Influence of pressure losses on compressor performance in a pressurized fuel cell air supply system for airplane applications

Low ambient pressures at elevated flight altitudes lead to power losses in fuel cell powered aircrafts. As countermeasure ambient air can be pressurized with a suitable fuel cell air supply system. In this study the influence of low ambient pressures and pressure losses within the system on the performance of two turbo compressors and the resulting stack power are examined theoretically and the findings validated experimentally. Results show that decreasing ambient pressures and pressure losses in front and after the compressor reduce the maximum pressure from 2.4 to 1.6 bar(a) in the examined system. Air compression may require a significant share of the fuel cell stack power and the maximum system power is reduced from 54 to 41 kW. For air pressures higher than 1.8 bar(a) the fuel cell stack power gain due to pressurization is found to be cancelled out by the increasing power required for air compression.     

Boundaries of local no-return sets for control systems

For control systems described by ordinary differential equations and given subsets A of the state space, the points on the boundary of A through which entrance and exit is possible are studied. We assume that solutions that leave A and remain close to it cannot return. Continuity properties of the entrance and exit boundaries when the control range is changed are described.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Low Dielectric Loss Ceramics of AlN Fine and Nanopowders

The dielectric properties of ceramics pressurelessly sintered from nano (10-100 nm) and fine (0.8-1.0 μm) aluminum nitride powders have been studied. High-purity aluminum nitride sintered from nanopowder of nearly-theoretical density has been found to exhibit a low dielectric constant ε = 6.3 and low loss, tgδ = 0.011 at 3.32 GHz. Ceramics sintered from a fine aluminum nitride powder doped with yttrium oxide have revealed ε = 7.7 and tgδ = 0.021 at 3.23 GHz. The dielectric properties have allowed us to use ceramics sintered from fine aluminum nitride powder as a material for microwave windows for the diagnosis and treatment of tumors of the larynx and breast.     

Thermooptical compensation methods for high-power lasers

Thermally induced optical effects can be exploited to generate adaptive optical devices such as self-adjusting lenses. An adaptive lens in a resonator can be used to compensate for the thermal lens in a high-power solid-state laser rod (LR) and herewith significantly improve the beam quality and increase the output-power range of solid-state lasers. With suitable materials and an appropriate design of the compensating device, resonators with self-balancing thermal lenses can be developed. In this paper, we review the material requirements for a self-adaptive compensating element and discuss a selection of suitable materials (glasses, liquids and curing gels) and schemes to compensate for the thermal lens of a Nd:YAG LR. Finally, we present a very simple and promising design of a thermooptically self-compensated laser amplifier.