• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.35 no.12
• /
• pp.1383-1389
• /
• 2011

The current study focuses on the consistent analysis of heat transfer in multichannel volumetric solar receivers used for concentrating solar power. Changes in the properties of the absorbing material and channel dimensions are considered in an optical model based on the Monte Carlo ray-tracing method and in a one-dimensional heat transfer model that includes conduction, convection, and radiation. The optical model results show that most of the solar radiation energy is absorbed within a very small channel length of around 15 mm because of the large length-to-radius ratio. Classification of radiation losses reveals that at low absorptivity, increased reflection losses cause reduction of the receiver efficiency, notwithstanding the decrease in the emission loss. As the average temperature increases because of the large channel radius or small mass flow rate, both emission and reflection losses increase but the effect of emission losses prevails.

• Journal of Astronomy and Space Sciences
• /
• v.21 no.2
• /
• pp.93-100
• /
• 2004

We have investigated characteristic solar wind dynamics associated with relativistic electron events at geosynchronous orbit. Most of the events for April, 1999 through December, 2002 are found to be accompanied by a prolonged solar quiet period which is characterized as low solar wind density, weak interplanetary magnetic field (IMF), and fast alfvenic fluctuations in IMF $B_z$. In a typical relativistic event, electron fluxes begin to increase by orders of magnitude when solar wind parameters drop to low values (e.g., $n_{sw}∼5 cm^{-3}$ and ｜$B_{IMF}$∼5 nT) after sharp peaks. Then the elevated electron fluxes stay at the high level during the solar quiet period. This observation may suggest the following scenario for the occurrence of a geosynchronous relativistic event: (ⅰ) Quiet solar winds can yield a stable and more dipole-like magnetospheric configurations in which the geosynchronous orbit locates well inside the trapping boundary of the energetic electrons. (ⅱ) If a large population of MeV electrons are generated (by whatever acceleration process(es)) in the inner magnetosphere, they can be trapped and effectively accumulated to a high intensity. (ⅲ) The high electron flux can persist for a number of days in the geosynchronous region as long as the solar wind dynamics stays quiet. Therefore the scenario indicates that the occurrence of a relativistic event would be a result of a delicate balance between the effects of electron acceleration and loss. In addition, the sensitive dependence of a relativistic event on the solar wind conditions makes the prediction of solar wind variability as important as understanding of electron acceleration processes in the forecast of a relativistic event.

• Journal of international Conference on Electrical Machines and Systems
• /
• v.3 no.1
• /
• pp.78-82
• /
• 2014

Power Transformers with more than one secondary winding are not uncommon in industrial applications. But new classes of applications where very large number of independent secondaries are used are becoming popular in controlled converters for medium and high voltage applications. Cascade H-bridge medium voltage drives and Pulse Step Modulation (PSM) based high voltage power supplies are such applications. Regulated high voltage power supplies (Fig. 1) with 35-100 kV, 5-10 MW output range with very fast dynamics (${\mu}S$ order) uses such transformers. Such power supplies are widely used in fusion research. Here series connection of isolated voltage sources with conventional switching semiconductor devices is achieved by large number of separate transformers or by single unit of multi-secondary transformer. Naturally, a transformer having numbers of secondary windings (~40) on single core is the preferred solution due to space and cost considerations. For design and simulation analysis of such a power supply, the model of a multi-secondary transformer poses special problem to any circuit analysis software as many simulation softwares provide transformer models with limited number (3-6) of secondary windings. Multi-Secondary transformer models with 3 different schemes are available. A comparison of test results from a practical Multi-secondary transformer with a simulation model using magnetic component is found to describe the behavior closer to observed test results. Earlier models assumed magnetising inductance in a linear loss less core model although in actual it is saturable core made-up of CRGO steel laminations. This article discusses a more detailed representation of flux coupled magnetic model with saturable core properties to simulate actual transformers very close to its observed parameters in test and actual usage.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.25 no.1
• /
• pp.35-38
• /
• 2015

LEDs are considered to be an alternative for enhancement of energy efficiency, applied for numerous areas such as display, automotive headlight not only lights. Yellow phosphor is generally utilized with blue LEDs to generate WLED, $Y_3Al_{5}O_{12}:Ce^{3+}$ is typically used as the yellow phosphor. The phosphor, mixed with epoxy resin, has been used by being spread and hardened on the blue LED chip. This paste-based packaging gives rise to problems of degradation of phosphor by heat and decrease of luminous efficiency. Although phosphor plate is used instead of the epoxy-phosphor mixture to solve these problems, loss of luminous efficacy by total internal reflection inside the plate also should be solved. In this study, we coated the side of the plate with silver as one of the solution.

• Journal of Welding and Joining
• /
• v.35 no.3
• /
• pp.68-74
• /
• 2017

Characteristics of dissimilar metal welds between alloy steel ASTM A387 Gr. 91 and carbon steel ASTM A516 Gr.70 made with Flux cored arc welding(FCAW) have been evaluated in terms of microstructure, mechanical strength, chemical analysis by EDS as well as corrosion test. Three heat inputs of 15.0, 22.5, 30.0kJ/cm were employed to make joints of dissimilar metals with E71T-1C wire. Post-weld heat treatment was carried out at $750^{\circ}C$ for 2.5 h. Based on microstructural examination, Intragranular polygonal ferrite and grainboundary ferrite were formed only in first layer of weld metal. Another layers consisted of acicular ferrite and $Widmannst{\ddot{a}}tten$ ferrite. The amount of acicular ferrite was increased with decreasing heat input and layer. Heat affected zone of alloy steel showed the highest hardness due to the formation of tempered martensite and lower bainite. Lower and upper bainite were formed in heat affected zone of carbon steel. Tensile strengths of dissimilar metal welds decreased with increasing heat inputs. Dissimilar metal welds showed a good hot cracking resistance due to the low HCS index below 4. The salt spray test of dissimilar metals welds showed that the weight loss rate by corrosion below 170 hours was decreased with increasing heat inputs due to the increase of the amount of acicular ferrite.

