2016 Diagnostic of Electrical Machines and Insulating Systems in Electrical Engineering (DEMISEE), 2016
The corrosiveness and thermal stability of five types of insulating oils - both in contact and co... more The corrosiveness and thermal stability of five types of insulating oils - both in contact and contactless with copper and S235J2G3 carbon steel - have been studied through gravimetric determination, XRF analysis and dynamic viscosity measurements. The experimental results showed that, after a heat treatment (700 hours) at 110± 30°C, the weight of the investigated samples by mineral oil drops (by volatilization) to be between 7 and 7.5%. For vegetable oils have not been registered remarkable changes in weight. The XRF analyzes revealed that the investigated new minerals oils have a sulfur content up to 0.03% and the used oil sample (30 years in transformer) up to 0.14%. Sulfur content in the vegetable oils has been not recorded. Corrosion of copper and carbon steel in the investigated oils increased due to increasing the sulfur content.
The paper presents the analytical solution used to calculate the depression which appears under d... more The paper presents the analytical solution used to calculate the depression which appears under different types of impellers. This solution can be useful as a preliminary calculus to estimate the magnitude of the depression created at different rotational speeds, in order not to damage the equipment from different systems, such as hydraulic turbines. The authors obtain the variable radius of the depression zone, the pressure inside the depression zone and the depression height. The theoretical approach, the calculus itself and the numerical results presented in the article are original. In order to validate the proposed calculus, experiments were carried out using two experimental setups. The performed measurements showed a good agreement of the experimental and theoretical results.
The paper presents the operation of a hydraulic closed circuit equipped with a butterfly valve wh... more The paper presents the operation of a hydraulic closed circuit equipped with a butterfly valve which can close with a step of 10o. Using four pressure transducers and one temperature transducer, the authors determined the volumetric flow rate, the average water velocity and the local pressure loss in the butterfly valve, the flow coefficient Kv and also the incipient cavitation coefficient. Recommendations for the disposal of the butterfly valve are made and conclusions are obtained regarding the range of opening degrees in which the butterfly valve is better to be used.
The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentrat... more The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentration of the suspension and low specific energy consumption (kW/m3) Keywords: mixing reactor, homogenization process, draft tube, fluid flow Mixing processes are used in numerous industrial branches (chemical, pharmaceutical, food and metallurgical process industries) as well as in municipal wastewater treatment, in dispersion applications such as liquid-solid, liquid-gas, liquid-solid-gas etc The homogenization of the substances leading to a uniform value of the concentration, the heat transfer and the shortening of the reaction time in certain industrial processes are achieved through mixing. This process also avoids suspension sedimentation (which leads to a decrease in the effective volume of the vessel), scum formation and particles agglomeration. Different studies were elaborated referring to mixing processes [2-4], industrial reactors In this paper the authors aim to determine the s...
The main objective of the paper is to determine the influence of the velocity induced by a grid o... more The main objective of the paper is to determine the influence of the velocity induced by a grid of bubble columns on homogenization and mass transfer. The paper studies experimentally the flow field induced by a grid of porous diffusers placed in a tank and the interaction among the bubble columns. In the cylindrical experimental tank five diffusers are immersed and distributed at certain distances so that the columns to not interact. The diffusers are plates Ø120, of sintered glass obtained from energetic wastes. To evaluate the effect of the flow induced at different air flow rates, the variation of dissolved oxygen (DO) with time, for a constant water volume is experimentally determined. The two sets of measurements are simultaneously performed using specific devices. For determining instantaneous velocities at different points in the water volume, a MicroADV operating on Doppler principle is used. After data processing the evolution of average velocities with variation of the su...
The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentrat... more The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentration of the suspension and low specific energy consumption (kW/m3). For a reactor with given geometry and volume, free or intubated impellers are used for the mixing of the suspension. In this work is studied the influence of the draft tube length. The streamlines corresponding to the flow of water are obtained through the numerical integration of Navier-Stokes equations. The increasing of the draft tube length modifies the flow, leading to the reduction of the number and intensity of the vortices inside the reactor (outside the draft tube). These vortices can lead to suspensions agglomeration, facilitating the sedimentation and thus increasing the energy consumption of the homogenization process.
