Jo Aug 27, 2024
In order to increase the life of blast furnaces, the wall of blast furnaces must be equipped with cooling equipment. The cooling stave is commonly manufactured with cast iron, steel and copper as cooling materials. Among them, copper is widely used as a cooling stave material in blast furnaces because it has good thermal conductivity and high cooling capacity.
The structure and size of the copper cooling stave depend on the structure, size and operation characteristics of the blast furnace. A HIsmelt (high-intensity smelt) furnace has some differences from conventional ones in the fuel used and in the structure.
In order to manufacture a copper cooling stave for the HIsmelt furnace, O Song Ryong, a student at the Faculty of Materials Science and Technology, has designed its optimal structure for safe operation and its casting process.
For the design of the copper cooling stave, he determined the diameter of a copper tube installed in it and its arrangement, the wall thickness and the flow rate of cooling water according to the size. He examined the stability of the copper cooling stave by simulating the cooling capacity under the HIsmelt furnace operating conditions using Fluid Flow (Fluent) of Ansys Workbench programme (version 15).
For the design of the casting process, he made a casting plan and simulated on ProCAST the occlusion of flow channel of the cooling water due to the melting of the copper pipe in the mold during casting and solidification of the molten metal and the supply of cooling agent to prevent the separation of the cooling stave and the copper pipe. The cooling agent was determined to be water (94%) + air (6%).
For more information, please refer to his paper “Structure and Casting Process Optimization of Inclined Copper Cooling Stave for Hismelt Furnace Using Process Simulation Based on Taguchi Method” in “International Journal of Metal casting” (SCI).
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Jo Aug 25, 2024
During hot working, material exhibits a complex nonlinear relationship between flow stress and thermo-mechanical processing parameters, due to the simultaneous occurrence of work hardening and softening. In order to express more accurately the nonlinear relationship between flow stress and thermo-mechanical processing parameters, a large number of constitutive models have been developed, which can be divided into three categories: a phenomenological constitutive model, a physically-based model and an artificial neural network model.
Phenomenological constitutive models are widely used to predict the hot deformation behavior of metallic materials including alloy steels and titanium alloys, due to their simple equations and small computational quantities. Physically-based constitutive models consider not only the thermo-mechanical processing parameters, but also the physical mechanism such as dislocation movement and thermal activation during hot working process. Compared with phenomenological constitutive models, physically-based models are more complex to establish. However, due to their high prediction accuracy, physically-based models are widely used in many commercial finite element analysis programs.
The Ti-6Al-2Sn-2Zr-3Mo-1.5Cr-2Nb (TC21) alloy is a 1 100MPa damage tolerance titanium alloy, with ultimate tensile strength of 1 130MPa, yield strength of 1 020MPa, elongation of 12%, toughness value of 80MPa·m1/2, and crack extension rate of 1.47×10-5 mm·cycle-1. Due to a good combination of strength and toughness, the application value of TC21 alloy is much higher than that of TC4 alloy.
So far, there have been many studies on the hot deformation behavior of TC21 alloy. However, few studies have been conducted or reported on other constitutive models for TC21 alloy including physically-based constitutive models.
Pak Hun, a student at the Faculty of Materials Science and Technology, has constructed phenomenological and physically-based models for predicting more accurately the hot deformation behavior of TC21 alloy, and compared their predictability. To this end, he carried out isothermal uniaxial compression tests at different strain rates in α + β two-phase and β phase fields. Based on the experimental data (flow stress curves), he developed a modified Johnson-Cook (m-JC) model, a strain-compensated Arrhenius type (sc-AT) model and a modified Zerilli-Armstrong (m-ZA) model for Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy. In addition, he compared the prediction accuracies of the three constitutive models using standard statistical parameters.
The results showed that the proposed m-ZA model is the preferred constitutive model to predict the hot deformation behavior of Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy.
You can find the details in his paper “Comparisons of phenomenological and physically-based constitutive models for Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy” in “Applied Physics” (SCI).
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Jo Aug 23, 2024
Today, electricity is not only the main power of industry but also the main energy source indispensable for the people’s lives, and the demand is in a steady rise. Therefore, it is important for the power industry to reliably ensure the operation of power systems in a scientific and technological way.
The trend of global power system development is to establish and operate a flexible AC transmission system (FACTS) that can rapidly control the main characteristics of power system like voltage, frequency, active and reactive power, etc.
One of the important issues in establishing and operating such FACTS is to reduce the total harmonic distortion rate in power system equipment including reactive power compensators, for power quality.
Ri Tong Chol, a researcher at the Faculty of Electrical Engineering, has established a fuzzy logic controller for controlling the DC bus voltage of the converter to improve the response in a distribution grid static synchronous compensator (DSTATCOM) which is widely used in FACTS and to ensure normal operations of the compensator under asymmetric conditions and grid voltage variations, and verified its effectiveness through computer simulation tests.
