초록 ▼

< 1차년도: 금속용융물층의 Rayleigh-Benard 자연대류 >
- 열속집중(Focusing) 현상
- 금속용율물층 상부의 열전달 조건에 따른 현상변화
- 이상유동 모사방법론 개발1

< 2차년도 : 혼합물층의 자연대류(고 Rayleigh수) >
- 각도에 따른 원자로 하반구의 열속 (Angle-dependent heat flux)
- 혼합물층의 하부과 상부로 방출되는 열속의 비
- 이상유동 모사방법론 개발

< 3차년도 : 원자로 외벽냉각 >
- 원자로외벽의 열속조건

< 1차년도: 금속용융물층의 Rayleigh-Benard 자연대류 >
- 열속집중(Focusing) 현상
- 금속용율물층 상부의 열전달 조건에 따른 현상변화
- 이상유동 모사방법론 개발1

< 2차년도 : 혼합물층의 자연대류(고 Rayleigh수) >
- 각도에 따른 원자로 하반구의 열속 (Angle-dependent heat flux)
- 혼합물층의 하부과 상부로 방출되는 열속의 비
- 이상유동 모사방법론 개발

< 3차년도 : 원자로 외벽냉각 >
- 원자로외벽의 열속조건와 단열체 기하구조에 따르는 유동의 형성 (단상유동)
- 비등열전달을 고려한 원자로 외벽냉각시 기하구조의 영향

(출처 : 요약서 3p)

Abstract ▼

Ⅳ. Result of Project
○ Task 1 : Natural convection heat transfer on metallic layer
1) Natural convection heat transfer according to the height and the existence of side walls
Rayleigh-Benard natural convection experiments were carried out as the preliminary experiment to simulate the natura

Ⅳ. Result of Project
○ Task 1 : Natural convection heat transfer on metallic layer
1) Natural convection heat transfer according to the height and the existence of side walls
Rayleigh-Benard natural convection experiments were carried out as the preliminary experiment to simulate the natural convection of the core melt at the severe accident conditions. This work focused on the influences of plate separation distance(S), the existence of the side walls. The test results measured for a single horizontal plate were in good agreement with the correlation reported by McAdams and those for two horizontal plates showed the similar trend to the existing Rayleigh-Benard heat transfer correlations developed by Dropkin and Somerscales, Globe and Dropkin. The measured heat transfer rate decreased with the increasing separation distance between the two plates and became similar to those for a single horizontal plate. For all cases, the heat transfer rates measured for open side walls are higher than those for closed ones. It can be explained by the circulating flow which causes the inflow of the cold fluid at side while the hot plume was rising.

2) Natural convection heat transfer according to the cooling condition The aim of this study is to investigate the heat focusing depending on the height(H) and heat transfer characteristic of upper boundary for applications related to severe accident phenomena. The test results for the three configurations (only top cooling, only side wall cooling and both walls cooling) presented that the heat transfer on side wall cooling condition without top cooling is biggest. Also, the heat transfer was enhanced by decreasing the aspect ratio(H/R). The enhancement of top cooling in a severe accident condition reduces the heat focusing to the side walls. Reversely, when the top cooling is not enough heat is focused to the side wall about 4 times.

3) Natural convection heat transfer according to Pr
Numerical and experimental analyses on laminar natural convection in the enclosure were performed to investigate the influence of Prandtl number and height (H) on the multidirectional heat flow condition. The numerical analyses were carried out using FLUENT 6.3 for a wide range of Pr and H. Based on the analogy concept between heat and mass transfer, mass transfer experiments were performed using CuSO4-H2SO4 electroplating system. The results of this study showed the similar trend with the existing studies. For each cooling condition, the numerical results agreed with the experimental results within 5%. When comparing cooling conditions, the condition of top and side cooling showed the enhanced heat transfer by the increase of cooling area compared to the top cooling only.

