3-DIMENSIONAL EVALUATION OF 2-PHASE FLOW IN BWR FUEL BUNDLES BASED ONCOMPRESSIBLE 2 FLUID-ONE PRESSURE AND KAPPA-EPSILON TURBULENCE MODELS

The three dimensional fluid dynamic code system was developed to simul ate a comprehensive two-phase flow field in fuel bundles of boiling wa ter reactors. The system is based on the compressible two fluid-one pr essure (six equations) model and is designed to be applicable to both detailed fully three dimensional geometries and porous medium sub-chan nel type geometries. In the detailed modeling, the turbulence effect i s considered by the additional four conservation equations of the k-ep silon turbulence model and the convective terms are formulated by the modified skewed upwind scheme. The sub-channel type modeling, in contr ast, is built from coarser meshes. The empirical void drift and turbul ence mixing models are introduced to replace the k-epsilon turbulence model and other additional constitutive models such as the local loss are added to facilitate efficient sub-channel-type calculations. Gener al problems were investigated regarding numerical methodologies in dis cretizing a vapor-liquid two-phase flow field based on the two fluid-o ne pressure model. The performances of several pressure iteration sche mes were compared in combination with the outer Newton-Raphson iterati on 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 k-epsilon model enhances the local build-up of vapor. Two sample calculations, one numerical and the other experimental verification, w ere performed to prove the performance of the code system. In the firs t simulation, flow fields of two typical spacer designs were compared and it was indicated that the vapor acceleration and its winding motio n can be changed notably by design modifications. In the second simula tion, a two-phase flow field of the 4x4 heated bundle test section was evaluated by the detailed model and the resultant macroscopic loss co efficients were reflected in the subsequent sub-channel type model. Th e void distribution was compared with the measurement by the X-ray CT scanner and it was indicated that not only the axial pressure loss, bu t also the transverse pressure loss, has a significant effect on the p lanar void distribution in the wake of the spacers. (C) 1997 Elsevier Science Ltd.