For effective operation of DSTATCOM, the DC-side capacitor voltage of the voltage-type converter should be kept constant, and for this purpose, a fuzzy PI controller was used.
The simulation model of DSTATCOM was designed in a three-phase distribution system using SIMULINK tool in MATLAB environment and the performance of DSTATCOM with fuzzy logic control algorithms was simulated and analyzed under distorted PCC voltage due to unbalanced nonlinear load.
The harmonic distortion factors (THD) of the power supply current, load current and PCC voltage on A were measured to be 3.6%, 26.0% and 4.4%, respectively, using the proposed fuzzy logic controller of DSTATCOM.
The results confirmed that the controller performance improved with a power supply current of 3.6%, while the THD of PCC voltage was 4.4%.
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Jo Aug 22, 2024
While water consumption increases steadily due to various socio-economic factors such as population concentration in cities, increase in industrial wastewater along with the rapid development of industry and the improvement of people’s living and cultural level, a large amount of water resources are contaminated, so it is vital to increase the amount and efficiency of wastewater treatment at sewage treatment plants.
The trend in the development of global wastewater treatment processes is the introduction of a large number of aeration facilities that provide biochemical oxidation of organic pollutants to the plant. The problem is that with the increase of sewage treatment plants, its operating cost, especially the power consumption of the fan, increases. Aeration is one of the most energy-intensive processes in the wastewater treatment plants and any improvement in it is likely to enhance the efficiency of the overall process.
Of course, 15~20% of electric power consumption can be reduced by controlling the blast volume according to the influx of sewage and its composition, but upgrading of the aerating facilities for improvement of the oxidation effect is more stable and effective.
Ji Chol Hyok, a researcher at the Nano Physics Engineering Institute, has proposed a sewage clarification method by micro-bubble aeration for improving the clarification efficiency and reducing its cost, and compared its effectiveness with conventional ones, thus demonstrating its superiority.
He chose an 80㎥ tank of a pig farm as an experimental reaction tank and measured some sewage treatment parameters.
The results showed that the proposed method ensured up to 90.8% of removal efficiency of COD (chemical oxygen demand) while the conventional ones guaranteed only 86.4%, and that the overall removal efficiency is over 20% higher than bottom aeration when the temperature is low.
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Jo Aug 20, 2024
The main purpose of laser surface melting is to obtain great strength of alloy surface and to ensure high productivity. The laser surface melting of AZ31B Mg alloy is very important in determining the microstructures and strength of alloy surface. The laser power and laser scanning speed in the laser surface melting play important roles in the temperature change and the morphology of the melting region. However, massive amounts of data on laser surface melting process take a lot of time and cost for detailed experiments of investigation.
To date, there has been an impressive amount of research on this topic, especially to investigate the melting flow and its thermal transfer by laser.
Kim Yong Il, a researcher at the Nano Physics Engineering Institute, has estimated the effects of laser power and laser scanning speed on the temperature distribution of AZ31B Mg alloy using COMSOL Multiphysics 5.3.
Firstly, based on finite element analysis software, he built a finite element model of laser surface melting Mg alloy. Then, he conducted a numerical computation with the variables used in the experiments of previous works. The numerical computation by simulation was done with different laser powers at the laser scanning speed of 360mm/min, and with different laser scanning speeds at the laser power of 2 000W.
The simulation results seem to be in good agreement with the experimental results based on infrared temperature measurement.
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Jo Aug 18, 2024
Determination and description problems are two basic problems of Formal Concept Analysis (FCA). Since most methods of determining the concept lattice are based on generation of non-neighbor concepts, the process of analyzing the neighboring relation between generated concepts is required to solve the description problem.
Pak Chol Hong, a researcher at the Faculty of Applied Mathematics, has proposed some new efficient algorithms for simultaneously describing the concept lattice and its hierarchy-matrix without this process. The hierarchy-matrix is a successful description of the concept lattice, by which any software can autonomously understand the information of hierarchy of the concepts. The focus of his algorithms is on generation of neighbor concepts tested for canonicity and registering the neighbor information in the hierarchy-matrix sequentially.
He drew the following conclusions.
First, the concepts of subcontexts induced by a given concept are lower (or upper) ones of the concept and all lower (or upper) neighbor concepts of the concept are denoted by the antitone Galois connections on the subcontexts.
Second, all lower (or upper) neighbor concepts of a given concept are generated by the antitone Galois connections on the subcontexts and restricted by the rank of the subcontex-matrices.
Third, the key aspects of the efficiency of the concept lattice and its hierarchy-matrix based on the generation of lower neighbor concepts (BCLHMLN) and the concept lattice and its hierarchy-matrix based on the generation of upper neighbor concepts (BCLHMUN) are generation of neighbor concepts on the subcontext, the equivalence class of the object or the attribute with respect to adjoin mappings of Galois connections, the design of canonicity test and the utilization of the histories of generated lower (or upper) concepts.
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