For Pr<1, due to the thicker thermal plumes, the cell patterns were formed regularly and stably. But for Pr>1, the thinner plumes were formed and moved randomly due to the interaction between these plumes and the fluid, therefore the flow become chaotic except for the lowest height(H). Eventually, the side wall temperature condition was carefully controlled to form the relatively simple twodimensional flow in the three-dimensional enclosure, the complex three-dimensional flow was still formed by the characteristic of plumes and chaotic behavior of flow for high Pr .

4) Development of measurement methodology of cupric ion concentration The experimental study for the focusing effect was carried out previously.
When the electroplating system is adopted as the mass transfer system, in order to simulate the different temperature conditions of top and side walls, an electrical resistance was attached to the top wall so that it is mimics hotter wall condition. Because the quantitative temperature conditions according to the electrical resistance were unknown, the methodology development is necessary.

The methods of concentration measurement are RGB, Brightness, ICP, PIV, and Interferometry. The key of RGB, Brightness and PIV method is the clear images of the concentration boundary layer corresponding the thermal boundary layer of heat transfer. The results for ICP method can be got by taking the trace sample of a solution, accurately. The formation of patterns in the interferometry is very important. The characteristics of the interference pattern depend on the nature of the light source, the precise orientation of the mirrors, etc.

○ Task 2 : Natural convection heat transfer on oxide layer
1) Previous studies
In a severe accident, the molten fuels could be stratified into 2-layer (upper light metal layer and lower oxide layer) or 3-layer (upper light metal layer, middle oxide layer and lower heavy metal layer) by the density differences. Especially, the light metal layer of the 3-layer configuration, becomes thinner than that of the 2-layer system, resulting in the intensification of heat focusing to the reactor vessel.

BALI, SIGMA CP, COPOⅠ, COPOⅡ and SIMECO experiments simulated the oxide layer for 2-layer configuration using 2D facilities. ACOPO, UCLA, LIVE and SIGMA 3D experiments used 3D facilities. They used water, freon gas, air and simulant materials as working fluids. The heat sources were simulated using joule heating or microwave. The modified Ra'H ranged from 106 to 1015. The heat transfer correlations were developed for upward NuH and downward NuH. All tests indicated that the angular heat flux increased with the angle. SIMECO experiments simulated stratified three layers using three simulants. They measured the angular heat distribution and upper heat flux ratio, but they couldn’t explain the focusing effect.

2) Heat transfer in a 2-Layer configuration according to heat source shape We simulated the oxide layer for two-layer configuration using 2D and 3D facilities(MassTER-OP2 and MassTER-OP3). Three different types of volumetric heat sources were used and the results were compared. The downward heat transfer was not affected by the heat source type. However, the upward heat transfer increased when the heat sources were long and densely distributed. This result may be explained by flow obstruction due to the volumetric heat sources.

3) Heat transfer in a 2-Layer configuration according to modified RaH
We investigated IVR phenomena using 2D and 3D facilities (MassTER-OP2 and MassTER-OP3) for three different Ra'H. The measured mean NuH’s of the curved surface were 37 % less, and those of the top plate were 35% and 47% greater than existing studies, due to the high Pr of this study. The correlation for mean NuH were developed. For both the MassTER-OP2 and MassTER-OP3, the angular NuH’s of the curved surface increased with its angle. In the upper section, Ra'H had an influence on the 3D results, but not on the 2D results. A correlation was developed for MassTER-OP2 and we suggested a multiplier that allowed conversion from 2D to 3D results. The local NuH’s for the MassTER-OP3 in the top plate decreased steadily, while those for the MassTER-OP2 were almost consistent, slightly decreasing near the edge.

4) Heat transfer in a 3-Layer configuration according to aspect ratio
We investigated the natural convective heat transfer of oxide layer in the 3-layer configurations for three different aspect ratios: 0.28, 0.56 and 0.78.

