Numerical simulation of single-phase heat transfer and nucleate boiling of liquid sodium in narrow annulus channel similar to that of SFR fuel subchannel

Abstract

The sodium cooled fast reactor (SFR) is a Generation IV nuclear reactor that uses fast neutrons for fission and liquid sodium as the coolant for fission heat removal. Compared to conventional fluids, such as water, liquid sodium shows superior performance in heat transfer because of its high thermal conductivity and low Prandtl number. Understanding of single-phase and two-phase heat transfer characteristics of liquid sodium is essential for the analysis of the normal operation and certain postulated accidental events of SFR. This study is intended towards the development of computational fluid dynamics (CFD) based multiphase models for the safety analysis of SFR using the finite volume method. The numerical simulations of single-phase and two-phase heat transfer processes have been performed for the basic two-dimensional domain that has resemblance with the flow subchannel of SFR fuel subassembly. This study deals with heat transfer considerations under low Peclet number at high temperature, and its boiling inception and further to the nucleate boiling regime in a narrow vertical annulus channel. The Realizable k-ε model with Kays correlation for the turbulent Prandtl number better predicts the single-phase heat transfer characteristics under low Peclet number. A correlation for Nusselt number is fitted from the numerical results for low Peclet number flow conditions in an annulus channel. The flow boiling inception and nucleate boiling are numerically predicted by the Eulerian-Eulerian multiphase model with non-equilibrium wall-boiling for wall heat flux partitioning. The turbulence is modeled using Realizable k-ε model with standard wall function. Under the flow and heat flux values considered the near steady-state boiling conditions are predicted without much pulsating behavior as observed in the reference experiments and during this, the two-phase flow pattern varied from inception to the nucleate boiling regime. The mechanistic model based heat transfer coefficient is evaluated in the boiling zone from the flux partitioning components and compared with the available empirical correlations.