Two dimensional flow deflection screen model

A flow deflection screen model is proposed and implemented to predict the influence of expanded metal screens on a turbulent wind tunnel flow. The screen model successfully predicted the mean velocity and turbulence distribution in the flow field downstream of different screen types and orientation in the wind tunnel flow.     

Tuberculous cerebellar abscess

We report uncultured rod-shaped magnetotactic bacteria from natural waters that biomineralize magnetic crystals in two different size ranges. Electron spectroscopic imaging of whole bacteria deposited over formvar-coated grids permitted a better visualization and measurement of the magnetosomes. All magnetosomes of individual bacteria could be observed by this technique. The magnetosomes formed one large chain, composed of three to four columns of crystals, disposed in parallel to the main axis of the bacteria. The magnetosomes ranged from 19 to 136 nm in length and 14 to 112 nm width. Smaller magnetosomes (less than 80 nm in length) localized mostly in extremities of the bacterial body while larger preferentially localized in the middle part of the cell. Electron spectroscopic diffraction and X-ray microanalysis indicate that both types of magnetosomes contain magnetite (Fe3O4). In projection, most magnetosomes seem to present pseudo-hexagonal morphologies described for magnetite. As the aspect ratios for smaller and larger magnetosomes are different, we suggest that different levels of control on biomineralization of magnetosomes may exist.     

Vertical two-phase flow: Part II. Holdup and pressure drop

Two-phase gas-liquid flow has been investigated in a 1-inch internal diameter vertical tube coil containing two risers and a downcomer all connected by “U” bends. Pressure drop, holdup and flow pattern data were successfully obtained simultaneously in the three vertical tubes, each 17.30 ft. long, for five different air-liquid systems at about 25 psia and 50°F-80°F over flow ranges of 0–700 lb_m air/min-ft² and 140--25300 lb_m liquid/min-ft². Pressure drops and liquid holdups were plotted against gas volume flowrate with liquid flowrate as a parameter. From these plots it was found that for a combination of an increase in liquid viscosity and density, and a decrease in surface tension, the frictional pressure drop increased in down flow and decreased in upflow. Holdup, on the other hand, increased for both types of vertical flow with respect to the same combination of parameters. The Lockhart-Martinelli scheme was satisfactory in correlating frictional pressure drop and holdup in all the flow regimes except the frothy-slug regime in upflow. In downflow however, the Lockhart-Martinelli scheme met with limited success because of a strong influence of liquid flowrate, physical properties and pipe orientation. Holdup in the falling film and falling bubbly film regimes in downflow were satisfactorily treated by the drift flux approach which emphasizes the relative motion of the two phases.     

Digital inequalities and implications for social inequalities: A study of Internet penetration amongst university students in South Africa

International and national data sources confirm the limited access to the Internet in South Africa. However, there is a need to study how this limited access is distributed amongst population groups in the country. This study investigates the pattern of Internet access amongst university students in South Africa. University students are a relevant population group for studying technology penetration; this population is a generation born in the current era of digital revolution, they are presumably early adopters of technology and are adept in using them. A survey conducted in ten South African universities reveals that beyond publicly available access on campuses – with its attendant constraints, personal/household Internet access is a challenge and it reflects a pattern of inequalities in South Africa. Bearing in mind that social and economic exclusion is the biggest challenge for South Africa, this study reveals that digital inequalities amongst students replicate the pattern of social inequalities in the country. This exacerbates existing social inequalities, specifically amongst university students in South Africa, with certain categories of students being disproportionately disadvantaged.     

Turbulence decay behind expanded metal screens

An experiment study of the turbulent flow behind expanded metal screens has been carried out in a low‐turbulence wind tunnel using an X‐probe hot‐wire anemometer system. The expanded metal screens turn flow due to a complex array of vaned elements. The flow turning was found to vary accordingly with the dimensions of the strands that make up the screen. The turbulence generated by the screens decays at a rate proportional to the downstream distance to the power ‐ 5/7, consistent with studies in the literature of conventional screens types, and was found to scale with the thickness of the screen strands. The mean velocity, pressure drop and turbulence characteristics of expanded metal screens are presented.     

Heat transfer in co-current vertical two-phase flow

Heat transfer in co-current two-phase upflow and downflow of air–water has been investigated in a 25.8 mm electrically heated vertical pipe at 172.3 kPa for water mass velocities of 54 to 172 kg/m²s and gas flow rates of 0 to 1.322 × 10⁻² m³/s. It was found that although the injection of air in the liquid flow increased the two-phase heat transfer coefficients significantly for both systems, upflow coefficients were generally higher than those for downflow for the same liquid flow rate. This could have important implications in the design of some chemical reactors and heat engineering processes. Changes in heat transfer rates were found to occur at the flow pattern transition boundaries. Two-phase heat transfer coefficients were well correlated by an expression based on dimensional analysis for both upflow and downflow.