• Nuclear Engineering and Technology
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• 제52권9호
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• pp.1955-1962
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• 2020

In a sudden shutdown of primary pump or coolant loss accident in a marine nuclear power plant, the primary flow decreases rapidly in a transition process from forced circulation (FC) to natural circulation (NC), and the lower flow enters the steam generator (SG) causing reverse flow in the U-tube. This can significantly compromise the safety of nuclear power plants. Based on the marine natural circulation steam generator (NCSG), an experimental loop is constructed to study the characteristics of reverse flow under middle-temperature and middle-pressure conditions. The transition from FC to NC is simulated experimentally, and the characteristics of SG reverse flow are studied. On this basis, the experimental loop is numerically modeled using RELAP5/MOD3.3 code for system analysis, and the accuracy of the model is verified according to the experimental data. The influence of the flow variation rate on the reverse flow phenomenon and flow distribution is investigated. The experimental and numerical results show that in comparison with the case of adjusting the mass flow discontinuously, the number of reverse flow tubes increases significantly during the transition from FC to NC, and the reverse flow has a more severe impact on the operating characteristics of the SG. With the increase of flow variation rate, the reverse flow is less likely to occur. The mass flow in the reverse flow U-tubes increases at first and then decreases. When the system is approximately stable, the reverse flow is slightly lower than obverse flow in the same U-tube, while the flow in the obverse flow U-tube increases.

• Fisheries and Aquatic Sciences
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• 제5권1호
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• pp.5-20
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• 2002

To investigate the effect of mechanical parameters on the circulation and its fluctuation in Sagami Bay, baroclinic model experiments were carried out by use of a two-layer source-sink flow in a rotating tank. In the experiment, a simple coastal topography with flat bottom was reproduced. The results show that the path of the Through Flow, which corresponds to the branch current of the Kuroshio, depends on external Rossby number (Ro) and internal Rossby number $(Ro^*)$, and divided into two regimes. For $Ro^*\leq1.0$ in which Rossby internal radius of deformation of the Through Flow is smaller than the width of the approaching channel, the current flows along the Oshima Island as a coastal boundary density current separated from the western boundary of the channel. For $Ro^*>1.0$ it changes to a jet flow along the western boundary of the channel, separated from the coast of Oshima Island. The current is independent on both Ro and Ro* in the regime of $Ro^*>1.0,\;Ro\geq0.06$ and $Ro^*\leq1.0,\;Ro\geq0.06$. The pattern of the cyclonic circulation in the inner part of the bay is also determined by Ro and Ro*. In case of $Ro^*\leq1.0$, frontal eddies are formed in the northern boundary of the Through Flow. These frontal eddies intrude into the inner part along the eastern boundary of the bay providing vorticity to form and maintain the inner cyclonic circulation. For $Ro^*>1.0$, the wakes from the Izu peninsula are superposed intensifying the cyclonic circulation. The pattern of the cyclonic circulation is divided into three types; 1) weak cyclonic circulation and the inner anticyclonic circulation $(Ro<0.12)$. 2) cyclonic circulation in the bay $(0.12\leq Ro<0.25)$. 3) cyclonic circulation with strong boundary current $(RO\geq0.25)$.

• Nuclear Engineering and Technology
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• 제54권5호
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• pp.1851-1859
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• 2022

Experimental research on flow characteristics of open natural circulation system was performed, to figure out the mechanism of the open natural circulation behaviors. The influence factors, such as the heating power, the inlet subcooled and the level of cooling tank on the flow characteristics of the system were examined. It was shown that within the scope of the experimental conditions, there are five flow types: single-phase stable flow, flash and geyser coexisting unstable flow, flash stable flow, flash unstable flow, and flash and boiling coexisting unstable flow. The geyser flow in flash and geyser coexisting unstable flow is different from classic geysers flow. The flow oscillation period and amplitude of the former are more regular, is a newly discovered flow pattern. By drawing the flow instability boundary diagram and sorting out the flow types, it is found that the two-phase unstable flow is mainly characterized by boiling and flash, which determine the behavior of open natural circulation respectively or jointly. Moreover, compared with full liquid level system, non-full liquid level system is more prone to boiling phenomenon, and the range of heat flux density and undercooling degree corresponding to unstable flow is larger.

• 한국추진공학회지
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• 제23권5호
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• pp.43-52
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• 2019

극저온 유체를 사용하는 발사체는 극저온 유체의 자연순환회로를 이용하여 발사체의 엔진 입구를 냉각한다. 자연순환회로의 질량유량은 순환시스템을 구성하는 배관의 길이 및 직경과 시스템으로 들어오는 열유입에 의하여 결정된다. 극저온 유체의 자연순환회로의 순환 검증 및 질량유량 측정을 위하여 실험을 진행하였으며, 이론적 계산 결과와 비교하였다. 비교 결과 12%의 오차가 있음을 확인하였다. 이 결과를 바탕으로 발사체 상단에서 저중력 구간 및 가속 구간에서의 자연순환 질량유량을 예측한 내용을 포함한다. 가속구간에서는 산화제탱크가 100 kPa 내외로 유지하는 것이 자연순환유량 증가에 이로웠으며, 저중력구간에서는 중력가속도의 크기에 따른 최적 압력으로 조절해야 자연순환유량의 최고값을 유지할 수 있었다.

