Some aspects of dynamic computational modelling of direct current plasma arc phenomena

Direct current arc furnaces see considerable use in modern industrial melting and smelting processes. Pyrometallurgical applications for this type of furnace are wide-ranging, and include commodities such as Ferrochrome, Ferronickel, Cobalt, Zinc, Magnesium, Titanium Dioxide, Platinum-group metals1, and others. Central to the operation of such furnaces is the direct current plasma arc, a sustained high temperature jet of ionised gas which is formed between the end of one or more graphite electrodes and the bath of molten process material below. Passage of electric current through the arc inputs energy and maintains the high temperatures necessary for ionisation via ohmic heating. This is balanced by various mechanisms of energy loss from the arc, including volumetric radiation and convection to the molten bath surface below. Much of this energy is delivered to a localised area directly beneath the arc, making it a very efficient means of heating the process material. Flow of plasma in the arc column is driven strongly by electromagnetic Lorentz forces resulting from the constriction of the conduction channel in the vicinity of the electrode. This constriction causes the arc to draw in gas from the surroundings and accelerate it away from the electrode surface, toward the molten bath below (the Maecker effect2). Much research has been conducted in the area of numerical modelling of arc phenomena, starting with Szekely and co-workers3 and becoming increasingly more sophisticated with the advent of better software, property data, and increased computing capability. However, the majority of arc modelling efforts concentrate on steady-state, axisymmetric systems. While valuable from an engineering standpoint these models are not able to describe any transient behaviour exhibited by the arc, or any evolution of the shape and structure of the arc which breaks the symmetry imposed by the model. Both of these aspects are important for a deeper understanding of direct current plasma arc behaviour

An Analysis of Pressure Fluctuations in a CFB of Heavy Minerals

Pressure fluctuations were measured at high frequencies in a CFB model operated with air at ambient conditions. The model comprises an 80-mm-ID, 5-m-tall riser with a blind-T exit, a cyclone, a 50-mm-ID standpipe, and an L-valve. Tests were conducted with particles of natural rutile (TiO2), a heavy mineral mined from coastal dunes. The particles fall into group B of Geldart’s classification. The solids inventory was kept at 25 kg. The superficial gas velocity ranged from 3 to 6 m/s. The solid circulation flux varied between 10 and 40 kg/m2.s. Profiles of solid concentrations in the riser are C-shaped. The amplitude of pressure fluctuations increases with increasing solids-circulation rate, and the increase appears to be linear. The amplitude does not correlate with solids concentration, however. The implication is that gas-solid interactions differ significantly at the bottom and the top of the riser, despite similar solids concentrations in these two zones. The analysis in the frequency domain shows that the power of signals resides in those of low frequencies (less than 2 Hz). The pressure fluctuations reflect white noise: there is no dominant frequency and no periodic component. Pressure waves move at 25–45 m/s up the riser, an order of magnitude greater than the superficial gas velocity

On the application of differences in intrinsic fluctuations of Cherenkov light images for separation of air showers

The sensitivity of ground-based imaging atmospheric Cherenkov gamma-ray observatories depends critically on the primary particle identification methods which are used to retain photon-initiated events and suppress the spurious background produced by cosmic rays. We suggest a new discrimination technique which utilizes differences in the fluctuations of the light intensity in the images of showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events for the proposed VERITAS observatory has been used to evaluate the efficiency of the new technique. Analysis has been performed for both a single VERITAS imaging telescope, and a system of these telescopes. We demonstrate that a discrimination efficiency of > 1.5 - 2.0 can be achieved in addition to traditional background rejection methods based on image shape parameters.Comment: 17 pages, 9 figures, accepted for publucation in Astropart. Phy

A New Analysis Method for Reconstructing the Arrival Direction of TeV Gamma-rays Using a Single Imaging Atmospheric Cherenkov Telescope

We present a method of atmospheric Cherenkov imaging which reconstructs the unique arrival direction of TeV gamma rays using a single telescope. The method is derived empirically and utilizes several features of gamma-ray induced air showers which determine, to a precision of 0.12 degrees, the arrival direction of photons, on an event-by-event basis. Data from the Whipple Observatory's 10 m gamma-ray telescope is utilized to test selection methods based on source location. The results compare these selection methods with traditional techniques and three different camera fields of view. The method will be discussed in the context of a search for a gamma-ray signal from a point source located anywhere within the field of view and from regions of extended emission.Comment: 24 pages, 16 figures, accepted for publication in Astroparticle Physics May 11, 200

Security and Privacy Qualities of Medical Devices: An Analysis of FDA Postmarket Surveillance

