• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.2
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• pp.141-149
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• 1993

Production of hydro-energy is random in its output amount due to the characteristics of the reservoir inflows. Therefore, it is necessary to provide the rationality in determining the amount of energy for a supply contract. This study presents a methodology for determining reasonably reliable amount of the energy supply considering the energy sale-incomes associated with the penalties which are subject to inflow-reliabilities. The objective function consists of the returns of energy sales and the risk-loss function to reflect statistically relevant risks. A range of the coefficient of the risk-loss function was figured out by its sensitivity analysis. The risk-loss herein means the penalty which should be paid by the energy supplier in case that the level of the energy supply is behind the contracted amount. And the reliability of reservoir inflow is defined by the exceedance probability of the inflow. The log-normal distribution was accepted as the probability density function of monthly inflows on the level of significance at 5%. Golden-ratio searching was applied to identify the optimal reliability and Incremental Dynamic Programming was used to maximize generation of the hydro-power energy in reservoir operation. The algorithm was the applied to the Daechung multi-purpose reservoir and hydro-power plant system in order to verify its usefulness.

• Plant Journal
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• v.15 no.1
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• pp.25-30
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• 2019

In this study, by varying flow patterns, which is one of the hydraulic factors of FAC, a strategy to reduce pipe wall thinning by mass transfer has been investigated. A similarity between heat transfer and mass transfer was verified via theoretical analysis, and local convective heat transfer coefficients were analyzed using a commercial numerical analysis program. When ribs were installed inside and outside of the internal surface in the straight section of the pipe, the maximum local heat transfer coefficient was shown to decrease substantially by up to 24.9% compared to the basic flow depending on the position and shape of ribs. If a guide vein was inserted in the pipe elbow, the maximum local heat transfer coefficient decreased by up to 26.7% compared to the basic flow depending on the internal surface area of the pipe by the guide vein.

• Journal of computational fluids engineering
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• v.19 no.2
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• pp.17-23
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• 2014

Cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency in devices, such as propellers, pump impellers, nozzles, injectors, torpedoes, etc. Thus, the cavitating flow simulation is of practical importance for many engineering systems. In the present work, a two-phase flow solver based on the homogeneous mixture model has been developed. The solver employs an implicit preconditioning, dual time stepping algorithm in curvilinear coordinates. The flow characteristics around Clark-Y hydrofoil were calculated and then validated by comparing with the experimental data. The lift and drag coefficients with changes of angle of attack and cavitation number were obtained. The results show that cavity length and lift, drag coefficient increase with increasing angle of attack.

• Nuclear Engineering and Technology
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• v.26 no.1
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• pp.19-31
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• 1994

The objective of this study is to conduct a thermal-hydraulic analysis on the spent fuel pool and to evaluate a parametric effect for the thermal-hydraulic analysis of spent fuel pool. The selected parameters are the Reynolds Number and the gap flow through the oater gap between fuel cell and fuel bundle. The simplified flow network for a path of fuel cells is used to analyze the natural circulation phenomenon. In the flow network analysis, the pressure drop for each assembly from the entrance of the fuel rack to the exit of the fuel assembly is balanced by the driving head due to the density difference between the pool fluid and the average fluid in each spent fuel assembly. The governing equations ore developed using this relation. But, since the parameters(flow rate, pressure loss coefficient, decay heat, density)are coupled each other, iteration method is used to obtain the solution. For the analysis of the YGN 3&4 spent fuel rack, 12 channels are considered and the inputs such as decay heat and pressure loss coefficient are determined conservatively. The results show the thermal-hydraulic characteristics(void fraction, density, boiling height)of the YGN 3&4 spent fuel rack. There occurs small amount of boiling in the cells. Fuel cladding temperature is lower than 343.3$^{\circ}C$. The evaluation of parametric effect indicates that flow resistances by geometric effect are very sensitive to Reynolds number in the transition region and the gap flow is negligible because of the larger flow resistance in the gap flow path than in the fuel bundle.

• Journal of the Korean Society of Propulsion Engineers
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• v.8 no.1
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• pp.54-60
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• 2004

Experimental study on combustion characteristics of double swirl coaxial injectors has been conducted for the assessment of critical injector design parameters. A reusable, subscale thrust chamber has been fabricated with a water-cooled copper nozzle. Two different configurations of injectors have been tested for the understanding of the effects of recess length on high pressure combustion. Clearly, the recess length drastically affects the combustion efficiency and hydraulic characteristics of an injector. Internal mixing of propellants in an injector with recess number of two increases a combustion efficiency and reveals sound combustion although a pressure drop required for the same amount of mass flow rates increases compared with an injector of recess number of one.

