Jo Dec 15, 2023
Drying process within a porous solid has two drying periods: a constant rate drying period during which free water content is removed and a falling rate drying period which occurs due to the capillary action and it is a complex process of simultaneous transfer of heat and mass. Simultaneous transfer of heat and mass through porous solids plays crucial roles in chemistry, drying, food processing, etc.
Numerical analysis of drying process of a porous solid is highly challenging because it involves different scaling parameters.
Most works commonly used a standard correlation to compute heat and mass transfer coefficients at the interface of products for numerical analysis of convective drying.
Jon Chol Jin, a researcher at the Faculty of Heat Engineering, has proposed a method for numerical simulation on the convective drying of moist clay-tiles for moderate Reynolds numbers.
He applied the energy equation and moisture transport equation based on Whitaker’s theory of drying for moist clay-tiles, and solved the Navier-Stokes equations, vapor diffusion equations, thermal diffusion equations, continuity equations and turbulence equations for the flow field of fluid.
He accomplished innovative connection between the turbulent flow of the air and moist clay-tiles by setting the boundary conditions for the continuity of state variables and their respective fluxes through the interface.
He simulated drying of the clay-tile under different operation conditions using ANSYS Fluent Computational Fluid Dynamics code. Then, he analyzed the effects of velocity, temperature and relative humidity of air on the temperature and moisture content within the clay-tile during the drying process.
His numerical simulation method can be used to analyze convective drying processes of moving porous solids and that of porous solid combined with ultrasonic wave and microwave.
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Jo Dec 14, 2023
A sine-cosine resolver, which has high measurement accuracy and reliability, is widely used as an angular position and speed sensor in many fields of industry. Attempts to further raise the robustness of a resolver have been made, and as a result, a pancake brushless resolver has been developed.
A pancake resolver has no traditional collector that brings energy into the rotor but feeds current into the rotor through a rotary transformer located at the stator edge. The advantage of such structure over the traditional resolver with a collector is absence of relative movement between mechanical parts that causes wear, vibration and noise. But the disadvantage is that a rotary transformer consumes most of the power that a pancake resolver needs.
Ro Yong Sim, a researcher at the Faculty of Electrical Engineering, has proposed a new structure of a rotary transformer for reducing consumed power of a pancake resolver. The core material she used was electrical steel containing silicon. This material has large resistivity, which can result in lowering core loss.
She has also proposed a scheme for cutting some air gaps in the core to block the path of eddy current and to further decrease core loss. She estimated the proper amount of silicon and the size of air gaps through field analysis by Ansoft Maxwell and experimental results.
The simulations by Ansoft Maxwell software and the experimental results in a resolver manufacturing plant have shown that a newly designed rotary transformer consumes less power (71.7%) than before.
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Jo Dec 12, 2023
Rotating machinery is widely used in industries and, in practice, it faces inevitable faults because of the errors in manufacturing, assembling and operating conditions.
Mass unbalance is one of the most common faults encountered in rotating machinery.
Rotors are classified into two types: rigid and flexible rotors. Flexible rotors are served at speeds exceeding their critical speeds and rigid ones are not. In general, flexible rotors are balanced at service speed because their dynamical characteristics change with speeds. Balancing of flexible rotors is relatively complicated, expensive and dangerous because they have to run at high speeds around critical speeds.
Therefore, methods for low-speed balancing of high-speed rotors have been developed and presented.
Ri Yong Ho, a researcher at the Science Engineering Institute, has proposed a new low-speed balancing method for high-speed rotors by using Component Mode Synthesis (CMS).
The proposed method was tested numerically by simulating harmonic response of a rotor bearing system which is constructed of a steel shaft, three discs and two rolling elements bearings. The shaft was supported at two ends by bearings and worked at the maximum speed of 9 000r/min. The system was modeled by FEM. The shaft was divided into two dimensional finite beam elements with two translational and one rotational DOF at one node and the discs were modeled as concentrated masses and inertia moments at the corresponding nodes. And the support bearings were modeled as spring elements and proportional damping was supposed.
He confirmed by his method that a high speed flexible rotor can be exactly and effectively balanced without test runs at speeds higher than critical speed.
