NUMERICAL SOLUTION OF HEAT TRANSFER PROCESS IN A PRISMATIC VHTR CONSIDERING CORE BYPASS AND CROSS FLOW

摘要

The existence of interstitial gaps between fuel blocks in a VHTR are inevitable due to tolerances in manufacturing and installation, thermal expansion and irradiation shrinkage. Both bypass flow and cross flow gaps exist in the core. The coolant mass flow rate distribution, temperature distribution and hot spot are significantly affected by bypass and cross flow. In the present study, three-dimensional CFD analysis is conducted for the thermal analysis in the reactor core. Validation study for the turbulence model are performed by comparing the friction coefficient with published correlations. A sensitive study for near wall mesh is conducted to ensure the mesh quality. Parametric study by changing the size of bypass gap and cross gap are performed with a one-twelfth sector of fuel block. Simulation results show the influence of bypass gap size on temperature distribution and coolant mass flow rate distribution in the prismatic core. It is shown that the maximum fuel and coolant channel outlet temperature increases with the increase of gap size which may lead to a risk on the structure of fuel block. The cross flow is divided to two kinds. One is the cross flow from bypass gap to coolant channels and another is the flow from high pressure coolant channels to low pressure coolant channels. These two kinds of flow have opposite influence on temperature gradient. It is found that the presence of the cross flow gaps may have a significant effect on the distribution of the coolant in the core due to flow mixing in the cross gaps.