Исследование динамических режимов бортового авиационного выпрямителя

Объектом исследования является бортовой авиационный выпрямитель на основе преобразователя с высокочастотным звеном. Цель работы – разработать имитационную модель бортового авиационного выпрямителя, провести проверку на адекватность полученной системы и исследовать динамические режимы выпрямителя.The object of research is an on-board aircraft rectifier based on a high-frequency converter. The purpose of the work is to develop a simulation model of an on-board aviation rectifier, to check the adequacy of the resulting system and explore the dynamic modes of the rectifier

Strength degradation mechanisms in NiAl alloy coated sapphire fibers

The efficiency of a gas turbine can be increased by increasing the combustion zone temperature. For this new materials are needed. NiAl strengthened with single crystal alpha-Al2O3 (sapphire) fibers is considered as a load bearing component in the combustion zone turbine blades. However, due to strength degradation of the fibers during composite fabrication, sufficient strengthening of NiAl can not be achieved. The goal of this thesis is to identify strength degradation relevant mechanisms in order to minimize the strength degradation during the production of Intermetallc Matrix Composites (IMC’s) in the future. NiAl and IP75 (Ni45Al45Cr7,5Ta2,5) were considered as matrix materials. The presence of an interlayer on the fiber strength was studied; hexagonal BN (h-BN) for a weak interface, Y and Hf for enhanced interface strength by compound formation. The strength of coated fibers was evaluated by tensile testing and compared to the strength of uncoated fibers. The effect of temperature during diffusion bonding was studied. None of the investigated coupling concepts yields the improvement in tensile strength of NiAl or IP75 necessary for high temperature structural applications due to process related fiber strength degradation. By systematic examination of fracture and fiber surfaces as well as chemical analysis, the following strength degradation mechanisms have been identified: Twinning of the rhombohedral plane (r-plane) of the sapphire crystal as a consequence of the evolving thermal stress during cooling. Chemical reactions between fiber and matrix material and/or impurities and surface diffusion of Al2O3 into irregularities in the adjacent matrix material like cracks in the h BN interlayer both lead to altering of the fiber surface morphology. In combination with the evolving thermal stress during cooling, fracture mirror formation at the surface flaws takes place, degrading the fiber strength. In this work the fiber strength degradation mechanisms active during individual process steps are identified. The significance of thermal stress induced fiber damage is emphasized and it is concluded that the elimination or a significant reduction thereof is the largest challenge which has to be addressed to explore the strengthening potential of sapphire fibers for IMC´s in the future. Based on the results presented here, a strategy for obtaining high strength IMC´s in the future is compiled. It is suggested that the strength degradation may be avoided or minimized by: lowering the thermal stress by increasing the fiber volume fraction and/or by using a ductile interlayer

Strength degradation mechanisms in NiAl alloy coated sapphire fibers

The efficiency of a gas turbine can be increased by increasing the combustion zone temperature. For this new materials are needed. NiAl strengthened with single crystal alpha-Al2O3 (sapphire) fibers is considered as a load bearing component in the combustion zone turbine blades. However, due to strength degradation of the fibers during composite fabrication, sufficient strengthening of NiAl can not be achieved. The goal of this thesis is to identify strength degradation relevant mechanisms in order to minimize the strength degradation during the production of Intermetallc Matrix Composites (IMC’s) in the future. NiAl and IP75 (Ni45Al45Cr7,5Ta2,5) were considered as matrix materials. The presence of an interlayer on the fiber strength was studied; hexagonal BN (h-BN) for a weak interface, Y and Hf for enhanced interface strength by compound formation. The strength of coated fibers was evaluated by tensile testing and compared to the strength of uncoated fibers. The effect of temperature during diffusion bonding was studied. None of the investigated coupling concepts yields the improvement in tensile strength of NiAl or IP75 necessary for high temperature structural applications due to process related fiber strength degradation. By systematic examination of fracture and fiber surfaces as well as chemical analysis, the following strength degradation mechanisms have been identified: Twinning of the rhombohedral plane (r-plane) of the sapphire crystal as a consequence of the evolving thermal stress during cooling. Chemical reactions between fiber and matrix material and/or impurities and surface diffusion of Al2O3 into irregularities in the adjacent matrix material like cracks in the h BN interlayer both lead to altering of the fiber surface morphology. In combination with the evolving thermal stress during cooling, fracture mirror formation at the surface flaws takes place, degrading the fiber strength. In this work the fiber strength degradation mechanisms active during individual process steps are identified. The significance of thermal stress induced fiber damage is emphasized and it is concluded that the elimination or a significant reduction thereof is the largest challenge which has to be addressed to explore the strengthening potential of sapphire fibers for IMC´s in the future. Based on the results presented here, a strategy for obtaining high strength IMC´s in the future is compiled. It is suggested that the strength degradation may be avoided or minimized by: lowering the thermal stress by increasing the fiber volume fraction and/or by using a ductile interlayer

