Jo Jul 23, 2024
Dielectric Barrier Discharge (DBD) has been applied in many fields such as ozone synthesis, degradation of VOCs, plasma etching, surface modification, actuators, sterilization of medical devices, etc., and it has been attracting a great deal of attention of researchers in recent years. In order to find and control the features of DBDs, a lot of efforts have been devoted to their theoretical, numerical and experimental investigation. The advance in numerical simulation encouraged the development of DBD technology.
DBD is generally produced between two electrodes, which have several shapes such as parallel plate, concentric cylinder, and so on. Parallel plate electrodes are preferred as it not only makes it easy to produce large area plasma, but is also flexible and convenient for treating film-like materials or gases.
So far, finite element method (FEM) has been dominant in simulating DBD plasma. Despite the fact that FEM is well-defined and powerful for unstructured grid, it is essentially non-conservative and complicated. In order to avoid checkboard instability due to unstructured grids, special care is needed with its integrals.
In recent years, the LB method has been successfully applied to some hydrodynamic problems such as Navier–Stokes and advection–diffusion equations. It is based on the well-known Boltzmann equation rather than hydrodynamic equations. Unlike other methods, the LB method chases distribution of particles within each discretized node, which ensures that non-linearity is local and non-locality is linear. Although the LB method is more memory-sensitive and requires weak compressibility, it is preferred for simulating multicomponent flow in complex geometries such as porous media. Parallel computation is available in this method, which means it is faster than others. LB method is not only simple, but also relatively stable to implement. Its stability depends mainly on lattice velocity and advection velocity, which allows us to adjust only two parameters for stability.
Choe Yong Son, a researcher at the Faculty of Physical Engineering, applied a lattice Boltzmann (LB) D1Q3 scheme to the numerical simulation of multi-component plasma by dielectric barrier discharge (DBD).
DBD in atmospheric argon is generated in a small gap of 0.5mm between two parallel plate electrodes, and is driven by high voltage AC of about 20kHz.
In order to find the characteristics of the DBD plasma, he coupled the LB numerical model with continuity and Poisson equations. The simulation results showed that LB method can be used for simulation of DBD plasma.
For more information, please refer to his paper “One-dimensional lattice Boltzmann simulation of parallel plate dielectric barrier discharge plasma in atmospheric argon” in “Mathematics and Computers in Simulation” (SCI).
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Jo Jul 22, 2024
Fasteners are widely used in all industrial sectors and the auto industry alone consumes 2 800 to 3 100 fasteners for assembly of a vehicle. Externally threaded fasteners comprise the bulk of fasteners used in these applications with over 90% of being produced by thread rolling.
Thread rolling is a cold forming or chipless machining operation where a matched set of dies with reverse form of thread displaces material to produce external threads on cylindrical or conical blanks with no material loss. It ensures specialization of production and increased production.
Hence, it is very important to determine reasonable process parameters by simulating a thread rolling process and analyzing the influence of process parameters, for the improvement of product accuracy and lengthening of a roll’s life. Actually, it is very difficult to survey the change of force acting on the roll and blank fitting apparatus. Therefore, the influence of parameters on thread rolling process cannot be correctly determined by real experiments. Thus, prior researchers simulated the thread rolling process by computer modeling. However, their simulations were restricted to 2D simulation due to the difficulties arising in 3D simulation of metric thread rolling process.
Pae Ung Chol, a researcher at the Faculty of Materials Science and Technology, has conducted a 3D simulation of thread rolling process to find influence factors and predict their tendency.
He analyzed the influence of process parameters such as blank diameter, roll diameter and fitting height of blank using a finite element code DEFORM.
The results of the study showed that the blank diameter has a little influence on product accuracy and the life of thread rolling die (roll) while roll diameter has a significant impact on its life, and that fitting height of blank has a remarkable influence on wear of roll screw thread and product accuracy.
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Jo Jul 19, 2024
In a three-phase power system, the voltages at the generation side are sinusoidal waves equal in magnitude with 120° phase difference. At the load side, however, voltages may become unbalanced due to phase angle deviations or unequal distribution of single phase loads caused by unequal voltage magnitudes of each phase. Voltage unbalance is a major power quality issue because a small unbalance in the phase voltages can even cause a larger unbalance in the phase currents.
A completely balanced three-phase system contains only positive sequence components of voltage, current and impedance, whereas an unbalanced system contains both positive and negative sequence components of voltages and currents. The negative sequence component of current in the unbalanced system increases temperature loss in the equipment. Hence, it is necessary to mitigate this problem by removing the negative sequence current at the load side and keep the source side balanced.
Reactive power compensation can be ensured by using Static Var Compensators (SVCs), DSTATCOMs, Static Synchronous Series Compensators (SSSCs) and Unified Power Quality Conditioners (UPQCs), thus improving power quality.
