No description available.

No description available.

Thesis(B.Sc.(Hons. ))--University of Adelaide, dept. of Mathematics, 1968.

In thermoacoustic devices, thermal energy is directly converted to an acoustic wave (mechanical energy) or an acoustic input is converted into thermal energy. This is a result of heat interaction between a solid material and adjacent gas, within the so-called ‘‘thermal penetration depth” of the compressible oscillatory flow. Thermoacoustic technology is receiving growing interest in research for its many advantages, such as having no moving parts, being environmentally friendly and the possibility of using renewable energy for its operation (Adeff and Hofler, 2000). However, this technology is still at the development stage and needs more research to produce feasible and practical devices that are ready for domestic and industrial applications. A looped-tube travelling-wave thermoacoustic engine was designed using DELTAEC (Design Environment for Low-amplitude ThermoAcoustic Energy Conversion). The device was equipped with a ceramic regenerator, which is commonly used in catalytic converters for automotive applications, with square channels. The results of preliminary testing of the device were compared with theoretical values estimated from the numerical model. Very close agreement was observed at the qualitative level and reasonable agreement was observed at the quantitative level. After the validation stage, the device was equipped with three selected low-cost porous materials for performance testing and studies. In addition to the ceramic regenerator that was tested before, regenerators made from stainless steel scourers, stainless steel wool and wire mesh screens were tested. This last type is widely available and commonly used in this application. To facilitate meaningful comparison, the regenerators were made in two sets: one having a common hydraulic radius of 200 μm and the other of 120 μm. In total, six regenerators were successfully tested. Before the performance experiments, all of the regenerators were tested in a steady air flow rig that was built for this purpose, to estimate their relative pressure drop due to viscous dissipation. The relative performance of the regenerators was then investigated. The testing focused on the onset temperature difference, the maximum pressure amplitude generated and the acoustic power output as a function of mean pressure as it varied from 0 to 10 bar gauge pressure. This comparative testing revealed a poor relative performance for the regenerators made of scourers and steel wool, while the cellular ceramic regenerator- 10 -seems to offer an alternative for traditional regenerator materials, which may reduce the overall system cost. The literature reports many different observations of nonlinear phenomena by various researchers, a fact which drove the candidate to carefully monitor the behaviour of the device at all stages and led to an interesting finding of a number of nonlinear behaviours during the start-up of the device. These behaviours included an “on-off” effect and “fishbone-like” oscillations in addition to the normal smooth start-up process. The new findings and the detailed observations are reported in chapter 6 of this thesis. The existence of these phenomena focused attention on identifying the key parameters affecting the existence and type of behaviour, which were found to be the mean pressure and the input power, in addition to the material of the regenerator. An attempt was also made to study the phenomena quantitatively. The observations suggest that there are strong interactions between the acoustic and temperature fields within the regenerator, which may be responsible for the reported quasi-periodic unsteady behaviour of the engine.

As a design and analysis aid for the development of an experimental TWT, a computer program is written which allows the small-signal gain to be computed for various operating conditions, such as various conditions of tube bias (beam voltage and current) and frequency. In order to arrive at a value for the gain, a number of parameters need first to be defined or calculated. Using the method (Approach II) of Jain and Basu [17] which is applicable to a helix with a free-space gap between it and circular dielectric support rods surrounded by a metal shell, the dielectric loading factor (DLF) for the structure is found and the dispersion relation then solved to obtain the radial propagation constant y and axial propagation constant B. The method is tested for a helix with measured data and found to be acceptably accurate. Helix losses are calculated for the low-loss input and output sections of the helix, using the procedures developed by Gilmour et al [14,18], from which values are found for the helix loss parameter d. Another value for d, obviously much larger, is also found for the lossy attenuator section of the helix. Here measured data for the attenuator is used as a basis for a polynomial which models the attenuator loss as a function of frequency. The Pierce gain parameter C is found using the well-known equations of Pierce [21,22,26], and then the space-charge parameter Q. Here knowledge of the space-charge reduction factor F is required to find Q, and a simple non-iterative method is presented for its calculation, with some results. From the other parameters already calculated the velocity parameter, b, is then found. since sufficient information is now available, the electronic equations are solved. These equations are in a modified form, better accounting for the effects of space-charge than the well-known standard forms. Results are compared and slight differences found to exist in the computed gain. Now that the x's and y's (respectively the real and imaginary parts of the complex propagation constants for the slow and fast space-charge waves) are known the launching loss can be calculated. Launching losses are found for the three space-charge waves, not just for the gaining wave. The gain of the TWT is not found from the asymptotic gain equation but from a model which includes the effects of internal feedback due to reflections at the ports and attenuator. Values of reflection coefficients are modelled on the results of time-domain measurements (attenuator) and found by calculation (ports). This model permits the unstable behaviour of the tube to be predicted for various conditions of beam current and voltage and anticipates the frequencies at which instability would be likely. Results from simulations are compared with experimental observations. The need to pulse the experimental tube under controlled conditions led to the development of a high-voltage solid state pulse modulator providing regulated output pulses of up to 5000V and 200mA directly, without the use of transformers. The pulse modulator design embodies two unusual features a) its operation is bipolar, delivering positive or negative output pulses, depending only on the polarity of the rectifier input, and b) the use of multiple regulating loops and stacked pass elements to achieve high-voltage operation. Some results are presented. / Thesis (Ph.D.)-University of Natal, Durban,1994.

