• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 514
  • 154
  • 93
  • 61
  • 53
  • 31
  • 14
  • 13
  • 11
  • 6
  • 5
  • 4
  • 4
  • 2
  • 2
  • Tagged with
  • 1128
  • 283
  • 282
  • 133
  • 117
  • 100
  • 90
  • 87
  • 84
  • 83
  • 68
  • 61
  • 61
  • 52
  • 52
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
111

Heat transfer enhancement during condensation in smooth tubes with helical wire inserts

Ji, Tianfu 17 July 2008 (has links)
D.Ing. (Mechanical Engineering) / In the past two decades the refrigeration, air-conditioning and heat pump industries began the conversion from chlorofluorocarbon (CFC) refrigerants to hydrochlorofluorocarbons (HCFCs) and to natural refrigerants. This changeover not only involves redesigning, re-optimizing and re-testing all new original equipment but also involves retrofitting many large existing systems. Combining this process with the goal of developing more accurate design methods and more energy-efficient cycles, heat transfer and, specifically, heat transfer enhancement, has become a very active research field and will probably continue to boom in the next decades as the HCFCs are also phased out of use. The most prominent alternative refrigerants are R134a and R407C to replace the present market dominating refrigerant R22. Many heat transfer enhanced techniques have simultaneously been developed for the improvement of energy consumption, material saving, size reduction and pumping power reduction. Helical wire inserts in tubes are a typical technique that offers a higher heat transfer increase and, at the same time, only a mild pressure drop penalty. This study investigates the heat transfer characteristics of a horizontal tube-in-tube heat exchanger with a helical wire inserted in the inner tube. The influence of the pitch (or helix angle) of such geometry on the heat transfer performance and pressure drop during condensation (having all other geometric parameters the same) was investigated experimentally. Firstly, three refrigerants were tested in three helical wire-inserted tubes with different pitches of 5, 7.77 and 11 mm. The local and average heat transfer coefficients, and semi-local and average pressure drops were studied systematically. The experimental results were compared not only with the referenced experimental data of the smooth tubes, but also with the results of micro-fin tubes. The heat transfer enhancement factors, pressure drop loss penalty factors and overall efficiencies of the tested condensers with helical wire-inserted geometry were calculated. The tube with a helical wire pitch of 5 mm inserts was found to have the highest enhancement factor and overall efficiency. Secondly, the heat transfer enhancement mechanism was studied and explained. It was found that the extension of the annular flow regime contributed mainly to this enhancement. The transitional qualities from annular flow to intermittent flow were derived and incorporated in a flow regime map. Thirdly, heat transfer coefficient and pressure drop correlations for this special heat transfer enhancement geometry were deduced, and they predicted the experiment data to within 80% and 78% respectively, within a deviation of  20%. Finally, the water flowing through helical wire-inserted tubes (glass) was demonstrated, providing a visual understanding of the heat transfer enhancement mechanism. / Prof. J.P. Meyer Prof. L. Liebenberg
112

Heat-transfer coefficients for film condensation of steam on an inclined cylinder

Garrett, Todd January 1960 (has links)
This thesis discusses an investigation of film condensation of steam on the outside surface of an inclined cylinder. The objectives of the investigation were (1) to obtain heat-transfer coefficients for steam condensing on an inclined cylinder, (2) to determine the effect of the angle of inclination of the cylinder on the condensing coefficient, and (3) to verify values of the coefficient obtained from an accompanying theoretical development. Experiments were conducted for steam condensing on a 12.45 in. length of the outside surface of a 3/4 in. diameter by 0.065 in. thick wall, water-cooled, copper tube. Condensing coefficients were obtained for inclinations to the horizontal from 0° to 90° at 15° intervals. It was found that these coefficients decreased in value as the angle of inclination increased from 0° to 90°. At the same time good agreement was obtained between the measured and theoretical coefficients. No single equation could be found which would correlate the results for all the inclinations tested, but general equations were obtained which apply for each of two zones. One zone included inclinations from 0° to 30°, the other inclinations from 45° to 90°. The zone from 30° to 45° requires more investigation before it can be satisfactorily correlated. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate
113

Experimental studies of electrokinetic phenomena in brine-saturated porous materials

Li, Sidney Xi 01 January 1996 (has links)
When a pressure difference is applied to the two ends of a capillary tube containing an electrolyte, the fluid flow will induce an electric current flow. Streaming potential (STP) will result across the capillary tube when there is no net electrical current flow. Conversely when a voltage difference is applied to the two ends of a capillary tube containing an electrolyte, the electric current flow will induce the fluid flow. This is called Electroosmosis (ELO). The electroosmotic counter pressure will result when there is no net fluid flow. We investigated experimentally streaming potential and electroosmosis associated with brine flowing through porous media. Rock and sintered glass beads samples are made in the form of porous plugs which are then saturated with aqueous NaCl solutions. The experimental set-up is automated through a computer interface. Lock-in amplification and FFT techniques are used and their advantages over conventional DC flow method are demonstrated. Streaming potential and electroosmosis coefficients are measured along with permeability and conductivity. These linear response coefficients give a complete description of electrical and fluid transport properties. Onsager's relations are verified in our experimental data. We then propose a new way of measuring the throat size and the permeability of the porous materials. We also find when we change the electrolyte concentration, the surface properties of different samples show a different characteristics. We will propose a method to detect the thickness of a mudcake using STP frequency spectrum.
114

