소듐-공기 열교환기 굴뚝 설계를 위한 자연대류 열전달에 관한 연구

Abstract

The SFR(Sodium-cooled Fast Reactor) has attracted increasing attention recently as it provides means for efficient utilization of uranium resources and reduction of radioactive wastes. In order to meet one of the Gen Ⅳ SFR technology goals of dramatic improvement of safety, a PDRC(Passive Decay Heat Removal Circuit) was adopted. Sodium-Air Heat Exchanger(AHX) is a part of the PDRC, which is one of the essential characteristic design concepts of the Gen Ⅳ SFR. The AHX is composed of helical tube banks in the chimney. The natural convective air flow driven by the heat transfer from the helical tube interacts with the chimney and leads to a complex phenomenological behavior. There were only a few experimental investigations on the natural convection heat transfer for chimney effect, and most of previous studies have been performed numerically. In this study, using analogy concept for heat transfer and mass transfer, the natural convective heat transfer phenomena in a chimney-system were simulated by mass transfer experiments. And the applicability of the experimental methodology was identified by numerical analysis using FLUENT. The present works are divided into four steps. Firstly, the velocity of flow in the chimney and Nusselt number were calculated theoretically using simplified balance equation approach. Secondly, to know whole phenomena, preliminary experiments were carried out. Through those two steps, it was found that the increase of the exit length caused by the chimney enhanced the heat transfer up to a certain chimney height(effective length). This effective length was determined by the balance of acceleration driven by buoyancy and the deceleration due to the friction between fluid and wall of the chimney. Thirdly, through the comparison between the mass transfer experiment and the numerical analysis, the validity and applicability for analogy experimental method was confirmed. The temperature and velocity profiles of the numerical simulations near the heated wall with and without the anodes in the middle of the chimney, was quite similar. Thus it was concluded that the mass transfer experiment with the anode in the middle of the chimney can predict the real phenomena. Finally, what happened to the effective length and the heat transfer in the chimney-system was demonstrated depending on extension and expansion ratios, heat input, and cross-sectional shape of the chimney. The heat transfer was enhanced with the increase in extension ratio up to a certain length (effective length). As the expansion ratio increases, the heat transfer rates decreases and the effective length increases. The effective length increases with the heat input. The effective length was same regardless of the cross-sectional shape of the chimney. This study has significance in a few senses. Firstly it is one of the rare studies using experimental means. Secondly, it provides theoretical background of extending the applicability of the analogy experimental method using the electroplating system. Thirdly, the experiments were performed for high Grashof numbers.