The occurrence of intermittences in electrical circuits can result in serious reliability problems particularly where low signal voltages and currents are used. It is well known that differential thermal expansion or vibration in electrical connectors can result in micromotion resulting in fretting corrosion. This can lead to an increase in contact resistance and eventual loss of electrical contact. The occurrence of electrical intermittences has been considered a precursor to contact failure associated with fretting corrosion and can cause disruption in digital circuit signals.We report the development of instrumentation that simultaneously measures the occurrence of electrical intermittences along with contact resistance during the fretting of electrical contacts. In addition, the measurement system records contact friction and normal force dynamically. Intermittences can be counted and timed with durations from 20ns to milliseconds as a function of fretting cycles and correlated with the increase in contact resistance. All systems are integrated under LABVIEW computer control software.Measurements were made on Cu-Cu and Sn-Sn ridedflat combinations. Results will be interpreted in terms of the influence of wear debris on the electrical properties of the contacts.
The Star Properties Concept Inventory ͑SPCI͒ is among the first instruments made available to assess student gains in a major section or "unit" of a traditional introductory astronomy course, in this case the unit on stars. This is in contrast to instruments for an entire course, such as the Astronomy Diagnostic Test or the Lunar Phases Concept Inventory that is for a single subject. One of the intended uses of the SPCI is for comparison of the effectiveness of different instructional methods. The following is a report on the results of a study in which the SPCI was used as a pretest and post test to compare gains in active and collaborative student-centered instruction using lecture tutorials to those in traditional instructor-centered instruction. 2. THE "STARS" UNIT By the fall semester of 2008, coverage of the stars had evolved to largely centering on lecture-tutorials from the manual "Lecture-Tutorials for Introductory Astronomy" ͑Adams et al. 2007͒. The following 11 tutorials were used over the course of the seven biweekly, 80-min class periods devoted to coverage of light, the sun and stars:
Among the critical components of an efficient thermoacoustic device are its heat exchangers. This investigation was undertaken to determine whether it is reasonable to apply values of heat-exchange coefficients, h, estimated from well-known forced-convection correlations obtained in steady flow. The method involves suspending an element of the heat exchanger at a velocity antinode in a standing acoustic wave. Using Joule heating, the element is heated several degrees above the surrounding gas. Upon switching off the heating current, the element’s temperature (deduced from its electrical resistance) decays toward ambient. The value of h is extracted both from the amplitude and from the decay time. Measurement of h of single wires was chosen for the initial application of this method. Preliminary results have shown that the coefficient h decreases with increasing frequency to values substantially smaller than those estimated from steady-flow forced-convection correlations.
A thermoacoustic refrigerator was built to explore scaling to large heat flux. The refrigerator was constructed according to a modular design so that various stack, heat exchanger, and resonator sections are easily interchangeable. The resonator is driven by a commercial 10-in. woofer. Initial tests, using pure helium gas as the working fluid and steel honeycomb (0.8-mm cell) for the stack, pumped 60 W of heat against a 10 °C temperature gradient. Measurements of heat flux and efficiency will be reported as functions of stack structure (e.g., pore size and shape) and will be compared with theoretical predictions.
Among the critical elements of a thermoacoustic device are its internal heat exchangers. This work quantifies the behavior of actual heat exchangers in situ in an operating thermoacoustic refrigerator. Each heat exchanger consisted of a specified arrangement of metal in contact both with oscillating gas adjacent to a thermoacoustic stack and also with water bearing tubes. Measured quantities were the temperature Ts of the adjacent end of the stack, the temperature Tw of the water flowing through the tubes, the change of water temperature δT between tube inlet and outlet, and the water flow rate. The heat-transfer rate Q was determined from the latter two quantities. The overall heat-transfer conductance G, defined by G=Q/(Ts−Tw), is determined by three quantities: the gas-to-metal temperature drop, the fin efficiency, and the metal-to-water temperature drop. Raw experimental results will be presented for several gas-side heat-exchanger geometries, including copper screen, reticulated aluminum foam, and bundled copper tubes. Where possible, comparisons will be made to simple theoretical expectations.
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