Development of a subgrid model with a fine moving grid overlying a coarse Eulerian mesh for modeling reflood heat transfer.

摘要

One of the major uncertainties in predicting the heat transfer during core reflood which follows a hypothetical Large Break LOCA is the modeling of the phenomena occurring at the quench front.; One of the major limitations of all the current best-estimate codes is the use of a relative coarse mesh for the solution of the complex fluid flow in the quench front region in a reactor core during reflood.; The work described in this thesis is the development of a subgrid model where a Fine Hydraulic Moving Grid overlies a coarse Eulerian mesh in the proximity of the quench front and froth region. The Fine Hydraulic Moving Grid (FHMG) software package was developed and implemented into the COBRA-TF computer code. The result is a new version of the code called COBRA-TF/FHMG.; The FHMG is based on one-dimensional, three-field description of the two-phase flow in proximity of the quench front. The model includes a two-dimensional heat conduction model for the rod as well as the heat transfer from the rod surface to the fluid. The model is based on a fully implicit scheme for both heat transfer and fluid flow.; The assessment of the COBRA-TF/FHMG was performed comparing the code results against FLECHT-SEASET reflood test data. The fine hydraulic mesh, which is used in proximity of the quench front, provides a better characterization of the quench front and froth region. The quench front velocity is well predicted. However the clad temperature and the vapor temperature appear to be under predicted by the code. Further analysis showed that the current entrainment and the transition boiling models are the limitation. These models were developed based on a relatively coarse mesh and currently do not fully benefit from the additional information provided by the FHMG that better describes the evolution of the flow and heat transfer in proximity of the quench front and froth region.; The new computational model offered by the FHMG provides the framework for the development of specific phenomenological models that better characterizes the phenomena at the quench front and froth region where current models has the largest uncertainty. (Abstract shortened by UMI.)