Time filter

Source Type

Mkwizu M.H.,Nelson Mandela African Institution of Science and Technology | Makinde O.D.,Stellenbosch University
Comptes Rendus - Mecanique | Year: 2015

The present work investigates the combined effects of thermophoresis, Brownian motion and variable viscosity on entropy generation in an unsteady flow of water-based nanofluids confined between two parallel plates with convective heat exchange with the ambient surrounding at the walls. Both first and second laws of thermodynamics are applied to analyse the problem. The nonlinear governing equations of continuity, momentum, energy, and nanoparticles concentration are tackled numerically using a semi-discretisation finite-difference method together with a Runge-Kutta-Fehlberg integration scheme. Numerical results for velocity, temperature, and nanoparticles concentration profiles are obtained and utilised to compute the skin friction, the Nusselt number, the entropy generation rate, the irreversibility ratio, and the Bejan number. Pertinent results are displayed graphically and discussed quantitatively. © 2014 Académie des sciences.

Mulibo G.D.,Pennsylvania State University | Mulibo G.D.,Nelson Mandela African Institution of Science and Technology | Nyblade A.A.,Pennsylvania State University
Geochemistry, Geophysics, Geosystems | Year: 2013

P and S relative arrival time residuals from teleseismic earthquakes recorded on over 60 temporary AfricaArray broadband seismic stations deployed in Uganda, Tanzania, and Zambia between 2007 and 2011 have been inverted, together with relative arrival time residuals from earthquakes recorded by previous deployments, for a tomographic image of mantle wave speed variations extending to a depth of 1200 km beneath eastern Africa. The image shows a low-wave speed anomaly (LWA) well developed at shallow depths (100-200 km) beneath the Eastern and Western branches of the Cenozoic East African rift system and northwestern Zambia, and a fast wave speed anomaly at depths ≤ 350 km beneath the central and northern parts of the East African Plateau and the eastern and central parts of Zambia. At depths ≥350 km the LWA is most prominent under the central and southern parts of the East African Plateau and dips to the southwest beneath northern Zambia, extending to a depth of at least 900 km. The amplitude of the LWA is consistent with a ∼150-300 K thermal perturbation, and its depth extent indicates that the African superplume, originally identified as a lower mantle anomaly, is likely a whole mantle structure. A superplume extending from the core-mantle boundary to the surface implies an origin for the Cenozoic extension, volcanism, and plateau uplift in eastern Africa rooted in the dynamics of the lower mantle. ©2013. American Geophysical Union. All Rights Reserved.

Jande Y.A.C.,Hanyang University | Jande Y.A.C.,Nelson Mandela African Institution of Science and Technology | Kim W.S.,Hanyang University
Desalination | Year: 2013

Capacitive deionization (CDI) is an emerging technology of desalinating brackish/seawater to attain freshwater. The process involves polarization of the two electrodes electrically using direct current; thus the cations and anions are attracted towards the oppositely charged electrode. So far most of the experiments/models involve the charging of the CDI cell at constant voltage. However, charging at constant voltage leads to having a shorter time in a given CDI cell cycle when the system has reached its lowest effluent concentration. This is undesired phenomena. To overcome this problem desalination process is preferred to be performed at constant current. The dynamic response model to describe the variation of the effluent concentration with time under constant current charging has been derived and validated. Also, the effect of processing parameters such as applied current, flow rate, CDI cell dead volume, and capacitance on the lowest effluent concentration is analyzed. © 2013 Elsevier B.V.

Kona J.R.,University of Connecticut | King'Ondu C.K.,University of Connecticut | King'Ondu C.K.,Nelson Mandela African Institution of Science and Technology | Howell A.R.,University of Connecticut | Suib S.L.,University of Connecticut
ChemCatChem | Year: 2014