The main objective of the paper is to solve two problems of the Clabucet Hydropower, caused by th... more The main objective of the paper is to solve two problems of the Clabucet Hydropower, caused by the inlet piping system. The authors are trying to eliminate the air emulsions and the sediments which enter in the main inlet pipe PecineaguClabucet. The paper is based on the experimental studies performed in the Hydroenergy laboratory of Power Engineering Faculty from Politehnica University of Bucharest. Two installations are used at a proper scale and the results are extended for real scale installations using the similitude theory. Both installations represent original models and are used to predict results at real scale.
This paper focuses on bubble size distribution (BSD) in a rectangular column of 0.3×0.3 m cross s... more This paper focuses on bubble size distribution (BSD) in a rectangular column of 0.3×0.3 m cross sectional area and 1.1 m height, equipped with a ceramic porous diffuser of 50 mm diameter. Different values of average bubble size and bubble rise velocities are reported in literature due to the differences in the distributor design, column diameter and range of gas velocities. BSD is an important parameter for determining the volumetric mass transfer coefficient. A high speed digital video camera is used for direct flow visualizations and, combined with image processing, for BSD. Experimental results show that the structure of air-water dispersed system changes with increasing superficial gas velocity, from a homogeneous regime characterized by small spherical bubbles and low bubble density, to a heterogeneous regime, where bubble deformation and density increase
The paper aims to present the investigation of H2 adsorption capacity in metal doped nanostructur... more The paper aims to present the investigation of H2 adsorption capacity in metal doped nanostructured materials, by using two methods. Carbonic materials are considered to be one of the most promising materials to be used for hydrogen adsorption and storage. They have different applications and one of the most important is considered to be fuel cells technology. By using metals for doping these materials, the adsorption capacity increases, thus approaching the target of 6.5% weight ratio of H2 adsorbed in a substrate. Within these investigations multi-wall nanotubes and poly-aniline have been used as substrates. The poly-aniline has been prepared and doped in laboratory while the nanotubes used in experiments have been purchased from the market and afterwards doped in laboratory. The doping procedure consists of a physical-chemical method which involves salts of the metal for doping and the use of ultrasounds in order to activate the substrate for doping. The adsorption capacity of th...
2016 Diagnostic of Electrical Machines and Insulating Systems in Electrical Engineering (DEMISEE), 2016
ZnO based surge protective devices have been made and characterized both morphological (SEM micro... more ZnO based surge protective devices have been made and characterized both morphological (SEM microscopy and interferometry) and electrical (U/I characteristic and dielectric spectroscopy). SEM microscopy analyzes revealed that the disks of metal oxides sintered at 12000C present a homogeneous distribution of ZnO grains, without pores and well-defined intergranular limits. It was also found that Ag electrodes (applied by Ag paste to 650°C) are well anchored in the structure of the sintered material. The interferometry measurements revealed that the sintered material surface roughness ranges within ± 3μm. The electrical measurements revealed that the threshold voltage of the achieved devices is proportional to the thickness of the disks of approximately 156V / mm. Reverse currents, measured at 0.8 of the threshold voltage were between 0.28 and 0.31mA. Also, the measurements of dielectric spectroscopy revealed that the dielectric loss is highly dependent on frequency, respectively about 0.15 at 40 Hz and decreases exponentially to 0.019 at 10 kHz.
The structural and physicochemical changes that lead to the aging of the bituminous insulation of... more The structural and physicochemical changes that lead to the aging of the bituminous insulation of underground metallic pipelines were determined through various investigations (XRD, dielectric spectroscopy and thermal analysis � coupled techniques TG + DTA + DTG). The resistance to moulds of the bituminous insulation was evaluated by using specific microbiological methods and the dominant microbiological species found in the soil where the investigated bituminous material was exposed for 19 years were identified. The measurements carried out on samples taken from site showed that the bituminous material used in the anti-corrosive isolation of steel pipes has a very low resistance to microorganisms; thus, the dielectric qualities of bitumen deteriorate substantially (tg� increases 3-4 times). The exposure of the bituminous insulation to different climatic conditions and to solar radiation (UV-IR) leads to a decrease in the dielectric losses in the insulation and to an increase in cry...
