• Corrosion Science and Technology
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• 제20권2호
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• pp.69-76
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• 2021

In this work, we have performed a corrosion failure analysis of a leaking tube connected to an upper header of a condensate pre-heater in a heat recovery steam generator. It was revealed that the leakage position in the tube was the location where the materials were easily vulnerable due to tensile residual stresses induced by the material manufacturing process and welding process. In addition to an imbalance in the module induced by temperature difference during operation of the pre-heater, the weight of the modules and thermal fatigue provoked a type of stress of tensile-tensile fatigue on the tube. Thus, the leakage position of the pre-heater was exposed to the tensile stress on the inner surface of the tube facing the gas, which rendered the unstable oxide layer susceptible to corrosion and the formation of pits on the water side. The cracks propagated along with the degraded microstructure in a transgranular cracking mode under fatigue loading and finally resulted in water leakage.

• 한국표면공학회지
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• 제56권6호
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• pp.443-450
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• 2023

SiC/SiC_f CMC is vulnerable to water vapor corrosion at a high temperature of 1500℃. So, EBC (Environmental Barrier Coating) materials are required to protect Si-based CMCs. Ytterbium silicates are reported to have coefficient of thermal expansion (CTE) similar to that of the base material, such as SiC/SiC_f CMC. When the EBC are materials exposed to high temperature environment, the interface between ytterbium silicates and SiC/SiC_f CMC is not separated, and the coating purpose can be safely achieved. For the perspective of EBC applications, thermally grown oxide (TGO) layer with different CTE is formed by the reaction with water vapor in EBC, which leads to a decrease in life time. In this study, we prepare two types of ytterbium silicates to observe the corrosion behavior during the expose to high temperature and water vapor. In order to observe this behavior, the steam-jet furnace is prepared. In addition, phase formation of these ytterbium silicates is analyzed with microstructures by the before/after steam-jet evaluation at 1500℃ for 100 h.

• 한국압력기기공학회 논문집
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• 제19권2호
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• pp.109-116
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• 2023

This study presents a CRUD modelling to simulate the thermal resistance behavior of CRUD, deposited on the surface of a cladding tube of a fuel assembly. When heat produced from fuels transfers to a coolant through a cladding tube, the CRUD acting as an additional thermal resistance is expressed as two layers, i.e., a solid oxide layer and an imaginary fluid layer, which are added to the experimental tube's heat structure of the MARS-KS input data. The validation calculation for the experiments performed in UNIST-DISNY facility showed that the center and surface temperatures of the cladding tube increased as the porosity and the steam amount inside pores of the CRUD got higher. In addition, the temperature gradient in the imaginary fluid layer was calculated to be larger than that in the solid oxide part, indicating that the steam amount inside the layer acted more largely as thermal resistance. It was also evaluated through sensitivity calculations that the cladding tube temperature was more sensitive to the CRUD porosity and the steam amount in pores than to the inlet flow rate of the coolant.

• Nuclear Engineering and Technology
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• 제56권7호
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• pp.2545-2556
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• 2024

In the present study, we investigated the velocity distribution, temperature distribution and condensation characteristics of steam jet issuing from four different orifice nozzles with a Reynolds number of approximately 79,000 using the phase Doppler particle analyzer system and a K-type thermocouple. The steam jet discharged from the orifice nozzle has a wider jet width compared to pipe nozzle because of the vena-contracta which can enhance the mixing of steam jet with the ambient air. Therefore, the orifice jet showed less condensation due to its wideness, resulting in small velocity decay rate and large temperature decay rate due to momentum conservation and decreased latent heat release compared to pipe nozzle, respectively. Also, the wider jet width of the orifice jet resulted in larger velocity and temperature spread rate compared to the pipe jet. In addition, the increase in the aspect ratio of the orifice jet led to more condensation and larger velocity spread rate and temperature spread rate due to both the vena-contracta and axis-switching effect, resulting in the increase of jet entrainment.

• 한국지구과학회지
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• 제41권1호
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• pp.75-91
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• 2020