Manganese oxide octahedral molecular sieve (OMS) materials with well-defined pores have been extensively studied over two decades due to their intriguing chemical and physical properties. OMS-2, the synthetic cryptomelane form of manganese oxide, was synthesized by a modified reflux method and was found to be highly active for obtaining α,β-unsaturated esters (up to 95 % yield and with high diastereoselectivities) from a variety of benzyl, heteroaryl, allyl and alkyl alcohols via one-pot alcohol oxidation-Wittig reaction. The transformation utilizes air as the stoichiometric oxidant, and the inexpensive catalyst can be recovered and reused. Filter and use again! Porous manganese oxide molecular sieve based catalysts were found to efficiently promote the oxidation of a variety of alcohols to the aldehydes, which reacted insitu with stabilized Wittig reagent, providing almost exclusively E-α,β-unsaturated esters in good to excellent yields. The heterogeneous catalyst used was made from inexpensive starting materials, and the recovered catalyst was found to be reusable with a modest loss in activity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kerrouche A.,Heriot - Watt University | Hardy D.A.,Heriot - Watt University | Ross D.,Heriot - Watt University | Richards B.S.,Heriot - Watt University | Richards B.S.,Nelson Mandela African Institution of Science and Technology
Solar Energy Materials and Solar Cells | Year: 2014

Luminescent solar concentrators (LSC) are a promising technology for building integrated photovoltaics (BIPV) given the wide variety of forms and colours that can be realised. Given the flexibility of the technology, the use of ray-trace modelling is indispensable in the design, performance evaluation, and optimisation of LSCs. This work begins by comparing a three dimensional (3D) ray-trace model of an LSC with experimental results. The study includes 70 samples - both square and circular LSCs, containing five different fluorescent organic dyes (BASF Lumogen) each at seven different concentrations. The figure-of-merit used for performance evaluation was the average power density determined at the LSC edge sheet, measured using an optical fibre connected to a spectrometer. The results demonstrate that 3D ray-trace results gives good agreement with the experimental measurements, to within around ±5% within a wide concentration range (optical density=0.05-8) and a maximum difference of ±13%. The wide range of colours achieved is presented in a CIE chart. Overall, the validated experimental results give confidence in the use of modelling for future larger LSCs for BIPV. Therefore, based on these results and the colours achievable, a model of a stained-glass window is constructed and its performance throughout a solar day is simulated. © 2013 Published by Elsevier B.V. All rights reserved.

Shen J.,Heriot - Watt University | Shen J.,Nelson Mandela African Institution of Science and Technology | Schafer A.,Nelson Mandela African Institution of Science and Technology | Schafer A.,Karlsruhe Institute of Technology
Chemosphere | Year: 2014

Inorganic contamination in drinking water, especially fluoride and uranium, has been recognized as a worldwide problem imposing a serious threat to human health. Among several treatment technologies applied for fluoride and uranium removal, nanofiltration (NF) and reverse osmosis (RO) have been studied extensively and proven to offer satisfactory results with high selectivity. In this review, a comprehensive summary and critical analysis of previous NF and RO applications on fluoride and uranium removal is presented. Fluoride retention is generally governed by size exclusion and charge interaction, while uranium retention is strongly affected by the speciation of uranium and size exclusion usually plays a predominant role for all species. Adsorption on the membrane occurs as some uranium species interact with membrane functional groups. The influence of operating conditions (pressure, crossflow velocity), water quality (concentration, solution pH), solute-solute interactions, membrane characteristics and membrane fouling on fluoride and uranium retention is critically reviewed. © 2014 Elsevier Ltd.

Poyraz A.S.,University of Connecticut | Kuo C.-H.,University of Connecticut | Biswas S.,University of Connecticut | King'Ondu C.K.,University of Connecticut | And 2 more authors.
Nature Communications | Year: 2013

Mesoporous oxides attract a great deal of interest in many fields, including energy, catalysis and separation, because of their tunable structural properties such as surface area, pore volume and size, and nanocrystalline walls. Here we report thermally stable, crystalline, thermally controlled monomodal pore size mesoporous materials. Generation of such materials involves the use of inverse micelles, elimination of solvent effects, minimizing the effect of water content and controlling the condensation of inorganic frameworks by NOx decomposition. Nanosize particles are formed in inverse micelles and are randomly packed to a mesoporous structure. The mesopores are created by interconnected intraparticle voids and can be tuned from 1.2 to 25 nm by controlling the nanoparticle size. Such phenomena allow the preparation of multiple phases of the same metal oxide and syntheses of materials having compositions throughout much of the periodic table, with different structures and thermal stabilities as high as 800°C. © 2013 Macmillan Publishers Limited.

