| Pressurizer surge line is a long curved pipeline which connects the pressurizer and the main loop of the reactor system,and plays an important role in maintaining the safe operation of the nuclear power plant.Because of the function and the structural feature of the surge line,the flow and heat transfer state of the fluid in the pipe is complex and varied,and thermal deformation and thermal fatigue can easily occur in the pipe structure.In this work,a coupled thermo-hydro-mechanical analysis method and a thermal fatigue assessment method are explored and proposed,and the study of the flow and heat transfer,the coupled thermal stress and strain,the fatigue damage rate and life of the pressurizer surge line are carried out with these methods.This research project derives fcrom the engineering practice,and has important academic value in coupled thermal-structural research.Firstly,a thermo-hydraulic similar model of the pressurizer surge line is determined.By comparing the results of the experiments and the numerical simulations,the validity and accuracy of the CFD method,the coupled CFD-FEM method and the fatigue assessment method are proved.Then,both the fluid flow and heat transfer in the surge line,such as the thermal stratification,the turbulent penetration,the thermal fluctuation et al,and the coupled thermal stress and strain of the surge line structure,such as the thermal deformation,the thermal stress fluctuation et al are studied deeply and detailedly.Ultimately,the relationship between the temperature fluctuation and the thermal fatigue damage rate is founded by data analysis,and the theoretical method which evaluates thermal fatigue by studying the temperature fluctuation is established.To sum up,the main work and the main conclusions are:(1)Establishing a coupled thermo-hydro-mechanical method and a thermal fatigue assessment method for solving the complicated thermal structural coupling problems in the pipeline of nuclear power plants.By developing the data batch processing program and thermal fatigue assessment program,the CFD simulation,the coupled CFD-FEM simulation and the fatigue damage evaluation are combined as a whole research method.(2)Studying the mechanism the influencing factors of the fluid flow and heat transfer phenomenon in the pressurizer surge line,and fitting the correlation function to connect the flow and heat transfer characteristics and the pipeline parameters,the working conditions et al.In this part,both the thermal stratification phenomenon and the turbulent penetration phenomenon under different temperature differences,different mass flow rate ratios,different pipe diameters and different pipeline arrangements are studied and analyzed deeply.The correlations of maximum normalized top-bottom temperature difference in the pipe section,the normalized length of the thermal stratification zone and the nonmalized length of the turbulent penetration zone are summarized and fitted by data analysis.Besides,the temperature fluctuation which induced by the turbulent penetration is studied emphatically.It can be found that the degree of temperature fluctuation is the biggest in the position where the normalized mean temperature equals to 0.5,and the maximum frequency of the temperature fluctuation is linearly positively correlated with the velocity of the fluid in the main pipe.(3)Determing the thermal deformation response mechanism of the surge line under different temperature load,different working condition and different pipeline arrangement,establishing criterions for evaluating the pipeline displacement and the cross-section deformations and seeking out the realationship between the pipeline arrangement and the flexibility performance of the pressurizer surge line.In this work,the space displacement is employed to reflect the pipeline displacement,and the diameter displacement is employed to show the deformation of the cross-section.By the coupled CFD-FEM simulation,it can be found that both the magnitude and the direction of the thermal deformation are determined by the degree and the distribution of the thermal stratification.By comparing the thermal deformation of the surge line under different working condition,it can be found the maximal thermal deformation under the In-Surge case is larger than that under the Out-Surge case,but the duration of thermal deformation is shorter.By comparing the thermal deformation of the surge line with different pipeline layout,it can be found that the sharp bend can enhance the piping flexibility of the surge line effectively,whereas the obtuse bend has no effect.(4)Founding the relationship between the temperature fluctuation,thermal stress fluctuation and the thermal fatigue damage rate,and establishing the theoretical method which evaluates thermal fatigue by studying the temperature fluctuation.In this part,the normalized mean temperature is found to be linearly positively related to mean stress,and the normalized RMS temperature is found to be linearly positively related to stress amplitude.By the fatigue damage assessment of the surge line,the fatigue damage rate is found to be an additive effect of the thermal fatigue damage rate and the structural fatigue damage rate.By analyzing and inducing the obtained results and conclusions,the surface functions of the austenitic stainless steel under different temperature difference which related to the normalized mean temperature,the normalized RMS temperature and the velocity of main pipe flow are fitted,and the thermal fatigue damage rate can be estimated directly by analyzing the temperature fluctuation characteristics.With the study of the pressurizer surge line in this paper,both the piping design and the state evaluation problems in the engineering practice can be responsed and solved.Besides,the research methods and conclusions in this study can provide a theoretical basis and a more accurate guidance for the integrity and safety design and assessment of the pipelines in nuclear power plant under the complex flow and heat transfer conditions. |
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