Jo May 30, 2024
Bundle-series initiation is currently used as a method for simultaneously transmitting blast signals from a detonator to a large number of shock tubes for blasting using shock tubes and its initiation modes affect the ability and reliability of signal transmission.
For building demolition and tunneling blasting, over 50, even 100 lines of shock tubes must be simultaneously initiated by a detonator. In the meantime, a large number of shock tubes often cause initiation failures. Therefore, for shock tubes, blasting system must have not only high ability of blast signal transmission (or initiation) but also high reliability of the signal transmission on shock tubes. The methods of bundle-series initiating a large number of shock tubes are subject to possible failure modes where the ability and reliability of signal transmission on each shock tube can be strongly affected by the coupling mode of a detonator and the number of shock tubes.
To the best of our knowledge, there are no reports in the literature regarding the experiments and numerical simulation to predict or estimate the dynamic blast load and the initiating probability according to the coupling structure of a detonator with many shock tubes.
Choe Yong Chol, a researcher at the Faculty of Mining Engineering, has studied the structural influence of initiation modes on the pressure impulse generated by a detonator and transmitted to the bundle of a large number of shock tubes and the signal transmission probability, and proved the advantage of frontal bundle-series initiating mode over lateral bundle-series initiating mode.
He confirmed that the numerically obtained pressure impulse for a bundle of shock tubes shows a good correlation with the experimentally obtained signal transmission probability of a bundle of shock tubes for a variety of bundle-series initiation modes of shock tubes.
You can find further details in his paper “Influence of Initiation Modes in the Bundle-Series Initiation of a Large Number of Shock Tubes by Detonators” in “Shock and Vibration” (SCI).
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Jo May 29, 2024
It can be said that many production means and consumer goods in use today are actually associated with comminution. For example, a machine tool is made of steel, the production of which starts from mining and comminution of iron ore. Also, the production of cement used for construction of houses and buildings where we live and work begins with crushing of limestone and ends with grinding of cement clinker. Coal comminution is still included in the production of electricity indispensable for modern production and life. Thus, it can be said that comminution is an essential part in many fields of economy, consuming a greater amount of energy. Therefore, it is very important to reduce energy consumption of a comminution machine by studying its working process on scientific basis.
Breaking in a hammer crusher, a kind of machine for comminution, occurs fundamentally due to collision between the hammer and agglomerate. So it is of great importance to study the collision breakage process between the hammer and agglomerate. In particular, accurate calculation of the energy loss quantity of a hammer in the collision breakage process is one of the key issues in determining the breakage power of a hammer crusher.
Previous studies on collision crushing in the hammer crusher by DEM considered the collision process between hammer modeling in the wall and agglomerate. However, the wall in the DEM model has no mass, so its energy cannot be considered. Thus, the aforementioned simulation methods cannot calculate the energy change of the hammer in the collision process. The aim of studying the collision crushing between the hammer and agglomerate in the hammer crusher is to estimate the breakage degree of agglomerate due to collision and to predict the energy consumption in a hammer crusher by obtaining the energy loss of hammer in the collision process.
Hong Sok Gun, a researcher at the Faculty of Mining Engineering, has obtained the energy change in the hammer by making a particle model with a combination of particles rather than walls, in accordance with the nature of the DEM. From the viewpoint of attaching greater importance to the internal structure rather than the external one of the breakage process in agglomerate modeling, he made an agglomerate model starting from one perspective on internal crystal structure of minerals discussed in crystal mineralogy.
This method allows the prediction of energy to be added to the breaking tool to break the agglomerate by directly measuring the energy loss of the breaking tool in the study on the breakage process by DEM.
You can find further details in his paper “Numerical Simulation of the Collision Breakage Process between the Agglomerate and Hammer in a Hammer Crusher Using DEM” in “Shock and Vibration” (SCI).
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Jo May 28, 2024
Analyzing coercivity mechanism is one of the key problems to understand magnetic behavior and to improve the magnetic properties of ferromagnetic materials including permanent magnets.
Up to now, many studies have been done to analyze the coercivity mechanisms of different kinds of permanent magnets, especially rare earth nanocrystalline permanent magnets based on Nd-Fe-B or Sm-Co alloys.
The coercivity mechanism of nanocrystalline magnets also depends on microstructures, especially on grain boundary phase. A study on coercivity of Nd-Fe-B sintered magnets showed that existence of amorphous Nd-rich grain boundary between Nd2Fe14B grains strengthens domain wall pinning so that coercivity is enhanced. And a study on the magnetic properties of high performance Dy-doped Nd-Fe-B sintered magnet concluded that the Dy-rich shell increases the nucleation field from the grain boundary and the Nd-rich layers weaken the inter-granular exchange coupling to enhance coercivity.
Kim Kyong Gi, a researcher at the Faculty of Physical Engineering, evaluated and analysed the relationship between coercivity and α-Fe layer thickness in an exchange coupled Nd2Fe14B/α-Fe nanocomposite multilayer system by comparing the results of previous experimental and micromagnetic studies.
The results showed that the coercivity decreases with increasing α-Fe layer thickness, which is coincident with the results of prior micromagnetic and experimental studies.
For further information, please refer to his paper “An analytic study on coercivity mechanism of exchange coupled Nd2Fe14B/α-Fe nanocomposite magnets” in “Journal of Magnetism and Magnetic Materials” (SCI).
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Jo May 26, 2024
Blockchain is a distributed ledger that is shared and maintained by all participants on the network based on a consensus protocol. The most widely used consensus mechanism is Proof-of-Work (PoW), which has been deployed in public blockchain networks like Bitcoin and Ethereum. In PoW, block generation requires solving a cryptographic math puzzle whose solution is easy to verify but extremely hard to solve. The participants in the blockchain network exhaust their computing resources to solve the puzzle. Here, block generation is called mining and participants are called miners.
