• 대한기계학회논문집B
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• 제21권5호
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• pp.670-678
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• 1997

A method to mitigate the thermal stratification phenomenon of pressurizer surge line is proposed by heating bottom outside of horizontal pipe. Unsteady two dimensional model has been used to numerically investigate an effect of heating the bottom of pipe. The dimensionless governing equations are solved by using the control volume formulation and SIMPLE algorithm. Temperature and streamline profiles of fluids and pipe walls with time are compared with the previous study result. The numerical result of this study shows that the outside heating can relaxate the thermal stratification flow of the pressurizer surge line. Maximum dimensionless temperature difference between hot and cold sections of the pipe inner wall which causes thermal stratification was reduced from 0.514 to 0.424 at dimensionless time 1, 632 and 1, 500 respectively.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2006년도 추계 학술대회논문집
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• pp.77-82
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• 2006

Evaluation of the elliptic blending turbulence model (EBM) together with the two-layer model, shear stress transport (SST) model and elliptic relaxation model (V2-F) is performed for a better prediction of natural convection and thermal stratification. For a natural convection problem the models are applied to the prediction of a natural convection in a rectangular cavity and the computed results are compared with the experimental data. It is shown that the elliptic blending model predicts as good as or better than the existing second moment differential stress and flux model for the mean velocity and turbulent quantities. For thermal stratification problem the models are applied to the thermal stratification in the upper plenum of liquid metal reactor. In this analysis there exist much differences between the turbulence models in predicting the temporal variation of temperature. The V2-F model and EBM better predict the steep gradient of temperature at the interface of thermal stratification, and the V2-F model and EBM predict properly the oscillation of temperature. The two-layer model and SST model fail to predict the temporal oscillation of temperature.

• 환경영향평가
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• 제22권4호
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• pp.329-343
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• 2013

According to previous studies, the increased air temperature can lead to change of thermal stratification structure of lakes and reservoirs. The changed thermal stratification may result in alteration of materials and energy flow. The objective of this study was to predict the effect of climate change on the water temperature and stratification structure of Daecheong Reservoir, located in Geum River basin of Korea, using a three-dimensional(3D) hydrodynamic model(ELCOM). A long-term(100 years) weather data set provided by the National Institute of Meteorological Research(NIMR) was used for forcing the 3D model. The model was applied to two different hydrological conditions, dry year(2001) and normal year(2004). It means that the effect of air temperature increase was only considered. Simulation results showed that the surface water temperature of the reservoir tend to increase in the future, and the establishment of thermal stratification can occur earlier and prolonged longer. As a result of heat flux analysis, the evaporative heat loss can increase in the future than now and before. However, the convective heat loss and net long wave radiation from water surface decreased due to increased air temperature.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.110-114
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• 2001

A numerical analysis has been perfonned to estimate the effect of turbulent penetration and thermal stratified flow in the branch lines piping. This phenomenon of thermal stratification are usually observed in the piping lines of the safety related systems and may be identified as the source of fatigue in the piping system due to the thermal stress loading which are associated with plant operating modes. The turbulent penetration length reaches to $1^{st}$ valve in safety injection piping from reactor coolant system (RCS) at normal operation for nuclear power plant when a coolant does not leak out through valve. At the time, therefore, the thermal stratification does not appear in the piping between RCS piping and $1^{st}$ valve of safety injection piping. When a coolant leak out through the $1^{st}$ valve by any damage, however, the thermal stratification can occur in the safety injection piping. At that time, the maximum temperature difference of fluid between top and bottom in the piping is estimated about $50^{\circ}C$.

• Nuclear Engineering and Technology
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• 제30권6호
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• pp.581-591
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• 1998

Numerical analysis was peformed for the two-dimensional turbulent natural convection for a long horizontal line with different end temperatures. The turbulent model has been applied a standard k-$\varepsilon$ two equation model of turbulence similar to that the proposed by the Launder and Spalding. The dimensionless governing equations are solved by using SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm which is developed using control volumes and staggered grids. The numerical results are verified by comparison with the operating PWR test data. The analysis focuses on the effects of variation of the heat transfer rates at the pipe surface, the thermal conductivities of the pipe material and the thickness of the pipe wall on the thermal stratification. The results show that the heat transfer rate at the pipe surface is the controlling parameter for mitigating of thermal stratification in the long horizontal pipe. A significant reduction and disappearance of the thermal stratification phenomenon is observed at the Biot number of 4.82$\times$10$^{-1}$ . The results also show that the increment of the thermal conductivity and thickness of the wall weakens a little the thermal stratification and somewhat reduces temperature gradient of y-direction in the pipe wall. These effects are however minor, when compared with those due to the variation of the heat transfer rates at the surface of the pipe wall.

