Numerical modeling of an in-vessel flow limiter using an immersed boundary approach

Abstract

This work is in the context of the mitigation of the consequences of a largebreak loss of coolant accident in a Pressurized Water Reactor. To minimize the flow leaving the vessel and prevent or delay the uncovering of the core, CEA has devised a device, named in-vessel flow limiter, limiting the flow of fluid from the vessel to the break. The goal is to interfere as little as possible with the nominal operation flow and maximize the fluid retained in the event of this kind of accident. In order to quickly perform a series of 3D-CFD simulations to optimize this device, it is imperative to have a simulation tool that provides sufficiently accurate results in a reasonable time. For this goal, an immersed boundary condition approach is retained. The solid obstacles constituted by the fins of the device are not extruded from the fluid domain, but included in the calculation domain itself. Their presence is considered by a first-order in space local Direct Forcing term using a penalty approach. Through 3D/1D up-scaling of CFD global quantities, local pressure-drop coefficients, induced by the in-vessel flow limiter, can be provided to Thermal-Hydraulic system safety codes. It allows safety studies of the thermal-hydraulic system taking into account the in-vessel flow limiter presence in a more realistic way.

A Finite Element Penalized Direct Forcing Immersed Boundary Method for infinitely thin obstacles in a dilatable flow

“…The obtained results for the head loss coefficients are gathered in Table 6. The values justify the preliminary study results [8] which use an approximated soft penalty added source term which is only present in the prediction equation. Moreover, those results show once again that the interpolation technique tends to enhance the spatial convergence (cf.…”

“…The outflow pressure is fixed at an ad hoc value of 50 bar. A Schlichting's scalar turbulence model is also used with a characteristic length of turbulence 𝐿 𝑇 = 0.3 m. For a more detailed description of the case, the interested reader can refer to [8].…”

“…postpone the core dewatering). For more information about the background, the interested reader can refer to [7,8].…”

“…Keeping in mind the statistical optimization goal, a Homogeneous Equilibrium Model (HEM) with disequilibrium closure laws [9] should be considered to preserve low computational times while taking into account two-phase aspects. However, for the sake of simplicity of the presentation, only one-phase dilatable Navier-Stokes equations are considered in this paper, (but two-phase flow illustration can be found in [8]).…”

A Finite Elements Penalized Direct Forcing Method to Take Into Account Infinitely Thin Immersed Boundaries in a Dilatable Flow

“…Thus, in order to carry out shape optimization and safety studies, a fast and robust numerical tool, able to faithfully model the new passive safety systems, is needed. For more information about the background, the interested reader can refer to [1]. The aimed applications involve a subsonic compressible twophase flow in non-equilibrium state.…”