• Nuclear Engineering and Technology
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• 제56권7호
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• pp.2881-2892
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• 2024

Annular flow refers to a special type of two-phase flow pattern in which liquid flows as a thin film at the periphery of a pipe, tube, or conduit, and gas with relatively high velocity flows at the center of the flow section. This gas also includes dispersed liquid droplets. The liquid film flow rate continuously changes inside the tube due to two processes-entrainment and deposition. To determine the liquid holdup, pressure drop, the onset of dryout, and heat transfer characteristics in annular flow, it is important to have proper knowledge of flow characteristics. Especially a better understanding of entrainment fraction is important for the heat transfer and safe operation of two-phase flow systems operating in an annular two-phase flow regime. Therefore, the objective of this work is to develop a computational model for the simulation of the annular two-phase flow regime and assess the various existing models for the entrainment rate. In this work, Computational Fluid Dynamics (CFD) in ANSYS FLUENT has been applied to determine annular flow characteristics such as liquid film thickness, film velocity, entrainment rate, deposition rate, and entrainment fraction for various gas-liquid flow conditions in a vertical upward tube. The gas core with droplets was simulated using the Discrete Phase Model (DPM) which is based on the Eulerian-Lagrangian approach. The Eulerian Wall Film (EWF) model was utilized to simulate liquid film on the tube wall. Three different models of Entrainment rate were implemented and assessed through user-defined functions (UDF) in ANSYS. Finally, entrainment for fully developed flow was determined and compared with the experimental data available in the literature. From the simulations, it was obtained that the Bertodano correlation performed best in predicting entrainment fraction and the results were within the ±30 % limit when compared to experimental data.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 1984년도 제26회 수공학연구발표회논문초록집
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• pp.7-21
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• 1984

Entrainment and deposition experiments were counducted in fresh water on four groups of sediments: three well-defined sediments of uniform composition and narrow-size distribution (1 to 9 um, 10 to 50 um, and 50 to 90 um), and a fourth group which was a mixture of these three sediments. In the entrainment experiments and at a particular stress, the steady-state suspended sediment concentration of the coarse group was the lowest while the concentrations of the fine and medium groups were higher that that of the coarse group but were similar to each other. Deposition experiments generally showed an exponential decrease of suspended sediment concentration with time with the decay time being a function of particle size and applied stress.

• Nuclear Engineering and Technology
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• 제47권4호
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• pp.389-397
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• 2015

The effects of mixing vanes (MVs) attached to a grid spacer on the characteristics of air-water annular flows were experimentally investigated. To know the effects, a grid spacer with or without MV was inserted in a vertical circular pipe of 16-mm internal diameter. For three cases (i.e., no spacer, spacer without MV, and spacer with MV), the liquid film thickness, liquid entrainment fraction, and deposition rate were measured by the constant current method, single liquid film extraction method, and double liquid film extraction method, respectively. The MVs significantly promote the re-deposition of liquid droplets in the gas core flow into the liquid film on the channel walls. The deposition mass transfer coefficient is three times higher for the spacer with MV than for the spacer without MV, even for cases 0.3-m downstream from the spacer. The liquid film thickness becomes thicker upstream and downstream for the spacer with MV, compared with the thickness for the spacer without MV and for the case with no spacer.

• 한국해양학회지
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• 제22권1호
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• pp.25-33
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• 1987

河口와 인접한 內 에서 海流에 의한 微細粒 堆積物質의 輸送, 擴散現象을 數値的으로 分析하였다. 사용된 모델은 水理實驗과 現場觀測의 최근 硏究結果를 근거로 開發되었으며 堆積物의 浮上, 堆積, 輸送過程을 보다 실제 상황에 근접하도 록 數値, 計算하였다. 江의 흐름과 潮汐에 의한 海水流動, 이에 따른 沿岸海域에서의 浮遊物質 濃度, 海底寢食 그리고 堆積分布에 대한 時間別 變化를 計算하였다. 初期 에는 浮上된 堆積物에 의해 浮遊物質의 濃度가 크게 增加되었으나, 時間이 경과됨 에 따라 外海로 輸送되어 江河口에서는 濃度가 크게 줄어들었다. 河口隣接 內 의 海底 堆積物의 寢食과 堆積狀態는 內 의 地形的 條件에 따른 海流의 流速分布와 堆積物 特性에 의해 주로 決定됨을 나타내었다.

• Nuclear Engineering and Technology
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• 제40권5호
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• pp.387-396
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• 2008

Many experimental analyses for annular film dryouts, which is one of the Critical Heat Flux (CHF) mechanisms, have been performed because of their importance. Numerical approaches must also be developed in order to assess the results from experiments and to perform pre-tests before experiments. Various thermal-hydraulic codes, such as RELAP, COBRATF, MARS, etc., have been used in the assessment of the results of dryout experiments and in experimental pre-tests. These thermal-hydraulic codes are general tools intended for the analysis of various phenomena that could appear in nuclear power plants, and many models applying these codes are unnecessarily complex for the focused analysis of dryout phenomena alone. In this study, a numerical model was developed for annular film dryout using the drift-flux model from uniform heated tube geometry. Several candidates of models that strongly affect dryout, such as the entrainment model, deposition model, and the criterion for the dryout point model, were tested as candidates for inclusion in an optimized annular film dryout model. The optimized model was developed by adopting the best combination of these candidate models, as determined through comparison with experimental data. This optimized model showed reasonable results, which were better than those of MARS code.

