No description available.

The present study deals with the development of a single-stage implosion-driven hypervelocity launcher. A thin-walled tube filled with helium surrounded by explosives acts as a driver for the launcher. Implosion of the tube drives a strong shock that reflects back and forth between the projectile and the implosion pinch, generating very high temperatures and pressures. Simple analytic models were used to approximate the performance of the pump tube and its use as a driver for a launcher. Experiments to evaluate the implosion dynamics and performance of the pump tube were carried out, and implosion-driven launcher experiments demonstrated muzzle velocities above 4 km/s with 5-mm-diameter aluminum projectiles. Projectile integrity was verified by high-speed photography. Disagreement of experimental data with the analytical models of performance is mostly due to failure to seal the chamber of the launcher, resulting in loss of driver gas, and pump tube expansion, which weakens the precursor shock.

In this thesis a fluidized bed containing a draft tube has been studied with the aim of developing the apparatus for coal gasification. The process has the capability of producing synthesis quality gas using air for combustion, and of being able to accomodate poor quality coal feeds containing heavy fines loads. These advantages arise from two special features of a draft tube fluidized bed. In the first place, the bed may be operated as two separate and independent reaction zones, one contained within the draft tube and the other in the annulus region surrounding it. As a result, the gasification reactions may be carried out in one compartment and the combustion reactions in the other, allowing the useful gasification products to be taken off separately and undiluted with the combustion flue gases. Secondly, the fluidized material in the bed may be induced to circulate up the draft tube and down the annulus. These circulating solids provide the heat carrier from the combustion to the gasification zones within the bed. Furthermore, circulation of the bed in this way leads to a much longer residence time of fine particles within the bed and results in a high fine coal utilization efficiency. In order to achieve these benefits in practice, it is necessary to separate the gases supplied to and emitted from the draft tube from those of the annulus, but at the same time allowing free movement of solids between these regions. The thesis deals with how this may be accomplished in three parts: Firstly, the principles underlying division of a fluidized bed with a draft tube into discrete reaction zones are formulated, and strategies for achieving zonal separation, based on these arguments, are experimentally tested. As a result a reactor configuration and operating conditions suitable for coal gasification have been empirically identified. Secondly, a model describing the bulk circulation of solid material in the bed is presented, for the draft tube operating in the slugging mode. This model allows the average solids residence time and the particle velocities in the annulus and draft tube to be predicted, provided that slug velocities and spacings are known. The necessary correlations between hydrodynamic behaviour and the system properties are available in the literature for round nosed and wall slugs, but not for square nosed slugs, which appear to be characteristic in the apparatus used here. The third part consequently examines the square nosed slugging regime, and a theory to describe this behaviour, based on interparticle stress analysis, is presented. This regime is identified as having significant advantage over other bubbling modes because of the high dense phase gas flow rates which are sustained, and the resulting improved gas-solid contacting. The three models together mathematically describe the operation of the draft tube fluidized bed, allowing gas partition between the annulus and the draft tube regions as well as solids circulation to be predicted, for different bed configurations and operating conditions. The predictions compare well with experimental results. The last part of the thesis deals with the application of the system to coal gasification on a one ton coal per day pilot plant. A high quality gas, containing up to 80% CO + H2, (balance CO2), has been produced by steam gasification in the draft tube, using air for the combustion reaction in the annulus. The H2/CO ratio can be varied from about 1 to 3, by changing the operating temperature of the reactor. / Thesis (Ph.D.)-University of Natal, Durban, 1984.

Travelling-wave distributed amplifiers are providing gain over broad frequency ranges for microwave applications. Similar concepts are applicable to distributed mixers and, with the use of controlled feedback, to a multifunction component simultaneously emulating a mixer, amplifier and an oscillator. The concept of this new travelling-wave frequency converter is introduced and data for a discrete component test circuit is presented. To facilitate the converter operation a new three-port travelling-wave mixer is introduced and characterized. Four-port scattering and wave scattering transformations are derived as a method of analysis of the four-port distributed structure. This enables sequential circuit analysis on a small computer. Practical applications unique to the advanced automatic network analyser, including time domain measurements, are presented to characterize test circuits as well as to develop ancillary equipment such as a transistor test fixture. Automated error corrected transistor measurements and de-embedding are also discussed. A piecewise linear quantum mechanical method of modelling the conduction channel of a short gate field effect transistor is given to aid the extrapolation of the distributed frequency converter concept to submicron and heterojunction structures. / Thesis (Ph.D.)-University of Natal, Durban, 1985.

