Hyeon-k Seo

Thermal-Flow Characteristics of Ferrofluids in a Rotating Eccentric Cylinder under External Magnetic Force

Heat dissipation has become an important issue due to the miniaturization of various electronic devices. Various methods such as spray and nozzle coolers, heat sinks and so on are used for heat dissipation. However, the emergence of ferrofluids drastically improves the operating characteristics of electromagnetic systems and devices. A ferrofluid is a suspension containing 10-nm magnetic particles in a colloidal solution. This material exhibits paramagnetic behavior and is sensitive to magnetic field and temperature. In this study, heat transfer characteristics of ferrofluids in a rotating eccentric cylinder were investigated using the commercial code, COMSOL Multiphysics. Numerical results of the local Nusselt number, magnetophoretic force and velocity distributions were obtained from various eccentricities of the cylinder, and the results were graphically depicted with various flow conditions.

Hydraulic Characteristics of the Francis Turbine with Various Groove Shapes of Draft Tube

The small hydropower is a renewable energy technology, because the energy resource ‘falling water’ is replenishable as the fuel (falling water) is part of the hydrological cycle. This technology is being a form of renewable energy with no gaseous emissions, the small hydropower systems are among the options for climate change mitigation and therefore, they are candidates for international carbon trading opportunities such as the Clean Development Mechanism. The draft tube is an important component of a Francis turbine which is one of the small hydropower systems, influences the hydraulic performance. Moreover, as the swirl flow in the draft tube of the Francis turbine decreases pressure at the inlet region, the suppression of swirl flow can be a useful method of minimizing the occurrence of cavitation. In this study, the flow characteristics in a Francis turbine on the 15 MW hydropower generations have been investigated numerically with various shapes of draft tube. Numerical analysis was conducted by using the commercial code, ANSYS CFX. Results showed that the grooved model has relatively uniform distribution of pressure field compared with basic model. Hence, the stability of flow is enhanced, which may attribute to the suppression of draft surge and cavitation.

High-current arcs simulation in a puffer-assited self-blast interrupter considered the mechanical stress

In SF6 puffer-assisted self-blast interrupters, which are being developed as the next-generation switching mechanism, the arc burns within insulating nozzles with an ablation of the nozzle material and leads to an incoming gas flow for the pressure chamber with a pressure-rise during the high-current phase. Close to current zero, the arc can be cooled down and quenched completely by the reverse gas flow from the chamber to the arcing zone according to the pressure gradient between them. Therefore, the interruption of fault currents is dependent on the thermal decay of arcs during the current-zero phase by the blowing-off force, which is generated by the pressure-rise of the chamber during the high-current phase. In this study, we calculated SF6 switching arcs caused by fault current interruption inside a SF6 puffer-assisted self-blast interrupter with the reversed stroke of electrode and investigated the thermal breakdown with thermal decay near current zero.

Magnetic field and thermal characteristics simulation of axial magnetic field contacts in a vacuum interrupter

In this study, we considered two different types of axial magnetic field (AMF) electrodes, the coil type and the cup type, which are used widely in industries, to investigate the effective arc area and the magnetohydrodynamic characteristics by high-current vacuum arcs in a vacuum interrupter (VI) through three-dimensional FEM analysis. In order to validate the present calculation, the calculated results were compared with the results obtained using a commercial package, MAXWELL 3D, which is a reliable analysis tool for electro-magnetic fields, to validate the present calculation. From the results of two different types of AMF electrodes, the maximum axial magnetic flux (Bz) was locally observed with the respective number of slots on the contact surfaces. Results also showed that the cup type electrode led to a more uniform AMF distribution and Joule heat concentration at the contact than the coil type electrode.

Effect of Hole Pitch Ratio and Compound Angle on the Thermal Flow Fields of Double-Jet Film Cooling Hole

In this study, the effect of a row of double-jet film-cooling hole configurations on the thermal-flow characteristics of gas turbine blades was examined. To investigate the effect of the interference of anti-kidney vortices, the ratios of the pitch distance and hole diameter (P/d=5, 6.25, 8.333) were considered with two different compound angles (λ=0°, 4°). The film cooling performance and the generated losses were studied. Then, the relevant mechanisms were identified and explained. A numerical study was performed using ANSYS CFX 14.5 with the shear stress transport (SST) turbulent model. The blowing ratio was kept at a constant value of M=1.5. The film cooling effectiveness and temperature distribution are graphically depicted with various geometrical configurations.Copyright

Numerical Study on the Thermal Characteristics of Large Sized Silicon Polycrystal Growth by DS Method

The development of the manufacturing processes of solar silicon ingot is one of the more important issues to guarantee the growth of the photovoltaic industry, because of the saving of manufacturing costs of ingots, wafers, solar cells and of solar modules. Several solidification processes have been developed by industries, including casting, heat exchanger method and electromagnetic casting. However, the market growth using mono- and polycrystalline Si wafers might be saturated due to the shortage of Si feedstock. One of the methods to solve this problem is to make higher quality polycrystalline Si wafers which are capable of producing higher efficiency solar cells. In this paper, the effects of changing several geometrical configurations of cooling path were evaluated to improve the directional solidification (DS) method and to achieve the main issues. The developed DS method has the advantages of the small heat loss, short cycle time and efficient directional solidification. Based on the CFD (computational fluid dynamics) model, the simulation was performed on the thermal characteristics during the DS process. Using a commercial CFD code, Fluent, the thermal characteristics in the DS system are calculated, and the results are graphically depicted.Copyright