• Geomechanics and Engineering
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• v.13 no.1
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• pp.1-23
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• 2017

A coupled liquid-gas-solid three-phase model, linking two numerical codes (TOUGH2/EOS3 and $FLAC^{3D}$), was firstly established and validated by simulating an in-situ air flow test in Essen. Then the coupled model was employed to investigate responses of multiphase flow and soil skeleton deformation to compressed air or freshwater injection using the same simulation conditions in an aquifer of Tianjin, China. The simulation results show that with injecting pressurized fluids, the vertical effective stress in some area decreases owing to the pore pressure increasing, an expansion of soil skeleton appears, and land uplift occurs due to support actions from lower deformed soils. After fluids injection stops, soil deformation decreases overall due to injecting fluids dissipating. With the same applied pressure, changes in multiphase flow and geo-mechanical deformation caused by compressed air injection are relatively greater than those by freshwater injection. Furthermore, the expansion of soil skeleton induced by compressed air injection transfers upward and laterally continuously with time, while during and after freshwater injection, this expansion reaches rapidly a quasi-steady state. These differences induced by two fluids injection are mainly because air could spread upward and laterally easily for its lower density and phase state transition appears for compressed air injection.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.03a
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• pp.213-220
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• 2010

This study presents a numerical simulation model of vertical ground heat exchangers, considering unsaturated hydro static ground conditions induced by the ground water table fluctuation. Heat transfer in ground and grout is modeled by a 3-D FEM transient conductive heat transfer model, where heat transfer between circulating fluid and heat exchanging pipe is treated as 1-D quasi steady state forced convective elements. To take into account the unsaturated ground condition, soil thermal conductivity and heat capacity which are dependent on the matric suction are applied to ground elements. Parametric studies considering various ground water table conditions are conducted to investigate the influence of unsaturated hydro static ground condition on the mean heat exchange rate of ground heat exchanger. Simulation results considering water table fluctuation show 60~100% of mean heat exchange rate for a saturated soil condition and 125~208% of that for a dry soil condition. Thus consideration of unsaturated soil condition is substantially recommended for more accurate design and performance evaluation for ground heat exchangers.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.11
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• pp.1089-1096
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• 2001

The cool-down performance after soaking is very important in an automotive air-conditioning system and is considered as the key design variable. Therefore, understanding of the overall transient characteristics of the system is essential to the preliminary design as well as steady-state characteristics. The objective of this study is to develop a computer simulation model and estimate theoretical1y the transient performance of an automotive air-conditioning system. To accomplish this, a mathematical modelling of each component, such as compressor, condenser, expansion valve, and evaporator, is presented first of all. For a detailed calculation, condenser and evaporator are divided into many subsections. Each sub-section is an elemental volume for modelling. In models of expansion valve and compressor, dynamic behaviors are not considered in an attempt to simplify the ana1ysis, but the quasi-static ones are just considered, such as the relation between mass flow rate and pressure drop in expansion device, polytropic process in compressor, etc. The developed simulation model is validated with a comparison to laboratory test data of an automotive air-conditioning system. The overall time-tracing properties of each component agreed fairly well wish those of test data in this case.

• Proceedings of the KIEE Conference
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• 2009.07a
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• pp.193_194
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• 2009

본 논문은 장기 전압 안정도 분석을 위한 프로그램 개발을 목적으로 한다. 장기 전압 불안정 현상은 전력 계통에 투입되어 있는 무효전력과 관련된 제어기 중에서 그 동작특성이 수 분에서 수십 분에 이르는 제어기에 의해 발생한다. 기존에 장기 전압 안정도 분석에는 상용툴의 Full-time Domain 알고리즘을 이용하였다. 하지만 기존의 Full-time Domain 알고리즘에 의해 장기 전압 안정도 분석을 하게 되면 그 연산 시간이 매우 오래 걸릴 뿐만 아니라, 시적분에 의한 수치 에러로 인하여 정확성을 보장할 수 없다. 본 논문에서는 장기 전압안정도 분석을 하기 위해 준동적 시모의 알고리즘(Quasi-Steady-State Algorithm)을 도입하였다. 준동적 시모의 알고리즘을 적용한 프로그램을 보이고, 이에 따fms 모의를 한구전력계통에 적용하였다.

• Proceedings of the KIEE Conference
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• 2005.07a
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• pp.24-26
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• 2005

This paper presents a framework for determining control strategies against unwanted tripping actions of relay operation that plays a very important role in cascading events leading to voltage collapse. The framework includes an algorithm for quick identification of possible zone 3 relay operation during voltage instability. The proposed approach comes up with control strategy of load shedding at the selected location with active power and relay margin criteria. In addition, Quasi Steady-State (QSS) simulation is employed to obtain time-related information which is valuable for both the timing and amount of control. The methodology is demonstrated through the modified New England 39-bus system.