Tensional Vacuum Vessels (TVV) are vessels constructed such that the walls are placed in tension rather than in compression as is the case with conventional vacuum vessels. TVVs have the advantage of being cost-effective, light weight in construction, and potentially portable. Tensional vessels have already been designed with regard to submarine applications. However, the use of this principle with regard to vacuum applications is believed to be novel. TVVs have two interlinked thin walled shells instead of the traditional single thick wall of conventional design. These shells are placed in tension by pressurising the intermediate space. This thesis outlines the theory of tensional vessels and describes the performance of a number of experimental chambers developed during this investigation. The fundamental theory of the TVV is outlined and developed in more detail with regard to cylindrical vessels. These include vessels constructed from longitudinal and circumferential tubes. The basic theory for any TVV can be derived from the equilibrium condition. This states that the force due to the gauge pressure on the outer shell must be greater than or equal to the force due to the absolute pressure on the inner shell. If the inward force predominates implosion will occur. Materials science considerations are also taken into account. If the tension in the walls exceeds that required for yield, the vessel will deform. The use of novel tensile materials is also explored. TVVs are potentially inflatable and theory is developed with regard to the possibility of buoyant vessels. The first experiments were based on earlier work performed at this institution with cylindrical TVVs constructed from longitudinal tubes. The tubes employed were soft drink cans which were sealed together with putty. The work described in this thesis outlines the development of larger versions and the instabilities which developed are noted. High vacuum experiments performed through the inclusion of a guard vessel are then described. This is followed by a further description of experiments performed with this basic tensional wall design in an attempt to gain a better understanding of its properties. These vessels were smaller and were gas pressurised in order to allow for increased flexibility with regard to pressure and volume variation. It is found that the compressional elements of such vessels cannot be ignored. A series of cylindrical TVVs with the walls constructed from circumferential tubes is then described, including high vacuum experiments, also performed through the inclusion of a guard vessel. The initial experiments were small in scale and made use of small bicycle tyres as the TVV walls. Larger vessels were then built, the walls being constructed from car tyres. These vessels are also inflatable and more stable than those constructed from longitudinal tubes. Also, the compressional elements do not play as great a role in these vessels. A fully tensional cylindrical vessel is then described which includes tensional end plates. Experiments performed with large bowls as the end plates are outlined. The theory of the deformation of a circular plate is also given including finite element analysis. Finally, a further novel vacuum vessel design is proposed. This is the spinning vacuum vessel. Proof of principle experiments are performed on a small scale vessel (a soft drink can with its interior reinforced with putty) which yields promising results. / Thesis (M.Sc.)-University of Natal, 1999.

I consider Dirichlet eigenvalues for an elliptic system in a region that consists of two domains joined by a thin tube. Under quite general conditions, I am able to give a rate on the convergence of the eigenvalues as the tube shrinks away. I make no assumption on the smoothness of the coefficients and only mild assumptions on the boundary of the domain. Also, I consider the Green function associated with the mixed problem on a Lipschitz domain with a general decomposition of the boundary. I show that the Green function is Hölder continuous, which shows how a solution to the mixed problem behaves.

A cathode structure was demonstrated that utilizes aligned carbon nanotubes (CNTs) to improve the thermionic electron emission by increasing the field enhancement of the cathode surface. Aligned CNTs were grown on the surface of a tungsten substrate by plasma enhanced chemical vapor deposition. The tungsten-CNT structure was further coated with a thin film of low work function emissive materials by magnetron sputtering. Numerous cathodes with varying CNT morphology and oxide layer thickness were created. The field and thermionic emission of the cathodes were tested in order to study the effects of the surface properties on the emission characteristics. It was observed that the introduction of CNTs into an oxide cathode structure improves both the thermionic and field emission, even in cathodes with relatively low field enhancement factors. Because of the high field enhancement factors that are available for CNTs, there remains a potential for dramatically improved electron emission. / Department of Physics and Astronomy

A continuous wave infrared CO laser has been used to monitor kinetic processes occurring behind shock waves. Studies have been made of both vibrational energy transfer and reaction kinetics. It has been demonstrated that, following shock-heating, the vibrational levels of CO and DC1 relax via a continuous series of Boltzmann distributions into the final Boltzmann equilibrium distribution at rhe translational temperature. This is shown to be in accord with the established theory, and it is proposed that all diatomic molecules relax in this manner. Previous results for HI, which suggested otherwise, are questioned, and the theoretical arguments used to explain these results are shown to be misconceived. A new method is developed to calculate vibrational relaxation times from any laser absorption trace. The initial vibrational distribution of CO formed in the unimolecular decomposition of OCS at 4000 K has been investigated. It has been found that at least 90 % of the CO is born in the lowest vibrational level v=0, when 50 % would be in v=0 at equilibrium. This result is explained in terms of the dynamics of the reaction. The kinetics of the isotope exchange reaction ¹²C¹⁸O + ¹³C<sup>16>O ⥋ ¹²C¹⁶O + ¹³C¹⁸O have been investigated. The results have been shown to be consistent with an atomic chain mechanism, in conflict with the conclusions of earlier work. Reasons for this are discussed. A general account of the principles of operation of the apparatus is given, and conclusions are arrived at for the most profitable directions of future work.

