The chemical nature of most geothermal fluids poses some severe technical constraints to the util... more The chemical nature of most geothermal fluids poses some severe technical constraints to the utilisation of geothermal energy, and especially of the high-enthalpy part. Geothermal fluids contain various quantities of soluble species (up to 300 g/kg of fluid) and dissolved gases, which under the thermodynamic changes occurring during their utilisation may result in scaling (or generally speaking fouling), corrosion of the metallic surfaces in contact with the fluids and even environmental problems (emission of harmful gases, liquid disposal etc.). Another intrinsic characteristic of geothermal energy is that the geothermal fluids vary greatly from site to site and for this reason measures taken to cope with these problems in one site may not be applicable to another site. The control and prevention of the corrosion and scaling problems has evolved over the past 50 years from the empirical approach and an ‘a posteriori’ treatment (chemical and/or mechanical removal of scales, replacem...
ABSTRACT The scope of this study was to improve the Andritsos-Hanratty model for estimating inter... more ABSTRACT The scope of this study was to improve the Andritsos-Hanratty model for estimating interfacial friction factor and pressure drop in horizontal stratified gas–liquid two-phase flow. New experimental results (focusing on the effects of gas density and surface tension), combined with experimental data available in the literature, permit the development of semi-theoretical correlations for the transition from smooth stratified to 2-D wave region and from the latter to large-amplitude wave region and of different empirical relations for the interfacial friction factor in the two wave regions. The transition correlations agree reasonably well with existing and new data obtained in this work and the modified relations give improved predictions for both liquid holdup and pressure drop during gas–liquid stratified flow in horizontal and slightly downward pipelines as deduced from a statistical analysis of the results.
ABSTRACT The presence of heavy metals in aqueous systems is an intense health and environmental p... more ABSTRACT The presence of heavy metals in aqueous systems is an intense health and environmental problem as implied by their harmful effects on human and other life forms. Among them, chromium is considered as an acutely hazardous compound contaminating the surface water from industrial wastes or entering the groundwater, the major source of drinking water, by leaching of chromite rocks. Chromium occurs in two stable oxidation states, Cr(III) and Cr(VI), with the hexavalent form being much more soluble and mobile in water having the ability to enter easily into living tissues or cells and thus become more toxic. Despite the established risks from Cr(VI)-containing water consumption and the increasing number of incidents, the E.U. tolerance limit for total chromium in potable water still stands at 50 μg/L. However, in the last years a worldwide debate concerning the establishment of a separate and very strict limit for the hexavalent form takes place. In practice, Cr(VI) is usually removed from water by various methods such as chemical coagulation/filtration, ion exchange, reverse osmosis and adsorption. Adsorption is considered as the simplest method which may become very effective if the process is facilitated by the incorporation of a Cr(VI) to Cr(III) reduction stage. This work studies the potential of using magnetic nanoparticles as adsorbing agents for Cr(VI) removal at the concentration levels met in contaminated drinking water. A variety of nanoparticles consisting of ferrites MFe2O4 (M=Fe, Co, Ni, Cu, Mn, Mg, Zn) were prepared by precipitating the corresponding bivalent or trivalent sulfate salts under controlled acidity and temperature. Electron microscopy and X-ray diffraction techniques were used to verify their crystal structure and determine the morphological characteristics. The mean particle size of the samples was found in the range 10-50 nm. Batch Cr(VI) removal tests were performed in aqueous nanoparticles dispersions showing the efficiency of ferrite nanoparticles to reduce Cr(VI) concentration below the regulation limit. The removal capacity is maximized for Fe3O4 nanoparticles due to the high reducing potential of the Fe2+ cations. Furthermore, their applicability was tested in a pilot-scale magnetic separator for the continuous flow removal of nanoparticles after water treatment that takes advantage of the magnetic properties. Acknowledgment This work was implemented within the framework of the Action «Supporting Postdoctoral Researchers» of the Operational Program "Education and Lifelong Learning" (Action's Beneficiary: General Secretariat for Research and Technology), and is co-financed by the European Social Fund (ESF) and the Greek State.
ABSTRACTMagnetite nanoparticles were produced by the chemical co-precipitation of iron sulfates a... more ABSTRACTMagnetite nanoparticles were produced by the chemical co-precipitation of iron sulfates at alkaline conditions and were tested as a Cr(VI) adsorbent from water. Batch adsorption experiments showed a high removal efficiency, which is maximized at pH values below 6. This behavior was also verified in a continuous flow reactor, where nanoparticles were in contact with the polluted water. In particular, using a particle concentration of 1 g/L in water containing 100 μg Cr(VI)/L, a contact time of at least 2 h was required to achieve complete removal of Cr(VI). The recovery of nanoparticles after their use was accomplished using their magnetic nature. Application of an external magnetic field at the sides of the tube in which the suspension was flowing was sufficient to completely collect the nanoparticles in the outflow of the contact reactor, thus, providing water free of Cr(VI) and a solid phase.
