One of the critical issues for reactor pressure vessel (RPV) structural integrity is related to the pressurized thermal shock (PTS) event. Therefore, within the framework of safety assessments special emphasis is given to the effect of PTS-loadings caused by the nonuniform azimuthal temperature distribution due to cold water plumes or stripes during emergency coolant injection. This paper describes the method used to predict the thermal mechanic boundary conditions (system pressure and wall temperature). Using a system code the pressure and global temperature distributions were calculated, systematically varying the leak size and the location of the coolant water injection. Spatial and temporal temperature distributions in the main circulation pipes and at the RPV wall were predicted by mixing analyses with a computational fluid dynamics (CFD) code. The model used for these calculations was validated by post-test calculations of a UPTF (upper plenum test facility) experiment simulating cold leg injection during a small break loss of coolant accident (LOCA). Comparison with measured temperatures showed that the modeling used is suitable to obtain enveloping results. Fracture mechanics analyses were carried out for circumferential flaw sizes in the weld joint near the core region and between the RPV shell and the flange, as well as for axial flaws in the nozzle corner. Stress intensity factors KI were calculated numerically using the finite element program ansys and analytically on the basis of weight and polynomial influence functions using stresses obtained from elastic finite element analyses. Benchmark tests revealed good agreement between the results from numerical and analytical calculations. For all regions of the RPV investigated and the most severe transients it was demonstrated that a large safety margin against brittle crack initiation exists and brittle fracture of the RPV can be excluded.
One of the critical issues for Reactor Pressure Vessel (RPV) structural integrity is related to the Pressurized Thermal Shock (PTS) event. Therefore, within the framework of safety assessments special emphasis is given to the effect of PTS-loadings caused by the non-uniform azimuthal temperature distribution due to cold water plumes or stripes during emergency coolant injection. The paper describes the method used to predict the thermal mechanic boundary conditions (system pressure, wall temperature). Using a system code the pressure and global temperature distributions were calculated, systematically varying the leak size and the location of the coolant water injection. Local and temporal temperature distributions in the main circulation pipes and at the RPV wall were predicted by mixing analyses with a Computational Fluid Dynamics (CFD) code. The model used for these calculations was validated by post-test calculations of a UPTF (Upper Plenum Test Facility) experiment simulating cold leg injection during a small break Loss of Coolant Accident (LOCA). Comparison with measured temperatures showed that the modelling used is suitable to obtain bounding results. Fracture mechanics analyses were carried out for circumferential flaw sizes in the weld joint near the core region and between the RPV shell and the flange, as well as for axial flaws in the nozzle corner. Stress intensity factors KI were calculated numerically using the finite element program ANSYS and analytically on the basis of weight and polynomial influence functions using stresses obtained from elastic finite element analyses. Benchmark tests revealed good agreement between the results from numerical and analytical calculations. In order to determine the worst case loading conditions a wide spectrum of thermal-hydraulic transients was considered. Since the resulting load paths decrease with lower temperatures after a maximum, the warm prestress (WPS) effect was employed. The fracture toughness curve determined by deeply notched specimens with high constraint is not representative of the nozzle corner due to the considerable loss of constraint at LOCA conditions. Hence the influence of constraint on fracture toughness was accounted applying the constraint modified master curve concept and the relationship between the T-stress and the reference temperature T0. According to ASME Code Cases N-629 and N-631 the reference temperatures RTNDT and RTT0 can be used alternatively for the adjustment of the KIC-curve. Therefore both the RTNDT- and the RTT0-concept were considered. For all regions of the RPV investigated and the most severe transients it was demonstrated that a large safety margin against crack initiation exists and brittle fracture of the RPV can be excluded.
In many technical fields, e.g. heat exchangers, circular cylinders are involved in Fluid Structure Interaction (FSI) problems. Therefore correct frequency and magnitude of fluid forces, respectively Strouhal number, drag and lift coefficient are needed. If fluid forces are evaluated with Computational Fluid Dynamics (CFD), mostly flow around a rigid cylinder is used to verify model and numerical methods. Unfortunately experimental as well as numerical results show great variation, making verification and testing of models difficult. Reynolds number is regarded as main influencing parameter for a rigid cylinder in cross flow. Most of experimental deviations can be related to other parameters, which differ from experiment to experiment. In this paper such parameters are specified and it is shown, that a closer look is needed, if one really wants to verify a model. Besides experimental results, which can be found in literature, some parameters are investigated by numerical simulation. Like experiments CFD (Computational Fluid Dynamics) simulations show a huge bandwidth of results, even when the same turbulence model is used. Flow around cylinders separates over a wide range of Reynolds numbers. It will be demonstrated that, using CFD, large deviations in fluid forces can often be related to miscalculation of the point of separation.
The shell-side cross-flow in tubular heat exchangers may cause vibrations leading to failure within hours or in long term. Design is still based on half-empirical correlation, based on the equation of Connors (1978). Overdesign (Kassera 1996) and singular cases of damage (Fischer and Strohmeier, 2002) are the result. Therefore a structural model for the tube motions has been developed further and coupled to the commercial flow simulation code ANSYS CFX. The predictive capability of such coupled methods is limited by the flow simulation. Still simplifications or modeling are needed, especially for turbulence. The paper starts with an overview of modeling assumptions used so far. In addition to simulations of flow around rigid circular cylinders (Reichel and Strohmeier, 2008) LDA-measurements of flow through rigid glass-bundles have been compared to flow simulations. The sample of results presented below demonstrates that, besides level of fluctuations is predicted far too low, the overall velocity distribution on the shell side is predicted well by the SST turbulence model (Menter 1994), making URANS models like SST worth a try, if mainly flow forces are needed. To capture the tube dynamics an Euler-Bernoulli beam model of Fischer (2001), discretized by central differences in space and Newmark’s method (1959) in time, has been extended and implemented into ANSYS CFX. Calculations will be presented, showing that simulations of initially deflected tubes almost perfectly match analytic predictions. To adapt the numerical grid for the flow calculations to the tube displacements, the code inherent standard methods at large displacements resulted in negative volumes and solver failure. Therefore the standard methods have been replaced by own routines for grid deformation. Even for grids with fine near wall resolution, this method is able to cope with large displacements. Finally, coupled simulations are conducted of a single cylinder and of a cantilevered tube bundle in cross flow. For the single cylinder amplitudes are extremely overpredicted as long as 2D-modeling is used. 3D-modeling shows a phase shift of vortex shedding along the cylinder, which results in noticeably lower tube deflections. But, using the SST-model, amplitudes are still higher than measured. A model extension for laminar to turbulent transition leads to further improvement. The same holds for the tube bundle. Onset velocity of instability is predicted too low, amplitudes are too high. Modeling transition and large scale 3D-effects moves results closer to experimental observations. Further improvements are expected taking small scale 3D-effects into account by introducing more grid layers along the tubes.
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