Second All-Union Seminar on Hydromechanics and Heat and Mass Exchange in Weightlessness, summaries of reports

Abstracts of reports are given which were presented at the Second All Union Seminar on Hydromechanics and Heat-Mass Transfer in Weightlessness. Topics include: (1) features of crystallization of semiconductor materials under conditions of microacceleration; (2) experimental results of crystallization of solid solutions of CDTE-HGTE under conditions of weightlessness; (3) impurities in crystals cultivated under conditions of weightlessness; and (4) a numerical investigation of the distribution of impurities during guided crystallization of a melt

Enhancing Convective Heat Transfer over a Surrogate Photovoltaic Panel

This research is particularly focused on studying heat transfer enhancement of a photovoltaic (PV) panel by putting an obstacle at the panel’s windward edge. The heat transfer enhancement is performed by disturbing the airflow over the surface and increasing the heat and momentum transfer. Different objects such as triangular, square, rectangular, and discrete rectangular ribs and partial grids were applied at the leading edge of a surrogate PV panel and flow and the heat transfer of the panel are investigated experimentally. This approach was selected to expand understanding of effect of these different objects on the flow and turbulence structures over a flat surface by analyzing the flow comprehensively. It is observed that, a transverse object at the plate’s leading edge would cause some flow blockage in the streamwise direction, but at the same time creates some velocity in the normal and cross stream directions. In addition to that, the obstacle generates some turbulence over the surface which persists for a long downstream distance. Also, among all studied objects, discrete rectangular ribs demonstrate the highest heat transfer rate enhancement (maximum Nu/Nu0 of 1.5). However, ribs with larger gap ratios are observed to be more effective at enhancing the heat transfer augmentation at closer distances to the rib, while at larger downstream distances from the rib, discrete ribs with smaller gap ratios are more effective. Furthermore, this work attempted to recognize the most influential flow parameters on the heat transfer enhancement of the surface. It is seen that the flow structure over a surface downstream of an object (flow separation-reattachment behaviour) has a significant effect on the heat transfer enhancement trend. Also, turbulence intensities are the most dominant parameters in enhancing the heat transfer rate from the surface; however, flow velocity (mostly normal velocity) is also an important factor

Optically pumped planar waveguide lasers, part I: fundamentals and fabrication techniques

The tremendous interest in the field of waveguide lasers in the past two decades is largely attributed to the geometry of the gain medium, which provides the possibility to store optical energy on a very small dimension in the form of an optical mode. This allows for realization of sources with enhanced optical gain, low lasing threshold, and small footprint and opens up exciting possibilities in the area of integrated optics by facilitating their on-chip integration with different functionalities and highly compact photonic circuits. Moreover, this geometrical concept is compatible with high-power diode pumping schemes as it provides exceptional thermal management, minimizing the impact of thermal loading on laser performance. The proliferation of techniques for fabrication and processing capable of producing high optical quality waveguides has greatly contributed to the growth of waveguide lasers from a topic of fundamental research to an area that encompasses a variety of practical applications. In this first part of the review on optically pumped waveguide lasers the properties that distinguish these sources from other classes of lasers will be discussed. Furthermore, the current state-of-the art in terms of fabrication tools used for producing waveguide lasers is reviewed from the aspects of the processes and the materials involved

Large Eddy Simulation of Labyrinth Seals and Rib Shapes for Internal Cooling Passges

The turbine is one of the key components in gas turbine engines. To prevent the turbine blades from being badly damaged by their harsh working environment, it is necessary to keep them cool. This can be achieved by enhancement of the heat transfer performance through internal cooling passages. However, the large quantity of flow within this internal cycle inevitably results in mass flow loss, which is a major source of loss in turbomachinery. Therefore labyrinth seals are also investigated in this study, attempting to reduce the flow leakage and further increase the turbine efficiency. Large Eddy Simulation ( LES ) is used for its capability to capture the complex unsteady flow features in this study. Different rib shapes in a fully developed ribbed channel are investigated, aiming to improve the heat transfer performance. An immersed boundary method ( IBM ) is used with LES to generate complex geometries. With the use of IBM , the range of geometries can be represented on a background Cartesian grid. To obtain the best sealing performance, an investigation is undertaken into the possibility of optimising labyrinth seal planforms using a genetic algorithm ( GA ). By making use of the large number of populations, a much faster calculation can be achieved toward the objective function. Three hundred LES calculations are carried out, and an optimised design is generated that maximises the sealing effectiveness. The optimised design shows a leakage reduction of about 27.6% compared to the baseline geometry. The optimisation process employing a GA will be continued. It is expected that automated optimisation as presented will become increasingly important in the design process of future turbomachines, particularly for flows with strong parameter interactions, with an aim to further improve the overall efficiency of gas turbines.CS

