• Nuclear Engineering and Technology
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• 제54권8호
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• pp.3166-3175
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• 2022

The most numerical investigations of the thermal-hydraulic phenomena following the loss of the residual heat removal capability during the mid-loop operation of the pressurized water reactor were performed according to simplifications and are not sufficiently accurate. To perform more accurate and more reliable predictions of thermal-hydraulic accidents in a nuclear power plant using computational fluid dynamics codes, a more detailed methodology is needed. Modelling results identified that thermal stratification and natural convection are observed. Temperatures of lower monitoring points remain low, while temperatures of upper monitoring points increase over time. The water in the heated region, in the upper unheated region and the pipe region was well mixed due to natural convection, meanwhile, there is no natural convection in the lower unheated region. Water temperature in the pipe region increased after a certain time delay due to circulation of flow induced by natural convection in the heated and upper unheated regions. The modelling results correspond to the experimental data. The developed computational fluid dynamics methodology could be applied for modelling of two-component single/two-phase natural convection and thermal stratification phenomena during the mid-loop operation of the pressurized water reactor or other nuclear and non-nuclear installations at similar conditions.

• 설비공학논문집
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• 제4권4호
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• pp.323-331
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• 1992

A two-dimensional Benard-convection system with a phase-change material inside has been analysed. The main purpose of the present study is to clarify the basic reason of the hysteresis found by the previous investigators. The interface between the solid and the liquid is assumed to be planar. The analysis was performed with heat transfer rates under the steady state on the interface. It was found that the hysteresis occurs due to the abrupt increase in the heat transfer rate at the onset of natural convection in the classical Benard-convection system. The spectral method was applied to obtain the steady solution of the natural convection for the specific material and to confirm the hysteresis phenomenon.

• Nuclear Engineering and Technology
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• 제52권1호
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• pp.27-36
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• 2020

Natural convection and thermal stratification phenomena are found in large water pools that are being used as heat sinks for decay heat removal from the reactor core using passive heat removal systems. In this study, the two-phase (water and air) natural convection and thermal stratification phenomena with conjugate heat transfer in the rectangular enclosure were investigated numerically using ANSYS Fluent 17.2 code. The transient numerical simulations of these phenomena in the full-scale computational domain of the experimental facility were performed. Generation of water vapour bubbles around the heater rod and evaporation phenomena were included in this numerical investigation. The results of numerical simulations are in good agreement with experimental measurements. This shows that the natural convection is formed in region above the heater rod and the water is thermally stratified in the region below the heater rod. The heat from higher region and from the heater rod is transferred to the lower region via conduction. The thermal stratification disappears and the water becomes well mixed, only after the water temperature reaches the saturation temperature and boiling starts. The developed modelling approach and obtained results provide guidelines for numerical investigations of thermal-hydraulic processes in the water pools for passive residual heat removal systems or spent nuclear fuel pools considering the concreate walls of the pool and main room above the pool.

• 대한기계학회논문집
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• 제16권1호
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• pp.142-155
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• 1992

본 연구에서는 실제 빙축열 시스템의 해석을 위한 기초 모델로서 수직원관 주 위의 응고과정에 대하여 밀도차와 자연대류 효과 및 최대 밀도점을 나타내는 밀도-온 도 관계식을 도입한 수치해석과 실험을 통하여 여러가지 매개변수들이 상변화에 미치 는 영향을 알아 보았다.

• 한국생산제조학회지
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• 제24권6호
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• pp.682-686
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• 2015

The objective of this study is to validate the performance of the current $k-{\epsilon}$ turbulence model for a single-phase turbulent natural convection, which has been considered an important phenomenon in nuclear safety. As a result, the natural convection problems in the 2D and 3D cavities previously studied are calculated by using the ANSYS Fluent software. The present results show that the current $k-{\epsilon}$ turbulent model accounting for the buoyancy effect is in good agreement with the previous results for the natural convection problems in the 2D and 3D cavities although some improvements should be required to get better prediction.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.107-108
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• 2008

The temperature rise of GIS (Gas Insulated Switchgear) busbar system is a vital factor that affects its performance. In this paper, a two-dimensional model is presented by commercial code CFX11 for the evaluation of natural convection in the busbar system. In the model, SF6 (Sulfur Hexafluoride) is used to insulate the high voltage device and improves the heat transfer rate. The power losses of a busbar calculated by the magnetic field analysis are used as the input data to predict the temperature rise by the nature convection analysis. The heat-transfer coefficients on the boundaries are analytically calculated by applying the Nusselt number considering material property and model geometry for the natural convection. The temperatures of the tank and conductors from CFX simulation and the experiment were compared. The results show a good agreement. In the future, we will calculate the 3-D model and try to reduce the temperature by adjusting some dimensional parameters.

