Numerical Assessment of Prandtl Number Effect on Transient Heat Flux Distribution Imposed on Nuclear Reactor Pressure Vessel by Application of PECM in a Volumetrically Heated Molten Pool

摘要

In the framework of this research, the principle focus is to analyze the effects of fluid Prandtl number (Pr) on natural convection heat transfer in a volumetrically heated molten pool. As a part of the work, numerical analysis is performed for hemispherical 3-D vessel slice to investigate the physics of the effect of Pr number on convective heat transfer characteristics in the melt pool. The investigation is based on ANSYS FLUET, where natural convection heat transfer effect is taken into consideration by Phase-change Effective Convectivity Model (PECM), which is implemented with FLUENT CFD as User Defined Function (UDF), programed by the user. The PECM is tested first by a benchmark test against CFD to gain confidence in its applicability as an analysis tool. Different simulant materials are used with their thermo-physical properties representing different Pr number as input for modelling for both single and double layer melt pool configuration. The selected modelling approach is validated against RASPLAV experimental result with respect to the inner temperature distribution that qualifies our model to run in the proceeding calculation. It is ensured that an isothermal boundary condition (343 K) is applied along vessel outer wall throughout the series of simulation cases. The corresponding Rayleigh number (Ra) ranges from 1014 - 1015 and Prandtl number (Pr) 3 - 5. It is found that the fluid Pr number has small effects on the averaged Nu numbers in the convection-dominated regions. The decrease in the Pr number may cause a decrease in the Nu numbers on the top and sidewalls of cavities. In the conduction dominated regions (stably stratified bottom parts of enclosure), the effect of fluid Pr number on heat transfer is more significant and it grows with increasing Ra number.