The diploma thesis is focused on the optical beam propagating in the atmosphere in a wireless communication optical link. The first part of the work explains the atmospheric transmission media with turbulence and its effects on reliability of the optical system. The second part introduces methods for turbulence determination based on a statistical approach to turbulence quantification are introduced. In the third part, method of equivalent temperature gradient is described with the advantage of immediate turbulence evaluation. The output of this thesis is the model of turbulent environment formed by the optical elements. Analysis of turbulent properties and non-reciprocal nature of turbulent channel is processed by a 2D simulator based on the mentioned model and method of equivalent temperature gradient.

Thermoelectric generators (TEGs) are currently a topic of interest in the field of energy harvesting for automobiles. In applying TEGs to the outside of the exhaust tailpipe of a vehicle, the difference in temperature between the hot exhaust gases and the automobile coolant can be used to generate a small amount of electrical power to be used in the vehicle. The amount of power is anticipated to be a few hundred watts based on the temperatures expected and the properties of the materials for the TEG. This study focuses on developing efficient heat exchanger modules for the cold side of the TEG through the analysis of experimental data. The experimental set up mimics conditions that were previously used in a computational fluid dynamics (CFD) model. This model tested several different geometries of cold side sections for the heat exchanger at standard coolant and exhaust temperatures for a typical car. The test section uses the same temperatures as the CFD model, but the geometry is a 1/5th scaled down model compared to an full-size engine and was fabricated using a metal-based rapid prototyping process. The temperatures from the CFD model are validated through thermocouple measurements, which provide the distribution of the temperatures across the TEG. All of these measurements are compared to the CFD model for trends and temperatures to ensure that the model is accurate. Two cold side geometries, a baseline geometry and an impingement geometry, are compared to determine which will produce the greater temperature gradient across the TEG. / Master of Science

Continental Paleogene climates have been well studied in Europe and North America, but very little is known from Asia because paleoclimatic results have only been reported from particular geological intervals. Here, based on 29 plant assemblages from 8 well age-controlled fossiliferous sites, we quantitatively reconstruct the climates through most of the Paleogene of northeast China and discuss related seasonal variations. Our results demonstrate that the mean annual temperature (MAT) gradient was fairly shallow (0.27 °C/1° latitude) during the Paleocene throughout this region. In the Eocene, seasonality was high in the region, indicated by marked differences in both temperature and precipitation between winters and summers of the sites. The paleo-East Asian monsoon must have had intensified at least in the late mid Eocene, shown by apparent differences in annual precipitation distribution at all the sites. Regarding the Paleogene climatic evolution of northeast China, our quantitative results suggest that MAT overall declined from warm in the Paleocene and Eocene to moderate in the Oligocene, generally consistent with the trend of marine records but with some distinctions. Two significant cooling events are recognized in the early and mid Eocene with MAT 3.4 °C and 3.8 °C lower, and winter temperature 5.8 °C and 4.7 °C lower, respectively, in similar magnitudes to corresponding variations in Europe and North America. Furthermore, the present results show that MAT rebounded in the late mid Eocene and then decreased until the Oligocene, a similar pattern demonstrated in Europe during the mid Eocene to Oligocene interval.

The easiest and most effective way to influence the separation process in gas chromatography (GC) is achieved by controlling the temperature of the chromatographic column. In conventional GC, the temperature along the length of the column is constant at any given time, T(t). In my research, I investigated the effects of temperature gradients on GC separations as a function of time and position, T(t,x), along the column. This separation mode is called thermal gradient GC (TGGC). The research reported in this dissertation highlights the fundamental principles of axial temperature gradients and the separation potential of the TGGC technique. These goals were achieved through the development of mathematical models and instrumentation that allowed study of the effects of axial temperature gradients. The use of mathematical models and computer simulation facilitated evaluation of different gradient profiles and separation strategies prior to development of the instrumentation, providing theoretical proof of concept. Three instruments capable of generating axial temperature gradients, based on convective cooling and resistive heating, were developed and evaluated. Unique axial temperature gradients, such as nonlinear and moving sawtooth temperature gradients with custom profiles were generated and evaluated. The results showed that moving sawtooth temperature gradients allowed continuous analysis and were well-suited for comprehensive GCxGC separations. The use of custom temperature profiles allowed unique control over the separation power of the system, improving separations, as well as selectively increasing the peak capacity and signal-to-noise. A direct comparison of TGGC with conventional GC methods showed that TGGC produces equivalent separations to temperature programmed GC. This technology holds great promise for performing smart separations in which the column volume is most efficiently utilized and optimum separations can be quickly achieved. Moreover, precise control of the elution of compounds can be used to greatly reduce method development time in GC. This feature can be automated using feedback to develop efficient separations with minimum user intervention. This technology is of special interest in micro-GC systems, which allows relatively easy incorporation of resistive heating elements in the micro-column design.

