Three dimensional evaluation of two-phase flow in BWR fuel bundles based on compressible two fluid-one pressure and κ-ε turbulence models

Abstract

The three dimensional fluid dynamic code system was developed to simulate a comprehensive two-phase flow field in fuel bundles of boiling water reactors. The system is based on the compressible two fluid-one pressure (six equations) model and is designed to be applicable to both detailed fully three dimensional geometries and porous medium sub-channel type geometries. In the detailed modeling, the turbulence effect is considered by the additional four conservation equations of the κ-ε turbulence model and the convective terms are formulated by the modified skewed upwind scheme. The sub-channel type modeling, in contrast, is built from coarser meshes. The empirical void drift and turbulence mixing models are introduced to replace the κ-ε turbulence model and other additional constitutive models such as the local loss are added to facilitate efficient sub-channel-type calculations. General problems were investigated regarding numerical methodologies in discretizing a vapor-liquid two-phase flow field based on the two fluid-one pressure model. The performances of several pressure iteration schemes were compared in combination with the outer Newton-Raphson iteration loop. Among them, the MILUCR scheme is found to be most stable and efficient. It was also observed that the turbulent effect simulated by the κ-ε model enhances the local build-up of vapor. Two sample calculations, one numerical and the other experimental verification, were performed to prove the performance of the code system. In the first simulation, flow fields of two typical spacer designs were compared and it was indicated that the vapor acceleration and its winding motion can be changed notably by design modifications. In the second simulation, a two-phase flow field of the 4×4 heated bundle test section was evaluated by the detailed model and the resultant macroscopic loss coefficients were reflected in the subsequent sub-channel type model. The void distribution was compared with the measurement by the X-ray CT scanner and it was indicated that not only the axial pressure loss, but also the transverse pressure loss, has a significant effect on the planar void distribution in the wake of the spacers.