| Pipeline system is one of the most important components in facilities of oil,chemistry,power industries.The high-frequency temperature changes of the fluid temperature in the pipeline can initiate material damage due to the mechanism of thermal fatigue,which is a potential risk to the operation of pipeline systems.Since the through-wall crack incident in the pipeline of the French nuclear power plant Civaux,many research investigations have been launched to reveal the mechanism causing thermal fatigue in thermal-mixing processes in pipelines.The latest work has found a new flow phenomenon in the pipeline,namely tangential oscillation of thermal stratification,which is able to create a periodical temperature to the piping material and lead to material damage due to thermal fatigue.In this study,experimental and numerical work will be performed to investigate the initiation of the tangential oscillation of thermal stratification and evaluate the potential for thermal fatigue due to this phenomenon.The major contents of this work are as follow.Firstly,an experiment facility was designed and constructed based on the similarity principle of the non-dimensional parameters.The thermal stratification was created by using the heating devices only at the upper part of the pipeline.A 90° pipe-elbow was installed in the pipeline,in which the tangential oscillation is initiated.Temperature in the near-wall fluid was captured with high-performance thermocouples at different position close the elbow.Temperature data was analyzed with their relevance of thermal fatigue.The results of frequency analysis confirm the initiation of the tangential oscillation.With this method,the tangential oscillation of thermal stratification was as the first time identified in an experiment.Secondly,numerical simulation was performed based on the thermal stratification experiment at the pipe-elbow.The simulation was validated with the experimental data.The temporal and spatial distributions of the temperature and velocity in the pipeline were generated based on the simulation data.The results show that the hot water moves to the intrados side of the elbow because of the centrifugal force,while swings back the extrados due to the buoyance effect at the end of the elbow.In addition,the simulation results indicate that the heat transfer within the thermal stratification is enhanced at the elbow,which leads to a complete mixing of the thermal stratification at the end of the elbow.Moreover,the potential of thermal fatigue at different positions of the pipe-elbow was evaluated with the help of frequency analysis.Finally,the mechanism of tangential oscillation of thermal stratification in the pipeline was revealed in combination of experimental and numerical work,which provides a theoretical basis for weakening or avoiding the phenomenon in the future.Tangential oscillation of thermal stratification in combination of various experimental conditions was quantitatively discussed,the influence of thermal mixing process of elbow and downstream of elbow on thermal fatigue was analyzed.Suggestions were provided for transport of fluid in different temperature and optimization of the structure of pipeline in thermal stratification. |
