Nowadays the availability of sophisticated Computational Fluid Dynamics (CFD) codes and of cheaper powerful computers gives the possibility to investigate with high degree of detail the several phenomena, for example mixing, occurring at small scale in a NPP or experimental facility. In the nuclear field the use of such codes has been for instance devoted to the study of the PTS (Pressurized Thermal Shock) phenomena occurring during the injection of the emergency systems, the determination of the boron mixing and distribution inside the cold leg and vessel, the distribution of hydrogen in the containment in the case of a severe accident. An interesting application of the CFD codes is constituted by the characterization of the pressure drop coefficients along the circuit in simple and complex configurations, changes of flow area and abrupt modifications of the geometry. The present work is focused in the analysis of the pressure drop across the junction between the cold leg and the vessel downcomer of a nuclear reactor by means of ANSYS-CFX-10 CFD code. The objective of the work is to show which are the parameters affecting the pressure drop in this simple configuration geometry. The mesh geometry has been developed based in the size of the reference LOBI integral test facility. The main objective is to investigate the pressure drop depending on the different tube fitting radius: perpendicular shape (90° angle, radius 0mm) or with a radial shape (radius of 3mm, 6mm, 9mm, 12 mm) and the velocity of the inlet flow. A series of sensitivity analyses have also been performed varying models of CFD solution and other boundary conditions. Both forward (Kfwd) and reverse (Krev) flow energy loss coefficient (used in system thermalhydraulic codes such as RELAP5) are later on calculated from the CFD results, since they are directly dependent on the inlet and outlet pressure drop in the cold leg-vessel downcomer junction.
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