The corrosion state of an underground metallic pipeline of �161 mm and 565 m length was assessed ... more The corrosion state of an underground metallic pipeline of �161 mm and 565 m length was assessed by specific electrical and electrochemical measurements. The investigated pipe, buried in 1997, was protected against corrosion by successive layers of bituminous material with a total thickness of 1 to 1.2 mm. The pipeline crosses three electrified railway lines (50 Hz - 28 kV), and then its route is approximately parallel to these lines; thus, the induced AC voltages between line and ground were calculated obtaining values between 4.05 and 7.1 Vrms, in good agreement with the values measured in the accessible points. The measurements regarding the insulation capacity against corrosion of the bituminous insulation, performed at one month and after 19 years of burial, showed an increase of the average cathode current density needed for obtaining the protection potential in the range �1.00 � �1.28 VCu/CuSO4 of approx. three times (from 6.65 up to 19.96 mA/m2), in good agreement with the e...
The behaviour of some insulating fluids in contact with copper foil and / or insulation paper und... more The behaviour of some insulating fluids in contact with copper foil and / or insulation paper under thermal stress (110�30C for 1000 hours), in a closed system (the access of atmospheric oxygen being limited), has been studied by determining the changes in viscosity and concentration of CO2 and CO. The experimental data revealed that, following the applied heat treatment, the change in viscosity of the esters-based insulating fluids (both synthetic and vegetable) is approximately 7 times lower than in the case of the investigated mineral oils. It has also been found that, following the thermal ageing, the gas content of the mineral oils is substantially higher than in the esters-based oils (8 times higher for CO2 and 4 times higher for CO, respectively). The experimental results indicate superior values for the thermal stability and compatibility with the insulation paper of ester-based insulating fluids.
This paper presents an exhaustive study regarding the behaviour of copper in various insulating f... more This paper presents an exhaustive study regarding the behaviour of copper in various insulating fluids when exposed to 1100C for 1000 hours in closed systems, with no access to atmospheric oxygen. Following the analysis carried out, it was found that the variation of oxygen concentration in the investigated oil samples, during the thermal treatment, may be best described using an exponential decrease versus time. Moreover, from the gas chromatography determinations with respect to the evolution of the CO and CO2 content in the oils during the thermal treatment one found out that these fit very well the Richards model, which corroborated with all the other analysis confirms the fact that the copper films undergo consistent morphological changes only when exposed to mineral oils, so that it is of paramount importance that, taking into account this aggressive character, the equipment placed in direct contact with these oils should be carefully inspected on a periodical basis. This beha...
2016 Diagnostic of Electrical Machines and Insulating Systems in Electrical Engineering (DEMISEE), 2016
The corrosiveness and thermal stability of five types of insulating oils - both in contact and co... more The corrosiveness and thermal stability of five types of insulating oils - both in contact and contactless with copper and S235J2G3 carbon steel - have been studied through gravimetric determination, XRF analysis and dynamic viscosity measurements. The experimental results showed that, after a heat treatment (700 hours) at 110± 30°C, the weight of the investigated samples by mineral oil drops (by volatilization) to be between 7 and 7.5%. For vegetable oils have not been registered remarkable changes in weight. The XRF analyzes revealed that the investigated new minerals oils have a sulfur content up to 0.03% and the used oil sample (30 years in transformer) up to 0.14%. Sulfur content in the vegetable oils has been not recorded. Corrosion of copper and carbon steel in the investigated oils increased due to increasing the sulfur content.
The paper presents the analytical solution used to calculate the depression which appears under d... more The paper presents the analytical solution used to calculate the depression which appears under different types of impellers. This solution can be useful as a preliminary calculus to estimate the magnitude of the depression created at different rotational speeds, in order not to damage the equipment from different systems, such as hydraulic turbines. The authors obtain the variable radius of the depression zone, the pressure inside the depression zone and the depression height. The theoretical approach, the calculus itself and the numerical results presented in the article are original. In order to validate the proposed calculus, experiments were carried out using two experimental setups. The performed measurements showed a good agreement of the experimental and theoretical results.
The paper presents the operation of a hydraulic closed circuit equipped with a butterfly valve wh... more The paper presents the operation of a hydraulic closed circuit equipped with a butterfly valve which can close with a step of 10o. Using four pressure transducers and one temperature transducer, the authors determined the volumetric flow rate, the average water velocity and the local pressure loss in the butterfly valve, the flow coefficient Kv and also the incipient cavitation coefficient. Recommendations for the disposal of the butterfly valve are made and conclusions are obtained regarding the range of opening degrees in which the butterfly valve is better to be used.
