Impinging jets are a flow geometry that is of interest in many chemical and processing engineering applications for a wide range of industries. Of particular interest in the current research is their application to particle re-suspension in nuclear reprocessing activities such as the HAS (highly active storage) tanks at Sellafield, UK. The challenging nature of these operations and their environment means that in-situ experimental work is impossible. Therefore, when designing and optimising equipment such as HAS tanks, engineers often turn to computational modelling to help gain an understanding about what effects certain modifications may have on the performance of the jet. The challenge then becomes obtaining physically realistic predictions using the methods available to industry. Impinging jets are complex and complicated flow geometries that have caused a number of problems for computational modellers over the years. Indeed, several turbulence models and approaches have been developed specifically with impinging jets in mind to help overcome some of the more difficult aspects of the flow. The work presented herein compares Reynolds-averaged Navier-Stokes (RANS) commercial codes readily available to industrial users for single- and multi-phase flows with RANS and large eddy simulation (LES) codes developed in an academic research environment. The intention is to contrast and compare and highlight where industrial-based computational models fall short and how these might be improved through implementing schemes with fewer simplified terms. The work conducted for this Engineering Doctorate has modelled a series of impinging jets with varying jet heights and Reynolds numbers using a range of RANS turbulence models within commercial and academic-based codes. This allows not only the discussion of the performance of the applied turbulence models, but also the effects of varying jet height. The predictions are validated against available experimental data for assessment of the performance of the scheme used. The degree of alignment with real, physical data is an indication of the performance of a model and is used to conclude where a particular model has failed or whether it is more suited than another. Different particle sizes have also been considered to determine the ability of different particle tracking schemes to predict particle behaviour based on their response to the continuous phase. Multi-phase data is also validated against limited available experimental data. Finally, LES has been used to demonstrate the next step in complexity in terms of simulation and prediction of continuous phase flows in difficult engineering applications and how these can greatly improve upon predictions from RANS methods.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / This work analyzes a voltage regulator applied to low-voltage single-phase distribution networks, especially long distribution rural distribution lines with problems to comply with voltage level standards. This voltage regulator is classified as D-STATCOM (Distribution Static Synchronous Compensator), since it is composed of a single-phase PWM inverter connected in parallel with the point common coupling through a second-order LC filter, composing a third order circuit (LCL circuit) with the line inductance. A digital control system is proposed for the D-STATCOM in this dissertation, composed of three voltage loops: the damping, the control loop of the AC voltage, which controls the voltage supplied by the regulator control loop and the DC voltage control loop, responsible for regulating the DC bus voltage. Moreover, a full-state feedback controller is used to achieve active damping of the LCL filter resonance. Finally, experimental results are presented to demonstrate the efficiency of the proposed control system / Neste trabalho é realizado um estudo acerca de um regulador de tensão aplicado a redes de distribuição monofásicas de baixa tensão, especialmente para longas linhas de distribuição rurais que apresentam problemas de adequação dos níveis de tensão. A topologia do regulador de tensão classifica-se como D-STATCOM (Distribution Static Synchronous Compensator), uma vez que é composto por um inversor PWM (Pulse Width Modulation) monofásico conectado em paralelo com o ponto de acoplamento comum através de um filtro LC, compondo um sistema de terceira ordem (LCL) com a indutância da rede. Um sistema de controle digital para o D-STATCOM é proposto nesta Dissertação, sendo composto por três malhas de tensão: a de amortecimento, a malha de controle da tensão CA, responsável pelo controle da tensão fornecida pelo regulador e a malha de controle da tensão CC, responsável pela regulação da tensão do barramento CC. Além disso, emprega-se uma retroação de estados a fim de realizar o amortecimento ativo do filtro LCL. Por fim, resultados experimentais são apresentados para demonstrar a eficiência do sistema de controle proposto.

