atw - International Journal for Nuclear Power | 05.2020

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atw Vol. 65 (2020) | Issue 5 ı May RESEARCH AND INNOVATION 272 Physical properties Inner diameter surge-line Outer diameter surge-line Length of surge line center arc Length of main pipe section Outer diameter main-pipe Inner diameter main-pipe Inner diameter surge-line Outer diameter surge-line Fluid Structure Interaction Analysis of a Surge-line Using Coupled CFD-FEM Muhammad Abdus Samad, Xiang bin li and Hong lei Ai The mixing with different-temperature water in the pressurizer surge line may result in thermal stratification, then the significant deformation of the solid structure due to different thermal expansion at different parts of the structure perhaps occur, which will be a threat for the plant safety. To better understand the coupling mechanism, the corresponding characteristics in a pressurizer surge line is analyzed using CFD software (ANSYS CFX) and FEM solver, (ANSYS MECHANICAL). The fluid temperature distribution is calculated first, then the corresponding thermal and mechanical characteristics are analyzed. It is found that a large steady state stress present at the edges of the main pipe and the pressurizer, the consequent deformation showed large displacement at the center of the surge line. Introduction The surge line is the pipe that connects the pressurizer with the hot leg of the primary loop. As the controlling of the pressure takes place in the pressurizer, surge line acts as the in between of main pipe and the pressurizer. As a consequence of pressure control the surge line experiences thermal stresses along its life time. These stresses are often cyclic in nature given the nature of the load that the power plant experiences, such cyclic stress over time cause material fatigue and in worst cases can cause significant damage and are therefore, a big factor for plant safety design. The deformation of these line can cause rupture and the subsequent leakage may have undesirable effects on plant working. There have been many reports on the damage of piping in PWR plants due to thermal stresses. In the US 284.20 mm 360 mm 19.187 m 4.23 m 870 mm 736 mm 284.20 mm 60 mm there have been reports from Trojan plants regarding unusually large piping displacements due to thermal stratification, which resulted in crushed insulations, decreased gaps among rupture restraints and heavier pipe support loads, Beaver valley 2 also had unexpected pipe displacements which caused the snubbers to stroke out. In Slovakia the surge line elbow at Bouhnice 3 had to be replaced because the calculated cumulative fatigue usage factor was high [1]. Piping in PWR plants have been undergoing unwanted thermal stress for quite a long time, the reports of unwanted movement in pipes as a result of inadequate calculations of | Fig. 1. Mesh of surge line structure. material and fluid interaction have been available in the literature from as early as 1995 when PWR plants in France reported experiencing thermal stratification due to the geometry which were un-accounted for in the design calculations. This stratification continued in steady state and the stresses were calculated by 1d-2d method developed by FRAMOTOME [2]. The same year a German PWR presented its own study on the existence of stratification in PWR reactors especially in the horizontal regions, in his paper they used ADINA code to calculate the stress in the surge line [3]. The Atomic Regulatory Board of India worked on developing an Analytical model for induced stress using intermixing layer they validated their model by testing it on a surge line [4]. In recent literature Korean Institute of Nuclear Safety worked on these stresses present in Surge lines in detail and performed several analysis to calculate the thermal stress in in-surge out-surge cases using commercially available ANSYS codes [5]. Also similar techniques were used at Beijing university of Chemical Engineering, Harbin University of Engineering, Xian Jiao tang University to evaluate thermal stresses and the consequent effects on the surge line [6–8]. | Tab. 1. Physical properties of surge line. Mesh Id Maximum Element Size Mesh cells Average Mesh quality 1 0.003 9011907 0.85209 2 0.004 3966425 0.85091 3 0.006 1371084 0.84793 4 0.008 739633 0.84512 | Tab. 2. Sensitivity Analysis. | Fig. 2. Mesh of fluid and solid structure. Research and Innovation Fluid Structure Interaction Analysis of a Surge-line Using Coupled CFD-FEM ı Muhammad Abdus Samad, Xiang bin li and Hong lei Ai
atw Vol. 65 (2020) | Issue 5 ı May Although much work has been done on the different cases of transient there is severe lack of work on steady state of thermal stratification present in the surge line as first observe in France. In this paper conjugate heat transfer analysis is performed on a surgeline PWR in ANSYS CFX, then the steady state temperature profile is then transferred to ANSYS Mechanical to calculate the stress acting on the surge line in steady state. Model Physical Model As shown in the Figure 1, the concerned structure is a pipe of diameter 360 mm connected to main pipe with a diameter 870 mm perpendicularly. The material of the pipe is stainless steel and it has physical parameters as defined in Table 1. For this simulation the working fluid is water. The water in surge line comes from the pressurizer where the temperature is around 270 °Celsius. While cold water at 120 °C flows through the main pipe at an average wave velocity of 15.6 m/s. The flow in the surge line is taken as 0.1 m/s towards the main pipe. In this study the steady state analysis is done on the surge line so the initial condition of the pipe are taken as the temperature of the main pipe. The simulation was tested in increasingly refined mesh to test the independence of the mesh. The results were then compared in ANSYS CFD. The sensitivity analysis as well as all the other data in this paper is plotted along the length of four lines on the surface of the structure, these lines run parallel to the axis of the surge line and are labeled by the angle at which they end near the pressurizer end of the surge line as seen in Figure 4. These lines start from the part of sure line near the main pipe, the data is plotted along the length of the line and at the end point the face of pipe is considered for their names. | Fig. 3. Position of lines with respect to pipe. RESEARCH AND INNOVATION 273 Meshing and Sensitivity Analysis The accuracy of the results in any discrete simulation depends significantly on the mesh. The solution space should be defined in such a manner that the simulation could be com pleted accurately and with low amount of utilized resources. For evaluating the temperature profile of the structure under discussion our region of interest was the connection connecting portion between the two the pipes. So a separate body was assigned and the mesh in that body was refined step by step until desired quality of results were achieved. The details of the mesh are provided in the table the mesh was made using ICEM, the connection of interest was sized using the body sizing function to achieve the max element size as shown in table. The fluid inside of the structure was meshed separately as it is required in conjugate heat transfer for the solid and fluid domains to be defined separately | Fig. 4. Sensitivity Analysis of mesh. | Fig. 5. Temperature of Surge line fluid and structure. Research and Innovation Fluid Structure Interaction Analysis of a Surge-line Using Coupled CFD-FEM ı Muhammad Abdus Samad, Xiang bin li and Hong lei Ai
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atw - International Journal for Nuclear Power | 05.2020

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