Jo Dec 20, 2023
A mine hoist plays an essential part in mine haulage.
In open pit mines, ore and overburden are transported from working places to the earth’s surface by incline hoists. In addition, this machine transports miners from the surface to the various underground levels of underground mines.
In recent years when open mining is widely conducted, dynamic analysis of skips during discharging of hoisting equipment for inclined shafts have an important bearing on the design of transport systems by skips for inclined shafts. It is particularly important to analyze the influence of the structure of a non-dumping large-size skip and the geometrical locus type of a dump scroll track during skip discharging on the output of hoisting equipment, that is, energy consumption of systems.
Paek In Chol, vice dean of the Faculty of Mining Engineering, has selected suitable geometrical locus of a cosine dump scroll track and a linear accretion dump scroll track and carried out a dynamic simulation and energy analysis of the discharging process by MSC Visual Nastran. On this basis, he proved that a cosine dump scroll track is better than a linear accretion dump scroll track. In other words, the cosine dump scroll track consumes less energy than the linear accretion dump scroll track.
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Jo Dec 19, 2023
Microgrid is a small-scale power system in which distributed generations (DGs) such as wind turbines and photovoltaic arrays combined with energy-storage systems are integrated to energize local loads and to deliver electricity to utility grid.
Microgrids now play a more crucial role than ever before concerning the system stability and power quality requirement, thanks to power electronic converters, information and communication technologies and distributed generators. The existence of nonlinear loads such as power electronic devices, electric arc furnaces, variable frequency motor drives, etc. may affect the voltage of the point of common coupling (PCC) due to the current harmonics.
To compensate voltage distortion, nonlinear load current should be shared among the DGs in islanded microgrids. However, harmonic power cannot be accurately shared among the DGs by droop control due to mismatched line impedances.
In order to cope with this problem, Kim Sung Hyok, a researcher at the Faculty of Electrical Engineering, has proposed a distributed harmonic sharing control using adaptive regulation of virtual harmonic impedance, when large amount of nonlinear loads are connected to the PCC.
According to his method, DGs make proportional distribution of harmonic power by using a consensus algorithm. The virtual harmonic impedance is obtained according to discovered harmonic power sharing error and the harmonic power sharing error is removed through interaction between the harmonic power re-sharing and the virtual harmonic impedance regulation in steady state.
The validity of the proposed method has been proved by MATLAB/Simulink simulation.
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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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