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.     

Model checks for parametric regression models

In the paper we deal with the problem of model selection among fixed-design regression models. We establish a new test that indicates whether or not the model fits the data. The test statistic is based on the difference between a parametric estimator for the model variance and a nonparametric difference-based estimator, see Hall et al. (Biometrika 77:521–528, 1990). The weights in the nonparametric estimator depend on n, and they are chosen by solving an optimisation problem in order to obtain a test with high power.     

Towards Reference Viscosities of Carbon Monoxide and Nitrogen at Low Density Using Measurements between 290K and 680K as well as Theoretically Calculated Viscosities

An all-quartz oscillating-disk viscometer of very high precision was used to determine the temperature dependence of the viscosities of carbon monoxide and nitrogen at low densities. The measurements were based on a single calibration at room temperature with a value theoretically calculated on the basis of an accurate ab initio pair potential for helium and the kinetic theory of dilute monatomic gases. The uncertainty of the experimental data is conservatively estimated to be ±0.15% at room temperature increasing to ±0.20% at the highest temperature of 682 K. The new data are compared with values recommended by the National Institute of Standards and Technology in the framework of its Standard Reference Data Program REFPROP as well as with experimental data from the literature. Whereas the REFPROP values for nitrogen can be considered as reference values, the new experimental data for carbon monoxide are up to 2% higher than the REFPROP values and should be taken into consideration for a new correlation. The temperature dependence of the viscosities calculated theoretically using ab initio intermolecular potential energy hypersurfaces for carbon monoxide and nitrogen and the kinetic theory of dilute molecular gases should be used for extrapolating the viscosity correlations of both gases to low and high temperatures. In addition, the viscosity ratio of carbon monoxide to nitrogen is investigated with the purpose to establish an improved correlation for carbon monoxide.     

Tracking and anti-sway control for boom cranes

This paper presents an anti-sway and tracking control for harbor mobile cranes. The control objective is the sway-free transportation of the crane's load taking the commands of the crane operator into account. Based on the mathematical model linearizing and stabilizing control laws for the slewing and luffing motion are derived using the input/output linearization approach. The operator's commands are smoothened online by a trajectory generator accounting for input and state constraints. The resulting optimal control problem is solved using the model predictive control approach. The efficiency of the control concept is illustrated with experiments on an industrial harbor mobile crane.