Lithium-sulfur(Li-S) batteries have generated significant attention due to their high theoretical specific capacity and energy density among a host of energy storage power devices.Nevertheless,the lithium polysulfide ...Lithium-sulfur(Li-S) batteries have generated significant attention due to their high theoretical specific capacity and energy density among a host of energy storage power devices.Nevertheless,the lithium polysulfide dissolution shuttle that occurs within Li-S batteries will lead to capacity deterioration and inadequate cycling stability.In the paper,we proposed a measure to deal with the above problems by modifying the separator with Nd_(2)O_(3)/graphene composite in Li-S batteries.Graphene's special chemical properties and structural qualities make it an excellent choice for Li-S batteries.Meanwhile,Nd_(2)O_(3) has a strong binding affinity with lithium polysulfide due to its low electronegativity,which exhibits Lewis' s acidity and forms strong interactions with lithium polysulfide,which is strongly Lewis basic.By utilizing these advantageous properties,Li-S batteries assembling Nd_(2)O_(3)-decorated reduced graphite oxide modified polypropylene separators(Nd_(2)O_(3)/RGO/PP) demonstrate outstanding electrochemical performance,including a mere 0.0525% capacity attenuation rate under 2C during 1000 cycles and with exceptional rate performance of 614 mAh/g even at 3C.This study presents valuable knowledge for effectively modifying separators using rare earth oxides to incorporate graphene,ultimately promoting the practical application of Li-S batteries.展开更多
The effect of Nd addition on the microstructure and mechanical properties of ZK20 magnesium alloy was investigated by room tensile test, optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SE...The effect of Nd addition on the microstructure and mechanical properties of ZK20 magnesium alloy was investigated by room tensile test, optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to develop a magnesium alloy with higher ductility. Results showed that the crystal grains of as-extruded ZK20+0.5%Nd magnesium alloy were effectively refined, and the alloy exhibited higher strength and ductility, with the UTS of 237 MPa and the elongation of 32.8%, increasing by 5...展开更多
3D printing technology, as one of the most subversive technologies in the 21st century, processes the entity through layer by layer superposition. It breaks through the design constraints and adapts to customized prod...3D printing technology, as one of the most subversive technologies in the 21st century, processes the entity through layer by layer superposition. It breaks through the design constraints and adapts to customized production with high flexibility, which is an important direction of industrial development in the future. 3C product has a very board application market and groups, so it has been widely valued over the world. The advantages of 3D printing technology are very suitable for the characteristics of 3C product, such as diversification, small-volume, personalized. Here, the impact of 3D printing technology on manufacturing industry is firstly discussed;3C product manufacturing system integrated with 3D printing is finally put proposed. The advantages of the proposed system possess shorter manufacturing cycle, supply chain and logistics. It makes easier to realize personalized 3C product, localized manufacturing and reduce cost. The development and wide application of 3D printing technology in 3C product industry will make the manufacture of low-cost small batch or even individual customized products into a general mode towards the trend of “light luxury + light customization”.展开更多
In order to study the mechanical and triboloical properties of powder metallurgy (PM) parts under different process parameters, the specimens were used in pack carburizing processes. These specimens made from industri...In order to study the mechanical and triboloical properties of powder metallurgy (PM) parts under different process parameters, the specimens were used in pack carburizing processes. These specimens made from industrial test pieces were carburized in a powder pack for about two to five hours at a temperature of about 850?C - 950?C. The effects of austenitization and quenching are investigated on some specimens. Also the wear tests are performed by means of a pin-on-disc tribotester using roll bearing steel as the counterface material. The results indicate that by appropriate selection of process parameters, it is possible to obtain high wear resistance along with moderate toughness. It is concluded that surface treatments increases the wear resistance and performance of PM parts in service conditions. By increasing the role of PM in industry which resulted from their ability to produce the complex shapes, high production rate, and dimension accuracy of final products, they need to be heat treated. Carburizing method was selected as a surface hardening method for PM parts. Results of wear and hardness show considerable enhancement in mechanical properties of PM parts.展开更多