Liquid-phase epitaxy of doped KY(WO4)2 layers for waveguide lasers

Rare-earth-ion doped KY(WO4)2 (hereafter KYW:RE) is a promising material for novel solid-state lasers. Its low laser threshold, high laser efficiency, and third-order nonlinear effects have stimulated research towards miniaturized thin-film waveguide lasers and amplifiers. A method of liquid-phase epitaxy (LPE) to produce KYW:RE thin layers with vertical substrate dipping has been developed. Undoped KYW crystals having laser-grade polished (010) faces served as the substrates. Two solvents, the tungstate K2W2O7 and the chloride NaCl–KCl–CsCl, were tested. The K2W2O7 solvent contains no impurity ions and is a good solvent for KYW, which is the only stable phase to be crystallized from the solution. The substrate position and rotation rate were optimized by numerical simulation of liquid flow in the crucible in order to obtain uniform layer thickness. A crystallization rate of 1.2 mg K-1 g-1 at the growth temperature of 900°C results in high-quality layers with thickness up to 100 µm and RE3+ concentrations ranging from 0.2 to 3 mol% with respect to Y3+. Dipping the substrate at 0.1–0.3 K above the saturation point helps to eliminate surface defects and assure a defect-free interface. An undoped overlay of KYW can subsequently be grown on KYW:RE layers to obtain buried structures. The chloride NaCl–KCl–CsCl solvent with its melting point of 480°C allows epitaxial growth at temperatures as low as 520°C, which can reduce thermal stress in heavily RE-doped layers. However, the LPE is complicated by the formation of parasitic phases and pronounced 3D island nucleation, which limit the maximum layer thickness to approx. 10 µm. The original concept of microchannel epitaxy (MCE) has been applied for the first time to produce channel structures with an oxide material. KYW:RE ribs, 40-200 µm wide and 3-20 µm high, can be grown from the K2W2O7 solvent on KYW substrates with a patterned gold or platinum mask deposited on the substrate surface. Surface and buried planar layers as well as channels of KYW:RE have been tested as optical waveguides. End-coupling and propagation of laser light at 633 nm or pumping at 981 nm results in excellent passive (633 nm) or active (e.g. 1030-nm Yb3+ fluorescence) waveguiding performance with propagation losses of only 0.1-0.2 dB cm-1. Continuous-wave (CW) lasing in both surface and buried KYW:Yb planar waveguides has been demonstrated at 1025 nm in the fundamental mode. The maximum output power is 290 mW and the slope efficiency is as high as 80.4%, which is, to the best of our knowledge, the highest value ever reported for a planar waveguide laser

The 15th Aerospace Mechanisms Symposium

Technological areas covered include: aerospace propulsion; aerodynamic devices; crew safety; space vehicle control; spacecraft deployment, positioning, and pointing; deployable antennas/reflectors; and large space structures. Devices for payload deployment, payload retention, and crew extravehicular activities on the space shuttle orbiter are also described

Microfabrication of photonic devices for mid-infrared optical applications

This thesis details research into the microfabrication of photonic devices for mid-infrared optical applications using the technique of ultrafast laser inscription. A diverse range of devices and materials is explored, including the first fabrication and development of an ultrafast laser inscribed mid-infrared waveguide laser source in thulium-doped sesquioxide ceramic gain media. The source produced 81 mW of output power at 1942 nm with a maximum slope efficiency of 9.5% demonstrating progress towards compact, low-threshold and efficient ultrafast laser written waveguide laser sources near 2 μm with the potential for high pulse repetition rate and ultrashort pulse operation. Also shown is the first demonstration of ultrafast laser inscription enabled selective chemical etching of chalcogenide glass. Investigations into the etching of modified regions in gallium lanthanum sulphide glass showed they could be etched at a rate ~13.3 times greater than the un-modified bulk. This result was explored further as a potential route to the production of optofluidic sensors for gas, liquid chemical or biomedical samples. The first demonstration and characterisation of ultrafast laser written waveguides in the chalcogenide glass GASIR-1 is also described. The waveguides were employed for chip scale supercontinuum generation producing the broadest and deepest Infrared supercontinuum from an ultrafast laser inscribed waveguide to-date, spanning ~4 μm from 2.5 to 6.5 μm, which has applications in remote sensing. Finally, the design, build and commissioning of an advanced laser processing setup suitable for ultrafast laser inscription is also detailed