The thesis work consists of two parts. First, the development of two-dimensional numerical models of a Bridgman unidirectional solidification furnace, and second, the construction work of the furnace at KTH. The aim is to build a Bridgman furnace which is capable of close control over temperature gradient and growth rate such that the solidification structures of a duplex stainless steel (SAF2507) could be replicated at a laboratory scale for different cooling rates.Two numerical models of Bridgman furnace are created using COMSOL Multiphysics. The models are used as predictive tools to simulate the locations of solidification front and the temperature gradients at the solidification fronts, which are parameters difficult to access during experiments. Different hot zone temperatures of the furnace (1500~1550 °C) and different sample pulling rates (0.5~10 mm/s) are studied in simulations. The major finding from modeled results is that the temperature gradient of the sample at the solidification fronts range from 5 ~ 17 K/mm, which are lower than the furnace temperature gradient of ~50 K/mm. The corresponding steady-state cooling rates range between 5 ~ 85 K/s. The next step is to validate the models with experimental temperature profiles of the furnace, and decide whether the furnace design should be modified to achieve the cooling rates of interests. / Examensarbetet består av två delar. Först utvecklingen av tvådimensionella numeriska modeller av en Bridgman enkelriktad stelningsugn, och för det andra konstruktionsarbetet för ugnen vid KTH. Syftet är att bygga en Bridgman-ugn som har förmåga att kontrollera temperaturgradienten och tillväxthastigheten så att stelningsstrukturerna i ett duplex-rostfritt stål (SAF2507) skulle kunna replikeras i laboratorieskala för olika kylningshastigheter. Två numeriska modeller av Bridgman-ugnen skapas med COMSOL Multiphysics. Modellerna används som prediktiva verktyg för att simulera placeringen av stelningsfronten och temperaturgradienterna vid stelningsfronterna, vilket är parametrar som är svåra att komma åt under experiment. Olika varmzonstemperaturer i ugnen (1500~1550 °C) och olika provdragningshastigheter (0.5~10 mm/s) studeras i simuleringar. Det viktigaste fyndet från modellerade resultat är att provets temperaturgradient vid stelningsfronterna sträcker sig från 5 ~17 K/mm, vilket är lägre än ugns temperaturgradient på ~ 50 K/mm. Motsvarande stabilitetskylningshastigheter varierar mellan 5 ~ 85 K/s. Nästa steg är att validera modellerna med experimentella temperaturprofiler för ugnen och bestämma om ugnsutformningen ska modifieras för att uppnå intressens kylningshastigheter.

No description available.

This dissertation presents a detailed account of the study undertaken on the subject of fouling of Exhaust Gas Recirculator (EGR) coolers. The fouling process in EGR coolers is identified to be due to two primary reasons — deposition of fine soot particles and condensation of hydrocarbons known as dry soot and wet soot fouling, respectively. Several numerical simulations are performed to study the fouling process. Preliminary analysis of the particle forces for representative conditions reveal that drag, thermophoresis and Brownian forces are the significant transport mechanisms and among them, the deposition process is dominated by thermophoresis. Soot deposition in a representative turbulent plain channel shows a direct relationship of the amount of deposition with the near-wall temperature gradient. Subsequently, periodic and developing flow simulations are performed on a wavy channel geometry, a common EGR design for various Reynolds numbers and thermal boundary conditions. Constant heat flux boundary condition is used in the periodic fully-developed calculations, which assist in establishing various deposition trends. The wavy nature of the walls is noted to affect the fouling process, resulting in specific deposition patterns. For the lower Reynolds number flows, significantly higher deposition is observed due to the higher particle residence times. On the other hand, the developing flow calculations facilitate the use of wall temperature distributions that typically exist in EGR coolers. The linear dependence of the amount of deposition on the near-wall temperature gradient or in other words, the heat flux, is ascertained. It is also observed in all the calculations, that for the sub-micron soot particles considered, the deposition process is almost independent of the particle size. In addition, the nature of the flow and heat transfer characteristics and the transition to turbulence in a developing wavy channel are studied in considerable detail. Finally, a study on the condensation of heavy hydrocarbons is undertaken as a post-processing step, which facilitates the prediction of the spatial distribution and time-growth of the combined fouling layer. From the calculations, the maximum thickness of the dry soot layer is observed to be near the entrance, whereas for the wet soot layer, the peak is found to be towards the exit of the EGR cooler. Further, parametric studies are carried out to investigate the effect of various physical properties and inlet conditions on the process of fouling. / Master of Science

