Jo Jul 11, 2025
Cold pilger mills include a roll cold-pipe rolling mill and a roller cold-pipe rolling mill. As the roll cold-pipe rolling mill involves difficult roll processing, the roller cold-pipe rolling mill is now in wide use.
Determination of rolling force during periodic rolling in roller cold pilger mills is one of the important problems in determining the design parameters. In particular, the roller cold pilger mill is subject to periodic reciprocating motion so that the deformation zone and rolling force continue to change during one cycle, and thus it is important to determine the maximum rolling force.
Han Myong Jin, a researcher at the Faculty of Materials Science and Technology, has determined the average pressure and contact area between the rolling roller and the workpiece as one of the basic data for determination of the design parameters of two-row roller cold pilger mills, and investigated the rolling force.
First, he derived a formula of the maximum rolling force and performed a comparison using the finite element analysis tool Deform-3D. Then, he installed a force sensor under the inclined support plate and compared it with the measured results to confirm the accuracy of calculation.
When producing a copper tube of ϕ24mm×0.5mm in size from a workpiece of ϕ30mm×3mm in size in a two-row roller cold-pipe rolling mill, the rolling force analysis by the computer-coupled rolling force measuring device and the rolling force calculation confirmed that the rolling force formula is relatively accurate, with 3% in the first row and 0.73% in the second row. In the same way, the comparison of the rolling force by the simulation analysis data with Deform-3D and the rolling force determined by the rolling force formula showed that the rolling force formula is relatively accurate, with 2.8% in the first row and 1.96% in the second row.
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Jo Jul 10, 2025
A turbogenerator is one of the main equipment for large-capacity power stations such as nuclear power plants. In order to design a turbogenerator of high operational reliability, thermal characteristics should be analyzed exactly.
The direct gas-cooled rotor winding in large power turbogenerators is cooled by gas (hydrogen) that flows along the U-type or radial ventilation duct. Many researchers and experts have published their research results on the temperature characteristics of winding of direct gas-cooled rotor in large turbine generators larger than 1 000MW. However, the detailed research results on the cooling duct dimension at the radial cooling rotor windings are rarely mentioned in the literature.
An Se Gwang, a researcher at the Faculty of Electrical Engineering, has determined the reasonable cross section of an axial ventilation duct that makes the distribution of flow velocity uniform in the radial cooling slot of a 1 000MW 4-pole turbogenerator and the reasonable value of the width coefficient of cooling duct and segment length.
First, he proposed a mathematical model to analyze the temperature feature of radial cooling rotors and confirmed the range of practical value to determine the reasonable number and dimension of radial ventilation ducts.
Then, he proved that the gas velocity distribution has an equal value along the axial length when the input cross section of the axial ventilation duct is increased 1.67 times the output cross section, with the effect of revolution in consideration.
The research result is helpful for the design standard of large 4-pole turbine generators, and it can be applied to the design and building of 1 000 MW 4-pole turbine generators.
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Jo Jul 8, 2025
Manganese is an important alloying element indispensable in the metallurgical industry and it is widely deposited in the form of oxides in nature.
The reduction reaction of manganese oxides in the production of metallic manganese or ferromanganese from these manganese raw materials is of great importance, and there have been many thermodynamic and kinetic studies on it. In particular, several kinetic models describing the carbothermic reduction reaction of MnO have been proposed and experimentally examined.
However, there are some errors in the calculation and experimental results in the models proposed by the previous authors, and no studies have yet applied the fractional order to the kinetic modeling of the reduction reaction of MnO by solid carbon.
Kim Un Dok, a researcher at the Faculty of Metal Engineering, has developed a kinetic model for the reduction process by solid carbon of MnO in high-carbon ferromanganese slags using fractional differential equations (FDE), determined the relationship between fractional order, fractional rate constant and temperature, and verified the accuracy of the fractional order model through the comparison with the results of previous studies.
Comparing the preset values calculated from the FDE and previous models with the experimental values, the root-mean-square errors (RMSE) were 0.005 and 0.029, respectively, and the coefficients of determination were 0.999 and 0.980, respectively, which shows that the proposed model is more accurate.
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Jo Jul 7, 2025
Aluminum and its alloys have light weight, good thermal and electrical properties, excellent mechanical properties and process ability, and low cost, so they are widely used in various fields of the national economy such as the mechanical industry, aerospace, daily necessities industry, electronic industry, building materials industry, etc. Aluminum and its alloys are easily oxidized in the atmosphere because of their strong chemical activity, resulting in a thin oxide film on the surface, which is protective because of its good corrosion resistance in dry air, but it is easily damaged in contaminated atmosphere, water, seawater, soil and various corrosive media, which is not protective. Therefore, surface treatment is carried out to enhance the surface performance of aluminum and its alloy, such as corrosion resistance, wear resistance and decorativeness.
At present, the sealing of anodized aluminum alloy is carried out at high temperature, which leads to high energy consumption and low productivity.