The formation and evolution laws of the defect temperature field,heat dissipation in the process of defect evolution were studied.On the basis,the formation and evolution laws of the defect temperature field were inve...The formation and evolution laws of the defect temperature field,heat dissipation in the process of defect evolution were studied.On the basis,the formation and evolution laws of the defect temperature field were investigated,the interaction among defects in the process of defect evolution was carried out.The numerical simulation of the temperature field of ABS was made.The results show that the process of defect evolution is one of energy dissipation,in which the defect temperature field forms due to that its heat dissipation possesses fractal property and its fractal dimension not only relates to the interaction among the defects,but also is the function of time,this incarnates the efficiency of coordinated actions of striding over the different gradations in the process of defect evolution and among gradations.The increase of the local temperature with the increase of deformation-induced heating effect in ABS is obvious.Moreover,the shape of plastic zone and inner heat source density function has big effect on the temperature field.展开更多
A magnetorheological (MR) fluids brake is a device to achieve brake by shear force of MR fluids. A MR rotary brake has the property that its braking torque changes quickly in response to an external magnetic field. In...A magnetorheological (MR) fluids brake is a device to achieve brake by shear force of MR fluids. A MR rotary brake has the property that its braking torque changes quickly in response to an external magnetic field. In this study, the design method of the circular plate MR fluids brake is investigated theoretically. The equation of the torque transmitted by the MR fluids in the brake is derived to provide the theoretical foundation in the design of the brake. Based on this equation, after mathematically manipulated, the calculations of the volume, thickness and width of the MR fluids within the circular plate MR fluids brake are yield.展开更多
The modelling of one kind of nonlinear parabolic distributed parameter control system with moving boundary, which had extensive applications was presented, Two methods were used to investigate the basic characteristic...The modelling of one kind of nonlinear parabolic distributed parameter control system with moving boundary, which had extensive applications was presented, Two methods were used to investigate the basic characteristics of the system: I) transforming the system it? the variable domain into that in the fixed domain; 2) transforming the distributed parameter system into the lumped parameter system. It is found that there are two critical values for the control variable : the larger one determines whether or not the boundary would move, while the smaller one determines whether or not the boundary, would atop automatically. For one-dimensional system of planar, cylindrical and spherical cases the definite solution problem can be expressed as a unified form. By means of the computer simulation the open-loop control system and close-cycle feedback control system have been investigated. Numerical results agree well with theoretical results. The computer simulation shows that the system is well posed, stable, measurable and controllable.展开更多
A type of mesh called a body-fi tted Cartesian mesh,very different from the traditional structured body-fi tted mesh,is established.At f irst,the right parallelepiped mesh is generated,then,a feature analysis is done ...A type of mesh called a body-fi tted Cartesian mesh,very different from the traditional structured body-fi tted mesh,is established.At f irst,the right parallelepiped mesh is generated,then,a feature analysis is done on the cross sections.These cross sections are the intersections of the casting shape with the right parallelepiped grids(under the Cartesian coordinate system).On the basis of the feature analysis,two sorts of body-f itted boundary grids,shape-keeping grids and shape-distortion grids,are def ined.Shape-distortion grids can be removed or weaken by increasing the number of grids or moving the coordinates of the mesh generation region,so actually the body-fitted Cartesian mesh generation is to get shape-keeping grids.A shape-keeping grid mainly consists of two sorts of surfaces(I type face and II type face),and each of them is joined by two types of points(I type point and II type point).If only these two types of points were given,the shape-keeping mesh would be constructed.In this paper,the cases of the above two boundary grids being generated were discussed.An algorithm was put forward to get the shape-keeping grids.Several body-fi tted Cartesian meshes generated on castings show the validity of the algorithm.The mesh generation examples show that the body-fi tted Cartesian mesh is more excellent than the right parallelepiped mesh in aspects of decreasing grids number and being closer to the shape of the casting solid.展开更多
The bubble radius at the early stage of discharge in water is investigated using high-speed photography. Some simulation results on the bubble radius are presented, which are in agreement with the experimental results...The bubble radius at the early stage of discharge in water is investigated using high-speed photography. Some simulation results on the bubble radius are presented, which are in agreement with the experimental results, with a maximum difference of about 10%. The reasons why the peak pressure of the first shock wave is only related to the energy released in the bubble during the first half period are addressed. The energy released in the bubble after the first half period increases the bubble pulsation period, but it produces no more than 10% under the peak pressure of the second shock wave.展开更多