The goal of this research was to create a microstructure graded glass-ceramic and investigate the resulting properties as a function of crystallization processing. The desired glass-ceramic was a lithium disilicate material that has a crystallization gradient across the sample, leading to functionally graded properties as a result of the microstructure gradient. Samples were prepared by melting and pouring glass at 1400°C, annealing at 400°C for 48 hours, and nucleating at 480°C for 2 hours. To ensure that crystallization would not occur homogeneously throughout the sample, a temperature gradient was imposed during crystallization. Samples were crystallized on a self-constructed resistance wire furnace that was open to air. Several crystallization processing parameters were tested, including high temperature for a short time and low temperature for a long time. Samples were ground and polished to 0.25 microns before characterization methods were performed. Scanning electron microscopy (SEM) showed the microstructure transition across the sample cross section, with crystals present on the crystalline side and only nuclei present on the glassy side. Raman spectroscopy showed a transformation of the characteristic spectra across the sample cross section, with defined, high-intensity peaks on the crystalline side and broad, low-intensity peaks on the glassy side. Microhardness showed a slight transition in hardness values across the sample cross section, however the variability was too great to draw any conclusions. The characterization methods showed that the desired material was created and the resulting properties were a function of the crystallization processing parameters. / Master of Science

Grootvlei Power Station’s thermal efficiency had been on a steady declining trend since it was re-commissioned in 2008, which had tremendous financial implications to the company at the time of writing. The main contributory factor to the thermal efficiency losses was identified to be the condenser backpressure losses that the station was experiencing. This loss was responsible for approximately 17% of the total efficiency losses. Therefore an investigation was conducted to determine the potential impact of the condenser backpressure loss on the thermal efficiency and the financial implications thereof. The deliverables were to determine the cause of the condenser backpressure loss and propose possible resolutions, to quantify the financial effect and to produce a cost benefit analysis in order to justify certain corrective actions. Grootvlei Power Station is one of the older power stations in South Africa and it was used as the first testing facility for dry-cooling in South Africa. It consists of six 200MW units, two of which are dry-cooled units. In 1990 it was mothballed and due to rising power demands in South Africa, it was re-commissioned in 2008. Thermal efficiency has been playing a great role due to the power constraints and therefore it was deemed necessary to conduct this study. The approach that was used was one of experimental and quantitative research and analyses, incorporating deductive reasoning in order to test various hypotheses of factors that could have been contributing to the backpressure losses. In order to do so, a logic diagram was designed which could be used to aid in the identification of possible causes of the condenser backpressure losses. The logic diagram was able to identify whether the problem had to do with the cooling tower or the condenser. It was able to identify which area on the condenser was defective i.e. whether the pumps were not performing, or whether the air ejectors were not performing. It was also able to indicate whether the inefficiency was due to air ingress or fouling. Alongside the logic diagram, a condenser efficiency analysis was used in order to strengthen and improve on the investigation. This analysis was able to identify whether the condenser was experiencing fouling conditions, air ingress, passing valves or low cooling water flow. After the investigation commenced, it was decided to focus on the two largest contributing units since the largest contributor was a dry-cooled unit and the second largest contributor was a wet-cooled unit, thus some comparison between the units was incorporated. The condenser efficiency analysis on Unit 3 (wet-cooled unit) indicated a low cooling water flow, fouling as well as air ingress. The logic diagram indicated poor cooling tower performance, high air ingress as well as fouling. Further tests and analyses as well as visual inspections confirmed these phenomena and condenser fouling was identified to be the largest contributor to the backpressure loss on this unit. The condenser efficiency analysis on Unit 6 indicated that air was entering the condenser. The logic diagram indicated that a segment of the backpressure loss was due to poor cooling tower performance. Inspection of the cooling tower indicated damage and leaks. A cooling tower performance test was conducted and the result of the test indicated that the tower was in need of cleaning. Further analyses according to the logic diagram indicated that the condenser was experiencing air ingress which concurred with the condenser efficiency analysis. A helium test, condensate extraction pump pressure test as well as a flood test was conducted on this unit and various air in-leakage points were identified. The financial implications of the backpressure losses were investigated and found to be costing millions each month. The condenser backpressure loss was contributing more than 2% to the thermal efficiency loss. The cost benefit analysis indicated that the cost of cleaning the condenser on Unit 3 would be made up within six months and a return on investment of 16,6% was calculated. The cost benefit analysis motivates for extended outage times for the purpose of cleaning the condensers from a financial perspective. Therefore, it was recommended to clean the condenser on Unit 3 and fix all known defects on the unit as well as on Unit 6. The cooling towers were recommended to be refurbished. Further investigation was recommended to determine the feasibility of installing an online cleaning system on the wet-cooled units’ condensers such as a Taprogge system. Alternative investigation methods were suggested such as smoke stick analyses for air ingress determination. It was also recommended to review the maintenance strategies that were being used since many of the defects were found to be maintenance related. If the identified problem areas are attended to, the condenser backpressure loss will decrease and the condensers transfer heat more efficiently which will lead to financial gains for Grootvlei Power Station as well as efficiency gains, plant reliability and availability gains. / MIng (Development and Management Engineering), North-West University, Potchefstroom Campus, 2015

