• Journal of Mechanical Science and Technology
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• 제15권9호
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• pp.1217-1225
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• 2001

The governing equations are derived for the analysis of a stepped labyrinth gas seal generally used in high performance compressors, gas turbines, and steam turbines. The bulk-flow is assumed for a single cavity control volume set up in a stepped labyrinth cavity and the flow is assumed to be completely turbulent in the circumferential direction. The Moodys wall-friction-factor model is used for the calculation of wall shear stresses in the single cavity control volume. For the reaction force developed by the stepped labyrinth gas seal, linearized zeroth-order and first-order perturbation equations are developed for small motion about a centered position. Integration of the resultant first-order pressure distribution along and around the seal defines the rotordynamic coefficients of the stepped labyrinth gas seal. The resulting leakage and rotordynamic characteristics of the stepped labyrinth gas seal are presented and compared with Scharrers theoretical analysis using Blasius wall-friction-factor model. The present analysis shows a good qualitative agreement of leakage characteristics with Scharrers analysis, but underpredicts by about 20%. For the rotordynamic coefficients, the present analysis generally yields smaller predictied values compared with Scharrers analysis.

• 한국농공학회논문집
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• 제57권1호
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• pp.79-87
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• 2015

A thermochemical equilibrium model was constructed for predicting composition of synthesis gas in biomass gasification. The model included estimation of equilibrium constants using Gibbs free energy. After constructing the model, the results were compared with the experimental values and predictions from a previous model. Gas compositions were reasonably well agreed with them and showed effects of operational and fuel condition. When the reaction temperature increased, the lower heating values decreased due to the decrease in CH4 concentrations. The methane concentrations were lower than those observed in experimental results. The model was used to predict the gas composition and heating values for the cases of mixed fuel of charcoal and un-dry woodchips. Although downdraft gasifiers require fuels less than 15% of moisture contents, the model results indicated that the mixed fuel with charcoal and woodchips which had over 25% of moisture contents could be used in the downdraft gasifiers. It might be explained by increase in energy density resulting from mixing charcoal. The results imply that the efforts and costs for drying biomass fuels could be reduced by mixing charcoal or fuels with higher calorific values.

• 자원ㆍ환경경제연구
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• 제23권1호
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• pp.133-156
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• 2014

경제구조를 표현하는 식들로 구성된 모형을 이용하여 도출되는 정량적 분석결과는 성과의 크기를 제시하므로 정책을 평가할 때에 유용하다. 이에 본 논문에서는 해외석유 가스개발사업의 국민경제에 대한 효과를 정량적으로 측정하기 위하여 해외석유 가스개발사업을 반영한 거시경제모형을 구축하고, 이를 이용하여 해외석유 가스개발사업으로부터의 회수액이 GDP, 경상수지, 실업률, 소비자물가, 환율 등에 미치는 효과를 분석하였다. 해외석유 가스개발 사업의 회수액은 우리나라 기업이 해외석유 가스개발사업에 참여하여 획득한 석유 또는 가스를 달러 형태로 금액화한 것이다. 해외석유 가스개발사업을 반영한 거시경제모형은 한국은행의 모형(1997, 2004, 2012)을 이용하여 구축하였으며, 개별 행태방정식의 통계적 적합성과 historical simulation을 통해 모형 전체의 안정성을 확인하였다. 분석결과에 따르면 시간이 경과하면서 해외석유 가스개발사업 회수액의 규모가 커지면서 우리 나라의 거시경제 변수에 미치는 효과의 크기도 커졌다. 변수별로 보면 해외석유 가스개발사업은 경상수지, 실질 GDP, 실업률에 대해서 긍정적인 효과를 유도하였다. 이에 비해서 소비자물가에 대해서는 상승하는, 그리고 환율에 대해서는 하락(원화가치의 평가절상)하는 효과를 보였다. 매해마다 효과의 크기는 다르지만, 2011년 기준으로 해외석유 가스개발사업 회수액은 경상수지를 2.226%, 실질 GDP를 0.401% 증가시키고, 실업률을 0.489%p 감소시켰다. 소비자물가는 0.101% 증가시키고, 원/달러환율은 0.379% 낮추었다.

• 자원ㆍ환경경제연구
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• 제26권4호
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• pp.519-547
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• 2017

본 연구는 지역 단위 도시가스 수요예측모형을 이용하여 전국 도시가스수요예측의 정확도를 향상할 수 있는지 여부를 살펴봤다. 지역별 수요예측모형을 구축하게 된 배경은 용도별 도시가스 수요의 행태가 분화되는 상황에서 자료의 제한으로 용도별 수요예측모형을 구축하기 어렵다는 것에 있다. 지역별 수요예측모형은 전국수요를 수도권과 지방으로 구분하여 별도의 예측모형을 구성하는 것으로, 시간변동계수를 갖는 공적분모형을 이용하였다. 지역모형에서 전국 도시가스수요예측은 지역별 수요전망치를 합산하여 산출하였다. 2013~2016년의 4년간 예측력 평가결과, 지역별 모형을 통한 전국 도시가스수요 예측이 전국단위 예측모형에 비하여 예측력이 전반적으로 우수한 것으로 나타났다. 지역모형에서는 수도권과 지방권 모형을 별도로 구축함으로써 해당 지역 수요의 특성을 반영한 예측모형이 가능했다. 수도권수요는 가정용수요 비중이 높아 기온에 보다 민감하게 반응하고, 전력수요와 경쟁관계가 있다. 이에 반해 지방권은 산업용수요 비중이 높아 전반적인 경기상황에 따른 수요변동이 크고, 수도권과 달리 벙커씨유와 LPG와 같은 산업용 연료와 대체관계를 보였다. 상기 결과는 성숙기에 접어든 도시가스산업에서 지역별 수요에 대한 세부적인 분석을 통해 전국 단위 수요예측의 정확도를 향상시킬 수 있다는 것을 보여주고, 이와 더불어 용도별 도시가수요 분석에도 유용한 정보를 제공할 것으로 기대한다.

