• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 17765
  • 1752
  • 1004
  • 529
  • 529
  • 529
  • 529
  • 529
  • 528
  • 439
  • 249
  • 167
  • 133
  • 63
  • 29
  • Tagged with
  • 25878
  • 25878
  • 3720
  • 1930
  • 1726
  • 1721
  • 1492
  • 1492
  • 1009
  • 1009
  • 975
  • 912
  • 863
  • 791
  • 782
  • 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.

The Performance of a Streamlined Cross-Section Heat Pipe

Moustapha, Hany Said 03 1900 (has links)
<p>A streamlined cross-section heat pipe manufactured from 4130 steel, using water as the working fluid and layers of 200 mesh stainless-steel screen for a wick was designed and tested. The experimental effort was directed toward two main objectives. The first objective was to determine the operational details of building a streamlined heat pipe system for experimental use. The second objective was to test the heat pipe at different inclinations to determine the effect of gravity on heat pipe performance.</p> <p>The axial temperature distributions, performance data and parameters for the streamlined heat pipe, at various power input and angles of inclination, were presented, analysed and compared with existing theories and previous investigations.</p> <p>The heat pipe transferred a maximum power of 250 watts while installed horizontally. As the evaporator was raised above the condenser level, the maximum heat decreased, and at the vertical orientation, no thermal equilibrium was attained, at a power input of 25 watts.</p> <p>The performance of the heat pipe was also compared with that of a solid conductor having the same shape. The equivalent thermal conductivity and specific output calculated, were found to be approximately four and thirteen times respectively greater, than that of a similar solid pure copper rod.</p> / Master of Engineering (ME)

A Study of Hairpin Bubble Generating Circuits and Strip Domain Shapes

Hutton, S. 10 1900 (has links)
<p>A mathematical model which describes the forces acting on the domain walls of a strip domain as they are pushed together under the influence of an externally applied field is developed and presented in the thesis.</p> <p>Photographs are presented of actual domains as their walls are pushed together by the field due to current flowing in a hairpin conductor circuit.</p> <p>The photographs of wall shape are compared with wall shapes predicted by a computer program which was based on the mathematical model.</p> <p>Possible extensions to the model are proposed which would make it a useful tool for designing an optimal bubble cutter.</p> / Master of Engineering (ME)

Drag Reduction in Aqueous Flow Systems

Czaban, Jan Zbigniew 02 1900 (has links)
<p>This thesis presents an experimental study of the drag reduction effects obtained from injecting additive solutions into a turbulent boundary layer developing over a flat plate submerged in water. Both direct injection, from a reservoir through a slit adjacent to the flat plate test section, and ablative coating methods of introducing the additive were studied.</p> <p>Drag reduction data were obtained for polymeric and micelle materials. The test conditions included varying the free stream velocity over the plate from 1.9 to 5.4 fps and injecting the additives in concentrations of up to 2000 wppm at rates up to 50 ml/sec over the test section of the flat plate.</p> <p>It was found that although optimal injection rates exist, drag reduction seems to be a function of how much additive is present in the flow over a test surface and not how it was delivered there. It was also found that ablative coatings of the type used for these experiments seem to have a long life expectancy and produce noticeable drag reduction.</p> / Master of Engineering (ME)

A Bifurcation Study of Wrinkling in Deep Drawing

Karima, Mohamed Nassouh Medhat 07 1900 (has links)
<p>This work is concerned with the wrinkling behaviour when deep drawing cylindrical cups from circular blanks. Wrinkling is a uniqueness problem, and the present work uses a bifurcation approach to predict its occurrence. The results are presented in terms of a critical ratio of blank diameter to thickness above which wrinkling commences, along with the number of waves into which the flange of the cup buckles.</p> <p>It is demonstrated that when the classical Prandtl-Reuss equations are incorporated into the bifurcation analysis, the theoretical predictions are at variance with the published experimental data.</p> <p>A number of ad-hoc modifications are made to the classical elastic-plastic model to make the predictions conform with the experimental results.</p> <p>A critical re-examination of both the flow and deformation theories of plasticity was carried out, leading to the proposal of a modified incremental theory. The modified constitutive equations is shown to reduce to an appropriate model for both elastic and rigid-plastic solids, as limiting cases. The consequence of the modified equations is non-coaxiality of the principal axes of stress and plastic strain increment, and this is supported by published experimental data. The proposed constitutive equations lead to a better prediction of the wrinkling behaviour vis a vis the other models discussed here-in.</p> <p>An experimental investigation of the wrinkling behaviour of a number of materials, drawn through a conical and a modified tractrix die, was undertaken. The study has resulted in proposals for certain material parameters as being beneficial for inhibiting wrinkling.</p> <p>A theoretical study of wrinkling when drawing through a conical die is also presented.</p> / Doctor of Philosophy (PhD)

