• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.532-537
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• 2003

Gas flow through orifice is encountered in many diverse fields of engineering applications. In order to investigate the critical gas flow through an orifice system, a computational analysis is performed using axisymmetric, compressible, Navier-Stokes equations which are numerically solved by a fully implicit finite volume method. In the present study, the discharge coefficients of two different types of orifices which are a straight-bore orifice and a sharp-edged orifice, are predicted to obtain the critical flow conditions. The present CFD data are compared with the previous experimental results. The present computational results show that the critical mass flow rate through orifice is well predicted and it is a strong function of Reynolds number. The discharge coefficient increases with the orifice diameter.

• 드라이브 ㆍ 컨트롤
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• 제10권4호
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• pp.29-33
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• 2013

Pneumatic system is widely applied in various industry because it have a many advantage(low cost, high safety, etc..). For design of pneumatic system, accurate flow measurement is required. In this study, compressible fluid flow measurement apparatus was designed and built. It uses an isothermal chamber that can approximate isothermal condition. Therefore, it can be measured for flow-rate using pressure response of isothermal chamber. As a result, this apparatus can be measured for sonic conductance and critical pressure ratio of pneumatic components and it required less time and energy than conventional flow meter. The effectiveness of the designed apparatus is proved by experimental result.

• Tuberculosis and Respiratory Diseases
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• 제85권4호
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• pp.332-340
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• 2022

Background: Cardiac dysfunction patients have long been considered at high risk of reintubation. However, it is based on past studies in which only conventional oxygen therapy was applied after extubation. We investigated association between cardiac dysfunction and reintubation rate in situation where high-flow nasal cannula (HFNC) was widely used during post-extubation period. Methods: We conducted a retrospective observational cohort study of patients treated with HFNC after planned extubation in medical intensive care unit of single tertiary center. Patients were divided into normal function group (ejection fraction [EF] ≥45%) and cardiac dysfunction group (EF <45%). The primary outcome was reintubation rate within 72 hours following extubation. Results: Of 270 patients, 35 (13%) had cardiac dysfunction. Baseline characteristics were similar in both groups. There were no differences in the changes in vital signs between the two groups during the first 12 hours after extubation except diastolic blood pressure. The reintubation rates were 20% and 17% for cardiac dysfunction group and normal function group, respectively (p=0.637). In a multivariate Cox regression analysis, cardiac dysfunction was not associated with an increased risk of reintubation within 72 hours following extubation (hazard ratio, 1.56; p=0.292). Conclusion: Cardiac dysfunction was not associated with increased reintubation rate within 72 hours when HFNC is immediately applied after planned extubation.

• Nuclear Engineering and Technology
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• 제34권4호
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• pp.382-395
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• 2002

An experimental study on transient critical heat flux (CHF) under flow coastdown has been performed for the water flow in a non-uniformly heated vertical annulus under low flow and a wide range of pressure conditions. The objectives of this study are to systematically investigate the effect of the flow transient on the CHF and to compare the transient CHF with steady-state CHF The transient CHF experiments have been performed for three kinds of flow transient modes based on the coastdown data of a nuclear power plant reactor coolant pump. At the same inlet subcooling, system pressure and heat flux, the effect of the initial mass flux on the critical mass flux can be negligible. However, the effect of the initial mass flux on the time-to- CHF becomes large as the heat flux decreases. The critical mass flux has the largest value for slow flow reduction rate. There is a pressure effect on the ratio of the transient CHF data to steady-state CHF data. Except under low system pressure conditions, the flow transient CHF was revealed to be conservative compared with the steady-state CHF data. Bowling CHF correlation and thermal hydraulic system code MARS show promising results for the prediction of CHF occurrence .

• Nuclear Engineering and Technology
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• 제13권3호
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• pp.121-129
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• 1981

많은 기존 공식중 세계의 가장 우수한 최대 유량공식을 찾아, 그 세개의 한계유량공식의 개요와, 냉각재상실사고시 격납용기격리에 실패하는 경우를 특정지을 수 있는 구멍의 크기와 격납용기의 압력 및 온도 등이 주어진 상태하에서 격납용기로부터 외부대기로 방출되는 개략적인 핵분열생성물의 양을 추정하기 위한 계산절차를 제시하였다. 이상기체의 임계유량공식과 이상유(two-phase flow)의 최대유량을 산출하기 위한 무디(Moody)의 도표를 이용하여 계산실예를 제시하였으며, 그 결과를 콘뎀프트-앨티(CONTEMPT-LT) 전산코드의 질량유출공식을 콘버징 노즐(converging nozzle)을 통과하는 음속류(sonic flow)의 경우에 적용하여 산출한 값과 비교하여 보았다. 이리하여 이상 기체의 임계유량공식은 무디(Moody)의 공식이 주는 값과 거의 비슷한 결과를 줌을 입증하였다. 또한 냉각재상실 사고시 격납용기로부터의 유출율을 추정하기 위해서는 콘템프트-앨티(CONTEMPT-LT)의 질량유출공식을 사용하는 것보다 이상 기체의 최대유량공식을 사용하는 것이 더 보수적인 방법임을 보여 주었다.

