• 한국분말재료학회지
• /
• 제17권5호
• /
• pp.379-384
• /
• 2010

In the present work, bismuth nanopowders with various particle size distributions were synthesized by controlling argon (Ar) gas flow rate and chamber pressure of a gas condensation (GC) apparatus. From the analyses of transmission electron microscopy (TEM) images and nitrogen gas adsorption results, it was found that as Ar gas flow rate increased, the specific surface area of bismuth increased and the average particles size decreased. On the other hand, as the chamber pressure increased, the specific surface area of bismuth decreased and the average particles size increased. The optimum gas flow rate and chamber pressure for the maximized electrochemical active surface area were determined to be 8 L/min and 50 torr, respectively. The bismuth nanopowders synthesized at the above condition exhibit 13.47 $m^2g^{-1}$ of specific surface area and 45.6 nm of average particles diameter.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2007년도 춘계학술대회B
• /
• pp.3086-3091
• /
• 2007

When a molten corium is relocated in a lower head of a reactor vessel, the ERVC (External Reactor Vessel Cooling) system is actuated as coolant is supplied into a reactor cavity to remove a decay heat from the molten corium during a severe accident. To achieve this severe accident mitigation strategy, the two-phase natural circulation flow in the annular gap between the external reactor vessel and the insulation should be formed sufficiently by designing the coolant inlet/outlet area and gap size adequately on the insulation device. For this reason, one-dimensional natural circulation flow tests were conducted to estimate the natural circulation flow under the ERVC condition of APR1400. The experimental facility is one-dimensional and scaled-down as the half height and 1/238 rectangular channel area of the APR1400 reactor vessel. As the water inlet area increased, the natural circulation mass flow rate asymptotically increased, that is, it converged at a specific value. And the circulation mass flow rate also increased as the outlet area, injected air flow rate, and outlet height increased. But the circulation mass flow rate was not changed along with the external water level variation if the water level was higher than the outlet height.

• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2006년도 제27회 추계학술대회논문집
• /
• pp.207-210
• /
• 2006

액체로켓엔진을 설계할 때 가장 중요한 부품중의 하나가 인젝터이며, 이를 어떻게 배치하느냐에 따라 엔진의 성능이 크게 달라진다 본 연구에서는 swirl형 인젝터를 적용한 UDMH와 액체산소를 추진제로 사용하는 추력 170톤급 액체로켓 엔진의 인젝터 헤드를 1차 설계할 때, swirl 인젝터의 유량 분포 특성을 수식화한 식으로부터 단위면적당 유량 분포 관계를 계산하였다. 또한 연소실 중앙과 막냉각이 있는 벽면에 이 관계식을 적용함으로써, 연소실 원주방향의 연소특성을 예측하였다.

• 한국원자력학회:학술대회논문집
• /
• 한국원자력학회 1996년도 춘계학술발표회논문집(2)
• /
• pp.241-246
• /
• 1996

An analytical model to calculate rate of vapor generation due to heterogeneous wall nucleation in flashing flow is developed. In the present model, an important parameter of the vapor generation term, i.e. nucleation site density is calculated by integrating its probability distribution function with respect to active cavity radius. The limits of integration are minimum and maximum active cavity radii, and these are formulated using an active cavity model for nucleate boiling. This formulation, therefore. can statistically account for the effect of surface specific thermo-physical and geometric conditions on the vapor generation rate and flashing inception. For verifying the adequacy of the present model, steady state two-fluid and the bubble transport equations are solved with applicable constitutive equations. The applicable region of the bubble transport equation is also extended to churn-turbulent flow regime to predict interfacial area concentration at high void fraction. Predicted results in terms of axial pressure and void fraction profiles along the channels are compared with experimental data of Super Moby Dick and BNL Reasonable agreements have been achieved and this shows the applicability of the present model to flashing flow analysis.

• 한국지하수토양환경학회:학술대회논문집
• /
• 한국지하수토양환경학회 2002년도 총회 및 춘계학술발표회
• /
• pp.351-356
• /
• 2002

Groundwater recharge rate was estimated by applying the groundwater level fluctuation method utilizing Theis (1937) approach with specific yield estimation technique of Shevenell (1996) and the temperature method using observed data from National Groundwater Observation Stations. Results based on analysis of water level observation data of 10 alluvium wells reveal that the recharge rates for 5 wells of Kum river area range 3.7~25.0% and those for 5 wells of Nakdong river area range 3.6~21.7%. Results obtained from the temperature method based on water temperature data indicated that the upward flow resulted from evapotranspiration is dominant for 4 wells of the Kum river area and 5 wells of the Nakdong river area. The other wells showed the downward flow which is related to groundwater recharge in these areas.