Microlayer Formation, Evaporation and Bubble Growth in Nucleate Boiling

Fath, El-Banna Saad Hassan January 1981 (has links)
<p>A numerical study of microlayer formation and evaporation is reported in which computer programs developed in order to predict the isothermal (initial) and the instantaneous microlayer thicknesses as well as the time varying mass evaporated from the microlayer for any specified bubble growth rate are presented. The results of these programs show that the isothermal microlayer thickness δ₀(r) is of the order of 1-5 μm and its profile agrees with the author's experimental data. The results of the microlayer evaporation show that the microlayer may contribute up to 100% of the bubble mass and that the instantaneous microlayer profile δ(r,t) agrees with some photographically measured data from the same reference for certain selected bubbles.</p> <p>Another computer program was developed to solve the Navier-Stokes equations, which account for all forces in the liquid acting on the bubble, in conjunction with the energy equation in order to study the entire hydrodynamic and thermodynamic fields around a growing bubble on a heating surface. The velocity, pressure and temperature distributions were analysed and the time varying mass evaporated from the bubble cap was computed. The results show that the mass evaporated from the bubble cap is less by an order of magnitude than the mass evaporated from the microlayer.</p> <p>Finally, a numerical model for bubble growth was constructed using the programs mentioned above, assuming the mass transfer across the liquid-vapour interface from both the microlayer and bubble cap to be the predominant mechanism for bubble growth. Some aspects of bubble growth are discussed including the time varying bubble mass, vapour pressure, bubble growth rate and bubble growth relationship. A comparison between this numerical model and the author's experimental data for the instantaneous microlayer thickness shows good agreement.</p> / Doctor of Philosophy (PhD)

Optimal Tolerance Allocation

Michael, Waheed K. 07 1900 (has links)
<p>This thesis addresses itself to one of the most general theoretical problems associated with the art of engineering design. Viewed in its entirety the proposed approach integrates the relation between the design and production engineers through the theory of nonlinear optimization. The conventional optimization problem is extended to include the optimal allocation of the upper and lower limits of the random variables of an engineering system. The approach is illustrated by an example using a sequence of increasingly generalized formulations, while the general mathematical theory is also provided. The method appears to offer a practical technique provided a satisfactory cost function can be defined.</p> <p>The thesis presents an analytical approach to full acceptability design conditions as well as less than full acceptability or scrap design conditions. An important distinction between the design and the manufacturing scrap has been introduced and illustrated through examples.</p> <p>The space regionalization technique is utilized to estimate the system design scrap. Optimization strategies are introduced to the mathematically defined upper and lower limits of the regionalization region. This region is then discretized into a number of cells depending upon the probabilistic characteristic of the system random variables.</p> <p>The analytical approach exhibited does not rely explicitly on evaluation of partial derivatives of either the system cost objective or any of its constraints at any point. Moreover, the technique could be applied to engineering systems with either convex or nonconvex feasible regions. It could also be exercised irrespective of the shape of the probabilistic distributions that describe the random variables variation.</p> <p>Industrially oriented design examples are furnished to justify the applicability of the theory in different engineering disciplines.</p> / Doctor of Philosophy (PhD)

Aerodynamic Loading on the Blades of a Helicopter Rotor in Steady Translational Flight

Gupta, Suresh K. 09 1900 (has links)
<p>One of the most important and complex problems in the design of helicopters is the estimation of aerodynamic loading on the blades of the lifting rotor in steady translation flight. It is essential for the designer to have a fairly good knowledge of this blade loading, especially its harmonic content. Such information is necessary in order to successfully evaluate and solve problems such as: the structural integrity of the rotor hub and the blades; the oscillatory forces in the control systems; and the vibrations of the entire aircraft. Many analytical and computational methods have been developed to predict the blade loading in forward flight. These methods invariably demand a knowledge of the rotor wake details and the sectional airfoil data. The trailing wake of the rotor is extremely complex and it has not yet been possible to include in the analysis all its details. Nor has it yet been possible to obtain the sectional airfoil data in a manner which is compatible with the unsteady three dimensional and compressible aerodynamic environment of the rotating blades. We are presenting, in this thesis, a new technique to calculate the blade loading in forward flight. The proposed technique does not require a knowledge of the rotor wake or the sectional airfoil data. We have used our proposed model to calculate the blade loading for a full scale NASA model rotor at an advance ratio of 0.29. Our results compare very well with those obtained experimentally by NASA. In fact, our results show better agreement with the experimental results that those obtained by using currently available computational models for rotor blade loading in forward flight.</p> / Doctor of Philosophy (PhD)

