3-16 Sensitivity of mass flow in natural circulation after each increasing power (FLiBe) property uncertainty. 4-15 Comparison of radial heater exit velocity profile in DOWTHERM RP with natural circulation.
Introduction
Background
Thus, the thermophysical properties and geometry of the natural circulation system that give significant effect to the force are important, especially for the heat transfer capability of the natural circulation. Considering the difference in thermophysical properties of molten fluorine salts from other coolants, natural circulation in molten salt systems is assumed to have a difference in heat transfer capability due to different Pr ranges.
Literature Review
- High Prandtl number (Pr) Molten Salt
- Molten Salts Application in Nuclear Reactors
- Molten Salts Natural Circulation in Passive Safety System
- Similarity Technique for Molten Salt Heat Transfer
This gives the important meaning of the study on the natural circulation heat transfer of the molten salts. However, the significance of the passive safety with the natural circulation can be maximized in the case of MSR.
Theoretical Similarity of Molten Salt Natural Circulation with High-Pr Heat Transfer Oil - 13
Application of Similarity to Molten Salt and Natural Circulation System
- Selection of Molten Salt and Natural Circulation System
- Selection of Simulant Fluid and Experimental Apparatus
According to the choice of the target molten salt, FLiBe, and its natural circulation system, simulant fluid and the experimental facilities were considered. Heating section is located on the lower part of the left side in the loops.
Results and Discussion
For the similarity of the natural circulation, matching of Gr was applied to the decoupled equations (6) and (7)21. The inner diameter of the primary loop was used as the characteristic fluid length scale, L.
Performance for Heat Transfer Capability of High-Pr Natural Circulation
Introduction
DOWTHERM RP Natural Circulation Experiment
Modeling of DOWTHERM RP Natural Circulation in MARS Code
The pressure-dependent equation with the addition of the temperature term was used for main properties as seen in equation (16). In the case of vapor phase thermal expansion, exponential fitting was used for more accuracy. The results of the fitting and calculation were compared with the original experimental data as seen in figures 3-2 and 3-3.
Transport properties, viscosity (Pa-s), thermal conductivity (W/m-K) and surface tension (N/m) were implemented to generate the implementation file. Surface tension was used as a form of correlation, which was developed from three point values of the experimental data as shown in Equation (21). The geometry and dimension conditions of the natural circulation loops are the same as in the experiment, as explained in Section 3.2.
Heat source is supplied to the outer surface of the heat structure of the heater. Because the experimental condition of the natural circulation was assumed to be in the laminar flow region, the friction factor was calculated using the Hagen-Poiseuille correlation as seen in Equation (22)34. From the reference of the 'K' factor tables, each locally geometric pressure loss coefficient was calculated35.
Sensitivity Study of Natural Circulation using High-Pr Fluids
Results and Discussion
- DOWTHERM RP Natural Circulation Experiment
- MARS Simulation of DOWTHERM RP Natural Circulation
- Sensitivity Study of Natural Circulation using High-Pr Fluids
The calculated values and the dimensionless numbers were used in the experimental comparison of the heat transfer coefficient (33). However, the results of the MARS simulation show the higher mass flow rate and lower temperature distribution compared to the experimental data. On the graphs it was identified that the trends of the increasing mass flow rate only changed according to the changes of K.
The first part evaluates the effect of the sensitivity of each four thermophysical properties on the mass flow rate, which is the main parameter in natural circulation. From the comparison of the change of the mass flow rate according to the measured uncertainty of each property, it was identified that viscosity and thermal conductivity had a relatively large effect. However, FLiBe molten salt showed clear variation of the mass flow rate regardless of the change of Pr and power.
From the various analyzes of the sensitivity study, it was found that the effect of the uncertainty of the properties affects the ability of natural circulation. To compare the effect of the friction factor on the mass flow rate, the change in the uncertainty tendency between them was also compared, as shown in Figure 3-21. From the results, it was confirmed that the value of Pr is not dominant for the uncertainty of the mass flow rate and the coefficient of friction.
The decreasing tendency of f and the increasing tendency of the mass flow rate are proportional. Comparison of friction coefficient between experimental data and MAR simulation of DOWTHERM RP in each section.
