Assessment of an advanced near-wall treatment for URANS computations of natural circulation loops

Three-dimensional fluid dynamics simulations of a rectangular Natural Convection Loop (NCL) for nuclear cooling purposes have been carried out. The heating/cooling arrangement consists of vertical heater and cooler on opposite side legs of the loop at modified Grasshof numbers between 1011 and 1013. The conditions chosen are representative of those of nuclear power plant operating conditions, in which the flow reaches a fully turbulent state. The transient RANS models assessed include low-Re and high-Re forms. The latter type is combined with a recently developed, numerical variant of the analytical wall function (AWF) proposed by Katsamis et al. (2022). All the single-phase flow simulations include conjugate heat transfer analysis to account for the effects of heat conduction within the pipe wall. The near-wall and 3-D effects of the flow are investigated to enhance further understanding of these closed systems, important during the design stage. Validation is performed against experimental and recently published LES data. The numerical variant of the AWF has proven a valuable tool in modelling these flows, with modest grid requirements and still able to capture all the main features of the flow and thermal development.