No description available.

The predictions of non-equilibrium radiation for a Titan aerocapture aeroshell vary significantly amongst Computational Fluid Dynamics (CFD) analyses and are limited by the physical models of the non-equilibrium flow. Of particular interest are the non-equilibrium processes associated with the cyanogen (CN) molecule which is known to be a strong radiator. It is therefore important to have experimental data for these radiating shock layers which will allow for validation of CFD models. Furthermore, a more detailed understanding of the chemical processes that lead to the formation of CN above equilibrium concentration is required. This thesis describes the modelling of the radiation behind a shock using a collisional-radiative (CR) model and presents measurements of radiation intensity behind a shock in simulated Titan and Martian atmospheres. The uncertainties in radiation is more significant at lower speeds (around 5-8 km/s) with these atmospheres when compared to Earth entry. This is due to the formation of CN and because of the highly non-equilibrium nature of the flow. The motivation for this work began with the successful landing of the Huygens probe on the surface of Titan which led to the renewed interest in inter-planetary missions. Thus radiative heating during atmospheric entry to Titan and Mars was the subject of several experimental campaigns and extensive computational analyses. In order to better understand the formation of CN, and the nonequilibrium radiation emitted under such atmospheric conditions, NASA Ames Research Center conducted a series of experiments on their Electric Arc Shock Tube facility, EAST. Furthermore, several research groups in Europe and the United States independently developed CR models to predict the measured levels of radiation. The results from these simulations showed some ma jor discrepancies and highlighted a lack of knowledge and understanding about the fundamental physics behind the formation and decay of the CN molecule and its associated excited states. Based on a comparison of the various simulations with the CR models and the EAST experimental data, it was concluded that the absolute level of peak radiation was well predicted, however, there was a significant discrepancy related to the decay rate of the radiation. Therefore, to add to the relatively small amount of experimental data for these highly non-equilibrium radiating flow conditions, experiments were performed on the X2 shock tube at The University of Queensland with the aim of producing a comprehensive set of benchmark data for Titan entry. The data obtained from these experiments have been used to validate the results from the NASA Ames testing, and due to the large parametric variation, as a source for code validation. In addition to the experimental component of this thesis, an investigation into the simulation of CN non-equilibrium radiation was conducted. It has been previously concluded that there was a significant discrepancy between the experimentally measured radiation decay rate and the predicted value from CR models. Therefore, the primary aim of the simulation work presented in this thesis is to explain the reason behind this discrepancy. Through a parametric study of important reactions combined with an analysis of the reaction set, it was concluded that the coupling between the dissociation of N2 and the formation of CN (through the reaction N2 + C ↔ CN + N) controlled the radiation decay rate. The reason for the super equilibrium concentrations was identified to be a result of the N2 + C ↔ CN + N reaction continuing to over-produce CN after nominal equilibrium values are reached. This is due to the slow build up of N to drive the reverse reaction. Thus it has been shown in this thesis that the behaviour of the CN concentration is controlled by the rate of N2 dissociation. This led to the implementation of a more thorough method for simulating the dissociation process of molecular nitrogen. Therefore, a mono-quantum vibration state specific model that includes excitation and de-excitation reactions for all the vibrational states of nitrogen was incorporated into the CR model developed by Magin et al. The nitrogen vibration state specific model that was implemented was developed by Pierott and is based on SSH theory. The model developed in this thesis is known as the ViSpeN CR model (Vibrationally Specific Nitrogen). The ViSpeN results show significantly better agreement with experimental data in terms of the decay rate, initial rise of the radiation and the overall trends in the data. However, the work in this thesis has shown there are still discrepancies in predicting the absolute level of radiation measured in shock tunnel experiments. This led to the development of a modification to the ViSpeN model (known as ViSpeN-L) which includes a proposed new value for the radiative lifetime of the CN violet transition. The agreement between the experimental data and the ViSpeN-L model is excellent for conditions relevant to Titan entry.