The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentrat... more The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentration of the suspension and low specific energy consumption (kW/m3) Keywords: mixing reactor, homogenization process, draft tube, fluid flow Mixing processes are used in numerous industrial branches (chemical, pharmaceutical, food and metallurgical process industries) as well as in municipal wastewater treatment, in dispersion applications such as liquid-solid, liquid-gas, liquid-solid-gas etc The homogenization of the substances leading to a uniform value of the concentration, the heat transfer and the shortening of the reaction time in certain industrial processes are achieved through mixing. This process also avoids suspension sedimentation (which leads to a decrease in the effective volume of the vessel), scum formation and particles agglomeration. Different studies were elaborated referring to mixing processes [2-4], industrial reactors In this paper the authors aim to determine the s...
The main objective of the paper is to determine the influence of the velocity induced by a grid o... more The main objective of the paper is to determine the influence of the velocity induced by a grid of bubble columns on homogenization and mass transfer. The paper studies experimentally the flow field induced by a grid of porous diffusers placed in a tank and the interaction among the bubble columns. In the cylindrical experimental tank five diffusers are immersed and distributed at certain distances so that the columns to not interact. The diffusers are plates Ø120, of sintered glass obtained from energetic wastes. To evaluate the effect of the flow induced at different air flow rates, the variation of dissolved oxygen (DO) with time, for a constant water volume is experimentally determined. The two sets of measurements are simultaneously performed using specific devices. For determining instantaneous velocities at different points in the water volume, a MicroADV operating on Doppler principle is used. After data processing the evolution of average velocities with variation of the su...
The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentrat... more The efficiency of a suspension homogenization/mixing reactor is evaluated by a uniform concentration of the suspension and low specific energy consumption (kW/m3). For a reactor with given geometry and volume, free or intubated impellers are used for the mixing of the suspension. In this work is studied the influence of the draft tube length. The streamlines corresponding to the flow of water are obtained through the numerical integration of Navier-Stokes equations. The increasing of the draft tube length modifies the flow, leading to the reduction of the number and intensity of the vortices inside the reactor (outside the draft tube). These vortices can lead to suspensions agglomeration, facilitating the sedimentation and thus increasing the energy consumption of the homogenization process.
The main objective of the paper is to solve two problems of the Clabucet Hydropower, caused by th... more The main objective of the paper is to solve two problems of the Clabucet Hydropower, caused by the inlet piping system. The authors are trying to eliminate the air emulsions and the sediments which enter in the main inlet pipe PecineaguClabucet. The paper is based on the experimental studies performed in the Hydroenergy laboratory of Power Engineering Faculty from Politehnica University of Bucharest. Two installations are used at a proper scale and the results are extended for real scale installations using the similitude theory. Both installations represent original models and are used to predict results at real scale.
This paper focuses on bubble size distribution (BSD) in a rectangular column of 0.3×0.3 m cross s... more This paper focuses on bubble size distribution (BSD) in a rectangular column of 0.3×0.3 m cross sectional area and 1.1 m height, equipped with a ceramic porous diffuser of 50 mm diameter. Different values of average bubble size and bubble rise velocities are reported in literature due to the differences in the distributor design, column diameter and range of gas velocities. BSD is an important parameter for determining the volumetric mass transfer coefficient. A high speed digital video camera is used for direct flow visualizations and, combined with image processing, for BSD. Experimental results show that the structure of air-water dispersed system changes with increasing superficial gas velocity, from a homogeneous regime characterized by small spherical bubbles and low bubble density, to a heterogeneous regime, where bubble deformation and density increase
The paper aims to present the investigation of H2 adsorption capacity in metal doped nanostructur... more The paper aims to present the investigation of H2 adsorption capacity in metal doped nanostructured materials, by using two methods. Carbonic materials are considered to be one of the most promising materials to be used for hydrogen adsorption and storage. They have different applications and one of the most important is considered to be fuel cells technology. By using metals for doping these materials, the adsorption capacity increases, thus approaching the target of 6.5% weight ratio of H2 adsorbed in a substrate. Within these investigations multi-wall nanotubes and poly-aniline have been used as substrates. The poly-aniline has been prepared and doped in laboratory while the nanotubes used in experiments have been purchased from the market and afterwards doped in laboratory. The doping procedure consists of a physical-chemical method which involves salts of the metal for doping and the use of ultrasounds in order to activate the substrate for doping. The adsorption capacity of th...
