자연대류 조건에서의 수직튜브 외벽 응축열전달에 대한 실험 및 해석 연구

Abstract

In the nuclear power industry, the importance of heat transfer by steam condensation has been emphasized more since the Fukushima accident and the earthquake in the Gyeong-ju, recently. Especially, development research of Passive Containment Cooling System (PCCS) that maintains the integrity of containment buildings centering on power generation companies is actively underway. The PCCS consisted of a heat exchanger consisting of a vertical tube bundle and a cooling water tank. A passive containment cooling tank was installed outside of the containment and heat exchangers were installed inside of the containment. The heat source such as the steam generated from the containment is condensed on the heat exchanger to maintain the integrity of containment. Therefore, PCCS is a passive safety system to remove the energy released to containment by using condensation heat transfer in case of the accident such as Loss of Coolant Accident (LOCA) or Main Steam Line Break (MSLB) and to guarantee the structural integrity of the containment. In this study, to evaluate the heat removal performance of PCCS, the condensation heat transfer coefficient according to the change of outer diameter and heat transfer area of the condenser tube was studied. From the results of the experiment, a new empirical correlation that could reflect the geometric change of the condensation tube was proposed and verification process was performed. The proposed correlation is applied to MARS-KS and compared with the Colburn-Hougen model. Finally, to evaluate the heat removal performance of the PCCS installed in the containment, the containment, and the PCCS were simulated and analyzed by using MARS-KS with the proposed correlation. There are three main points conducting the experiments. The first is the identification of the steam-air stratification inside of the test section. For this purpose, the temperature of the gas mixture is measured in the axial and radial directions, and it is confirmed that the distribution according to the position is uniform. The second was the control of the condenser tube wall temperature. By circulating the coolant in the tube at high speed, the axial variation of the wall temperature was kept as small as possible. It makes the uniform temperature over the effective heat transfer length, and to minimize measurement errors that results from significant axial variation of wall temperatures. Third, we tried to control the wall subcooling. In the previous experiments, wall subcooling was recognized as a main variable for condensation heat transfer. However, since the influence of this parameter was not well understood, the wall temperature was controlled by using coolant. Based on the results, the trend of the condensation heat transfer is same as those of previous work. The condensation heat transfer coefficient increased with increasing pressure and decreased with increasing air mass fraction. The effect of wall subcooling showed that condensation heat transfer coefficient tended to decrease as wall subcooling increased. The variation of the condensation heat transfer coefficient using the condenser tube with different outer diameter in similar conditions was also confirmed, but the variation of the condensation heat transfer coefficient was independent of air mass fraction. Therefore, further experiments and analyses will be necessary for this part by using another condenser tube. Based on the experimental results, a new empirical correlation that could reflect the effects of pressure, air mass fraction, wall subcooling and outer diameter was derived and verification was performed. In the case of the finned tube, it was confirmed that the condensation heat transfer increases with the increase of the effective heat transfer area by the fin. It was found that this was due to the geometrical effect of the annular pin. The applicability of the PCCS to the annular finned tube was also evaluated. To verify the model of Colburn-Hougen, the condensation experiment facility in Jeju National University(JNU) was simulated by using MARS-KS with the proposed correlation. The JNU nodalization was designed to reflect the natural convection flow which may occur in the test chamber. In the comparison with the experimental results, the analytical results using the proposed correlations are well predicted compared with the existing results. From the results, to evaluate the realistic heat removal performance of PCCS and thermal-hydraulic analysis, the actual containment with the PCCS were simulated and analyzed by using MARS-KS with proposed correlation. The condensation experiment database and the safety analysis of the containment with the PCCS from this study are very useful value. Also, the proposed correlations in this study are expected to be applied positively in various fields because it reflects the various condition of steam-air mixture and the various outer diameter of condenser tube. The results of the analysis of the containment could also be applied to the present PCCS research and development.