• Nuclear Engineering and Technology
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• 제50권3호
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• pp.379-388
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• 2018

Models for the rate of atomization and deposition of droplets for stratified and annular flow in horizontal pipes are presented. The entrained fraction is the result of a balance between the rate of atomization of the liquid layer that is in contact with air and the rate of deposition of droplets. The rate of deposition is strongly affected by gravity in horizontal pipes. The gravitational settling of droplets is influenced by droplet size: heavier droplets deposit more rapidly. Model calculation and simulation results are compared with experimental data from various diameter pipes. Validation for the suggested models was performed by comparing the Safety and Performance Analysis Code for Nuclear Power Plants calculation results with the droplet experimental data obtained in various diameter horizontal pipes.

• 한국해양학회지
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• 제18권1호
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• pp.21-28
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• 1983

堆積物 流動의 再現과 豫測을 위한 數値模型의 開發에는 堆積物의 浮上機作 과 沈澱過程의 究明이 先決要件이 된다. 따라서 이의 究明을 爲한 一蓮 의 水理實驗이 企圖하였다. 硏究結果, 堆積物의 浮上은 전단응력과 퇴적물에 함유된 수분량 및 堆積物의 구성종류에 따라 좌우되고 있음이 밝혀 졌으며, 침전은 퇴적물의 농도와 점토광물질의 함유량에 따라 크게 변하고 있음이 밝혀 졌다.

• 대한토목학회논문집
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• 제33권3호
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• pp.965-974
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• 2013

2011년 7월 27일 우면산 유역에서 일련의 토석류가 발생하여 많은 인명과 재산 피해를 야기했다. 이러한 토석류 피해를 저감하기 위해서는 토석류가 어떻게 이동하고 퇴적되어 하류부에 피해를 주는지를 예측할 수 있는 기술이 필요하다. 이 연구에서는 우면산 토석류에 대해서 FLO-2D 수치모형을 적용하여, 모형의 성능을 평가하고, 이들 토석류의 유동 매개변수를 분석하였다. 우면산 북측사면에서 발생한 토석류의 현장 관측자료와 FLO-2D 모의 결과 분석을 통해서 역산된 우면산 토석류의 대표 항복응력은 1,022 Pa 그리고 대표 동점성은 652 $Pa{\cdot}s$ 인 것으로 나타났다. 선정된 매개변수를 이용하여 우면산 토석류의 퇴적영역을 산정한 결과, 래미안 유역의 경우 63 - 85% 정확도의 피해유역을 재현됨으로서 FLO-2D 모형은 피해유역 산정에 적합한 것으로 나타났다. 하지만, FLO-2D 모형이 침식물질과 우수의 연행작용을 고려할 수 없는 고유의 한계 때문에 연행작용이 현저한 래미안 및 신동아 유역 토석류의 유하속도는 과소평가하는 것으로 나타났다. 유하부에서 침식에 의한 연행작용이 활발하지 않은 형촌마을 유역 토석류는 수로와 저수지 상부 부근에서의 퇴적을 우수한 정확도로 모의되었다. 민감도 분석 결과 유사체적농도와 바닥조도계수 모두 토석류의 흐름두께와 유속 모두에 상당한 영향을 미치는 것으로 나났으므로, 이들 매개변수의 선정에 신중해야 함을 보여주었다.

• 한국항만학회지
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• 제9권1호
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• pp.45-56
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• 1995

Investigation of the physical behavior of dredged material disposal in coastal water includes estimations of water column concentration in the receiving water, exposure time, the initial deposition pattern as well as thickness of material at the dumping fields near the estuary area. Calculation based on vertical setting and horizontal advection of single particles ignore the effects of bulk properties of the disposed material, vertical and horizontal diffusion, and material dilution due to the entrainment of ambient water during descent. This paper focuses on the spatial and temporal changes in the dumping fields for the water column and bottom at a hypothetically confined coastal water, where the ambient time-invariant velocity and density profiles are applied, within the initial time period following the instantaneous release of the dredged material. This model accounts the behavior of material after release divided into three phases: convective descent, dynamic collapse and long-term passive dispersion

• 물과 미래
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• 제17권3호
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• pp.191-196
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• 1984

퇴적물의 입자 크기가 부상과 퇴적과정에 미치는 영향을 규명하기 위하여 네 종류의 각기 다른 입도분 포성분으로 구성된 퇴적물을 사용하여 수리실험을 하였다. 사용된 세 가지의 퇴적물은 입도분포가 조밀한 동질의 입도로 구성되었으며 네 번째 것은 위의 세 종류의 퇴적물을 한데 섞은 복합적 구성요소를 지닌 것이다. 또한 최적물 표층에 가해지는 1~14dynes/$\textrm{cm}^2$ 범위의 전단응력을 사용하여 각 군의 퇴적물마다 부상과 퇴적에 대한 실험을 하였으며 그 결과를 상호 비교하였다.