• Progress in Superconductivity
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• 제9권1호
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• pp.102-106
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• 2007

YBCO films have been synthesized using a spray pyrolysis method. We used nitrates of Y, Ba, Cu as precursors. Deposition was made on $LaAlO_3$ (100) single crystal substrate by spraying the mist of aqueous precursor solution generated by a concentric nozzle. The distance between concentric nozzle and substrate was 15 cm. C-axis oriented films were obtained at deposition temperature of $740{\sim}800^{\circ}C$ and working pressure of 20 Torr. Oxygen partial pressure was 3 Torr and substrate was transported with the speed ranging from 0.23 cm/min to 0.7 cm/min by reel to reel. Scanning electron microscope (SEM) and X-ray diffraction (XRD) observation revealed that films are smooth and highly textured with (001) planes parallel to substrate. Highest critical current density (Jc) was $1.38\;MA/cm^2$ at 77K and self-field for the film with a thickness of $0.5\;{\mu}m$ prepared at a substrate temperature of $780^{\circ}C$ and $PO_2\;=3\;Torr$. The effect of temperature on the microstructure and YBCO phase formation will be discussed.

• 한국지하수토양환경학회지:지하수토양환경
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• 제22권3호
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• pp.42-49
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• 2017

TOUGH2 was used to simulate the migration of $CO_2$ injected into a sandy aquifer. A series of numerical simulations was performed to investigate the effects of a low-permeability layer (LPL) embedded in the aquifer on the injection rate and the pressure distribution of $CO_2$. The results show that the size and location of the LPL greatly affected the spread of $CO_2$. The pressure difference between two points in the aquifer, one each below and above the LPL, increased as the size of the LPL increased, showing a critical value at 200 m, above which the size effect was diminished. The location of the LPL with respect to the injection well also affected the migration of $CO_2$. When the injection well was at the center of the LPL, the injection rate of $CO_2$ decreased by 5.0% compared to the case with no LPL. However, when the injection well was at the edge of the LPL, the injection rate was decreased by only 1.6%. The vertical distance between the injection point and the LPL also affected the injection rate. The closer the LPL was to the injection point, the lower the injection rate was, by up to 8.3%. Conclusively, in planning geologic storage of $CO_2$, the optimal location of the injection well should be determined considering the distribution of the LPL in the aquifer.

• 한국자동차공학회논문집
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• 제22권1호
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• pp.148-156
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• 2014

The objective of this study is to compare NOx emissions from light duty diesel vehicles measured from on-road tests that conducted under various driving routes and seasonal conditions. We measured real-driving NOx emissions using PEMS, portable emissions measurement system, under the urban, rural and motorway road traffic conditions. On-road tests were repeated at summer, fall and winter season. The accumulated driving distance is more than 1,200 km per each vehicle. Route average NOx emission factors were compared among nine route-season combinations. The emission characteristics of each combinations were investigated using time series mass emission rates and vehicle operation-based emission rates and activities, which is based on U.S. EPA's MOVES model. Most concerned route-season combination is "urban road condition at summer", which shows two to eleven times higher NOx emissions than other combinations. The emission rates and activities under low speed operating conditions should be managed in order to reduce urban-summer NOx. From a NOx control strategy perspective, the exhaust gas recirculation, EGR, is observed to be properly operated under wide range of vehicle driving conditions in Euro-5 vehicles, even if the air conditioner turns on. In high power demanding conditions, the effect of overspeeding could be more critical than that of air conditioner activation.

• Membrane and Water Treatment
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• 제9권1호
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• pp.43-51
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• 2018