To realize the goal of harvesting energy from pavement vibration on a large scale, a new type of piezoelectric harvesting units as the energy transducer has been proposed. The piezoelectric harvesting units are paved ...To realize the goal of harvesting energy from pavement vibration on a large scale, a new type of piezoelectric harvesting units as the energy transducer has been proposed. The piezoelectric harvesting units are paved 40 mm below the asphalt, which is the same as thickness of the top layer of typical asphalt pavement in China. The spacing distance is 2200 mm, which is the same as the one between two tires of a normal vehicle. A mathematical model of the unit is deduced on Meda empirical formula and Hamilton principle and piezoelectric equations. Effects of the external vehicle load on its harvesting energy properties and pavement deformation and stress are analyzed with the finite element method. The results show that the excited voltage is linearly variation with contact pressure while the harvested electrical energy exponential varies with contact pressure. The more the contact pressure is, the larger the harvested electrical energy and the deformation and stress are. The harvested electrical energy also increases with the load frequency. At least 100 mJ of electrical energy can be collected with the proposed piezoelectric harvesting unit. It shows that the technology application of the piezoelectric harvesting energy from pavement is promising.展开更多
A general theory of optical parametric generation that accounts for pump depletion, loss, phase mismatch, group-velocity mismatch among the pump, signal and idler pulses, and intrapulse group-velocity dispersion is pr...A general theory of optical parametric generation that accounts for pump depletion, loss, phase mismatch, group-velocity mismatch among the pump, signal and idler pulses, and intrapulse group-velocity dispersion is proposed for coherent ultrashort pulses with arbitrary shapes and carrier chirps. The coupled differential equations are numerically solved using a symmetric split step beam-propagation method. The general solutions of these equations are obtained and the optical parametric generation process is theoretically investigated. Results show that the major factors, which remarkably affect the optical parametric conversion efficiency and durations of the pulses in phase-matched structure, are the group velocity mismatch and the intrapulse group velocity dispersion.展开更多
With the consideration of slip deformation mechanism and various slip systems of body centered cubic （BCC） metals, Taylor-type and finite element polycrystal models were embedded into the commercial finite element c...With the consideration of slip deformation mechanism and various slip systems of body centered cubic （BCC） metals, Taylor-type and finite element polycrystal models were embedded into the commercial finite element code ABAQUS to realize crystal plasticity finite element modeling, based on the rate dependent crystal constitutive equations. Initial orientations measured by electron backscatter diffraction （EBSD） were directly input into the crystal plasticity finite element model to simulate the develop- ment of rolling texture of interstitial-free steel （IF steel） at various reductions. The modeled results show a good agreement with the experimental results. With increasing reduction, the predicted and experimental rolling textures tend to sharper, and the results simulated by the Taylor-type model are stronger than those simulated by finite element model.＇Conclusions are obtained that rolling textures calculated with 48 { 110} 〈 111 〉＋ { 112 } 〈 111〉＋ { 123 } 〈 111 〉 slip systems are more approximate to EBSD results.展开更多
It is of great significance for developing self-powered micro-devices to explore the research of piezoelectric effect in conversion of wind energy into electricity. Based on the different excitation modes, the existin...It is of great significance for developing self-powered micro-devices to explore the research of piezoelectric effect in conversion of wind energy into electricity. Based on the different excitation modes, the existing wind energy piezoelectric generators are firstly classified. The research status of wind piezoelectric generators is further analyzed, and characteristics of various types of wind energy piezoelectric generators are summarized. Finally, the future research direction and emphasis of wind energy piezoelectric generators is proposed to carry out its miniaturization, lightweight and integration.展开更多
The mechanical properties of recycled aggregates concrete from demolition have been studied for several years. It has been documented that rheological properties of these concrete are generally affected by use of recy...The mechanical properties of recycled aggregates concrete from demolition have been studied for several years. It has been documented that rheological properties of these concrete are generally affected by use of recycled aggregates. They could present mechanical properties less affected by the presence of recycled aggregates if the initial concrete were of good quality. However, manufacturing problems, mainly attributed to the angular character of these aggregates and to the granulometry of recycled sand, limit their industrial use. The worth point of this study consists in the optimization of the concrete formulation using specific admixture, adapted to this aggregates in order to facilitate its manufacturing. It shows that the new generation of superplasticizers containing some copolymer polycarboxylate makes it possible to significantly improve the fluidity of the recycled aggregates concrete in its fresh state. The aim of this research is to control rheologica] properties of fresh recycled aggregates concrete with fine and coarse recycled aggregates to limit the negative influence of aggregates on mechanical properties of concrete.展开更多