• 대한기계학회논문집B
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• 제24권5호
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• pp.615-622
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• 2000

Numerical simulation is performed with Denton's code to get pressure loss coefficients in wide range of reverse flow incidence(from -90 degree to +85 degree) for an axial compressor cascade. As a results, it is found that the pressure loss coefficient is increased with incidence and there exist critical incidence which corresponds to the maximum pressure loss coefficient. Pressure loss coefficient with bigger incidence than its critical value is decreased. The effect of increasing incidence in a cascade extremely reduce the mass flow rate by the large flow separation region. Consequently this effect reduce the portion of dynamic pressure in the total pressure loss and beyond the critical incidence the pressure loss coefficient decrease.

• Membrane and Water Treatment
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• 제13권6호
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• pp.313-320
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• 2022

Theoretical calculation results are presented for the enhancement of the water mass flow rate through the hydrophobic micro/nano pores in the membrane respectively on the micrometer and nanometer scales. The water-pore wall interfacial slippage is considered. When the pore diameter is critically low (less than 1.82nm), the water flow in the nanopore is non-continuum and described by the nanoscale flow equation; Otherwise, the water flow is essentially multiscale consisting of both the adsorbed boundary layer flow and the intermediate continuum water flow, and it is described by the multiscale flow equation. For no wall slippage, the calculated water flow rate through the pore is very close to the classical hydrodynamic theory calculation if the pore diameter (d) is larger than 1.0nm, however it is considerably smaller than the conventional calculation if d is less than 1.0nm because of the non-continuum effect of the water film. When the driving power loss on the pore is larger than the critical value, the wall slippage occurs, and it results in the different scales of the enhancement of the water flow rate through the pore which are strongly dependent on both the pore diameter and the driving power loss on the pore. Both the pressure drop and the critical power loss on the pore for starting the wall slippage are also strongly dependent on the pore diameter.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3003-3008
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• 2007

Critical nozzle has been frequently employed to measure the flow rate of various gases, but hydrogen gas, especially being at high-pressure condition, was not nearly dealt with the critical nozzle due to treatment danger. According to a few experimental data obtained recently, it was reported that the discharge coefficient of hydrogen gas through the critical nozzle exceeds unity in a specific range of Reynolds number. No detailed explanation on such an unreasonable value was made, but it was vaguely inferred as real gas effects. For the purpose of practical use of high-pressure hydrogen gas, systematic research is required to clarify the critical nozzle flow of high-pressure hydrogen gas. In the present study, a computational fluid dynamics(CFD) method has been applied to predict the critical nozzle flow of high-pressure hydrogen gas. Redlich-Kwong equation of state that take account for the forces and volume of molecules of hydrogen gas were incorporated into the axisymmetric, compressible Navier-Stokes equations. A fully implicit finite volume scheme was used to numerically solve the governing equations. The computational results were validated with some experimental data available. The results show that the coefficient of discharge coefficient is mainly influenced by the compressibility factor and the specific heat ratio, which appear more remarkable as the inlet total pressure of hydrogen gas increases.

• 한국전산유체공학회지
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• 제19권1호
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• pp.34-40
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• 2014

Conjugate heat transfer of an automotive oil cooler with offset-strip fins was numerically investigated to predict the performance of the oil cooler for various flow-rates. The simulations were conducted by directly modeling offset-strip fins with unstructured meshes. The incompressible Navier-Stokes equations coupled with energy equation were used for the present simulations. Heat transfer characteristics of the oil cooler was compared well with experimental results and the errors were approximately within 5 percents. It was found that the performance of the oil cooler increased as the flow-rate increased up to the flow-rate of 12 L/min, but the performance seemed to be saturated beyond a critical flow-rate, which was estimated as 15 L/min. Furthermore, it was confirmed that compared to the performance without fins, that of the oil cooler with offset-strip fins was increased by about 75 percents.

• 반도체디스플레이기술학회지
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• 제23권2호
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• pp.65-70
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• 2024

Using a computational fluid dynamics(CFD) simulation tool, we have provided a coating guideline for slot-die coating with a double layer slot die head. We have analyzed the fluid dynamics in terms of the coating speed, flow rate ratio, and viscosity ratio, which are critical for the stability of coating meniscus. We have identified the common coating defects such as break-up, air entrainment, and leakage by varying the coating speeds. The flow rate ratio is the critical parameter determining the wet film thickness of the top and bottom layers. It is shown that when the flow rate ratio exceeds or equals 1.8, air entrainment occurs due to insufficient hydraulic pressure in the bottom layer, even though the total flow rate remains constant. Furthermore, we have found that the flow of the bottom layer is significantly affected by the viscosity of top layer. The viscosity ratio of 4 or higher obstructs the flow of the bottom layer due to the increased hydraulic resistance, resulting in leakage. Finally, we have demonstrated that as the viscosity ratio increases from 0.1 to 10, the maximum coating speed rises from 0.4 mm/s to 1.6 mm/s, and the minimum wet film thickness decreases from 800 ㎛ to 200 ㎛.