Hollow fiber (HF) membranes are gaining wide interest over flat membranes due to their compaction and high area to surface volume ratio. This work addresses the fabrication of HF from polysulfone (PS) and polyethersulfone (PES) using N-methylpyrrolidone (NMP) as solvent in addition to other additives to achieve desired characteristics. The semi-pilot spinning system includes jacketed vessel, four spinneret block, coagulation and washing baths in addition to dryer and winder. Different parameters affecting dry-wet spinning phase inversion process were investigated. Dope compositions of PES, NMP and polyvinyl pyrrolidone (PVP) of varying molecular weights as additive were addressed. Some critical parameters of importance were also investigated. Those include dope flow rate, air gap, coagulation & washing baths and drying temperatures. The measured dope viscosity was in the range from 1.7 to 36.5 Pa.s. Air gap distance was adjusted from 20 to 45 cm and coagulation bath temperature from 20 to $46^{\circ}C$. The HF membranes were characterized by scanning electron microscope (SEM), atomic force microscope (AFM) and mechanical properties. Results indicated prevalence of finger like structure and average surface roughness from about 29 to 78.3 nm. Profile of stress strain characteristics revealed suitability of the fibers for downstream interventions for fabrication of thin film composite membrane. Different empirical correlations were formulated which enable deeper understanding of the interaction of the above mentioned variables. Data of pure water permeability (PWP) confirmed that the fabricated samples fall within the microfiltration (MF)-ultrafiltration (UF) range of membrane separation.

• 한국항공우주학회지
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• 제45권12호
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• pp.1084-1093
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• 2017

전장에서 임무를 수행하는 항공기의 생존성이 위협을 받음에 따라 항공기의 생존성을 향상시키기 위한 연구의 필요성이 커지고 있다. 본 연구에서는 항공기 Plume IR 신호에 관련된 무인전투기의 생존성을 분석하였다. 항공기의 생존성을 분석하기 위해 먼저 항공기의 IR 탐지거리인 Lock-on Range와 미사일 격추 범위인 Lethal Envelope 개념을 도입하였다. 또한 수직면 기준 분석을 포함하는 지대공 미사일에 대한 Lethal Envelope를 계산하는 기법을 개발하였다. 계산결과 무인전투기의 Red Zone이 고도 위 방향과 아래 방향의 특성 뿐만 아니라 정량적인 Zone 사이즈에서 상당한 차이가 생김을 확인하였다.

• 한국컴퓨터정보학회논문지
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• 제17권11호
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• pp.73-81
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• 2012

본 논문에서는 애드 혹 네트워크의 노드 위치 정보와 홉 수를 이용하여 캡슐화 웜홀 공격과 터널링 웜홀 공격을 탐지하는 ETWAD(Encapsulation and Tunneling Wormhole Attack Detection) 기법을 설계하였다. ETWAD 탐지 기법은 애드 혹 네트워크 내의 노드 ID와 그룹 키로 노드의 신분을 확인할 수 있는 GAK(Group Authentication Key)를 생성하여 RREQ와 RREP에 추가하여 애드 혹 네트워크의 구성원임으로 인증할 수 있도록 설계하였다. 또한, ETWAD 탐지 기법은 RREP 메시지 내의 홉 수를 카운트하고, 근원지 노드 S와 목적지 노드 D의 거리를 계산하여 임계치와 홉 수를 이용하여 캡슐화 웜홀 공격, 터널링 공격을 탐지하는 GeoWAD 알고리즘을 설계하였다. 그 결과, 평균 웜홀 공격 탐지율이 91%, 평균 FPR이 4.4%로 평가되므로 ETWAD 탐지 기법은 웜홀 공격 탐지율과 웜홀 공격 탐지의 신뢰성을 향상시켰다고 볼 수 있다.

• 대한임베디드공학회논문지
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• 제7권4호
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• pp.163-172
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• 2012

A driver assistance system is critical to improve a convenience and stability of vehicle driving. Several systems have been already commercialized such as adaptive cruise control system and forward collision warning system. Efficient vehicle detection is very important to improve such driver assistance systems. Most existing vehicle detection systems are based on a radar system, which measures distance between a host and leading (or oncoming) vehicles under various weather conditions. However, it requires high deployment cost and complexity overload when there are many vehicles. A camera based vehicle detection technique is also good alternative method because of low cost and simple implementation. In general, night time vehicle detection is more complicated than day time vehicle detection, because it is much more difficult to distinguish the vehicle's features such as outline and color under the dim environment. This paper proposes a method to detect vehicles at night time using analysis of a captured color space with reduction of reflection and other light sources in images. Four colors spaces, namely RGB, YCbCr, normalized RGB and Ruta-RGB, are compared each other and evaluated. A suboptimal threshold value is determined by Otsu algorithm and applied to extract candidates of taillights of leading vehicles. Statistical features such as mean, variance, skewness, kurtosis, and entropy are extracted from the candidate regions and used as feature vector for SVM(Support Vector Machine) classifier. According to our simulation results, the proposed statistical feature based SVM provides relatively high performances of leading vehicle detection with various distances in variable nighttime environments.

