Jo Dec 18, 2023
Subcooled flow boiling is used in many cooling systems due to its high cooling effect. In these systems, different substances such as water and refrigerants can be used as working fluid. However, most synthetic refrigerants such as CFCs and HCFCs have undesirable impact on the environment, so natural refrigerants like CO2 are often used in different refrigeration and heat pump systems worldwide. CO2 can also be used as working fluid in the cooling system based on subcooled boiling, and it is very important to accurately analyze the heat transfer behavior of CO2.
In contrast to previous Computational Fluid Dynamics (CFD) studies concerned with subcooled flow boiling of water or synthetic refrigerants in a vertical tube, Ma Kuk Chol, a researcher at the Faculty of Heat Engineering, has proposed a mathematical model to analyze subcooled flow boiling characteristics of CO2 in a horizontal tube.
He built a mathematical model by using the Eulerian multi-phase model coupled with the RPI model, and utilized the IAC model included in ANSYS FLUENT v.19.3 in order to consider the coalescence-breakup and evaporation-condensation effects of bubbles.
The important thing in building the CFD model for subcooled flow boiling heat transfer of CO2 was to determine correct values of bubble waiting time coefficient and critical Weber number. The analysis indicated that the former 0.8 and the latter 0.25 produced the most accurate results for subcooled flow boiling of CO2.
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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 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 4, 2023
With downscaling of MOSFET to nanometers, gate induced drain leakage (GIDL), short channel effect (SCE), etc. greatly affect its characteristics.
Adaptation of a double-gate (DG) in MOSFETs can improve the SCE and GIDL can be alleviated by a structure of dual materials. DMDG-MOSFETs have been proposed and studied in some aspects.
Ri Hui Chol, a section head at the Semiconductor Institute, has proposed a model for potential distribution and subthreshold swing through the entire channel. In the DMDG-MOSFET structure, he adopted the structure of two gate materials that have different work functions. This structure enables potential to increase through two steps so that the potential change rate may be decreased. Thus, the intensity of the electric field on the position close to the drain can get lower. This feature is useful for improving electron mobility and for hot electron generation.
When VGS was relatively small, the simulation on the model and the previous results agreed well. But as the applied bias increased, so did the deviation.
It is because he employed depletion approximation that only accounts for ion concentration. When VGS is relatively small, carrier concentration is low, so the influence of neglecting this term is not so significant. But VGS reaches a certain extent, the influence of the gate voltage on channel potential is eliminated by inversed charge so that it may result in deviation.
Especially, under the double-material gate configuration the band voltage of part M2 is relatively small and the effective gate voltage is big, so the inversed charge can be generated more easily. Thus, though both models (double-material gate model and single-material gate model) are obtained in the same method, the accuracy is different, that is, the accuracy of the DMDG-MOSFET model may be lower than SMDG-MOSFET’s.
The simulation results indicate that the subthreshold swing increases with the decrease in the channel length like general DG MOSFETs, and that the subthreshold feature is deteriorated with the increase in the thickness of the gate oxide and the channel.
If further information is needed, please refer to his paper “Simulation for characteristics of the Dual-Material Double-Gate MOSFET” in “Journal of Analog and Digital Devices” (EI).
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