The feeding ecology of bellbirds at Craigieburn

This thesis investigated the feeding ecology of bell birds, Anthornis melanura (Aves: Meliphagidae), at Craigieburn Forest Park to find out what, if any, aspect of the bellbirds' ecology may be limiting the pollination and possibly dispersal of two mistletoe species (peraxilla tetrapetala and Alepis flavida) in the area (shown by Ladley and Kelly, 1995b, 1996; Robertson et al., in press). Two hypotheses were tested to explain why bellbirds may not be making an adequate number of visits to mistletoe plants. The first is that there are non-mistletoe foods which are more energetically valuable to bellbirds than mistletoe fruits and nectar and so make up a larger proportion of the bellbird diet during mistletoe fruiting and flowering seasons. The second is that bellbirds concentrated on mistletoe foods when available but the numbers of bell birds at Craigiebum are too low to allow sufficient pollination and dispersal. To answer these questions, the bellbird diet, the energetic value of their food resources, and bellbird numbers were sampled over a twelve month period. Direct observation of the bellbird diet showed that they are annual generalists, on invertebrates (annual mean of 54% of bell bird diet, range 22-85%) and honeydew (annual mean of 22% of the bellbird diet, range 2-45%), and seasonal specialists, on mistletoe fruit (mean of 40% of the bellbird diet while available, range 18-60%) and nectar (mean of 39% of the bellbird diet while available, range 27-58%). The energy value of invertebrates dominated the available food energy with an annual mean of 14,255 kJ/ha (range 8,695-22,876 kJ/ha). Honeydew was the only other food source that was available all year with an annual mean energy value of 2,518 kJ/ha (range 1,199-3,283 kJ/ha). Mistletoe fruit and nectar featured prominently in the bellbird diet (18-60%) in the fruiting and flowering season even though their energy values were low with a seasonal mean energy of 2,867 kJ/ha (range 1,467-5,125 kJ/ha) for mistletoe fruit, and 3,658 kJ/ha (range 400-10,050 kJ/ha) for mistletoe nectar. This suggested that mistletoe fruit and nectar could be more valuable to the bellbirds than indicated by the measured energy values (kJ/ha) alone. One third of the variation in the bellbird diet could be explained by changing energy values of major food resources which suggests that the bellbird diet responds to the energy value of foods. As mistletoe fruit and nectar were preferred foods when in season, and the bellbird index of density was low at Craigieburn when compared to other sites, I concluded that it was the possible low number of bell birds in the area, and not their choice of diet, which was limiting mistletoe pollination, and possibly dispersal The possible low number of bell birds in the area could have been explained by the population being food limited. The time bellbirds spent foraging (feeding plus locomotion time) was a low percentage of their time budget (mean of 39%, range of 31-60%). This did not change significantly from winter to spring. If the values for late January and February were dropped, because of possible pressures from feeding nestlings, there was a significant decrease in the foraging percentage of the bellbird's time budget from winter to summer. However, when corrected for change in daylength, the hours spent foraging each day were not significantly different between seasons (mean of 4.7 hours foraging and a range of 3.6-5.7 hours) suggesting that bellbirds were not seriously limited by the availability of food. The possible low bellbird density could be a result of introduced vertebrate predators such as stoats which have had a negative impact on the populations of other native birds in New Zealand (pierce, 1993; Elliott, 1996; Elliott et al., 1996; O'Donnell, 1996; Wilson et al., 1998)