Interfacial tension in systems with a supercritical component at high pressures

Interfacial tension of the binary systems pelargonic acid/ethane, pelargonic acid/carbon dioxide, linoleic acid/ethane, and linoleic acid/carbon dioxide was measured at 313, 333, and 353 K and pressures ranging up to 250 bar, using the capillary rise and Du Nouy ring methods. Both these methods yield similar values. The surface excess of supercritical components is deduced from the measurement results with the aid of Gibbs adsorption equation. The surface excess concentration increases with increasing gas activity, runs through a pronounced maximum, later decreasing rapidly to very small values. Near the surface excess maximum, a falling film of fatty acids disintegrates into a mist of tiny droplets.     

Die Mikroflora des Sauerteiges

Zusammenfassung Es wurde dem Verhalten heterofermentativer Sauerteigbakterien bei gemeinsamer Kultur mit Hefen nachgegangen. Diese Untersuchungen erstreckten sich aufLactobacillus brevis, L. brevis var. lindneri undL. fermentum einerseits, sowic aus Sauerteigen isolierte Hefen (Candida krusei, Pichia saitoi, Torulopsis holmii, Saccharomyces cerevisiae) andererseits. Das Verhalten der Mischkulturen wurde an Sauerteigen aus Roggenmehl (Type 997, Teigausbeute 180, 27–28 °C, 18 Std) studiert. Je nach den zueinander in Wechselbezichung tretenden Sauerteigbakterien und Hefen ergab sich eine unterschiedliche Beeinflussung der Säurebildung. WährendTorulopsis holmii insbesondere die vonLactobacillus brevis undL. brevis var. lindneri ausgehende Teigsäuerung um bis zu fünf Einheiten erhöhte, bedingteSaccharomyces cerevisiae eine Förderung der vonL. brevis var. lindneri undL. fermentum ausgehenden Säuerung des Sauerteiges.

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The microflora of sourdoughXV. Communication: On the behaviour of heterofermentative sourdough bacteria and yeasts in mixed culture

Summary The behaviour of heterofermentative sourdough bacteria in mixed culture with yeasts was investigated. The investigations includedLactobacillus brevis, L. brevis var. lindneri andL. fermentum as well as several yeasts, isolated from sourdough (Candida krusei, Pichia saitoi, Torulopsis holmii, Saccharomyces cerevisiae). The behaviour of the mixed cultures was studied in ryeflour doughs (Type 997, dough yield 180, 27–28 °C; 18 h). Depending on specific interrelationships between types of sourdough bacteria and yeasts influence on acidification varied.Torulopsis holmii raised the dough acidification produced byLactobacillus brevis andL. brevis var. lindneri by up to five units;Saccharomyces cerevisiae enhanced the acidification byL. brevis var. lindneri andL. fermentum.

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Nr. 4676 der Veröffentlichungen der Bundesforschungsanstalt für Getreide- und Kartoffelverarbeitung, Detmold

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Diese Untersuchungen wurden ermöglicht durch eine finanzielle Förderung seitens des Bundesministeriums für Forschung und Technologie im Rahmen des Programms Biologic and Technologic     

Oberflächenstrukturierung polymerer Fasern durch UV-Laserbestrahlung. 4. Der Einfluß äußerer Spannungsfelder

This paper reports the results of UV-laser-irradiations of elastomer fibres under application of varying external tensions on the fibres and under varying laser conditions. It is known that tension as well as temperature fields can lead to these synergetic effects that are supposed to cause the surface structuring by UV-laser-irradiation. Therefore the purpose of the reported experiments was to evaluate whether tension or temperature as the source of the system instabilities eventually lead to surface structuring.     

Temperature dependence of the ultrasonic transmission through electrical resistance heated imperfect metal–metal interfaces

Because of surface roughness, the area of contact between real surfaces is less than the geometrical area. For this reason the known rules of acoustic reflection and transmission have to be modified for real interfaces. Ultrasonic transmission through imperfect interfaces is commonly described in terms of the contact stiffness model which assumes distributed springs between the surfaces in contact [Proc. R. Soc. Lond. A 202 244; J. Acoust. Soc. Am. 89 (1991) 503ff; J. Acoust. Soc. Am. 68 1516; J. Nondestr. Eval. 4 177; J. Geophys. Res. 94 (1989) 17681ff].Several authors [Trans. ASME 123 8ff; J. Acoust. Soc. Am. 103 657ff; Ultrasonics 38 513] theoretically and experimentally show the pressure and frequency dependence of the ultrasonic transmission through such interfaces. Our paper will document, that the temperature of the interface has significant influence on the ultrasonic transmission as well.In the experimental approach, a CuSn8-rod was clamped between the electrodes of a resistance welding unit. Either longitudinally or transversally polarised ultrasonic pulses were generated by a transmitter built in the upper electrode. The transmitted ultrasonic signal was subsequently detected by the receiver integrated in the lower electrode. After the welding current was turned on, a strong decrease in ultrasonic transmission has been observed due to resistance heating of the interfaces between the welding electrodes and the rod.To explain this extraordinary strong temperature effect, a simple quasistatic analytical thermal–electrical model of the contacting area was used to give a rough estimation of the maximum interfacial temperature and the temperature distribution during the flow of the welding current. In a second step, the temperature dependence of the material data in the acoustic contact stiffness model was used to calculate the changes in ultrasonic transmission caused by the welding current. It is shown, that the observed decrease in ultrasonic transmission with increasing temperature is mainly caused by the temperature dependence of the elastic constants of the metals and is much stronger than expected for ideal interfaces.The calculated ultrasonic transmission is in very good agreement with the experimental data and seems to be interpreted correctly by the acoustic contact stiffness model.     

Impacts of fouling and cleaning on the performance of plate fin and spine fin heat exchangers

An experimental study was conducted to investigate the effects of air-side fouling and cleaning on the performances of various condenser coils used in unitary air-conditioning systems. A total of six condenser coils with different fin geometry and row number were tested. Performance tests were performed at three different conditions: clean-as-received, after fouling, and after cleaning. In all cases, it was observed that the fouling was mostly confined to the frontal face of the heat exchanger as reported in the previous investigations. The amount of deposited dust was more dependent on fin geometry for the single-row heat exchangers than for the double-row heat exchangers. The predominant effect of fouling was to cause a more significant increase in air-side pressure drop than a degradation in heat transfer performance. For the single-row heat exchangers, the pressure drop increased by 28 to 31%, while the heat transfer performance decreased by 7 to 12% at the standard air face velocity of 1.53 m/s depending on fin shape. For the double-row heat exchangers, the pressure drop increased by 22 to 37%, and heat transfer performance decreased by only 4-5% at the same air face velocity. Once the contaminated coils were cleaned according to the given cleaning procedure the original performance of the heat exchangers could almost be recovered completely. The pressure drop could be restored within 1 to 7% and the heat transfer performance could be recovered to within 1 to 5% of the originally clean heat exchangers. Therefore, it is concluded that a periodic application of the specified cleaning technique will be effective in maintaining the thermal performance of the condenser coils.     

Optimization of electrostatically actuated miniature compressors for electronics cooling

This paper explores the feasibility of using electrostatically actuated diaphragm compressors in a miniature-scale refrigeration system for electronics cooling. A previously developed experimentally validated analytical model for the diaphragm compressor is used in conjunction with an optimization approach to determine the required dimensions for the compressor. The analysis reveals that the pressure rise and volume flow rate required for the electronics cooling application are not achieved using a single compressor because of material property limitations. A three-dimensional array of compressors is proposed instead with which the cooling requirements and the size restrictions for electronics cooling applications may be simultaneously satisfied.