Due to their excellent thermal and mechanical properties silicon‐based ceramics and composites are prime candidates for high temperature structural applications. In this communication the authors report for the first time that amorphous silicoaluminum carbonitride (SiAlCN) ceramics possess anomalously high resistance to oxidation and hot‐corrosion. A mechanism underlying the observed phenomena is discussed.
The tangential momentum accommodation coefficient (TMAC) is used to improve the accuracy of fluid flow calculations in the slip flow regime where the continuum assumption of zero fluid velocity at the surface is inaccurate because fluid “slip” occurs. Molecular dynamics techniques are used to study impacts of individual gas atoms upon solid surfaces to understand how approach velocity, crystal geometry, interatomic forces, and adsorbed layers affect the scattering of gas atoms, and their tangential momentum. It is a logical step in development of techniques estimating total TMAC values for investigating flows in micro- and nano-channels or orbital spacecraft where slip flow occurs. TMAC can also help analysis in transitional or free molecular flow regimes. The impacts were modeled using Lennard-Jones potentials. Solid surfaces were modeled approximately three atoms wide by three atoms deep by 40 or more atoms long face centered cubic (100) crystals. The gas was modeled as individual free atoms. Gas approach angles were varied from 10 to 70deg from normal. Gas speed was either specified directly or using a ratio relationship with the Lennard-Jones energy potential (energy ratio). To adequately model the trajectories and maintain conservation of energy, very small time steps (approximately 0.0005 of the natural time unit) were used. For each impact the initial and final tangential momenta were determined and after many atoms, TMAC was calculated. The modeling was validated with available experimental data for He gas atoms at 1770m∕s impacting Cu at the given angles. The model agreed within 3% of experimental values and correctly predicted that TMAC changes with angle. Molecular Dynamics results estimate TMAC values from high of 1.2 to low of 0.25, generally estimating higher coefficients at the smaller angles. TMAC values above 1.0 indicate backscattering, which numerous experiments have observed. The ratio of final to initial momentum, when plotted for a gas atom sequence spaced across a lattice cycle typically follows a discontinuous curve, with continuous portions forward and backscattering and discontinuous portions indicating multiple bounces. Increasing the energy ratio above a value of 5 tends to decrease TMAC at all angles. Adsorbed layers atop a surface influence the TMAC in accordance with their energy ratio. Even a single adsorbed layer can have a substantial effect, changing TMAC +∕−20%. The results provide encouragement to continue model development and next evaluate gas flows with Maxwell temperature distributions involving numerous impact angles simultaneously.
Buoyancy-driven bidirectional pulsating exchange flow through a vent in a horizontal partition is studied experimentally using a brine/water system. The associated transient and pulsating exchange flows were studied by densimetric measurements, flow visualization, and laser Doppler velocimetry (LDV) measurements for three different vent length-to-diameter ratios: 0.106, 0.0376, and 0.008. A time scale, based on the rate of decay of the density difference between the two compartments, is developed that collapses all experimental data regarding the decay of density in the top compartment into one curve. Flow visualization was used to understand the flow features contributing to the pulsating flow and to provide a quantitative measure of the major pulsation frequency. Interfacial instability between brine and water at the vent was found to contribute to the pulsation. The pulsation frequencies and their decay were determined from the power spectrum of LDV measurements. For the small length-to-diameter ratios (0.008 and 0.0376) there are two different frequencies that decay at different rates, suggesting multiple flow processes that contribute to flow pulsations.
A new collaborative design scheme of a super-high-speed permanent-magnet synchronous motor (PMSM) and its digital controller is presented, which provides a low-cost but highly efficient motor system with guaranteed stability and performance. Since the systematic design of the PMSM can ensure its stability over the full operating speed range, a simple and reliable open-loop controller can be designed for the super-high-speed motor. With stability assurance, an optimal digital control is also designed in order to enhance the efficiency and performance of the PMSM. The unique feature in the proposed optimal V/f control is its design consideration to the stator resistor, which is generally neglected in most V/f controls but cannot be neglected in the super-high-speed motor owing to the extra small-size requirement. The simulation and stability analysis for various design options are provided. Finally, simulation and experimental results validate the design technique and its effectiveness.
Totally enclosed air to air cooled (TEAAC) generator with IC6A1A6 (as per IEC 60034-6) cooling is a widely accepted generator cooling solution for squirrel cage induction generators (SCIG) used in wind power generation, where the generator has two cooling systems, internal and external. The internal cooling is a closed loop system, where a shaft mounted mechanical fan helps in recirculating air inside the generator, and transfers the heat from the generator into an air-to-air heat exchanger. The external cooling system is a separate ventilation system creating airflow from the nacelle through the heat exchanger and removing the heat outside the nacelle with the help of an electrical fan mounted near the non-drive end of the generator. Cooling improvement of generator and bearing windings for performance enhancement is a well known research topic in turbo-generation as well as wind energy. Various winding design effort has been made in past for efficient generator cooling for turbogenerator as well as for wind turbine generator. With the increasing demand of power output, winding and bearing temperature class reaching its limit. Challenge is to come up with a solution for improving the performance of generator in terms of reducing temperature experienced by windings and at the same time reducing the cost. Detailed testing has been done on a test turbine to compare results obtained from open ventilated solution IC3A1 (as per IEC60034-6) and IC6A1A6 cooling for the squirrel cage induction generator. Paper presents advantages in case of using IC3A1 cooling where temperature of windings reduced substantially in comparison with IC6A1A6 cooling. To avoid contamination led windings and bearings hotspots, presented IC3A1 cooling configuration uses a unique design with inlet duct, filters, outlet duct and wire mesh.
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