Jo May 15, 2024
The vortex street flow meter is an instrument that measures a flow rate in the pipe line by measuring the frequency of vortex streets in the wake of a vortex generator located perpendicular to the flow direction in the pipe line. When Reynolds number reaches a specific value in the pipe line through which fluid runs, alternate vortex street occurs in the wake of the vortex generator and the generating frequency of this vortex street is in proportion to flow velocity. Therefore, the generating frequency is used for prediction of flow rate. The frequency depends on the type of generator.
The vortex street flowmeter has good accuracy and stability in flow measurement, but they are not widely used for low flow rate due to the small vortex frequency signal when the diameter of a pipe line is small or flow rate is low. In industrial pipe lines, flows with Re below 2×104 are often found, so in such cases, the vortex street flowmeter cannot guarantee measurement accuracy.
Compared to other types of vortex generators, a triangular prism-shaped vortex generator is now used in most vortex street flowmeters because it is easier to make and has better stability in vortex separation.
Jong Ji Song, a researcher at the Faculty of Mechanical Science and Technology, has solved and optimized hydrodynamic problems arising in the design of a triangular prism-shaped vortex generator available in the low discharge domain with minimum pressure loss.
First, he analyzed the influence of the structural parameters of the vortex generator such as thickness and width on the vortex generation and fluid flow by CFD simulation of the vortex flow around the triangular prism-shaped vortex generator. As a result, he found that with great thickness, the vortex generator easily generates vortex but has large flow resistance, and its great width is not good for generation of vortex and its narrow width cannot guarantee the stability of vortex. Next, on the basis of this analysis of the vortex generation and the flow loss in the pipe line, he determined the structural parameters of the triangular prism-shaped vortex generator available in the low discharge domain by optimization with BP neural network and genetic algorithm.
The proposed CFD simulation method and the optimization of structural parameters can also be applied in the design of vortex generators of any shape.
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Jo May 14, 2024
As the rotary angular sensor is important for measuring accurate positions and directions, it plays an important role in various applications such as robotic controller, camera and industrial machines.
At present, lots of angular position sensors such as simple resistive potentiometers, capacitive potentiometers, optical sensors and magnetic sensors are in use, and optical sensors which operate in visible light or infrared light offer the advantages of contactless measurement and insensitivity to electric and magnetic fields. So, an optical angular sensor has become recognized as an indispensable displacement/position sensor due to its high resolution, light weight and excellent immunity to electromagnetic interface.
Jo Myong Jin, a researcher at the Semiconductor Institute, has proposed an absolute rotary angular sensor with a nonlinear transparent disc. This rotary angular sensor is of transparent type and it has a nonlinear transparent disc between light source and LDRs. This sensor consists of five elements, that is, light source, a shaft coupled nonlinear transparent disc, a lens, a pair of light dependent resistors (LDR) and a signal processing circuit. In this sensor, absolute rotary angle is measured by output resistance of double LDRs which has a linear change by nonlinear transparency of the disc in the whole measurement range (from 0° to 360°) according to the characteristics of LDR’s resistance via irradiance. Its operation principle is similar to that of a resistive potentiometer.
The advantage of the sensor is that it is immune to shock and vibration as it has a nonlinear transparent disc instead of a binary coded disc and a large gap distance between the disc and the optical sensor. Another advantage is that as the output signal of LDRs is analogue, its resolution is determined by the A/D converter in the signal processing circuit. Therefore, this rotary angular sensor provides a high resolution in the range of 0~360°.
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Jo May 10, 2024
Nowadays numerical simulation is an essential tool for stability analysis of mining engineering, and constructing a more realistic numerical model is important to ensure the accuracy and reliability of mining engineering design.
In recent years, many researchers have made significant progress in the study to quickly obtain more realistic structural elements of stopes and to apply them to production practice by performing numerical simulations of mining areas with FLAC3D. However, the model construction parts in these programs are quite difficult to use for building large and complex 3D mining models, particularly, for multi-seam and multi-boundary geological bodies.
Han Un Chol, a researcher at the Science Engineering Institute has proposed a new method of quickly converting a 3D solid model into a numerical one for stability analysis of geotechnical and mining structures by combining ANSYS, FLAC3D and SURPAC.
First, he carried out a detailed analysis of the data structure used in these three programs and presented a transformation technique between the data. Then, he converted the integrated 3D solid model made by SURPAC into a numerical model of ANSYS, and meshed it as tetrahedral elements, and consecutively, converted the meshed model of ANSYS into a grid model of FLAC3D for numerical simulation.
Finally, he applied the proposed method to numerical analysis of pillar stability in the sublevel open stopes in the target mine. It was shown that the model transformation time can be reduced to 1/50 and the accuracy of the numerical model can be increased by more than 20%.
