• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 3374
  • 1184
  • 1031
  • 495
  • 203
  • 91
  • 83
  • 83
  • 83
  • 83
  • 83
  • 82
  • 82
  • 82
  • 78
  • Tagged with
  • 8431
  • 942
  • 940
  • 884
  • 867
  • 867
  • 821
  • 714
  • 696
  • 524
  • 478
  • 417
  • 414
  • 387
  • 385
  • 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.
441

Heat transfer and pressure drop characteristics of angled spiralling tape inserts in a heat exchanger annulus

Coetzee, Hennie 21 November 2011 (has links)
M.Ing. / There are different types of heat transfer enhancement techniques that can be used, but some of these techniques are expensive and cannot be afforded by small manufacturing firms. An easy and affordable technique considered in this paper has an angled spiralling tape inserted into the annulus of a tube-in-tube heat exchanger. The purpose of this paper was to determine the single phase heat transfer and pressure drop characteristics in the annulus. Experimental measurements were taken on four set-ups; a normal tube-in-tube heat exchanger used as a reference and three heat exchangers with different angled spiralling tape inserts. From the results correlations were developed that can be used to predict the heat transfer and pressure drop characteristics. It was concluded that the angled spiralling tape inserts resulted in an increase in the heat transfer and pressure drop characteristics as can be expected.
442

An investigation into the effects of using limited precision integer arithmetic in digital modems

Hale, Roger G. January 1990 (has links)
The main aim of this thesis is to study the effects of using a reduced level of arithmetical precision (as found in a 16-bit microprocessor) whilst running various algorithms in the detection stages of a digital modem. The reason for using a lower precision is to see if these algorithms will run on a limited precision device, such as a Texas Instruments TMS320C25 digital signal processor, in real time.
443

Effets gyroscopiques et non-linéarités dans les vitesses critiques des arbres mécaniques en rotation

Laurent, Jean-Pierre January 1969 (has links)
Une première partie montre l'étude des "vibrations de flexion d'un arbre mécanique en rotation sur lequel est monté un disque, et qui n'est soumis qu'a des effets mécaniques linéaires, en régime permanent. Deux particularités ont été spécialement étudiées; - l'influence de l'effet gyroscopique du disque. - la résolution mathématique exacte du système en tenant compte de la masse répartie de l'arbre, La complexité du problème est due à la résolution d'une équation différentielle du quatrième ordre. Dans la deuxième partie, l'arbre précédent est soumis, en plus, à des effets mécaniques extérieurs non-linéaires (influence d'une force non-linéaire due à la présence d'un ressort à caractéristique du troisième degré). La particularité étudiée est l'influence de l'effet gyroscopique, juxtaposée à celle des forces non-linéaires. La complexité du problème est due aux résolutions d'équations différentielles non-linéaires; ces résolutions ne peuvent être qu'approchées et illustrent le phénomène dit de "saut".
444

An interferometric study of heat transfer through gases

Brooks, Reginald Gwyn January 1970 (has links)
No description available.
445

Extracting room acoustic parameters from received speech signals using artificial neural networks

Li, Francis Feng January 2002 (has links)
Quantitative room acoustics over a century has accumulated a knowledge base centred around objective acoustic parameters. Realistic and accurate measurements are essential in room acoustics. Occupied measurements are difficult to undertake with current technology, yet it is well established that occupancy changes acoustics. For this reason, new measurement techniques are sought. This thesis concerns anew, machine learning based approach for measuring room acoustic parameters, which is particularly useful for occupied in-situ measurements. A set of artificial neural networks, associated pre-processors and machine learning regimes are developed to extract Reverberation Time (RT), Early Decay Time (EDT) and Speech Transmission Index (STI) from received speech signals. Utilising naturalistic sounds - speech - as excitations, the developed methods circumvent the use of unpleasant noisy test signals and therefore measurements can be made in occupied spaces in a non-invasive fashion. Given the non-invasive nature and achievable accuracy, the new methods can facilitate occupied measurements, providing an alternative to traditional methods to better quantify acoustics of spaces where speech communication is important. Much of the development work of the neural network methods focuses on the preprocessors which produce data reduced and pre-conditioned signals for the neural networks. Two different speech scenarios, separate utterances and continuous running speech are considered, leading to the development of four major neural network methods: 1. Time domain method to extract RTIEDT from separate utterances. 2. Straightforward FFT method to extract STI from short-time speech. 3. Frequency domain method to extract STI from long-time running speech. 4. Frequency domain method to extract RTIEDT from long-time running speech. These methods are all based on supervised learning. Unsupervised models, representing another important class of neural networks, are also investigated in the context of this study and are found useful as pre-processors. The model development and validations are carried out through computer simulations. Results show that better than O.ls and 0.02 resolutions in reverberation time and STI extractions are achievable based on a "one-net-one-speech" machine learning regime: a neural network trains on a particular anechoic speech to extract a designated objective parameter under the excitation of that speech. Neural network systems extracting acoustic parameters from received arbitrary speech signals without using prior knowledge of the speech stimuli, termed source independent measurements, are explored. Although the achieved accuracy is not as good as that of the standard methods and the developed neural network methods on the one-net-one-speech basis, the source independent extraction is potentially more useful in practical systems. Improving the accuracy of the source independent measurements and extending the developed methods to music signals are seemingly the most significant further work of this study.
446