This study consists of an experimental investigation into the fluid flow and heat transfer aspects of microchannels. Rectangular copper microchannels of hydraulic diameters 1.05 mm, 0.85 mm and 0.57 mm were considered. Using water as the working fluid, heat transfer and pressure drop characteristics were determined under a constant surface heat flux for different inlet configurations in the laminar and transitional regimes. Three inlet geometries were experimentally investigated: a sudden contraction inlet, a bellmouth inlet and a swirl-generating inlet. The influence of the inlet conditions on the pressure drop, Nusselt number and critical Reynolds number was determined experimentally. Pressure drop results showed good agreement with existing correlations for adiabatic conditions. Diabatic friction factor results for the sudden contraction and bellmouth inlets were overpredicted when using the friction factor results from literature. It is noted that a relationship between the pressure drop and heat flux existed in the laminar regime, where an increase in the heat input resulted in a decrease in the friction factor. The bellmouth inlet condition showed an enhancement of the heat transfer in the transition regime compared with the sudden contraction inlet. The critical Reynolds number for the onset of transition for the sudden contraction inlet was found to be approximately 1 950, with a sharp rise to the turbulent regime thereafter. The bellmouth inlet influenced the originating point of the transition regime, which commenced at a Reynolds number of approximately 1 600. A smoother and more gradual increase to the turbulent regime was observed as an effect of the bellmouth inlet over the sudden contraction inlet. The swirl-generating inlet condition produced higher friction factor results in all three flow regimes. Transition occurred at a Reynolds number of approximately 1 500 and the turbulent regime was quickly ii reached thereafter. The turbulent regime friction factor was found to be significantly higher with the swirl inlet compared with both the sudden contraction and bellmouth inlets. Nusselt numbers continued to increase until the onset of the transition regime, and did not converge to a constant value as stated in theory. Similar enhancement of the transition regime with the bellmouth inlet was observed for the Nusselt numbers as with the friction factors. The initial turbulent regime results followed the trend of the theory for both the sudden contraction and bellmouth inlet conditions for most of the data sets, with deviation occurring in some of the 0.57 mm test cases. The swirl inlet Nusselt number results were significantly underpredicted by the theory in the early turbulent regime. / Dissertation (MEng)--University of Pretoria, 2014. / gm2014 / Mechanical and Aeronautical Engineering / unrestricted

This thesis gives an overview on the fault of high-voltage cable transmission lines in the distribution system of the E.ON company. The first part of this thesis introduces reader to the topic of cable transmission lines, types of commonly used cables and the most commonly occurring faults. In the second part of this thesis, the methods of locating faults in cable transmission lines by professional personnel are explained. The third part deals with the analysis of a group of chosen occurred faults. The final part focuses on the statistics of fault occurrence on South Moravian region from 1.1.2017 until 1.1.2019.

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Single phase switched reluctance machines (SRMs) have a special place in the emerging high-volume, low-cost and low-performance applications in appliances and also in high-speed low-power motor drives in various industrial applications. Single phase SRMs have a number of drawbacks: low power density as they have only 50% utilization of windings, lack of self-starting feature unless otherwise built in to the machine, most of the times with permanent magnets or sometimes with distinct and special machine rotor configurations or additional mechanisms. Many of these approaches are expensive or make the manufacturing process more difficult. In order to overcome such disadvantages a method involving interpoles and windings is discussed in this research. Also, a new and novel converter topology requiring only a single switch and a single diode is realized. This research tests the concepts and feasibility of this new single-phase SRM motor topology and converter in one quadrant operation. The converter electronics and a simple minimum component, minimum cost analog converter are designed and implemented. The entire system is simulated and evaluated on its advantages and disadvantages. Simple testing without load is performed. This system has a large number of possibilities for development. Due to its lightweight, compact design and efficient, variable high-speed operation, the system might find many applications in pumps, fans, and drills. / Master of Science

