Jo Dec 26, 2023
Diffusion combustion is widely used in conventional diesel engines, and heat release rate and formation of harmful substances are determined by the mixing characteristics of diesel spray.
Both fuel atomization and spray characteristics are keys to improving diesel combustion and emission performance. In a combustion chamber with certain geometry, the process of fuel spray and mixing has a great impact on the combustion and emissions.
In a DI diesel engine, the centers of the combustion chamber and the injector are eccentric and the injector inclines gently. Therefore, in order to improve the fuel/air equivalence ratio distribution, it is important to analyze the effect of spray parameters such as spray angle, spray direction and number of spray holes suitable for the combustion chamber type.
In order to improve the quality of mixture, to boost combustion effects and to reduce the emission of exhaust gas, Hwang Kon, an institute head at the Faculty of Shipbuilding and Ocean Engineering, has analyzed the effect of spray angle in a reentrant combustion chamber and determined the best spray angle.
He simulated and investigated mixture formation and combustion processes at different spray angles in diesel engines by using AVL-FIRE code.
The main findings can be summarized as follows:
(a) Spray angle has a great influence on the fuel/air equivalence ratio in the combustion chamber and in-cylinder temperature distribution, and it also concerns the combustion process and emissions.
(b) The combustion performance parameters imply that increase in spray angle up to 160° is suitable and at that time emission is minimized.
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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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