Performance for Flow Characteristics of High-Pr Natural Circulation
Introduction
In reality, the effect of Pr is taken into account when the complex natural circulation is theoretically approached for analysis. The boundary layer theory derived from the approximate governing equations proves the correlation between the boundary layers and Pr. The Pr of the exact or approximate value influences the ratio of the velocity and the thickness of the thermal boundary layer8.
This correlation can also be used as one of the approximations of natural convection in terms of flow characteristics due to boundary layer development. Therefore, observing the characteristics of the flow developed by convective boundary layers in the natural circulation is expected to be significantly related to Pr.
PIV Visualization of DOWTHERM RP Natural Circulation
CFD Simulation of High-Pr Natural Circulation
For the real-time observation of the temperature and velocity change, several monitoring points have also been introduced. The location was considered to be the center of the inlet and outlet of the heating and cooling sections. For the additional observation of the radial distribution, the local positions of the upper heating section with several points in a radial direction were also applied.
Regarding the material, the thermophysical properties of DOWTHERM RP are not included in the basic code. Thus, these values are entered based on the experimental data from the reference data sheet9. First is the basic investigation of the high-Pr natural circulation in the developing heating section.
We compared it with PIV visualization based on the assumption of flow characteristics. The second is the analysis of the flow characteristics in relation to the ratio with Pr at the wall. The third part is the influence of the value of thermophysical properties and the type of heat sources.
Results and Discussion
On the upper part of the heating section, the spiral rise was observed as shown in the enlarged figure in Figure 4-10. For the analysis of the velocity near the wall, the radial velocity evolution on the heating and cooling sections was observed. These results tell the importance of the CFD simulation because of the measurement limit in a real experiment.
To investigate the velocity development on the heating section, velocity with increasing height on the upper part of the heating section was compared. Based on the conditions, the modified concept of the temperature and velocity boundary layer thickness was calculated from the CFD simulation results. In this way, a simple comparison of the natural circulation between the local-external and local-internal heat sources was also done.
However, a clearer rate difference was observed in the case of the temperature-dependent property condition. On the other hand, the difference in the state of the heat source affected the formation of different temperature and velocity distributions. Grid convergence for the distribution of temperature (top) and velocity (bottom) on the cross-section of the upper vertical part of the heating section.
Velocity distribution on the horizontal cross section of the upper heating part of the natural circulation of DOWTHERM RP. vertical height increases with an interval of 0.1 m). Comparison of the heater exhaust velocity profile in the radial direction in the natural circulation of DOWTHERM RP.
Conclusions
저의 걱정과 불안에도 불구하고 친절한 조언과 가르침으로 석사과정을 잘 이끌어주셔서 정말 감사드립니다. 또한, 연구자로서 적극적인 사고와 행동, 그리고 함께 성장할 수 있도록 도와주신 덕분에 석사과정 2년을 후회 없이 성장할 수 있는 기회를 가질 수 있었습니다. 졸업장을 준비하는 과정에서 교수님들의 조언과 가르침 덕분에 졸업 주제에 대해 좀 더 다양하게 생각하고 발전시킬 수 있었습니다.
비록 짧은 기간이었지만 연구자로 활동하면서 개인적인 진로와 관심분야에 대해 많은 생각을 하고 방향을 정할 수 있었습니다. 그리고 가족들보다 더 많은 시간을 밤낮없이 함께하며 함께 일해준 선후배들, 연구실 동료들에게도 감사의 마음을 전하고 싶습니다. Dr. Dr.에게 감사드리고 싶습니다. 연구실 선배로서 열심히 일하며 얻은 경험과 지식을 아낌없이 가르쳐주신 라지 선생님과 제가 연구나 생활에 어려움이 있을 때 늘 걱정해주시고 도와주신 박성대님께.
연구실장이자 연구실의 최고령으로서 많은 연구실 업무를 담당하고 있지만, 후배들을 챙겨주시는 서한 선배님, 경모 선배님께 늘 감사한 마음을 갖고 있습니다. 저에게 가장 기본적인 것부터 많은 것을 가르쳐 주신 영신 장로님, 오랜 연구실 생활 덕분에 저에게 직업적으로나 개인적으로 많은 도움을 주시고 의지해 주신 허효 장로님, 민호 장로님께 감사의 말씀을 전하고 싶습니다. 또한, 학부 및 대학원 생활 동안 즐거운 학교생활을 할 수 있도록 도와주신 원자력학과 선배님들과 동기들에게도 감사의 말씀을 전하고 싶습니다.
