A Blind, Numerical Benchmark Study on Supercritical Water Heat Transfer Experiments in a 7-Rod Bundle

Abstract

Heat transfer in supercritical water reactors (SCWRs) shows a complex behavior, especially when the temperatures of the water are near the pseudocritical value. For example, a significant deterioration of heat transfer may occur, resulting in unacceptably high cladding temperatures. The underlying physics and thermodynamics behind this behavior are not well understood yet. To assist the worldwide development in SCWRs, it is therefore of paramount importance to assess the limits and capabilities of currently available models, despite the fact that most of these models were not meant to describe supercritical heat transfer (SCHT). For this reason, the Gen-IV International Forum initiated the present blind, numerical benchmark, primarily aiming to show the predictive ability of currently available models when applied to a real-life application with flow conditions that resemble those of an SCWR. This paper describes the outcomes of ten independent numerical investigations and their comparison with wall temperatures measured at different positions in a 7-rod bundle with spacer grids in a supercritical water test facility at JAEA. The wall temperatures were not known beforehand to guarantee the blindness of the study. A number of models have been used, ranging from a one-dimensional (1-D) analytical approach with heat transfer correlations to a RANS simulation with the SST turbulence model on a mesh consisting of 62 million cells. None of the numerical simulations accurately predicted the wall temperature for the test case in which deterioration of heat transfer occurred. Furthermore, the predictive capabilities of the subchannel analysis were found to be comparable to those of more laborious approaches. It has been concluded that predictions of SCHT in rod bundles with the help of currently available numerical tools and models should be treated with caution.

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

“…Heat transfer to water at supercritical pressures is complex even in the simplest geometries, such as a straight vertically installed smooth bore tube ( [1][2][3][4]). This is attributed to three different heat transfer regimes that can be encountered: the normal heat transfer (NHT), enhanced heat transfer (EHT), deteriorated heat transfer (DHT) regimes.…”

“…Among these regimes, the deteriorated heat transfer regime is of the most concern because the heated solid surface could exceed its temperature limit (it is maximum 620 C for the most advanced nuclear grade materials, for, e.g., Inconel alloy). The most of the heat-transfer correlations have been developed for normal and enhanced heat transfer regimes ( [1,4]) while properly estimating the results for heat-transfer deterioration cases remains impossible. The advanced computational fluid dynamics (CFD) models have been applied in support of the understanding of the deteriorated heat transfer phenomena ( [3,5]).…”

“…The advanced computational fluid dynamics (CFD) models have been applied in support of the understanding of the deteriorated heat transfer phenomena ( [3,5]). At this point, the accurate calculation of this phenomenon remains challenging and there is no consensus on the choice of models to be used for modeling ( [3,4]).…”

“…Under distinct conditions ( [1,8]), the heat transfer deteriorates and undesirably high wall temperatures (hot spots) could occur on the heated wall of structural materials. Therefore, the accurate simulation capability of supercritical water is essential for the detailed design of supercritical-water-cooled reactors (SCWRs) ( [1][2][3][4]).…”

“…In case of such correlations, the extrapolation is not recommended as these correlations mimic the experimental data but do not cover the complex heat transfer Table 1 Thermal hydraulic parameter of the two CIAE experimental cases p ref (MPa) q w a (kW/m 2 ) G (kg/m 2 s) T pc ( C) T in ( C) T out ( C) q w /G (J/kg) Gr/Re a The heat flux defined at the inner surface of the tube. mechanism of supercritical water ( [4]). A more fundamental approach is needed to understand the phenomena and extend the prediction capability beyond the current experimental database in support of the SCWR development.…”