• 한국자동차공학회논문집
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• 제12권1호
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• pp.55-60
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• 2004

Residual gas acts as a diluent which results in reducing the in-cylinder temperature as well as the flame speed, significantly affecting fuel economy, NOx emissions and combustion stability. Therefore it is important to determine the residual gas fraction as a function of the engine operating parameters accurately. However, the determination of the residual gas fraction is very sophisticated due to the unsteady state of induction and exhaust process. There has been little work toward the development of a generally applicable model for quantitative predictions of residual gas fraction. In this paper, a simple model for calculating the residual gas fraction in SI engines was suggested. The amount of fresh air was evaluated through AFR and fuel consumption. After this, from the intake temperature and pressure, the amount of total cylinder-charging gas was estimated. The residual gas fraction was derived by comparing the total charging and fresh air. This results coincide with measured EGR value very well.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1997년도 춘계학술발표회논문집(1)
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• pp.184-189
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• 1997

A prompt fission gas release model incorporating the resistance to gas flow in the gap was developed and the effects of gap resistance and failure location on prompt fission gas release from the cladding breach were assessed. The process of prompt fission gas release from the plenum and gap into the coolant was modeled in accordance with three major phenomena: (1) transient gas flow in the gap, (2) the growth of the fission gas bubble while it is still attached to the breach, and (3) the detachment of the fission gas bubble from the breach and mixing with the coolant. The cumulative mass release fraction by the present model was calculated for the case of Young-Gwang 3 & 4 nuclear fuel rod as a typical example. The results showed that the release behavior of prompt fission gas with time was different from the frictionless model which has frequently been used in a simplified approach, and that the location of cladding failure was another key factor for the prompt fission gas release process due to the resistance in the gap.

• 한국가스학회지
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• 제10권4호
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• pp.1-5
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• 2006

가스 소비의 급증으로 천연가스의 수송수단인 배관의 역할이 증가하고 있다. 일반적으로 배관으로 가스를 수송할 경우, 주로 컨덴세이트 형성에 기인하는 일부 액체가 관내에 형성되어 가스 산업현장에서 복잡한 조업 상의 문제점을 발생시킨다. 그러므로, 가스 컨덴세이트의 존재가 조업 효율에 미치는 영향을 예측할 수 있는 적절한 방법이 필요하다. 본 연구에서는 통합된 단상/2상 유동개념을 사용하여 가스 컨덴세이트의 발생 시작점과 그 발생량의 영향이 출구 압력에 미치는 영향을 분석할 수 있는 통계 모델을 개발하였다. 또한 컨덴세이트 발생 시작점 이후, 2상 유동지역에서 유동형태의 변화가 출구 압력에 미치는 영향도 분석되었다.

• 한국분무공학회지
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• 제17권2호
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• pp.77-85
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• 2012

The CFD simulation of diesel spray tip penetrations were compared with 0-D simulation for experimental data obtained with common rail injection system. The simulated four injection patterns include single, pilot and split injections. The CFD simulation of the spray penetration over these injection patterns was performed using the KIVA-3V code, which was implemented with both the standard KIVA spray and original gas jet sub-models. 0-D simulation of the spray tip penetration with time-varying injection profiles was formulated based on the effective injection velocity concept as an extension of steady gas jet theory. Both the CFD simulation of the spray tip penetration with the standard KIVA spray model and 0-D simulation matched better with the experimental data than the results of the gas jet model for the entire fuel injection patterns.

• 한국압력기기공학회 논문집
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• 제4권2호
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• pp.1-6
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• 2008

Very High Temperature Gas Cooled Reactor (VHTR) has been selected as a high energy heat source for nuclear hydrogen generation. The VHTR can produce hydrogen from heat and water by using a thermo-chemical process or from heat, water, and natural gas by steam reformer technology. A co-axial double-tube primary hot gas duct (HGD) is a key component connecting the reactor pressure vessel and the intermediate heat exchanger (IHX) for the VHTR. In this study, a preliminary design analysis for the primary HGD of the nuclear hydrogen system was carried out. These preliminary design activities include a determination of the size, a strength evaluation and an appropriate material selection. The determination of the size was undertaken based on various engineering concepts, such as a constant flow velocity model, a constant flow rate model, a constant hydraulic head model, and finally a heat balanced model.

• 드라이브 ㆍ 컨트롤
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• 제14권3호
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• pp.32-39
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• 2017

In this study, a mathematical model of the pneumatic part in a gas blow-down system is proposed. The mathematical model consists of four major parts: pressure vessel, reservoir, pressure regulator and pipe-line. To ensure accuracy in long-time simulations, heat transfer between gas and pressure vessel is considered. The model is validated by comparing simulation results with experimental data. Experiments are conducted on the ground, where free convection can be assumed. Simulation results indicate the proposed model can accurately describe behavior of a gas blow-down system. Therefore, it may be used for design and analysis of similar systems with a slight modification.