Browsing by Author "Alexandrova S."

Now showing 1 - 7 of 7
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    Comparison of citrus peels as a source of valuable components - Polyphenols and antioxidants
    (2013-11-05) Karsheva M.; Kirova E.; Alexandrova S.; Georgieva S.
    The aim of this study is to compare some citrus fruits' peels: mandarins and grapefruits, as sources of polyphenolic compounds, to determine the antioxidant activity of the extracts and to find the effect of the operational conditions on the final extract properties. It was found that the both materials are rich of polyphenols and exhibit high antioxidant activity; better results are obtained for the grapefruit peels. The flavonoids' content in both materials studied is quite close. The solid-to-liquid ratio plays an important role in the extraction process. The better results are obtained for the ratio of 1:10. The results of this study confirm that the citrus peels are promising source of natural antioxidants and polyphenols.
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    Effect of the viscosity ratio on the mass transfer into a spherical drop in liquid-liquid dispersions
    (2014-01-01) Alexandrova S.; Karsheva M.; Saboni A.; Gourdon C.
    Numerical solutions of the Navier-Stokes equations of motion and the equations of mass transfer have been obtained for the unsteady state transfer into a spherical drop moving in an unbounded fluid medium of different viscosity. The effect of the viscosity ratio, k, between the dispersed phase and the continuous phase, on the mass transfer into a spherical drop, is investigated for different ranges of viscosity (0 < k < 10), Reynolds number (0 < Re < 100) and Peclet number (0 < Pe < 104).
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    Effects of interface contamination on mass transfer into a spherical bubble
    (2015-01-01) Saboni A.; Alexandrova S.; Karsheva M.
    A numerical study has been conducted to investigate the effects of contamination on mass transfer into a spherical bubble. The influence of the contamination on the mass transfer inside the bubble was studied for different ranges of dispersed phase Reynolds numbers (0.1 < Red < 10), Schmidt numbers (0.7
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    Heat transfer from a moving fluid sphere with internal heat generation
    (2014-01-01) Alexandrova S.; Karsheva M.; Saboni A.; Gourdon C.
    In this work, we solve numerically the unsteady conduction-convection equation including heat generation inside a fluid sphere. The results of a numerical study in which the Nusselt numbers from a spherical fluid volume were computed for different ranges of Reynolds number (0 < Re < 100), Peclet number (0 < Pe < 10000) and viscosity ratio (0 < κ < 10), are presented. For a circulating drop with Re → 0, steady creeping flow is assumed around and inside the sphere. In this case, the average temperatures computed from our numerical analysis are compared with those from literature and a very good agreement is found. For higher Reynolds number (0 < Re < 100), the Navier-Stokes equations are solved inside and outside the fluid sphere as well as the unsteady conduction-convection equation including heat generation inside the fluid sphere. It is proved that the viscosity ratio κ (κ = = μd/μc) influences significantly the heat transfer from the sphere. The average Nusselt number decreases with increasing κ for a fixed Peclet number and a given Reynolds number. It is also observed that the average Nusselt number is increasing as Peclet number increases for a fixed Re and a fixed κ.
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    Mass transfer into a spherical bubble
    (2016-10-02) Saboni A.; Alexandrova S.; Karsheva M.; Gourdon C.
    A numerical study has been conducted to investigate the mass transfer inside a spherical bubble at low to moderate Reynolds numbers. The Navier-Stokes and diffusion-convection equations were solved numerically by a finite difference method. The effect of the bubble Schmidt number (over the range 0.1
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    Natural antioxidants from citrus mandarin peels. extraction of polyphenols; effect of operational conditions on total polyphenols contents and antioxidant activity
    (2013-05-22) Karsheva M.; Kirova E.; Alexandrova S.
    The main waste of the citrus fruits after processing is the citrus peel. The extraction of valuable components from mandarin peels with ethanol-in-water solutions was investigated. The effect of the operational conditions on the total extracted amount, the totalpolyphenols content and antioxidant activity was studied. The conditions varied were: theethanol concentration, the particle size and the temperature. Taking into account the thermolability of the polyphenols, vitamin C, etc., the influence of the drying temperature on the properties of extracts after re-dissolution was studied. It was found that the increase in drying temperature leads to decrease in TPPC in the extracts after re-dissolution. The worst results were obtained for drying temperature of 60°C - total polyphenols contents after drying and re-dissolution decreased almost twice (2.22 times). The comparison in TPPC and AOA of the mandarin peels' extracts with other citrus peels shows that the source studied is a perspective for possible use in food and cosmetic industries due to thevaluable components in it.
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    The influence of surfactants on the drainage and rupture of mobile liquid films between drops: A parametric numerical study
    (2018-01-01) Alexandrova S.; Karsheva M.; Spasic A.M.; Saboni A.
    Film drainage and rupture during coalescence between two drops approaching each other under a constant force in the presence of insoluble surfactants affected by the van der Waals forces is studied numerically. The mathematical problem is based on the coupled equations of flow in each phase and the convection-diffusion equation governing the surfactant distribution at the interface as well as the related Marangoni effects. The latter are subject to the boundary conditions in the limit of gentle interactions (small-deformations) for which the drops are nearly spherical, except in the near-contact region, where a flattened thin film forms. The finite difference method is used to discretize the lubrication equation in the gap and the interfacial diffusion-convection equation while the boundary integral method is employed to solve the flow in the drops. In this work, a parametric study is carried out by a numerical simulation for Peclet numbers (Pe) ranging from 102 to 104, initial dimensionless surfactant concentrations (Γ0*) from 0 to 10 and dimensionless Hamaker parameter (A*) from 10-7 to 10. The results indicate that the critical film thickness (hc) is strongly dependent on the combination of the three parameters which determines the predominance of van der Waals forces or those of Marangoni.