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Jo May 6, 2024
To estimate the lifetime of welding joints of steam pipes working at high temperatures and high pressures is of great importance for the stability and maximum economic efficiency of steam power plants. Long-term exposure of heat-resistant steels at high temperature leads to deterioration in the mechanical properties because of the changes in the microstructure. Alloying elements and microstructures significantly affect the heat resistance in the welds of low alloy heat resistant steel at high temperature.
In order to illustrate the changing nature of heat resistance, Ri Won Jun, a researcher at the Faculty of Materials Science and Technology, investigated the mechanical properties (ultimate strength, yield strength, elongation, hardness, etc.) of pipe materials and their welding joints with respect to the content of elements in carbides, their morphology and change in the metallic structures. Also, he conducted an experiment by using zirconium instead of vanadium for electrodes to evaluate the microstructure of deposited metal, mechanical properties at room temperature, metallic structures analysis, carbide analysis, scale resistance, short-term creep strength and long-term creep strength.
As a result, he drew the following conclusions.
First, the metallographic study showed that Zr series has less migrant tendency of the grain boundary than V series at high temperatures for a long time. And the comparison of mechanical properties with hybrid carbides showed that Zr series is superior to V series in all indices.
Second, the experiment for oxidation inclination at high temperature showed that the value of scale oxidation of Zr series is 1.4 times higher than that of V series, indicating that Zr series is more stable at high temperature.
Third, the metal deposited by the stick electrode coated with Zr has no ferrite segregation and the primary structure is finer than that with V.
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Jo Apr 29, 2024
Investment casting is widely used to make complex castings with high dimensional accuracy at low cost. The impellers of centrifugal pumps have many thin-walled regions and structures of uneven thickness in parts, so shrinkage defects can occur in these areas.
Also, their complex internal cavities cause excessive variations in the velocities of molten metal flowing into the shell mold. This results in gas and non-metal inclusion defects in castings. Therefore, it is important to avoid these casting defects occurring in investment casting of impellers.
Kim Yu Chol, a section head at the Faculty of Materials Science and Technology, has succeeded in making centrifugal pump impellers with no defects by optimal design of runner/riser system and mold tilt angle with ProCast software, thus enhancing the quality of castings and reducing production hours. The sprue was used as a kind of riser to feed shrinkage of casting and enhance casting yield. The shell mold was tilted at various tilt angles to eliminate gas and inclusions during pouring.
You can find the details in his paper “Improvement of quality and yield for investment casting of centrifugal pump impeller by tilting mold and optimizing runner/riser system” in “The International Journal of Advanced Manufacturing Technology” (SCI).
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Jo Apr 26, 2024
Preparation of copper nanoparticles have depended on liquid-phase reduction, where copper sulfate as raw material is dissolved in deionized water and a certain amount of dispersant is added before they were heated under stirring to reach reduction reaction temperature and reducing agent is dropped and mixed to produce copper nanoparticles.
Then, mixing reducing agents takes long while the reduction rate of copper ions is very fast (reduction reaction is already initiated before required amount of reducing agent is mixed). This results in non-uniform nucleation-crystalline growth, thus copper nanoparticles unhomogeneous in size being obtained.
After the reduction reaction, the copper nanoparticles dispersed in liquid phase are separated from the liquid phase using a high-speed centrifuge, which are diluted again in deionized water, and centrifugation washing process is repeated several times, so oxidation by atmospheric oxygen is easily achieved.
Kim Song Chol, a researcher at the Faculty of Materials Science and Technology, prepared relatively uniform-size copper nanoparticles by thoroughly mixing the reactants (copper salt solution + reducing agent solution) at room temperature in the liquid-phase reduction process (based on the fact that reduction reaction never occurs even when copper sulfate aqueous solution and reducing agent solution are mixed below 40ºC) and heating to the reaction temperature under stirring, allowing simultaneous and homogeneous nucleation of crystalline nuclei to be formed in the reaction system.
In addition, he realized separation and wash of produced copper nanoparticles from aqueous solution without a centrifuge, and instead introduced volatile organic solvent into reaction system to encapsulate the produced copper nanoparticles. By doing so, he ensured their surfaces are protected as soon as the copper nanoparticles are produced, and also simplified the separation and washing process.
The proposed technique can reduce the production cost of copper nanoparticles and increase the rate of recovery, particle size homogeneity and oxidation stability. In addition, it is a great potential for practical applications as producing copper nanoparticles as there is no need for expensive high-speed centrifuges. It is also useful for mass production of copper nanoparticles as lubricating oil additives.
You can find more information in his paper “A new method to improve homogeneity and oxidation stability of Cu nanoparticles for lubricant additive in liquid phase reduction process” in “ Materials Research Express ” (SCI).
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