No description available.

The last decades have seen a surge in the construction of tall buildings all over the world. Due to their, often, innovative and complex layouts, tall buildings can pose unique challenges to architects and engineers. Previous tall building failures raised significant concerns on the applicability of prescriptive fire design for these structures. The use of structural fire engineering can enhance the safety of a tall building under fire by strengthening any vulnerable areas in the structure and at the same time reduce the costs of fire protection by removing it when unnecessary. Commercial finite element and specialist structural fire engineering software have their advantages and disadvantages. In this thesis, the object-oriented and open-source finite element software OpenSees is presented along with its development with structural fire capabilities by the author and other researchers at the University of Edinburgh. Specifically, new pattern, element, section and material classes have been introduced. All the developed code follows the object-oriented paradigm and is consistent with the ethos of the existing framework. Verification and validation studies of the developed code are also presented. Several procedures including that for dynamic analysis of structures in fire for the collapse assessment of structures are discussed. The development of OpenSees with structural fire capabilities allows the collaboration of engineers across geographical boundaries and disciplines using a community tool. In this work, the behaviour of tall buildings under different fire scenarios has been modelled using the developed OpenSees code. Firstly, the collapse mechanisms of generic tall buildings are investigated, namely the strong and weak floor mechanisms are demonstrated, and criteria are established on when each of these mechanisms occurs. The parametric study performed demonstrated that the weak floor collapse is less probable for generic composite buildings however this type of failure can become easier to appear as the number of floors in fire increase. The effect of vertically travelling fires on these mechanisms is also examined. The results of the study show that slower travelling rates delay or avoid the global failure of a tall building compared to quicker travelling rates allowing for the time required for steel members to regain their strength during cooling to ambient temperature. However, it was seen that higher tensile membrane forces were observed in the floors as the travelling rates increased which could result in possible connection failure. Most of the research and design codes, such as Eurocode, typically assume a uniform thermal environment across the floor area of a structure when defining the design fire. However, in reality fires are more likely to travel in large enclosures, hence there is a need to understand how tall buildings behave under more realistic fire conditions such as travelling fires. A methodology for defining the thermal environment of large enclosures using travelling fires has been recently developed at the University of Edinburgh. Taking into account OpenSees' programmable architecture and its recent inclusion with heat transfer capabilities by other researchers, there was a collaborative effort in order to understand the thermal and structural response of a generic composite tall building under horizontally travelling fires. The findings of the study showed that larger travelling fire sizes produce quicker heating to the steel beams while smaller fire sizes produce higher peak temperatures in the concrete slab. The structural analysis also demonstrated that travelling fires produced higher midspan deflections in comparison to Eurocode parametric fires and higher plastic deformations which is an indication of higher damage. Further work focused on looking at the behaviour of tall buildings under the combined scenario of horizontally and vertically travelling fires. The results of the study showed that the travelling fires produce lower maximum compressive and tensile membrane forces in the composite floor compared to the Eurocode parametric fires for the building examined and thus in a multi-floor scenario the columns are pulling-in less after large deflections develop in the floor. More specifically, the short-hot fire produced the most demanding response. This suggests that in long floors where uniform heating is really impossible, the time of failure predicted by parametric fires in a multi-floor scenario can be more onerous. The outcomes of this work can aid designers when considering the structural fire response of tall buildings in a performance based design context. It was demonstrated that multi-floor fires could be a threat for tall buildings, and thus this possibility should be considered in design. The use of more realistic fire definition for large enclosures, such as travelling fires, should also be considered. The travelling fire methodology can provide an enhanced level of confidence for the safety of a building since it can represent a range of similar fires to those that may occur in a real fire scenario.

Travelling wave based fault location techniques work well for line commutated converter (LCC) based high voltage direct current (HVDC) transmission lines, but the large capacitors at the DC line terminals makes application of the same techniques for voltage source converter (VSC) based HVDC schemes challenging. A range of possible signals for detecting the fault generated travelling wave arrival times was investigated. Considering a typical VSC HVDC system topology and based on the study, an efficient detection scheme was proposed. In this scheme, the rate of change of the current through the surge capacitor located at each line terminal is measured by using a Rogowski coil and compared with a threshold to detect the wave fronts. Simulation studies in PSCAD showed that fault location accuracy of ±100 m is achievable for a 300 km long cable and 1000 km long overhead line. Experimental measurements in a practical HVDC converter station confirmed the viability of the proposed measurement scheme.

No description available.

Travelling wave based fault location techniques work well for line commutated converter (LCC) based high voltage direct current (HVDC) transmission lines, but the large capacitors at the DC line terminals makes application of the same techniques for voltage source converter (VSC) based HVDC schemes challenging. A range of possible signals for detecting the fault generated travelling wave arrival times was investigated. Considering a typical VSC HVDC system topology and based on the study, an efficient detection scheme was proposed. In this scheme, the rate of change of the current through the surge capacitor located at each line terminal is measured by using a Rogowski coil and compared with a threshold to detect the wave fronts. Simulation studies in PSCAD showed that fault location accuracy of ±100 m is achievable for a 300 km long cable and 1000 km long overhead line. Experimental measurements in a practical HVDC converter station confirmed the viability of the proposed measurement scheme.