The effects of molecular architecture and conformational asymmetry on block copolymer morphology

Pochan, Darrin John 01 January 1997 (has links)
The effects of molecular architecture and block conformational asymmetry on the equilibrium bulk morphological behavior of strongly phase-separated, amorphous block copolymers have been studied. Transmission electron microscopy techniques and small angle x-ray scattering, as well as small angle neutron scattering, were primarily utilized to characterize the block copolymer morphologies. Both architecture and block conformational characteristics are found to be molecular parameters, in addition to the relative volume fractions of the constituent blocks, with which one can controllably manipulate the bulk morphological behavior. The effects of novel molecular architecture were discerned via a systematic morphological study of a series of simple graft A$\sb2$B, or "Y", block copolymers where A=polyisoprene (PI) and B=polystyrene (PS). In the microphase separated state a 2:1 A to B arm number asymmetry is introduced across the AB interface due to the simple graft architecture. This arm number asymmetry causes significant deviations in the volume fraction dependence of the morphologies formed by the A$\sb2$B series as compared to the volume fraction dependence of linear diblock morphology. In addition, at a unique volume fraction in the A$\sb2$B series where the two PI arms per molecule are first forced to the concave side of the interface, a new morphology in neat block copolymers is observed which has not been predicted by theory. The bulk morphological behavior of a series of poly(isoprene-block-tert-butylmethacrylate) linear block copolymers was characterized. The larger unperturbed dimension of PtBMA, due to its larger statistical segment length relative to PI, provides for a lower PtBMA entropic chain stretching penalty in the microphase separated state. This also causes the relative volume fraction windows in which morphologies are observed to shift to higher relative volume fractions of the more easily stretched PtBMA block than found in conformationally symmetric AB linear diblocks. In addition, initial morphology studies on more complicated graft architectures and linear diblocks with tunable conformational asymmetry are presented.
115

Competition between phase separation and crystallization in polyolefin blends

Akpalu, Yvonne A 01 January 1998 (has links)
In this thesis, the crystallization and melting of blends of high and low density homogeneous ethylene-1-octene copolymers with appended long chain branches have been investigated in real time by means of time-resolved SALS under cross-polarized and parallel-polarized optical alignments using a charge-coupled device camera (CCD) system, simultaneous small angle-X-ray and wide angle X-ray measurements using synchrotron radiation and differential scanning calorimetry (DSC). For the highest density material studied, our data show that in the case of crystallization at low supercooling, spherulitic growth (primary crystallization) occurs first while the apparent degree of crystallinity is less than 2%. Over 90% of the crystallinity develops after the primary crystallization process. When the average branch content of the blend is 14 branches per 1000 carbons complete spherulites are observed. The internal spherulite disorder is unchanged relative to that obtained for the high density component. When the average branch content of the blend is increased to 58 branches per 1000 carbons, the crystallization rate is faster than that of the moderate increase but the morphology suggested from SALS is consistent with incomplete spherulites. For this case, our data suggest that the domain sizes resulting from the incipient melt phase separation is the likely cause of the accelerated crystal growth. We propose that the incomplete spherulites formed may be a consequence of the competition between amorphous phase separation in the residual melt and crystallization. The results of a numerical study investigating the dynamics of spinodal decomposition in blends of linear Gaussian chains in three dimensions were used to evaluate the effect of branch content on the phase separation kinetics for mixtures of linear Gaussian chains and branched chains. The phase separation kinetics in the branched systems are identical to that of linear mixtures with larger domain sizes. Hence, it may not be possible to detect differences in the time dependence of the structure factor and domain size from scattering measurements if one varies the branch content of the blend. Blends of linear and branched polymers can be treated as blends of linear Gaussian chains even when the branch content is very high.
116