Background: Medical devices increasingly depend on computing functions such as wireless communication and Internet connectivity for software-based control of therapies and network-based transmission of patients’ stored medical information. These computing capabilities introduce security and privacy risks, yet little is known about the prevalence of such risks within the clinical setting. Methods: We used three comprehensive, publicly available databases maintained by the Food and Drug Administration (FDA) to evaluate recalls and adverse events related to security and privacy risks of medical devices. Results: Review of weekly enforcement reports identified 1,845 recalls; 605 (32.8%) of these included computers, 35 (1.9%) stored patient data, and 31 (1.7%) were capable of wireless communication. Searches of databases specific to recalls and adverse events identified only one event with a specific connection to security or privacy. Software-related recalls were relatively common, and most (81.8%) mentioned the possibility of upgrades, though only half of these provided specific instructions for the update mechanism. Conclusions: Our review of recalls and adverse events from federal government databases reveals sharp inconsistencies with databases at individual providers with respect to security and privacy risks. Recalls related to software may increase security risks because of unprotected update and correction mechanisms. To detect signals of security and privacy problems that adversely affect public health, federal postmarket surveillance strategies should rethink how to effectively and efficiently collect data on security and privacy problems in devices that increasingly depend on computing systems susceptible to malware

The Photo Essay: A Visual Research Method for Educating Obstetricians and Other Health Care Professionals

When it comes to issues related to low-income women seeking early, adequate, or continuous prenatal ca re, the public health and medical communities continue to tell women to take responsibility for their actions. Rarely are messages aimed at providers. To help physicians see how factors in their offices and clinics can affect service utilization, the photo essay, a visual qualitative research strategy was developed using low- income minority and disenfranchised women who had recently given birth or were near to giving birth. Eight photo essays were completed. Together, the narratives, in collaboration with the photos, provided an opportunity for physicians to hear and observe women, as consumers, as they expanded their descriptions of their prenatal care experience

Selenium hyperaccumulation offers protection from cell disruptor herbivores

<p>Abstract</p> <p>Background</p> <p>Hyperaccumulation, the rare capacity of certain plant species to accumulate toxic trace elements to levels several orders of magnitude higher than other species growing on the same site, is thought to be an elemental defense mechanism against herbivores and pathogens. Previous research has shown that selenium (Se) hyperaccumulation protects plants from a variety of herbivores and pathogens. Selenium hyperaccumulating plants sequester Se in discrete locations in the leaf periphery, making them potentially more susceptible to some herbivore feeding modes than others. In this study we investigate the protective function of Se in the Se hyperaccumulators <it>Stanleya pinnata </it>and <it>Astragalus bisulcatus </it>against two cell disrupting herbivores, the western flower thrips (<it>Frankliniella occidentalis</it>) and the two-spotted spider mite (<it>Tetranychus urticae</it>).</p> <p>Results</p> <p><it>Astragalus bisulcatus </it>and <it>S. pinnata </it>with high Se concentrations (greater than 650 mg Se kg^-1) were less subject to thrips herbivory than plants with low Se levels (less than 150 mg Se kg^-1). Furthermore, in plants containing elevated Se levels, leaves with higher concentrations of Se suffered less herbivory than leaves with less Se. Spider mites also preferred to feed on low-Se <it>A. bisulcatus </it>and <it>S. pinnata </it>plants rather than high-Se plants. Spider mite populations on <it>A. bisulcatus </it>decreased after plants were given a higher concentration of Se. Interestingly, spider mites could colonize <it>A. bisulcatus </it>plants containing up to 200 mg Se kg^-1dry weight, concentrations which are toxic to many other herbivores. Selenium distribution and speciation studies using micro-focused X-ray fluorescence (μXRF) mapping and Se K-edge X-ray absorption spectroscopy revealed that the spider mites accumulated primarily methylselenocysteine, the relatively non-toxic form of Se that is also the predominant form of Se in hyperaccumulators.</p> <p>Conclusions</p> <p>This is the first reported study investigating the protective effect of hyperaccumulated Se against cell-disrupting herbivores. The finding that Se protected the two hyperaccumulator species from both cell disruptors lends further support to the elemental defense hypothesis and increases the number of herbivores and feeding modes against which Se has shown a protective effect. Because western flower thrips and two-spotted spider mites are widespread and economically important herbivores, the results from this study also have potential applications in agriculture or horticulture, and implications for the management of Se-rich crops.</p

Exposure to Household Air Pollution from Biomass Cookstoves and Levels of Fractional Exhaled Nitric Oxide (FeNO) among Honduran Women

Household air pollution is estimated to be responsible for nearly three million premature deaths annually. Measuring fractional exhaled nitric oxide (FeNO) may improve the limited understanding of the association of household air pollution and airway inflammation. We evaluated the cross-sectional association of FeNO with exposure to household air pollution (24-h average kitchen and personal fine particulate matter and black carbon; stove type) among 139 women in rural Honduras using traditional stoves or cleaner-burning Justastoves. We additionally evaluated interaction by age. Results were generally consistent with a null association; we did not observe a consistent pattern for interaction by age. Evidence from ambient and household air pollution regarding FeNO is inconsistent, and may be attributable to differing study populations, exposures, and FeNO measurement procedures (e.g., the flow rate used to measure FeNO)