With the decrease of the aspect ratio, the upward heat ratios increased due to the reduced side wall cooling. The upward heat ratios were higher for 3-layer configuration than for 2-layer configuration which means that any of 3-layer configuration is severer than 2-layer configuration in terms of intensification of heat focusing at the light metal layer. The local heat transfer at the side wall decreased when the angle decreased in all cases. At the top plate, the local heat transfer decreased towards the edge where the slope is steeper with the high aspect ratio. On the other hand, the results for aspect ratio of 0.28 peaked near the middle of the top plate, that could be explained by the formation of multi-cell flows in the flat geometry.

○ Task 3 : Estimation of critical heat flux during external reactor vessel cooling
1) Basic experiments for two-phase flow using mass transfer system
Basic experiments were conducted to evaluate the possibility of simulation of two-phase flow by using electroplating experiment based on the mass transfer method. As a basic experiment, we carried out two experiments for the inside vertical pipe and reduced APR1400 as 1/10 scaling. The boiling simulation experiments were deemed feasible as the phenomena observed that the hydrogen bubbles reduced from the solution as the current applied to the device were merged and leaved from the surface.

2) Development of experimental methodology to simulate two-phase flow using mass transfer system
Two-phase simulation methodology using mass transfer system was developed through the comparative analysis of boiling heat transfer phenomena and CHF models. Mass transfer system in sulfuric acid aqueous solution was adopted to simulate the CHF with non-heating method. The behavior of hydrogen bubbles generated due to the reduction as the current applied and showed similarity to that of the water. Resultingly, A CCD (Critical Current Density) similar to the CHF phenomenon was measured.

Then, hydrogen volume generation rate was calculated using measured CCD, and the equation for converting it into CHF, based on the calculation was proposed. Additionally, The factors were selected to compensate the difference between CHF of heat transfer and CHF, transformed from CCD through the analysis of CHF models. The analytical factors were correspond to the surface tension, buoyancy force, vapor density, working fluid temperature, vapor column area, receding contact angle, and heater porosity. The formular for calculating the heat transfer CHF is presented through the correction factor derived from the selected analytical factor.

3) Simulation of critical heat flux on pool boiling upward-facing disk CHF simulation experiments were conducted for the upward-facing disk plate in pool boiling condition. In this study, CHF simulation methodology using a sulfuric acid aqueous solution as mass transfer system was purposed and boiling phenomena was visualized using the high speed camera. Measured bubble departure diameter was 20 times smaller than water one. However, the bubble behavior near the CCD showed a similar tendency that vapor bubble behavior in the heat transfer experiment performed on the same sized heating plate. The CHF transformed from two-phase flow simulation method through the mass transfer system and correction factor was calculated from about 150 times smaller to 150 times later than the CHF in the heat transfer experiment depending on the correction factor.

4) Simulation of critical current density on pool boiling downward-facing plate subject to the orientation
Experimental study for the CHF according to the surface orientation was carried out. The width and length of downward-facing plate, simulated boiling surface were fixed and the orientation was varied. CHF according to the orientation was measured and the vapor behavior was visualized using the high speed camera. The measured CHF decreased as the plate become closer to the totally downward-facing, and decreased sharply after a certain orientation. This is because the generated bubbles are obstructed by the plate during the ascending by the buoyancy, and the degree of escaping is reduced, so that the bubbles form vapor film by merging with the surrounding bubbles easily. The measured CHF showed a similar tendency to the existing heat transfer correlations and visualized bubble behavior was also similar.

5) Simulation of critical heat flux on the channel boiling
가) Influence of the gap size
Experimental studied for the CHF was conducted to evaluate the influence of insulator when boiling occurs on the heated surface. The experiment was carried out by varying the gap size with respect to the downward plate of all orientations. Except the totally downward-facing plate, CHF increased as the gap size increased. However it showed a tendency to decrease over a certain gap size. This is because the CHF increased due to the increase of the flow velocity as the chimney effect with the size of the gap increases to a specific size, but the CHF decreased as the chimney effect impaired due to the decrease of the driving force as channel volume increased when the gap size increased.

