Adaptation of the Turbulence Model to the Heat Transfer in a Hexagonal Rod Bundle

Fast neutron reactors using liquid metal or gas coolants are one of the most actively studied fields of research. Increased safety requirements and insufficient knowledge of the thermohydraulic characteristics of these coolants require validation of RANS simulations via information from experiments or DNS. This information can also be applied to improve the RANS turbulence model if necessary. In this paper, a series of thermohydraulic DNS simulations of the uniformly heated hexagonal rod bundle with a relative pitch of 1.4 has been performed to improve RANS. A total of 12 test cases with four different Prandtl numbers of Pr=0.0043, 0.0154, 0.0324, and 0.71, which correspond to liquid metals and gas, and three Reynolds numbers of Re=2500, 5000, and 10000 are considered. The DNS calculations show good agreement with the experimental data for velocity distribution, turbulence intensity, Nusselt number, and friction factor. An intermittent flow regime was observed at Re=2500. Furthermore, the turbulent Prandtl number is structured with the maximum of up to y + ≈25, and from y + ≈50 onwards, in most cases takes on a constant value, while increasing the Prandtl or Reynolds number, the average value tends to 1.0. It was also revealed that the turbulent contribution to heat transfer is linear to Reynolds number for the selected coolant. As a result, the selected RANS model has been improved (the maximum integral error dropped from 49% to 4%) by the new model of the turbulent Prandtl number and by introducing model coefficients from DNS.