Grootvlei Power Station’s thermal efficiency had been on a steady declining trend since it was re-commissioned in 2008, which had tremendous financial implications to the company at the time of writing. The main contributory factor to the thermal efficiency losses was identified to be the condenser backpressure losses that the station was experiencing. This loss was responsible for approximately 17% of the total efficiency losses. Therefore an investigation was conducted to determine the potential impact of the condenser backpressure loss on the thermal efficiency and the financial implications thereof. The deliverables were to determine the cause of the condenser backpressure loss and propose possible resolutions, to quantify the financial effect and to produce a cost benefit analysis in order to justify certain corrective actions. Grootvlei Power Station is one of the older power stations in South Africa and it was used as the first testing facility for dry-cooling in South Africa. It consists of six 200MW units, two of which are dry-cooled units. In 1990 it was mothballed and due to rising power demands in South Africa, it was re-commissioned in 2008. Thermal efficiency has been playing a great role due to the power constraints and therefore it was deemed necessary to conduct this study. The approach that was used was one of experimental and quantitative research and analyses, incorporating deductive reasoning in order to test various hypotheses of factors that could have been contributing to the backpressure losses. In order to do so, a logic diagram was designed which could be used to aid in the identification of possible causes of the condenser backpressure losses. The logic diagram was able to identify whether the problem had to do with the cooling tower or the condenser. It was able to identify which area on the condenser was defective i.e. whether the pumps were not performing, or whether the air ejectors were not performing. It was also able to indicate whether the inefficiency was due to air ingress or fouling. Alongside the logic diagram, a condenser efficiency analysis was used in order to strengthen and improve on the investigation. This analysis was able to identify whether the condenser was experiencing fouling conditions, air ingress, passing valves or low cooling water flow. After the investigation commenced, it was decided to focus on the two largest contributing units since the largest contributor was a dry-cooled unit and the second largest contributor was a wet-cooled unit, thus some comparison between the units was incorporated. The condenser efficiency analysis on Unit 3 (wet-cooled unit) indicated a low cooling water flow, fouling as well as air ingress. The logic diagram indicated poor cooling tower performance, high air ingress as well as fouling. Further tests and analyses as well as visual inspections confirmed these phenomena and condenser fouling was identified to be the largest contributor to the backpressure loss on this unit. The condenser efficiency analysis on Unit 6 indicated that air was entering the condenser. The logic diagram indicated that a segment of the backpressure loss was due to poor cooling tower performance. Inspection of the cooling tower indicated damage and leaks. A cooling tower performance test was conducted and the result of the test indicated that the tower was in need of cleaning. Further analyses according to the logic diagram indicated that the condenser was experiencing air ingress which concurred with the condenser efficiency analysis. A helium test, condensate extraction pump pressure test as well as a flood test was conducted on this unit and various air in-leakage points were identified. The financial implications of the backpressure losses were investigated and found to be costing millions each month. The condenser backpressure loss was contributing more than 2% to the thermal efficiency loss. The cost benefit analysis indicated that the cost of cleaning the condenser on Unit 3 would be made up within six months and a return on investment of 16,6% was calculated. The cost benefit analysis motivates for extended outage times for the purpose of cleaning the condensers from a financial perspective. Therefore, it was recommended to clean the condenser on Unit 3 and fix all known defects on the unit as well as on Unit 6. The cooling towers were recommended to be refurbished. Further investigation was recommended to determine the feasibility of installing an online cleaning system on the wet-cooled units’ condensers such as a Taprogge system. Alternative investigation methods were suggested such as smoke stick analyses for air ingress determination. It was also recommended to review the maintenance strategies that were being used since many of the defects were found to be maintenance related. If the identified problem areas are attended to, the condenser backpressure loss will decrease and the condensers transfer heat more efficiently which will lead to financial gains for Grootvlei Power Station as well as efficiency gains, plant reliability and availability gains. / MIng (Development and Management Engineering), North-West University, Potchefstroom Campus, 2015