The thermal stratification plays an important role in the aging of the piping because of the excessive stresses caused by the temperature differences in the cross section of the pipe. These stresses can limit the lifetime of the piping, causing deformation, support failure, thermal fatigue, cracking, etc. Several nuclear power plants have experienced such mechanical damages. This phenomenon has been addressed as one of the significant safety issues in nuclear power plant operation. Research of the thermal stratification phenomenon by both Computational Fluid Dynamics (CFD) method and experimental method are performed in this paper. It is observed that in the thermal stratification region, the cold and hot fluid in the pipe flow conversely in the numerical analysis. This phenomenon is caused by the flow characteristic that the surge flow pushes the fluid of the whole cross section downstream, causing pressure drops at the upper or bottom section of the surge flow. The fluid downstream flows backwards because of the pressure difference. In the experiment the converse flow when thermal stratification happens in the pipe is confirmed, which causes a sudden change of the tendency of transient temperature evolutions and the fluctuation of the temperature. Further experimental analysis and validation are needed as this phenomenon enlarges the temperature difference of the pipelines.

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