A hypothetical experiment and Monte Carlo simulations were used to examine the effectiveness of statistical design of experiments methods in identifying from the experimental data the correct terms in postulated regression models for a variety of experimental conditions. Two analysis of variance techniques (components of variance and pooled mean square error) combined with F-test statistics were investigated with first-order and second-order regression models. It was concluded that there are experimental conditions for which one or the other of the procedures results in model identification with high confidence, but there are also other conditions in which neither procedure is successful. The ability of the statistical approaches to identify the correct models varies so drastically, depending on experimental conditions, that it seems unlikely that arbitrarily choosing a method and applying it will lead to identification of the effects that are significant with a reasonable degree of confidence. It is concluded that before designing and conducting an experiment, one should use simulations of the proposed experiment with postulated truths in order to determine which statistical design of experiments approach, if any, will identify the correct model from the experimental data with an acceptable degree of confidence. In addition, no significant change in the effectiveness of the methods in identifying the correct model was observed when systematic uncertainties of up to 10 percent in the independent variables and in the response were introduced into the simulations. An explanation is that the systematic errors in the simulation data caused a shift of the whole response surface up or down from the true value, without a significant change in shape. [S0098-2202(00)03102-3]
The working characteristic of a new hollow cathode, thermionic-arc dc discharge ionization device was investigated experimentally. Under typical operating conditions the device produces a steady, field-free plasma plume by ionizing a flowing gas column. Experiments were performed with argon, nitrogen, and helium, the typical discharge pressures being 0.2–0.6 Torr. The discharge exploits the inherent stability of the point to plane geometry and the ionization efficiency of the hollow cathode. Electrically, the discharge is fed from a constant–voltage power supply and maintained below the interelectrode gap breakdown threshold. Thus the discharge current is given by Ohm’s law and is a function of the applied field and local plasma parameters. This modus operandi increases the ionization efficiency by maintaining a local thermal nonequilibrium in the discharge. The degree of nonequilibrium maintained in the plume downstream the cathode was higher in nitrogen (Te/Tgas≅8–10) than in argon (Te/Tgas≅3–4), and is attributed to a lower collision frequency in the nitrogen plume. At an input power level of 0.9 kW plasmas with electron densities of 1017 part/m3 in nitrogen and 1018 part/m3 in argon were recorded in the plume outside the high-field cathode region. Helium discharge displayed the highest nonequilibrium level (Te/Tgas≅100) but at the same time showed the largest degree of instability and the lowest electron density levels 1015 part/m3. The experiments indicate that the stability and the ionization efficiency of the discharge are enhanced by a supersonic flow field. Flow surveys have shown that the more stable argon and nitrogen plumes were slightly supersonic in the range of pressure investigated while helium flow remained subsonic throughout. The important consequence to the supersonic flow is the development of a normal shock in front of the cathode filament. Analysis of the data indicates that the normal shock is an effective stabilizing boundary.
Bladeless turbines were first proposed and demonstrated almost 100 years ago, but they have not found widespread application. They have the potential for converting flow energy into rotational energy at high efficiencies. This paper discusses an effort to understand the factors affecting bladeless turbine efficiency, and the related experimental program performed. A theoretical model of turbine performance was developed and used to predict the performance of a small scale test unit. The model inputs include geometric parameters and inlet conditions including pressure and flow rate. The test unit was designed and assembled. Rapid prototyping techniques were used to facilitate part fabrication. It was coupled to an experimental set up including appropriate instrumentation. A comparison of model predictions and test results is presented.
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