• 청정기술
• /
• 제16권2호
• /
• pp.124-131
• /
• 2010

슬러리 기포탑을 이용하여 수산화칼슘 농도(0.16~0.64 wt%), 계면활성제 농도(2~16 wt%), 총 부피유량(3~6 L/min) 및 $CO_2$ 유량의 부피분율(0.3~0.6)이 탄산칼슘의 morphology, 결정구조, 입자의 크기, 입자 간의 응집화, 비표면적에 미치는 영향을 알아보았다. 실험에 사용한 반응기는 높이가 1.0 m이고 직경이 0.11m, 그리고 중앙에는 직경 4 cm인 튜브가 들어있는 슬러리 기포탑이다. 실험에 사용한 음이온 계면활성제 Dispex N40은 탄산칼슘 합성에 있어 반응 속도에 영향을 주어 반응 종결 시간을 감소시켰다. Dispex N40의 농도가 2 wt%일 때 수산화칼슘의 포화농도인 0.16 wt%에서 이산화탄소의 유량에 따른 침강성 탄산칼슘의 morphology를 살펴보면 반응 속도가 증가할수록 결정의 형태는 단일 결정으로 존재하는 입자들이 많아졌다. Dispex N40은 탄산칼슘의 crystal의 성장과 입자와 입자간의 응집 현상에 영향을 주어 탄산칼슘의 평균 입도를 변화시켰다. 또한 $0.9 L/min\;CO_2$ 유량에서 수산화칼슘의 농도가 0.64 wt%일 때 2 wt%의 계면활성제 첨가로 인해 비표면적을 $35m^2/g$에서 $44m^2/g$로 크게 증가시켰다.

• 한국자동차공학회논문집
• /
• 제15권2호
• /
• pp.65-73
• /
• 2007

The use of an Exhaust Gas Recirculation(EGR) for a diesel engine with variable geometry turbocharger(VGT) has confronted how to obtain the amount of EGR for NOx reduction requirement at wide operating range and less side effect. Through a combined effort of modeling(wave action simulation) and experiment, an investigation into the effect of EGR area ratio and pipe length on EGR characteristics of common rail diesel engine with VGT has been performed. For accurate computation, calibration of constants involved in empirical and semi-empirical correlations has been performed at a specific operating point, before of its use for engine simulation. From the results of this study, it was found that EGR rate is sharply increased with increasing EGR area ratio until area ratio of 0.3. However, the effect of EGR area ratio on EGR rate is negligible beyond this criteria. This study also investigates the effect of EGR pipe length on a EGR amount and pulsating flow characteristics at EGR junction. The results showed that the longer EGR pipe length, the lower EGR amount was achieved due to the flow loss resulting in lower amplitude of pressure wave.

• 한국항공우주학회지
• /
• 제37권5호
• /
• pp.476-482
• /
• 2009

본 논문에서는 Fluent의 Spalart-Allmaras 난류모델을 적용하여 연소실 내부에 설치된 핀틀 형상이 핀틀 추진기관 추력 크기에 미치는 영향을 수치해석으로 분석하였다. 핀틀이 존재하면 노즐목을 지나는 질량 유량율은 이론적으로 예측된 값 보다 항상 작았으며, 핀틀 직경이 커질수록 노즐목에서 경계층 두께가 차지하는 비율이 증가되어 노즐목의 질량 유량율이 더욱 감소하였다. 핀틀 직경이 커질수록 핀틀 팁에 나타나는 재순환 영역의 압력에 의한 추력은 증가하지만 노즐 및 연소실 압력에 의한 추력은 감소하여 총 추력은 핀틀 직경이 작은 것 보다 감소하였다. 핀틀 추진기관의 비추력은 큰 차이가 없었다.

• 대한기계학회논문집B
• /
• 제26권6호
• /
• pp.779-787
• /
• 2002

The cooling heat transfer by impinging water spray jets on a rotating roll with a relatively large diameter has been investigated under various experimental conditions with 3 different sizes of flat type nozzle. The local heat transfer coefficients were calculated by finite difference method using measured surface temperatures of the circular cylinder as boundary conditions. Results show that a peak value of the heat transfer coefficient is located at the center of sprayed area and there may be a secondary peak at the downstream. The average heat transfer coefficients on the sprayed area were found to be 10 to 22 ㎾/$m^2$$^{\circ}C$, and were not related to spraying pressure, but approximately linearly to flow rate of sprayed water. Also it is found that increasing the distance from roll to nozzle could improve the cooling efficiency by increasing the sprayed area.

• 한국세라믹학회지
• /
• 제37권5호
• /
• pp.410-417
• /
• 2000

In this study the effects of specifics surface area of cement, addition amount of gypsum and substitution ratio of gypsum anhydrite ont he physical properties of cement mortars were investigated by measruements of setting time, flow, compressive strength and hydration heat evolution rate. The results showed that fluidity of mortars was increased by 40 wt.% of maximum flow change with the decreasing specific surface area of cement from 3,500$\textrm{cm}^2$/g to 3,300${\pm}$50$\textrm{cm}^2$/g and affected by the relationship between the cement and balancing between the chemical activityof cement and solubility of calcium sulfate are desirable to prevent the fluidity of concrete from decreasing by high temperature in summer season.