• Nuclear Engineering and Technology
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• 제54권1호
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• pp.72-83
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• 2022

This paper presents a numerical investigation of two-phase natural circulation flows established when external reactor vessel cooling is applied to a severe accident of the APR1400 reactor for the in-vessel retention of the core melt. The coolability limit due to external reactor vessel cooling is associated with the natural circulation flow rate around the lower head of the reactor vessel. For an elaborate prediction of the natural circulation flow rate using a thermal-hydraulic system code, MARS-KS1.5, a three-dimensional computational fluid dynamics (CFD) simulation is conducted to estimate the flow rate and pressure distribution of a liquid-state coolant at the brink of significant void generation. The CFD calculation results are used to determine the loss coefficient at major flow junctions, where substantial pressure losses are expected, in the nodalization scheme of the MARS-KS code such that the single-phase flow rate is the same as that predicted via CFD simulations. Subsequently, the MARS-KS analysis is performed for the two-phase natural circulation regime, and the transient behavior of the main thermal-hydraulic variables is investigated.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2018년도 학술발표회
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• pp.355-355
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• 2018

Natural hydrology systems, including high flow and low flow events, are important for aquatic ecosystem health and are essential for controlling the structure and function of ecological processes in river ecosystems. Ecosystem responses to flow changes have been studied in a variety of ways, but little attention has been given to how episodic typhoons and atmospheric circulation patterns can change these hydrologic regime-ecological response relationships. In this diagnostic study, we use an empirical approach to investigate the salient features of interactions between atmospheric circulation, climate, and runoff in the five major Korean river basins.

• 한국시뮬레이션학회논문지
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• 제7권2호
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• pp.17-32
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• 1998

The flow fields in Gas Bubble Driven Circulation Systems were numerically analyzed. In various gas flow rate and bubble size, the flow characteristics were predicted. Eulerian-Eulerian approach was used for the formulation of both the continuous and dispersed phases. The modification of the general purpose computer program PHOENICS code was employed to predict the mean flow fields, turbulent characteristics, gas dispersion, volume fraction. The predicted shows very satisfactory agreement with experimental results for all regions of ladle. The results are of interest in the design and operation of wide variety of material processing.

• 한국해양공학회지
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• 제15권3호
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• pp.35-42
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• 2001

The characteristics of tidal circulation with the flow-control structures using the three-dimensional numerical model (POM, Princeton Ocean Model) of Chinhae Bay, Korea were investigated. To confirm th efficiencies of flow-control structures, the training wall and submerged training wall were constructed at the mouth and narrow channel in Chinhae Bay. On the basis of the present investigation, the tidal circulation induced by the construction of flow-control structures could enhance the water exchange improvement appropriately. And, th training wall at the central is more dominated than the other structures for the efficient of water exchange. The sites and types of structure and flow patterns seem to be very sensitive in tidal simulation and changes in flow fields.

• Nuclear Engineering and Technology
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• 제54권3호
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• pp.826-833
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• 2022

A two-phase natural circulation test using SMART integral test loop (SMART-ITL) was conducted to explore thermo-hydraulic phenomena of two-phase natural circulation in the SMART reactor. Specifically, the test examined the natural circulation in the primary loop under a stepwise coolant inventory loss while keeping the core power constant at 5% of the scaled full power. Based on the test results, three flow regimes were observed: single-phase natural circulation (SPNC), two-phase natural circulation (TPNC), and boiler-condenser natural circulation (BCNC). The flow rate remained steady in the SPNC, slightly increased in the TPNC, and dropped abruptly and maintained in the BCNC. Using a natural circulation flow map, the natural circulation characteristic in the SMART-ITL was compared with those in pressurized water reactor simulators. In the SMART-ITL, a BCNC regime appeared instead of siphon condensation and reflux condensation regimes because of the use of once-through steam generators.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1999년도 추계 학술대회논문집
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• pp.152-157
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• 1999

For the simulation of the blood flow in coronary artery, the system modeling of coronary hemodynamics is combined with CFD technique. The blood flow in coronary artery interacts with the global coronary circulation. Especially in case of the coronary artery with stenosis, the interaction plays an important role in the hemodynamics of the circulation. In this study we present a combined numerical approach using both the CFD technique for flow simulation and the global system model of coronary circulation. We use a lumped parameter model for the global simulation of coronary circulation whereas the finite element method is employed to compute the viscous flow field in stenosed coronary artery, The time variation of the pressure drop due to stenosis is obtained from the proposed numerical method. Numerical results shows that the flow resistance and pressure drop due to stenosis has a relatively large value in systole.