Polyethylene oxide solutions have a behavioral flexibility that provides researchers with the opportunity to use constitutive law models in a variety of ways for their MRI characterization. Our results obtained in num...Polyethylene oxide solutions have a behavioral flexibility that provides researchers with the opportunity to use constitutive law models in a variety of ways for their MRI characterization. Our results obtained in numerical simulation carried out in 2D and 3D for speed profiles of a solution of PEO deployed by the simple method of the cylindrical Couette geometry considering the fluid Newtonian defect, allowed to identify the behavior of fluid complex (rheo-fluidifying threshold fluid). The relevance and the interest of the method are examined by analyzing these results generated by the numerical data obtained, since these profiles depend on the non-Newtonian properties of the fluid which one does not know a priori and which one seeks to measure by postulating first to the power law of Ostwald, then to the truncated power law.展开更多
This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat...This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat exchangers with individually finned heat pipes. The fundamental parameters used for performance analysis were the number of fins per heat pipe, the number of heat pipes, the inlet temperatures, and the flow rates of hot and cold fluids. The heat exchanger under analysis uses Freon 404A as a working fluid in an air conditioning system for cooling in the Evaporator and energy recovery in the Condenser. The theoretical model is localized and applied individually to the Evaporator, Condenser, and heat exchanger regions. The results obtained through the simulation are compared with experimental results that use a global approach for the heat exchanger. The thermal quantities obtained through the theoretical model in the mentioned regions are air velocity, Nusselt number, thermal effectiveness, heat transfer rate, and outlet temperature. The comparisons made with global experimental results are in excellent agreement, demonstrating that the localized theoretical approach developed is consistent and can be used as a comprehensive analysis tool for heat exchangers using heat pipes.展开更多
This work aims to establish comparisons between two models used for the performance of heat exchangers. The chosen system, in this case, consists of a heat exchanger used in automotive radiators flat finned tube type....This work aims to establish comparisons between two models used for the performance of heat exchangers. The chosen system, in this case, consists of a heat exchanger used in automotive radiators flat finned tube type. Water and ethylene glycol compound as base fluid and volume fractions of iron oxide nanoparticles (Fe<sub>3</sub>O<sub>4</sub>) are used as a refrigerant. The quantities determined in this work are the nanofluid exit temperature, the air exit temperature, the absolute error between the models for heat transfer rate, and Effectiveness. The quantities that constitute parameters, independent variables, are the airflow, represented by the Reynolds number, and the iron oxide volume fraction. Ethylene Glycol 50% compound has slightly better thermal performance than pure water and reduces the reactive effect of water on the environment, increasing the average life of the equipment. The absolute relative error between the models is less than 20% and presents maximum values with the increase of the nanoparticle volume fraction and growth in the Reynolds number for the air.展开更多
In this study, it shows how the main indicators of torsional vibrations of the crankshaft change when its materials change. In the study, the crankshaft of diesel engine with four cylinders in line was taken, in which...In this study, it shows how the main indicators of torsional vibrations of the crankshaft change when its materials change. In the study, the crankshaft of diesel engine with four cylinders in line was taken, in which the material is changing from steel to cast iron, due to the technological possibilities of production. For study of torsional vibration of crankshaft system, the construction of equivalent reduced scheme is carried. Reduced inertia moments of discs are determined for each crank of crankshaft, by receiving the impact of piston group and rod mass. Reduced rigidities of crank are determined by experimental method. The results show that the rigidity of crank for the same crankshaft varies up to 6%, while the change between crankshafts goes up 10%. At the end, frequency and vibration forms are calculated using Tole-Holxer method. From calculations, it results that the frequencies are 19% smaller, while the vibration forms varies slightly. In this case, the change of the materials leads to the first frequency in the area of engine rotations and it can worsen the level of torsional vibration, therefore, it should check the resonance areas and vibration amplitude.展开更多
The study of damage in rotating machineries is of fundamental interest in the fields of machine and structure design. A rotating system, supported by bearings and under some dynamic conditions, can generate a variety ...The study of damage in rotating machineries is of fundamental interest in the fields of machine and structure design. A rotating system, supported by bearings and under some dynamic conditions, can generate a variety of problems that are encountered in many different types of rotating machines. One of these problems is the unbalance due to non-homogeneous mass distribution along the shaft. One of the techniques which are widespread today is the identification of parameters and excitation forces that may well followed by monitoring the evolution and change of possible variations of these parameters. Although several methods for the identification of