Numerical Study of Supercritical Water Heat Transfer in Vertical Bare Tubes Using Fluent CFD Code

Abstract

In this paper, a numerical study of heat transfer in supercritical water flowing upwards in vertical bare tubes using the Computational Fluid Dynamics (CFD) code FLUENT-12 is presented. A large dataset was collected for conditions similar to those of SuperCritical Water-cooled Reactors (SCWRs) at the Institute for Physics and Power Engineering at Obninsk, Russia.  This set includes 80 runs in a 4-m long, 10-mm inside diameter vertical bare tube within a wide range of operating parameters: pressure of 24 MPa, inlet temperatures from 320 to 350°C, values of mass flux ranged from 200 – 1500 kg/(m²s) and heat fluxes up to 1250 kW/m², for several combinations of wall and bulk-fluid temperatures, which were below, at, or above the pseudocritical temperature (381^oC at 24 MPa).Complete analysis of the SCW properties in the tube was done using an axisymmetric 2D-model of the tube with 10,000 nodes along the length of the tube for optimal results.  Two viscous models were used in the process: 1) k-ε model with enhanced wall treatment and pressure gradient and thermal effects, and 2) k-ω SST model with low Re corrections and viscous heating.  Results show a good fit for most low/mid range operating conditions with noticeable deviations at high range primarily in the deteriorated heat-transfer regime, with overall better fit for k-ε model.FLUENT showed a better fit for experimental results for the low heat and mass fluxes than empirical correlations, but FLUENT still shares the same problem in predicting the deteriorated heat-transfer regime accurately.  FLUENT also shows some deviation from the experimental data within the entrance region for high heat and mass fluxes, associated with the level of flow development in the tube which is attributable to entrance turbulence modelling.