Jong Il Bok, a researcher at the Faculty of Electrical Engineering, proposed a controller to determine the difference value between load current and positive sequence current as the standard value of compensating current and a proportional complex integral controller to control the current of each phase for compensating asymmetry and harmonics of load using a distribution static compensator (DSTATCOM). Then, he studied the performance of the controller by simulating the entire system in the MATLAB/Simulink environment.
This technique makes it possible to compensate asymmetry or harmonics of reactive power of load and load current. In addition, it makes switching frequency constant and improves the current tracking ability.
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Jo Jul 17, 2024
Spray nozzles are now used in various fields such as agriculture, chemical industry, etc., especially for cooling and dusting devices.
The most important aspect of the characteristics of spray nozzles is droplet size and spray volume distribution. In general, the purpose of almost all spraying is to spray uniformly over the area of spraying. However, it is important to ensure uniform distribution because the spray volume distribution radially from the spray center is not uniform and different distribution patterns are found depending on the flow rate.
Kim Song Won, a researcher at the Faculty of Mechanical Science and Technology, conducted an experimental study on the jet characteristics of a pressure swirling nozzle used for the design of a large cooling tower spraying system, and analyzed the effects of spray water flow rate and the structure of a nozzle outlet and a vortex vane on the spray characteristics.
The study confirmed the following.
First, the droplet size required for the large cooling tower can be achieved by a type X vortex vane in the range of 500 μm to 1000 μm, and the spray volume distribution becomes almost uniform when the vane angle is 120° and the spray flow rate is large.
Second, the smaller the flow rate is, the greater spray volume is concentrated in the spray center, and the larger the flow rate is, the lower the spray volume concentration peak gets and it moves radially.
Third, the length of the nozzle exit section does not significantly affect the spray characteristics in the pressure swirling atomization nozzle.
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Jo Jul 16, 2024
Cuprous oxide (Cu2O) possesses special optical, photoelectric and catalytic properties and it is widely used as a raw material in paints, glass and ceramics industries, and as a catalyst in agriculture and organic chemical industries. Specifically, it is used as a fungicide in orchards. Meanwhile, research works are under way to use it as a poison in the production of antifouling coatings against marine organisms such as mussels and scallops attached to marine structures like ships.
According to the literature, Cu2O is mainly prepared by wet chemistry, electrochemistry, radiation irradiation, polyol, etc. In these methods, operation is complicated and a number of wastes harmful to the environment are released during reactions. Hence, preparation of cuprous oxide by mechanochemical method, a simple and environmentally friendly preparation method, is being investigated.
Mechanochemistry studies physicochemical changes and reactions when energy by mechanoaction is given to the material. In other words, mechanical energy is converted to chemical energy that is needed to drive a thermodynamically impossible reaction with △G>0. Therefore, mechanochemical reactions have some features that are different from ordinary chemical reactions, and the reaction mechanism and thermodynamic and kinetic features are also significantly different from ordinary thermochemical reactions.
Several kinds of reducing agents including sodium sulfite, hydrazine, sodium hypophosphite, sodium borohydride, glucose, etc. are used to prepare cuprous oxide from copper compounds. All of them except glucose are environmentally harmful or hard to purchase. Meanwhile, few reports are related to the preparation of cuprous oxide from copper sulfate by mechanochemical method.
Kim Yong Chol, a researcher at the Faculty of Chemistry, investigated the possibility of preparing cuprous oxide ultrafine particles by the soft mechanochemical method in which copper sulfate and sodium hydroxide were mixed and glucose and sodium dodecyl sulfate (SDS) were added as a reducing agent and a dispersing agent, respectively, before undergoing milling. Then, he observed the effects of reaction time and molar ratio of sodium hydroxide and copper sulfate on the preparation of Cu2O by this method.
He found that cubic ultra-fine cuprous oxide with good dispersion property and initial particles of 36nm can be prepared when the additive amount of sodium hydroxide is 1.2 times as much as the theoretical value and the crushing time is 30 minutes.
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Jo Jul 14, 2024
The fracture occurring in many metallic structures made of steel is mainly fatigue fracture. Fatigue fracture is a dangerous one that decreases the safety of structures. Fatigue fracture occurring in metallic structures is predicted by fracture mechanics method.
Paris suggested a propagation equation for fatigue crack and many researchers found other crack propagation equations by correcting and supplementing Paris’s equation.
However, very few studies have focused on the prediction of the fatigue life of structures and components from the characteristic data of structures comprising metallic materials. For example, the relationship between the metallographic parameters of steel like hardness, volume ratio, grain size and distribution of non-metallic inclusion, etc., and its fatigue life was not quantified.
Kim Mun Chol, a researcher at the Faculty of Materials Science and Technology, simulated the fatigue procedure of a steel structure using a crack-tip sliding model based on continuous distribution dislocation theory and calculated the rest fatigue life of an inertia train structure.
Through the simulation, he found that the hardness of microstructure exerts decisive effects on the fatigue life of steel structures of ferrite-pearlite structure while the grain size and volume ratio hardly affect it, and slip-band crack sharply decreases as inclusion grain size decreases below 7㎛.
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