2016 Diagnostic of Electrical Machines and Insulating Systems in Electrical Engineering (DEMISEE), 2016
ZnO based surge protective devices have been made and characterized both morphological (SEM micro... more ZnO based surge protective devices have been made and characterized both morphological (SEM microscopy and interferometry) and electrical (U/I characteristic and dielectric spectroscopy). SEM microscopy analyzes revealed that the disks of metal oxides sintered at 12000C present a homogeneous distribution of ZnO grains, without pores and well-defined intergranular limits. It was also found that Ag electrodes (applied by Ag paste to 650°C) are well anchored in the structure of the sintered material. The interferometry measurements revealed that the sintered material surface roughness ranges within ± 3μm. The electrical measurements revealed that the threshold voltage of the achieved devices is proportional to the thickness of the disks of approximately 156V / mm. Reverse currents, measured at 0.8 of the threshold voltage were between 0.28 and 0.31mA. Also, the measurements of dielectric spectroscopy revealed that the dielectric loss is highly dependent on frequency, respectively about 0.15 at 40 Hz and decreases exponentially to 0.019 at 10 kHz.
The structural and physicochemical changes that lead to the aging of the bituminous insulation of... more The structural and physicochemical changes that lead to the aging of the bituminous insulation of underground metallic pipelines were determined through various investigations (XRD, dielectric spectroscopy and thermal analysis � coupled techniques TG + DTA + DTG). The resistance to moulds of the bituminous insulation was evaluated by using specific microbiological methods and the dominant microbiological species found in the soil where the investigated bituminous material was exposed for 19 years were identified. The measurements carried out on samples taken from site showed that the bituminous material used in the anti-corrosive isolation of steel pipes has a very low resistance to microorganisms; thus, the dielectric qualities of bitumen deteriorate substantially (tg� increases 3-4 times). The exposure of the bituminous insulation to different climatic conditions and to solar radiation (UV-IR) leads to a decrease in the dielectric losses in the insulation and to an increase in cry...
The corrosion state of an underground metallic pipeline of �161 mm and 565 m length was assessed ... more The corrosion state of an underground metallic pipeline of �161 mm and 565 m length was assessed by specific electrical and electrochemical measurements. The investigated pipe, buried in 1997, was protected against corrosion by successive layers of bituminous material with a total thickness of 1 to 1.2 mm. The pipeline crosses three electrified railway lines (50 Hz - 28 kV), and then its route is approximately parallel to these lines; thus, the induced AC voltages between line and ground were calculated obtaining values between 4.05 and 7.1 Vrms, in good agreement with the values measured in the accessible points. The measurements regarding the insulation capacity against corrosion of the bituminous insulation, performed at one month and after 19 years of burial, showed an increase of the average cathode current density needed for obtaining the protection potential in the range �1.00 � �1.28 VCu/CuSO4 of approx. three times (from 6.65 up to 19.96 mA/m2), in good agreement with the e...
The behaviour of some insulating fluids in contact with copper foil and / or insulation paper und... more The behaviour of some insulating fluids in contact with copper foil and / or insulation paper under thermal stress (110�30C for 1000 hours), in a closed system (the access of atmospheric oxygen being limited), has been studied by determining the changes in viscosity and concentration of CO2 and CO. The experimental data revealed that, following the applied heat treatment, the change in viscosity of the esters-based insulating fluids (both synthetic and vegetable) is approximately 7 times lower than in the case of the investigated mineral oils. It has also been found that, following the thermal ageing, the gas content of the mineral oils is substantially higher than in the esters-based oils (8 times higher for CO2 and 4 times higher for CO, respectively). The experimental results indicate superior values for the thermal stability and compatibility with the insulation paper of ester-based insulating fluids.
This paper presents an exhaustive study regarding the behaviour of copper in various insulating f... more This paper presents an exhaustive study regarding the behaviour of copper in various insulating fluids when exposed to 1100C for 1000 hours in closed systems, with no access to atmospheric oxygen. Following the analysis carried out, it was found that the variation of oxygen concentration in the investigated oil samples, during the thermal treatment, may be best described using an exponential decrease versus time. Moreover, from the gas chromatography determinations with respect to the evolution of the CO and CO2 content in the oils during the thermal treatment one found out that these fit very well the Richards model, which corroborated with all the other analysis confirms the fact that the copper films undergo consistent morphological changes only when exposed to mineral oils, so that it is of paramount importance that, taking into account this aggressive character, the equipment placed in direct contact with these oils should be carefully inspected on a periodical basis. This beha...
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Papers by Gabriela Oprina