나) Influence of the orientation
CHF showed a tendency to decrease as the orientation of the plate increased regardless of the gap size. This is because the escaping velocity of hydrogen bubble decreased due to increase of friction as surface orientation increased. As the boiling surface close to the fully downward-facing, the bubbles become more hard to rise so that the bubbles are trapped under the plate and formed vapor film easily. Thus the early CHF was occurred and measured.

다) Influence of the upstream and downstream
The effects of upstream and downstream were investigated through the experiment. For the experimental results with optimal gap size, the effect of upstream and downstream was simulated by the bubble behavior generated by the addition of electrodes at the front and rear of the measuring electrode.

In this regard, the hydrogen bubbles generated in the electrodes were controlled by adjusting the applied current. In the case of applying to the top and bottom, the CHF measured at the measuring electrode showed little difference within the error of 2%. The CHF increased when 50% of the current applied to the measuring electrode was applied. On the other hand, CHF did not increased when 80% of current was applied. This is because the supplementation of the constant flow increases the flow rate and improves the CHF, but the occurrence of a large amount of bubbles interferes with the behavior and hinders the improvement of the CHF.

(출처 : SUMMARY 13p)

목차 Contents

• 표지 ... 1
• 제 출 문 ... 2
• 보고서 요약서 ... 3
• 요 약 문 ... 4
• SUMMARY ... 12
• CONTENTS ... 21
• 목차 ... 24
• 제 1 장 연구개발과제의 개요 ... 27
• 제 1 절 연구 배경 및 필요성 ... 27
• 제 2 절 연구 내용 및 범위 ... 27
• 1. 금속용융물층에서의 자연대류 열전달 ... 28
• 2. 혼합물층에서의 자연대류 열전달 ... 31
• 3. 원자로 외벽냉각 시 임계열유속 모사 ... 34
• 제 2 장 국내외 기술개발 현황 ... 38
• 제 1 절 국내연구개발수준 ... 38
• 제 2 절 국내외 연구현황 ... 38
• 1. 지금까지의 연구개발 실적 ... 38
• 2. 현기술상태의 취약성 ... 39
• 제 3 장 연구개발수행 내용 및 결과 ... 41
• 제 1 절 금속용융물층에서의 자연대류 열전달 ... 41
• 1. 높이(H), 측면벽 유무에 따른 열전달 변화 ... 41
• 2. 냉각 조건에 따른 열전달 변화 ... 56
• 3. Pr 수에 따른 열전달 변화 ... 77
• 4. 구리이온 농도 측정법 개발 ... 85
• 제 2 절 혼합물층에서의 자연대류 열전달 ... 96
• 1. 이론적 배경 ... 96
• 2. 2층 구조에서 열원 형상에 따른 열전달 ... 127
• 3. 2층 구조에서 수정 RaH 수에 따른 열전달 ... 138
• 4. 3층 구조에서 높이비 변화에 따른 열전달 ... 146
• 제 3 절 원자로 외벽냉각 시 임계열유속 평가 ... 155
• 1. 물질전달계를 이용한 이상유동 기초 실험 ... 155
• 2. 물질전달계를 이용한 이상유동 모사방법론 개발 ... 158
• 3. 풀비등 상향 원판에서의 임계열유속 모사 ... 183
• 4. 풀비등 하향 평판의 기울기에 따른 임계열유속(CHF) 모사 ... 197
• 5. 채널비등에서의 임계열유속 모사 ... 206
• 제 4 장 목표달성도 및 관련분야에의 기여도 ... 236
• 제 1 절 연구목표달성도 ... 236
• 제 2 절 관련분야에의 기여도 ... 236
• 제 5 장 연구개발결과의 활용계획 ... 237
• 제 6 장 연구개발과정에서 수집한 해외과학기술정보 ... 238
• 제 7 장 연구개발성과의 보안등급 ... 239
• 제 8 장 연구장비의 구축 및 활용 결과 ... 240
• 제 9 장 연구개발과제 수행에 따른 연구실 등의 안전 조치 이행 실적 ... 241
• 제 10 장 참고문헌 ... 243
• 끝페이지 ... 255