Synchronous machines (SMs) are a vital part of today's world, and precise modeling is important for studying their stability. In this thesis, a small-signal analysis is done on the d-q frame for the AC SM. Starting from formulating the SM's abc frame equations, a d-q transformation is done based on the SM rotor frame so that the inductance from the abc frame representation will be fixed. Then, the SM's fundamental parameters are obtained from standstill frequency response testing. In the interest of having the most complete model possible, a governor controller and an exciter controller have been designed, and their performance has been evaluated according to the International Standard ISO-8628. A d-q steady-state analysis has been carried out and the resulting small-signal perturbation has been added to the steady-state equations. The model has been analyzed with exciter control only, with governor control only, and with both controllers, and in the end, the small-signal d-q impedance model for the SM has been verified by simulations and experiments in the Center for Electronics Systems (CPES) at Virginia Tech. The impedance measurement unit (IMU) was built by CPES. It is designed for measuring three-phase AC power systems and DC systems. Nevertheless, even though the single-phase system can be connected to the IMU, the impedance result in the end is not correct. Modifications have been made to the IMU so that it is able to calculate the single-phase AC power system impedance. The experimental results demonstrate that implementation has been completed. / Master of Science / Emergency diesel generators are critical in the event of widespread blackouts. Generators are typically synchronous machines (SMs), and stability studies are crucial from the standpoint of the power system. With knowledge gained through stability analysis, if instability occurs, a solution can be quickly formulated based on phenomena that has been observed during the event. In order to study the system stability, an accurate model is essential. The small-signal d-q impedance model will be derived by the Center for Power Electronics Systems (CPES) at Virginia Tech based on an AC generator that is rated at 28 kW, and the impedance results will be verified by both MATLAB/Simulink simulations and experiments. The impedance measurement unit (IMU) will serve as the equipment to do the measurement. However, the IMU can only correctly calculate impedance for a three-phase power system. For the single-phase AC power system, due to the three-phase phase locker loop (PLL) calculating the wrong angle, the IMU cannot properly determine the impedance. However, modifications have been applied to the IMU so that it can automatically calculate the impedance for a single-phase AC power system.

This thesis proposes one comparison methodology for single and three-phase high power high frequency transformers in power conversion systems. The objective is to compare the volume of the transformers. And single and three-phase Dual Active Bridge Converter (DAB1 and DAB3) topologies with single and three-phase isolating transformers are selected for the transformer comparison. Design optimization of power transformer has been studied and simplified models have been built for the single and three-phase transformer design optimization in this work, including assumptions for core shapes, materials, winding structures and thermal model. Two design methods have been proposed according to different design constraints, named T – B Method and J – B Method separately. T – B Method is based on feature of the core, which has the major limits of maximum flux density and temperature rise. The flux density should not reach the saturation value of the core, and temperature rise should meet specifications in different applications to assure the performance of the core (permeability, saturation flux density, and core loss) and the insulation of the wire. And J – B Method starts from the comparison of area product in conventional design method. The relationship between area product of transformer cores and the flux and current of the transformer in design is analyzed. There is specified relationship between area product of single and three-phase transformers if flux and current densities are specified for both. Thus J – B Method is proposed with the design constraints of specified current and flux density. Both design methods include both single and three-phase transformer design. One example case for single and three-phase transformer comparison is selected as high power high frequency DAB conversion system. Operation principles are studied for both DAB1 and DAB3 based on previous work. And transformer design based on the T – B and J – B Methods are carried out and transformer volumes are compared. And results show that three-phase transformer has little benefit in volume or thermal than single-phase transformer, when they are utilized in single-phase DAB and three-phase DAB converters separately. Scaled-down single and three-phase DAB systems have been built and volume and thermal tests have been carried out. / Master of Science

Distributed generation for consumer applications is a relatively new field and it is difficult to satisfy both cost and performance targets. High expectations coupled with extreme cost cutting to compete with traditional technologies make converter design difficult. As power electronics mature more opportunities arise for entry into this lucrative area. An excellent understanding of converter dynamics is crucial in producing a well performing and cost competitive system. The six-switch single-phase inverter proposed in this thesis is a prime candidate for use in single households and small businesses. Its compact size and compatibility with existing electrical standards make its integration easy. However, little work is available on characterizing the system from a controls point of view. In particular balancing the two outputs with an uneven load is a concern. This thesis uses nodal and loop analysis to formulate a mathematical model of the six-switch single-phase inverter. A non-linear time invariant model is constructed for circuit simulation; details found in real circuits are added. A hardware-in-the-loop (HIL) configuration is used for more accurate simulation. In fact, its use makes for an almost seamless transition between simulation and hardware experimentation. A detailed explanation of the HIL system developed is presented. The system is simulated under various load conditions. Uneven loads and lightly loaded conditions are thoroughly examined. Controllers are verified in simulation and then are tested on real hardware using the HIL system. DC bus disturbance rejection and non-linear loads are also investigated. Acceptable inverter performance is demonstrated without expensive current sensors or high sampling frequency. / Master of Science