A numerical model that simulates the relaxation process for surface potential at the surface of a charged oil in a grounded metal vessel is developed. In the model, constant (ohmic) conductivity of the oil is assumed, and a surface conduction mechanism for the dissipation of electric surface charge is introduced. Calculations for a cylindrical vessel of diameter D/sub t/, height H/sub t/, and fill fraction F show that the charge relaxation is influenced by vessel geometry and fill level, as expected, and by the absolute size of the vessel as well. In fact, it is found that the effective relaxation time of the surface potential correlates quite well to a dimensionless quantity D/sub t/ kappa / lambda , where kappa (in Siemens per meter) is the bulk conductivity of the oil, and lambda (in Siemens) is the effective surface conductivity at the oil/air interface.< >
Gas sensors operated with a small battery is an exciting objective. The authors tried it with a thin film SnO{sub 2} semiconductor as a sensing element and IC-technics, and the other with a sintered SnO{sub 2} and various pulse driving methods. These tries have both advantages and disadvantages. Thin-film sensors suffer from low stability and low sensitivity to certain gases. SnO{sub 2} sensors have more favorable operating characteristics, but they require much power and have long response times. The authors corrected these shortcomings with good success through miniaturization of a SnO{sub 2} sensor.
Our goal in this study was to analyze the relationship between the shape of the non-spiral scan sensitivity profile on the Z-axis (SSPz) and the shape of spiral scan SSPz and image noise of spiral pitch with an asymmetric-type detector in multi-slice CT. In the first experiment, we used a bead phantom of 0.1 mm in diameter with non-spiral SSPz and spiral SSPz. Measuring non-spiral SSPz, we moved the bead phantom by manual control and measured the effect of partitions on each slice collimation, the full width at half maximum (FWHM), and the full width at tenth maximum (FWTH). When we experienced spiral scan, we measured FWHM and FWTM at various pitches and collimations. In the subsequent experiments, we used a water phantom of 200 mm in diameter to image noise. We measured the standard deviation (SD) of the CT number at the center of the water phantom at various pitches and collimations. The results of non-spiral SSPz showed that partitions had an effect on the shape of SSPz. On the other hand, at the same collimation, the results of spiral SSPz showed that spiral pitches had no effect on FWHM. However, spiral pitches had a slight effect on FWTM and the base of SSPz. The results of image-noise showed that, if the same collimation was used, spiral pitches had little effect on SD because of tube current control. Thus our results were different from the those obtained with a matrix-type detector in multi-slice CT.
Transient decay of surface potential in an upright cylindrical vessel with a central fill pipe is investigated. The transient problem is solved analytically for the ideal case of the fill pipe extending fully to the vessel bottom. For the practical case when the fill pipe does not reach to the vessel bottom, this problem is studied by numerical calculation. In both solutions, constant ohmic resistivity is assumed and a surface conduction mechanism is introduced for the dissipation of surface charge. It is revealed that the existence of a fill pipe beyond the liquid surface increases modestly the relaxation rate of surface potential. However, for the case when the fill pipe is located just above the liquid surface, decay of surface potential is dramatically slowed.< >
An experimental investigation of transient charge decay in a vessel partially filled with electrically charged liquid is reported. The test facility is an upright cylindrical tank with diameter and height equal to 50 cm. It is instrumented to permit continuous measurement of both the bulk charge density in the liquid and the electrostatic field during experimental runs. By correlating the time course of the bulk charge density to that of the electrostatic field, the transient redistribution of electrostatic charge can be characterized. One objective of this research is to ascertain the role played by surface charge in controlling the transient decay.< >
Transient decay of the surface potential in an upright cylindrical vessel with a central fill pipe is investigated. The transient problem is solved analytically for the ideal case of the fill pipe extending fully to the vessel bottom. For the practical case in which the fill pipe does not reach the vessel bottom, this problem is studied by numerical calculation. In both solutions, constant ohmic resistivity is assumed and a surface conduction mechanism is introduced for the dissipation of surface charge. It is found that the existence of a fill pipe extending below the liquid surface modestly increases the relaxation rate of surface potential. However, when the fill pipe is located just above the liquid surface, the decay of surface potential is greatly slowed.< >
Experiments with a single fire source were carried out in an unconfined space to characterize the effects of cross-winds on apparent flame heights and flame tilt angles. Three sets of propane burners were used as model fire sources. One was a small circular burner, the other was large square burners, and the other was rectangular burners, having the aspect ratio ranging from 1 to 6. A refined empirical model on the apparent flame heights of the inclined flames modified by heat release rate and aspect ratio of fire source was presented. We also developed an empirical model of the flame tilt angles based on the balance of mass between the fluxes given by the upward hot current and the cross-winds. These models are correlations with respect to dimensionless heat release rates, Froude numbers and aspect ratios of fire sources. The values of empirical coefficients and exponents of the correlations were derived from the experimental results.