• 한국전자파학회논문지
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• 제23권6호
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• pp.669-681
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• 2012

본 논문에서는 헬리컬 코일을 이용하는 공진형 무선 전력 전송 시스템에서 새로운 상호 인덕턴스 계산식을 이용한 해석적 계산을 제안한다. 상호 인덕턴스 계산 과정 중에 중요한 변수의 look-up table을 수식화 하였다. 송 수신이 대칭인 구조에서 두 헬리컬 코일 간의 거리를 53 mm부터 500 mm까지 10 mm 간격으로 거리를 증가시키며, 공진 주파수와 삽입 손실에 대한 계산 결과와 실험 결과를 비교하였다. 실험 결과와 계산 결과의 공진 주파는 평균적으로 5.63 %의 차이를 보였다. 삽입 손실은 실험 결과와 290 mm에서 0.33 dB의 가장 작은 차이를 보였고, 평균적으로 2.25 dB의 차이를 보였다. 발룬을 사용하지 않았을 때의 실험 결과가 계산 결과와 더 근접한 결과를 보이는 것을 확인했다.

• 대한기계학회논문집B
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• 제27권5호
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• pp.668-676
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• 2003

The effect of nozzle height on heat transfer of a hot steel plate cooled by an impinging liquid jet is not well understood. Previous studies have been based on the dimensionless parameter z/d. To test the validity of this dimensionless parameter and to investigate gravitational effects on the jet, stagnation velocity of an impinging liquid jet were measured and the cooling experiments of a hot steel plate were conducted for z/d from 6.7 to 75, and an inverse heat conduction method is applied for the quantitative comparison. Also, the critical instability point of a liquid jet was examined over a range of flow rates. The experimental velocity data for the liquid jet were well correlated with the dimensionless number 1/F $r_{z}$$^2$based on distance. It was thought that the z/d parameter was not valid for heat transfer to an impinging liquid jet under gravitational forces. In the cooling experiments, heat transfer was independent of z when 1/F $r_{z}$$^2$< 0.187(z/d = 6.7). However, it was found that the heat transfer quantity for 1/F $r_{z}$$^2$=0.523(z/d = 70) is larger 11% than that in the region for 1/F $r_{z}$$^2$=0.187. The discrepancy between these results and previous research is likely due to the instability of liquid jet.uid jet.

• Journal of Advanced Marine Engineering and Technology
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• 제10권2호
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• pp.156-164
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• 1986

In the case of boiling on high temperature wall, vapor film covers fully or parcially the surface. This phenomenon, film boiling or transition boiling, is very important in the surface heat treatment of metal, design of cryogenic heat exchanger and emergency cooling of nuclear reactor. Mainly supposed hydraulic-thermal accidents in nuclear reactor are LCCA (Loss of Coolant Accident) and PCM (Power-Cooling Mismatch). Recently, world-wide studies on reflooding of high temperature rod bundles after the occurrence of the above accidents focus attention on wall temperature history and required time in transient cooling process, wall superheat at rewet point, heat flux-wall superheat relationship beyond the transition boiling region, and two-phase flow state near the surface. It is considered that the further systematical study in this field will be in need in spite of the previous results in ref. (2), (3), (4). The paper is the study about the fast transient cooling process following the wall temperature excursion under the CHF (Critical Heat Flux) condition in a forced convective subcooled boiling system. The test section is a vertically arranged concentric annulus of 800 mm long and 10 mm hydraulic diameter. The inner tube, SUS 304 of 400 mm long, 8 mm I.D, and 7 mm O.D., is heated uniformly by the low voltage AC power. The wall temperature measurements were performed at the axial distance from the inlet of the heating tube, z=390 mm. 6 chromel- alumel thermocouples of 76 .mu.m were press fitted to the inner surface of the heating tube periphery. To investigate the heat transfer characteristics during the fast transient cooling process, the outer surface (fluid side) temperature and the surface heat flux are computed from the measured inner surface temperature history by means of a numerical method for inverse problems of transient heat conduction. Present cooling (boiling) curve is sufficiently compared with the previous results.