Mulibo G.D.,Pennsylvania State University | Mulibo G.D.,Nelson Mandela African Institution of Science and Technology | Nyblade A.A.,Pennsylvania State University
Geophysical Research Letters | Year: 2013

P to S conversions from the 410 and 660 km discontinuities observed in receiver function stacks reveal a mantle transition zone that is ~30-40 km thinner than the global average in a region ~200-400 km wide extending in a SW-NE direction from central Zambia, across Tanzania and into Kenya. The thinning of the transition zone indicates a ~190-300 K thermal anomaly in the same location where seismic tomography models suggest that the lower mantle African superplume structure connects to thermally perturbed upper mantle beneath eastern Africa. This finding provides compelling evidence for the existence of a continuous thermal structure extending from the core-mantle boundary to the surface associated with the African superplume. © 2013. American Geophysical Union. All Rights Reserved.

Jande Y.A.C.,Hanyang University | Jande Y.A.C.,Nelson Mandela African Institution of Science and Technology | Kim W.S.,Hanyang University
Separation and Purification Technology | Year: 2013

Capacitive deionization (CDI) is a promising technology for desalination of brackish water with different applications such as in the pharmaceutical industry, semiconductor manufacturing, and domestic use. The CDI cell utilizes an electric potential across two electrodes in which one of the electrodes becomes positively charged and the other becomes negatively charged. Cations and anions are attracted towards the anode and cathode, respectively. The adsorption and desorption mechanism within the CDI cell determines the amount of salt in the effluent stream. Modeling the dynamic response of the effluent concentration is vital to understanding the water purity level. In this paper, the equations predicting the lowest concentration time and lowest concentration have been found using the adsorption cycle mathematical model. During purification process the effluent concentration reaches the highest purity level after a certain period of time. We define the time it takes to reach the highest purity level as lowest concentration time and the corresponding instantaneous effluent purer water is what we call lowest concentration. While the lowest concentration depends on all of the CDI operating parameters i.e., applied potential, capacitance, flow rate, feed concentration, dead volume, and spacer volume, the lowest concentration time depends only on flow rate, dead volume, and capacitance. Using a genetic algorithm, it was found that seawater (32,702 ppm) could be desalinated to as low as 2.1 ppm; which is within the standards for drinking water set by the world health organization. © 2013 Elsevier B.V. All rights reserved.

FLOWERED objective is to contribute to the development of a sustainable water management system in areas affected by fluoride (F) contamination in water, soils and food in the African Rift Valley countries (Ethiopia, Kenya, Tanzania), thus to improve living standards (environmental, health and food security) of its population. FLOWERED aims to study, test and implement innovative defluoridation technologies for drinking and irrigation water that will mainly operate at small village scale and to develop an integrated, sustainable and participative water and agriculture management at a cross-boundary catchment scale. On the basis of the complexity of the issue of water de-fluoridation, the proposed scientific approach in FLOWERED is based on a detailed knowledge of the geological and hydrogeological setting that controls contamination of water that constitute the prerequisite for the implementation of a sustainable water management and for the proposal of sustainable and suitable strategies for water sanitation and agricultural system. Innovative agricultural practices will be assessed, aiming to mitigate the impacts of F contamination of water and soil on productivity of selected food and forage crops and dairy cattle health and production. The development of an innovative and shared Geo-data system will support the integrated, sustainable and participative management system. FLOWERED, focusing on innovative technologies and practices and taking into account local experiences, will implement an integrated water and agriculture management system and will enable local communities to manage water resources, starting from using efficient defluoridation techniques and applying sustainable agricultural practices. The integrated approaches improve knowledge for EU partners, local researchers, farmers and decision makers. The Project through the involvement of SMEs will strengthen the development co-innovative demonstration processes as well as new market opportunities.

Loading Nelson Mandela African Institution of Science and Technology collaborators
Loading Nelson Mandela African Institution of Science and Technology collaborators