헬리컬 튜브의 자연대류 열전달 특성에 관한 연구

Abstract

The needs for securing clean energy source and stable energy supply raise the research need for Sodium-cooled Fast Reactor(SFR) which can decrease radioactive waste and radioactivity toxicity. As the SFR uses fast neutrons for nuclear fissions and has the characteristics of converting fertile material to fissile material, the SFR has been developed from early nuclear development period for the efficient utilization of uranium resources. The SFR is targeting the goal of dramatical improvement of safety and technological verification and adopts PDRC(Passive Decay Heat Removal Circuit). Sodium-Air Heat Exchanger(AHX) is a part of PDRC, which is one of the essential characteristic design concepts of the Sodium-cooled Fast Reactor. The AHX is composed of helical tube banks inside a stack. The hot sodium flows down inside the helical tubes and cold air cools the outside of the helical tubes. The natural convective air flow driven by the heat transfer from the helical tube interacts with the stack and results in a complex phenomenological behavior. The heat removal capacity of the AHX are to be proven by the experimental means in order to get the Design Certificate from the regulatory body. The studies on natural convection heat transfer on helical tubes are limited. Furthermore, most of the available investigations are concerned with flow and heat transfer inside the helical tube. A study on natural convection heat transfer phenomena of a helical tube was performed as a part of work to verify heat removal capability of th SFR. The study is aiming at the determination of governing parameters for the helical tube phenomena, which will be later utilized for the developed of experimental facilities. In this study, natural convection heat transfer phenomena of helical tube were investigated experimentally. With the analogy concept, heat transfer systems were replaced by mass transfer systems. the copper sulfate electroplating system was employed as the mass transfer system. A series of preliminary tests were performed. Natural convection experiments on horizontal cylinder were performed and mass transfer rates were measured as well as the heat transfer pattern was visualized by electroplating technique. Natural convection heat transfer tests on inclined cylinders reveal that when the inclination angle is less than 30° from the horizontal, the measured heat transfer rates were similar to that of the horizontal cylinder. Natural convection experiments of vertical in-line double horizontal cylinders showed that bottom cylinder is not influenced by top cylinder but top cylinder is influenced with pitch-to-diameter ratio between top and bottom cylinders. Main tests for helical tube were performed. The diameters of helical tube were varied from 0.001m to 0.008m, the total lengths of helical tube were varied from 0.157m to 1.571m, the heights of helical tube were varied from 0.033m to 0.289m, and the pitchs of helical tube were varied from 0.004m to 0.1m, which correspond to the Rayleigh numbers by diameter of 5.5×105 to 9.4×108, Rayleigh numbers by total length of 6.54×1011 to 6.54×1014, and Rayleigh numbers by height of 6.09×109 to 3.62×1012. The study suggested that the heat transfer corelations by diameter, total length, and height of helical tube from experimental results. From natural convection heat transfer phenomena of helical tube, heat transfer rate was decreased with diameter of helical tube at laminar flow, but total length and height of helical tube did not influence the heat transfer rates. And the diameter of helical tube was effective characteristic length when pitch-to-diameter ratio is larger than 5 and pitch-to-radius is smaller than 2.3. In the AHX, the helical tube is located inside a stack. Due to the chimney effect inside the stack, the total driving force will be affected by the length or height of the tube. Therefore, the experiments on helical tube in duct should be performed to confirm the AHX phenomena study.