unbalance excitation force are available in the literature, none of them can be considered unrestricted to be applied for all rotating systems. In this study, two methodologies to identify unknown excitations, such as unbalance, have been proposed. This project refers to the analysis of unbalanced forces from displacement parameters and speed by using methods of identification by Fourier series and Legendre polynomials together with the finite element method, state observers in reasons of the problem of absence of signs of rotational displacement, bandpass filter were used to noise suppression of the data collected from the experimental part, Quasi-Newton method to minimize a function in which the bearing stiffness and its damping are unknowns, and also the experimental verification of the methodology, using for this system owned by a rotary mechanical vibrations of the Department of Mechanical Engineering of Faculty of Engineering, campus of llha Solteira.展开更多
This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat...This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat exchangers with individually finned heat pipes. The fundamental parameters used for performance analysis were the number of fins per heat pipe, the number of heat pipes, the inlet temperatures, and the flow rates of hot and cold fluids. The heat exchanger under analysis uses Freon 404A as a working fluid in an air conditioning system for cooling in the Evaporator and energy recovery in the Condenser. The theoretical model is localized and applied individually to the Evaporator, Condenser, and heat exchanger regions. The results obtained through the simulation are compared with experimental results that use a global approach for the heat exchanger. The thermal quantities obtained through the theoretical model in the mentioned regions are air velocity, Nusselt number, thermal effectiveness, heat transfer rate, and outlet temperature. The comparisons made with global experimental results are in excellent agreement, demonstrating that the localized theoretical approach developed is consistent and can be used as a comprehensive analysis tool for heat exchangers using heat pipes.展开更多
基金Project supported by Guangzhou Science and Technology Project,China (201803020038)Guangdong Science and Technology Commissioner Project (GDKTP2020046700)Innovation and Entrepreneurship Training Program for University Students (202324040)。
文摘Lithium-sulfur(Li-S) batteries have generated significant attention due to their high theoretical specific capacity and energy density among a host of energy storage power devices.Nevertheless,the lithium polysulfide dissolution shuttle that occurs within Li-S batteries will lead to capacity deterioration and inadequate cycling stability.In the paper,we proposed a measure to deal with the above problems by modifying the separator with Nd_(2)O_(3)/graphene composite in Li-S batteries.Graphene's special chemical properties and structural qualities make it an excellent choice for Li-S batteries.Meanwhile,Nd_(2)O_(3) has a strong binding affinity with lithium polysulfide due to its low electronegativity,which exhibits Lewis' s acidity and forms strong interactions with lithium polysulfide,which is strongly Lewis basic.By utilizing these advantageous properties,Li-S batteries assembling Nd_(2)O_(3)-decorated reduced graphite oxide modified polypropylene separators(Nd_(2)O_(3)/RGO/PP) demonstrate outstanding electrochemical performance,including a mere 0.0525% capacity attenuation rate under 2C during 1000 cycles and with exceptional rate performance of 614 mAh/g even at 3C.This study presents valuable knowledge for effectively modifying separators using rare earth oxides to incorporate graphene,ultimately promoting the practical application of Li-S batteries.
基金Project supported by the Major State Basic Reasearch and Development Program of China (973) (2007CB613700)National Natural Science Foundation of China (50725413) National Key Technologies R&D Program of China (2007BAQ00134-04)
文摘The effect of Nd addition on the microstructure and mechanical properties of ZK20 magnesium alloy was investigated by room tensile test, optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to develop a magnesium alloy with higher ductility. Results showed that the crystal grains of as-extruded ZK20+0.5%Nd magnesium alloy were effectively refined, and the alloy exhibited higher strength and ductility, with the UTS of 237 MPa and the elongation of 32.8%, increasing by 5...
文摘3D printing technology, as one of the most subversive technologies in the 21st century, processes the entity through layer by layer superposition. It breaks through the design constraints and adapts to customized production with high flexibility, which is an important direction of industrial development in the future. 3C product has a very board application market and groups, so it has been widely valued over the world. The advantages of 3D printing technology are very suitable for the characteristics of 3C product, such as diversification, small-volume, personalized. Here, the impact of 3D printing technology on manufacturing industry is firstly discussed;3C product manufacturing system integrated with 3D printing is finally put proposed. The advantages of the proposed system possess shorter manufacturing cycle, supply chain and logistics. It makes easier to realize personalized 3C product, localized manufacturing and reduce cost. The development and wide application of 3D printing technology in 3C product industry will make the manufacture of low-cost small batch or even individual customized products into a general mode towards the trend of “light luxury + light customization”.
