volume fractions due to the cross-mode correlation between the potential and the virial flux. A strong solid-fluid coupling also introduces an amorphous-like structural transition and a pronounced cage effect that significantly reduces the self diffusion of the nano-clusters. These attendant structural and diffusive effects, unlike the self correlation of the potential flux, are amenable to experimental observations. The cluster-fluid interface is characterized by large fluctuations in the potential energy which is indicative of an unusual exchange of potential energy among the interfacial fluid atoms. For small nano-clusters...

Computer simulation and topological modeling of radiation effects in zircon

Zhang, Yi, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 298 p.
Português
Relevância na Pesquisa
1386.6803%
The purpose of this study is to understand on atomic level the structural response of zircon (ZrSiO4) to irradiation using molecular dynamics (MD) computer simulations, and to develop topological models that can describe these structural changes. Topological signatures, encoded using the concepts of primitive-rings and local clusters, were developed and used to differentiate crystalline and non-crystalline atoms in various zircon structures. Since primitive-rings and local clusters are general concepts applicable to all materials, and the algorithms to systematically identify them are well-established, topological signatures based on them are easy to implement and the method of topological signatures is applicable to all structures. The method of topological signatures is better than the Wigner-Seitz cell method, which depends on the original crystalline reference grid that is unusable in heavily damaged structures or regions; it is also better than those methods based only on local structures limited to first coordination shell, since one can decide whether or not to include ring contents of large rings into the topological signatures, effectively controlling the range of the topological signatures. The early-stage evolution of non-crystalline disorder and the subsequent recrystallization in zircon collision cascade simulations were successfully modeled by using the topological signatures to identify non-crystalline atoms. Simply using the number of displaced atoms was unable to correctly show the initial peak of structural damage followed by the subsequent annealing stage. Using the topological signatures...

Radiation and litigation : analyses of the ALARA principle and low dose radiation in the courts, and the future of radiation in court cases; Analyses of the ALARA principle and low dose radiation in the courts, and the future of radiation in court cases

Esparza, Enrique
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 39 leaves
Português
Relevância na Pesquisa
1386.4247%
Currently there are a growing number of radiation workers. In order to ensure the safety of the employees, regulations have been established by the federal government and state governments to limit the dose equivalent to radiation workers. The most well known strategy for reducing radiation doses in the work place is the ALARA principle which stands for "as low as reasonably achievable". Within the phrase, "reasonably achievable" there is an implied element of subjectivity. Because "reasonably achievable" can vary in meaning for different people, this paper will analyze the ALARA principle in detail. Also, the manner in which inconclusive data on low dose radiation are treated in the court rooms will be evaluated. A secondary part of the paper will deal with what happens when accidents occur to radiation workers. Specifically, this paper will deal with the accidents at Kerr-McGee, Three Mile Island and SONGS. The thesis will delve into the litigation that followed the radiation accidents and analyses of the rulings, and will look at where current radiation litigation is heading.; by Enrique Esparza.; Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006.; Includes bibliographical references (leaves 36-39).

Conceptual design of nuclear systems for hydrogen production

Hohnholt, Katherine J
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 81 p.
Português
Relevância na Pesquisa
1389.944%
Demand for hydrogen in the transportation energy sector is expected to keep growing in the coming decades; in the short term for refining heavy oils and in the long term for powering fuel cells. However, hydrogen cannot be harvested from natural sources like other fuels, it must be industrially produced. In the United States, the vast majority of hydrogen is produced today by reforming methane, a carbon-based fuel. Due to environmental and fuel source concerns, non-carbon alternatives for producing hydrogen from water are being explored using different combinations of thermal, chemical, and electrical energy. This work explores some of the non-carbon alternatives, specifically using a nuclear reactor for providing heat and electricity for high temperature steam electrolysis and a hybrid electrolysis-chemical sulfur cycle. Also addressed is the sensitivity of production and efficiency of these cycles to process conditions. For a desired hydrogen distribution pressure of 3MPa, high system pressures increase the efficiency of high temperature steam electrolysis because of the decreased post-cycle compression energy requirements. High system pressures for the hybrid sulfur cycle, however, decrease the equilibrium thermal acid decomposition necessary to the process. High temperature steam electrolysis may also be used to provide variable hydrogen production when coupled with an electricity generation system. Increased hydrogen production decreases the efficiency of the electricity production...