• 한국물환경학회지
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• 제36권1호
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• pp.1-13
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• 2020

The purpose of this study was to investigate the thermal and chemical stratification in Lake Paldang 2013-2018 weekly using Schmidt's stability index (SSI) and the index of chemical stratification (IC-i). The annual average for SSI was 19.1 g cm/㎠ with the maximum value of 45.3 g cm/㎠ in the summer and the minimum value of 4.8 g cm/㎠ in fall-winter showing seasonal differences as well as increased vertical mixing in the summer. The lake stability increased higher in 2016 as compared with the other period. The most influential factors of thermal stratification were temperature and heavy rainfall. Especially, high water temperature and a prolonged residence duration caused by reduced rainfall and inflows could result in an increase of the stratification period. While decreasing inflow and outflow at the end of the rainfall, the thermal stratification was restrengthened within 7-14 days, and then stabilized rapidly before the rainfall. IC-DO increased with high air temperature in the spring and fall-winter. However increasing sunshine duration and residence time and decreasing rate of outflow caused an increase of IC-DO in the summer. Rainfall (less than 800 mm/year) and discharge (less than 200 CMS) significantly declined in 2015 resulting in IC-DO (0.77) increased more than three times over the other years and bottom water hypoxia occurred. The SSI and IC-i used in this study could be applied to other lakes to understand changes in stratification and mixing dynamics.

• 설비공학논문집
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• 제5권3호
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• pp.187-197
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• 1993

In this study, the numerical analysis and experiments of the hot water storage using the thermal stratification techniques were carried out. The CPU time for a typical run of the the thermal stratification up to 900 seconds took one week for a $81{\times}31$ mesh size and 10 days for a $118{\times}31$ mesh size, respectively, for a cylindrical shape of the storage. In the initial stage, the numerical results were in favorable agreement with the experimental results, but it showed that the temperature gradients in the storage decreased gradually with time. It was also found that the increase of ${\delta}t$ decreased the convergent speed due to the intensive fluctuation of the velocity field in every iteration. The increase of numbers of grids is projected to forecast a more accurate result, but it made the computing time longer and woul slow down convergence. At the experiments of the flow visualization, it was confirmed that the thermal stratification was apparently built up due to the installation of diffuser at the lower part of the storage. Thus, the thermal performance of the storage could be improved by installing the diffusers at the inlet and outlet.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제23회 추계학술대회 논문집
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• pp.178-183
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• 2004

액체산소 탱크 내에서의 열적 성층화(thermal stratification) 현상은 대기로부터의 열 투입과 탱크 내에서의 극저온 액체의 열적 비평형에 의해 발생된다. 열적 성층화 현상은 벤트 시스템, 탱크 단열. 펌프 설계에 영향을 미치게 되므로 정교한 해석 및 시험적 검증이 필요하다. 본 논문에서는 side-wall에서의 열 투입에 의해 발생되는 경계층 유동을 해석적 방법으로 1차원 모델링하여 시간에 따른 성충화 부피의 증가 및 탱크 내에서의 높이에 따른 온도 분포를 묘사한다.

• 한국태양에너지학회:학술대회논문집
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• 한국태양에너지학회 2008년도 춘계학술발표대회 논문집
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• pp.300-303
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• 2008

The stratification effect was investigated with four different types of diffuser shape in a thermal storage tank. For this study, experimental facility was constructed, which was composed of experimental thermal storage tank, hot and cold water storage tanks, boiler, chiller, data acquisition system, etc.. Visualization and lab scale experimental result showed that radial curved type diffuser was the highest degree of stratification among the four diffuser shapes.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.729-734
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• 2001

Thermal stratification due to turbulence penetration and in-leakage of valve cause the large thermal stress, which lead to fatigue crack of the piping system of nuclear power plant. So it is needed that numerical and experimental study for the phenomenon is conducted because there have not yet been sufficient study for the relationship between turbulence penetration and thermal stratification. Therefore numerical analysis is done here and respected to give a fundamental method of the approach to the phenomenon.