• 대한공업교육학회지
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• 제33권2호
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• pp.300-311
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• 2008

두 벽면이 서로 다른 평균 온도와 일정한 위상 차이 하에서 작은 파동수의 정현적인 온도 분포를 갖는 시스템에서의 자연 대류를 조사한다. 벽면 온도 분포의 파동수 k=0.5이고 위상차는 ㄱ/2 이며, 고려하는 유체는 Pr =0.7인 공기이다. 작은 Rayleigh 수의 전도 영역에서는 한 주기에 걸쳐 약간 기울어진 형태의 두 개의 큰 셀이 형성된다. 그러나 Rayleigh 수가 증가하면 열적으로 불안정한 영역에서 여러 개의 셀이 형성되는 다수 셀 유동이 일어난다. 다수 셀 영역에서 과도기적인 기간에서는 일시적으로 공간적인 점대칭성이 깨어지기도 하지만 정상 상태의 유동장은 언제나 대칭성을 만족한다. 유동 형태의 천이가 일어나는 임계 Rayleigh 수 부근에서는 Nusselt 수가 급격하게 증가한다.

• 한국안전학회지
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• 제15권3호
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• pp.14-22
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• 2000

A numerical analysis has been performed on the two-dimensional rectangular gallium melting problem using the enthalpy method. The major advantage of this method is that the physical domain is discretized with fixed grids without transforming variables and the interface conditions of phase change are accounted for the definition of suitable source terms in the governing equations. But in the fixed method, there is some ambiguity in defining the porosity constant which has no physical interpretation. If the velocity correction is included in the momentum equation, for the appropriate range of porosity constant, the realistic predictions are obtained. The object of the present work is to predict the phase change within the molten steel with thin riser slab using the modified enthalpy-porosity method. The computational procedures for predicting velocity and temperature are based on the finite volume method and the non-staggered grid system. The influence of natural convection on the melting process is considered. A comparison with the experimental results shows that the modified method is better than the previous one.

• 대한기계학회논문집
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• 제19권9호
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• pp.2353-2364
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• 1995

The ignition phenomena of a solid fuel plate of polymethyl-methacrylate(PMMA), which is vertically positioned and exposed to a thermal radiation source, is numerically studied here. A two-dimensional transient model includes such various aspects as thermal decomposition of PMMA, gas phase radiation absorption, gas phase chemical reaction and air entrainment by natural convection. Whereas the previous studies considers the problem approximately in a one-dimensional form by neglecting the natural convection, the present model takes account of the two-dimensional effect of radiation and air entrainment. The inert heating of the solid fuel is also taken into consideration. Radiative heat transfer is incorporated by th Discrete Ordinates Method(DOM) with the absorption coefficient evaluated using gas species concentration. The thermal history of the solid fuel plate shows a good agreement compared with experimental results. Despite of induced natural convective flow that induces heat loss from the fuel surface, the locally absorbed radiant energy, which is converted to the internal energy, is found to play an important role in the onset of gas phase ignition. The ignition is considered to occur when the rate of variation of gas phase reaction rate reaches its maximum value. Once the ignition takes place, the flame propagates downward.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2008년도 하계학술발표대회 논문집
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• pp.1155-1160
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• 2008

In the present study investigated the heat dissipation characteristics of the natural convection type radiator by using the latent heat from a solid-liquid PCM(Phase Change Material). Total radiator volume size is $423{\times}295{\times}83\;mm$ and PCM tank size is $398{\times}270{\times}26\;mm$. The objective was elapsed time lower than maximum operating temperature. Experimental condition, in order to study the effects of the phase-change phenomenon, carried out the various mass flow rate, input electric power, and heat of fusion temperature of two type PCMs. For the above experimental conditions, the cooling performance by using the latent heat showed that heat absorption rate performs for about 3 hours from using PCM $38^{\circ}C$. However, cooling performance by using PCM $50^{\circ}C$ showed higher than surface temperature of heater block because of heat of fusion.