• Nuclear Engineering and Technology
• /
• v.25 no.1
• /
• pp.8-19
• /
• 1993

The natural convection model of the consolidated system has been developed to make sure the removal of decay heat generated in the spent fuel for the loss of forced cooling accident. The numerical technique employed was based on the ADI scheme. The calculation of heat generation rate in the spent fuel was peformed by the ANS-79 decay heat model, and the nonuniform surface heat flux is assumed with a chopped sine curve for the conservative decay heat generation input. The sensitivity study was performed to examine the possibility of the pool bulk boiling by varying the various parameters, i.e. inter-fuel spacing ratio, heat generation power, and radius of the fuel rod. The application results of this model show that the natural circulation flow through compacted spent fuel bundles enables the pool temperature to control in a safe and effective manner, after the required cooling time. The corresponding acceptance criteria of the cooling time for rearranging the spent fuel rods were also found.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.17 no.1
• /
• pp.181-187
• /
• 2016

Silica is an essential material in the electronics industries of LCDs and OLEDs, which particularly require high purity. This study attempted to find the optimal design of a magnetic separator for silica sand containing iron compounds using CFD simulation. Three designs of magnetic separation were prepared and their efficiency was examined. As a result of the evaluation, the sufficient contact of particulate silica with the surface of magnetic emitters improved the magnetic separation effects. In addition, the loss of $SiO_2$ and the removal rate of $Fe_2O_3$ depended strongly on the particle size, flow rate and magnetic flux density. In addition, magnetic separation is quite effective for a particle size of $10{\mu}m$ with a 0.2 m/s flow rate.

• The Transactions of the Korean Institute of Electrical Engineers
• /
• v.38 no.3
• /
• pp.155-165
• /
• 1989

In this paper, design equations for the calculation of the dimensions and characteristics of the ferrite magnet DC motor are derived. Through the computer iterative calculation applying the parameter survey method with those equations, the design method is presented. The following facts and the propriety of the design method are identified by comparing with the computer simulation results and dimension elements for the proposed motor. (1) The dimension ratio and the pole arc ratio as simulation parameters are in close connection with the dimension elements and motor performance, and those values of the parameter are important factor in determining the reasonable dimension of the motor. (2) It is proved that the minimization of the ferrite magnet volume is possible by representing the permeance coefficient as a function of the flux density ratio only. (3) It is shown that the torque equation suggested by introducing the copper loss area density of the conductors located in the slot is available in the determination of motor dimensions.

• Journal of the Korean association of regional geographers
• /
• v.10 no.3
• /
• pp.667-681
• /
• 2004

This research is aimed to investigate the relationship of the effective wind velocity and the morphological change in coastal dune at Sindu-ri in Korea. Sediment flux was estimated by the measurement of elevation change along eight transects deployed in the study area from July 1999 to July 2000. The results of this study indicated that. first. based on the occurrence of morphological change and winds effective for sand movement. two distinct seasons were identified: a season of deposition and that of erosion. During the depositional season, spanning from November to April, effective winds were dominant and sand accumulation occurred mainly in foredunes and sequentially in dune plains. During the erosional season, from May through October, volume change was small and erosion or sand loss occurred mostly at the dunefoot of foredunes. Secondly, the research revealed that the sediment budget of Sindu coastal dune turned out to be surplus on the whole, but there are some regional differences. Deficit budgets were observed partly in secondary dunes. The utmost northern part of Sindu coastal dune was provided with abundant sand, whereas the central and northern parts were poorly supplied.

• Economic and Environmental Geology
• /
• v.3 no.1
• /
• pp.17-24
• /
• 1970

The purpose of this report is to prepare a data for the economic evaluation on the Goobong Limestone Mine which is located at the south-eastern corner of the Yongchun Quadrangle scaled in 1:50,000. The accessibility from the mine to railroad was considered in two ways. One is to Dodam Station on Central Railway Line and the other is to reach Songjung-ni village which is near Sangyong Station on Hamback Railway Line. The distance of the former way is 26.7km and the later is 24.2km. Geologically the mine is situated near the base of the Greast Limestone Series which strikes generally $N25^{\circ}{\sim}30^{\circ}E$. The series comprises six different formations from older to younger; Pungchon Limestone Formation and Whajol Formation of Cambrian age, and Dongjum Quartzite Formation, Dumudong Formation, Maggol Limestone Formation and Goseong Formation of lower to middle Ordovician age. 82 samples; 48 from Pungchon Limestone Formation, 11 from Dumudong Formation, 15 from Maggol Limestone Formation and 8 from Goseong Formation, were taken from the series in the crossed direction to the general trend of the series as shown in geological map. They were chemically analyzed on the components of CaO, MgO, $SiO_2$, $R_2O_3(Al_2O_3+Fe_2O_3)$ and ignition loss as shown in table 2, table 3, table 4, and table 5. As seen from the tables, among the formations of the series, middle to upper parts of the Pungchon Limestone Formation and middle and upper parts of the Dumudong Formation have chemical composition as available source for the raw material of cement industry, not only that but also the part of the Pungchon Formation was highly evaluated as source for the flux of iron smelting and the raw material of carbide manufacturing because of its high purity of calcium carbonate.