Thermally uninsulated dead-leg pipes connected to the main flow of a hot fluid exhibit complex thermohydraulic behavior. At the T-junction, a turbulent swirl may develop and penetrate into the pipe, while thermal stratification and natural convection develop in the externally cooled section. Such configurations are found in certain nuclear power plants, including the Krško Nuclear Power Plant (NEK), where a dead-leg pipe forms part of the safety injection system. Their transient, three-dimensional, and anisotropic flow makes numerical prediction challenging, with CFD results strongly dependent on turbulence models, boundary conditions, mesh resolution, and numerical methods used.
This master's thesis investigates flow and heat transfer in a dead-leg pipe of the NEK safety injection system using computational fluid dynamics (CFD). The objective was to investigate whether the main flow characteristics could be adequately predicted using a geometrically reduced model, in which the reactor vessel downcomer is not explicitly simulated and its influence is represented by an inlet boundary condition. This reduces the computational domain and, consequently, the computational cost of the simulation.
Among the investigated boundary conditions, prescribing the inlet pressure field proved most suitable, as it allows the flow to respond dynamically to the instantaneous state of the computational domain. The reduced model reproduced the main characteristics of the investigated flow regime. However, the results were highly sensitive to mesh density, particularly regarding turbulent swirl penetration depth and the thermal response of the pipe. The findings indicate that the reduced model is a promising approach for less computationally demanding analyses of the main thermohydraulic characteristics of dead-leg pipes, although significant numerical uncertainty remains.
|
