Jo Jun 13, 2024
Powder injection molding is advantageous for hard metal components with complex shapes or large h/d. In the production of cemented carbide by powder injection molding, the change in the carbon content of the alloy is greater due to the higher content of the moulding agent than in conventional die molding.
The carbon content in cemented carbides is a key factor in determining the properties of alloys. In general, carbon content is controlled by the composition of raw material, the atmosphere of thermal degreasing and the atmosphere of sintering.
Kim Song Il, a researcher at the Faculty of Materials Science and Technology, has succeeded in forming η phase on the microstructure by controlling the binder content and the solvent parameters of debinding process in WC-8Co hard alloy, which leads to the prolongation of life of alloy and cost reduction.
He studied the effect of binder content on the property of products by measuring the carbon content, shrinkage, hardness, transverse rupture strength (TRS) while changing the binder content between 40 and 50vol%. He performed solvent debinding in the mixed solvent of benzole and acetone in the ratio 3:2 at an extraction temperature of 45℃ with an immersion time of two to six hours. Thermal debinding was conducted together with sintering, and the final sintering was done at 1 450℃.
The hardness value and TRS were HV1 560 and 1 580MPa respectively, and the service life was extended by 1.3 times.
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Jo Jun 11, 2024
A distributed air microbubble generator can produce microbubble solutions of uniform bubble diameter and high concentration. Two-phase fluid mixers of rotor-stator structure are usually used for generating microbubbles by distributed air. The structure design of its stator and rotor depends on the composition of two-phase medium to disperse. Both the stator and the rotor have independent effects on the characteristics of the two-phase fluid mixing device.
Rotor-stator mixers are of several types including the Silverson mixer, which is often used for homogenization and dissolution in liquid-liquid two-phase fluids. However, it reveals some defects in gas-liquid two-phase fluid mixtures. It is because the viscosity of gas-liquid two-phase fluid is relatively low compared to liquid-liquid two-phase fluid and the difference in gas and liquid density is very large. It is also because gas dispersion in the liquid is very difficult, and the conventional rotor-stator mixer configuration cannot provide sufficient pressure gradients in the shear gap, an important indicator of microbubble generation, due to the self-characteristics of gas-liquid two-phase fluid flow.
In order to increase the pressure gradient in the shear gap, Cha Su Song, a researcher at the Nano Physics Engineering Institute, has proposed a stator structure with a shear blade and rotor-stator engagement relationship. Then, he has evaluated the bubble diameter distribution formed in gas-liquid two-phase fluid flowing through the generator by the CFD-PBM method and compared it with experimental results.
Both the rotor and stator are of a shear blade structure. He analyzed the bubble diameter distribution in the fluid analysis program ANSYS Fluent 18.1. Then, he built a microbubble generator with the same structure as in the analysis and did some experiments. After that, he measured bubble diameter distribution by zeta potential analyzer (Malvern Zetasizer Nano ZS) and compared it with the analytical results.
The average bubble diameters obtained by CFD analysis and experiments were about 107nm and 113nm, respectively, which were in good agreement. In conclusion, the improvement of stator structure in the distributed air microbubble generator has a significant effect on generator characteristics.
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Jo Jun 10, 2024
In recent years, motion analysis technology based on inertial measurement units (IMU) has been widely used in the field of sports science due to its advantages such as its low cost and good adaptability in both indoor and outdoor environments. Xsens MVN BIOMECH, one of the motion analysis technologies that are being widely used all over the world, generally uses several IMUs and processes the rotary motion or position changes of body segments quantitatively.
This technology has been applied to motion analysis in several sporting events including horse riding, ski, basketball, tennis, etc.
Choe Nam Chol, a researcher at the Faculty of Electronics, has applied it to the motion analysis of a table tennis player. As the upper limb plays an important role in the table tennis player’s motion using a racket, he set it as a target.
Every player has their own modality in 3D-changes of wrist, elbow and shoulder joints during the stroke of each type. Those unique modalities would be very important for managers to assess and improve each player’s performance. Therefore, he analyzed the modalities of individual players by using the motion analysis system.