• Proceedings of the KIEE Conference
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• 2005.07a
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• pp.12-14
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• 2005

Security problem has been a fundamental issue in the operation and planning of power system. Voltage instability is widely recognized as an important issue of power system blackout. As far as real-time operation is concerned, there is a need for appropriate tools to identify dangerous contingencies, assess security margins and suggest corrective actions. In this paper, we propose the framework design of Voltage Security Assessment(VSA) using QSS(Quasi Steady-State) analysis method in order to implement fast time domain simulation engine as a major part of VSA.

• Proceedings of the KIEE Conference
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• 1998.11b
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• pp.438-441
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• 1998

In this paper, we proposes an adaptive flux observer to estimate initial values of rotor fluxes and unknown rotor resistance. The error system between the model of induction motor and a proposed observer is devided as a fast subsystem and a slow one by a singular perturbation system. The fast subsystem is exponentially convergent on a boundary-layer. And the overall error system is reduced to a quasi-steady-state system. The adaptive law for an unknown rotor resistance is designed to stabilize the approximate error system. As computer simulation results show, the proposed adaptive flux observer estimates fast initial values of rotor fluxes and unknown rotor resistance.

• Korean Chemical Engineering Research
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• v.59 no.1
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• pp.138-151
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• 2021

The effect of a reactant ratio on the growth of a buoyancy-driven instability in an irreversible A+B→C reaction system is analyzed theoretically and numerically. Taking a non-stoichiometric reactant ratio into account, new linear stability equations are derived without the quasi-steady state assumption (QSSA) and solved analytically. It is found that the main parameters to explain the present system are the Damköhler number, the dimensionless density difference of chemical species and the ratio of reactants. The present initial grow rate analysis without QSSA shows that the system is initially unconditionally stable regardless of the parameter values; however, the previous initial growth rate analysis based on the QSSA predicted the system is unstable if the system is physically unstable. For time evolving cases, the present growth rates obtained from the spectral analysis and pseudo-spectral method support each other, but quite differently from that obtained under the conventional QSSA. Adopting the result of the linear stability analysis as an initial condition, fully nonlinear direct numerical simulations are conducted. Both the linear analysis and the nonlinear simulation show that the reactant ratio plays an important role in the onset and the growth of the instability motion.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.9
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• pp.923-928
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• 2015

The FDS is one of the most used programs for fire analysis and needs an optimal grid selection for an accurate analysis. This study selected various grid aspect ratios (ARs) for selection of optimal grid and analyzed them with FDS v 6.1.2. A calculation time of 10 min. was used, which is enough to obtain the time average value of temperature changes. Temperature, visibility, and the time average value of mass balance are obtained from 200-600 s, which is a period of maintaining quasi-steady state. Two polyurethane fires of 1 [MW] and 2 [MW] in two enclosures of $10{\times}10{\times}3[m^3]$ and $20{\times}20{\times}3[m^3]$ were considered. Time variations of heat release rates, temperature, visibility, and mass balance were compared for ARs from 1-6. The heat release rates were accurate for all aspect ratios regardless of fire and enclosure sizes. The quasi-steady state temperature and visibility were well predicted for $AR{\leq}5$. Temperature drop and skewness of mass conservation, however, increased with increasing aspect ratio. Therefore, careful investigation of the grid size is recommended in performance-based design when $AR{\geq}3$, where temperature and visibility in early stage of a fire are important parameters. For accurate simulations of enclosure fires, grid sizes of 0.1~0.2 [m] and smaller in the vertical direction and $AR{\leq}2$ are recommended.

• Journal of Navigation and Port Research
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• v.30 no.10 s.116
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• pp.861-867
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• 2006

In connection with redevelopment of Busan North Port, there has been lots of studies and efforts for the development of superannuated North general piers into a center of marine tourism and waterfront for the citizens of Busan. Recently it has moved to the stage of execution, after several trials to find concrete solutions. On the other hand, the change of flow field and tidal exchange cuased by redevelopment is one of the important investigation subjects. This study deals with the change of flow field and water exchange after redevelopment using numerical simulation technique, based on the general data which were collected and analyzed. As a result of simulation, the speed of tidal currents are tended to decrease near the North and inner-port and increase at the main waterway. Furthermore, the tidal exchange had a tendency to be small both before and after redevelopment by about 77% in a quasi steady state, which is about 15 days after.