• Nuclear Engineering and Technology
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• v.27 no.6
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• pp.811-824
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• 1995

The CANDU element bowing is attributed to actions of both the thermally induced bending moments and the bending moment due to hydraulic drag and mechanical loads, where the bowing is defined as the lateral deflection of an element from the axial centerline. This paper consider only the thermally-induced bending moments which are generated both within the sheath and the fuel and sheath by an asymmetric temperature distribution with respect to the axis of an element The generalized and explicit analytical formula for the thermally-induced bending is presented in con-sideration of 1) bending of an empty tube treated by neglecting the fuel/sheath mechanical interaction and 2) fuel/sheath interaction due to the pellet and sheath temperature variations, where in each case the temperature asymmetries in sheath are modelled to be caused by the combined effects of (i) non-uniform coolant temperature due to imperfect coolant mixing, (ii) variable sheath/coolant heat transfer coefficient, (iii) asymmetric heat generation due to neutron flux gradients across an element and so as to inclusively cover the uniform temperature distributions within the fuel and sheath with respect to the axial centerline. As the results of the sensitivity calculations of the element bowing with the variations of the parameters in the formula, it is found that the element bowing is greatly affected relatively with the variations or changes of element length, sheath inside diameter, average coolant temperature and its variation factor, pellet/sheath mechanical interaction factor, neutron flux depression factor, pellet thermal expansion coefficient, pellet/sheath heat transfer coefficient in comparison with those of other parameters such as sheath thickness, film heat transfer coefficient, sheath thermal expansion coefficient and sheath and pellet thermal conductivities.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05b
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• pp.391-396
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• 1996

내부부수로, 벽면부수로, 모서리부수로를 포함하는 가압경수로형 원자로의 핵연료집합체를 모의하는 3$\times$3 봉다발을 모델로 수치해석을 통해 봉다발 주변의 유동특성을 알아보고 각 봉에서의 원주방향 위치에 따른 국소열전달 특성에 관해 고찰하였다. 봉다발에서 열전달계수의 분포는 벽면영향으로 인한 각 부수로에서의 유속분포와 밀접한 관계가 있으며 내부부수로에 인접한 봉에서 가장 높았고, 그 다음이 벽면부수로, 모서리부수로에 인접한 봉에서는 가장 작게 나타났다. 현재 핵연료의 열수력 설계시에 적용하고 있는 부수고 내의 모든 열수력학적 변수가 일정하다고 가정하는 부수로 해석방법은 봉다발내의 실제 열전달 현상과는 상당한 차이가 있음을 보여주었다.

• Korean Chemical Engineering Research
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• v.46 no.6
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• pp.1087-1094
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• 2008

This article presents a multi-scale simulation approach starting from the molecular level for the adsorption process development, specifically, in n-hexane adsorption on activated carbon. A grand canonical Monte-Carlo(GCMC) method is used for the prediction of adsorption isotherms of n-hexane on activated carbon at the molecular level. Geometric effects and hydrodynamic properties of the adsorption column are examined by means of the two dimensional CFD(computational fluid dynamics) simulation. The adsorption isotherms from the molecular simulation and the axial diffusivity from the CFD simulation are exploited for the process simulation where the elution curve of n-hexane is obtained. For the first moment(mean residence time) of the pulse-response with respect to temperature and flowrate, the process simulation results obtained from this three-steps multiscale simulation approach show a good agreement with experimental data within 20% of maximum difference. The multi-scale simulation approach addressed in this study will be useful to accelerate the adsorption process development, while reducing the number of experiments required.

• Nuclear Engineering and Technology
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• v.18 no.3
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• pp.200-208
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• 1986

The loss of coolant accident based on a double-ended cold leg break is analyzed with the discharge coefficient (Ca) of 0.4. This analysis covers the whole transient period from the start of depressurization to the complete refilling of the core by using RELAP4/MOD6-EM and RELAP4/ MOD6-HOT CHANNEL for the system thermal-hydraulics and the fuel performance during the blowdown phase respectively, and RELAP4/MOD6-FLOOD and TOODEE2 during the reflood phase. A simple analytical method has been developed to account for the lower plenum filling by approximating steam-water countercurrent flows and superheated wall effects at the downcomer during the refill period. Based on the informations. at the time of EOB (end-of-bypass), the refill duration time and the initial reflooding temperature were estimated and compared with the results from the RELAP4/MOD6, resulting in a good agreement. In addition, some parametric studies on the EOB were performed. The form loss coefficient between upper head and upper downcomer was found to be sensitive to the occurrence of the spurious EOB. Appropriate form loss coefficients should be taken into account to avoid the flow oscillations at the downcomer. The analyses with the six and three volume core nodalizations, respectively, show much similar trends in the system thermal-hydraulic performance, but the former case is recommended to obtain good results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.1
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• pp.97-107
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• 2015

An analytical model was developed to estimate the viscous and squeeze-film damping ratios of heat exchanger tubes subjected to a two-phase cross-flow. Damping information is required to analyze the flow-induced vibration problem for heat exchange tubes. In heat exchange tubes, the most important energy dissipation mechanisms are related to the dynamic interaction between structures such as the tube and support and the liquid. The present model was formulated considering the added mass coefficient, based on an approximate model by Sim (1997). An approximate analytical method was developed to estimate the hydrodynamic forces acting on an oscillating inner cylinder with a concentric annulus. The forces, including the damping force, were calculated using two models developed for relatively high and low oscillatory Reynolds numbers, respectively. The equivalent diameters for the tube bundles and tube support, and the penetration depth, are important parameters to calculate the viscous damping force acting on tube bundles and the squeeze-film damping forces on the tube support, respectively. To calculate the void fraction of a two-phase flow, a homogeneous model was used. To verify the present model, the analytical results were compared to the results given by existing theories. It was found that the present model was applicable to estimate the viscous damping ratio and squeeze-film damping ratio.