Once a miner successfully mines a block, he becomes eligible to receive a reward. For this reason, miners competitively participate in the mining process. To reduce the average block generation time, individual miners aggregate their computing powers into a mining pool, where all participating miners solve the puzzle together and share the reward. In a mining pool, the pool manager distributes less difficult partial PoW puzzles (pPoW) than the original full PoW puzzle (fPoW) to individual miners. If enough miners solve pPoW, some of these solutions are likely to become the solution to fPoW because each solution to pPoW has a probability of yielding a solution to the fPoW. However, mining pools undermine the decentralization and security of Blockchain. Especially, if a single mining pool owns more than half of the entire network mining capacity, this pool can generate the blocks faster than any other pool, reaping all rewards and choices of transactions to confirm (51% attack).
Meanwhile, in order to be used in bitcoin practically, any solution to the centralized mining problem must preserve the existing blockchain; preserve large investments many miners have made and are planning to make in their equipment; provide a seamless transition from the existing system to the new one, providing adjustable knobs that can be fine-tuned for a desired trade-off that fits the community’s needs.
However, none of the proposals satisfies all the requirements above. In other words, preceding techniques require some changes in the design of the cryptocurrency and so, they are not compatible with the current bitcoin system.
Kim Kyu Chol, a section head at the Faculty of Information Science and Technology, has proposed a hybrid PoW consensus protocol in order to discourage centralization and tackle the 51% attack. In the proposed scheme, miners can generate the block by solving either an original outsourceable bitcoin puzzle or a non-outsourceable puzzle.
The main feature of his system is that it is fully compatible with current bitcoin designs, i.e., it can be implemented right now, because it preserves both the existing blockchain and investments which have been made in mining hardware. In contrast to the current bitcoin system and any other preceding protocols, his scheme presents two puzzles, but is still single-tiered.
You can find further details in his paper “Single-Tiered Hybrid PoW Consensus Protocol to Encourage Decentralization in Bitcoin” in “Security and Communication Networks” (SCI).
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Jo May 24, 2024
Water inrush in mining fields is considered as one of the major dangerous factors for safe production and workers’ lives in coal mines. To forecast the water inrush in coal mines, various software related to flow simulation has been used in recent years. Especially, Visual MODFLOW (VMOD) is one of the powerful software that provides the tools for building three-dimensional groundwater conceptual and numerical models.
Visual MODFLOW (VMOD) can simulate the water flow distribution state with the hydrogeological conditions and detailed data in accordance with pumping well tests of the mine. In numerical simulations for underwater flow, repeated measurements of water level is essential for forecasting water inrush, so the control point drawdown must be clearly calculated in accordance with the mining construction plans, dewatering period and drainage time.
Song Myong Song, a researcher at the Faculty of Mining Engineering, has simulated the water flow distribution state considering the hydrogeological conditions and detailed data in accordance with pumping well tests of a mine.
First, he built a mathematical model to simulate the water inrush along the major faults of the mine by using three-dimensional simulation of groundwater flow analysis software (Visual MODFLOW). Then, on the basis of the full analysis of the geological and hydrogeological conditions, he set several major faults as boundary conditions and evaluated the water level distribution under steady flow conditions using the in-situ well pumping test data. After that, he determined optimal parameters in relation to the water inrush forecasting using the PEST-ASP module in Visual MODFLOW in order to get the accurate groundwater flow distribution at the control points including normal and maximum water inrush.
Comparing the simulation results with in-situ data of water inrush, the relative error was lower than 0.74%, showing that this method is the most effective for forecasting water inrush in coal mines.
For further details, please refer to his paper “Forecast of Mine Inflow using In-situ Well Test and Visual Modflow in an Underground Mine” in “CHALLENGES IN ROCK MECHANICS & ROCK ENGINEERING”, proceedings of the 15th ISRM Congress 2023 & 72nd Geomechanics Colloquium.
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Jo May 22, 2024
Today, demand for energy has been increasing and the environment pollution has become serious with the rapid growth of economy day by day. The reality urgently requires reduction of fossil fuel consumption and wide use of renewable energy resources.
Geothermal energy, a kind of the renewable energies such as solar energy and waste heat from the industry, has been widely used for power generation as an energy source unaffected by seasons and environment. Organic Rankine cycle (ORC) is recognized as a practical solution for small power production using low grade geothermal resources thanks to its simplicity, reliability, and flexibility.
Selection of the working fluid, cycle configuration and operation parameters most suitable for given temperature of heat source is a key process to improve the performance of ORC. For this reason, many researchers have reviewed the optimal combination of ORC configurations, working fluid and cycle parameters under the conditions of heat source above 100℃ and below 100℃. However, it is difficult to identify the optimal ORC configuration, working fluid and cycle parameters for particular areas from the reported data since the best working fluid and cycle parameters vary with the temperatures of heat and cooling source and the system configuration.
Ri Hung Nam, a section head at the Faculty of Heat Engineering, has performed thermodynamic optimization of ORC based on the net work output and exergy efficiency in areas with different geothermal and cooling water temperatures.
He determined optimal ORC system configuration, best working fluid, optimal evaporation temperature and the optimum area through comparative analysis of 8 different ORC systems using 8 working fluids in the two areas. He proposed a multi-objective function combined with the net work output and exergy efficiency to evaluate the overall thermodynamic performances of ORC.
As a result, he found that even though the exergy efficiency is relatively low in the area with lower temperatures of geothermal and cooling water, the net work output is greater and the overall thermodynamic performance is more powerful. It was shown that the area where geothermal and cooling water temperatures are relatively low is the optimal one in terms of overall thermodynamic performance of ORC.
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