ABSTRACT Traveling waves in inclined film flow in channels of finite width are never truly two-di... more ABSTRACT Traveling waves in inclined film flow in channels of finite width are never truly two-dimensional (2D) because of a long-range effect of sidewalls. The present study documents the characteristics of the first waves that are observed beyond the primary instability (termed nominally 2D) by taking measurements in a 3000 mm long inclined facility with adjustable width up to 450 mm using a fluorescence imaging technique. It is observed that nominally 2D waves are very persistent structures with their crests attaining a parabolic shape, which is symmetric with respect to the channel centerplane irrespective of the 3D content of the inlet forcing. The apex curvature of the parabola varies inversely with channel width and Reynolds number. The wave height is maximum at the centerplane and decreases to zero at the sidewalls, irrespective of the wetting properties of the system. The linear phase velocity of nominally 2D waves is always lower than predicted by the theory for small amplitude, 2D waves, and significantly in narrow channels and/or small inclinations. The above characteristics are shown to explain discrepancies between theory and observations, in particular the recently reported deviation of the onset of the primary instability from the classical prediction [M. Vlachogiannis et al., Phys. Fluids 22, 012106 (2010)].
The quality of a final thin film is essentially determined by the processes taking place at incip... more The quality of a final thin film is essentially determined by the processes taking place at incipient CdS deposition, which in turn are strongly influenced by the physicochemical properties of the substrate and liquid in contact. SEM pictures of deposits formed through steady flow of a supersaturated (with respect to CdS) solution suggest that initially nuclei are continuously generated on the substrate and grow as discrete "surface" particles. With time, these particles tend to "coalesce" with neighboring ones, while new nuclei keep forming and growing, leading to the formation of a coherent film. There is evidence that similar growth patterns prevail in CdS deposition via the chemical bath deposition (CBD) process. Based on experimental observations, a simple model is developed, which is capable of predicting macroscopically determined film characteristics such as the temporal thickness evolution including the "induction period." Two cases of the growth pattern are examined theoretically; one based on instantaneous surface nucleation (due to its simplicity) and another with a constant surface nucleation rate, which appears to be closer to experimental observations.
The chemical nature of most geothermal fluids poses some severe technical constraints to the util... more The chemical nature of most geothermal fluids poses some severe technical constraints to the utilisation of geothermal energy, and especially of the high-enthalpy part. Geothermal fluids contain various quantities of soluble species (up to 300 g/kg of fluid) and dissolved gases, which under the thermodynamic changes occurring during their utilisation may result in scaling (or generally speaking fouling), corrosion of the metallic surfaces in contact with the fluids and even environmental problems (emission of harmful gases, liquid disposal etc.). Another intrinsic characteristic of geothermal energy is that the geothermal fluids vary greatly from site to site and for this reason measures taken to cope with these problems in one site may not be applicable to another site. The control and prevention of the corrosion and scaling problems has evolved over the past 50 years from the empirical approach and an ‘a posteriori’ treatment (chemical and/or mechanical removal of scales, replacem...
ABSTRACT The scope of this study was to improve the Andritsos-Hanratty model for estimating inter... more ABSTRACT The scope of this study was to improve the Andritsos-Hanratty model for estimating interfacial friction factor and pressure drop in horizontal stratified gas–liquid two-phase flow. New experimental results (focusing on the effects of gas density and surface tension), combined with experimental data available in the literature, permit the development of semi-theoretical correlations for the transition from smooth stratified to 2-D wave region and from the latter to large-amplitude wave region and of different empirical relations for the interfacial friction factor in the two wave regions. The transition correlations agree reasonably well with existing and new data obtained in this work and the modified relations give improved predictions for both liquid holdup and pressure drop during gas–liquid stratified flow in horizontal and slightly downward pipelines as deduced from a statistical analysis of the results.