文摘In order to study the mechanical and triboloical properties of powder metallurgy (PM) parts under different process parameters, the specimens were used in pack carburizing processes. These specimens made from industrial test pieces were carburized in a powder pack for about two to five hours at a temperature of about 850?C - 950?C. The effects of austenitization and quenching are investigated on some specimens. Also the wear tests are performed by means of a pin-on-disc tribotester using roll bearing steel as the counterface material. The results indicate that by appropriate selection of process parameters, it is possible to obtain high wear resistance along with moderate toughness. It is concluded that surface treatments increases the wear resistance and performance of PM parts in service conditions. By increasing the role of PM in industry which resulted from their ability to produce the complex shapes, high production rate, and dimension accuracy of final products, they need to be heat treated. Carburizing method was selected as a surface hardening method for PM parts. Results of wear and hardness show considerable enhancement in mechanical properties of PM parts.
基金Project(10372074) supported by the National Natural Science Foundation of China
文摘The formation and evolution laws of the defect temperature field,heat dissipation in the process of defect evolution were studied.On the basis,the formation and evolution laws of the defect temperature field were investigated,the interaction among defects in the process of defect evolution was carried out.The numerical simulation of the temperature field of ABS was made.The results show that the process of defect evolution is one of energy dissipation,in which the defect temperature field forms due to that its heat dissipation possesses fractal property and its fractal dimension not only relates to the interaction among the defects,but also is the function of time,this incarnates the efficiency of coordinated actions of striding over the different gradations in the process of defect evolution and among gradations.The increase of the local temperature with the increase of deformation-induced heating effect in ABS is obvious.Moreover,the shape of plastic zone and inner heat source density function has big effect on the temperature field.
文摘A magnetorheological (MR) fluids brake is a device to achieve brake by shear force of MR fluids. A MR rotary brake has the property that its braking torque changes quickly in response to an external magnetic field. In this study, the design method of the circular plate MR fluids brake is investigated theoretically. The equation of the torque transmitted by the MR fluids in the brake is derived to provide the theoretical foundation in the design of the brake. Based on this equation, after mathematically manipulated, the calculations of the volume, thickness and width of the MR fluids within the circular plate MR fluids brake are yield.
文摘The modelling of one kind of nonlinear parabolic distributed parameter control system with moving boundary, which had extensive applications was presented, Two methods were used to investigate the basic characteristics of the system: I) transforming the system it? the variable domain into that in the fixed domain; 2) transforming the distributed parameter system into the lumped parameter system. It is found that there are two critical values for the control variable : the larger one determines whether or not the boundary would move, while the smaller one determines whether or not the boundary, would atop automatically. For one-dimensional system of planar, cylindrical and spherical cases the definite solution problem can be expressed as a unified form. By means of the computer simulation the open-loop control system and close-cycle feedback control system have been investigated. Numerical results agree well with theoretical results. The computer simulation shows that the system is well posed, stable, measurable and controllable.
基金financially supported by the fund of the State Key Laboratory of Solidif ication Processing in NWPU(No:SKLSP201006)the fund of the National Basic Research Program of China(No:2011CB610402)
文摘A type of mesh called a body-fi tted Cartesian mesh,very different from the traditional structured body-fi tted mesh,is established.At f irst,the right parallelepiped mesh is generated,then,a feature analysis is done on the cross sections.These cross sections are the intersections of the casting shape with the right parallelepiped grids(under the Cartesian coordinate system).On the basis of the feature analysis,two sorts of body-f itted boundary grids,shape-keeping grids and shape-distortion grids,are def ined.Shape-distortion grids can be removed or weaken by increasing the number of grids or moving the coordinates of the mesh generation region,so actually the body-fitted Cartesian mesh generation is to get shape-keeping grids.A shape-keeping grid mainly consists of two sorts of surfaces(I type face and II type face),and each of them is joined by two types of points(I type point and II type point).If only these two types of points were given,the shape-keeping mesh would be constructed.In this paper,the cases of the above two boundary grids being generated were discussed.An algorithm was put forward to get the shape-keeping grids.Several body-fi tted Cartesian meshes generated on castings show the validity of the algorithm.The mesh generation examples show that the body-fi tted Cartesian mesh is more excellent than the right parallelepiped mesh in aspects of decreasing grids number and being closer to the shape of the casting solid.
文摘The bubble radius at the early stage of discharge in water is investigated using high-speed photography. Some simulation results on the bubble radius are presented, which are in agreement with the experimental results, with a maximum difference of about 10%. The reasons why the peak pressure of the first shock wave is only related to the energy released in the bubble during the first half period are addressed. The energy released in the bubble after the first half period increases the bubble pulsation period, but it produces no more than 10% under the peak pressure of the second shock wave.
