Andrea Pucciarelli

Summary on the Results of Two Computational Fluid Dynamic Benchmarks of Tube and Different Channel Geometries

Two computational fluid dynamic (CFD) benchmarks have been performed to assess the prediction accuracy and sensitivity of CFD codes for heat transfer in different geometries. The first benchmark focused on heat transfer to water in a tube (first benchmark), while the second benchmark covered heat transfer to water in two different channel geometries (second benchmark) at supercritical pressures. In the first round with the experimental data unknown to the participants (i.e., blind calculations), CFD calculations were conducted with initial boundary conditions and simpler CFD models. After assessment against measurements, the calculations were repeated with the refined boundary conditions and material properties in the follow-up calculation phase. Overall, the CFD codes seem to be able to capture the general trend of heat transfer in the tube and the annular channel but further improvements are required in order to enhance the prediction accuracy. Finally, sensitivity analyses on the numerical mesh and the boundary conditions were performed. It was found that the prediction accuracy has not been improved with the introduction of finer meshes and the effect of mass flux on the result is the strongest among various investigated boundary conditions. [DOI: 10.1115/1.4038162]

Computational Fluid Dynamics Prediction of Heat Transfer in Rod Bundles With Water at Supercritical Pressure

The paper further explores the application of computational fluid dynamics (CFD) codes for the study of the heat-transfer phenomena involved when working with fluids at supercritical pressure; bundle analysis is considered here in particular. As for previous simulations performed by the authors considering heat-transfer deterioration inside heated tubes, this application points out the limited capabilities of the most commonly used Reynolds-averaged Navier–Stokes models when approaching the heat-transfer deterioration phenomenon. It must be noted that some of the considered experimental conditions, which are very close to the pseudocritical temperature, represent at the same time one of the most challenging situations for the CFD codes and a very common situation if supercritical water-cooled reactors (SCWRs) will be developed. Improvements of the currently available turbulence models are then needed. The paper analyzes the most likely causes of the observed insufficient quality of the obtained predictions. In addition to comparing the measured and calculated wall temperature trends, the effect of the presence of the spacer grids on the turbulent flow is considered. Spacers are in fact very important to assure the structural stability of fuel, though they also affect the flow, generally improving the turbulence conditions in their neighborhood and slightly impairing it in the downstream region. A comparison between predictions performed including or not including the spacers is also performed.