Hydrodynamische Einlauflänge und Widerstandsgesetz von reibungsmindernden kationschen Tensidlösungen

Zusammenfassung Durch Zugabe geeigneter Tensidsysteme in Wasser kann der Str?mungsdruckverlust in geraden Rohren erheblich gesenkt werden. Die diesem Effekt zugrunde liegenden Mechanismen wurden in früheren Arbeiten eingehend diskutiert. In dieser Arbeit werden neue Ergebnisse vorgestellt, die sowohl die Bestimmung der hydrodynamischen Einlaufl?nge als auch die Ermittlung der Widerstandsbeiwerte im ausgebildeten Str?mungszustand erm?glichen. Für den Bereich der ausgebildeten Rohrstr?mung wird eine neue Berechnungsgleichung vorgeschlagen. Durch Erweiterung der bisherigen Modellvorstellungen ergibt sich ein Widerstandsgesetz für den gesamten Wirkungsbereich des Tensids. Die Ergebnisse stützen sich auf Untersuchungen mit zwei verschiedenen Additivsystemen, bei denen die Parameter Str?mungsgeschwindigkeit, Temperatur und Einsatzkonzentration variiert wurden.     

Gas/liquid interfacial area per unit volume and volumetric mass transfer coefficient in stirred slurry reactors

In three-phase systems, where the liquid constitutes the continuous phase, solid is the catalyst and gas represents the dispersed phase, there are decisive criteria which have to be observed in reactor design. These are e.g. the interfacial area per unit volume between gas and liquid, the volumetric mass transfer coefficient and the mass transfer coefficient. The basic aim of the present work was therefore the investigation of these parameters in relation to the main influencing parameters. Process parameters stirrer speed and superficial gas velocity were varied as well as the physical properties such as liquid viscosity, solids concentration, particle diameter and a geometrical parameter, i.e. reactor diameter. The sulphite method was employed for the determination of these values. The test results confirmed the known relationships of power consumption and superficial gas velocity. An increase in the liquid viscosity leads to a decrease in all the tested values. In most cases, suspended solid particles lead to a lowering of the test values. The influence of the tank diameter on the plots of the test values against specific power consumption turned out to be invariant so that a scale-up of geometrically similar systems can be carried out at constant power consumption, superficial gas velocity and liquid viscosity.     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Reduction of counts of Listeria monocytogenes in cheese by means of high hydrostatic pressure

Inactivation of Listeria monocytogenes (strains NCTC 11994 and Scott A) was evaluated in model cheeses submitted to 10 min HHP treatments of 300, 400 or 500 MPa at 5 or 20 degrees C. Counts were measured immediately after high hydrostatic pressure (HHP) treatment (day 1) and after 2, 15 and 30 days of storage at 8 degrees C. Both strains behaved significantly different after 400 and 500 MPa, being NCTC 11994 more sensitive. Scarce differences were found among final values at both HHP treatment temperatures. Initial reductions (log cfu/g) for 400 MPa at 20 degrees C were 2.9 +/- 0.2 for strain NCTC 11994 and 1.5 +/- 0.2 for Scott A. They reached after 30-day storage 5.3 +/- 0.2 and 4.6 +/- 0.4 log cfu/g for NCTC 11994 and Scott A, respectively. For 500 MPa treatments, day-1 reductions of both strains were around 5-log cfu/g, and counts fell below quantification limit after 30 days. Injured cells (around 0.8-log cfu/g) were mostly observed in 400 MPa treated samples on days 1 and 2. Starter cells suffered higher inactivation and injury. For 20 degrees C treatments, its final counts (log cfu/g) at 300, 400 and 500 MPa were: 8.5 +/- 0.2, 5.4 +/- 0.3 and 2.5 +/- 0.1, respectively. These figures evidence the HHP potential to improve safety of cheese products.     

Carbon speciation of diesel exhaust and urban particulate matter NIST standard reference materials with C(1s) NEXAFS spectroscopy

The U.S. National Institute of Standards and Technology (NIST) provides a number of particulate matter (PM) standard reference materials (SRM) for use in environmental and toxicological methodology and research. We present here the first analysis with respect to the molecular structure of the carbon in three such NIST SRM samples, i.e., diesel engine exhaust soot from heavy duty equipment engines (SRM 1650), diesel soot from a forklift engine (SRM 2975), and urban PM collected in St. Louis, MO (SRM 1648), with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectra of the two diesel soot samples appear quite similar, while they differ significantly from the urban PM spectrum, in agreement with X-ray diffraction data published recently. Such comparison is made in terms of aromatic and aliphatic carbon species, as well as by a general comparison with graphitic materials. Both diesel soot SRM samples contain basic graphitic structures, but the presence of exciton resonance and extended X-ray absorption fine structure oscillations in SRM 1650 and the lack therof in SRM 2975 suggest that SRM 1650 is the more graphitic material.The presence of polycyclic aromatic hydrocarbons, which have a characteristic NEXAFS resonance at the same position as graphite, can obscure the graphitic character of soot, unless an extraction of the organic matter is made. Our NEXAFS data do not suggest that the urban PM sample SRM 1648 contains a substantial amount of graphite-like material.