• Nuclear Engineering and Technology
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• 제28권4호
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• pp.405-414
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• 1996

In this paper, the unsteady 2-dimensional turbulent flow model for thermal stratification in a pressurizer surge line of PWR plant is proposed to numerically investigate the heat transfer and flow characteristics. The turbulence model is adapted to the low Reynolds number K-$\varepsilon$ model (Davidson model). The dimensionless governing equations are solved by using the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm. The results are compared with simulated experimental results of TEMR Test. The time-dependent temperature profiles in the fluid and pipe nil are shown with the thermal stratification occurring in the horizontal section of the pipe. The corresponding thermal stresses are also presented. The numerical result for thermal stratification by the outsurge during heatup operation of PWR shows that the maximum dimensionless temperature difference is about 0.83 between hot and cold sections of pipe well and the maximum thermal stress is calculated about 322MPa at the dimensionless time 28.5 under given conditions.

• Communications for Statistical Applications and Methods
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• 제22권3호
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• pp.255-264
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• 2015

This paper addressed the problem of estimation of finite population mean in double sampling for stratification. This paper proposed a generalized ratio-cum-product type estimator of population mean. The bias and mean square error of the proposed estimator has been obtained upto the first degree of approximation. A particular member of the proposed generalized estimator was identified and studied from a comparison point of view. It is observed that the identified particular estimator is more efficient than usual unbiased estimator and Ige and Tripathi (1987) estimators. An empirical study was conducted in support of the theoretical findings.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2005년도 추계 학술대회논문집
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• pp.105-110
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• 2005

Evaluation of 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 thermal stratification in an upper plenum of a liquid metal reactor by applying them to the experiment conducted at JNC. The algebraic flux model is used for treating the turbulent heat flux. There exist much differences between turbulence models in predicting the temporal variation of temperature. The V2-F model and the 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.

• 한국자동차공학회논문집
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• 제9권3호
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• pp.9-17
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• 2001

This paper is the second of 3 companion papers which investigate axial stratification process. In-cylinder fuel behavior has been investigated in the port injected Sl engine by visualizing for the purpose of understanding stratification. Planar laser light sheet from an Nd:YAG laser has been illuminated through the transparent quartz cylinder of the single cylinder optical engine and the Mie scattered light has been captured through the quartz window in the piston head with an ICCD camera. Fuel has been replaced with an air-ethanol mixture to utilize atomized fuel spray fur the visualization purposes. This results have been compared with steady flow concentration measurement. For low/medium swirl port, the early injection makes such a fuel distribution state that is upper-rich, middle-lean and lower-rich along the combustion chamber and cylinder by tumbling motion. On the other hand, the late injection induces upper-rich, middle-lean and lower-rich state due to the short fuel penetration.

• 한국자동차공학회논문집
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• 제9권3호
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• pp.18-26
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• 2001

This paper is the third of 3 companion papers which investigate axial stratification process. In-cylinder fuel behavior has been investigated in the port injected SI engine by visualizing for the purpose of understanding stratification. Planar laser light sheet from an Nd:YAG laser has been illuminated through the transparent quartz cylinder of the single cylinder optical engine and the Mie scattered light has been captured through the quartz window in the piston head with an ICCD camera. Fuel has been replaced with an air-ethanol mixture to utilize atomized fuel spray for the visualization purposes. This results have been compared with steady flow concentration measurement. In high swirl port, the most fuel remains at combustion chamber and upper cylinder region without being affected by injection timing. The macro-distributed state is not changed but the difference of the amount of fuel around the spark plug varies according to injection timing, which determines LML.

• 대한기계학회논문집
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• 제18권9호
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• pp.2454-2462
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a heated circular cylinder were examined in a wind tunnel. Turbulent intensities, rms values of temperature and turbulent convective heat flux distributions in the heated cylinder wake with and without thermal stratification were measured by using a hot-wire and cold-wire combination probe. A phase averaging method was also used to estimated coherent motion in the near wake. It is found that the vertical turbulent motion in the stably stratified flow case dissipates faster than that of the neutral case, i.e., vertical growth of vortical structure is suppressed under the strongly stratified condition. The coherent motion of temperature makes a large contribution like velocity coherent motion. However, the coherent motions of temperature fluctuation become very different with the change of experimental conditions, though the velocity coherent motions are quite similar in all experimental conditions.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1597-1602
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• 2003

Numerical analysis is carried out to assess the temperature distribution on the mixing tee line of Residual Heat Removal System (RHRS). In RHRS, hot and cold fluids of main and bypass piping are mixed and unmixed by the flow rate or piping layout. Thermal stratification phenomenon is a cause of major degradation on RHRS piping. According to the analysis for each operation modes, maximum temperature difference between top and bottom of piping were evaluated about 60K when the flow rate of main and bypass lines is same. Temperature difference will be decreased at the elbow on RHRS piping if the length of vertical piping is increased.

• 대한기계학회논문집
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• 제19권8호
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• pp.2064-2070
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• 1995

Characteristics of thermal flow stratification were studied experimentally by using the small scale pressurizer-surge line model. Thermal flow stratifications in the horizontal section of surge line were analyzed by the relation between the maximum temperature difference at any cross section in surge line and the Froude number representing the boundary conditions, i.e., in/out surge flow velocity and temperature difference of system. Thermal flow stratifications in outsurge flow decreased inversely proportional to the Froude number and did not exist for Fr>1. In insurge flow thermal flow stratifications disappeared near Fr=1.5, but resulted in the higher temperature difference than the case of outsurge flow.

• 설비공학논문집
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• 제15권1호
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• pp.32-39
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• 2003

Thermal comfort in the Rotunda which is high wide visiting space of the new national museum of Korea has been numerically investigated in this paper. To evaluate thor-mal comfort of the Rotunda, well-known indices, PMV and PPD were introduced. The results of present investigation show that thermal comfort is satisfied at the breathing zone of the visiting space. However a thermal stratification with $9^{\circ}C$ of temperature difference occurs along the height of the Rotunda which makes the thermal environment worse. For example, the PPD value reaches up to 50% in the 6th floor connection passage. Consequently, additional HVAC design factors should be considered in order to reduce the large thermal stratification.

• 설비공학논문집
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• 제26권1호
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• pp.8-14
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• 2014

We have performed experiments to enhance the stratification in a storage tank in order to raise the collector efficiency and solar fraction in solar thermal systems. The storage tank with a spiral jacket in the side wall has a scroll-shaped heat exchanger coil added to the upper part. The performance was compared between only the side and upper-side heating part through simulation using TRNSYS under the same weather conditions and initial conditions. As a result, the upper-side heating has a 4.2% advantage in solar fraction, but almost no increase in collector efficiency.