文摘To realize the goal of harvesting energy from pavement vibration on a large scale, a new type of piezoelectric harvesting units as the energy transducer has been proposed. The piezoelectric harvesting units are paved 40 mm below the asphalt, which is the same as thickness of the top layer of typical asphalt pavement in China. The spacing distance is 2200 mm, which is the same as the one between two tires of a normal vehicle. A mathematical model of the unit is deduced on Meda empirical formula and Hamilton principle and piezoelectric equations. Effects of the external vehicle load on its harvesting energy properties and pavement deformation and stress are analyzed with the finite element method. The results show that the excited voltage is linearly variation with contact pressure while the harvested electrical energy exponential varies with contact pressure. The more the contact pressure is, the larger the harvested electrical energy and the deformation and stress are. The harvested electrical energy also increases with the load frequency. At least 100 mJ of electrical energy can be collected with the proposed piezoelectric harvesting unit. It shows that the technology application of the piezoelectric harvesting energy from pavement is promising.
文摘A general theory of optical parametric generation that accounts for pump depletion, loss, phase mismatch, group-velocity mismatch among the pump, signal and idler pulses, and intrapulse group-velocity dispersion is proposed for coherent ultrashort pulses with arbitrary shapes and carrier chirps. The coupled differential equations are numerically solved using a symmetric split step beam-propagation method. The general solutions of these equations are obtained and the optical parametric generation process is theoretically investigated. Results show that the major factors, which remarkably affect the optical parametric conversion efficiency and durations of the pulses in phase-matched structure, are the group velocity mismatch and the intrapulse group velocity dispersion.
文摘With the consideration of slip deformation mechanism and various slip systems of body centered cubic （BCC） metals, Taylor-type and finite element polycrystal models were embedded into the commercial finite element code ABAQUS to realize crystal plasticity finite element modeling, based on the rate dependent crystal constitutive equations. Initial orientations measured by electron backscatter diffraction （EBSD） were directly input into the crystal plasticity finite element model to simulate the develop- ment of rolling texture of interstitial-free steel （IF steel） at various reductions. The modeled results show a good agreement with the experimental results. With increasing reduction, the predicted and experimental rolling textures tend to sharper, and the results simulated by the Taylor-type model are stronger than those simulated by finite element model.＇Conclusions are obtained that rolling textures calculated with 48 { 110} 〈 111 〉＋ { 112 } 〈 111〉＋ { 123 } 〈 111 〉 slip systems are more approximate to EBSD results.
文摘It is of great significance for developing self-powered micro-devices to explore the research of piezoelectric effect in conversion of wind energy into electricity. Based on the different excitation modes, the existing wind energy piezoelectric generators are firstly classified. The research status of wind piezoelectric generators is further analyzed, and characteristics of various types of wind energy piezoelectric generators are summarized. Finally, the future research direction and emphasis of wind energy piezoelectric generators is proposed to carry out its miniaturization, lightweight and integration.
文摘The mechanical properties of recycled aggregates concrete from demolition have been studied for several years. It has been documented that rheological properties of these concrete are generally affected by use of recycled aggregates. They could present mechanical properties less affected by the presence of recycled aggregates if the initial concrete were of good quality. However, manufacturing problems, mainly attributed to the angular character of these aggregates and to the granulometry of recycled sand, limit their industrial use. The worth point of this study consists in the optimization of the concrete formulation using specific admixture, adapted to this aggregates in order to facilitate its manufacturing. It shows that the new generation of superplasticizers containing some copolymer polycarboxylate makes it possible to significantly improve the fluidity of the recycled aggregates concrete in its fresh state. The aim of this research is to control rheologica] properties of fresh recycled aggregates concrete with fine and coarse recycled aggregates to limit the negative influence of aggregates on mechanical properties of concrete.
文摘Polyethylene oxide solutions have a behavioral flexibility that provides researchers with the opportunity to use constitutive law models in a variety of ways for their MRI characterization. Our results obtained in numerical simulation carried out in 2D and 3D for speed profiles of a solution of PEO deployed by the simple method of the cylindrical Couette geometry considering the fluid Newtonian defect, allowed to identify the behavior of fluid complex (rheo-fluidifying threshold fluid). The relevance and the interest of the method are examined by analyzing these results generated by the numerical data obtained, since these profiles depend on the non-Newtonian properties of the fluid which one does not know a priori and which one seeks to measure by postulating first to the power law of Ostwald, then to the truncated power law.
