Theory of competition between breakup and coalescence of droplets in flowing polymer blends

The Smoluchowski equation for the breakup and coalescence of dispersed droplets has been solved for flowing polymer blends. A scaling form for the distribution of droplet sized derived and published for a system of clusters with fragmentation and coagualation was used in our dervation. Equations are developed here for the average droplet size and for the characteristic time of transition to steady state flow of blends with a high content of the dispersed phase. Expressions reasonably describing the average size of droplets for all concentrations were obtained by a theory modification. Measured dependences of droplet size on the blend composition can be matched only if simultaneous collisions of three and more droplets are considered. The results of the theory indicate that the mechanism of droplet breakup (formation of pieces with the same or different volumes) has only a small effect on their average size in concentrated systems. The dependence of droplet size on the shear rate in flow is determined by properties of the blend components, and is generally nonmonotonic.     

Production of phosphoric acid with low content of impurities

The present work is to study the possibility for controlled distribution of the impurities when they are decomposed by sulphuric acid to wet phosphoric acid and phosphogypsum, as well as generating technological conditions for production of clean wet phosphoric acid and its derivatives - phosphoric fertilizers, food phosphates and other phosphoric products.A coefficient of distribution D_A is introduced as a general criterion for elements dispersion. The study includes relationships between the weight ratio sulphuric acid/natural phosphate and D_A; the period of decomposition and D_A; the particle's size content of the natural phosphate and D_A. The obtained experimental data and relationships about the influence of the above discussed technological parameters on dispersion of more than 23 elements allow for control of that process in wet phosphoric acid and phosphogypsum by selecting specific technological conditions. Thus, depending on the requirements of acid's consumers and procedures for secondary processing of phosphogypsum to end-products, a selective decomposition of the natural phosphates to phosphoric acid and phosphogypsum can be carried out with content of the impurities in those products varying in narrow limits.     

Backyard shacks,informality and the urban housing crisis in South Africa: stopgap or prototype solution?

Rapid urbanisation in the South has contributed to the growth of informal housing on a large scale. South Africa’s experience is somewhat unusual in that the growth of informality appears to have taken the form of backyard shacks in established townships rather than free-standing shacks in squatter settlements. This is potentially important for household well-being (e.g. better access to services) and for the efficient functioning of urban areas. The paper develops a framework for assessing the impacts and applies it to the country’s leading metropolitan region, Gauteng. It finds that people are slightly better-off in backyards than in shacks elsewhere, although the wider benefits for urban areas are equivocal. In some respects backyard shacks are a stopgap for poor households desperate for somewhere to live. In other respects they represent a kind of prototype solution to the urban housing crisis. The government could do more to improve basic dwelling conditions and to relieve the extra pressure on local services.     

Book reviews

Managing Cities. The New Urban Context. P. Healey, S. Cameron, S. Davoudi, S. Graham & A. Madani‐Pour (Eds) Chichester, John Wiley and Sons, 1995, £14.95 pb, ISBN 0–471–95533–7

Industrial Property: Policy and Economic Development. Rick Ball & Andy Pratt (Eds). London, Routledge, 1994, £45.00 hb, ISBN 0–415–09152–7

Grassroots Environmental Action: People's Participation in Sustainable Development. Dharam P. Ghai & Jessica M. Vivian (Eds). London and New York, Routledge, 1995 (pb), 347 pp., ISBN 0–415–12703–3

Services and Space: Key Aspects of Urban and Regional Development. N. Marshall & P. Wood. Essex, Longman Scientific and Technical, 1995, 294 pp., £17.99 pb, ISBN 0–582–25162–1

Servicing The Middle Classes: Class, Gender and Waged Domestic Labour in Contemporary Britain. Nicky Gregson & Michelle Lowe. Routledge, 1994, vii + 327 pp., £13.99 pb, ISBN 0–419–08531–4

Urban Development Planning: Lessons for the Economic Reconstruction of South Africa's Cities. Richard Tomlinson. Johannesburg, Witwatersrand University Press, 1994, 277 pp., £14.95 pb, ISBN 1–868–14258–2     

An investigation into the surface fatigue of materials and thin layers using a sliding diamond spherical segment

Methods for testing the surface fatigue of materials or thin layers normally involve configurations with two or more rolling balls or discs. Such equipment and test specimens are quite complicated, and for a large number of experiments, costly. Testing could be considerably simplified if the stressing of the material surface were performed with a sliding natural diamond spherical segment. A diamond on steel (or any different material) friction pair has the particular attribute that sliding wear is absent. Under these circumstances, after a certain number of strain cycles, fatigue failure of the investigated surface or layers appears. A particularly relevant application is investigation of the adhesion of a DLC layer.     