ABSTRACT The presence of heavy metals in aqueous systems is an intense health and environmental p... more ABSTRACT The presence of heavy metals in aqueous systems is an intense health and environmental problem as implied by their harmful effects on human and other life forms. Among them, chromium is considered as an acutely hazardous compound contaminating the surface water from industrial wastes or entering the groundwater, the major source of drinking water, by leaching of chromite rocks. Chromium occurs in two stable oxidation states, Cr(III) and Cr(VI), with the hexavalent form being much more soluble and mobile in water having the ability to enter easily into living tissues or cells and thus become more toxic. Despite the established risks from Cr(VI)-containing water consumption and the increasing number of incidents, the E.U. tolerance limit for total chromium in potable water still stands at 50 μg/L. However, in the last years a worldwide debate concerning the establishment of a separate and very strict limit for the hexavalent form takes place. In practice, Cr(VI) is usually removed from water by various methods such as chemical coagulation/filtration, ion exchange, reverse osmosis and adsorption. Adsorption is considered as the simplest method which may become very effective if the process is facilitated by the incorporation of a Cr(VI) to Cr(III) reduction stage. This work studies the potential of using magnetic nanoparticles as adsorbing agents for Cr(VI) removal at the concentration levels met in contaminated drinking water. A variety of nanoparticles consisting of ferrites MFe2O4 (M=Fe, Co, Ni, Cu, Mn, Mg, Zn) were prepared by precipitating the corresponding bivalent or trivalent sulfate salts under controlled acidity and temperature. Electron microscopy and X-ray diffraction techniques were used to verify their crystal structure and determine the morphological characteristics. The mean particle size of the samples was found in the range 10-50 nm. Batch Cr(VI) removal tests were performed in aqueous nanoparticles dispersions showing the efficiency of ferrite nanoparticles to reduce Cr(VI) concentration below the regulation limit. The removal capacity is maximized for Fe3O4 nanoparticles due to the high reducing potential of the Fe2+ cations. Furthermore, their applicability was tested in a pilot-scale magnetic separator for the continuous flow removal of nanoparticles after water treatment that takes advantage of the magnetic properties. Acknowledgment This work was implemented within the framework of the Action «Supporting Postdoctoral Researchers» of the Operational Program "Education and Lifelong Learning" (Action's Beneficiary: General Secretariat for Research and Technology), and is co-financed by the European Social Fund (ESF) and the Greek State.
ABSTRACTMagnetite nanoparticles were produced by the chemical co-precipitation of iron sulfates a... more ABSTRACTMagnetite nanoparticles were produced by the chemical co-precipitation of iron sulfates at alkaline conditions and were tested as a Cr(VI) adsorbent from water. Batch adsorption experiments showed a high removal efficiency, which is maximized at pH values below 6. This behavior was also verified in a continuous flow reactor, where nanoparticles were in contact with the polluted water. In particular, using a particle concentration of 1 g/L in water containing 100 μg Cr(VI)/L, a contact time of at least 2 h was required to achieve complete removal of Cr(VI). The recovery of nanoparticles after their use was accomplished using their magnetic nature. Application of an external magnetic field at the sides of the tube in which the suspension was flowing was sufficient to completely collect the nanoparticles in the outflow of the contact reactor, thus, providing water free of Cr(VI) and a solid phase.
ABSTRACT Traveling waves in inclined film flow in channels of finite width are never truly two-di... more ABSTRACT Traveling waves in inclined film flow in channels of finite width are never truly two-dimensional (2D) because of a long-range effect of sidewalls. The present study documents the characteristics of the first waves that are observed beyond the primary instability (termed nominally 2D) by taking measurements in a 3000 mm long inclined facility with adjustable width up to 450 mm using a fluorescence imaging technique. It is observed that nominally 2D waves are very persistent structures with their crests attaining a parabolic shape, which is symmetric with respect to the channel centerplane irrespective of the 3D content of the inlet forcing. The apex curvature of the parabola varies inversely with channel width and Reynolds number. The wave height is maximum at the centerplane and decreases to zero at the sidewalls, irrespective of the wetting properties of the system. The linear phase velocity of nominally 2D waves is always lower than predicted by the theory for small amplitude, 2D waves, and significantly in narrow channels and/or small inclinations. The above characteristics are shown to explain discrepancies between theory and observations, in particular the recently reported deviation of the onset of the primary instability from the classical prediction [M. Vlachogiannis et al., Phys. Fluids 22, 012106 (2010)].
The quality of a final thin film is essentially determined by the processes taking place at incip... more The quality of a final thin film is essentially determined by the processes taking place at incipient CdS deposition, which in turn are strongly influenced by the physicochemical properties of the substrate and liquid in contact. SEM pictures of deposits formed through steady flow of a supersaturated (with respect to CdS) solution suggest that initially nuclei are continuously generated on the substrate and grow as discrete "surface" particles. With time, these particles tend to "coalesce" with neighboring ones, while new nuclei keep forming and growing, leading to the formation of a coherent film. There is evidence that similar growth patterns prevail in CdS deposition via the chemical bath deposition (CBD) process. Based on experimental observations, a simple model is developed, which is capable of predicting macroscopically determined film characteristics such as the temporal thickness evolution including the "induction period." Two cases of the growth pattern are examined theoretically; one based on instantaneous surface nucleation (due to its simplicity) and another with a constant surface nucleation rate, which appears to be closer to experimental observations.
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