Heat transfer coefficients in concentric annuli

Dirker, Jaco 05 September 2012 (has links)
M.Ing / The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, as cross section, the diameter ratio of the annular space plays an important role. The purpose of this investigation was to find a . correlation that will accurately predict heat transfer coefficients at the inner wall of smooth concentric annuli for the flow of water. Experiments were conducted on water under turbulent flow conditions for a wide range of diameter ratios. The Wilson plot method was used to determine the heat transfer coefficients from which a correlation was developed that could be used to predict the heat transfer coefficients. It was found that the correlation predicted Nusselt numbers accurately within 3% of measured values for diameter ratios between a = 1.7 and a = 5.1 and a Reynolds numbers range of 4 000 to 30 000.
447

Aspekte van vibrasies in romp- en buis hitteruilers

Jordaan, Christiaan Cornelius 12 February 2015 (has links)
M.Ing. (Mechanical Engineering) / Shell and tube heat exchangers are commonly used in industrial processes. In the heat exchangers the tubes incur flow induced vibrations. These vibrations lead to fretting on the perimeter of the tube at the baffle plates and to fatigue failure at the tube sheets. Various studies were and are done to qualify and quantify these vibrations. This is then used to develop design criteria and methods for lowering the vibration amplitudes of the tubes. In the thesis the response of a tube bundle in cross flow is shown. The effect of increasing cross flow velocity on the vibration amplitude of the tubes is determined in a low speed wind tunnel. The results show similarity to the work done by Gorman [27]. By raising the natural frequency of the tubes in the first row upstream it is shown that the vibration peaks exhibited by the unmodified model.at cross flow velocities lower than the critical flow speed do not materialize in the modified tube bundle. This modification can be implemented by decreasing the unsupported length of the tubes in the first row. The inclusion of fluid damping when calculating the total damping of the tubes according to Blevins [24] is shown in the experiment. From experimental results it is shown that the prediction of the critical velocity according to Blevins [24] results in a better prediction of this velocity. Numerical solutions of two dimensional flow over a single cylinder and a pair of cylinders are obtained by simulating the flow on the STAR CD flow simulation package. The effect of neighbouring tubes on one another is determined by qualifying and quantifying the flow over and the forces on the tubes. The vortex shedding frequency determined from the simulation coincides with experimental results. This shows great potential in future simulation of flow over a tube bundle. A computerised design model is also developed. Themodel determines the vibration potential of the tube bundle in a shell and tube heat exchanger. If needed, modifications to new designs and built heat exchangers can be done and the vibration potential is easily determined with the model. The results of the experimental work are used to further develop the design criteria preventing detrimental vibrations in heat exchangers. Promising results to decrease vibration amplitudes of tubes in tube bundles are obtained in the experiments done. This and the results obtained in the simulation of the vortex shedding frequencies that coincide with the results in the experimental work, show great potential for flow simulation and development of methods to decrease flow induced vibrations of tubes in shell and tube heat exchangers.
448

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
449

Travelling wave-based fault location algorithm for power systems

Ogbonnaya Ibe, Anthony January 1984 (has links)
This thesis describes the development of an accurate method of fault location in transmission lines and cables. While the presence of a high frequency transient superimposed on the 50 Hz f a u l t voltage and current waveforms must be removed for most of the transmission line protection methods, the method presented in this thesis, which draws from travelling wave principles, locates the fault position using recordings from the fault waveform independent of the frequency components present. Using the telegraph equations as a line model, voltage and current samples taken at one end of a line within the first 5 ms of fault inception were used to generate instantaneous voltage and current profiles for the rest of the transmission line. The voltage and current estimation were based on the solution of the equations of the line model by the method of characteristics. Different criteria functions, involving the computed variables were applied to determine the fault positions. The basic functions involve any one of the square of the voltage, the square of the current or the product of the two. Fault position is determined by the turning or inflexionary point in the functions. In a further development the variation of the tangent to the functions described was considered and the fault position was found to be indicated by the peak of the scatter diagram produced. This latter function finds special application for resistive faults and faults in teed networks where the turning point given by the earlier functions are not usually well defined. The algorithm has been tested both with laboratory simulations and digital computer simulated data on typical 33 kV, 132 kV and 400 kV systems. Fault location accuracies of between 0.1% to 3.3% were achieved. Following the success with laboratory tests on models, a proposal has been put forward for an on-line fault locator.
450

The adaptive adjustment of digital data receivers using pre-detection filter

Ameen, Siddeeq Yousif January 1990 (has links)
This thesis is concerned with the adaptive adjustment of digital data receivers employed m synchronous serial data transmission systems that use quadrature amplitude modulation The receiver employs a pre-detection filter that forms the first part of a decision feedback equalizer or else is used ahead of a near maximum-likelihood detector. The filter is ideally adjusted such that the sampled impulse response of the channel and filter is minimum phase.

Page generated in 0.4469 seconds