Development and application of a steady state code for supercritical carbon dioxide cycles

Legault, David M. (David Michael)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 63 leaves
Português
Relevância na Pesquisa
1388.7736%
The supercritical CO2 power conversion system is of interest for advanced nuclear reactor applications because the same efficiencies are obtained as for the most developed of the closed gas-turbine cycles (helium-Brayton), but at lower temperatures and higher pressures. The original in-house code, named CYCLES, could potentially be used by others who are researching the S-CO2 cycle, but it has its shortcomings. In particular, CYCLES does not factor in the pressure drops due to pipes and plena. Also, for new users, it takes a significant amount of time to fully understand how to use the code. The objectives of this thesis were to modify CYCLES to ensure that pipe and plena effects were included, and to improve the readability and functionality of the code. Changes to CYCLES are included in the rewritten code, named CYCLES II, and are also documented in this thesis. Furthermore, documentation of the program input and output is given, along with a flow chart of the algorithm logic. Two applications of the code are provided to show the effect of the pipes and plena on cycle performance. In comparing the cycle efficiency with and without the effects of the pipes and plena, for a 300 MWe S-CO2 Brayton power conversion system, the results indicate that the net cycle efficiency drops from 49% to 45% when pipes and plena of reasonable dimensions are included in the calculations. The losses are dominated by the low pressure pipe and plena segments. However...

Effects of ionizing radiation on normal and tumor-associated lymphatic vessels

Lobo, Jennifer D
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 43 p.
Português
Relevância na Pesquisa
1386.5138%
Lymphatic vessels play a crucial role in both the pathophysiology of tumors and in the spread cancer cells to lymph nodes. The effects of radiation on these vessels, however, are largely unknown. Here, we seek to describe the effects of ionizing radiation on normal and tumor-associated lymphatic vessels in vitro and in vivo. Clonogenic assays were employed to study the radiation dose response of lymphatic endothelial cells. Putative lymphatic endothelial cell mitogens and antiproliferative agents, including vascular endothelial growth factor-A (VEGF-A), VEGF-C and AZD2171, a tyrosine kinase inhibitor of the VEGF receptors, were tested as radiation sensitizers and protectors. Our results indicate that VEGF-A and VEGF-C are radiosensitizers while AZD2171 did not modulate the radioresponse. In vivo, normal lymphatics were studied with the experimental group receiving a single fraction of 8 Gy and the control group receiving no radiation. We observed no difference in the average lymphatic vessel diameter between these two groups over the course of 6 months. VEGF-C overexpressing tumor-associated lymphatic vessels were studied in vivo with four treatment groups: control animals (no irradiation), 8 Gy two weeks prior to implantation, 8 Gy at the time of implantation and 16 Gy given in two fractions before implantation (two weeks prior to and at the time of implantation). The average lymphatic vessel diameter and frequency of lymph node metastasis in these four groups indicates that the ability of radiation to prevent VEGF-C driven lymph node metastases is time-dependent; radiation must be delivered in close proximity to VEGF-C overexpressing tumor cell implantation to impact nodal metastases. This suggests that VEGF-C may be a functional lymphatic vessel radiosensitizer in vivo.; (cont.) However...

