Jo Dec 23, 2023
Coal is a main energy resource and many households in our country use honeycomb briquettes made of coal for both heating and cooking. But because there is no necessity for heating in summer, a lot of coal is wasted when honeycomb briquettes are used as fuel for cooking.
What is important in making ignition honeycomb briquette as fuel for cooking is to reduce the consumption of ignition energy to the minimum, to shorten ignition time and to raise the combustion rate as high as LPG (Liquefied Petroleum Gas) so that cooking can be finished in a short time. Therefore, many researchers have conducted study on lowering an ignition temperature and raising a combustion rate by lowering combustion activation energy of coal with addition of combustion additive to coal or mixing of coal and biomass.
Kim Mun Hyok, a section head at the Faculty of Mining Engineering, has developed a double layer ignition honeycomb briquette with high combustion rate that can be ignited with only a sheet of paper. For this, he first calculated the minimum mass of a double layer honeycomb briquette for cooking a meal for a household (5 people) with coal and charcoal as main materials and sodium ferrate solution as a combustion additive. Then, he determined the mixing ratio and the crushing size of raw materials of the ignition layer, and the additive amount of combustion additives by the orthogonal experimental method.
The combustion rate of the double layer ignition honeycomb briquette is as high as 84.18% of LPG.
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Jo Dec 22, 2023
Hydrofoil craft have been continuously built and used for various purposes because of their high speed performance, and many experimental investigations and theoretical researches have been made for the development of them. Shallowly immersed hydrofoils near free surface have large lift-drag ratio and their structure is rather simple. These hydrofoils are applied to many hydrofoil craft with Tandem hydrofoils. In order to apply Tandem hydrofoil system, the interaction between hydrofoils and the stability of vertical plane motion should be solved.
Jang Tong Hyon, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has built a mathematical model of finite hydrofoil near free surface by using horseshoe vortex lattice method and proposed another equation for determining the lift coefficients of hydrofoils by using this model.
This equation is convenient for differentiation and integral with respect to the immersed depth of hydrofoils. Therefore, it needs no model tests to calculate the very approximate lift coefficients of the hydrofoils and it can effectively be applied to designing Tandem hydrofoil craft with stability of vertical plane motion. He verified it by applying it to designing and building hydrofoil craft without any model tests.
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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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