文摘This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat exchangers with individually finned heat pipes. The fundamental parameters used for performance analysis were the number of fins per heat pipe, the number of heat pipes, the inlet temperatures, and the flow rates of hot and cold fluids. The heat exchanger under analysis uses Freon 404A as a working fluid in an air conditioning system for cooling in the Evaporator and energy recovery in the Condenser. The theoretical model is localized and applied individually to the Evaporator, Condenser, and heat exchanger regions. The results obtained through the simulation are compared with experimental results that use a global approach for the heat exchanger. The thermal quantities obtained through the theoretical model in the mentioned regions are air velocity, Nusselt number, thermal effectiveness, heat transfer rate, and outlet temperature. The comparisons made with global experimental results are in excellent agreement, demonstrating that the localized theoretical approach developed is consistent and can be used as a comprehensive analysis tool for heat exchangers using heat pipes.
文摘This work aims to establish comparisons between two models used for the performance of heat exchangers. The chosen system, in this case, consists of a heat exchanger used in automotive radiators flat finned tube type. Water and ethylene glycol compound as base fluid and volume fractions of iron oxide nanoparticles (Fe<sub>3</sub>O<sub>4</sub>) are used as a refrigerant. The quantities determined in this work are the nanofluid exit temperature, the air exit temperature, the absolute error between the models for heat transfer rate, and Effectiveness. The quantities that constitute parameters, independent variables, are the airflow, represented by the Reynolds number, and the iron oxide volume fraction. Ethylene Glycol 50% compound has slightly better thermal performance than pure water and reduces the reactive effect of water on the environment, increasing the average life of the equipment. The absolute relative error between the models is less than 20% and presents maximum values with the increase of the nanoparticle volume fraction and growth in the Reynolds number for the air.
文摘In this study, it shows how the main indicators of torsional vibrations of the crankshaft change when its materials change. In the study, the crankshaft of diesel engine with four cylinders in line was taken, in which the material is changing from steel to cast iron, due to the technological possibilities of production. For study of torsional vibration of crankshaft system, the construction of equivalent reduced scheme is carried. Reduced inertia moments of discs are determined for each crank of crankshaft, by receiving the impact of piston group and rod mass. Reduced rigidities of crank are determined by experimental method. The results show that the rigidity of crank for the same crankshaft varies up to 6%, while the change between crankshafts goes up 10%. At the end, frequency and vibration forms are calculated using Tole-Holxer method. From calculations, it results that the frequencies are 19% smaller, while the vibration forms varies slightly. In this case, the change of the materials leads to the first frequency in the area of engine rotations and it can worsen the level of torsional vibration, therefore, it should check the resonance areas and vibration amplitude.
文摘The study of damage in rotating machineries is of fundamental interest in the fields of machine and structure design. A rotating system, supported by bearings and under some dynamic conditions, can generate a variety of problems that are encountered in many different types of rotating machines. One of these problems is the unbalance due to non-homogeneous mass distribution along the shaft. One of the techniques which are widespread today is the identification of parameters and excitation forces that may well followed by monitoring the evolution and change of possible variations of these parameters. Although several methods for the identification of unbalance excitation force are available in the literature, none of them can be considered unrestricted to be applied for all rotating systems. In this study, two methodologies to identify unknown excitations, such as unbalance, have been proposed. This project refers to the analysis of unbalanced forces from displacement parameters and speed by using methods of identification by Fourier series and Legendre polynomials together with the finite element method, state observers in reasons of the problem of absence of signs of rotational displacement, bandpass filter were used to noise suppression of the data collected from the experimental part, Quasi-Newton method to minimize a function in which the bearing stiffness and its damping are unknowns, and also the experimental verification of the methodology, using for this system owned by a rotary mechanical vibrations of the Department of Mechanical Engineering of Faculty of Engineering, campus of llha Solteira.
文摘This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat exchangers with individually finned heat pipes. The fundamental parameters used for performance analysis were the number of fins per heat pipe, the number of heat pipes, the inlet temperatures, and the flow rates of hot and cold fluids. The heat exchanger under analysis uses Freon 404A as a working fluid in an air conditioning system for cooling in the Evaporator and energy recovery in the Condenser. The theoretical model is localized and applied individually to the Evaporator, Condenser, and heat exchanger regions. The results obtained through the simulation are compared with experimental results that use a global approach for the heat exchanger. The thermal quantities obtained through the theoretical model in the mentioned regions are air velocity, Nusselt number, thermal effectiveness, heat transfer rate, and outlet temperature. The comparisons made with global experimental results are in excellent agreement, demonstrating that the localized theoretical approach developed is consistent and can be used as a comprehensive analysis tool for heat exchangers using heat pipes.