Modeling of [¹⁸F]-FHBG in tumor and normal tissues

Slutsky, Emily Dale
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 40 p.
Português
Relevância na Pesquisa
1386.5366%
The development of new, non-invasive approaches for the treatment of tumors has led to the emergence of oncolytic virus therapy. Viruses have been engineered to preferentially target tumor cells. The efficiency and safety of this cancer treatment is dependent upon selective viral replication within cancer cells. In order for viral oncolysis to be successful in the clinical setting, the biodistribution of viral replication must be quantified. This study has used an enzyme-based positron emission tomography (PET) reporter system to trace the viral replication of herpes simplex virus (HSV)-l. [¹⁸F]FHBG was used as the substrate for the HSV-1 enzyme product - thymidine kinase (TK) - in order for PET imaging technique to identity sites of HSV-1 TK activity. The imaged mice were divided into three groups: a control group with no tumor growth and no viral injection, a control group with no tumor growth and viral injection, and an experimental group with both tumor growth and viral injection. The time-activity curves of [¹⁸F]FHBG accumulation in the heart, muscle, liver, kidneys, brain, and tumor were plotted for all three groups. A 3-Compartmental Model for the kinetics of [¹⁸F]FHBG accumulation within each of the organs was coded using MATLAB...

Bacterial influence on uranium oxidation reduction reactions : implications for environmental remediation and isotopic composition

Mullen, Lisa Maureen
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 248 p.
Português
Relevância na Pesquisa
1386.5567%
The bacterial influence on the chemistry and speciation of uranium has some important impacts on the environment, and can be exploited usefully for the purposes of environmental remediation of uranium waste contamination. It is important to understand both from a scientific and environmental perspective how different types of bacteria can affect the chemistry and speciation of uranium. Analysis of the kinetics of uranium reduction, to determine the influence of external governing factors, can help us to understand the mechanisms of uranium reduction in vitro and aid in the design of more effective uranium remediation schemes in the environment. Bacterial reduction kinetics are found to fit well to a first order exponential decay model. Using this model we have determined the dependence of the rate of bacterial uranium reduction on several parameters, including bacterial density and pH. Understanding the reduction kinetics is also an important step in the determination of the extent of isotopic separation that occurs as a result of the bacterial reduction process. Here, we demonstrate that isotopes of uranium, the heaviest naturally occurring element, are subject to fractionation when uranium serves as a terminal electron acceptor during anaerobic bacterial respiration...

Study of chemistry and irradiation effects on nanofluids to be used in light water reactor accident cooling

Lucas, Timothy R
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 75 leaves
Português
Relevância na Pesquisa
1388.7188%
Nanofluids, colloidal dispersions of nanoparticles in a base fluid, have shown enhancements in both pool boiling and flow boiling critical heat flux (CHF) in laboratory tests. The applicability of this aspect to nuclear reactor post-accident cooling is promising. This study investigates various parameters that should be considered for such applications. Dilute alumina nanofluids were tested in terms of radiation durability, chemical stability, and CHF enhancement capability in wire and plate geometries. They have been found to be stable under high doses of gamma radiation and in the chemical environment associated with emergency core cooling. However, exposure to tri-sodium phosphate, a chemical used in in-vessel retention systems, resulted in significant agglomeration of nanoparticles. CHF value increases were obtained for both nanofluid and primary coolant chemistry conditions. Additionally radiation induced surface activation (RISA) effect on CHF was evaluated for alumina and titania nanoparticle depositions on plate geometries. Pool Boiling experiments were conducted with both wire and plate heaters. Wire test results showed CHF enhancement of 23% to 74% for dilute alumina nanofluids of 0.001v% to 0.1v%, with the highest enhancement obtained for lowest concentration alumina nanofluid. With the addition of boric acid in alumina nanofluid...

