5,292 research outputs found

    Application of Laplace transforms for the solution of transient mass- and heat-transfer problems in flow systems

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    A fast numerical technique for the solution of partial differential equations describing timedependent two- or three-dimensional transport phenomena is developed. It is based on transforming the original time-domain equations into the Laplace domain where numerical integration is performed and by subsequent numerical inverse transformation the final solution can be obtained. The computation time is thus reduced by more than one order of magnitude in comparison with the conventional finite-difference techniques. The effectiveness of the proposed technique is demonstrated by illustrative examples

    Mathematical modelling of a flow-injection system with a membrane separation module

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    A mathematical model for a flow-injection system with a membrane separation module based on the axially dispersed plug flow model was developed. It takes into account the geometrical dimensions and dispersion properties of the main sections of the manifold, the mass transfer in the channels of the separation module and the characteristics of the membrane (thickness and diffusion coefficient within it). The model was solved analytically in the Laplace domain. The inverse transformation was found to give satisfactory results for reactor Peclet numbers less than 120. Otherwise a numerical solution based on the implicit alternating-direction finite difference method was preferred. The adequacy of the model was confirmed experimentally on a flow-injection manifold with a parallel-plate dialysis module. The unknown flow and membrane parameters were determined by curve fitting. The membrane parameters were determined also by steady-state measurements. Fairly good agreement between the dynamic and steady-state results and with results given in the literature was observed, which, together with other experimental results, supported the validity of the model and showed that it can be used successfully for the mathematical description and optimization of flow-injection systems with membrane separation modules. In this connection, the influence of the reactor parameters and the sample volume on the performance of such a system were investigated and conclusions for improving its sensitivity and sample throughput were drawn. Other possible applications of the model are in membrane technology for characterizing of various membranes and in process engineering for investigating the mass transfer in different dialysers

    Unstable coronal loops : numerical simulations with predicted observational signatures

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    We present numerical studies of the nonlinear, resistive magnetohydrodynamic (MHD) evolution of coronal loops. For these simulations we assume that the loops carry no net current, as might be expected if the loop had evolved due to vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical flux tube with line-tied ends. For a given amount of twist in the magnetic field it is well known that once such a loop exceeds a critical length it becomes unstableto ideal MHD instabilities. The early evolution of these instabilities generates large current concentrations. Firstly we show that these current concentrations are consistent with the formation of a current sheet. Magnetic reconnection can only occur in the vicinity of these current concentrations and we therefore couple the resistivity to the local current density. This has the advantage of avoiding resistive diffusion in regions where it should be negligible. We demonstrate the importance of this procedure by comparison with simulations based on a uniform resistivity. From our numerical experiments we are able to estimate some observational signatures for unstable coronal loops. These signatures include: the timescale of the loop brightening; the temperature increase; the energy released and the predicted observable flow speeds. Finally we discuss to what extent these observational signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure

    Feasibility and effectiveness of combination antiretroviral therapy in HIV-infected infants in Pietermaritzburg, South Africa

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    Background : In the absence of treatment, 50% of HIV-infected children will die before 2 years of age. In a recent randomized controlled trial, a 76% decrease in mortality was observed in infants receiving early combination antiretroviral therapy [1]. The World Health Organization now recommends starting all HIV-infected infants on combination antiretroviral therapy on diagnosis [2]. However, few data are available outside a well-controlled research setting. Purpose of the study : To show the feasibility and effectiveness of treating HIV-infected infants in a state-funded clinic located in a poorly resourced South African township. Methods : A retrospective chart review was performed of all HIV-1 infected infants initiated on combination antiretroviral therapy (cART) between 1st May 2005 and 31st May 2008 at the Edendale Family Clinic, Pietermaritzburg, South Africa. All HIV-1 infected infants who were less than 1 year of age when antiretroviral therapy was initiated, and who had completed at least 6 months of treatment, were included. Weight for age Z scores, CD4 %, viral loads (VL) and haemoglobin were collected on initiation of treatment and at 6-monthly intervals thereafter. Virological success was defined as VL25%. Z scores were analyzed using Epi-Info. Summary of results : Of 129 treated infants, 94 completed 6 months of cART; 60 completed 12 months and 39 completed 18 months of treatment. Mean age at initiation was 8 months (range 2.1-11.7). 77.2% had advanced disease (WHO Stage 3 or 4). The infants were severely malnourished, with a mean Z-score of -2.4 (range -6.1 - +0.8). Mean baseline VL was 4700 000 copies/ml. After 6 months of treatment, 52.3% of babies had an undetectable VL, with 75% having a VL of < 400 copies/ml. Viral suppression was achieved in 34 (56.9%) out of the 60 infants who completed 1 year of cART and 79.3% had a VL <400 copies/ml. Undetectable VL was found in 78.8% of the 39 children who received 18 months of treatment. Weight for age Z score increased from a mean of -2.4 (<3rd centile) at initiation of treatment to -0.3 (38th centile) for the children who received 18 months of cART. The CD4% increased from a mean of 16.5% at the start to 31.9% at 18 months. Conclusions : This study from a township in Kwazulu-Natal shows a good clinical, immunological and virological response to cART in HIV-infected infants, despite high baseline viral loads and advanced disease
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