Study on flow characteristics of high-Pr heat transfer fluid near the wall in a rectangular natural circulation loop

https://doi.org/10.1016/j.ijheatmasstransfer.2018.01.064Get rights and content

Highlights

Abstract

Among the promising advanced nuclear reactors, molten salt reactor employs various types of passive heat transfer system using the single-phase natural circulation of molten salts. The unique feature of molten salt, which has high Prandtl number, gives distinct heat transfer characteristics compared to other candidate fluids. To understand the heat transfer characteristics of molten salts, the similarity technique with a new simulant fluid was introduced based on the match of Prandtl number, in the previous studies. Extended from the previous studies, the present study investigated the unique thermal-hydraulic characteristics of high Prandtl number fluid in the natural circulation through the rectangular loop. Especially, the distinct flow phenomena of high Prandtl number fluid near the wall around the heating section were analyzed through both experimental and numerical approaches. The experimental approach employed direct observation of flow pattern at the upper part of the heating section using particle image velocimetry (PIV) technique. The visualized velocity profiles and gradients gave the clear evidence of unique flow development. Furthermore, a computational fluid dynamics (CFD) simulation using the ANSYS-CFX commercial CFD code verified the unique flow pattern of high Prandtl number fluid. With the aid of theoretical development based on the boundary layer theory, the unique flow phenomena was attributed to the enhanced local natural convection induced by high Prandtl number.

Introduction

After the Fukushima accident in 2011, the concepts of inherent and passive safety became the more important factors for nuclear reactor development. Then various working fluids as a coolant have been introduced and studied for the improved nuclear reactor systems. Among them, molten salts, which are based on fluoride or chloride salts, are the most extensively studied because of their superior properties in terms of both inherent and passive safety, which can prevent expanded or secondary disasters following nuclear accidents. This research trend even refocused attention on the molten salt reactor (MSR). As an effort to apply molten salts in the nuclear industry, many studies on the heat transfer characteristics of molten salts have been performed [1], [2], [3]. In particular, the fluoride molten salts, which were introduced as the most promising molten salt coolant or fuel in the original MSR, are characterized by high-Prandtl (Pr) number compared to other candidate coolants, as shown in Table 1. The heat transfer of high Pr number fluid is dominated by the convective heat transfer, which is different from that of low Pr number fluid such as liquid metal. Therefore, the understanding of heat transfer characteristics of high Pr number fluid in natural circulation is required to retain the reliability of diverse passive heat removal systems of nuclear reactors. The previous study in UNIST presented the heat transfer capability and characteristics of high-Pr number fluid using simulant [5]. The similarity technique was used because it is difficult to experiment with molten salt, owing to its characteristics such as high operating temperature, high temperature corrosion, and toxicity. The simulant fluids based on the similarity technique could reproduce the thermal behavior and fluid dynamics of molten salts, at a reduced temperature, pressure, dimension, and power scale [6]. The targeted molten salt was FLiBe (2LiF-BeF2) in the study. The matched Pr number and Grashof (Gr) number with those of target molten salt, ensured the similarity in the heat transfer characteristics in natural circulation. Fig. 1 shows that both DOWTHERM A and DOWTHERM RP heat transfer oil can cover Pr number range of major molten salt coolants including FLiBe. The previous study in UNIST [5] employed DOWTHERM RP as a simulant fluid, because it can treat the upper range of Pr and also is non-toxic fluid compared to the other candidates. Using the simulant fluid, a set of natural circulation experiments through rectangular loop was conducted. As a result, a laminar heat transfer correlation for the natural circulation of high Pr number fluid was developed based on Nu-Ra relationship, which gave the feasibility of similarity technique to the natural circulation capability for molten salt applications, especially for the passive safety systems [5].

Extended from the previous studies, the present study revealed the unique flow patterns and flow characteristics of high Pr number fluid in the natural circulation. Especially, the distinct flow phenomena of high Prandtl number fluid near the wall around the heating section were analyzed through both experimental and numerical approaches. The experimental approach employed direct observation of flow pattern at the upper part of the heating section using particle image velocimetry (PIV) technique. Furthermore, a computational fluid dynamics (CFD) simulation using the ANSYS-CFX commercial CFD code verified the unique flow pattern of high Prandtl number fluid. With the aid of theoretical development based on the boundary layer theory, the unique flow phenomena were analyzed.

Section snippets

Flow characteristics of high-Pr natural circulation

Single-phase natural circulation is driven by the buoyancy force, which is induced by local temperature gradients and resulting density differences in working fluids. While various parameters give the effects on the natural circulation characteristics, thermal-hydraulic parameters including wall thermal conductivity, fluid viscosity, power and loop inclinations, geometry, or the orientation of the heating and cooling sections determine the intensity and stability of the natural circulation [10]

Natural circulation model with high-Pr heat transfer fluid

The natural circulation experiments with visualization technique were conducted using DOWTHERM RP to observe the flow characteristics of the high Pr number fluid. Particle image velocimetry (PIV) technique was employed for the visualization of velocity profiles and gradients. Fig. 3 and Table 3 show the experimental facilities of the rectangular natural circulation loop with their specifications. The experimental facility consists of three main parts: vertical heating section, heat exchangers,

Results and discussion

Before the analysis of the flow characteristics of high-Pr natural circulation, the velocity distributions in both the experiment data and the CFD simulation were compared to identify the reliable results. Fig. 11 and Table 6 show the velocity distribution at the upper part, where the visualizing section was located. From the comparison, the CFD results was validated.

Conclusions

In this study, the unique thermal-hydraulic characteristics of high Prandtl number fluid in the natural circulation through the rectangular loop were investigated. Especially, the distinct flow phenomena of high Prandtl number fluid near the wall around the heating section were analyzed through both experimental and numerical approaches.

In the experiments with PIV visualization, the zigzag up-flow was observed at the visualizing section located in the upper part of the heating section. Based on

Acknowledgements

This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174030201430), “Nuclear Energy Research Program” through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (2016M2A8A6900481), and “Basic Science Research Program” through the National Research Foundation of Korea

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