Advances in MRI to probe the functional and structural network of the macaque brain; Advances in magnetic resonance imaging to probe the functional and structural network of the macaque brain

Khachaturian, Mark Haig, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 106 leaves
Português
Relevância na Pesquisa
1386.672%
Diffusion MRI and fMRI have provided neuroscientists with non-invasive tools to probe the functional and structural network of the brain. Diffusion MRI is a neuroimaging technique capable of measuring the diffusion of water in neural tissue. It can reveal histological architecture irresolvable by conventional magnetic resonance imaging methods and has emerged as a powerful tool to investigate a wide range of neuropathologies. fMRI is a neuroimaging technique sensitive to hemodynamics which is indirectly linked to neural activity. Despite the applications of diffusion MRI and fMRI in basic and clinical neuroscience, the underlying biophysical mechanisms of cerebral diffusion and the hemodynamic response remain largely unknown. Also, these neurotechnqiues suffer from low SNR compared to conventional MRI. The challenges associated with the acquisition and interpretation of diffusion MRI and fMRI limit the application of these powerful non-invasive neuroimaging tools to study the functional and structural network of the brain. The purpose of this thesis is three fold; (1) improve the acquisition and reconstruction of the diffusion MRI and fMRI signals and (2) develop an MR-compatible cortical cooling system to reversibly deactivate cerebral glucose metabolism...

A comparison of crystalline and molten structures of zirconolite (CaZrTi₂O₇), a potential plutonium wasteform medium, by molecular dynamics simulation and topological analysis

Rich, Sarah Celeste
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 52 p.
Português
Relevância na Pesquisa
1386.5138%
Molecular dynamics simulations of the ceramic compound zirconolite (CaZrTi₂O₇), a potential crystalline wasteform host for plutonium, were carried out for ideal and experimental crystalline forms and a simulated molten state, and the connectivities of the resulting structures were compared. Local primitive-ring topological clusters were determined for individual atoms, and averages of ring counts were calculated for atom types within each form of zirconolite. The ideal crystalline structure and the best experimental structure, deduced by Rossell from neutron diffraction data, proved very similar, though the Rossell local clusters contained small variations from the ideal. Molten zirconolite appeared very different; it exhibited much larger ring counts and local clusters, together with a tendency for Ca and Ti (but not Zr) cation clustering. The technique of looking at ring counts for individual atoms was found to be very sensitive to small changes in the structure, though more suited to comparison of the two crystalline structures because of their uniformity. Significant connectivity differences and heterogeneity in the molten structure were best compared by considering the average local cluster. The structure of metamict zirconolite...

Long range x-ray imaging utilizing coded aperture techniques and dynamic reconstruction

Lee, Tiffany (Tiffany Ting)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 50 p.
Português
Relevância na Pesquisa
1386.5366%
Improvised explosive devices (IED) pose a very serious threat to civilians and military forces around the world, and new technologies must be developed for the early detection of these objects. Because of the high concentrations of low atomic number material such as nitrogen and hydrogen present in these explosives, x-ray backscattering provides a viable method of collecting information about these targets by analyzing their shape. Furthermore, a coded aperture used in conjunction with dynamic reconstruction algorithms offers high sensitivity and resolution even while the target is moving towards the detector. This paper describes a lab-based system that simulated a source-target-detector arrangement to be utilized in a radiation detecting vehicle in order to test dynamic reconstruction methods. Using a 225 kVp x-ray tube as the source, a medical CT-system camera fitted with a drill mask of 50% fill factor as the detector, and both radioisotope sources and low Z backscatter targets, images were acquired and reconstructed. The geometry of the experimental setup was optimized to reduce background noise from air scatter and environmental sources, as well as to prevent incident photons from directly reaching the detector from the x-ray tube. Measurements of a Co-60 point source and Co-57 area source with high activity generated high contrast images for which the shapes of the sources were clearly resolved. Acquisitions with varying target-detector distance of low Z materials...