The evolution of order in liquid crystals and polymer crystals

Liu, Chester 01 January 1998 (has links)
This dissertation describes computer simulations and theoretical analyses of ordering processes in liquid crystals and polymers. One ordering process in liquid crystals occurs during the isotropic to nematic transition in which point and line defects form and annihilate. Monte Carlo simulations support the newly-derived scaling of the defect density, which was found to scale with time as $-$6/7 in both two and three dimensions, contrary to the mean-field result of $-$1. Frustration was determined to be a key factor in this difference. Another ordering process in liquid crystals is the evolution of inversion walls and loops. Analogous to the model of a shrinking elastic loop in a viscous medium which has been theoretically investigated by deGennes and Brochard, our lattice Monte Carlo simulations of inversion walls and loops show the same scaling behavior with time and the same predicted dependence of shrinkage rate on the orientational diffusion coefficient. Polymer crystallization constitutes the other major topic of this dissertation. The initial stages of crystallization are difficult to study experimentally but by using the united-atom Langevin dynamics method, single and multi-chain crystallization can be simulated. The simulations show that random fluctuations nucleate regions of higher order which in turn can induce further crystallization. Lamellar thicknesses obtained from simulations at various undercoolings showed the same scaling behavior as experimental data from literature. Besides homogeneous nucleation, secondary nucleation was also observed, with the newly-attached chains continuing to undergo conformational changes on the crystal surface. Finally, the phenomenon of lamellar thickening was investigated. Lamellar thickening was observed to occur cooperatively in a stepwise, quantized manner.
117

The morphological behavior of miktoarm star and multiple-graft block copolymers

Beyer, Frederick Louis 01 January 1999 (has links)
The effects of molecular architecture on block copolymer morphological behavior for two distinct types of architectures have been investigated. The first, AnBm-type stars, have a single, centrally located junction point from which blocks of two polymer species radiate, and are referred to as miktoarm stars. The extremes of a model proposed by Milner were investigated using three miktoarm systems, A2B2 stars, A8B 8 “Vergina” stars, and A5B stars. Samples having a low molecular asymmetry agreed in general with the predictions of this model, although bicontinuous morphologies were not observed. The A5B miktoarm samples, which have a high level of molecular asymmetry, exhibited behavior not predicted by the model, but which was consistent with a trend of discrepancies observed in prior studies. The effect of the junction point on chain stretching behavior in miktoarm stars was noted by comparing the lamellar period of several A2B2 and A8B8 stars to comparable AB diblocks. These materials were found to have significantly increased lamellar periods, thought to result from the increased chain stretching at and near the junction point. The second type of molecular architecture investigated was the multiple-graft architecture. Three series of multigrafts, were characterized: regular multigrafts with tetrafunctional branch points, random multigrafts with trifunctional branch points, and random multigrafts; with tetrafunctional branch points. Using the constituting block copolymer hypothesis, the behavior of these molecules was predicted using existing theories for block copolymers with simpler architectures. Use of this hypothesis is justified herein, and illustrates that the behavior of block copolymers with complex architectures is dictated by the preferred behavior of smaller architectural subunits from which the multigraft is comprised. The morphological behavior of multiple-graft block copolymers was shown to be influenced by branch point functionality, branch point location, and the number of branch points per molecule. Architectural heterogeneity was found to impair self-assembly behavior of the multigrafts. Small-angle scattering data indicating the formation of microphase separated domains of specific shapes were observed for non-lamellar morphologies, which are able to form and fill space without ordering on a lattice. Finally, lamellar grain size and shape a the series of regular multigrafts was investigated. Grain size was seen to be influenced by the total molecular weight of the multigraft. Grain anisotropy was found to increase as the lamellar grains increase in size, indicating that the growth of lamellar grains is anisotropic, occurring more readily in the direction normal to the plane of the lamellae.
118

Controlling polymer thin film structures by tuning interfacial interactions

Huang, Elbert E 01 January 2000 (has links)
The utilization of poly(styrene-random-methyl methacrylate), P(S-r-MMA), random copolymer brush surfaces to control the structure of polymer films is demonstrated. Random copolymer brush layers were generated by end anchoring the copolymers onto silicon substrates. By dictating the chemical composition of the random copolymer, the resultant brush layer could be precisely tuned to have properties ranging from pure PS to pure PMMA. This was determined by experiments where PS and PMMA homopolymer films showed dewetting behavior characteristic of the interactions between the homopolymer film and the underlying brush layer. Brushes having a styrene fraction of 0.58 have balanced interactions with PS and PMMA and constitute surfaces that are nonpreferential or neutral to PS and PMMA. The influence of interactions between polymer films and random copolymer brush layers is addressed for three systems. First, the interfacial structures of dPS and dPMMA homopolymer films with the brush layer were examined with neutron reflectivity (NR). The interfacial width between the two layers strongly depended upon the interactions between them. Furthermore, it is observed that penetration of dPMMA to the silicon substrate can occur for cases where the brush layer does not provide a sufficient enthalpic or entropic barrier. Second, atomic force microscopy studies (AFM) show that PS/PMMA demixed films had morphologies that varied greatly with substrate interactions. With annealing, some of these structures rearranged significantly while others remained relatively unchanged. The third type of polymer film examined in this study, diblock copolymers constitutes the main focus of this work. Normally, preferential interactions of one block at an interface induce a parallel orientation of the block copolymer domains. By utilizing neutral random copolymers, preferential segregation of each block is eliminated resulting in a perpendicular orientation of block copolymer domains. This was shown for poly(styrene-block-methyl methacrylate), P(S-b-MMA), block copolymers having lamellar and cylindrical morphologies. Using a wide variety of techniques the structural dependence of these films with annealing time and the effects of commensurability were examined. By controlling the growth of these domains, the generation of novel film structures comprised of perpendicular lamellar and cylindrical domains was achieved.
119