On the Numerical Solution of Nonlinear Problems of Continua

Gadala, Shehata Mohamed 11 1900 (has links)
<p>A state of the art survey of formulation aspects of geometric and material nonlinearity problem is given. The survey covers the formulation methods, the solution of nonlinear equilibrium equations, the incompressibility constraint, and, finally, the software aspects. A consistent Lagrangian, updated Lagrangian, and Eulerian formulation are derived from the energy balance equation transformed to the proper reference configuration. Difficulties opposed the pre-existence of a consistent Eulerian formulation in the literature are critically discussed. The proposed Eulerian formulation includes specific approximations which simplify the numerical treatment, and may restrict the applicability of the formulation to specific classes of problems, but do not alter the nature of the formulation. Differences between the presented Lagrangian and updated Lagrangian formulations and similar ones in the literature are found to be in specific geometric nonlinear terms in the final incremental equation as well as in the definition of the load increment vector. These differences are assessed within the framework of the basic equations of continuum mechanics. Specific forms of constitutive equations for elastic and elastoplastic materials are presented. For elastoplastic applications, it is shown that the use of proper frame indifferent stress rate leads to a constitutive equation which is a function of the incremental displacements and not the corresponding incremental strains.</p> <p>A concise discussion of software aspects is provided which leads to a suggestion of switching from the program package to the programming system concept. A programming system to account for material nonlinear behaviour is developed and tested.</p> <p>In the application, a plasticity theory for porous metals is proposed. A simple model of porous material is analysed by the finite element method. An assessment is made of an existing yield criterion for porous metals. A modified yield criterion and plastic potential function, and consequently, different plasticity equations are given. Reasonable agreement is obtained between the present numerical results and previous experimental and analytical results in the literature.</p> / Doctor of Philosophy (PhD)

Discrete Atomic Simulation of Fracture in Iron

Mullins, Mayes L. 05 1900 (has links)
<p>The thesis is concerned with improving the understanding of materials at the atomic level by computer simulation. The work has centered on three areas: a study of the efficiency of various computer algorithms used to carry out these simulations; the development of a new and very versatile boundary condition scheme for such problems; and the application of the results of these two studies to the simulation of (001) plane fracture in α-iron.</p> <p>Tests were performed comparing various solution methods used for atomic level computer modelling. The results are presented and indicate the most efficient method to be chosen for various problems. This may allow a reduction in computer cost by factors of two or three over that of other, often used, methods.</p> <p>The new boundary scheme which was developed involves the use of the finite element method. This offers several advantages over previous methods.</p> <p>The new boundary scheme was applied to the (001) plane crack in α-iron. Two dimensional cracks with crack line directions of [010] and [110] were modelled. Significant differences in lattice trapping limits and crack propagation speeds were observed between these two cases. Information on the magnitude and shape of the non-linear component of the crack tip displacement field is provided for both cases. No dislocations were emitted at low temperatures from these models, but warming the [010] model to 400 K apparently resulted in the emission of a dislocation from the crack tip. Experimental information available in the literature pertaining to these points is discussed and possible future work is described.</p> / Doctor of Philosophy (PhD)

Studies on Dispersion and Turbulence in Liquid Flows

Riedy, Osama K. 11 1900 (has links)
<p>This thesis describes an experimental study of the dispersion and turbulence of Newtonian fluids and dilute aqueous solutions of a drag reducing polymer, Reten 423, when ejected from a thin wall slot or point source into either developing or fully developed turbulent internal water flow at relatively high Reynolds numbers.</p> <p>Two small water tunnels with injection facilities were constructed in which concentration and energy spectra measurements were made using a spectrophotometer and a Laser Doppler Anemometer. The investigation was carried out with a maximum flow velocity of the order of 4.5 m/s for various injection flow rates and with injection concentrations in the range 0 to 1000 w.p.p.m.</p> <p>It was found that the dispersion rate for polymer solutions is reduced when compared to Newtonian diffusion. It was also observed that the velocity profiles for developing flows with polymer injection are fuller than those for water alone. Furthermore, there was no noticeable change in energy spectrum for low polymer concentration. However, for high injection concentration there was an increase of the energy within the low frequency end of the spectrum and a reduction of energy for the high frequency end of the spectrum.</p> <p>Correlations for the concentration profiles, diffusion boundary layer growth, maximum concentration and eddy diffusivity are presented. The results indicate that a universal diffusion correlation exists which represents the data for both Newtonian and polymer solutions.</p> <p>A very interesting phenomenon was observed for hetrogeneous polymer flow fields in which the sampling rate had an effect on the measured polymer solution concentration. The apparent concentration on the measured concentration approached the correct value when the sampling velocity approached that for the main stream, i.e. the isokinetic condition. This phenomenon is important and should be taken into consideration, for example, when using sampling techniques to obtain concentration profiles in nonhomogeneous polymer solution flow fields.</p> / Doctor of Philosophy (PhD)

Page generated in 0.1324 seconds