Measurement of near-surface void fraction and macrolayer thickness in boiling water and silica-based nanofluid

Lerch Andrew (Andrew J.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 49 leaves
Português
Relevância na Pesquisa
1386.4586%
Nanofluids are engineered fluids that contain a suspension of nanoparticles in a pure substance. Nanoparticles can be any variety of metals, metal oxides, or ceramics. They have been shown to increase heat transfer properties such as thermal conductivity, convective heat transfer, and critical heat flux(CHF). An optical probe used to detect phase was used to measure the void fraction during boiling, from which the macrolayer thickness can be derived. The optical probe was verified to have an error of 11.9% and 10.4% for measuring bubble diameter in water and R-123, respectively, and an error of 5.2% and 7.1% for measuring velocity in water and R-123. The macrolayer dryout theory of CHF was tested by investigating the change in macrolayer thickness for different heat fluxes in de-ionized (DI) water and 0.01% (by volume) SiO₂nanofluid. A current controlled power source heated a sandblasted, stainless steel plate resting in an isothermal bath. The silica nanofluid had a CHF enhancement of 82% over the DI water along with a slightly higher (20% enhancement) heat transfer coefficient. The macrolayer thickness, as measured by the optical probe, at a comparable heat flux was much larger than the DI water, possibly due to the increased wettability of the heater caused by the deposition of nanoparticles on the heater. This trend is in agreement with prediction of existing theory.; by Andrew Lerch.; Thesis (S.B.)--Massachusetts Institute of Technology...

Experimental and theoretical investigation of transport phenomena in nanoparticle colloids (nanofluids)

Williams, Wesley Charles, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 255 p.
Português
Relevância na Pesquisa
1388.6703%
This study investigates the thermal transport behavior of nanoparticle colloids or nanofluids. The major efforts are: to determine methods to characterize a nanoparticle colloid's mass loading, chemical constituents, particle size, and pH; to determine temperature and loading dependent viscosity and thermal conductivity; to determine convective heat transfer coefficient and viscous pressure losses in an isothermal and heated horizontal tube; and finally to determine the feasibility for potential use as enhanced coolants in energy transport systems, with focus on nuclear application. The efforts result in proving that the two selected nanofluids, alumina in water and zirconia in water, have behavior that can be predicted by existing single phase convective heat transfer coefficient and viscous pressure loss correlations from the literature. The main consideration is that these models must use the measured mixture thermophysical properties. With the acquired knowledge of the experiments, investigation into the potential use or optimization of a nanofluid as an enhanced coolant is further explored. The ultimate goal of contributing to the understanding of the mechanisms of nanoparticle colloid behavior, as well as, to broaden the experimental database of these new heat transfer media is fulfilled.; by Wesley Charles Williams.; This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

Theoretical prediction of [tau]E and [beta]⁻ in a large aspect ratio LDX

Kouznetsov, Alexei (Alexei Alexey)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 153 leaves
Português
Relevância na Pesquisa
1386.5567%
The Levitated Dipole Experiment (LDX) is a novel experiment to study the confinement of a high-temperature plasma in the magnetic field of a superconducting ring of wire. The levitated magnet produces a poloidal closed-line magnetic field characteristic of an ideal point dipole or a hard core Z-pinch magnetic configuration. The point dipole and hard core Z-pinch configurations share similar physics and may be respectively considered to be the zero and large aspect ratio approximations to LDX. The present work focuses on a hard-core Z-pinch magnetic configuration. An analysis is presented that theoretically predicts (1) the maximum pressure p., (2) the energy confinement time TE and (3) the average beta / by solving a proposed self-consistent model of plasma. The model makes the optimistic assumption that transport is purely classical in the region of the profile that is magnetohydrodynamically (MHD) stable against interchange modes. For the interchange unstable region, a quasilinear MHD transport model is developed. The analysis of MHD quasilinear transport starts with an assessment of stability corrections due to axial flows. The axial flows are taken as an approximation to the LDX toroidal flows, expected to appear due to non-ambipolar transport. It is shown that the subsonic axial flows create only negligible correction to the plasma stability and the MHD transport analysis is performed for a static plasma. The evolution of the particle density...