Applicability Evaluation of Turbulence Models for Gas-cooled Heat Transfer of Open Lattice Structure

The open lattice gas-cooled reactor presents a lightweight option for high-power space reactor power systems. The background of this study is a space reactor featuring a rod bundle core structure, with helium as the coolant. The typical Reynolds number at the core inlet is around 2 000. Reynolds average numerical simulation (RANS) is a commonly used computational fluid dynamics (CFD) method. The essence of the RANS method lies in turbulence models. Each turbulence model has its particular useful scenarios and needs to be chosen based on the specific working conditions. The helium-cooled rod bundle reactor is distinguished by its tight lattice structure and low flow Reynolds number. These features influence the flow and heat transfer characteristics in the reactor core. Consequently, when performing thermal-hydraulic analysis using CFD, it is essential to evaluate the applicability of turbulence models. Experiments of flow and heat transfer in 37-rod bundle structure were conducted, using electrically heated rods of the same size as the fuel rods and nitrogen as the experimental coolant. Based on these experiments, the convective heat transfer within the test section was numerically simulated using ANSYS Fluent, selecting four turbulence models: Realizable k-ε with enhanced wall treatment, SST k-ω, transition SST, and Reynolds stress model with enhanced wall treatment. The operating conditions for numerical calculations had inlet Reynolds numbers ranging from 688 to 2 986, all with uniform power distribution. By comparing the experimental measurements and calculated values of the heating rod cladding temperatures, the applicability of the four turbulence models was evaluated. Simultaneously, the differences in local flow field simulations by these models were observed, and an analysis was performed to understand the reasons behind the discrepancies in cladding temperature calculations among the different models. The results show that all four turbulence models generally underpredict the rod cladding temperatures. Among these models, the transition SST model exhibits the closest agreement with experimental data, with an overall average deviation of −2.0%. It effectively captures the crossflow characteristics between the rod bundle and is suitable for thermal-hydraulic simulations of open lattice gas-cooled reactor with Reynolds number around 2 000. This study confirms that the crossflow is an important factor affecting the flow and heat transfer in open lattice structures. Subsequent researches are needed to further investigate the factors and patterns influencing crossflow, in order to minimize its adverse effects on the heat transfer in the reactor core. The findings of this paper provide a reference for the numerical simulation and design of rod bundle gas-cooled reactors.