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Jo Dec 8, 2023
Our country has a huge number of cultural relics and remains worth boasting to the world.
The 80 000 blocks of the Complete Collection of Buddhist Scriptures, which is on show in Pohyon Temple on Mt. Myohyang, is one of the precious national treasures demonstrating the advanced level of typography in our country.
The 80 000 blocks of the Complete Collection of Buddhist Scriptures are a systematic Buddhist library of Buddhist scriptures and the books concerned with Buddhism.
As it was compiled during the Koryo dynasty, it is also called the Koryo Tripitaka.
The very first complete collection of Buddhist scriptures had already been born in our country in the first half of the eleventh century but it was burnt away by foreign aggressors.
The present collection was completed in over 1 530 kinds and 6 793 volumes from 1236 to 1251.
It is permeated with the creative wisdom of people including lots of typographers. The excellent typography employed for it makes it one of the proud cultural legacies of our people and the world’s cultural assets.
In those days, the people and handicraftsmen managed to make more than 80 000 blocks of pakdal trees and birch trees.
Each block has 23 lines with 14 letters each. Wooden ties are attached at both ends of blocks to avoid twist and the surfaces are lacquered to keep worms away.
The wood blocks are elaborate enough to resemble a fine sculpture.
The collection, a precious cultural legacy of our nation, shows the level of development and promotion of woodblock printing. It serves as a material evidence demonstrating that firm technical foundations for invention and use of metal types for the first time in the world were laid in Koryo at that time.
The collection is now recognized as the most perfect standard collection of all the old collections of Buddhist scriptures existing in the world.
The complete collection is shining brilliantly as our priceless national treasure and as a world cultural legacy, proudly boasting of the wisdom and resourcefulness of our nation.
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Jo Dec 7, 2023
Lumen degradation is a common failure mode in white LED light sources. Lumen maintenance life, defined as the time when the maintained percentages of the initial light output fall below a failure threshold, is a key characteristic for assessing the reliability of white LED light sources.
Most widely studied among various reliability tests for LEDs has been thermal degradation because heat is known to be one of the main factors affecting the LEDs performance and durability. However, the effect of humidity on LED lifetime is also important as many high-power LEDs including street lamps and floodlights are used in outdoor conditions with high humidity.
Ri Pyong Sol, a section head at the Faculty of Electronics, has conducted research into the reliability of high-power white LEDs. He aimed to predict the rated lifetime and to estimate the reliability of 50W white LEDs using their degradation data so that he could find an available method for testing the accelerated life of 50W high power white LEDs.
According to his research results, the relative error in the predicted value of lifetime by temperature accelerated test is 17.4%, which is 4.1% greater than that by temperature-humidity accelerated test.
By his method, the rated lifetime of LEDs can be predicted quite accurately within one day at the scene of high-power LED production.
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Jo Dec 6, 2023
Janggu dance, one of the typical folk dances of our country, is rich in national colour. Dancers do janggu dance with a janggu, a national instrument, on their shoulders.
Originated from the appearance and wide playing of janggu in our country, janggu dance has a long history.
The murals in Jiban Five Tombs of Koguryo show some dancers dancing and playing the janggu.
It indicates that janggu was already used in dancing as a prop and janggu dance was widely spread in the Koguryo age.
Later, janggu dance with such a long history became more fashionable with development of a peasant dance in the latter half of the feudal Joson dynasty, when playing skills and dancing skills were combined.
Janggu dance was found in several folk games and dances as well as in a peasant dance. In modern ages, it was made artistic by professional dancers and put on the stage, too.
Janggu dance that was loved by our people for a long period of time was reproduced in the form of a solo dance to be congenial to the aesthetic tastes of the era under the wise guidance of our Party.
As janggu serves as a main prop of janggu dance, skills in playing the janggu are given great importance in the dance composition.
With a variety of beating techniques and rich and sophisticated tones, janggu has a high possibility of expression. Therefore, janggu dance produces unique descriptive charm with the beating techniques of janggu and artistic rhythms of a dance combined harmoniously with each other.
Under the scrupulous guidance of our Party, janggu dance has been improved into a great diversity from a solo to an ensemble, a group dance, a large group dance, etc. and it has now become a dance of strong national coloring that preserves the flavor of folk dances.
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