• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.7
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• pp.895-904
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• 2001

Flow fields, temperature distributions, and particle trajectories in a 2-stage entrained-flow gasifier are calculated using a CFD code, FLUENT. Realizable k-$\xi$ model is used as a turbulent model. Because of swirling flow there appear recirculation regions near the burners. The characteristics of flow fields and temperature distributions in the gasifier are dependent on the swirl number of the system. Mean residence time of the particles in the reductor is inversely proportional to particle size, particle density and swirl number. As the swirl number is increasing, the particles injected from the combustor burners approach the wall near the combustor burners, which prevents the particles from entering the reductor and thus attatching the reductor wall. If the lower combustor burner angle is larger than the higher combustor burner angle for a given swirl number, the particles may move toward the reductor and cause ash/slag deposition problem.

• Journal of the Korean Society of Combustion
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• v.20 no.1
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• pp.43-51
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• 2015

A study on laminar jet flames in coflow air diluted with helium has been conducted to investigate self-excitations for various propane mole fractions and nozzle exit velocities. The stability map was represented as a function of nozzle exit velocity and fuel mole fraction for propane. The results show that two types of self-excitation were observed : (1) buoyancy-driven self-excitation (hereafter called BDSE) and (2) Lewis-number induced-self-excitation coupled with (1) (hereafter called LCB) near extinction limit for 9.4 mm nozzle diameter. It was shown that with 0.95 mm nozzle diameter, Lewis-number-induced self-excitation (hereafter LISE) and BDSE could be separated. The differences between the two self-excitations were shown and discussed.

• Journal of Mechanical Science and Technology
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• v.20 no.1
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• pp.158-166
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• 2006

Extended Graetz problem in microtube is analyzed by using eigenfunction expansion to solve the energy equation. For the eigenvalue problem we applied the shooting method and Galerkin method. The hydrodynamically isothermal developed flow is assumed to enter the microtube with uniform temperature or uniform heat flux boundary condition. The effects of velocity and temperature jump boundary condition on the microtube wall, axial conduction and viscous dissipation are included. From the temperature field obtained, the local Nusselt number distributions on the tube wall are obtained as the dimensionless parameters (Peclet number, Knudsen number, Brinkman number) vary. The fully developed Nusselt number for each boundary condition is obtained also in terms of these parameters.

• Journal of Korea Technology Innovation Society
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• v.7 no.3
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• pp.555-580
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• 2004

We analyzed the effect of 6-month lagged number portability policy and introduction of common number among carriers on Korean mobile market. Our result shows that the policy change contributed to both the increase in number of mobile subscribers and decrease in dominant carrier's market share. We found that unified prefix among mobile operators is more effective than the lagged number portability policy. The carriers' profit, however, decreased a lot due to excessive marketing expenditure. We also estimated market shares of carriers up to year 2005 when full number portability policy is adopted. The results show that dominant carrier's monopolistic power will be recovered by the end of 2005 implying government's number policy itself cannot guarantee the effective competition in Korean mobile market.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.20 no.5
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• pp.510-521
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• 2008

In the study, experiments are performed in the interaction region combined with wave and current to investigate the characteristics of local scour around submarine pipelines. Wave generator and current generator are used for the experiments and two current directions were used; co-direction and counter direction to the wave. The local scour depths around the pipeline are obtained according to the various pipe diameters(D), wave periods(T), wave heights(H), and current velocities(V). The experiments show that the maximum equilibrium local scour depth increases with pipe diameter, wave period, wave height, and current velocity. Using the experimental results, the correlations of scour depth and parameters such as Shields parameter($\theta$), Froude number(Fr), period parameter, Keulegan-Carpenter number(KC), Ursell number($U_R$), modified Ursell number($U_{RP}$) and ratio of velocities($U_{c}/(U_{c}+U_{m})$) are analyzed. In the interaction region combined with waves and currents, Froude number and Shields parameter are found the main parameters to cause the local scour around the submarine pipelines and this means that current governs the scour within any limits of the currents.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2005.11a
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• pp.178-179
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• 2005

Numerical and experimental investigation of incompressible turbulent flow and heat transfer through square channels with varying number of ribbed walls were conducted to determined pressure drop and heat transfer. The CFX solver used for the computation. The rough walls have a $45^0$ inclined square rib. Uniform heat flux is maintained on whole inner heat transfer channel area. The numerical results agreed well with experimental data that obtained for 7600$D_h$) of 0.0667. The results show that values of local heat transfer coefficient and friction factor increase with an increasing number of ribbed walls.

• Journal of Environmental Science International
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• v.18 no.10
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• pp.1143-1153
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• 2009

The purpose of this study is to propose an appropriate method for marking out a boundary line of area damaged by aircraft noise. In an actual situation, there are many gaps between the boundary line of aircraft noise contour and the boundary line of an occupant of a house. Three cases faced in practice are considered in this study. Case I is considered the land number together with sub-number. The Tong and Ban (the residential district number in Korea) are considered in Case II. In Case III, the configuration of ground is examined. The authors expect that the authority of aviation affairs should choose one of these cases and put in force in the near future.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.49 no.11
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• pp.632-636
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• 2000

Fuzzy controllers show good performance in case of the systems being nonlinear and difficult to solve. But these fuzzy controllers have problems which have to decide suitable rules and membership functions. In general, we decide those using the heuristic methods or the experience of experts. Recently, G.A. have been studied in this field. The number of rules increase exponentially when the number of input and output increase. It also makes hard to decide the rules and membership functions even though we use G.A. In this paper, we suggest parallel fuzzy controllers, and also the method to decrease the number of rules. The excellent performance of these methods is confirmed through simulations.

• Membrane and Water Treatment
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• v.3 no.1
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• pp.51-62
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• 2012

The permeate flux declination along an ultrafilter membrane is due mainly to the concentration-polarization resistance increment and the decline in transmembrane pressure. It was found in previous works that the concentration polarization resistance could be reduced in a ring-rod tubular membrane ultrafilter using the turbulent behavior. In the present study, the performance was further improved by properly and gradually decreasing the baffled-ring distance along the cross-flow channel coupled with properly adjusting the number of baffled rings. This theoretical analysis is based on the mass and momentum balances as well as the application of the resistance-in-series model. The correlation predictions are confirmed with the experimental results for dextran T500 aqueous solution ultrafiltration.

• International Journal of Contents
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• v.16 no.4
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• pp.26-38
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• 2020

Determining the optimal levels of the technical attributes (TAs) of a product to achieve a high level of customer satisfaction is the main activity in the planning process for quality function deployment (QFD). In real applications, the number of customer requirements for developing a single product is quite large, and the number of converted TAs is also high so the size of the house of quality (HoQ) becomes huge. Furthermore, the TA levels are often discrete instead of continuous and the product market can be divided into several market segments corresponding to the number of HoQ, which also unacceptably increases the size of the QFD optimization problem and the time spent on making decisions. This paper proposed a genetic algorithm (GA) solution approach to finding the optimum set of TAs in QFD in the above situation. A numerical example is provided for illustrating the proposed approach. To assess the computational performance of the GA, tests were performed on problems of various sizes using a fractional factorial design.