이 연구의 목적은 중학생을 위한 시스템 사고 향상에 적합한 사전 교육 및 STEAM 교육 프로그램을 개발하고 중학생에게 적용하여 프로그램의 효과를 분석하는 것이다. 이 연구는 중학생을 대상으로 시스템 사고에 대한 사전 교육프로그램과 시스템 사고를 향상시키기 위한 STEAM 교육 프로그램을 개발하는 것이다. 개발된 프로그램은 경상북도에 위치한 소규모 중학교 1학년 4명, 3학년 4명을 대상으로 투입하여 효과를 분석하였다. 시스템 사고의 분석은 Hung(2008)에 의해 개발된 루브릭이 활용되었다. 학생들의 시스템 사고에 대한 신뢰도와 루브릭의 타당도를 확인하기 위해 전문가들이 연구 결과를 검토하였다. 시스템 사고의 효과를 확인하기 위해 루브릭, 시스템 사고에 대한 학생들의 인지, 단어 간 관계 분석, 인과 지도의 검사를 체계적으로 적용하였다. 연구 결과를 정리하면 다음과 같다. 첫째, 시스템 사고 사전 교육 프로그램과 교사용 지도서를 개발하였다. 이는 학생의 시스템 사고에 대한 개념과 인과 지도 작성법에 대한 기초 지식을 학습시키기 위한 것이다. 개발된 사전 교육 프로그램은 '지구 온난화'를 주제로 한 무한도전이라는 친숙한 예능 프로그램을 이용하여 인과 지도 작성 방법을 학습하도록 구성되었다. 사전 교육프로그램은 대상 학생들이 친숙하게 접근할 수 있도록 지구 온난화를 주제로 한 예능 프로그램을 기반으로 하였으며, 이를 통해 인과 지도 작성 방법을 학습할 수 있도록 구성하였다. 효과성 검사를 적용한 결과, 학생들의 시스템 사고 향상을 위한 사전 교육 프로그램은 효과적인 것으로 분석되었다. 둘째, '분석-설계-제작-평가-시스템 사고'의 5단계로 구성된 ADBAS 모형을 적용하여 '공기 소화기' 주제의 시스템 사고 기반 STEAM 교육 프로그램을 개발하였다. 개발된 프로그램은 시스템의 이해 능력, 시스템 내 관계 파악 능력, 시스템 일반화 능력에 긍정적인 효과가 있는 것으로 나타났다. 이 연구를 통해 개발된 사전 교육 프로그램 및 STEAM 교육 프로그램은 학생의 시스템 사고 향상을 돕는데 적합하며, 시스템 사고와 STEAM 교육의 접목이라는 점에서 중요한 의미를 가진다. 또한 시스템 사고 향상을 위해 이 프로그램을 투입하기에 적절한 것으로 판단되어 '창의융합인재'를 양성하는데 기여할 것이다. 또한 시스템 사고 향상을 위한 위 프로그램의 적절한 투입은 창의융합형 인재를 양성하는 교육 목표를 달성하는 데 있어 기여를 할 것으로 판단된다.

• 대한공업교육학회지
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• 제40권2호
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• pp.239-258
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• 2015

이 연구의 목적은 융합인재교육의 학습성과에 대한 합리적인 교육평가에 활용될 수 있는 기초자료를 제공하는 것으로, 융합인재교육에서 학습자가 함양해야 할 핵심 역량 준거의 상대적 가중치를 포함하는 학습성과 평가 방안을 제시하였다. 연구 방법은 융합인재교육 전문가를 대상으로 핵심역량에 대한 이원비교 설문조사를 실시하였고 핵심역량 요인의 우선순위를 분석하기 위하여 계층적 분석기법(AHP)을 적용하였다. 이에 따른 연구의 결론은 다음과 같다. 첫째, 핵심역량 1계층에서 융합수행능력과 융합인지능력의 중요도는 각각 39.4%, 36.8% 순으로 분석되었다. 융합인재교육의 교육평가에서 융합수행능력과 융합인지능력은 유사한 중요도 수준을 보였으며, 융합태도능력(23.8%)보다는 중요하게 고려해야 하는 요인으로 평가되었다. 둘째, 핵심역량 2계층에서 문제해결(20.0%)의 상대적 중요도가 가장 높게 나타났고, 다음으로는 창의적 사고력(18.3%), 책임감(15.3%), 융합지식 이해(11.0%) 순으로 분석되었다. 셋째, 핵심역량의 상대적 가중치 분석 결과는 융합인재 교육프로그램의 교육목표 달성 여부를 상대적 가중치에 따라 합리적으로 평가하는 역량평가 체제의 토대가 될 것이다. 더불어 융합인재교육의 교육평가뿐만 아니라 교수 학습 과정 개선과 학습효과를 높일 수 있는 교육프로그램 개발에 가이드 역할을 할 것으로 기대된다.

• 한국초등과학교육학회지:초등과학교육
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• 제40권1호
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• pp.113-125
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• 2021

