Effect of blade tip geometry on the tip leakage flow in axial turbine cascades

The phenomenon of tip leakage has been studied in two linear cascades of turbine blades. The investigation includes an examination of the performance of the cascades with a variety of tip geometries. The effects of using plain tips, suction side squealers, and pressure side squealers are reported. Traverses of the exit flow field were made in order to determine the overall performance. A method of calculating the tip discharge coefficients for squealer geometries is put forward. In linking the tip discharge coefficient and cascade losses, a procedure for predicting the relative performance of tip geometries is developed. The model is used to examine the results obtained using the different tip treatments and to highlight the important aspects of the loss generation process.

Building damage and repair due to leakage in a deep excavation

The north-south line in Amsterdam is being built underneath the historic centre of the city. Three deep stations are being constructed in deep excavations supported by diaphragm walls. During the excavation for Vijzelgracht station, leakage through the wall resulted in large settlements and damage to historic buildings, which threatened continuation of the project. The authors analysed the cause of the leakage and the damage to the buildings. With the application of robust preventative measures at two of the deep excavations it was possible to continue the project. This paper reports on the cause of the events, the damage to the buildings and the counter-measures taken. It includes lessons learned for the project and for the foundations industry.

Effects of Arsenate on Microcystin Content and Leakage of Microcystis Strain PCC7806 Under Various Phosphate Regimes

Both arsenic pollution and eutrophication are prominent environmental issues when considering the problem of global water pollution. It is important to reveal the effects of arsenic species on cyanobacterial growth and toxin yields to assess ecological risk of arsenic pollution or at least understand naturally occurring blooms. The sensitivity of cyanobacteria to arsenate has often been linked to the structural similarities of arsenate and phosphate. Thus, we approached the effect of arsenate with concentrations from 10(-8) to 10(-4) M on Microcystis strain PCC7806 under various phosphate regimes. The present study showed that Microcystis strain PCC7806 was arsenate tolerant up to 10(-4) M. And such tolerance was without reference to both content of intra- and extra-cellular phosphate. It seems that arsenate involved the regulation of microcystin synthesis and cellular polyphosphate contributed to microcystin production of Microcystis responding to arsenate, since there was a positive linear correlation of the cellular microcystin quota with the exposure concentration of arsenate when the cells were not preconditioned to phosphate starvation. It is presumed that arsenate could help to actively export microcystins from living Microcystis cells when preconditioned to phosphate starvation and incubated with the medium containing 1 mu M phosphate. This study firstly provided evidence that microcystin content and/or release of Microcystis might be impacted by arsenate if it exists in harmful algal blooms. (C) 2008 Wiley Periodicals, Inc. Environ Toxicol 24:97 94, 2009.

The influence of boundary conditions on tip leakage flow

Most of the current understanding of tip leakage flows has been derived from detailed cascade experiments. However, the cascade model is inherently approximate since it is difficult to simulate the boundary conditions present in a real machine, particularly the secondary flows convecting from the upstream stator row and the relative motion of the casing and blade. This problem is further complicated when considering the high pressure turbine rotors of aero engines, where the high Mach numbers must also be matched in order to correctly model the aerodynamics and heat transfer. More realistic tests can be performed on high-speed turbines, but the experimental fidelity and resolution achievable in such set-ups is limited. In order to examine the differences between cascade models and real-engine behavior, the influence of boundary conditions on the tip leakage flow in an unshrouded high pressure turbine rotor is investigated using RANS calculations. This study examines the influence of the rotor inlet condition and relative casing motion. A baseline calculation with a simplified inlet condition and no relative endwall motion exhibits similar behavior to cascade studies. Only minor changes to the leakage flow are induced by introducing either a more realistic inlet condition or relative casing motion. However when both of these conditions are applied simultaneously the pattern of leakage flow is very different, with ingestion of flow over much of the early suction surface. The paper explores the physical processes driving this change and the impact on leakage losses and modeling requirements. Copyright © 2013 by ASME.

Predominant occurrence of apical cell divisions in Oedogoniam pakistanense and its phylogenetic significance

A rare terrestrial species, Oedogonium pakistanense, was first recorded from Hubei Province, south-central China. Morphological characters. including the predominant occurrence of apical cell division and unique lateral apical caps, are described. The growth of the filaments in O. pakistanense from China is usually the result of the repeated divisions of the apical cells and intercalary divisions are rare. It is suggested that this species may represent an evolutionary transition between Oedogonium and Oedocladium, the latter being a terrestrial genus with branched filaments and cell division more often terminal than intercalary.

Effect of surface treatment of GaN based light emitting diode wafers on the leakage current of light emitting diode devices

To form low-resistance Ohmic contact to p-type GaN, InGaN/GaN multiple quantum well light emitting diode wafers are treated with boiled aqua regia prior to Ni/Au (5 nm/5 nm) film deposition. The surface morphology of wafers and the current-voltage characteristics of fabricated light emitting diode devices are investigated. It is shown that surface treatment with boiled aqua regia could effectively remove oxide from the surface of the p-GaN layer, and reveal defect-pits whose density is almost the same as the screw dislocation density estimated by x-ray rocking curve measurement. It suggests that the metal atoms of the Ni/Au transparent electrode of light emitting diode devices may diffuse into the p-GaN layer along threading dislocation lines and form additional leakage current channels. Therefore, the surface treatment time with boiled aqua regia should not be too long so as to avoid the increase of threading dislocation-induced leakage current and the degradation of electrical properties of light emitting diodes

Role of Ga vacancies in enhancing the leakage current of GaN Schottky barrier ultraviolet photodetectors

The leakage current of GaN Schottky barrier ultraviolet photodetectors is investigated. It is found that the photodetectors adopting undoped GaN instead of lightly Si-doped GaN as an active layer show a much lower leakage current even when they have a higher dislocation density. It is also found that the density of Ga vacancies in undoped GaN is much lower than in Si-doped GaN. The Ga vacancies may enhance tunneling and reduce effective Schottky barrier height, leading to an increase of leakage current. It suggests that when undoped GaN is used as the active layer, it is necessary to reduce the leakage current of GaN Schottky barrier ultraviolet photodetector.

Influence of penetrating V-pits on leakage current of GaN based p-i-n UV detector

The leakage mechanism of GaN-based p-i-n (p-AlGaN/i-GaN/n-GaN) UV detector has been investigated. With the same dislocation density, devices made from material with higher density of V-pits on surface produce larger leakage current. SEM images show that some V-pits penetrate into i-GaN layer, sometimes even the n-GaN layer. If p-ohmic contact metal (Ni/Au) deposits in the V-pits, Schottky contact would be formed at the interface of metal and i-GaN, or form ohmic contact at the interface of metal and n-GaN. The existence of parallel Schottky junction and ohmic contact resistance enhances the leakage current greatly.

Influence of indium-tin-oxide thin-film quality on reverse leakage current of indium-tin-oxide/n-GaN Schottky contacts

Indium-tin-oxide (ITO)/n-GaN Schottky contacts were prepared by e-beam evaporation at 200 degrees C under various partial pressures of oxygen. X-ray photoemission spectroscopy and positron beam measurements were employed to obtain chemical and structural information of the deposited ITO films. The results indicated that the observed variation in the reverse leakage current of the Schottky contact and the optical transmittance of the ITO films were strongly dependent on the quality of the ITO film. The high concentration of point defects at the ITO-GaN interface is suggested to be responsible for the large observed leakage current of the ITO/n-GaN Schottky contacts. (c) 2006 American Institute of Physics.