First, he mounted four IMUs on the thoracic spine, the upper limb, the forearm and the back of the hand of a player to measure in real time the 3D-angles of rotation of three joints during the forehand, backhand and topspin forehand strokes. Next, he analyzed the motion pattern and the range of motion (ROM) of wrist, elbow and shoulder joints on the basis of the measured data.
The results showed that the proposed analysis method is effective for improvement of technical skills of players.
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Jo Jun 7, 2024
With the development of large CFB boilers, a cyclone separator with a downward gas exit that provides convenience for the construction and compact layout of boilers appeared. Later, a square cyclone separator was suggested for Π-arrangement in CFB boilers and for convenient installation of heat transfer surfaces on the inner separator walls.
Previous studies of the flow field in cyclone separators with a downward gas exit that focused only on the space between the vertex and the gas exit failed to get flow characteristics in each section of the separator.
Ryom Jun Ho, a researcher at the Faculty of Heat Engineering, has conducted a detailed simulation of the flow characteristics in a square cyclone separator with a downward gas exit.
He used the computational fluid dynamic (CFD) method to predict and evaluate the flow field and collection performance inside the square cyclone separator. In addition, he employed the Reynolds stress model (RSM) for flow field analysis in two cyclone separators with different dust exits.
The results demonstrate that the flow characteristics and collection performance of the square cyclone separator with a vertical dust exit are better than that with an inclined exit.
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Jo Jun 5, 2024
A research team led by Kim Mun Hui, a researcher at the Faculty of Electrical Engineering, has developed a small wind power generation system (SWPGS) for producing maximum power using a permanent magnet generator with two coils.
The generator with two coils ensures not only higher efficiency but also lower expense of the SWPGS than that with one coil, because it can automatically make the load curve more closely approach to the maximum power characteristic curve of a wind turbine without any control circuits.
The system consists of a three-blade horizontal axis wind turbine, a permanent magnet generator (PMG) with two coils, a rectifier, a battery, a load, etc.
PMG with two coils ensures higher accuracy of power control than that with one coil.
In the PMG, only the first coil works at low wind speed, and if the wind speed becomes higher, the second coil works, too.
As a result, the power generation system is driven at its maximum power because it is able to steadily approximate the load curve to the maximum power curve of the wind turbine.
Thus, this system can produce more electric power from the same wind resources in low wind speed areas.
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Jo May 30, 2024
Bundle-series initiation is currently used as a method for simultaneously transmitting blast signals from a detonator to a large number of shock tubes for blasting using shock tubes and its initiation modes affect the ability and reliability of signal transmission.
For building demolition and tunneling blasting, over 50, even 100 lines of shock tubes must be simultaneously initiated by a detonator. In the meantime, a large number of shock tubes often cause initiation failures. Therefore, for shock tubes, blasting system must have not only high ability of blast signal transmission (or initiation) but also high reliability of the signal transmission on shock tubes. The methods of bundle-series initiating a large number of shock tubes are subject to possible failure modes where the ability and reliability of signal transmission on each shock tube can be strongly affected by the coupling mode of a detonator and the number of shock tubes.
To the best of our knowledge, there are no reports in the literature regarding the experiments and numerical simulation to predict or estimate the dynamic blast load and the initiating probability according to the coupling structure of a detonator with many shock tubes.
Choe Yong Chol, a researcher at the Faculty of Mining Engineering, has studied the structural influence of initiation modes on the pressure impulse generated by a detonator and transmitted to the bundle of a large number of shock tubes and the signal transmission probability, and proved the advantage of frontal bundle-series initiating mode over lateral bundle-series initiating mode.
He confirmed that the numerically obtained pressure impulse for a bundle of shock tubes shows a good correlation with the experimentally obtained signal transmission probability of a bundle of shock tubes for a variety of bundle-series initiation modes of shock tubes.
You can find further details in his paper “Influence of Initiation Modes in the Bundle-Series Initiation of a Large Number of Shock Tubes by Detonators” in “Shock and Vibration” (SCI).
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