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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Jo Jul 12, 2024
Foam glass has been used as insulation material since 1930s. The most important properties of foam glass are light weight, mechanical stability, low thermal conductivity, freeze-thaw cycle resistance, nonflammability and chemical inertness.
There are two methods of producing foam glass according to the kinds of raw materials. One uses glass while the other uses waste glass as raw material. The second method is very simple and consumes less electricity and fuel.
Anthracite fine, graphite powder, soot, calcite, calcium carbide and silicon carbide can be used as a foaming agent. The quantity of foaming agent is 1~7% of finished products.
Previous papers considered thermo-physical properties of foamed glass using a single-system foaming agent.
Pae Kyong Ho, a researcher at the Faculty of Metal Engineering, conducted experiments under different conditions by using a mixture of calcite and hard coal in order to produce low density foam glass with both closed and open porosities and compressive strength of up to 1.8MPa.
The results show that addition of 5wt.% of calcite powder and 2wt.% of anthracite powder ensures the density of foam glass of below 240kg/m3, the compression strength of over 1.8MPa and the heat conductivity of below 0.21W/(m·K).
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Jo Jul 10, 2024
Lagoon Monggum is a scenic spot on the coast in Ryongyon County, South Hwanghae Province.
Blue waves lapping against the shore, white silvery sands stretching on the beach, thick pine woods and red sweet briers in full bloom on the sands blend well to produce spectacular scenery. The name Lagoon Monggum (where mong means ‘dream’ and gum means ‘gold’) is derived from an old tale about having a dream on this beautiful beach.
Its original names were Paeksachongsong (white sands and green pine trees), Paeksajong (port near the white sands) and Kumsasipri (10 ri stretch of golden sands).
In the south of the lagoon are rocky cliffs and rocks of fantastic shape like Elephant Rock, a living monument, which add to the scenic beauty.
The sands of Lagoon Monggum are of so good quality as to be well known to the world. The sandy beach runs up to the width of two kilometers and the length of eight kilometers.
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