Kim Chang Sok, a researcher at the Science Engineering Institute, has investigated the sealing mechanism of Ni-F system to realize room temperature sealing, one of the ways to reduce energy consumption and shorten the production cycle in the sealing process for surface treatment of aluminum alloy, and investigated the influence of various factors on the room temperature sealing quality to confirm a reasonable room temperature sealing process index.
Through the room temperature sealing experiments of aluminum alloy 6061, he has found that the optimum conditions are Ni2+ concentration of 1.5g/L, F- concentration of 2g/L, sealing temperature of 20~30℃, sealing time of 15~20min and pH of 5.5~6.5.
The surface properties of the samples sealed at room temperature are superior to the surface properties of the existing sealed samples at high temperature. In addition, the sealing temperature can be lowered by over 70℃ and the time for sealing can be shortened by 15 minutes, which leads to the decrease in energy consumption for heating sealing solution and the increase in productivity.
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Jo Jul 4, 2025
Multistage centrifugal pumps are commonly-used pieces of turbomachinery for applications with high head and flow duty requirements. So, the improvement of their technological indexes is greatly significant for saving energy, ensuring production and raising economic benefit.
Since the axial force in high-pressure multistage pumps reaches tens of tons, it is essential to balance hydraulic axial thrust. Even then, an axial bearing is required; and its size depends largely on the prediction accuracy of axial thrust and on the type of the balancing device. Therefore, the size of bearings and the axial thrust balance systems of multistage pumps are frequently determined on the basis of axial force measurements carried out on each particular type and hydraulic pressure of pumps.
In general, balancing devices are used for balancing the axial thrust acting on the rotors of multistage centrifugal pumps. Balancing devices create forces that oppose the axial thrust generated by the impellers. When dimensioned correctly, balancing devices are able to completely balance the axial thrust, so the required pressure automatically builds up in the radial gap due to an axial displacement of the rotor. Therefore, the axial clearance at the device depends on the axial force acting on the rotor, i.e. on the operation point of the pump.
Balancing devices have two gaps: a radial gap and an axial clearance. The mutual coupling of these two gaps allows automatic balancing of the axial force. According to the variation of operation point of a centrifugal pump, the axial thrust changes. The rotor moves left and right in the axial direction to be in a new equilibrium state, when the axial clearance changes with the movement.
If the axial displacement of the rotor is too large, the balancing device may contact with the casing to cause wear. Thus, the rotor vibrates and the stability of operation is destroyed. Therefore, when balancing device is designed, even small axial displacement must yield large changes in the balancing force. In previous studies, this performance was estimated by sensitivity coefficient.
Sensitivity factor represents the variation of balancing force according to the pressure distribution in front and rear shrouds of impeller, but does not represent variation of balancing force when a rotor moves. Therefore, it can be more correct to estimate the automatic balancing ability of a balancing device by the parameter representing the variation of balancing force when a rotor moves a unit length.
Han Pok Nam, an institute head at the Faculty of Mechanical Science and Technology, has decided leakage through radial and axial clearance, pressure difference and balancing force based on the theoretical analysis of the gap flow of a balancing device. Then, he has defined a new concept of stiffness coefficient and analyzed the relationship between stiffness coefficient and geometrical parameters.
Thus, he has found that the stiffness coefficient decreases with too large or too small sensitivity coefficient, and a reasonable sensitivity coefficient should be chosen around 0.5.
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Jo Jul 3, 2025
It is well known that TEA-CO2 laser is a kind of high-power laser with enough pulse peak output, and study on TEA-CO2 laser has been consistently conducted for a long period of time. The design of electrode structure, which emits field electrons in TEA-CO2 laser, is a question of finding an effective method for increasing laser power by raising the emission efficiency of field electrons and their utilization.
In practice, the tips of cold-cathode are made in the shape of a needle or a blade. Such tips emit field electrons at low discharge voltage by raising the amplification degree of electric field. When the electric field strength at the tips is too high, the lifetime of tips will be shortened owing to the cathode sputtering. The field electron generator that contains cold-cathode provides high vacuum of 10-4Pa.
The Marx generator supplies electric power to the generator of field electrons. Here, such geometric parameters as the height and basal width of tips and the size of anode cells are all related to the amplification degree of electric field. The electric field strength at the tips is affected by not only the shape and size of each tip but also their arrangement. When high voltage is applied to the cathode, the existent electric fields in the vicinity of the tips interact with each other, which obviously influences the electric field strength at the tips.
Pak Sin Hyok, a researcher at the Institute of Nano Science and Technology, has designed an electrode structure of cold cathode and grid anode with a high amplification degree of electric field strength at the tip, in consideration of the tip’s parameters, the arrangement of electrode structure, etc.
He designed the tips as a triangle, and simulated the electric field in the discharge region by the COMSOL Multiphysics software while changing the size and array interstice of tips and the dimension of grid anode corresponding to the tips. On this basis, he found out the optimum design parameters of electrode for raising the amplification degree of electric field strength at a tip to the maximum.
The simulation result shows that the optimum interstice of tips that provides the maximum amplification degree of electric field was 8mm, and the amplification degree of electric field strength was 1.2.
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