Testing facility for experimental evaluation of non‐structural components under full‐scale floor motions

The seismic vulnerability of non‐structural components and equipment with their expensive recovery and/or replacement costs has been demonstrated during past earthquakes. With the exception of the nuclear industry, the limited data collected from past earthquakes are not sufficient to completely characterize the seismic behaviour of non‐structural components and to develop effective mitigation measures. To address these limitations, the University at Buffalo has commissioned a dedicated non‐structural component simulator (University at Buffalo non‐structural component simulator (UB‐NCS)) composed of a two‐level testing frame capable of simultaneously subjecting both displacement and acceleration‐sensitive non‐structural components to realistic full‐scale floor motions expected within multi‐storey buildings. Current codes now require seismic qualification of important non‐structural components, and protocols for experimental qualification have been developed considering existing laboratory limitations. The new testing capabilities provided by the UB‐NCS are described to demonstrate the improved and more realistic qualification procedures that can be achieved. Results are presented from a test on two parallel partition walls aimed to verify the capabilities of the UB‐NCS. Copyright © 2008 John Wiley & Sons, Ltd.     

Life Cycle Assessment of urban wastewater reuse with ozonation as tertiary treatment: A focus on toxicity-related impacts

Life Cycle Assessment has been used to compare different scenarios involving wastewater reuse, with special focus on toxicity-related impact categories. The study is based on bench-scale experiments applying ozone and ozone in combination with hydrogen peroxide to a wastewater effluent from a Spanish sewage treatment plant. Two alternative characterisation models have been used to account for toxicity of chemical substances, namely USES-LCA and EDIP97. Four alternative scenarios have been assessed: wastewater discharge plus desalination supply, wastewater reuse without tertiary treatment, wastewater reuse after applying a tertiary treatment consisting on ozonation, and wastewater reuse after applying ozonation in combination with hydrogen peroxide. The results highlight the importance of including wastewater pollutants in LCA of wastewater systems assessing toxicity, since the contribution of wastewater pollutants to the overall toxicity scores in this case study can be above 90%. Key pollutants here are not only heavy metals and other priority pollutants, but also non-regulated pollutants such as pharmaceuticals and personal care products. Wastewater reuse after applying any of the tertiary treatments considered appears as the best choice from an ecotoxicity perspective. As for human toxicity, differences between scenarios are smaller, and taking into account the experimental and modelling uncertainty, the benefits of tertiary treatment are not so clear. From a global warming potential perspective, tertiary treatments involve a potential 85% reduction of greenhouse gas emissions when compared with desalination.     

Newtonian quantitative elastohydrodynamic film thickness with linear piezoviscosity

Examples are provided for linear pressure-viscosity response at low pressures, which are said to exist in the concentrated contact inlet, including lubricants at high temperature, low viscosity liquids and water/glycol solutions. Full isothermal, Newtonian simulations of the EHL problem reveal film-forming, which is quite similar to that of exponential liquids with the exception of the reduced sensitivity to the Dowson material parameter, G, or the Moes parameter, L. Film thickness formulas are offered, which have been experimentally validated. In a significant departure from most investigations of this kind, the viscosity of the experimental liquids was not adjusted to provide successful comparisons with measurement.     

Semi-discretization for stability analysis of in-feed cylindrical grinding with continuous workpiece speed variation

This paper presents the advances of semi-discretization approach for the dynamic stability analysis of in-feed cylindrical grinding process. The corresponding mathematical model is an autonomous delay-differential equation and the application of continuous workpiece speed variation (CWSV) in the process leads to a time-varying delay. Discretization techniques are a good way of dealing with differential equations for which the solution cannot be given in closed forms. Therefore, the semi-discretization method is proposed to analyze the equation. Stability maps are devised to study the influence of the CWSV application. Contrary to milling and turning processes, stability in grinding is very much influenced by the residual flexibility due to the deformation of the grinding wheel-workpiece system, so this term has been included in the approach. The validation has been carried out experimentally and good correlation between test and simulation results has been achieved.     

Aerodynamic design and analysis of a small-scale vertical axis wind turbine

Small-scale vertical axis wind turbines are regarded today as an attractive source of green energy, still insufficiently implemented and tested. This paper presents a fast design methodology of such a VAWT, in terms of choosing its main parameters: airfoil, rotor diameter and solidity. For obtaining generated power of each considered geometrical model at different undisturbed wind velocities and rotor angular velocities two models were used — momentum and vortex-wake model, combined with experimentally measured airfoil data (airfoil lift and drag coefficients). Even though the former model is simpler, it is the most utilized model, known to provide good results in stationary working regimes. Both models still present fairly accurate and fast tools for computation and optimization, particularly useful in the phase of conceptual design. In this research, the use of the momentum model resulted in determination of the maximal power coefficient, optimal- and minimal freestream velocity for every considered VAWT model. From these output parameters, a selection of the optimal geometric model was done, and a more detailed transient analysis and flow representation around the selected solution was obtained by the vortex-wake model. The results obtained by the two used computational models coincide satisfactorily.