In this study, a problem-centered learning based STEAM field experience learning program was developed and the effects of applying it were investigated. The program was composed of 8 sessions by using problem-centered learning education method and integrating STEAM elements between disciplines. The contents of program are as follow. In the step of sharing problems and making a problem-solving plan, they understood the various examples and meanings of endangered species, explored the project activities, and made an inquiry plan. In the search and re-exploration phase, a smart device was used to investigate the appearance, habitat environment and cause of extinction for Clithon retropictus, and a site inquiry plan was established for each group. Then, they moved to the field to explore brackish-headed gallops and discuss ways to protect endangered species. In the step of creating a solution, a web-based report was produced as the final product using smart devices based on the results of the inquiry. In the presentation and evaluation stage, the produced web-based report was used to present each group, conduct mutual evaluation, and organize project activities. The developed program was applied to 6th grade 29 students enrolled in the J University Gifted Education Center. In order to find out the effectiveness of the program, tests of science process skill, creative problem-solving ability, and scientific attitude were conducted before and after of program learning, and the results were statistically analyzed by t-test. In addition, a STEAM program satisfaction test was conducted after project in order to find out the satisfaction of the class. As a result of application of the program, the results were significantly improved in openness, criticism, and creativity among the sub-factors of creative problem-solving ability and scientific attitude. Satisfaction with the STEAM program was also high, but no significant result was found in science process skill. Therefore, the program of this study could be influenced on improvement of creative problem-solving ability and scientific attitude of gifted students in elementary science.

• 청정기술
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• 제28권1호
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• pp.1-8
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• 2022

미활용 저급자원인 석유코크스를 대상으로 고순도의 수소 생산을 위한 수성가스전이반응에 적용가능성을 확인하기 위하여 Cu/ZnO/MgO/Al₂O₃ (CZMA) 촉매를 공침법을 사용하여 제조하였다. 제조된 촉매는 BET, H₂-TPR을 사용하여 분석되었다. 촉매의 반응성 테스트는 고농도의 CO를 포함하는 합성가스로부터 단일 Low Temperature Shift 반응을 거치는 경우와 High Temperature Shift 반응을 거친 후 스팀의 응축 없이 즉시 LTS 반응을 거치는 두 가지의 경우를 비교 및 분석하였다. 두 조건에서 steam/CO 비, 유량 및 유속, 온도에 따른 반응특성을 확인하였다. 전환된 저농도의 CO와 스팀이 응축 없이 LTS로 즉시 주입되는 경우 많은 양의 스팀이 존재함에도 불구하고 대부분의 조건에서 다소 낮은 CO 전환율을 나타냈다. 또한 steam/CO비, 온도 및 유속에 대한 영향이 크게 나타나 최적의 조업조건을 결정하기에 추가적인 분석이 요구되었다. 반면, 고농도의 CO 기체를 포함하는 조건에서는 탄소침적 또는 촉매의 활성 저하가 나타나지 않았으며 대부분의 조건에서 높은 CO 전환율을 나타내었다. 결론적으로 Cu/ZnO/MgO/Al₂O₃ 촉매를 적용하여 고농도의 CO를 포함하는 합성가스 조성에서 적절한 조업조건을 적용시키면 단일 LTS 반응을 적용해도 고농도의 CO를 CO₂로 충분히 전환 가능함을 확인하였다.

• 무역학회지
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• 제44권2호
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• pp.127-141
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• 2019

In this study, the relationship between Baltic Dry Index(BDI) and maritime trade volume in the dry cargo market was verified using the vector autoregressive (VAR) model. Data was analyzed from 1992 to 2018 for iron ore, steam coal, coking coal, grain, and minor bulks of maritime trade volume and BDI. Granger causality analysis showed that the BDI affects the trade volume of coking coal and minor bulks but the trade volume of iron ore, steam coal and grain do not correlate with the BDI freight index. Impulse response analysis showed that the shock of BDI had the greatest impact on coking coal at the two years lag and the impact was negligible at the ten years lag. In addition, the shock of BDI on minor cargoes was strongest at the three years lag, and were negligible at the ten years lag. This study examined the relationship between maritime trade volume and BDI in the dry bulk shipping market in which uncertainty is high. As a result of this study, there is an economic aspect of sustainability that has helped the risk management of shipping companies. In addition, it is significant from an academic point of view that the long-term relationship between the two time series was analyzed through the causality test between variables. However, it is necessary to develop a forecasting model that will help decision makers in maritime markets using more sophisticated methods such as the Bayesian VAR model.

• Corrosion Science and Technology
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• 제14권2호
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• pp.76-84
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• 2015

The components in ultra super critical (USC) steam turbine, which is under development for high efficient power generation, are encountering harsher solid particle erosion by iron oxide scales than ones in the existing steam turbines. Therefore, the currently used boride coating will not be able to hold effective protection from particle erosion in USC system and should be replaced by new particle erosion resistant coatings. One of the best protective coatings developed for USC steam turbine parts was found to be vanadium-boride (V-boride) coating which has a hardness of about 3000 HV, much higher than that of boride, 1600~2000 HV. In order to evaluate particle erosion resistance of the various coatings such as V-boride, boride and Cr-carbide coatings at high temperature, particle erosion test equipments were designed and manufactured. In addition, erosion particle velocity was simulated using FLUENT software based on semi-implicity method for pressure linked equations revised (SIMPLER). Based on experimental results of this work, the vanadium-boride coating was found to be superior to others and to be a candidate coating to replace the boride coating.