NMR studies of granular media and two-phase flow in porous media

Yang, Xiaoyu 01 January 2004 (has links)
This dissertation describes two experimental studies of a vibrofluidized granular medium and a preliminary study of two-phase fluid flow in a porous medium using Nuclear Magnetic Resonance (NMR). The first study of granular medium is to test a scaling law of the rise in center of mass in a three-dimensional vibrofluidized granular system. Our granular system consisted of mustard seeds vibrated vertically at 40 Hz from 0g to 14g. We used Magnetic Resonance Imaging (MRI) to measure density profile in vibrated direction. We observed that the rise in center of mass scaled as ν 0α/Nlβ with α = 1.0 ± 0.2 and β = 0.5 ± 0.1, where ν 0 is the vibration velocity and Nl is the number of layers of grains in the container. A simple theory was proposed to explain the scaling exponents. In the second study we measured both density and velocity information in the same setup of the first study. Pulsed Field Gradient (PFG)-NMR combined with MRI was used to do this measurement. The granular system was fully fluidized at 14.85g 50 Hz with Nl ≤ 4. The velocity distributions at horizontal and vertical direction at different height were measured. The distributions were nearly-Gaussian far from sample bottom and non-Gaussian near sample bottom. Granular temperature profiles were calculated from the velocity distributions. The density and temperature profile were fit to a hydrodynamic theory. The theory agreed with experiments very well. A temperature inversion near top was also observed and explained by additional transport coefficient from granular hydrodynamics. The third study was the preliminary density measurement of invading phase profile in a two-phase flow in porous media. The purpose of this study was to test an invasion percolation with gradient (IPG) theory in two-phase flow of porous media. Two phases are dodecane and water doped with CuSO4. The porous medium was packed glass beads. The front tail width σ and front width of invading phase were extracted from fitting of the invading front profile. The front tail scaled as σ∞Ca −α, where Ca is capillary number and α is 0.4 ± 0.08. The result is very close to IPG predication 0.25.
120

Multiscale modeling of semiconductor nanocrystal synthesis in templating media

Kostova, Borislava 01 January 2006 (has links)
Several liquid-phase and vapor-phase techniques are reported in the literature for growing nanostructured materials. The use of templates in the synthesis of nanostructured materials is attractive as it combines precise control of size and shape with easy scale up for industrial production. This work aims to elucidate the underlying mechanisms controlling the growth rate and morphology of II-VI compound semiconductor nanocrystals (also called quantum dots) in templating media by employing theory, modeling, and simulation. The focus of the work is on zinc selenide (ZnSe), which can form nanocrystals emitting in the blue and violet part of the visible spectrum when excited by ultra violet radiation. We developed a lattice Monte Carlo simulation technique to describe the formation of ZnSe quantum dots by reacting diethylzinc with hydrogen selenide in the spherical nanodroplets of a microemulsion formed by self-assembly of a ternary system consisting of an amphiphilic block copolymer, a polar continuous phase (formamide) and a non-polar dispersed phase (heptane) [4]. The stochastic model describes diffusion of diethylzinc molecules in heptane, nucleation of ZnSe through a fast reaction between diethylzinc and hydrogen selenide at the interface between the droplet and the continuous phase, as well as diffusion and coalescence of ZnSe clusters inside the nanodroplet, eventually leading to the formation of a single nanocrystal per nanodroplet. The motion of molecules and clusters in the lattice is programmed according to their diffusivity, which is estimated by the Stokes-Einstein equation. A deterministic diffusion-reaction model describing diethylzinc depletion in a spherical droplet due to a fast interfacial reaction was used to investigate different growth regimes and compare its predictions with those of the stochastic model. In the early stages of the nanocrystal formation process, slow diffusion of hydrogen selenide through the surfactant layer is the rate determining step. The zinc precursor is progressively depleted inside the nanodroplet and its diffusion to the interface becomes the rate controlling step. This transition can be tracked precisely by our stochastic model without any assumptions, but not by the aforementioned deterministic model. The formation of stable clusters (also called "magic clusters") of ZnSe with a fullerene-like close-caged structure has been included in the stochastic simulations. The predicted size variation of the final nanocrystals due to the formation of such clusters has been studied using this stochastic model. (Abstract shortened by UMI.)

Page generated in 0.0913 seconds